WO1993016477A1 - Nonproliferative light water nuclear reactor with economic use of thorium - Google Patents

Abstract

A light water nuclear reactor, which derives most of its energy from thorium, utilizes a seed-blanket core arrangement and a nonparasitic and mechanically simple control system. Neither the initial fuel loading nor the fuel discharged from the reactor is useable for nuclear weapons purposes. The initial fissile fuel is enriched uranium, U-235/U-238 (20:80), which is known to be nonproliferative. The discharged fissile fuel consists of (1) uranium with about ten percent U-235 content, (2) less than one percent of the amount of plutonium produced in conventional light water reactors, (3) U-233 denatured by being uniformly mixed with more than twice as much U-238 and (4) the remaining thorium. About seventy-five to eighty percent of the reactor energy is derived by fissioning in place the U-233 formed in the thorium, thus avoiding the very expensive process of extracting and fabricating the highly gamma-active U-233 into fuel elements.

Description

NONPROLIFERATIVE LIGHT WATER NUCLEAR REACTOR WITH ECONOMIC USE OF THORIUM

BACKGROUND OF THE INVENTION

Although thorium is known to be at least three times as plentiful as uranium in the earth's core, no economic method of producing nuclear power from thorium, with or without proliferative fuels, has been found. The term "economic" is used herein to mean that most of the nuclear reactor energy is generated from thorium without the very expensive process of extracting the highly gamma-active U-233 and fabricating it into fuel elements.

The fundamental difficulty in utilizing thorium as a nuclear fuel is that it contains no natural fissionable material. Thorium can be made to produce energy only by (1) an initial addition of fissionable material, as is described in the report entitled "Thorium Utilization in PWRS", ' ^■ Kernforschungsanlage Jύlich GmbH (1988) , or (2) providing a neutron current into the thorium regions of the core, using a "seed-blanket" arrangement, as described in the "CRC Handbook of Nuclear Reactor Calculations", 1986, Volume III, pp. 365-448.

These two known approaches are summarized briefly below:

1. In a Brazilian-German collaboration extending from 1979 to 1988 and reported in "Thorium Utilization in PWRS", supra, the entire reactor core was assumed to consist of thorium with the uniform addition of fissile material. The most favorable results in the study were for cases where the thorium was initially enriched with plutonium, an element which is well known to be proliferative. According to calculations, gains over a conventional uranium reactor were obtainable only by repeatedly extracting the U-233 formed in the thorium, fabricating it into fuel elements, and reinserting it into fresh thorium. Another possibility that was considered was to begin with U-235/U-238 in the volume ratio of 20:80 as the initial fissile fuel for the thorium. However, so much of this fuel would be required to provide a sufficient reactivity for the accepted time between reloads, twelve to eighteen months, that the amount of plutonium formed in the U-238 into the thorium would be appreciable. Again, extraction, fabrication, and reinsertion of the highly gamma-active U-233 into the thorium would be necessary.

2. In the second approach referred to above, seed- blanket core arrangements have been used as described in the "CRC Handbook of Nuclear Reactor Calculations", supra. Such cores consist of seed regions which have multiplication (criticality) factors greater than one and blanket regions with multiplication factors less than one. In the arrangements which have been studied the blanket regions have been constructed primarily of natural thorium and the seeds have contained either U-235 or U-233 of weapons grade quality. In these studies the cores have been controlled typically by upward motion of each seed region from a position well below the core. This method of control has resulted in severe mechanical problems because of the heavy weight of the seeds to be moved. Furthermore, heat removal is difficult because of great variations in the power levels throughout the length and width of the core.

SUMMARY OF THE INVENTION

It is a principal object of the present invention to provide a nuclear reactor which is "non proliferative"; that is, a nuclear reactor for which neither the initial fuel loading nor the discharged. spent fuel can be used to make nuclear weapons.

It is a further object of the present invention to provide a nuclear reactor which makes economic use of thorium as a fuel.

It is a further object of the present invention to provide a nuclear reactor which has an extra margin of safety over conventional reactors.

It is a further object of the present invention to provide a nuclear reactor which discharges substantially less high level nuclear waste than conventional reactors.

These objects, as well as other objects which will become apparent from the discussion that follows, are achieved, in accordance with the present invention by providing a nuclear reactor core having one or more seed regions containing seed fuel elements essentially comprising U-235 and U-238 in the maximum ratio which is nonproliferative; a blanket region surrounding the seed region(s) containing blanket fuel elements essentially comprising Th-232 with a small percentage of nonproliferative uranium; and a nonparasitic mechanically simplified control system, all of which are described in detail below.

1. Seed Regions: These regions contain fuel elements of U-235/U-238, preferably in the ratio of 20:80, in the shape of rods and/or plates consisting of uranium-zirconium alloy. The water to fuel element volume ratio is in the range of six to approximately ten, far above the accepted norms of approximately two to one in conventional reactors. The high water content results in a resonance escape probability of above 0.95 in the U-238. The reduction of plutonium output comes first of all from the change in enrichment. A change in enrichment from the conventional value of U-235/U-238 (3:97) to U-235/U-238 (20:80) reduces plutonium production by a factor of seven. See "Optimization of Once-Through Uranium Cycle for Pressurized Light Water Reactors", by A. Radkowsky, et al., Nuclear Science and Engineering, 75, pp 265-274 (1980) . The high value of the resonance escape probability of the seed fuel further reduces the rate of plutonium production by a factor of six. The high value of the resonance escape probability also results in a high value of the seed multiplication factor, which increases the proportion of energy obtained from the blanket to the range of seventy-five to eighty percent of the total core power. Taking into account that the seed regions produce only twenty to twenty-five percent of the core power, it is evident that the rate of production of plutonium in the seed regions is well below one percent of that in a conventional reactor. The seed regions also contain some blanket fuel elements and are referred to as "composite seed-blanket regions".

2. Blanket Region: The blanket region contains fuel elements of mixed thorium-uranium oxide rods and/or plates. The uranium oxide volume content in the thorium-uranium mixture is in the range of six to approximately ten percent. The uranium oxide is U-235/U-238 in the ratio of 20:80. The water to fuel volume ratio is in the range of .8 to 1.5. With this choice of parameters, the blanket multiplication factor stays approximately constant during an irradiation of about 100,000 MWD/T. An irradiation of this magnitude has been shown to be feasible by experiments in Oak Ridge, Tennessee. See "Irradiation Behavior of Thorium-Uranium Alloys and Compounds" by A.R. Olsen, et al., International Atomic Energy Report (1977) . The approximate constancy of the blanket multiplication factor is necessary for two reasons: (1) so that the blanket will produce its appropriate share of the core power from the beginning of core life and (2) for the proper functioning of the control system as explained below.

For economic power it is necessary that the blanket be left in the core for a long irradiation. Otherwise, each time that a new blanket is inserted, fissile uranium fuel must be added to avoid a large expenditure of seed neutrons to build up the thorium reactivity. The U-238 inserted in the thorium serves a further purpose by being mixed uniformly with and thus denaturing the U-233 remnant in, the thorium at the end of the blanket life. The plutonium production rate will be, at most, 0.6 percent of that of a conventional core (eight percent U-238 content times seventy-five percent blanket power share divided by ten to twelve years of the blanket residence in the core) .

The blanket fuel elements may be of solid cylindrical shape or of annular shape with the center hole open to the water. For the same fuel volume the annular shape has superior nuclear and heat removal characteristics, but this shape requires internal as well as external cladding.

In addition to the blanket region internal to the core, the term "blanket" is also used to describe the regions in the reflector around the core which are utilized primarily to reduce neutron leakage from the core. Such blankets will have fuel compositions and fuel element shapes similar to those described above, except that depleted uranium would be used instead of the U-235/U-238 (20:80). The purpose of the depleted uranium is to ensure that any U-233 formed in these reflective blanket regions will be denatured by U-238.

3. Nonparasitic Control System: A nonparasitic control system is provided to increase safety and maximize the amount of core energy obtainable from the thorium. This control system ensures that all neutrons available from the seed are utilized usefully in the core blanket region, thus minimizing the number of fissions required in the seed regions. This is in contrast to conventional cores in which all excess neutrons are wasted by absorption in parasitic control materials.

The control system requires a uniform motion of the control rods of only approximately forty-five centimeters throughout the core length, as contrasted with the travel over the whole core length, typically about twelve feet, of conventional control rods.

The operating principle of the control system according to the invention depends upon the fact that the seed regions have a high multiplication factor, with correspondingly high neutron leakage, such that the core reactivity is greatly affected by small changes in effective seed dimensions.

The preferred embodiments of the present invention will now be described with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic diagram of a pressurized water reactor power system of the type to which the present invention relates.

Fig. 2 is a schematic diagram of a seed/blanket core of a nuclear reactor of the type to which the present invention relates.

Fig. 3 is a diagram showing the neutron absorption probability of U-238 over a spectrum of neutron energies.

Fig. 4 is a diagram showing the multiplication factor of a natural thorium oxide blanket with respect to time as compared to that of a thorium oxide blanket having some initial fissile fuel.

Fig. 5 is a diagram showing the blanket energy production of various thorium and uranium blankets for given inputs of seed neutrons.

Fig. 6 is a diagram showing the wasted neutrons over time in a nuclear reactor core controlled by conventional means.

Fig. 7, comprising Figs. 7a-7d are schematic diagrams of a single seed/control/blanket assembly illustrating the principle of the nonparasitic control system of the invention. These Figs, show the maximum and minimum reactivity positions, respectively, of the control system.

In Figs. 7a and 7b, the control system depicted indicates the movement of both seed type fuel (20% Uranium- 235, 80% Uranium-238) and blanket fuel (thorium-uranium oxide) in the operation of the control system. In Figs. 7c and 7d, seed type fuel elements only are moved in the operation of the control system.

Figs. 8a and 8b are horizontal sections (plan views) of a portion of a nuclear reactor core according to the invention showing respectively two equally preferred embodiments, which will be referred to for convenience as first and second preferred embodiments.

Figs. 9a and 9b are vertical sections (elevational views) of one-half a nuclear reactor core showing the first and second preferred embodiments of Figs. 8a and 8b, respectfully, for the first seed cycle and each subsequent odd numbered seed cycle. Similarly, Figs. 9c and 9d apply to the second cycle and each subsequent even numbered seed cycle.

Figs. 10a and 10b, corresponding to Figs. 9a and 9b, are representational elevational views showing a portion of the control regions in their maximum reactivity positions. Figs. 10c and lOd apply similarly to Figs. 9c and 9d.

Figs. 11a to lid, corresponding to Figs. 10a to lOd, are representational elevational views showing the control regions in their minimum reactivity positions.

If the control scheme shown in Figs. 7c and 7d is utilized, Figs. 9 to 11 apply except that movable blanket tyupe fuel elements are omitted. DESCRIPTION OF THE PREFERRED EMBODIMENTS

The essential concepts as well as the preferred embodiments of the present invention will now be described with reference to Figs. 1-11 of the drawings.

Fig. 1 schematically illustrates a pressurized light water nuclear reactor power system (pressurized water reactor or "PWR") of the type to which the present invention relates. As may be seen, this system comprises two fluid circuits between the nuclear reactor, which is the heat source, and a steam turbine which drives an electric generator. The primary fluid circuit maintains ordinary (light) water under pressure to prevent the formation of steam. This water is heated in the nuclear reactor pressure vessel and supplied to a steam generator which transfers heat energy to ordinary (light) water of the secondary fluid circuit. The water in the secondary circuit is converted to steam which is used to drive the steam turbine. Systems of this type are well known and are described in detail, for example, in Nuclear Fuel Management, H.W. Graves, Jr. , John Wiley & Sons, New York (1980) .

The present invention relates specifically to the nature of the nuclear reactor core. As is well known, the reactor core is fueled by a fissionable (fissile) material such as the isotope uranium-235 (U-235) . Since natural uranium contains only about 0.7 percent U-235, the rest being nonfiεsionable U-238, this natural uranium is "enriched" until the U-235 is about 3 to 4 percent of the total. In a conventional reactor, a sufficient amount of such enriched uranium fuel can provide enough energy for a year to eighteen months of reactor operation.

Since the element uranium corrodes with almost explosive force when coming in contact with the hot water used for cooling, the uranium cannot be used in metallic form. Instead, uranium oxide is used, usually in the form of 1 cm. diameter rods clad in zirconium, a metal which has good corrosion resistance and very little neutron absorption. It is also possible to use a metallic alloy of uranium and zirconium, either in the form of rods or plates.

There are two possible arrangements for the uranium oxide fuel elements in the nuclear reactor core. The most common arrangement is for all the uranium rods or plates to have the same enrichment. Another arrangement, which is illustrated in Fig. 2, includes a number of small islands of moderately enriched uranium, having a reactivity greater than one, surrounded by regions of fertile material which have a reactivity less than one: for example, natural uranium or thorium.

This type of arrangement has come to be called a "seed- blanket" core, the islands being called "seeds" and the surrounding region the "blankets". Since the blanket regions have a reactivity of less than one and the seed regions a reactivity greater than one, the seeds supply the neutrons needed to keep the blanket neutron population at a high enough level to generate the fissions necessary for the rated power. Seed-blanket cores have operated successfully for over 30 years at the world's first commercial nuclear power plant at Shippingport, Pennsylvania.

There are several advantages of a seed-blanket core over a conventional, uniform core: (1) less total enrichment is needed; (2) control rods are required only in the seed regions since the blanket region is subcritical; and (3) at each refueling (normally each year or 18 month period) only the seeds have to be replaced. The major part of the core - that is, the blanket region - can remain in place for a number of years (normally 10 to 12 years) . As a result, there is a saving in fuel manufacturing cost.

So far, all efforts to utilize thorium economically have been unsatisfactory, even without attempting to be nonproliferative.

The aforementioned ten-year Brazilian-German cooperation program on thorium utilization is typical of past attempts. Since thorium has no natural fissionable content, the first remedy would be to add some U-235; however, much more U-235 would be needed than in natural uranium because of the higher thorium absorption probability. Pure U-235 is undesirable because it iε proliferative; i.e., it can be used in nuclear weapons. A low enrichment of uranium could be used, but then so much space would be needed for the accompanying U-238 that there would be little space left for the thorium. (Thorium oxide and uranium oxide have about the same density.)

Consequently, it was proposed to add plutonium oxide to the thorium oxide, since the plutonium has no accompanying U-238. Plutonium can be obtained from conventional reactor discharged fuel. The German-Brazilian concept was to start operation with plutonium for about a year, reprocess the thorium to recover the U-233, which had been created in the meantime, fabricate the U-233 into fuel elements and then use these elements with fresh thorium and a reduced amount of plutonium. This operation could be continued and gradually the reactor could be run almost entirely on U-233. Such a procedure is, of course, (1) very expensive because of the high cost of making the U-233 and plutonium fuel elements, as has been explained above, and (2) proliferative at every stage. Another aspect in the proposed program was that no advantage would be taken of the high metallurgical resistance of thorium oxide, since the thorium was to be melted down for reprocessing after each year or so of operation. The Brazilian-German effort was eventually discontinued because Brazil decided not to reprocess plutonium from fuel discharged from its reactors.

With the present invention, first of all, the U-233 formed in the thorium is fissioned ("burnt") in place so that it is not necessary to fabricate U-233 fuel elements. Second, for economic reasons as much energy as possible is obtained from the thorium. Third, to fulfill both economic and nonproliferative objectives, the thorium in the form of oxide is retained in the core for its full metallurgical lifetime. If fissionable material were added to the thorium to make it critical (reactivity greater than one) for such a long lifetime, so much would be required that there would be no space for the thorium. The present invention therefore employs a seed-blanket core arrangement, as shown in Fig. 2, so that the thorium in the form of oxide can be left as a blanket in the core for 10 or more years, and only the seed regions need be replaced at the end of a normal refueling period. The blanket is always subcritical with a reactivity of about 0.9, which is designed to be nearly constant during operation. The seed regions must therefore supply about 10% of the blanket neutron population.

For the seed regions, an objective of the present invention is to keep the plutonium production rate to a minimum: to about 1 to 2% of that of a conventional reactor core. The seed regions therefore utilize 20% enriched uranium, (20% U-235 and 80% U-238) ; that is, approximately the highest enrichment of uranium which is nonproliferative.

The enrichment in the seeds is made as high as possible for two reasons. First, every neutron absorbed in U-238 eventually results in plutonium. The high amount of U-235 competes with the U-238 and reduces the number of neutrons going into U-238. This also makes available more neutrons for the blanket. Second, about four times as much cooling water is used in the seed region as is used in a conventional reactor core. Fig. 3 shows the neutron absorption of U-238 versus neutron energy, evidencing that U-238 has sharp lines, called resonances, at higher energies, where the absorption of neutrons, to make plutonium, is most intense. By providing a very large amount of water in the seed regions and thus slowing the neutrons, the high energy fission neutrons are reduced to low energies, bypassing the resonances. In addition, because thorium has resonances similar to those of U-238, the low energy neutrons coming from the seed regions to the blanket regions bypass the blanket resonances and are thus used more efficiently. While the water to fuel volume ratio in the seed regions is higher than in a conventional core, that in the blanket regions is lower, so that over-all core volume is no greater than that of a conventional core of the same power output.

To summarize, two objectives are served by the relatively high (20%) enrichment of the seed fuel: (1) the reduction to a very low level of the amount of plutonium created in the seed regions, and (2) (for a given power generated in the seed regions) maximizing the number of neutrons into the blanket so as to increase the amount of energy generated from the thorium.

In regard to the blanket design, instead of using pure thorium oxide, a few percent of 20% enriched uranium oxide is initially added to the fuel elements. This again has two purposes. Without the uranium, the thorium would be "dead" at the beginning, since it contains no fissionable material. Consequently, all the power would have to be generated in the small seed regions, and overheating would result. By enriching the thorium, the blanket immediately starts to generate power and, as the U-233 content builds up, the blanket maintains an almost constant reactivity for very high burn-up, over a period of 10 to 12 years. This effect is illustrated by the two curves in Fig. 4. The blanket power is maintained by burning the U-233 as it is formed in place. At the end of blanket life, the original U-235 content will have long since fissioned, but the nonfissionable U-238 will have remained and combined uniformly with the remanent U-233 to make it useless for weapons purposes. At the same time, there will be too little U-238 to make any appreciable amount of plutonium. Thus, there will be no incentive to reprocess the blanket, and it will be discarded, like other nuclear waste.

As shown in Fig. 5, for a given input of neutrons from the seed, a thorium blanket produces nearly twice as much energy as does a natural uranium blanket. Also, the thorium blanket with a small amount of U-235, as in the present case, starts much higher and remains higher in energy output than a natural thorium blanket.

An important aspect of the present invention is the system of control which results in major gains in safety and in reduction of costs, as well as advancing the objective of nonproliferation. This control system actually overcomes a basic defect in the control method of conventional power reactors. First, it must be understood that in any reactor it is necessary for practical purposes to add enough fuel to the reactor core at the start of a cycle so that it will last at least a year or 18 months, until shutdown for refueling. Consequently, the core initially must contain much more than the amount of enriched uranium needed to just sustain a chain reaction (reactivity of 1.0). In order to prevent the extra fuel from being operative until needed, "control" materials with high neutron absorption are inserted into the core. These materials simply absorb neutrons wastefully, as is illustrated in Fig. 6. For example, boric acid, which has a very high neutron absorption, is added to the water in the core at the beginning and gradually removed during the core lifetime. Not only does the use of boron waste neutrons, but small boron leaks cause safety problems, such as interfering with the operation of vital valves. See NRC letter IN 86-108, Supplement 2 of November 19, 1987. In addition. conventional control systems use control rods for rapidly inserting a neutron absorber into the core. Such control systems are subject to mechanical difficulties since the control rods are commonly 36 feet long and must be able to insert thin pins, about 1 cm. in diameter, a distance of 12 feet into the core. Any bending or distortion of the pins can prevent a control rod from entering the core, thereby causing a safety problem.

The control system according to the present invention is mechanically simple and ensures that all neutrons originating in the seed are absorbed usefully in the thorium to make U-233. In particular, the control system is entirely "nonparasitic"; i.e., nonwasteful of neutrons.

The control system according to the present invention may be visualized as a kind of "Venetian blind" in which each control element has to move only a small distance to go from "light to dark", from high reactivity to shutdown. In contrast, the control rods in a conventional core are like a "window shade" in having to traverse the whole length of the core to go from maximum to minimum reactivity.

Fig. 7 illustrates schematically the method of operation of the nonparasitic control system. The seed is divided into vertical layers each approximately 45 cm. long. If we number successive layers as #14 and #15, each #14 layer has higher fuel density in the seed fuel elements than in the #15 layer. Fig. 7a shows the position of maximum reactivity. Movable seed fuel elements in the center of the seed on the #14 layers are connected by zirconium extensions, located in the #15 layers. Movable blanket (mixed thorium uranium oxide) fuel elements in the center of the seed on the #15 layers are connected by zirconium extensions, located in the #14 layers. The movable blanket fuel elements are positioned on either side of the movable seed extensions.

Fig. 7b shows the position of minimum reactivity (shutdown) . The movable seed elements are now located in the #15 layers, and the movable blanket elements are now located in the #14 layers between the stationary seed fuel. The reactivity of the core has been decreased because: (1) the movable high density seed fuel has moved to a volume of lower multiplication factor; and (2) The regions of stationary high density seed fuel elements are now separated by blanket fuel, causing these regions to have a lower effective thickness and thus much higher leakage of neutrons to blanket fuel.

It will be seen that all excess neutrons from the seed are usefully absorbed in the thorium to create U-233, and there is no parasitically absorbing control material. Since no neutrons are wasted, the necessary seed power is reduced and the blanket power increased, which fulfills the objectives of the nuclear reactor. Seed power is expensive and produces a small amount of plutonium. Blanket (thorium) power is inexpensive and does not produce plutonium.

The control system according to the present invention is also much simpler mechanically than conventional control systems for nuclear reactor cores. In this connection, the pressure vessel is one of the most expensive items in a nuclear power plant. The present control system enables the pressure vessel height to be reduced with consequent lower cost. Thus, in addition to the nuclear gains, the present control system both improves safety and reduces the initial construction cost.

Fig. 8 shows two preferred- geometries for the composite seed-blanket regions according to the present invention: In Fig. 8a relatively small annuli and in Fig. 8b much larger and relatively narrower annuli. Seed fuel elements 11 are surrounded by blanket fuel elements 12. The control assemblies 13 are located in the center of the annuli.

Figs. 9a and 9b show the vertical structures of the stationary portions of the composite seed-blanket assemblies of Figs. 8a and 8b, respectively. These assemblies are made up of alternating forty-five centimeter thick layers 14 and 15. Layer 14 consists primarily of seed fuel elements. Layer 15 consists of blanket fuel elements and seed fuel elements of reduced uranium content. Since it is necessary to refuel the seed at intervals of twelve to eighteen months while the blanket fuel remains in the core for ten to twelve years, the following construction is adopted to permit separate removal of the seed fuel. Advantage is taken of the large spacing of e seed fuel elements. As shown in Figs. 10 and 11, stationary seed fuel elements 16 consist of a sequence of forty-five centimeter lengths of uranium- zirconium alloy 17 alternating with forty-five centimeter lengths of reduced content uranium-zirconium alloy 18 throughout the length of the core. Thus all the seed fuel elements 16 can be removed from the core and replaced by fresh fuel, while leaving all the blanket fuel elements in place.

Figs. 10 and 11 also show the details of the nonparasitic control system. The movable seed fuel elements 19 of the control assembly 13 consist of a sequence of forty-five centimeter lengths 20 of uranium-zirconium alloy alternating with forty-five centimeter lengths 21 of pure zircalloy throughout the length of the core. The movable blanket fuel elements 22 of the control assembly 13 consist of a sequence of forty-five centimeter lengths 23 of thorium-uranium oxide alternating with forty-five centimeter lengths 24 of pure zircalloy throughout the length of the core. These blanket fuel elements 22 extend between the seed fuel elements 16 and 19. The spacing of uranium- zirconium lengths 20, when opposite the layers 14, takes into account the water displaced by zircalloy connectors 24. To operate the control system in order to reduce reactivity to the minimum value, the seed fuel elements 19 of the control assembly are moved down forty-five centimeters from layers 14 to layers 15. As the seed fuel elements 19 move down, the blanket fuel elements 22 move from layers 15 to 14. Just the opposite motion is used to increase reactivity.

Both the blanket and seed fuel elements of the control system have yoked drives 25 and 26 (Fig. 9) , which move together while the reactor is in operation. During shutdown for seed refueling the drives can be unyoked and the seed fuel elements removed and replaced without disturbing the blanket fuel elements of the control system.

An important feature of the invention is the provision of uniform axial depletion of the blanket fuel. It is evident that, since the seed fuel is of lower density in layer #15 than in layer #14, there will be lower seed power in layer #15 and hence fewer neutrons supplied to the blanket, resulting in lower blanket power on that level.

For the movable blanket fuel there is no problem. When a fresh seed is inserted (seed reactivity a maximum) , the moving blanket fuel will be located in layer #14. As the seed depletes, the moving blanket fuel will gradually descend to layer #15. Thus in the course of a seed lifetime, the moving blanket fuel will experience approximately equal exposure to the seed fuel on both layers.

For the stationary blanket fuel, in order to ensure uniform axial depletion, each successive seed has the relative positions of the #14 and #15 layers reversed, as shown in Figs. 9c and 9d, 10c and lOd, and lie and lid. For the proper functioning of the control system, it is necessary only to raise or lower the drive for the moving blanket fuel by the approximately 45 cm. length of each layer. Thus in the course of the blanket lifetime, which will involve many seed replacements, each layer of the stationary blanket will experience ap^proximately equal depletion.

For the annuli of Fig. 8a, either a separate control drive 28 may be provided for each annulus, or a common control drive may be provided for two or more annuli. In the annuli of Fig. 8b, a number of separate control drives 28 may be provided as shown.

It should be noted that it is important that the multiplication factor of the blanket fuel remain approximately constant throughout core operation. Otherwise the effectiveness of the control system would have wide variations as the thorium multiplication factor increases from nearly zero to a value close to one.

Typical dimensions for the preferred embodiments illustrated in Figs. 8-11 are set forth in Table I below: TABLE I

TYPICAL DIMENSIONS

Figure 8a:

Seed Fuel Assembly

Distance Across Flats, cm 20

Number of Assemblies in core 69

Volume Ratio; Inner to Outer Region 25%

Inner Reflector, cm 7.5

Outer Reflector, cm 15

Figure 8b:

Seed Fuel Annulus

Thickness, cm 14

Number of Seed Annuli 3

Inner Reflection, cm 7.5

Figures 9a:

Active Core Height, cm 360

Number of Axial Layers 8

Height of Axial Layer, cm 45

Number of Control Mechanism 69

Figures 9b:

Active Core Height, cm 360

Number of Axial Layers 8

Height of Axial Layer, cm 45

Number of Control Mechanisms 48

Figures 10 and 11:

Parts 16 and 19, Diameter = ,mm 7.2 Parts 23 and 27, Thickness = mm 3.5

Cladding Thickness, mm 0.5

Composite Thickness, mm 2.5 min Part 24 minimum required by mechanical considerations

All of the above dimensions are to be considered relatively important in the respective embodiments since they affect (1) the control characteristics and (2) the neutron currents between the seed and blanket regions and the neutron leakage from the core, which in turn affect the fraction of core power produced by the blanket.

The dimensions of each of the seed regions are set by a compromise between minimizing the number of seeds so as to simplify the core design, yet having enough seeds to provide as uniform a power distribution as possible within the blanket.

The height of the axial layers, which is also the length of the stroke of the control mechanism, is set by a compromise between making the control stroke as small as possible, yet not having the sensitivity (change of reactivity per unit length) so large as to cause problems in the control drive mechanism.

Table II sets forth typical operating parameters for a 1300 megawatt electric pressurized water reactor employing the principles of the present invention.

TABLE II OPERATING PARAMETERS FOR A TYPICAL 1300 MWe PWR

Seed Fuel Pin Diameter, mm 7.2

Blanket Fuel Pin Diameter, mm

Outer 14.4

Inner 0.5

Cladding (zircalloy) Thickness, mm 0.56

Moderator-to-Fuel Volume Ratio

Seed 8.0

Blanket 1.2

Temperatures °K (°F)

Fuel 980 (1305)

Coolant 567 (560)

Cladding 630 (675)

Power Density

Kw/1 of core 90 w/cm core height 250

Equivalent Core Radius, cm 186

Active Core Height, cm 360

Core Material Densities 95% theoretical

MATHEMATICAL BASIS OF THE INVENTION

The mathematical basis for the present invention is described in the chapter entitled "Seed-Blanket Reactors", CRC Handbook of Nuclear Reactor Calculations, Volume III, CRC Press, pp. 365-448 (1986). It should be recognized that with the advent of high speed computers explicit mathematical formulae are no longer in common use today for practical reactor core design calculations. However, such formulae do provide physical insights and are therefore included below where they may be helpful.

Instead of such formulae, elaborate reactor codes of high accuracy, checked by experiment, are in general use. The principal codes employed in the development of the present invention were WIMS, RABBLE, DOT and ANISN. (See J.R. Askew, F.J. Fayers, and P.B. Kenshell, "A General Description of the Lattice Code WIMS", J. Br. Nucl. Energy Soc.. 5(4) 571 (1966); P.H. Kier, and A.A. Robbs, "RABBLE, A Program for Computation of Resonance Absorption in Multiregion Reactor Cells", ANL-7326, Argonne National Laboratory, Argonne, 111. (1967); W.A. Rhoads, et al., "DOT- Two Dimensional Discrete Ordinates Radiation Transport Code", ORNL CCC-276, Oak Ridge Laboratory, Oak Ridge, Tenn., (1976) and W.W. Engle, Jr., "ANISN - A One-dimensional Discrete Ordinates", Transport Code with Anisotropic Scattering, K-1699, Oak Ridge National Laboratory, Oak Ridge, Tenn., (1967). Seed Region: a. The principal source of plutonium in the seed is the capture of neutrons by the resonances of the U-238, which forms eighty percent of the uranium fuel of the seed. Of all neutrons created by fission, the fraction of neutrons which escape such capture by U-238 may be denoted by p, the resonance escape probability. Then 1 - p is the fraction of neutrons captured by the U-238, resulting in the formation of plutonium. p may be written approximately as: p = e-( _F / _w) , where A is a constant depending on the fuel element composition, V_F is the fraction of fuel volume and V_w is the fraction of water volume. It is evident that as V_F/V_W decreases, p approaches the value of 1. With the present invention, with a range of V_F/V_H between 6 to 10, the minimum value of p is 0.95 so that 1 - p = 0.05. Thus, the production rate of plutonium in the seed region is extremely low. b. The seed multiplication factor, k_s, is given by the traditional four-factor formula: k_s = η f P e , where η is 2.06, being the number of neutrons emitted per neutron capture by U-235, e is the so-called "fast effect" and is close to unity and f is the thermal utilization whose value varies with the amount of seed uranium and the fraction of burnup. p is the resonance escape probability, as noted above. Thus k_s reaches a maximum as p approaches unity. Blanket Region: a. The water to fuel volume ratio in the blanket (in the range of 0.8 to 1.5) and the fraction (in the range of 6 to 10 percent) of uranium oxide (U-235/U-238 in the ratio of 20:80) are chosen so as to keep the blanket multiplication factor, k_B, as high and as constant as possible over the entire blanket lifetime of 100,000 MWD/T. The blanket multiplication factor k_B is defined as usual as the number of neutrons produced per neutron absorbed. Many complex factors are involved so that the optimum choices must be determined by computer calculations. Representative curves are given on pp 384-5 in "Seed-Blanket Reactors", CRC Handbook of Nuclear Reactor Calculations, Volume III, CRC Press, (1986) . However, it is clear that the water to fuel volume ratio must not be so small as to present cooling problems and not so large that too many neutrons are captured by the water or protactinium. b. The ratio of blanket to seed power is of prime importance in determining the energy derived from thorium. A simplified formula which is quite accurate for large reactors that have only small neutron leakage out of the core is as follows: P_s k_s 1 - k_B k_BS

Here P_B is the power in the blanket, P_s is the power in the seed, k_β is the multiplication factor of the blanket and k_s is the multiplication factor of the seed, *k_BS is related to the current of thermal neutrons from the blanket to the seed.

In the prior known seed-blanket reactors the sign of ⁵k_BS is negative; however, with the present invention, because of the very high water content of the seed, the sign of ⁵k_BS is positive. The magnitude of ⁵k_BS is about 0.25, but it strongly influences the ratio of blanket to seed power, as will be seen in the following numerical example. The lowest value of k_s (when the seeds are about to be discharged) is about 1.4. The average value of P_B is about 0.93. Due to the inclusion of the *k_BS term, the ratio of P_B to (P_B + P_s) is over 0.8, so that more than eighty percent of the core power is derived from the blanket. c. To calculate the plutonium production in the blanket, it is assumed that the U-238 will absorb about as many neutrons as a similar amount of U-238 in a conventional uranium reactor core. The maximum amount of U-238 in the blanket is eight percent (taking the upper range of ten percent uranium content in the blanket) . Since the blanket will stay in the core at least ten years, the plutonium production rate per year will be 0.8 percent of that of a conventional core. The rate of production is actually about 0.6 percent of a conventional core (i.e., 0.8 x 0.75) since the blanket produces approximately seventy-five percent of the power of a conventional core. Nonparasitic Control System:

The control system motion of approximately forty-five centimeters was calculated on the basis of highly accurate codes ANISN and DOT 4.2, utilizing fifteen energy groups.

The neutrons in a reactor are distributed over a wide spectrum of energies ranging from over a million volts to a fraction of one electron volt. To make sure that all these neutron energies are properly treated, the spectrum of neutron energies is divided into a large number of groups. In the present calculations, it was found that increasing the number of groups above fifteen made no appreciable difference in the results. Thus, it was concluded that the use of fifteen neutron energy groups was adequate.

The calculation results showed that increasing the motion of the control system above forty-five centimeters did not increase the amount of control available and would merely add mechanical complexity. Reducing the stroke below forty-five centimeters rapidly decreases the amount of control available, and increases the change of reactivity per centimeter. This necessitates finer control of the control system motion and again adds to mechanical complexity. Thus, approximately forty-five centimeters has been found to be the ideal length for the control rod motion.

THORIUM FUEL USED IN THE INVENTION

The nuclear reactor core according to the present invention obtains about seventy-five percent of its power from thorium or Th-232. Therefore, some words of explanation about this fuel are appropriate.

Thorium is quite widespread in nature. The ores of interest contain five to eight percent thorium, as contrasted with one to four percent for uranium ores.

The thorium utilized in the present reactor core blanket is in the form of oxide, just as uranium oxide is utilized in conventional cores. The manufacturing processes for thorium oxide and uranium oxide are very similar. Thus no new techniques or tools are required for manufacturing thorium fuel elements.

The important ways in which thorium differs from uranium are:

1. Thorium is at least three times as abundant as uranium. There are major supplies in India and Brazil. Very little prospecting for thorium has been done since its market price is very low.

2. Natural thorium contains absolutely no fissionable material. 3. Thorium has about three times the neutron absorption probability of U-238.

4. When thorium absorbs a neutron, after about one month it transmutes to U-233, a fissionable form of uranium. The U-233 can be used for weapons, just as U-235 and Pu-239. For reactor use, U-233 is superior since it emits about 10% more neutrons per neutron absorbed than either U-235 or Pu- 239.

5. One disadvantage of U-233 is that it emits intense gamma radiation. For this reason, fabrication of U-233 into fuel elements must be done remotely, behind heavy shielding, a very expensive process. In contrast, U-235 can be handled without any special precautions. The handling of plutonium requires the use of face masks to prevent inhalation, so that plutonium fabrication is more expensive than for U-235, but much less expensive than for U-233.

6. Thorium oxide has superior metallurgical properties to uranium oxide, in that thorium oxide can withstand 10% or more of the atoms fissioned, more than twice as much as for uranium oxide. This is because thorium oxide forms a perfect cubic lattice, which is very strong, while uranium oxide has a structure with many irregularities. The present invention takes advantage of this property of thorium.

7. Thorium oxide has a higher melting temperature, as well as better thermal conductivity, than uranium oxide, which results in a greater resistance to meltdown in case of a loss of coolant accident.

ADVANTAGES OF THE INVENTION The principal advantages of the present invention over conventional nuclear reactors may be categorized as follows:

1. Nonproliferation: The United States Department of Defense is understandably concerned about the tonnages of plutonium generated by today's reactors. An even greater danger is posed by countries like Japan, which are planning to build sodium cooled fast breeder reactors that will produce vast quantities of weapons grade plutonium, only few kilograms of which are needed for a nuclear bomb.

2. Economics: The main item in the cost of operating a conventional nuclear reactor today is the uranium fuel. The cost of fueling a core constructed in accordance with the present invention will be reduced by at least 2/3 since only 20 to 25% of the useable energy will be obtained from uranium. The cost of fueling the core will also be reduced because 3/4 of the core (the thorium blanket region) will last for 10 to 12 years instead of the three years of a conventional core. Other substantial savings are also available in the initial cost of constructing the core.

3. Safety: Conventional nuclear reactor cores can be described as "waiting for an accident to happen". Both the soluble boron control system and the mechanical control system of conventional cores present quite obvious dangers.

4. Nuclear Waste: The nuclear reactor according to the present invention discharges less than half the high level nuclear waste than conventional reactors.

Each of these four categories will be discussed below in detail. 1. Nonproliferation

The seed fuel employed in the reactor core according to the present invention is 20% U-235/80% U-238. This is the type of fuel which the U.S. Department σf Energy specifies for all research reactors, since even an infinite quantity of this fuel could not produce a nuclear-explosion. As this fuel burns, the ratio of U-235 to U-238 is reduced.

The fuel discharged from the blanket cannot be used for nuclear bombs for two reasons: a. The only fissionable fuel created in the blanket is U-233, but it will be denatured by being uniformly mixed with relatively large amounts of nonfissionable isotopes which are: the U-238 that was included in the blanket at the start, and U-232 and U-234, which are created during operation. b. The U-233 discharged from the blanket will be accompanied by extremely intense gamma radiation. For this reason alone it would be impracticable to build a useful nuclear weapon from the U-233 because of the great weight of gamma shielding required for handling and personnel protection.

After U-233 is created in thorium, its gamma activity increases with time. This is an exception to the general rule that radioactivity decreases with time. The reason is that the gamma activity is not really due to the U-233 itself, but to the isotope U-232 which builds up by secondary reactions leading to products which have high gamma activity. The total amount of high level nuclear waste radioactivity discharged from the thorium is still well below the proportionate amount from a conventional core, as is explained below in connection with nuclear waste. 2. Economics

The economics of nuclear power are made up of two components: operating costs and capital costs.

As regards fuel costs at present, a conventional light water nuclear electrical power plant spends about $90,000,000 a year to replace one third of the reactor core. In the core according to the present invention, where only the relatively small seed regions are replaced each 12 to 18 months, this amount is reduced by half. The blanket will be replaced once in ten to twelve years. Its cost will be much less than that of the seed regions and will be spread over many years. The basic reason for this economic advantage is that about seventy to eighty percent of the energy is obtained from the thorium which, at present, is essentially "free". This load factor is achieved by using a nonparasitic control system, which greatly reduces the number of neutrons required from the seed. Another point is that the fuel discharged from a conventional reactor is discarded, because most of its fissile content is plutonium which is too expensive to use and the processing of which was prohibited by the U.S. Government. With the present invention, the discharged seed fuel will still have about ten percent U-235 content, and almost no plutonium. This fuel can be readily stripped of fission products and reenriched to twenty percent U-235 with very little cost in separative work. Other costs of conventional cores that the present invention avoids are the replacement of control absorbers and the rearrangement of fuel assemblies.

In the area of capital costs, the core design according to the present invention results in a saving of about fifteen to twenty percent of the total plant costs. This saving is attributed to (1) the elimination of the soluble boron system with thousands of feet of pipe, mixing tanks, filters, injectors, etc. ; (2) the reduction in the cost and complexity of the control rod drives; (3) the reduced height of the pressure vessel and (4) the resultant reduced size of the containment. As is well known, the so-called "load following" in a conventional reactor core is both slow and cumbersome due to the soluble boron control system. This is particularly so at the beginning of an operating cycle when there is a lot of boron in the core. In the core according to the present invention, on the other hand, the so-called "throttle control" technique can be used. This means that if there is an increase in power demand, the throttle is opened allowing more cold water to flow into the core increasing the reactivity and then the power level. With a conventional core the cooling water increases the density and the concentration of the dissolved boron reducing the reactivity. To overcome this difficulty in conventional cores, additional special control rods ("half" rods and "gray" rods) are installed at considerable extra expense. Further, the slow response to power demand changes means that some power is wasted, increasing operating expenses. 3. Safety

The reactor core concept according to the present invention is superior from the safety standpoint to conventional light water reactor cores in the following respects:

In conventional light water reactors, control rods and drive mechanisms extend approximately three times the core height of about twelve feet (i.e., a total of thirty-six feet) for a 1000 MWe rating. Each typical rod terminates in twenty-seven absorbing pins, each twelve feet long and one centimeter in diameter, which must be inserted into holes in the fuel assemblies. It is evident that driving such thin pins from more than twenty-four feet away involves a risk that the pins will suffer some distortion which could prevent them from penetrating the core. Furthermore, to shut down the core quickly, as in the case of a so-called "loss of flow accident" (LOFA) , the rods must go all the way into the core.

In contrast, the control system according to the present invention requires a movement of only about forty- five centimeters and therefore will shut down the core much more quickly. The present arrangement is also such that distortion is much less likely.

In case of a loss of flow accident (LOFA) , the core according to the present invention has several points of superiority. The seed regions with their high neutron leakage will behave much like small cores. The water in the seeds will start to boil first, resulting in a quick reduction of reactivity. The fuel elements in the seed regions are preferably of metallic, uranium-zirconium alloy, which have much less stored heat than the ceramic, uranium oxide fuel elements of conventional reactors. Even the blanket region of the present core has an advantage over conventional cores, since thorium oxide has higher thermal conductivity than uranium oxide. Conventional light water reactors now utilize boric acid in the coolant to control the reactivity and power level of the core during operation. As previously stated, it has been found that small boron leaks accumulate and corrode high strength steel parts such as those used in cooling pumps and valves. The presence of boron in the coolant interferes with efficient load following. Nevertheless, the industry has not been able to eliminate soluble boron control, probably because such elimination would entail the addition of many more control rods with the attendant mechanical complexity, described above. In the reactor core according to the present invention, in which soluble boron control is not required, the reactor control system is nevertheless much simpler mechanically than that of conventional reactors.

Another problem with soluble boron control is that, in case of a LOFA, the emergency coolant supply might be left unborated, thus pouring fresh water into the core and resulting in a severe reactivity surge.

Although conventional light water reactors, if properly designed and constructed, present virtually no risk of spreading radioactivity in case of an accident, there are still a number of weak points which could result in a meltdown and a major economic loss. With the reactor core according to the invention, the probability of such a failure is greatly reduced. 4. Nuclear Waste

There are two categories of nuclear waste to consider: low level and high level waste.

For low level waste, the reactor core according to the present invention has no advantage over conventional cores since the quantity of such waste depends only upon the total energy generated.

However, with regard to high level waste, the amount of radioactivity the present core will discharge will be less than half the amount from a conventional reactor core.

The explanation is as follows: The seed regions, which are refueled every twelve to eighteen months, will discharge high level waste at the same proportionate rate as a conventional reactor, but only twenty to twenty-five percent of the total energy is generated in the seeds. In the blanket region, which stays in the core for ten to twelve years, the radioactivity of the high level wastes will decrease by at least a factor of seven, simply because these wastes disintegrate rapidly and form residues with much smaller amounts of radioactivity. This process will be aided by neutron absorption in the high level waste while it is in the core, which also results in transmutation to nuclei which are less radioactive. Thus, the radioactivity of the high level waste discharged from the blanket will be less by at least a factor of seven than the proportionate amount discharged from a conventional reactor core. If the amount of radioactivity produced from both the seed and blanket regions is weighted by the amount of energy produced from each region (twenty to twenty-five % from the seed regions, eighty to seventy-five % from the blanket) , the total radioactive waste discharged can be shown to be well below half of the high level waste discharged from a conventional core.

In conclusion, therefore, a novel nonproliferative light water nuclear reactor has been shown and described which fulfills all the objects and advantages sought. Many changes, modifications, variations and other uses and applications of the subject invention will, however, become apparent to those skilled in the art after considering this specification and the accompanying drawings which disclose the preferred embodiments thereof. All such changes, modifications, variations and other uses and applications which do not depart from the spirit and scope of the invention are deemed to be covered by the invention, which is to be limited only by the claims which follow.

Claims

C L A I M S What is claimed is:

1. A nuclear reactor having a core comprising:

(a) at least one seed region, each containing seed fuel elements having a critical mass of fissionable material, said seed fuel elements essentially comprising U-235 and U- 238 which are initially in the ratio range from 10:90 to the maximum ratio which is nonproliferative; and

(b) a blanket region surrounding said seed region and containing blanket fuel elements comprising predominantly Th-232.

2. The nuclear reactor defined in claim 1, wherein the initial ratio of U-235 to U-238 in said seed region is approximately 20:80.

3. The nuclear reactor defined in claim 1, wherein there are a plurality of seed regions substantially uniformly distributed within said blanket region and wherein each seed region can sustain a neutronic chain reaction without substantial neutronic interaction with another seed region.

4. The nuclear reactor defined in claim 1, wherein the fuel elements contained in the blanket region are initially enriched with uranium to provide an initial volume percentage of uranium in the range of 2 to 12 percent.

5. The nuclear reactor defined in claim 4, wherein the initial volume percentage of uranium in said blanket region is from 6 to 10 percent.

6. The nuclear reactor defined in claim 5, wherein the initial volume percentage of uranium in said blanket region is approximately 8 percent.

7. The nuclear reactor defined in claim 4, wherein the uranium initially provided in said blanket region comprises U-235 and U-238 in the initial ratio range from 10:90 to the maximum ratio which is nonproliferative.

8. The nuclear reactor defined in claim 7, wherein the initial volume ratio of U-235 to U-238 in said blanket region is approximately 20:80.

9. The nuclear reactor defined in claim 1, wherein the uranium initially provided in said seed region is in the form of uranium oxide.

10. The nuclear reactor defined in claim 1, wherein the uranium initially provided in said seed region is in the form of uranium zirconium alloy.

11. The nuclear reactor defined in claim 1, wherein the thorium initially provided in said blanket region is in the form of thorium oxide.

12. The nuclear reactor defined in claim 4, wherein the uranium initially provided in said blanket region is in the form of uranium oxide.

13. The nuclear reactor defined in claim 1, further comprising means for replacing the fuel elements in said seed region without disturbing the fuel elements in said blanket region.

14. The nuclear reactor defined in claim 1 , further comprising means for replacing the fuel elements in said blanket region without disturbing the fuel elements in said seed region.

15. The nuclear reactor defined in claim 1, further comprising means for cooling said core using water exclusively as the heat transfer medium.

16. The nuclear reactor defined in claim 15, wherein said water is light water.

17. The nuclear reactor defined in claim 15, wherein said core cooling means includes a reactor pressure vessel containing said core and means for supplying said water to said vessel under pressure so as to inhibit the formation of steam at the operating temperature of said core.

18. The nuclear reactor defined in claim 15, wherein the volume ratio of water to fuel in said seed region is at least three times as high as in said blanket region.

19. The nuclear reactor defined in claim 15, wherein the volume ratio of water to fuel in said seed region is in the range from 3:1 to 10:1.

20. The nuclear reactor defined in claim 15, wherein the volume ratio of water to fuel in said blanket region is in the range from 0.8 to 1.5.

21. The nuclear reactor defined in claim 19, wherein the uranium in said seed region is in the form of uranium zirconium alloy.

22. The nuclear reactor defined in claim 1, further comprising a neutron reflector region surrounding said core, and wherein said reflector region contains fuel elements comprising predominantly Th-232.

23. The nuclear reactor defined in claim 22, wherein said fuel elements contained in said reflector region are initially enriched with U-238.

24. The nuclear reactor defined in claim 1, further comprising nonparasitic means for controlling the rate of the nuclear reaction in said core.

25. The nuclear reactor defined in claim 24, wherein said control means exclusively controls the rate of nuclear reaction in said seed region.

26. The nuclear reactor defined in claim 1, wherein said seed fuel elements are elongate and vertically oriented in said core, and wherein said reactor further comprises:

(c) means for controlling the rate of nuclear reaction in said core, said control means including:

(1) a plurality of elongate, vertically oriented control elements arranged parallel to said seed fuel elements, each control element having, from one end to the other, a plurality of sections of prescribed length; and (2) means for moving at least some of said control elements in the vertical direction over a distance substantially equal to said prescribed length.

27. The nuclear reactor defined in claim 26, wherein said control elements are nonparasitic.

28. The nuclear reactor defined in claim 26, wherein said control elements are arranged in a control region between said seed region and said blanket region.

29. The nuclear reactor defined in claim 28, wherein said control region surrounds said seed region.

30. The nuclear reactor defined in claim 26, wherein said control elements include a plurality of stationary control elements and a plurality of movable control elements connected to said moving means, said stationary control elements being arranged immediately adjacent to said movable control elements.

31. The nuclear reactor defined in claim 30, wherein at least some of said stationary control elements are disposed between said seed region and said movable control elements.

32. The nuclear reactor defined in claim 30, wherein at least some of said stationary control elements are disposed between said blanket region and said movable control elements.

33. The nuclear reactor defined in claim 30, wherein said stationary control elements are disposed on opposite sides of said movable control elements.

34. The nuclear reactor defined in claim 33, wherein said control elements are arranged in a control region between said seed region and said blanket region, there being first stationary control elements, on one side of said movable control elements, arranged adjacent said seed region, and second stationary control elements, on the opposite side of said movable control elements, arranged adjacent said blanket region.

35. The nuclear reactor defined in claim 26, wherein said sections of at least some of said control elements contain, in succession from one end to the other, alternately, fissionable material and nonfissionable material.

36. The nuclear reactor defined in claim 35, wherein said nonfissionable material is substantially transparent to neutrons.

37. The nuclear reactor defined in claim 35, wherein said nonfissionable material is neutron fertile material.

38. The nuclear reactor defined in claim 26, wherein said sections of at least some of said control elements contain, in succession from one end to the other, alternately, neutron fertile material and neutron transparent material.

39. The nuclear reactor defined in claim 37, wherein said fertile material is thorium.

40. The nuclear reactor defined in claim 38, wherein said fertile material is thorium.

41. The nuclear reactor defined in claim 26, wherein said prescribed length is substantially equal to 45 cm.

42. The nuclear reactor defined in claim 26, wherein each control element has eight sections of said prescribed length.

43. The nuclear reactor defined in claim 26, wherein said blanket fuel elements are elongate, vertically oriented elements arranged parallel to said seed fuel elements and said control elements.

44. The nuclear reactor defined in claim 43, wherein said seed and blanket fuel elements are substantially the same length, and wherein said control elements extend substantially the entire vertical dimension of said seed and blanket fuel elements.

45. The nuclear reactor defined in claim 26, wherein said moving means is operative to move said control elements to any desired vertical position along said distance.

46. A nuclear reactor having a core comprising:

(a) at least one seed region, each seed region containing a first plurality of elongate, vertically oriented seed fuel elements having a reactivity greater than one;

(b) at least one control region, each control region containing elongate, vertically arranged control elements oriented parallel to said seed fuel elements, each control element having, from one end to the other, a plurality of sections of prescribed length;

(c) at least one blanket region, each blanket region surrounding a seed region and containing blanket fuel elements having a reactivity less than one; and

(d) means for moving at least some of said control elements in the vertical direction over a distance substantially equal to said prescribed length.

47. The nuclear reactor defined in claim 46, wherein said control elements are nonparasitic.

48. The nuclear reactor defined in claim 46, wherein said control region is arranged between said seed region and said blanket region.

49. The nuclear reactor defined in claim 48, wherein said control region surrounds said seed region.

50. The nuclear reactor defined in claim 46, wherein said control elements include a plurality of stationary control elements and a plurality of movable control elements connected to said moving means, said stationary control elements being arranged immediately adjacent said movable control elements.

51. The nuclear reactor defined in claim 50, wherein at least some of said stationary control elements are disposed between said seed region and said movable control elements.

52. The nuclear reactor defined in claim 50, wherein at least some of said stationary control elements are disposed between said blanket region and said movable control elements.

53. The nuclear reactor defined in claim 50, wherein said stationary control elements are disposed on opposite sides of said movable control elements.

'- 54. The nuclear reactor defined in claim 53, wherein said control elements are arranged in a control region between said seed region and said blanket region, there being first stationary control elements, on one side of said movable control elements, arranged adjacent said seed region, and second stationary control elements, on the opposite side of said movable control elements, arranged adjacent said blanket region.

55. The nuclear reactor defined in claim 46, wherein said sections of at least some of said control elements contain, in succession from one end to the other, alternately, fissionable material and nonfissionable material.

56. The nuclear reactor defined in claim 55, wherein said nonfissionable material is substantially transparent to neutrons.

57. The nuclear reactor defined in claim 55, wherein said ^Q nonfissionable material is neutron fertile material.

58. The nuclear reactor defined in claim 46, wherein said sections of at least some of said control elements contain, in succession from one end to the other, alternately, neutron fertile material and neutron transparent material.

59. The nuclear reactor defined in claim 57, wherein said fertile material is thorium.

60. The nuclear reactor defined in claim 58, wherein said fertile material is thorium.

61. . The nuclear reactor defined in claim 46, wherein said prescribed length is substantially equal to 45 cm.

62. The nuclear reactor defined in claim 46, wherein each control element has eight sections of said prescribed length.

63. The nuclear reactor defined in claim 46, wherein said blanket fuel elements are elongate, vertically oriented elements arranged parallel to said seed fuel elements and said control elements.

64. The nuclear reactor defined in claim 63, wherein said seed and blanket fuel elements are substantially the same length, and wherein said control elements extend substantially the entire vertical dimension of said seed and blanket fuel elements.

65. The nuclear reactor defined in claim 46, wherein said moving means is operative to move said control elements to any desired vertical position along said distance.

66. The nuclear reactor defined in claim 46, wherein said first plurality of control elements includes a second plurality of control elements which comprise blanket fuel elements, and a third plurality of control elements which comprise seed fuel elements.

67. The nuclear reactor defined in claim 66, wherein said blanket fuel elements in said second plurality of control elements are disposed in alternate ones of said sections thereof.

68. The nuclear reactor defined in claim 67, wherein said moving means includes means for gradually moving said second plurality of control elements during the lifetime of said seed elements, over a distance substantially equal to said prescribed length.

69. The nuclear reactor defined in claim 66, wherein said seed fuel elements in said third plurality of control elements are disposed in alternate ones of εaid sections thereof.

70. The nuclear reactor defined in claim 69, wherein said seed fuel elements are successively loaded into alternate ones of said sections thereof, whereby in order to ensure axial depletion of said core, succeεsive loading of new seed elements has the relative positions of adjacent sections reversed.

71. The nuclear reactor defined in claim 70, wherein said moving means includes means for alternately raising and lowering said third plurality of control elements by a distance substantially equal to said prescribed length at the time of replacement of said seed elements.