CA2042896A1 - Passive safety shutdown system for nuclear reactors - Google Patents
Passive safety shutdown system for nuclear reactorsInfo
- Publication number
- CA2042896A1 CA2042896A1 CA002042896A CA2042896A CA2042896A1 CA 2042896 A1 CA2042896 A1 CA 2042896A1 CA 002042896 A CA002042896 A CA 002042896A CA 2042896 A CA2042896 A CA 2042896A CA 2042896 A1 CA2042896 A1 CA 2042896A1
- Authority
- CA
- Canada
- Prior art keywords
- safety device
- fuel assembly
- fuel
- neutron absorber
- neutron
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
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Classifications
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E30/00—Energy generation of nuclear origin
- Y02E30/30—Nuclear fission reactors
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- Structure Of Emergency Protection For Nuclear Reactors (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A passive nuclear reactor safety shutdown device includes a neutron absorber alloy, normally poised above the core, which melts and flows down by gravity into hollow tubes in each fuel bundle to shut down the reactor. The shutdown, in one form of the safety device, occurs within a minute of the heat transfer fluid reaching the trip temperature. The device cannot be removed when fuel bundle is in the core due to a special interlock system.
A passive nuclear reactor safety shutdown device includes a neutron absorber alloy, normally poised above the core, which melts and flows down by gravity into hollow tubes in each fuel bundle to shut down the reactor. The shutdown, in one form of the safety device, occurs within a minute of the heat transfer fluid reaching the trip temperature. The device cannot be removed when fuel bundle is in the core due to a special interlock system.
Description
204~896 PASSIVE SAFETY SHUTDOWN SYSTEM FOR NUCLEAR REACTORS
BACKGROUND OF THE INVENTION
This invention relates to nuclear reactor components employing passive safety systems for slowing or shutting down the nuclear reaction when the temperature of the heat transfer fluid leaving the reactor core exceeds a predetermined value.
It is essential to provide highly reliable systems for safely shutting down nuclear reactors in the event that control system failure should occur in an unsafe manner. Any such accident could cause overheating of fuel and release of radioactivity. It is well known that this could pose a hazard both to the operator and to the public. This could also damage the nuclear plant and/or result in the loss of the operating licence, leading to financial losses as well.
Several types of active safety shutdown systems are known in the art. An active shutdown system operates in response to a control signal, and normally inserts a neutron-absorbing material into the reactor core to stop the fission chain reaction. Such systems generally employ electronic eguipment such as temperature and pressure sensors, amplifiers, alarm units, power supplies, relay logic systems, manual controls, indicators, and computers as well as process equipment including valves, pumps and the like. Active systems are considered to be less reliable than passive methods of shutdown because they are more complex and have more failure modes. Active systems employ redundant "trip channels" which must be monitored and tested frequently to demonstrate that availability targets are met. Active systems also require much more attention by a nuclear operator which is a problem for small reactors where operating costs must be kept low.
Safety depends on proper operator action according to prescribed procedures and principles and hence active systems are more susceptible to human error. Generally, 204289~
active systems are relatively costly to purchase, install and maintain.
It will be app~rent from the above that it has ' become very desirable to provide passive safety shutdown systems for nuclear reactors which employ phenomena and devices which behave passively as a result of natural properties and forces within the reactor without any dependence on external controls and sources of power.
In order to be effective these passive systems must be extremely reliable without the need for frequent monitoring and testing and any other human involvement to confirm full operability.
There are three principal types of passive safety systems:
(A) those which do not require moving mechanical parts nor moving working fluid;
(B) those which involve a moving working fluid, but no moving mechanical parts; and (C) those which depend on moving mechanical parts.
The first type is the most preferred since it avoids concerns about potential impairments of the required motion such as might be ~reated by friction or leakage.
The present invention is concerned with passive systems of the second type noted above, involving the movement of a liquid thermaI neutron absorber into the reactor core at a specific predetermined temperature and under the natural force of gravity.
~0 The prior art has provided a number of passive shutdown systems which purport to operate when excessive temperatures in the core of the reactor are reached.
One such system is described in Schively U.S. Patent 3,795,580 filed October 19, 1972 which refers to the ~5 use of a lithium alloy abso~ber which melts at the required trip temperature and falls passively into the .... ,. ~ ~ .
"
204~896 core region to shut down the reaction. However, it appears that the Schively system has a significant disadvantage since the many hollow compartments needed in the core to accommodate this particular system would tend to allow a significant leakage of neutrons outwardly of the core during normal operation thus resulting in less than optimum fuel utilization or requiring a larger core size. Another potential problem with the Schively proposal is its relatively slow response to an accident due to the time needed to supply the heat of fusion necessary to melt the absorber alloy at the passive trip temperature. The actual reactor trip or shutdown could occur at a te~perature substantially above the melting point because the accident could progress substantially during the time the neutron absorber is changing from the solid to the liquid state.
It is well known that a neutron absorber affects the neutron flux in its immediate vicinity. To shut down a reactor adeguately the absorbers must be inserted throughout the core. The passive safety devices should be installed in each fuel assembly or bundle within the core. Ideally, each safety device must be unfailingly capable of releasing its absorber when the local temperature exceeds the passive trip temperature thus depressing each local flux peak until the reactor is stabilized at an acceptable power level or forced to shut down altogether. The safety devices are reguired to act only in the very rare event of an accident wherein the reactor control system has malfunctioned unsafely and the active safety shutdown system has not prevented the heat transfer fluid from exceeding active trip temperature.
SUMMARY OF THE INVENTIO~
It is a general object of the present invention to provide an improved passive safety shutdown .
.
:, :
, .
BACKGROUND OF THE INVENTION
This invention relates to nuclear reactor components employing passive safety systems for slowing or shutting down the nuclear reaction when the temperature of the heat transfer fluid leaving the reactor core exceeds a predetermined value.
It is essential to provide highly reliable systems for safely shutting down nuclear reactors in the event that control system failure should occur in an unsafe manner. Any such accident could cause overheating of fuel and release of radioactivity. It is well known that this could pose a hazard both to the operator and to the public. This could also damage the nuclear plant and/or result in the loss of the operating licence, leading to financial losses as well.
Several types of active safety shutdown systems are known in the art. An active shutdown system operates in response to a control signal, and normally inserts a neutron-absorbing material into the reactor core to stop the fission chain reaction. Such systems generally employ electronic eguipment such as temperature and pressure sensors, amplifiers, alarm units, power supplies, relay logic systems, manual controls, indicators, and computers as well as process equipment including valves, pumps and the like. Active systems are considered to be less reliable than passive methods of shutdown because they are more complex and have more failure modes. Active systems employ redundant "trip channels" which must be monitored and tested frequently to demonstrate that availability targets are met. Active systems also require much more attention by a nuclear operator which is a problem for small reactors where operating costs must be kept low.
Safety depends on proper operator action according to prescribed procedures and principles and hence active systems are more susceptible to human error. Generally, 204289~
active systems are relatively costly to purchase, install and maintain.
It will be app~rent from the above that it has ' become very desirable to provide passive safety shutdown systems for nuclear reactors which employ phenomena and devices which behave passively as a result of natural properties and forces within the reactor without any dependence on external controls and sources of power.
In order to be effective these passive systems must be extremely reliable without the need for frequent monitoring and testing and any other human involvement to confirm full operability.
There are three principal types of passive safety systems:
(A) those which do not require moving mechanical parts nor moving working fluid;
(B) those which involve a moving working fluid, but no moving mechanical parts; and (C) those which depend on moving mechanical parts.
The first type is the most preferred since it avoids concerns about potential impairments of the required motion such as might be ~reated by friction or leakage.
The present invention is concerned with passive systems of the second type noted above, involving the movement of a liquid thermaI neutron absorber into the reactor core at a specific predetermined temperature and under the natural force of gravity.
~0 The prior art has provided a number of passive shutdown systems which purport to operate when excessive temperatures in the core of the reactor are reached.
One such system is described in Schively U.S. Patent 3,795,580 filed October 19, 1972 which refers to the ~5 use of a lithium alloy abso~ber which melts at the required trip temperature and falls passively into the .... ,. ~ ~ .
"
204~896 core region to shut down the reaction. However, it appears that the Schively system has a significant disadvantage since the many hollow compartments needed in the core to accommodate this particular system would tend to allow a significant leakage of neutrons outwardly of the core during normal operation thus resulting in less than optimum fuel utilization or requiring a larger core size. Another potential problem with the Schively proposal is its relatively slow response to an accident due to the time needed to supply the heat of fusion necessary to melt the absorber alloy at the passive trip temperature. The actual reactor trip or shutdown could occur at a te~perature substantially above the melting point because the accident could progress substantially during the time the neutron absorber is changing from the solid to the liquid state.
It is well known that a neutron absorber affects the neutron flux in its immediate vicinity. To shut down a reactor adeguately the absorbers must be inserted throughout the core. The passive safety devices should be installed in each fuel assembly or bundle within the core. Ideally, each safety device must be unfailingly capable of releasing its absorber when the local temperature exceeds the passive trip temperature thus depressing each local flux peak until the reactor is stabilized at an acceptable power level or forced to shut down altogether. The safety devices are reguired to act only in the very rare event of an accident wherein the reactor control system has malfunctioned unsafely and the active safety shutdown system has not prevented the heat transfer fluid from exceeding active trip temperature.
SUMMARY OF THE INVENTIO~
It is a general object of the present invention to provide an improved passive safety shutdown .
.
:, :
, .
2(~4289~
system or device for nuclear reactors which is capable of satisfying the criteria for such passive systems as noted above.
The present invention accordingly relates to a fuel assembly adapted to form a part of a nuclear reactor core section, said fuel assembly including an array of fuel pins disposed in pre-selected spaced relation to one another. The fuel assembly is arranged to permit a liquid heat transer medium and moderator, (and which is referred to hereafter, and after first mention thereof in the detailed description and claims, simply as a heat transfer medium,) to flow upwardly in contact with the fuel pins to remove heat therefrom. At least one passive safety device is disposed in the fuel assembly within the array of fuel pins. The safety device has an upper portion lscated above the le~el of the array of fuel pins so as to be exposed to the upward flow of heat transfer medium which has been heated during passage through the array of fuel pins, and a lower portion disposed within the array of fuel pins. A
neutron absorber is normally disposed in said upper portion of the safety device and has a melting point such that when the temperature of the heat transfer medium to which said upper portion of the safety device is exposed exceeds a selected temperature, the neutron absorber melts such that it has the capability of flowing downwardly by gravity into said lower portion of the safety device to absorb neutrons and depress the neutron flux and the heat being generated in the vicinity of the neutron absorber.
In accordance with one aspect of the invention, the lower portion of the safety device comprises an elongated tubular body arranged such that it contains or is substantially filled with a neutron moderator during normal use to reduce or prevent neutron leakage along the safety device outwardly of the core .
: :
.. ~,' '.: - ' ' .
, ~0~2896 section.
As a further feature of the invention, the tubular body noted above may be vented so that it is filled with the liquid heat transfer medium when in normal use to reduce the neutron leakage along the safety device. In this situation the neutron absorber is of a material which does not react chemically with the heat transfer medium. When the safety device is activated by an over-temperature condition, the downwardly flowing neutron absorber displaces the liquid outwardly of the tubular body so that the latter becomes filled to the required level with the neutron absorber material.
In an alternate form of the invention, the above-noted tubular body may be provided with an axially arranged rod of moderator material disposed therein such that an annular space is provided between the rod and the tubular body to receive and contain the molten neutron absorber flowing downwardly from the upper portion of the safety device in response to an over-temperature condition. This alternative is used in situations wherein the neutron absorber material would react with the liquid heat transfer medium and hence under normal circumstances the above-noted annular space is filled with an inert gas such as helium.
In both of the alternatives described above, during normal operation, the liquid filled tubular body or, alternatively, the rod of moderating material, acts to scatter the neutrons hence reducing losses of neutrons from the core along the safety device and thus enhancing the efficiency of the reactor as compared, for example, with the above-noted Schively arrangement.
In accordance with a further aspect of the invention, said upper portion of the safety device defines a compartment for holding the neutron absorber, said compartment having a lower exit opening through - ~ .
.; . ~' . . : ~
2~2~396 which the molten neutron absorber can flow by gravity.
In a preferred form of the invention a fuse element of relatively small mass as compared with the mass of the neutron absorber is disposed in said exit opening, said fuse element having a higher melting temperature than does the body of neutron absorber in said compartment so that when the temperature of the heat transfer medium exceeds a first predetermined temperature below the trip temperature the body of neutron absorber melts but is retained in the compartment by the fuse element, said fuse element being arranged to melt at the trip temperature such that when the latter is reached the exit opening is unblocked and the previously melted neutron absorber flows down into said lower portion of the safety device.
The advantage of the system described above is that it provides a very rapid response time as compared, for example, with the Schively system. Since the fuse is of relatlvely small mass, it melts fairly quickly once the trip temperature is reached thus allowing the previously melted and much larger mass of neutron absorber to move downwardly by gravity into the lower portion of the safety device.
Typically, fins are provided on the exterior of the above-noted compartment so as to enhance the rate of heat transfer from the liguid heat transfer medium into the neutron absorber within this compartment.
In accordance with a still further aspect of the invention, said safety device is arranged such that it can be inserted endwise into each fuel assembly from above with shoulder means and fastener to define the axial location of the safety device within the fuel assembly, and an interlock means to secure the safety device against inadvertent withdrawal from the fuel assembly when it is in the core section, said interlock being inaccessible from above the core section such that ':
204;~89fi the fuel assembly must be removed from the reactor before the interlock can be released and the safety device then removed from the fuel assembly.
In a preferred form of the invention, the interlock means includes a release element which normally projects below a lower portion of the fuel assembly and is arranged such that depression of same occurring on placement of the fuel assembly on a surface after removal from the reactor effects release of the interlock and allows withdrawal of the safety device from the fuel assembly.
Various metals are suitable for use as the neutron absorber. Several suitable eutectic alloys are referred to hereafter. These are of course selected in accordance with their neutron absorbing characteristics and their melting points. Certain pure metals may also be suitable depending upon the melting temperature required.
The principles of the present invention are applicable to a wide variety of reactors provided that these reactors are ones in which accidents can only occur relatively slowly, i.e. in excess of a few minutes. The present system is not contemplated for use in reactors wherein accidents can occur quickly such as in certain pressurized power reactors wherein a serious failure can occur within seconds, as for example in the event of breakage of a pressurized pipe. These systems require fast shutdown arrangements which are designed to insert shutoff rods or a liquid absorber very rapidly.
Preferred embodiments of the invention will now be described with reference to the accompanying drawings.
BRIEF DESCRIPTION OF VIEWS OF DRAWINS
Fig. 1 is a longitudinal section view of a typical nuclear fuel assembly with the passive safety - ~ -. .
~ - :
20~;2 89fi device positioned therein;
Fig. 2 is a horizontal cross-section view of the fuel assembly of Fig. 1 with the passive safety device positioned therein;
Fig. 3 is a view similar to that of Fig. 1 and partially in section with a somewhat modified form of passive safety device in position;
Fig. 4 is a cross-section view of the fuel assembly and passive safety device as illustrated in Fig. 3; and Fig. 5 is a diagrammatic plan view of a typical reactor core layout showing the individual fuel assemblies, control absorbers and the like.
DETAILED DESCRIPTION OF_?HE PREFERRED EMBODIMENTS
Referring firstly to Figs. 1 and 2 there is shown a fuel assembly or bundle 10 for use in a nuclear reactor core section, the fuel bundle 10 including a multiplicity of fuel pins 12 each connected to and extending between a spaced apart parallel grid plate 14 and inlet nozzle assembly 15. The grid plate and nozzle assembly 15 are provided with a large number of closely spaced flow openings 16 thereby to permit a liquid heat transfer medium and moderator to flow upwardly in contact with the fuel pins 12 to remove the heat therefrom which is generated by the nuclear fission reaction.
In one particular reactor arrangement as illustrated in Fig. 5 (~nown as the AECL SES-10 reactor) each fuel pin 12 has a diameter of about 13 mm, the fuel pins being disposed in an 8X8 array as best seen in Fig.
2 with the outside dimensions of the array being approximately 140 mm x 140 mm. As illustrated in Fig.
5, 32 of these fuel bundles are arranged in a 6X6 array with the four corner fuel assemblies omitted, thus giving 32 fuel assemblies in all. The fuel pins 12 each are 750 mm long, and are each filled with pellets of 204~ 3fi g uranium dioxide (UO2). This particular reactor is capable of producing an output of approximately 10 megawatts (MW~. Normal operating temperature of the water coolant passing upwardly along and around the fuel pins of this reactor is from 75 to about 95C. For a further description of the SES-10 reactor, reference may be had to "Design of SES-10 Nuclear Reactor for District Heating" AECL-10222 by J.M. Cuttler. Prepared for presentation at the International Conference on Conventional and Nuclear District Heating, Lausanne, Switzerland, 1991 March 18-21.
As best illustrated in Fig. 1, the safety device 18 in accordance with the invention includes an upper portion 20 located above the upper grid plate 14 and above the array of fuel pins 12 such as to be exposed to the upward flow of liquid heat transfer medium which has been previously heated during p~ssage through the array of fuel pins 12 located generally between the inlet nozzles 15 and the grid plates 14.
This safety device 18 also has a lower tubular portion 22 disposed within the fuel pin array i.e. it extends downwardly through the grid plate 14 with its lower end portion being seated in the inlet nozzle assembly 15 in a manner to be described more fully hereinafter.
In the embodiment illustrated in Fig. 1, the upper portion 20 of the safety device includes a compartment 24 defining an annular space within which is disposed an annular body of a metallic neutron absorber 26. The interior of the compartment 24 is provided with an internal annular partition 28 defining a central axial passageway 30 having vent openings 32 at the upper end thereof in communication with vent opening 34 provided in the upper end of the compartment 24. These vent openings 32 and 34 permit the interior of the 3~ safety device 18, including the lower portion 22 which is in the form of an elongated tubular body, to become " '' ~
204~8 completely filled with the liquid heat transfer medium (water) when the reactor is in normal operation. As previously noted, the presence of water within the lower portion 22 of this safety device 18 inhibits escape of neutrons outwardly of the core along the interior of the safety device thus reducing neutron losses and assisting in maintaining efficient usage of the nuclear fuel during operation.
The lower end portion of the annular partition 28 is shaped to provide a generally U-shaped annular cup portion 36 which co-operates with an annular inverted L-shaped portion 38 fixed to the wall of compartment 24 with these two portions together defining an annular exit opening from the interior of the compartment while at the same time together defining what might be termed a liguid "trap" type of arrangement for allowing escape of the neutron absorber when in the molten condition under the circumstances to be described below.
Disposed between the outer wall of the compartment 24 and the outer wall portion of the cup 36 is an annular fuse 40. An annular gas lock 42 separates the metal of the neutron absorber 26 from the metal which defines the fuse 40 thus preventin~ any unwanted alloying from occurring therebetween.
In one particular application of the above-described reactor for hot water heating, the neutron absorber alloy selected is 44% indium, 42% tin and 14%
cadmium, this alloy having a melting point of 93C. The fuse 40 is made of an alloy comprising 54% bismuth, 26%
tin and 20% cadmium, this alloy having a meltin~ point of 103C.
In the embodiment of Fig. 1, the mass or amount of neutron absorber 26 must be sufPicient as to three-quarters fill the lower tubular portion 22 of the safety device 18, i.e. sufficient absorber is needed to fill this tubular portion up to a level three-guarters ~ :
: '~ `: ~ ..
2~ 89~
~ 1 the level of the upper grid plate 14. In a typical AECL
SES-10 reactor noted above the length of the filled portion is in the order of 600 mm. ~ith a typical inside diameter for the lower tubular portion 22 in the order of 29 mm, the total volume of neutron absorber for this particular application can be readily calculated.
For the sake of safety it is obviously better to overfill than to underfill.
Since the mass of the annular metal fuse 40 is very small in relation to the overall mass of the neutron absorber 26, the response time of the configuration illustrated in Fig. 1 is relatively short.
As noted above, the neutron absorber 26 melts at 93C.
and is available for release through the annular exit opening described above just as soon as the relatively small fuse 40 melts at the above-noted temperature of 103C. The response time of the particular embodiment described above can be in the order of one minute.
In order to enhance the rate of transfer of heat energy from the heat transfer medium or coolant into the compartment 24, the latter is provided with a multiplicity of outwardly directed fins 44.
A well known expression in nuclear engineering is that known as the "reactivity worth" of a shutdown system. For the configuration illustrated in Fig. 5 the total reactivity worth is approximately 90 mk, i.e.
about 3 mk per safety device on average.
It should of course be realized that in applications where a substantial response time ls permissible, the separate annular fuse 40 may be omitted altogether and the neutron absorber 26 selected from those alloys havin~ a melting temperature corresponding to the selected trip point. One suitable eutectic alloy here would comprise 54% bismuth, 26% tin and 20% cadmium to provide a melting temperature (and trip point) of 103C. This alloy has a heat of fusion of about 13 .. ~ ' .
204~89fi calories/gram and the time required to melt the mass of metal required would be in the order of 10 minutes for the S~S-10 reactor safety device described above.
The embodiment illustrated in Fig. 3 is utilized in situations wherein the alloy or alloys selected for the neutron absorber 26 and/or annular fuse 40 would tend to react with the heat transfer fluid. In these cases the alloy or alloys must remain dry at all times. In this dry configuration, the lower tubular portion 22 of the safety device is provided with an axially disposed rod 50 of a suitable moderating material such as carbon or beryllium. In the SES-10 reactor configuration under consideration this rod 50 has a diameter of 25 millimeters while, as noted previously, the inside diameter of the lower tubular portion 22 is 29 millimeters thus giving a 2 mm radial gap or annular space between rod 50 and the inner wall of lower tubular portion 22. Clearly, the amount of neutron absorber 26 reguired in this configuration to fill this annular space is relatively small as compared with the volume required for the configuration of Fig.
1. Nevertheless, the same principles still apply and a further discussion appears unnecessary at this point.
The rod 50 of moderating material functions during normal usage to inhibit excessive flow of neutrons axially upwardly along the interior of the safety device thus reducing neutron loss and improving fuel utilization. The annular space is normally filled with an inert gas such as helium.
It was noted previously that the safety device 18 is arranged such that it can be inserted endwise into the fuel bundle from above the core. This will be readily apparent from Figs~ 1 and 3 wherein it will be seen that the grid plate 14 is sized to accommodate the tubular lower portion 22 while the inlet nozzle assembly 15 is sized to accommodate a stepped neck 60 formed on , . . :
.' ' , ' . ' ''' ;
'- .: , ' ' ' , ~ . .
~04~896 the lowermost end of the lower tubular portion 22. This neck 60 is provided with a shoulder 62 and a screw thread which define the axial location of the safety device 18 and retain it within the fuel bundle. The neck 60 is also provided with an interlock 64 to secure the safety device against inadvertent withdrawal from the fuel bundle while still in the reactor core. This interlock 64 is designed so as to be inaccessible from above the core and it is arranged so that the fuel bundle must be removed from the reactor before the interlock 64 can be released and the safety device 18 unscrewed from the fuel bundle.
As shown in Figs. 1 and 3, the interlock 64 employs an actuating rod 66 having an enlarged head portion 68. This head portion 68 bears against a plurality of balls 70 which are seated in radial apertures in the lower end of the neck 60. When the balls 70 are in their outwardly disposed locking positions, they bear against the lower edge of the aperture in the inlet nozzle assembly 15 through which the neck 60 projects thus preventing axial movement of neck 60 upwardly relative to the nozzle assembly.
However, when the actuating rod 66 is forced upwardly in the direction of the arrow A, the head portion 68 moves upwardly beyond the plane of the balls 70 such that the balls 70 are then free to move radially inwardly by a short distance thereby allowing the entire safety device 18 to be unscrewed and withdrawn axially outwardly from the fuel bundle from above. A ~ompression spring 72 bears against the upper end of the head portion 68 thus ensuring that the interlock is maintained in the locking condition at all times except when a positive force sufficient to overcome the pressure of spring 72 is applied to the lower end of rod 66. Hence, the lower end of rod 66 is made sufficiently long that it projects a short distance downwardly below the lower edge of the : .
- ~
204Z89fi inlet nozzle 15 of the fuel bundle. Thus when the fuel bundle is lifted outwardly from the reactor core via the lifting lug 80 on the upper end of the assembly, and thereafter subsequently positioned on a flat surface, the rod 66 is depressed hence automatically releasing the interlock and allowing the safety device 18 to be unscrewed and lifted outwardly therefrom and transferred to a temporary holding rack in the reactor. It will thus be seen that the safety device 18 is relatively inaccessible to tampering by unauthorized people and damage by external events. This obviates the need to protect it from hazards thus reducing capital costs.
It is believed that those skilled in this art will readily appreciate the many advantages inherent in the passive safety shutdown system described above. The system can be readily adapted for a substantial variety of situations and the invention is not to be limited to the particular embodiments described above. Numerous modifications and variations can of course be made while still remaining within the spirit and scope of the invention. For definitions of the invention reference is to be had to the claims appended hereto.
--,:
. ~
system or device for nuclear reactors which is capable of satisfying the criteria for such passive systems as noted above.
The present invention accordingly relates to a fuel assembly adapted to form a part of a nuclear reactor core section, said fuel assembly including an array of fuel pins disposed in pre-selected spaced relation to one another. The fuel assembly is arranged to permit a liquid heat transer medium and moderator, (and which is referred to hereafter, and after first mention thereof in the detailed description and claims, simply as a heat transfer medium,) to flow upwardly in contact with the fuel pins to remove heat therefrom. At least one passive safety device is disposed in the fuel assembly within the array of fuel pins. The safety device has an upper portion lscated above the le~el of the array of fuel pins so as to be exposed to the upward flow of heat transfer medium which has been heated during passage through the array of fuel pins, and a lower portion disposed within the array of fuel pins. A
neutron absorber is normally disposed in said upper portion of the safety device and has a melting point such that when the temperature of the heat transfer medium to which said upper portion of the safety device is exposed exceeds a selected temperature, the neutron absorber melts such that it has the capability of flowing downwardly by gravity into said lower portion of the safety device to absorb neutrons and depress the neutron flux and the heat being generated in the vicinity of the neutron absorber.
In accordance with one aspect of the invention, the lower portion of the safety device comprises an elongated tubular body arranged such that it contains or is substantially filled with a neutron moderator during normal use to reduce or prevent neutron leakage along the safety device outwardly of the core .
: :
.. ~,' '.: - ' ' .
, ~0~2896 section.
As a further feature of the invention, the tubular body noted above may be vented so that it is filled with the liquid heat transfer medium when in normal use to reduce the neutron leakage along the safety device. In this situation the neutron absorber is of a material which does not react chemically with the heat transfer medium. When the safety device is activated by an over-temperature condition, the downwardly flowing neutron absorber displaces the liquid outwardly of the tubular body so that the latter becomes filled to the required level with the neutron absorber material.
In an alternate form of the invention, the above-noted tubular body may be provided with an axially arranged rod of moderator material disposed therein such that an annular space is provided between the rod and the tubular body to receive and contain the molten neutron absorber flowing downwardly from the upper portion of the safety device in response to an over-temperature condition. This alternative is used in situations wherein the neutron absorber material would react with the liquid heat transfer medium and hence under normal circumstances the above-noted annular space is filled with an inert gas such as helium.
In both of the alternatives described above, during normal operation, the liquid filled tubular body or, alternatively, the rod of moderating material, acts to scatter the neutrons hence reducing losses of neutrons from the core along the safety device and thus enhancing the efficiency of the reactor as compared, for example, with the above-noted Schively arrangement.
In accordance with a further aspect of the invention, said upper portion of the safety device defines a compartment for holding the neutron absorber, said compartment having a lower exit opening through - ~ .
.; . ~' . . : ~
2~2~396 which the molten neutron absorber can flow by gravity.
In a preferred form of the invention a fuse element of relatively small mass as compared with the mass of the neutron absorber is disposed in said exit opening, said fuse element having a higher melting temperature than does the body of neutron absorber in said compartment so that when the temperature of the heat transfer medium exceeds a first predetermined temperature below the trip temperature the body of neutron absorber melts but is retained in the compartment by the fuse element, said fuse element being arranged to melt at the trip temperature such that when the latter is reached the exit opening is unblocked and the previously melted neutron absorber flows down into said lower portion of the safety device.
The advantage of the system described above is that it provides a very rapid response time as compared, for example, with the Schively system. Since the fuse is of relatlvely small mass, it melts fairly quickly once the trip temperature is reached thus allowing the previously melted and much larger mass of neutron absorber to move downwardly by gravity into the lower portion of the safety device.
Typically, fins are provided on the exterior of the above-noted compartment so as to enhance the rate of heat transfer from the liguid heat transfer medium into the neutron absorber within this compartment.
In accordance with a still further aspect of the invention, said safety device is arranged such that it can be inserted endwise into each fuel assembly from above with shoulder means and fastener to define the axial location of the safety device within the fuel assembly, and an interlock means to secure the safety device against inadvertent withdrawal from the fuel assembly when it is in the core section, said interlock being inaccessible from above the core section such that ':
204;~89fi the fuel assembly must be removed from the reactor before the interlock can be released and the safety device then removed from the fuel assembly.
In a preferred form of the invention, the interlock means includes a release element which normally projects below a lower portion of the fuel assembly and is arranged such that depression of same occurring on placement of the fuel assembly on a surface after removal from the reactor effects release of the interlock and allows withdrawal of the safety device from the fuel assembly.
Various metals are suitable for use as the neutron absorber. Several suitable eutectic alloys are referred to hereafter. These are of course selected in accordance with their neutron absorbing characteristics and their melting points. Certain pure metals may also be suitable depending upon the melting temperature required.
The principles of the present invention are applicable to a wide variety of reactors provided that these reactors are ones in which accidents can only occur relatively slowly, i.e. in excess of a few minutes. The present system is not contemplated for use in reactors wherein accidents can occur quickly such as in certain pressurized power reactors wherein a serious failure can occur within seconds, as for example in the event of breakage of a pressurized pipe. These systems require fast shutdown arrangements which are designed to insert shutoff rods or a liquid absorber very rapidly.
Preferred embodiments of the invention will now be described with reference to the accompanying drawings.
BRIEF DESCRIPTION OF VIEWS OF DRAWINS
Fig. 1 is a longitudinal section view of a typical nuclear fuel assembly with the passive safety - ~ -. .
~ - :
20~;2 89fi device positioned therein;
Fig. 2 is a horizontal cross-section view of the fuel assembly of Fig. 1 with the passive safety device positioned therein;
Fig. 3 is a view similar to that of Fig. 1 and partially in section with a somewhat modified form of passive safety device in position;
Fig. 4 is a cross-section view of the fuel assembly and passive safety device as illustrated in Fig. 3; and Fig. 5 is a diagrammatic plan view of a typical reactor core layout showing the individual fuel assemblies, control absorbers and the like.
DETAILED DESCRIPTION OF_?HE PREFERRED EMBODIMENTS
Referring firstly to Figs. 1 and 2 there is shown a fuel assembly or bundle 10 for use in a nuclear reactor core section, the fuel bundle 10 including a multiplicity of fuel pins 12 each connected to and extending between a spaced apart parallel grid plate 14 and inlet nozzle assembly 15. The grid plate and nozzle assembly 15 are provided with a large number of closely spaced flow openings 16 thereby to permit a liquid heat transfer medium and moderator to flow upwardly in contact with the fuel pins 12 to remove the heat therefrom which is generated by the nuclear fission reaction.
In one particular reactor arrangement as illustrated in Fig. 5 (~nown as the AECL SES-10 reactor) each fuel pin 12 has a diameter of about 13 mm, the fuel pins being disposed in an 8X8 array as best seen in Fig.
2 with the outside dimensions of the array being approximately 140 mm x 140 mm. As illustrated in Fig.
5, 32 of these fuel bundles are arranged in a 6X6 array with the four corner fuel assemblies omitted, thus giving 32 fuel assemblies in all. The fuel pins 12 each are 750 mm long, and are each filled with pellets of 204~ 3fi g uranium dioxide (UO2). This particular reactor is capable of producing an output of approximately 10 megawatts (MW~. Normal operating temperature of the water coolant passing upwardly along and around the fuel pins of this reactor is from 75 to about 95C. For a further description of the SES-10 reactor, reference may be had to "Design of SES-10 Nuclear Reactor for District Heating" AECL-10222 by J.M. Cuttler. Prepared for presentation at the International Conference on Conventional and Nuclear District Heating, Lausanne, Switzerland, 1991 March 18-21.
As best illustrated in Fig. 1, the safety device 18 in accordance with the invention includes an upper portion 20 located above the upper grid plate 14 and above the array of fuel pins 12 such as to be exposed to the upward flow of liquid heat transfer medium which has been previously heated during p~ssage through the array of fuel pins 12 located generally between the inlet nozzles 15 and the grid plates 14.
This safety device 18 also has a lower tubular portion 22 disposed within the fuel pin array i.e. it extends downwardly through the grid plate 14 with its lower end portion being seated in the inlet nozzle assembly 15 in a manner to be described more fully hereinafter.
In the embodiment illustrated in Fig. 1, the upper portion 20 of the safety device includes a compartment 24 defining an annular space within which is disposed an annular body of a metallic neutron absorber 26. The interior of the compartment 24 is provided with an internal annular partition 28 defining a central axial passageway 30 having vent openings 32 at the upper end thereof in communication with vent opening 34 provided in the upper end of the compartment 24. These vent openings 32 and 34 permit the interior of the 3~ safety device 18, including the lower portion 22 which is in the form of an elongated tubular body, to become " '' ~
204~8 completely filled with the liquid heat transfer medium (water) when the reactor is in normal operation. As previously noted, the presence of water within the lower portion 22 of this safety device 18 inhibits escape of neutrons outwardly of the core along the interior of the safety device thus reducing neutron losses and assisting in maintaining efficient usage of the nuclear fuel during operation.
The lower end portion of the annular partition 28 is shaped to provide a generally U-shaped annular cup portion 36 which co-operates with an annular inverted L-shaped portion 38 fixed to the wall of compartment 24 with these two portions together defining an annular exit opening from the interior of the compartment while at the same time together defining what might be termed a liguid "trap" type of arrangement for allowing escape of the neutron absorber when in the molten condition under the circumstances to be described below.
Disposed between the outer wall of the compartment 24 and the outer wall portion of the cup 36 is an annular fuse 40. An annular gas lock 42 separates the metal of the neutron absorber 26 from the metal which defines the fuse 40 thus preventin~ any unwanted alloying from occurring therebetween.
In one particular application of the above-described reactor for hot water heating, the neutron absorber alloy selected is 44% indium, 42% tin and 14%
cadmium, this alloy having a melting point of 93C. The fuse 40 is made of an alloy comprising 54% bismuth, 26%
tin and 20% cadmium, this alloy having a meltin~ point of 103C.
In the embodiment of Fig. 1, the mass or amount of neutron absorber 26 must be sufPicient as to three-quarters fill the lower tubular portion 22 of the safety device 18, i.e. sufficient absorber is needed to fill this tubular portion up to a level three-guarters ~ :
: '~ `: ~ ..
2~ 89~
~ 1 the level of the upper grid plate 14. In a typical AECL
SES-10 reactor noted above the length of the filled portion is in the order of 600 mm. ~ith a typical inside diameter for the lower tubular portion 22 in the order of 29 mm, the total volume of neutron absorber for this particular application can be readily calculated.
For the sake of safety it is obviously better to overfill than to underfill.
Since the mass of the annular metal fuse 40 is very small in relation to the overall mass of the neutron absorber 26, the response time of the configuration illustrated in Fig. 1 is relatively short.
As noted above, the neutron absorber 26 melts at 93C.
and is available for release through the annular exit opening described above just as soon as the relatively small fuse 40 melts at the above-noted temperature of 103C. The response time of the particular embodiment described above can be in the order of one minute.
In order to enhance the rate of transfer of heat energy from the heat transfer medium or coolant into the compartment 24, the latter is provided with a multiplicity of outwardly directed fins 44.
A well known expression in nuclear engineering is that known as the "reactivity worth" of a shutdown system. For the configuration illustrated in Fig. 5 the total reactivity worth is approximately 90 mk, i.e.
about 3 mk per safety device on average.
It should of course be realized that in applications where a substantial response time ls permissible, the separate annular fuse 40 may be omitted altogether and the neutron absorber 26 selected from those alloys havin~ a melting temperature corresponding to the selected trip point. One suitable eutectic alloy here would comprise 54% bismuth, 26% tin and 20% cadmium to provide a melting temperature (and trip point) of 103C. This alloy has a heat of fusion of about 13 .. ~ ' .
204~89fi calories/gram and the time required to melt the mass of metal required would be in the order of 10 minutes for the S~S-10 reactor safety device described above.
The embodiment illustrated in Fig. 3 is utilized in situations wherein the alloy or alloys selected for the neutron absorber 26 and/or annular fuse 40 would tend to react with the heat transfer fluid. In these cases the alloy or alloys must remain dry at all times. In this dry configuration, the lower tubular portion 22 of the safety device is provided with an axially disposed rod 50 of a suitable moderating material such as carbon or beryllium. In the SES-10 reactor configuration under consideration this rod 50 has a diameter of 25 millimeters while, as noted previously, the inside diameter of the lower tubular portion 22 is 29 millimeters thus giving a 2 mm radial gap or annular space between rod 50 and the inner wall of lower tubular portion 22. Clearly, the amount of neutron absorber 26 reguired in this configuration to fill this annular space is relatively small as compared with the volume required for the configuration of Fig.
1. Nevertheless, the same principles still apply and a further discussion appears unnecessary at this point.
The rod 50 of moderating material functions during normal usage to inhibit excessive flow of neutrons axially upwardly along the interior of the safety device thus reducing neutron loss and improving fuel utilization. The annular space is normally filled with an inert gas such as helium.
It was noted previously that the safety device 18 is arranged such that it can be inserted endwise into the fuel bundle from above the core. This will be readily apparent from Figs~ 1 and 3 wherein it will be seen that the grid plate 14 is sized to accommodate the tubular lower portion 22 while the inlet nozzle assembly 15 is sized to accommodate a stepped neck 60 formed on , . . :
.' ' , ' . ' ''' ;
'- .: , ' ' ' , ~ . .
~04~896 the lowermost end of the lower tubular portion 22. This neck 60 is provided with a shoulder 62 and a screw thread which define the axial location of the safety device 18 and retain it within the fuel bundle. The neck 60 is also provided with an interlock 64 to secure the safety device against inadvertent withdrawal from the fuel bundle while still in the reactor core. This interlock 64 is designed so as to be inaccessible from above the core and it is arranged so that the fuel bundle must be removed from the reactor before the interlock 64 can be released and the safety device 18 unscrewed from the fuel bundle.
As shown in Figs. 1 and 3, the interlock 64 employs an actuating rod 66 having an enlarged head portion 68. This head portion 68 bears against a plurality of balls 70 which are seated in radial apertures in the lower end of the neck 60. When the balls 70 are in their outwardly disposed locking positions, they bear against the lower edge of the aperture in the inlet nozzle assembly 15 through which the neck 60 projects thus preventing axial movement of neck 60 upwardly relative to the nozzle assembly.
However, when the actuating rod 66 is forced upwardly in the direction of the arrow A, the head portion 68 moves upwardly beyond the plane of the balls 70 such that the balls 70 are then free to move radially inwardly by a short distance thereby allowing the entire safety device 18 to be unscrewed and withdrawn axially outwardly from the fuel bundle from above. A ~ompression spring 72 bears against the upper end of the head portion 68 thus ensuring that the interlock is maintained in the locking condition at all times except when a positive force sufficient to overcome the pressure of spring 72 is applied to the lower end of rod 66. Hence, the lower end of rod 66 is made sufficiently long that it projects a short distance downwardly below the lower edge of the : .
- ~
204Z89fi inlet nozzle 15 of the fuel bundle. Thus when the fuel bundle is lifted outwardly from the reactor core via the lifting lug 80 on the upper end of the assembly, and thereafter subsequently positioned on a flat surface, the rod 66 is depressed hence automatically releasing the interlock and allowing the safety device 18 to be unscrewed and lifted outwardly therefrom and transferred to a temporary holding rack in the reactor. It will thus be seen that the safety device 18 is relatively inaccessible to tampering by unauthorized people and damage by external events. This obviates the need to protect it from hazards thus reducing capital costs.
It is believed that those skilled in this art will readily appreciate the many advantages inherent in the passive safety shutdown system described above. The system can be readily adapted for a substantial variety of situations and the invention is not to be limited to the particular embodiments described above. Numerous modifications and variations can of course be made while still remaining within the spirit and scope of the invention. For definitions of the invention reference is to be had to the claims appended hereto.
--,:
. ~
Claims (18)
1. A fuel assembly adapted to form a part of a nuclear reactor core section, said fuel assembly including an array of fuel pins disposed in pre-selected spaced relation to one another, the fuel assembly being arranged to permit a liquid heat transfer medium and moderator to flow upwardly in contact with the fuel pins to remove heat therefrom, characterized by at least one passive safety device disposed within the array of fuel pins, said safety device having an upper portion located above the level of the array of fuel pins so as to be exposed to the upward flow of heat transfer medium which has been heated during passage through the array of fuel pins and a lower portion disposed within the array of fuel pins, and a neutron absorber normally disposed in said upper portion of the safety device and having a melting point such that when the temperature of the heat transfer medium to which said upper portion of the safety device is exposed exceeds a selected temperature the neutron absorber melts such that it has the capability of flowing downwardly by gravity into said lower portion of the safety device to absorb neutrons and depress the neutron flux and the heat being generated in the vicinity of the neutron absorber, and wherein said lower portion of the safety device comprises an elongated tubular body arranged such that it contains a neutron moderator during normal use to reduce or prevent neutron leakage along the safety device outwardly from the reactor core section.
2. The fuel assembly of claim 1 wherein said tubular body is vented so as to be filled with the liquid heat transfer medium when in normal use to reduce the neutron leakage along the safety device, the neutron absorber being of a material which does not react chemically with the heat transfer medium.
3. The fuel assembly of claim 1 wherein said tubular body has an axially arranged rod of neutron moderating material therein to effect the reduction in neutron leakage, said rod being arranged so that an annular space is provided between the rod and the tubular body to receive and contain the molten neutron absorber flowing downwardly from the upper portion of the safety device.
4. The fuel assembly of claim 1 wherein said upper portion of the safety device defines a compartment for holding the neutron absorber, said compartment having a lower exit opening through which the molten neutron absorber can flow by gravity.
5. The fuel assembly of claim 4 wherein a fuse element of relatively small mass as compared with the mass of the neutron absorber is disposed in said exit opening, said fuse element having a higher melting temperature than does the body of neutron absorber in said compartment so that when the temperature of the heat transfer medium exceeds a first predetermined temperature below a selected trip temperature the body of neutron absorber melts but is retained in the compartment by the fuse element, said fuse element being arranged to melt at the selected trip temperature such that when the latter is reached the exit opening is unblocked and the previously melted neutron absorber flows down into said lower portion of the safety device.
6. The fuel assembly section of claim 1 wherein said safety device is arranged such that it can be inserted endwise and fastened into each fuel assembly from above with means to define the axial location of the safety device within the fuel assembly and an interlock means to secure the safety device against inadvertent withdrawal from the fuel assembly when it is in the core section, said interlock being inaccessible from above the core section such that the fuel assembly must be removed from the reactor before the interlock can be released and the safety device then removed from the fuel assembly.
7. The fuel assembly of claim 6 wherein the interlock means includes a release element which normally projects below a lower portion of the fuel bundle and is arranged such that depression of same occurring on placement of the fuel bundle on a surface after removal from the reactor effects release of the interlock and allows withdrawal of the safety device from the fuel bundle.
8. A fuel assembly adapted to form a part of a nuclear reactor core section, said fuel assembly including an array of fuel pins disposed in pre-selected spaced relation to one another, the fuel assembly being arranged to permit a liquid heat transfer medium and moderator to flow upwardly in contact with the fuel pins to remove heat therefrom, characterized by at least one passive safety device disposed within the array of fuel pins, said safety device having an upper portion located above the level of the array of fuel pins so as to be exposed to the upward flow of heat transfer medium which has been heated during passage through the array of fuel pins and a lower portion disposed within the array of fuel pins, and a neutron absorber normally disposed in said upper portion of the safety device and having a melting point such that when the temperature of the heat transfer medium to which said upper portion of the safety device is exposed exceeds a selected temperature the neutron absorber melts such that it has the capability of flowing downwardly by gravity into said lower portion of the safety device to absorb neutrons and depress the neutron flux and the heat being generated in the vicinity of the neutron absorber, and wherein said upper portion of the safety device defines a compartment for holding the neutron absorber, said compartment having a lower exit opening through which the molten neutron absorber can flow, and wherein a fuse element of relatively small mass as compared with the mass of the neutron absorber is disposed in said exit opening, said fuse element having a higher melting temperature than does the body of neutron absorber in said compartment so that when the temperature of the heat transfer medium exceeds the selected temperature which is below a predetermined trip temperature the body of neutron absorber melts but is retained in the compartment by the fuse element, said fuse element being arranged to melt at the higher trip temperature such that when the latter is reached the exit opening is unblocked and the previously melted neutron absorber flows down into said lower portion of the safety device.
9. The fuel assembly of claim 8 including fins on the exterior of said compartment to enhance the rate of heat transfer into the neutron absorber.
10. The fuel assembly of claim 8 wherein said safety device is arranged such that it can be inserted endwise and fastened into each fuel bundle from above with means to define the axial location of the safety device within the fuel assembly and an interlock means to secure the safety device against inadvertent withdrawal from the fuel assembly when it is in the core section, said interlock being inaccessible from above the core section such that the fuel assembly must be removed from the reactor before the interlock can be released and the safety device then removed from the fuel assembly.
11. The fuel assembly of claim 10 wherein the interlock means includes a release element which normally projects below a lower portion of the fuel assembly and is arranged such that depression of same occurring on placement of the fuel assembly on a surface after removal from the reactor effects release of the interlock and allows withdrawal of the safety device from the fuel assembly.
12. A fuel assembly adapted to form a part of a nuclear reactor core section, said fuel assembly including an array of fuel pins disposed in pre-selected spaced relation to one another, the fuel assembly being arranged to permit a liquid heat transfer medium and moderator to flow upwardly in contact with the fuel pins to remove heat therefrom, characterized by at least one passive safety device disposed within the array of fuel pins, said safety device having an upper portion located above the level of the array of fuel pins so as to be exposed to the upward flow of heat transfer medium which has been heated during passage through the array of fuel pins and a lower portion disposed within the array of fuel pins, and a neutron absorber normally disposed in said upper portion of the safety device and having a melting point such that when the temperature of the heat transfer medium to which said upper portion of the safety device is exposed exceeds a selected temperature the neutron absorber melts such that it has the capability of flowing downwardly by gravity into said lower portion of the safety device to absorb neutrons and depress the neutron flux and the heat being generated in the vicinity of the neutron absorber, and wherein said safety device is arranged such that it can be inserted endwise into each fuel assembly from above with means to define the axial location of the safety device within the fuel assembly and an interlock means to secure the safety device against inadvertent withdrawal from the fuel assembly when it is in the core section, said interlock being inaccessible from above the core section such that the fuel assembly must be removed from the reactor before the interlock can be released and the safety device then removed from the fuel assembly.
13. The fuel assembly of claim 12 wherein said lower portion of the safety device comprises an elongated tubular body arranged such that it contains a neutron moderator during use to reduce neutron leakage from the active region of the core.
14. The fuel assembly of claim 13 wherein said tubular body is vented so as to be filled with the liquid heat transfer medium when in use to reduce the neutron leakage along the safety device, the neutron absorber being of a material which does not react with the heat transfer medium.
15. The fuel assembly of claim 13 wherein said tubular body has an axially arranged rod of moderating material therein to effect the reduction in neutron leakage, said rod being arranged so that an annular space is provided between the rod and the tubular body to receive and contain the molten neutron absorber flowing downwardly from the upper portion of the safety device.
16. The fuel assembly of claim 13 wherein said upper portion of the safety device defines a compartment for holding the neutron absorber, said compartment having a lower exit opening through which the molten neutron absorber can flow by gravity.
17. The fuel assembly of claim 16 wherein a fuse element of relatively small mass as compared with the mass of the neutron absorber is disposed in said exit opening, said fuse element having a higher melting temperature than does the body of neutron absorber in said compartment so that when the temperature of the heat transfer medium exceeds a first predetermined temperature below a selected trip temperature the body of neutron absorber melts but is retained in the compartment by the fuse element, said fuse element being arranged to melt at the selected trip temperature such that when the latter is reached the exit opening is unblocked and the previously melted neutron absorber flows down into said lower portion of the safety device.
18. The fuel assembly of claim 12 wherein the interlock means includes a release element which normally projects below a lower portion of the fuel bundle and is arranged such that depression of same occurring on placement of the fuel bundle on a surface after removal from the reactor effects release of the interlock and allows withdrawal of the safety device from the fuel bundle.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002042896A CA2042896A1 (en) | 1991-05-17 | 1991-05-17 | Passive safety shutdown system for nuclear reactors |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002042896A CA2042896A1 (en) | 1991-05-17 | 1991-05-17 | Passive safety shutdown system for nuclear reactors |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2042896A1 true CA2042896A1 (en) | 1992-11-18 |
Family
ID=4147622
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002042896A Abandoned CA2042896A1 (en) | 1991-05-17 | 1991-05-17 | Passive safety shutdown system for nuclear reactors |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2042896A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112366011A (en) * | 2020-10-27 | 2021-02-12 | 中国核动力研究设计院 | Control method and system for nuclear power of heat pipe reactor |
-
1991
- 1991-05-17 CA CA002042896A patent/CA2042896A1/en not_active Abandoned
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN112366011A (en) * | 2020-10-27 | 2021-02-12 | 中国核动力研究设计院 | Control method and system for nuclear power of heat pipe reactor |
| CN112366011B (en) * | 2020-10-27 | 2022-03-01 | 中国核动力研究设计院 | Control method and system for nuclear power of heat pipe reactor |
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