GB2545030A

GB2545030A - Rectangular nuclear reactor core

Info

Publication number: GB2545030A
Application number: GB1521490.1A
Authority: GB
Inventors: Richard Scott Ian
Original assignee: Individual
Current assignee: Individual
Priority date: 2015-12-06
Filing date: 2015-12-06
Publication date: 2017-06-07
Also published as: GB201521490D0

Abstract

A nuclear reactor core comprising a array of fuel assemblies containing a plurality of fuel tubes containing nuclear fuel. The fuel assemblies are aligned in rows, with the fuel assemblies migrating along the rows during operation. At the end of a row, spent fuel assemblies are removed, and from the opposing end of the same row, fresh fuel assemblies are added. During operation adjacent rows of fuel assemblies are migrated in opposite directions, along one dimension of the core, which may be square or rectangular thereby achieving a relatively uniform fission rate across the reactor core. The fuel assemblies may be migrated while the reactor is operational, alternatively the reactor can shut down, or control blades may decrease the power level in areas of the core whilst allowing the remainder of the core to remain critical. Preferably the fuel tubes contain molten salt nuclear fuel, and the assemblies are immersed in a coolant medium. Several rows of assemblies may be assembled into rectangular modules that can be linked end to end to create a rectangular reactor core, where the length of the traversing row of assemblies may be of an indefinite length.

Description

RECTANGULAR NUCLEAR REACTOR CORE

Technical field

The present invention relates to a simple procedure to maintain reactivity in a nuclear reactor core as fissile isotopes are consumed by replacement of spent fuel assemblies with fresh ones.

Background

The cores of nuclear reactors are generally cylindrical in shape so as to minimise the ratio of surface area to volume and hence the rate of neutron leakage. Rectangular cores have been proposed, for example the Russian RBMK2400 (A. P. Aleksandrov, N. A. Dollezhal Soviet Atomic Energy 43,985) but have rarely if ever been constructed.

Nuclear reactors containing cores formed from assemblies of tubes containing molten salt fuel have been described by Scott (GB2508537). Such reactors have substantial advantages over solid fuelled reactors. Replacement of spent fuel assemblies with fresh ones is one mechanism used to maintain the reactivity of the core as fissile isotopes are consumed and methods to do this without raising the fuel assemblies out of the core were described in PCT/GB2015/050484. In that patent, gradual migration of the fuel assemblies towards the centre of the reactor core, followed by relatively rapid movement out of the core along an "exit row" was described. Achieving this required a fuel assembly moving apparatus capable of moving assemblies in two horizontal directions, a relatively mechanically complex procedure. A method of maintaining reactivity by moving fuel assemblies in a single direction would be advantageous due to the greater mechanical simplicity of a system required to do that but would be most inefficient in a cylindrical reactor core, not permitting uniform or high burnup of the nuclear fuel.

Summary

According to one aspect of the present invention, a nuclear reactor using molten salt filled fuel tubes in fuel assemblies is constructed with the core being rectangular in form and fuel assemblies being square or rectangular in cross section. Fuel assemblies within one row of fuel assemblies are migrated across the rectangular core in a single direction. Adjacent rows of assemblies are migrated in opposite directions. This is illustrated in figure 1.

According to a second aspect of the invention several rows of assemblies are assembled into rectangular modules which can be linked end to end to create a rectangular reactor core of width equal to the length of the traversing row of assemblies but of indefinite length. The core width can be set such that the rectangular core is transportable intact by road or rail.

Brief description of the drawings

Figure 1 is a top view of a rectangular reactor core demonstrating the migration of fuel assemblies in opposite directions in alternate rows.

Figure 2 is a side view of an example of a bottom catch being a spike at the bottom of the fuel assembly that locates into a corresponding hole is a supporting diagrid.

Figure 3 shows an example of a top catch that allows spring pressure to hold down and positively locate the assembly under normal conditions while allowing its movement when required.

Figure 4 shows an example of how fission rate and average fissile concentration vary across a reactor core

Figure 5 shows a cross section of a fuel assembly containing a graphite core

Figure 6 shows the power distribution across a rectangular reactor core where 66% fuel burnup was achieved

Figure 7 shows an illustration of a reactor structure showing the mechanisms for moving fuel assemblies and removing or inserting them into the core

Detailed description

The reactor core consists of a number of fuel assemblies comprising a multiplicity of tubes containing the molten salt nuclear fuel. The assemblies are at least partly immersed in a coolant medium, which may be a second molten salt or may be another liquid coolant such as a molten metal such as sodium, potassium, lead, bismuth or mixtures thereof. The coolant is at such a level as to completely cover the fuel filled regions of the fuel assembly. The structure of the assembly can be similar to those extensively developed for solid fuelled reactors. The assemblies are of approximately square or rectangular cross section, although other cross sections permitting assemblies to fit closely together while being able to be moved along the row of assemblies are possible, including triangular sections.

The fuel assemblies are arranged in a series of rows as shown in figure 1. The fuel assemblies are securely supported from the top and bottom by top and bottom catches. Fuel assemblies are moved across the row by disengaging the top and bottom catches then moving the assembly to the next location in the row. Many mechanisms can be envisioned for achieving this, one is illustrated in figure 2 where the bottom catch is a conical/pyramidal "spike" at the bottom of the assembly which locates in a corresponding hole in a support grid, the diagrid, below the core. The catch is disengaged by raising the fuel assembly a short distance, prior to moving it laterally and reinserting the spike into the next hole in the diagrid. A suitable mechanism for the top catch is shown in figure 3. This mechanism incorporates springs which hold the fuel assembly firmly in the diagrid against any buoyancy forces together with pegs that firmly anchor the top of the assembly into the supporting top grid structure. The pegs are disengaged and the springs fully compressed by vertical pressure from the fuel assembly moving machine which then locks the springs in the fully compressed position prior to raising the fuel assembly sufficiently to disengage the bottom catch and then moving the assembly laterally.

Spent fuel assemblies are removed from the end of their row of assemblies by the same movement system. They are then moved away from the core. Optionally they can be moved laterally sufficiently far from the core to be out of the intense neutron flux and allowed to cool while still immersed in the coolant until decay heat has fallen sufficiently for them to be safely raised out of the coolant and the reactor tank. When a spent fuel assembly has been removed, the remaining fuel assemblies in that row are migrated by one position leaving a gap at the opposite end of the row. A fresh fuel assembly is then inserted in that gap.

Control of the core reactivity can be entirely by passive means, based on the high negative temperature coefficient of reactivity of molten salt fuel. However it can be convenient to provide neutron absorbing shut down or control elements. These can be located as blades of neutron absorbing material which can be inserted between adjacent rows of fuel assemblies. Standard fail safe electromagnetic systems as used in most reactor control rods can be used to control the blade position.

Movement of fuel assemblies can be carried out while the reactor is operational provided that adequate heat removal from the fuel assembly can be maintained during the process. Alternatively the reactor can be shut down during the fuel changing process or control blades can be used to decrease the power level in a particular row or rows of fuel assemblies while allowing the core as a whole to remain critical.

Movement of the fuel assemblies can be simplified by providing the fuel assemblies with an upper region above the level of the fuel tubes which is narrower that the main part of the fuel assembly. This provides space for support structures separating the rows of fuel assemblies from their neighbouring rows. It also creates space above the fuel filled portion of the reactor core for instrumentation including neutron and temperature sensors to be placed close to the active region of the core.

The principal purpose of the migration of adjacent fuel assembly rows in opposite directions is to maintain an approximately uniform average concentration of fissile isotopes within the reactor core. Migration in a single direction would result in high power and neutron flux on one side of the core and low power and neutron flux on the other side.

Figure 4 illustrates how this is achieved. Curve 401 shows the distribution of neutron flux across the core. Curve 402 and 403 show the fall in fissile isotope concentration across two adjacent rows of fuel assemblies. Curve 404 and 405 show the fission rate across the two adjacent rows. Curve 406 shows the average fission rate between two assemblies in adjacent rows across the core. It can be seen that the consequence of the migration of the rows of assemblies in opposite directions is a relatively flat average fission rate across the core.

Multiple rows of fuel assemblies can be incorporated into modules which can contain the fuel assembly support structure, assembly moving apparatus, heat exchangers, pumps, instrumentation etc. These modules can be assembled into longer rectangular reactors providing a simple method to create reactors of differing power levels with similar, potentially factory manufactured and assembled, modules.

Fuel assemblies may optionally contain neutron moderating materials such as graphite or zirconium hydride. Such reactors will operate in a thermal or epithermal neutron mode. Replacement of the moderator at the same time as the fuel overcomes the otherwise substantial problem of short life of materials such as graphite and zirconium hydride in intense neutron fields. An example of such a fuel assembly is provided in figure 5. EXAMPLE 1 A rectangular fast reactor core was constructed in a neutronic computer model. The analysis was performed using MCNPX for neutron transport simulation. Neutron scattering cross sections are sampled from the ENDF/B-VII.1 libraries. Fission product composition was calculated from MCNPX simulations using the ENDF/B-VII.O which have CINDER90 transmutation library information. The simulation results were analysed and plotted utilizing the CERN ROOT Framework.

The fuel tubes have an external diameter of 10 mm and a separation distance of a minimum of 1 mm in the lattice is ensured by a 1 mm diameter helically wrapped wire. The tube wall thickness is 0.316 mm. The fuel tubes are modelled as 204 cm tall tubes (outer)containing 160 cm fuel and a 40 cm void (gas plenum) above and two 2 cm thick tube end plugs. The helically wrapped wire is modelled as a vertical cylinder of diameter 1 mm aligned along the tube.

Besides of the coolant salt between tubes, a 100 cm coolant salt layer is modelled above and below the fuel tubes to act as a reflector. Both the tube and the wire are made from the metal Nimonic PE16. In this study the following low concentration elements are omitted from the PE16 material model: S, Ag,Bi, Pb, and Zr (boron is modelled, despite its low concentration). Material temperatures and densities are modelled as constant everywhere. Coolant salt is modelled as 41ZrF4-lZrF2-10NaF-48KF with a density of 2.77 g cm-3, and cross sections are based on a 600 KENDF/B-VII.O scattering database Doppler broadened to 773 K. Structural PE16 uses a 900 K ENDF/B-VII.O database with no broadening and has a density of 8.00 g cm-3.

Fuel salt is modelled at a density of 3.1748 g cm-3 using a 900 K database Doppler broadened to 1103 K. The fuel salt is a close-to-eutectic mixture of 60%NaCI with different fractions of UCI3, PUCI3, and fission products depending on initial composition and burnup level. No thermal neutron treatment scattering kernel has been applied, the effect of such thermal treatment is expected to be insignificant as this reactor is a fast reactor. The fuel assemblies are modelled as 201 x 199.0 mm2 hexagonal lattice, containing fuel tubes arranged in a tightly packed hexagonal array of 18x21. The core tubes in two neighbouring assemblies have a minimum separation distance of 2 mm. The core was modelled as a cuboid consisting of 10 x 19 assemblies (10 'wide', 19 'long'). 1/4 symmetry was assumed, using reflective boundaries. A lm layer of coolant salt is modelled on all sides of the core.

The simulation was based on a 66% consumption of initial fissile atoms during the period when the fuel assembly moved across the core (10 steps). Initial fuel composition was 16 mol% reactor grade plutonium trichloride, 24% natural uranium trichloride and 60% sodium chloride. Initial fissile concentration was 11.5% Pu-239/241 and this reduced to 3.8% in the spent fuel.

Figure 6 shows the average power density in the 10 assemblies along one row of assemblies (Row A) together with the power density in the neighbouring row (B) where assemblies are migrated in the opposite direction. Also shown is the sum of the power densities in the adjacent rows which indicates the distribution of average power density across the width of the entire core.

Power density for the individual fuel assembly is sustained at a relatively constant level until the assembly passes the mid point of the core. After this, power density falls significantly, over 50%, due to the combination of falling fissile isotope concentration and reduced neutron flux. Averaged over the adjacent rows however the power density peaks at the centre of the core but declines only by 33% at the edges of the core, an acceptably flat power distribution. At higher fissile isotope burnup, the average power does not peak at the centre line of the core but in two regions either side of the centre line. EXAMPLE 2

Figure 7 shows a possible configuration of a rectangular core reactor with counter flow movement of fuel assemblies. Narrow slots in the reactor lid above the fuel assemblies are used to positively locate the upper catch of the fuel assembly while permitting lateral movement of the assembly when the catch is disengaged. Wider slots at each end of the narrow slots allow fuel assemblies to be inserted into and removed from the reactor tank.

Claims

CLAIMS 1) A nuclear reactor core comprising a rectangular array of fuel assemblies containing a plurality of fuel tubes containing nuclear fuel, where fuel assemblies are migrated during operation along single rows along one dimension of the rectangular core with fresh fuel assemblies being added to one end of the row and spent fuel assemblies removed from the other end of the same row, where adjacent rows of fuel assemblies are migrated in opposite directions.