FI81214B - Nuclear reactor with increased efficiency - Google Patents

Description

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Higher efficiency nuclear reactor

The invention relates to a higher efficiency nuclear reactor which is better able to utilize the fuel of the core elements.

Light-pressure water-cooled and slow-deceleration nuclear reactors have a vessel with a reactor core inside that is immersed in the pressurized water that fills the vessel. The core of the reactor has elements 10 high in relation to its cross-sectional area, which are arranged vertically and side by side. The elements themselves consist of fissionable fuel rod bundles which are in external contact with the reactor cooling water.

A set of control rods connected to some elements of the core 15 is used to control the operation of the reactor. These control rods consist of parallel rods of neutron-absorbing material which can be moved vertically inside the guide tubes to replace some of the fuel rods 20 in the core-forming elements.

One of the major problems associated with the operation of nuclear reactors is the achievement of high power for the nuclear fuel use of the elements. This fuel is generally uranium as uranium oxide, containing primarily fertile uranium-238 and some uranium-235, the fissionability of which varies depending on the degree of enrichment of the fuel. As the reactor operates, the fission fuel dilutes, so at least some of the elements of the core have to be replaced after a certain period of operation.

30 The cost of the enrichment, storage and exchange of the fuel used in the reactor and its treatment is very high, which is why it is desirable to make the best possible use of the fuel placed in the reactor core in order to improve the economy of the reactor. 35 In particular, the aim is to achieve the most complete combustion of the uranium contained in the elemental material.

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By improving the combustion of uranium, one can either increase the lifetime of the heart per given fisslourane initiation charge or decrease the initial lifetime charge of fissile uranium per specified lifetime. In the former case 5, the operating costs of the reactor are reduced by performing recharging less frequently. In the second case, for example, it is possible to either reduce the total volume or mass of the fuel rods of the core or to use a fuel with a lower degree of enrichment. 10 This reduces the cost of refueling.

To control the reactor, i.e. to control the reactivity of the core, neutron absorbing agents are used either as control rods which are placed in the core of the reactor or as substances dissolved in the cooling and deceleration water of the 15 reactors. After charging the core, its reactivity is high, which means that larger amounts of absorbents have to be used to control the operation of the reactor. For example, rod-20 wadding containing consumable toxins is placed in the guide tubes of some elements of the heart, or considerable amounts of neutron-absorbing toxins are added to the cooling water.

As the excess reactivity decreases over time, the concentrations of dissolved neutron-absorbing toxins are reduced at the same time. These neutron-absorbing toxins, which are needed to regulate the operation of the reactor in its initial stages, are inherently expensive and reduce the energy efficiency of the fissionable fuel contained in the core.

It has been proposed to use early cardiac excess reactivity to produce fissile fuel (Plutonium-239) from uranium-238 in fuel cells. To this end, the energy spectrum of neutrons is shifted in the heart towards high-energy neutrons, by reducing the slow ratio of ink volume to fuel volume in the heart, in the first 3,81214 parts of the fuel cycle. When the residual reactivity over the fuel is practically zero, the ratio of the retarder volume to the fuel volume is restored to a value where the neutron spectrum is brought to the range where it is normally in pressurized water reactors. Neutrons are then referred to as “thermal” or “slow.” This results in a new excess of reactivity that extends the life of the fuel.

EP-A-54 787 relates to a so-called "energy spectral transfer reactor" in which the relationship between retarder volume and fuel volume is influenced by placing rods of neutron-permeable substances in the first part of the fuel cycle in some guide tubes of the heart elements. In this way, the water in these tubes is displaced and the retarder volume in the heart is reduced by the same amount.

In order to achieve a significant effect, about 20% of the cooling water has to be transferred during almost 60% of the life of the heart. To do this, it is necessary to use a large number of neutron-permeable rods and push them into all the guide tubes of the core elements, except those used to control the absorbing control rods of the reactor.

This greatly complicates the design and construction of the reactor. Namely, all the devices inside which the reactor core is located have to be dimensioned in such a way that it is possible to perform the control of the reactor operation above the core and the transfer control of the spectrum transfer rods. In this case, a large number of guide tubes have to be placed in the part of the equipment inside the reactor from which the caloric water usually comes out, to the detriment of the water economy of the reactor. In this case, a new arrangement of the cooling water flow in the reaction vessel has to be resorted to. In addition, the silr-35 control of these rods forces the placement of a very large number of control mechanisms on the lid of the reaction vessel, which have to be placed between the mechanisms of the already existing control bundles for controlling the operation of the reactor. In particular, all these coercive measures lead to an increase in the size of the reactor vessel compared to the conventional reactor 5, while the power remains the same.

In addition, the transfer of the neutron spectrum to high energies pain causes more neutrons to escape outside the reactor and the steel in the reactor vessel to become more brittle.

It is therefore an object of the invention to provide a high efficiency nuclear reactor having a vessel containing a core consisting of adjacent and vertically arranged carbonaceous fuel elements and immersed in reactor retarder-15 which can be moved vertically in the core to control the power of the reactor, and a more efficient nuclear reactor with a vessel containing a core of fission-20 eligible fuel elements arranged side by side and vertically and immersed in the reactor coolant and coolant, neutron absorbing rods which can be moved vertically in the core to control the power of the reactor 25, and neutron energy spectrum transfer rods connected to devices capable of both they are displaced vertically and can be pushed or pulled out of at least a portion of the core elements to control the retarder volume and fis-30 fluid volume in the heart and to transfer the neutron energy spectrum with these rods distributed over the entire cross-sectional area of the heart. This nuclear reactor makes it possible to make better use of the fuel contained in the elements and to reduce neutron leakage and thus the embrittlement of the steel of the reactor vessel, while at the same time being simple in structure and design.

To this end, the nuclear reactor according to the invention is characterized in that it comprises 5 - a massive metal partition structure of high energy neutron reflecting material arranged on the outer circumference of the core over its entire height, - two layers of low energy neutron absorbing and containing material 10 the lower part of the heart and the other upper part over its entire cross-sectional area, and - that the energy spectrum transfer rods (27) are a low-energy neutron absorbing material.

According to a preferred embodiment, the spectral transfer 15X rods are connected to every other fuel element in the heart like an oak board.

In order that the invention may be better understood, the following is a non-limiting example of a high power reactor according to the invention.

Figure 1 is a sectional view of a vertical plane of symmetry as seen from a pressure vessel in a nuclear reactor.

Figure 2 is a sectional view taken along line AA of Figure 1.

Fig. 3 is a sectional view taken along line BB of Fig. 1.

Fig. 4 is an enlarged view of a portion of the cross-section of the core shown in Fig. 3, showing the arrangement of the adjustment and spectrum transfer rods.

Figure 1 shows a reactor vessel 1 closed by a convex lid 2.

As shown in Figures 1 and 2, the pressure vessel has four pressurized water inlets 4 and four dewatering openings 5. The openings 4 are connected to the cold branches of the primary reactor cycle and

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Inside the container 1 are suspended internal structures, which comprise in particular a ring 8 which forms the sheath of the core 7 and in the lower part of which there is a base plate 9 of the core. Holes corresponding to the elements of the core are made in the base 9. As shown in Figure 3, the core has 193 elements of rectangular cross-section resting on the base 9.

Above the core 7 are the upper internal structures 11 of the reactor. These upper internal structures 11 comprise rod guide tubes 12 which act as cross-supports and connect the spacer 13 to the core plate 10 to which they are connected. The internal structures also include a top plate 14 to which the tops of the tubes 12 are attached. The plates 13 and 14 are attached to rings coaxial with the ring 8 and remain, such as ring 8, between the jacket 2 and the container 1. Inside the tubes 12 there are control cards 15 and continuous control devices with which the rods can be held and guided as they are moved in a vertical direction in the heart. At the bottom of the tubes 12 there are openings 16 through which the water 20 circulating in the elements provided with fuel bundles can escape towards the outlet of the vessel. The tubes 12 are in two parts, the upper part of the tubes hangs from the upper plate 14 and their lower part acts as a cross-support between the plates 10 and 13.

Between the annular space core 7, the cross-section of which is seen in Figure 3, and the core jacket 8, it is filled with a massive stainless steel partition structure 18, which acts in particular as a reflector of the high-energy neutrons generated in the heart. The massive septum 30 structure virtually fills the entire space between the heart and the myocardium.

The fuel rods of the elements are long zirconium tubes with uranium-235-enriched uranium oxide tablets inside. At each of the 35 ends of the zirconium tubes are tablets of uranium oxide (UO2) with lean uranium-235 about ten centimeters long, which replace the tablets with enriched uranium oxide. Thus, two virtually continuous layers of lean uranium 19 and 20 are formed at the upper end and at the lower end, respectively.

5 These layers 19 and 20 can absorb low-energy neutrons and produce uranium-238, which can be converted to plutonium-239 when bombarded with high-energy neutrons.

The stainless steel partition structure 18 10 and the lean layers 19 and 20 thus cause the escape of neutrons outside the core to a minimum, which improves the reactor output.

In addition, this reduces the leakage of neutrons into the vessel environment.

Figure 2 shows all the guide tubes 12 with which the rods can be guided in the reactor core. For the whole core, 97 absorbent rod bundles are used, each bundle of which can be inserted into the guide tubes of one element.

Figure 1 shows such a bundle of absorbent rods 20, i.e. an adjusting rod 24 in the upper position, attached to a guide arm 25, which in turn moves in a tubular space 26 which communicates inside the vessel. A transfer mechanism (not shown) of the arm 25 is located above the space 26. With such a hook mechanism of the conventional type 25, the adjusting rod 24 can be moved in the vertical direction and very precisely with respect to the tube 12a inside the guide tubes of the element 6a arranged perpendicular to the tube 12a.

Figure 1 also shows a series of neutron energy-30 spectrum transfer rods 27 inserted into the guide tubes of the element 6a. These neutron energy spectrum transfer rods consist of bundles of zirconium alloy tubes filled with lean uranium tablets along their entire length.

35 The control bundles as well as the neutron energy spectrum control tubes are as long as the elements.

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The neutron spectrum transfer rods 27 may be in one or the other of two positions, one being the fully retracted position shown in Figure 1 and the other being the fully retracted position corresponding to the position shown in Figure 1 of the control bundle 5 24. With the transfer device connected to the spectrum transfer rod 27, the rod can be moved from one position to another. When in the fully retracted position, the rods forming the spectrum transfer rod are in their entire length in the element 6a.

It can be seen from Figures 2 and 3 that 96 guide tubes 12 are used in a core with 193 elements, each of which can simultaneously control both one control rod 24 and one spectral transfer rod 27. It can also be seen that these guide tubes 12 are arranged directly in the fuel-15 elements 30 which, like an oak board, are fitted to the cross-sectional area of the heart. Every other element is equipped with both one control rod and one lean uranium spectrum transfer rod.

The elements 31 adjacent to the elements 30 provided with the control rods and the spectrum transfer rods do not come with either type of absorbent rod.

Figure 4 shows the arrangement of the guide tubes 32 in which the control rods are to be placed and the guide tubes 33 in which the spectrum transfer rods are to be placed. Each fuel-25 material element has 56 control tubes, of which 16 tubes will have control rods and 40 tubes will have spectrum transfer rods.

In fact, these rods are placed in the guide tubes of the elements 30 while the reactor is running, while a bunch of plugs are placed in the guide tubes of the elements 31 to provide a pressure drop equal to that caused by the rods placed in the guide tubes 32 and 33 of the elements 30.

The transverse distribution of the bundle assembly 24 forming the control rod is similar to the distribution of the guide tubes 32 35 shown in Fig. 4, while the distribution of the bundle assembly 9 81214 is similar to the distribution of the guide tubes 33 shown in Fig. 4.

At the beginning of the fuel assembly, i.e. after charging, the transfer mechanism of the spectral transfer rods and the lean uranium rods of lime are used to position them at maximum pushed into the fuel elements 30, whereby the ratio of retarder volume to fuel volume in the reactor core is significantly reduced. In addition, each lean uranium rod of the spectral transfer rods 10 absorbs low-energy neutrons locally, causing a stronger shift of the neutron spectrum toward high energies.

This local effect, which is repeated throughout the core evenly due to the spectral transfer rods fitted to every other fuel element 15, causes the general curing of the spectrum inside the reactor core.

In this way, at the same time as the retarder volume decreases and the low-energy neutrons are absorbed, sufficient spectral shift is obtained to cause a significant conversion of the fuel uranium-238 to plutonium-239.

Lean uranium rods absorb those extra slow neutrons that are not needed to maintain a chain reaction in the heart and cause the formation of plutonium-239 when bombarded with higher energy neutrons.

When operation has continued with the spectral transfer rods pushed into the fuel elements for a significant portion of the reactor cycle, these rods are withdrawn. After the spectral transfer rods have been removed, the fissile material generated by the bombardment of high-energy neutrons during the first stage is consumed inside the reactor core.

Due to the additional spectral shift due to lean uranium absorbing rods and when using a stainless steel circumferential reflector and lean uranium absorbing layers on both sides, bottom and top of the core, only every other fuel element in the core can be provided with a total of 5 , comprising at the same time both one control rod and one spectral silicon rod.

It can be seen from Figure 3 that of the 193 fuel elements forming the core, there are 96 elements 30, which become a control rod and a spectral scattering rod, and 97 elements 31, 10, which only become a plug end.

Thus, it is possible to place on the lid 2 of the container 1 control rod transfer mechanisms comprising a coaxial push and remove mechanism of the spectrum transfer rods, whereas instead it would have been very cumbersome to fit such mechanisms perpendicular to each element and would call into question the vessel, its lid and interior design.

The advantage of the device according to the invention is thus that the assembly formed by the control rods and the spectrum transfer rods can be put in place without having to significantly change the reaction vessel of a pressurized water type nuclear reactor. On the other hand, the reflection effect of the circumferential septum of the core, the absorbing effect of the upper and lower layers of lean uranium of the heart, and the greater spectral shift caused by the neutron spectral transfer rods greatly improve the reactor core fuel utilization conditions.

These significant improvements, which provide significant savings in fuel cost, are achieved with relatively minor changes in the structure of the 30 reactors.

The invention is not limited to the embodiment described herein; on the contrary, it encompasses all possible variations.

Thus, for example, a different distribution in each fuel element 30 of the control 35 tubes 32 and 33 to which the control rod and the spectral shifter 81214 will be conceivable is also conceivable.

5 Likewise, one can think of arrangements in which these two kinds of rods are not placed in the same elements of the heart. It is also conceivable to form bundles forming control rods and spectral silicon rods with different numbers of absorbing rods.

10 In the case of spectral scattering rods, it is conceivable to use various absorbents of lean uranium, such that the substance either contains or contains a useful substance which is capable of being converted to a fissile-capable substance by neutron bombardment.

It is also conceivable to have devices of any type, mechanical, hydraulic or pneumatic, for moving the control rods and pushing the spectrum rods completely into or out of the element elements.

The invention is applicable to all types of light pressure water cooled and retarded nuclear reactors having vertically arranged fuel elements within which control rods for controlling the reactor are moved in a vertical direction.

Claims (8)

A nuclear reactor with increased efficiency, comprising a tank (1), in which is a core (7), consisting of fissionable fuel elements (6), which are placed next to each other and vertically and are immersed in a light pressure water constituting the reactor's moderator and cooling fluid, control rods (24) of a neutron absorbing material which can be moved vertically in the core to control the reactor's power, and rods (27) for transmitting neutron energy spectrum, which rods are connected to devices by which they are moved vertically and can be pushed in at least in part of the core element (6) or pulled out completely to control the relationship between the moderator volume and the fission material volume in the core (7) and to transmit the neutron energy spectrum, rods (27) are distributed over the entire cross-sectional area of the core, characterized in that it comprises 20. a solid, metallic partition structure (18) of a material ial reflecting high energy neutrons, the structure of which is applied to the circumference of the core (7) along its entire height, - two layers (19) and (20) of a material which absorbs position energy neutrons and contains utility material, one of which is applied to the lower the end of the core (7) and the other at its upper end over its entire cross-sectional area, and that the energy spectrum transfer rods (27) are of a material that absorbs position energy neutrons. 30 2. A nuclear reactor according to claim 1, characterized in that the solid metallic intermediate wall structure (18) arranged on the outer edge of the core is of stainless steel. Nuclear reactor according to claim 1 or 2, characterized in that the layers (19,20), which ice 81214 is applied to the lower and upper part of the hard and are of a neutron absorbing material, are of uranium, whose uranium-235 is depleted . 4. A nuclear reactor according to claim 3, characterized in that the layers (19 and 20) of depleted uranium in the lower and upper part of the core consist of depleted uranium oxide pads which are applied to both ends of the tubular shells of the fuel rods. Nuclear reactor according to any of claims 1-4, 10, characterized in that the rods (27) for the neutron spectrum transmission consist of rod bundles which are arranged so that they are placed along a whole length (33) of the elements (6). control tubes, wherein each spectrum transfer rod (27) is connected to a single element (6). A nuclear reactor according to claim 5, characterized in that the spectrum transfer rods (27) are connected to each other element (6) in the core in an order corresponding to a lady board across the entire cross-sectional area of this core (7). Nuclear reactor according to claim 6, characterized in that, to each element to which a spectrum transfer rod (27) is connected, there is also connected a control rod (24) which comprises sheets of absorbent material and which access all the control tubes. (32) for elements which do not have any spectrum transfer bar (27), while all other elements (6) of the core will be without both spectrum transfer bar (27) and control rod (24). Nuclear reactor according to claim 7, characterized in that the control rod (24) and the spectrum transfer rod (27) connected to the same element (6) are connected to a common displacement mechanism.