DE1814641B2 - FUEL ELEMENT BUNDLE FOR A THERMAL NUCLEAR REACTOR - Google Patents

Description

The nuclear properties of plutonium is a mixture of several plutonium substances depend on the isotope content of the plutonium isotopes including the fissile Pu 239 and from, d. H. on the ratio of the quantities of Pu 239, 55 Pu 241, breeding material Pu 240 and Pu 242, which in Pu 240, Pu 241 and Pu 242. The proportion of Pu 240 a thermal reactor is a parasitic material is especially important because of its great or poisonous nature. An isotopic combination of Neutron capture resonance at around 1 eV. Neutron plutonium, typical of exhausted uranium deposits in Pu 240 produce Pu 241, which is extracted from a boiling water reactor by means of low-fuel. is consequential energetic thermal neutrons is fissile. 60 so:

The use of plutonium fuel all in one
Reactor originally designed for the use of

Uranium fuel has been sized, requires the isotope

Consideration of differences in reactor operation due to the differences in the nuclear properties 65 Pu 239

shafts of the two fuels. Among the most important- Pu 240

most differences with regard to reactor operation are Pu 241

hear the following: Pu 242

Atomic part

0.590 0.257 0.121 0.032

! 814 641

It is desirable to increase the cost of separating these chemically similar isotopes of plutonium avoid.

There are already fuel element bundles for a thermal nuclear reactor known (French patent 1 314 168), which are arranged in several groups including an inner group and at least one Aufiengrr ^ pe of fuel elements essentially concentrically around the inner group, the fuel elements at least one group of fissile material and Contain thorium 232 in a predetermined initial enrichment and wherein the initial thorium content of the fuel of the fuel assemblies of one group differs significantly from the initial thorium content of the fuel assemblies of every other group. The use of thorium 232 as a breeding material is intended to improve the line distribution in local areas with a high neutron flux. This known proposal, however, is not readily suitable to design a fuel element bundle of the type mentioned eo that the groups of the fuel elements under consideration of the problems mentioned have a suitable different Gehall of fissile uranium and plutonium.

Regarding the use of plutonium The resulting conditions in fuel element bundles of the type mentioned at the beginning are already there known ("Plutonium as a Reactor Fuel", 1967, pp. 3 to 26 and pp. 571 to 583), an uneven distribution of the plutonium content via the fuel elements of the bundle cross-section. The one there basically The indicated solution concerns the proposal (p. 18). the grid spacing or the diameter of the Modify fuel rods. In particular, the fuel elements of the outer group should be there show different levels of enrichment in plutonium. On the other hand, however, goes from the pre-publication shows that, because of the existing difficulties of the type mentioned, it has not been practically so far was possible. Plutonium in combination with uranium in fuel assemblies of the type mentioned at the beginning to use. In particular, there becomes a difficulty in mismatching bundles Bundles discussed (p. 25 below) when the surrounding bundles initially contain plutonium instead of uranium. The difficulties that exist in this connection are intended as far as possible by the invention largely avoided.

It is therefore the task of the invention to create a fuel system of the type mentioned above, taking into account the difficulties mentioned so to train that such an isotopic composition of plutonium burning clay "which is typical for plutonium extracted from irradiated nuclear fuel.

In a fuel element bundle of the type mentioned at the outset, this object is achieved according to the invention in that the fuel elements are divided into several concentric groups with regard to their fuel elements, that the fuel elements of the outer group initially contain fissile uranium but no plutonium, and that the fuel elements contain at least two other groups initially contain fissile plutonium and that the initial plutonium content of the fuel of the plutonium-containing groups differs in that way. that the highest plutonium content is in the innermost group.

In an advantageous development of the invention, the radial change in the initial plutonium enrichment of the fuel assemblies is transverse to the bundle at least 1.5 atomic percent.

Such an arrangement allows, in particular in boiling water reactors, an optimization of the performance and the critical heat flow curve by the

ίο Advantage of increasing the ratio of the thermal gap cross-section of fissile plutonium to the thermal gap cross-section of fissile uranium is used, as well as the higher heat resistance of the central fuel elements.

The invention will be explained in more detail with reference to the drawing. It shows

F i g. 1 shows a plan view of a reactor core of a boiling water reactor,
F i g. Fig. 2 is a perspective view of a typical

of fuel elements,

F i g. 3 shows a cross section through a fuel element bundle with a fuel issue according to the invention and

F i g. 4 the course of the local peak factor for the. Example of embodiment of the fuel element bundle according to the invention.

The exemplary embodiment of the invention is to be explained in connection with a boiling water reactor, the reactor core of which is formed by fuel element bundles 19, each of which surrounds control rods 13 in groups of four. Narrow spaces or gaps N are between the groups of fuel bundles, while wider spaces or gaps W are required between the fuel bundles of each group to accommodate the cruciform control rods. Thus, two sides of each fuel bundle are near control rod arm surfaces while the other two sides are near narrow coolant or water gaps (excluding those around the perimeter of the core). Each bundle 19 can be removed separately from the core 11 . With a typical refueling, around a quarter of the fuel bundles are replaced, ζ. Β one from each group.

F i g. Figure 2 is a perspective view of a typical detachable fuel bundle 19. It has a tubular flow channel 2ü, which is open at the ends, and several elongated fuel assemblies or rods 21, which are held between a lower holding plate 22 and an upper holding plate 23. the Fuel rods 21 run through a plurality of fuel spacers 24 which provide intermediate support. A lower opening 26 takes the current de; The cooling water up past the fuel rods on. For various reasons, e.g. B. because of de; Spacer resistance and the friction of the cooling niiltels along the inner walls of the flow channel 20. the coolant flow is greatest through the middle section (past the internal fuel rods the arrangement.

In Fig. 3 is a typical schematic in plan view. Fuel assembly shown in a fuel bundle. Although the invention does not exist is limited. the bundle has a matrix of 49 fuel rods arranged in a 7 7 matrix are. The fuel rods are also arranged in several essentially concentrically arranged groups!

5 6

divided, whereby the fuel rods in each group are named differently (abbreviated CHF). (The critical heat flow

contain different amounts and types of fissile material. is the heat flow at which there is a large increase

For example, the fuel element bundle contains in the fuel rod cladding temperature at a

from F i g. 3 three groups of fuel rods, namely a small further increase in the heat flux occurs. In

a middle group of fuel rods C, a mid-5 a boiling water reactor that is the point of change

group of fuel rods / and an external or change of seed boiling in stratified boilers.)

Circumferential group of bars P. When the plutonium fuel in the inner

The plutonium is concentrated transversely to the fuel rods, which is graded in the execution of the arrangement so that the plutonium content is the case according to the invention, it is the largest depending on the void space in the inner fuel rods C and the outside trend in the ratio of the inner rod (Pu) -Lei- or circumferential fuel rods P initially contain no pluto power to outer rod (U) power in the same nium. For example, the middle and direction, like the empty space-dependent trend of the combustion rods of groups C and I, can be split up into a fissile ratio of inner rod CHF to outer rod CHF. Plutonium in an amount of 1 to 2 atomic percent. These trends in performance and in critical heat mixed with uranium of 0.3 to 2 atomic percent, effluent are shown in FIG. 4 is shown, where the void fraction stops, while the circumferential fuel rods of group P of the coolant can contain 2 to 3 atomic percent fissile uranium by means of the local steam quality. So if the void fraction (that is. This arrangement ensures increased steam quality) increases with increasing reactor output - heat loading capacity and is particularly advantageous for increases, then the output in the inner fuel sticks increases for use in a boiling water reactor. 20 ben made of plutonium fuel are relatively stronger than that

In a boiling water reactor, the steam power is generated in the uranium outer fuel rods. blow voids in the moderator along the This greater increase in the performance of the center fuel rods. If the reactor output is increased, rods made of plutonium fuel can increase through the height the proportion of moderator vacancies too. The ren critical heat flow of the central combustion rods on the gap cross-section of uranium fuel takes 25 more quickly to be trapped, so that the heat resistance of the as the voids increase, compared to the reactor is increased.

Fissure cross-section of plutonium fuel, which contains uranium fuel in addition to fissile

doesn't change significantly with the white space. Hence U 235 breeding material U 238, which is split into fissile Pu 239

takes the ratio of the power in the internal combustion is implemented during the reactor operation. if

plutonium fuel rods ~ u the power in the 30 plutonium fuel evenly in all fuel rods

Outer fuel rods made of uranium-based fuel with the distributed, presses the relatively large fissure and

growing voids, which are caused by the increased cross-section of plutonium the conversion of

Reactor power arise. U 238 through its competing capture

The inner rods of a fuel element bundle of neutrons. Hence there is another advantage of the

have a higher heat flow capacity than the 35 concentration of plutonium fuel in the

Outer or circumferential rods, because of the green fuel rods in them that the transformation of

The outer coolant flow through the center U 238 in the outer or circumferential rods remains high,

section of the arrangement. The operating limit of those located near the water gaps in areas

Fuel rod heat flux will be critical heat flux at high Neutror.e.i flux.

For this purpose 2 sheets of drawings

Claims (2)

ι 2 1. The capture and gap cross-section for thermal Claims: Neutrons of fission isotopes Pu 239 and Pu 241 is larger than that of the fissile uranium isotope U 235. If 1. Fuel element bundle for a thermal plutonium fuel in a fuel element assembly see nuclear reactor, which is originally designed for the sole use of uranium, which also uses a breeding material such as U 238, then plutonium enters groups with respect to the neutrons of the fuel elements a stronger competition with U 238 than would be the case with borrowed content of fissile uranium and plutonium U 235. Having the plutonium fuel , characterized, therefore reduces the production of new Pu 239 that the fuel elements are made from the breeding material U 238 with regard to their io. The fact that the tonium fuel is present, the generation of fuel elements of the outer group, starting from fissile Pu 241, can more than compensate for the reduction in convertible fissile uranium, but no plutonium euthalation from U 238 to Pu 2? 9. Because of the fact that the fuel elements contain at least two of the initial large exhaustion of the fissile material of other groups initially fissile plutonium rials in a fuel assembly that contain plutonium fuel and that the initial plutonium contains, the mean fuel assembly content of the fuel contains the plutonium gap cross-section with the fuel irradiation differentiates the groups more strongly in such a way that the highest plutonium content in the innermost group 20 is present in the innermost group 20, the only enriched uranium burnt. contains substance. However, if the plutonium fuel 2. Fuel element bundle according to claim 1, containing enough breeding material Pu 240 (z. B. Pluto characterized in that the radial change nium of an isotope mixture, as is typical for erder Initial plutonium enrichment of the fuel rods - scooped uranium fuel of a power factor ;, is). ments across the bundle at least 1.5 atoms 25 can then start the production of new fissile Percent is. Pu 241 from Pu 240 have the consequence that the mean ratio of fissures due to thermal Neutrons to absorption by thermal neutrons (fission · captures) less quickly with the 30 fuel irradiation changes than is the case with a The invention relates to a fuel assembly which is only enriched for a thermal nuclear reactor originally containing uranium fuel. was designed for use with uranium alone 2. Excess hot reactivity is due to and in the group of fuel elements, one of the neutron captures in the one high quei different content of fissile uranium and 35 sectional gross isotope Pu 240 reduced. Have plutonium. At the same time Pluloniunibrennsioff experiences one The exhausted fuel that comes out of the reactor - less change in reactivity during irradiation, is removed, which was originally for the sole use of 3. The proportion of delayed neutrons of Pluto- manure with uranium contains a valuable nium fuel is less than that of L'ranbrenn amount of the original fissile material, a significant 40. ting amount of plutonium including fissile 4. The control bar value is due to the PIu Pu 239 and Pu 241 as well as breeding material Pu 240. This tonium fuel caused ι greater weather exhaustion As is well known, fuel can be processed by burning neutrons in comparison to uranium, reduced by uranium and plutonium for one substance. to regain new use. 45 5. With regard to a boiling water reactor, Pluto- When the cost of reclaimed plutonium fuel is a major negative void fuel with the cost of uranium fuel, the coefficient of reactivity as uranium fuel (for be equal, it is the same water-fuel ratio) above all for economic reasons desirable to use such plutonium fuel because of the moderator density dependence of the reso fuel supply of the reactor and / or for nanzabsorption in Pu 240. The plutonium, which is produced from exhausted or irradiated turn around. system of fuel from a thermal reactor.