FI80539B - Method for operation of a nuclear reactor moderated and cooled by light water - Google Patents

Description

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Method for operating a light water retarded and cooled nuclear reactor

The invention relates to a method for operating a light water retarder and a cooled nuclear reactor, which reactor comprises a core consisting of fuel elements immersed in light water for deceleration and cooling, which are exchanged at least in part for certain intervals corresponding to reactor operating cycles. energy spectrum transducers evenly distributed in the core are held in the heart to reduce the ratio between retarder volume and fissile material volume and shift the neutron energy spectrum toward higher energy concentrations, and in the second step of the cycle the energy spectrum transducers are removed to operate at lower energy. The invention also relates to a nuclear reactor used in the process.

In known types of light pressure water retarded and cooled nuclear reactors, at the beginning of the fission chain reaction phase, a surplus reactivity is generated at the beginning of the cycle, the amount of which depends on the degree of enrichment of the nuclear fuel, which must be limited to avoid "robbery". Conventional neutron-absorbing toxins are either used as dissolved or consumable, i.e., life-limited toxins, or neutron-absorbing rods, which are then used to adjust cardiac output.

In this way, energy is wasted which could be utilized, for example, to produce a fissionable fuel such as plutonium, thus reducing the need for uranium and its enrichment costs.

35 It has previously been proposed to take advantage of the excess reactivity present at the start of the reaction 2 80539 by reducing the degree of neutron deceleration provided by the cardiac retarder and coolant, allowing the transmitted neutrons with higher energy to reach the 5 useful atoms in the fuel.

When the utility absorbs these high-energy neutrons, it becomes fissionable and in this way an excess of fuel is obtained, resulting in better consumption of natural uranium.

10 The transfer of the neutron spectrum towards higher energies was achieved by reducing the volume of the retarder, whereby its braking effect disturbs the trajectory of fast neutrons as little as possible. It is known to use a method for reducing the volume of this retarder if, at the beginning of the life of the fuel, part of the light water forming the condenser and retarder of the pressurized water boiler is replaced by a certain amount of heavy water. When the retardation capacity of heavy water is greater than that of light water, the reactor is then made to operate in an amplified spectrum, i.e., a spectrum in which high-energy neutrons are used to convert a useful material such as U 238 to a fissile material such as Pu 239. In these reactors, the volume of light water was increased as the fuel became thinner, i.e., as its reactivity decreased, thereby reducing the volume of heavy water, increasing the retarder braking force and consequently shifting the spectrum to lower energies (thermal neutrons). continuation of the fission chain reaction-30 sex by itself.

This retarder volume conversion operation by first adding and then removing heavy water significantly increases the cost due to the cost of the heavy water itself and the need to provide laborious additional cycles for the dilution and recovery steps.

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Another method for modifying the retarder volume in pressurized water-cooled nuclear reactors has been proposed: it uses rod bundles 5 made of a neutron-permeable material (zirconium alloy) and placed in the control tubes of the partial fuel elements to prevent the coolant and retarder from entering them.

These bundles of spectral transfer rods or rods are kept inserted into the guide tubes during the early life of the heart, allowing the neutrons to be absorbed as the Suur-10 energies as the retarding effect of the retarder decreases. While the reactor is running, these bundles are withdrawn so that the retarder-coolant can penetrate the control tubes, thereby sliding towards lower energies, continuing the operation of the reactor in a similar energy spectrum (thermal neutron spectrum) as currently known light water power reactors.

However, such a mechanical modification, in contrast to heavy water-light water dilution, entails considerable stresses for the equipment. In this case, it is important to provide the best possible conversion efficiency of the beneficial agent into a fissile material in order to compensate for the additional equipment costs.

In addition, it is now known in the other reactors under consideration to use the spectrum of high-energy, i.e. fast, 25 neutrons throughout the life of the core, which has a high conversion factor for this core and thus produces a very considerable amount of fissile material for new refueling. For this purpose, the utility rod or element bundles are permanently attached either to the fuel elements of fissile material or to the edge portion of the zone formed by the less permanently fissile material.

It follows from the known methods described above that all studies focus on optimizing the utilization of neutrons 35 so that the absorption of neutrons released in the spectrum of fast neutrons 4 80539 is as productive as possible for conversion to a fissile material such as plutonium. However, although significant results are obtained with known methods for early transfer of the reactor spectrum, the production of plutonium consumed during the second part of the reactor life, when operating in the thermal neutron spectrum, still has a significant limitation for the neutron economy due to the slow the number of extra fissures generated locally by the neutrons, with the reactor operating in the spectrum transferred to the fast neutrons.

It is an object of the present invention to obviate the above disadvantages and limitations. The invention is characterized in that the spectral conversion rods contain a fuel which absorbs slow or low energy neutrons in order to locally effect a strong shift of the neutron energy spectrum towards higher energy concentrations, resulting in a greater conversion of the fuel to fission fuel. The method of the invention can increase the conversion of the beneficial agent to a fissile material and make the best use of cardiac neutron flux.

According to the invention, the absorbent is selected from the group consisting of absorbing slow neutrons that are extra to maintain the fission chain reaction.

The invention will be better understood from the following description and the accompanying figures.

Figures 1a, Ib, 1e show the curves of the neutron spectra as a function of their energy, the curve of the fission effective area of U-235 and Pu-239 used as a fission agent as a function of the energy of the neutrons, and the degree of conversion to plutonium as a function of the energy of the neutrons.

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Figure 2 shows a series of curves depicting the dilution of the fuel fissile material in the different energy spectra used during the cardiac cycle.

Figure 1a shows the population of neutrons required to keep the chain reaction running depending on the energy of the neutrons (the ordinate represents the flow, the abscissa the energy).

Curve A shows the spectrum of neutrons 10 emitted from the fissile material contained in the fuel rods of the core-forming elements of the current type of pressurized water core reactor. The retarder inhibits fast neutrons and it is observed that the population of neutrons (per unit volume) required to keep the fission chain reaction running consists mainly of thermal neutrons with an energy of almost 0.025 eV, which is schematically shown by Ax 15 in Fig. 1a.

Curve B depicts the neutron spectrum of prior art reactors in which the degree of neutron deceleration is reduced by reducing the retarder volume, as schematically indicated by reference numeral D, 20 using spectral transfer rods made of a neutron permeable material. The energy shift of neutrons is shown schematically by Bx.

This shift toward more energetic neutrons corresponds to a spectral cure that can improve the degree of conversion, as shown by B1 in Figure 1e, and consequently obtain more plutonium, which reduces the amount of fissile material that must be used from the original fuel.

However, if the chain reaction is maintained simultaneously with the fission of Pu-239 generated simultaneously in the fission of U-235 atoms, part of the neutron population will be thermalized. This part of the population is thermalized because a certain number of neutrons whose energy is too large to be absorbed and involved in the conversion of the beneficial agent to fissile material slows as they pass through the slow-release tin material. As a result, this population of slow neutrons proves to be extra when the reactivity of the fuel is high and a large amount of consumable toxins still have to be used to keep the growth factor still the same.

Curve C depicts the neutron spectrum used in this invention and their energy is shown schematically by reference numeral Cx.

Based on the fact that the production of plutonium depends on the conversion ratio, the present invention proposes to further reduce the number of slow surplus neutrons by placing lean uranium oxide rods in their path to contribute locally to the local absorption of these slow neutrons, thereby counteracting the relative proportion of the total population of neutrons and consequently the spectrum further hardens, i.e. the fission produced by the higher energy neutrons is obtained as indicated by F in Fig. 1a.

Local curing by temporarily placing lean uranium oxide rods in the fuel elements significantly increases, as shown in Figure 1e (Section C1), the conversion of the uranium-25 ni-238 contained in the fuel elements to plutonium-239, since when these the rods are evenly distributed in the core, there is a general curing of the spectrum in the region of the reactor core.

In addition, the effect of these rods, which are themselves exposed to the flow of fast neutrons, produces an additional amount of plutonium-239, which participates in maintaining the fission reaction when the rods are inserted into the fuel elements. This formation of plutonium-239 is complemented by the conversion of U-235 residues in these rods, which are exposed to the absorbed slow neutrons 35.

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The curves shown in Figure 2 illustrate different dilutions in a core with the same initial degree of fuel enrichment in the different spectra shown in Figure 1a.

5 Curve A depicts 100% theoretical dilution in the heart operating in the thermal neutron spectrum.

Curve B depicts the dilution of the same core, but when spectral curing occurs solely by reducing the retarder volume (D) during the first 10 parts of the fission process, using rod bundles of zirconium alloy. The first part of the curve corresponds to the period during which the rods are inserted into the guide tubes of the fuel elements, which explains the decrease in reactivity seen at the beginning of the curve.

It should be noted, as can be seen from Figure Ib, that the hardening of the spectrum brings with it a reduction in the effective range of U 235. However, as the conversion ratio increases, a larger amount of Pu-239 is formed. As shown in Figure Ib. The effective impact area of fission of Pu-239 is substantially higher in the energy range considered than that of U-235, and the reactivity of the fuel is then generally better.

Thus, the decrease in reactivity of the first part of curve B in Figure 2 is smaller than that of curve A.

25 Approaching the limit of reactivity, after which the growth factors are in danger of falling below the value required to keep the chain reaction running, where the number of radiating neutrons is no longer sufficient, the rods are removed from the guide tubes, as indicated in G. 30 When the rods are removed, the coolant-retarder substance can flow into the guide tubes, increasing the braking power and softening the neutron spectrum and returning to the low-energy neutrons. This removal of the rods causes a return to reactivity due to the increase in the effective fission area of the U 235 Pu 239 35, as can be seen from Figure Ib.

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In addition, the fission agent generated during the first stage of operation, while maintaining the initial degree of enrichment, allows the fission reaction to continue for a longer time, as indicated at H.

Curve C depicts the dilution of the same heart in the method of the invention.

Thus, when rods formed of lean uranium oxide tablets contained in zirconium alloy capsules are placed in the guide tubes, a reduction in retarder volume (as indicated by reference numeral D in Fig. 1a) and slow neutron absorption are achieved, causing local spectral curing.

This procedure thus causes the neutron spectrum to shift completely (since the lean UO2 rods are evenly distributed in the heart-forming fuel elements) to the high-energy neutrons side (as indicated by reference numeral F in Figure 1).

In addition, when the conversion ratio is higher (point Cx in Fig. 1e), a larger amount of Pu 20 239 is produced at this stage, thus contributing to the fission reaction, which causes the starting fuel to dilute, as indicated by C, to maintain the growth factor at the fission reaction. indicated, slower than in the thermal spectrum (curve A) and in the spectrum indicated by reference numeral B.

When the rods are removed (point I, Fig. 2), the spectrum returns to the thermal neutrons and the reactivity recovery resulting from the increase in the effective areas of fission of U 235 and Pu 239 is used, as indicated by reference numeral J at 30.

Referring to Figure 2, the efficiency of using lean uranium oxide retarder rods when the excess reactivity of the new fuel is utilized and still affected during operation with the rods inserted is not detrimental to the overall neutron economy of the nuclear 980539 reactor.

Thus, it is possible to achieve substantially greater savings than using neutron-transmitting rods by shifting the spectrum even further, i.e. by curing it locally, thus increasing the degree of conversion as much as possible, but maintaining neutron economy while ensuring reactor operation and control throughout the cycle. is shifted to the side of fast neutrons.

10 In addition, production of plutonium-239 will increase and may be reused as fuel in future periods. Namely, these rods no longer participate in the chain reaction when they are removed because they are in the devices above the heart and therefore do not thin.

The invention is not limited to the embodiment described herein in which lean uranium is used as the absorbent. Other absorbents that may or may not contain the beneficial agent may be used. Thus, it is possible to use plutonium-232 as a beneficial agent, which produces 20 fission agents as U-233.

The invention is applicable to all water retardant reactors with neutron spectral transfer rods.

Claims (8)

10 80539 A method of operating a light water retarded and cooled nuclear reactor, the reactor comprising a core consisting of fuel elements immersed in light water for deceleration and cooling, which are exchanged at least in part at predetermined intervals corresponding to the reactor operating cycles, the first phase of the cycle 10 vats evenly distributed in the heart are held in the heart to reduce the ratio between retarder volume and fissile material volume and shift the neutron energy spectrum toward higher energy concentrations, and in the second step of the cycle which absorbs slow, i.e. low-energy neutrons, to locally produce 20 energies of neutrons a strong shift of the spectrum towards higher energy concentrations, resulting in a greater conversion of useful fuel to fission fuel. Process according to Claim 1, characterized in that the absorbent used for the spectral change rods is a substance which causes the absorption of slow neutrons which are extra in order to maintain the fission chain reaction. Process according to Claim 1 or 2, characterized in that the spectral conversion rods contain the isotope 235 of poor uranium or thorium. A method according to claim 3, characterized in that the spectral conversion rods are bundled into rods consisting of lean uranium oxide tablets arranged in tubular capsules of zirconium alloy. 11 80539 Method according to one of the preceding claims, characterized in that the fuel elements simultaneously contain a fission material and a useful material and in that the energy spectrum transfer rods essentially comprise a useful material consisting of uranium 238 and / or thorium. Method according to one of the preceding claims, characterized in that the energy spectrum change rods consist of rod bundles which are removed from the 10 cores relative to the operation in the second stage of the cycle by pulling them out of guide tubes arranged in the fuel elements. 7. A light water retarded and cooled nuclear reactor comprising a core consisting of fuel elements embedded in light water, each fuel element containing fission material and useful material and provided with a guide tube; rod bundles for absorbing neutrons to control heart rate; and rod bundles for changing the energy spectrum, which bundles can be moved inside the reactor via at least some control tubes distributed in the core between two positions, one of which bundles are placed in the core in the first operating step of the reaction so that the neutron energy spectrum shifts towards higher deceleration ratios; and in the second, the rods are located in devices arranged above the core, characterized in that the rods of the energy spectrum transformer rod bundles contain traces of useful material possibly with fission material to provide further curing of the energy spectrum as a result of enhanced absorption of slow neutrons of the material. Nuclear reactor according to Claim 7, characterized in that the useful material contained in the energy spectrum conversion rod bundles is lean uranium or thorium. i2 80 539