RU2537013C2 - Fuel composition for water-cooled thermal-neutron reactors - Google Patents

Abstract

FIELD: physics, atomic power.

SUBSTANCE: invention relates to nuclear engineering, particularly thermal-neutron reactor fuel. The fuel composition for water-cooled thermal-neutron reactors includes a mixture of regenerated plutonium and enriched uranium in the form of oxides, wherein the enriched uranium used is enriched regenerated uranium, with the ratio of components defined by energy potential equal to the potential of freshly prepared fuel from enriched natural uranium which provides 100% reactor core loading.

EFFECT: invention enables to fully and simultaneously recycle regenerated uranium and plutonium extracted from spent nuclear fuel.

6 cl, 4 ex

Description

The invention relates to the field of nuclear technology and, in particular, to nuclear fuel using thermal neutrons.

Currently, water-cooled reactors of nuclear power plants, most of which are pressure reactors (PWR, VVER), are loaded with a uranium dioxide UO2 fuel composition containing 3.5–5% of the ²³⁵ U isotope (30–50 kg / t U for reactors) ) This is necessary to ensure an average burnup of irradiated nuclear fuel (SNF) of 30-50 GW * day / t HM (heavy metals + fission products - PD) in the load with a tendency to increase it to 70 GW * day / t HM. Such high burnup is achieved by the rational movement of spent nuclear fuel through the reactor zones, and the number of such annual overloads during a 1 year reactor campaign increases with burnup.

VVER-1000 SNF with an initial enrichment of 4.33% (43.3 kg / t) ²³⁵ U and a burnup of 50 GW * day / t contains 8.6 kg / t ²³⁵ U with 5.7 kg / t ²³⁶ U, which is moderate neutron absorber, as well as 11 kg / t of Pu, including 7.7 kg / t of the sum of the odd (fissile) isotopes ^{239 + 241} Pu. Excluding compensation of even isotopes, the energy potential of fissile isotopes for SNF VVER and PWR-1300 is ~ 30-35% of the initial one and can in principle be reused after reprocessing the SNF of the NPP. However, for this there are various kinds of limitations both in the design of reactors and in biological protection in the manufacture of regenerated fuel for reactors.

Currently, in Russia, RBMK and VVER-1000 reactors partially use regenerated uranium, and mixed uranium-plutonium regenerated fuel in pressure reactors is partially used only in France, with uranium and plutonium being separated during reprocessing of spent nuclear fuel. Plutonium of the indicated or close isotopic composition with a content of 60 kg / t is used as oxide fuel mixed with depleted uranium (2.5 kg / t ²³⁵ U), which load 30% of the zone of obsolete PWR-900 reactors, which is 40% of the energy capacity of nuclear power plants, that is, using regenerated plutonium, only 12% of the electricity produced at nuclear power plants is produced. The corresponding amount of SNF is processed at the UP-2 plant in France, and the rest of the SNF (~ 40%) is stored. In France, the separated regenerated uranium is enriched experimentally with the loading of the active zone (AZ) of two reactors (3% of capacity) with the prospect of increasing to 24% of capacity over 5 years due to the accumulated regenerate of previous years. SNF from reclaimed materials is not serially processed.

It should be noted that MOX fuel, consisting of regenerated plutonium and depleted natural uranium, is the only type of mixed oxide fuel used one way or another on an industrial scale. The reasons why plutonium is blended with depleted uranium is outside the scope of this process.

Calculations proving the possibility of only partial loading of VVER-1000 reactors with MOX fuel give a similar picture, as a result of which this solution has not yet been applied at Russian nuclear power plants.

A serious additional complication in the implementation of the program for using regenerated materials is the need to produce fuel from them in protective equipment (including the enrichment of regenerated uranium) due to the high toxicity of plutonium and sufficiently strong gamma radiation of daughter actinides as by-products of nuclear reactions.

To overcome the drawback noted above, such as incomplete loading of the AZ reactor with MOX fuel, a MICS fuel composition was proposed, which provides for a reduction in the amount of plutonium in the starting MOX fuel composition (regenerated plutonium with depleted uranium) when it is fed with the estimated amount of natural enriched uranium (Youinou G., Delpech M., Guillet JL, Puil A., Aniel S. Plutonium Management and Multirecycling in LWRs Using an Enriched Uranium Support. Proc. Int. Conf. Global′99, (USA, 1999)). Such fuel can load 100% of the core of the PWR-1300 reactor with the usual annual overload circuit, which allows it to be used unlimitedly in the nuclear fuel cycle (NFC) of thermal neutron reactors. Moreover, such fuel can be processed and used in cyclic mode with the corresponding recharge of ²³⁵ U between cycles. This composition is chosen by us for the prototype. It should be noted that the use of primarily regenerated uranium was not covered by the prototype.

We justified a similar composition for the VVER-1000 reactor (Pavlovichev AM, Pavlov V.I., Semchenkov Yu.M., Fedorov Yu.S., Bibichev B.A., Zilberman B.Ya. Neutron-physical characteristics of the core of the VVER reactor -1000 with 100% fuel loading from a mixture of regenerated uranium, plutonium and enriched uranium. -Atomic Energy, 2008, v. 104, No. 4, p. 196-198) with the difference that the starting mixture is an undivided mixture of uranium and plutonium recovered during reprocessing of VVER-1000 spent nuclear fuel, while maintaining previously allocated regenerated plutonium, as well as quenched natural uranium (the latter is similar to the prototype). The cycling of this composition was not considered, however, in an earlier work (Fedorov Yu.S., Bibichev B.A., Zilberman B.Ya., Kudryavtsev E.G. Use of regenerated uranium and plutonium in thermal reactors. Atomic energy. 2005. t 99, issue 2, pp. 136-141), multiple cycling of an undivided mixture of regenerated uranium and plutonium upon replenishment with enriched natural uranium was described. Thus, uranium regenerated from spent nuclear fuel was involved in the nuclear fuel cycle (NFC). Regenerated nuclear fuel, including in one form or another a mixture of regenerated uranium and plutonium, was called REMIX fuel.

A common drawback of all these compositions is the lack of reduction in the storage facilities of the previously accumulated SNF of the NPP with some “swelling” of its amount during cycling due to the inadmissibility of feeding the composition with enriched natural weapons-grade uranium or close to it (enrichment of 80% ²³⁵ U or more) due to limitations non-proliferation of weapons-grade nuclear materials. In the prototype, this is compounded by the fact that excess depleted uranium from the starting MOX fuel is involved in the cycle. The second drawback arising from the first is the ever-expanding production of plutonium-containing regenerated fuel, which requires a protective execution of the production, with a corresponding reduction in the production of original nuclear fuel from natural materials in the usual version, up to the complete closure of such production or the release of only starter loading kits for new nuclear power plants. Obviously, such a NFC will never reach a stationary state.

The objective of the claimed invention is to develop a fuel composition of the type REMIX, which allows to completely and simultaneously utilize the regenerated uranium and plutonium extracted from spent nuclear fuel at 100% loading of the reactor zone.

This is achieved by the fact that enriched regenerated uranium or its mixture with enriched natural uranium is introduced into the fuel oxide composition based on regenerated plutonium, with a ratio of components determined by the energy potential equal to the potential of freshly prepared nuclear fuel from enriched natural uranium, providing 100% loading of the reactor core . At the same time, depleted regenerated uranium of low nuclear quality is disposed of as intermediate level waste (SAW) for disposal.

Such a result can be achieved in several excellent ways. For example, a composition can be obtained in the form of a mixture of plutonium oxides and a portion of regenerated uranium within the limits of non-proliferation of nuclear materials (less than 19% of the amount of fissile nuclides in the mixture), which is then mixed with enriched regenerated uranium that satisfies the same requirements. At the same time, the recovered materials can be separated at different times from SNF of various batches and burnups, including from different reactors, and they are completed on the basis of calculations of equipotentiality (approximate equivalence of energy production and characteristics of the formed core).

The term “equal energy potential” means the amount of energy (burnup, GW * day / t U) that can produce nuclear fuel in a reactor of a certain type until reactivity (ability to maintain a chain reaction) is lost while loading the entire zone of the reactor or a certain part of it, which is determined by the balance of neutrons and their characteristics. This is ensured by a certain content of fissile nuclides (odd isotopes of uranium and plutonium) with compensation for the even isotopes of these elements contained in them. Therefore, fuel from regenerated materials is formally more enriched in fissile isotopes than the original fuel of nuclear power plants from natural enriched uranium. Nuclear fuel from enriched natural uranium often contains additives of one or another amount of a moderator, that is, an absorber of excess neutrons at the start of a campaign; such in the VVER-1000 reactor is gadolinium. In fuel from regenerated materials, even isotopes of uranium and plutonium play the role of moderators. If necessary, fine adjustment of the composition is achieved by introducing a smaller amount of enriched natural uranium into it.

It should be noted that the classic MOX fuel is not equipotential fuel from natural enriched uranium, as it is operated according to a special scheme with less final burnup and fewer annual overloads. In addition, only 30-40% of the area of second generation reactors with excess reactivity (mentioned above PWR-900 in France) can be loaded with such fuel.

In the VVER-1000 reactor, starting enrichment during a full campaign of a stationary reactor with 4 annual overloads with a total burnup of 50 GW * day / t is 4.34%; with a burn-out of 40 GW * day / t, the initial enrichment was 3.6%, and for VVER-TOI it will be 5.8% with a burn-out of 70 GW * day / t. For the RBMK-1000 reactor, the starting concentration is 2.8% ²³⁵ U with a burnup of 28 GW * day / t,

The calculations of equipotential fuel compositions in relation to specific reactor zones are carried out using standard IAEA codes (for example, LA - UR = 03-1987. MCNP - A General Monte Carlo Code, Version 5. License RI C00710MNYCP01 # 7E83-67A4).

The inventive composition is obtained, for example, after reprocessing the spent nuclear fuel of a nuclear power plant using the Purex process in the variant of producing a plutonium reextract in a mixture with part of the regenerated uranium obtained in a subsequent operation, the uranium reextract is subjected to evaporation and denitration, the uranium oxide is fluorinated and the obtained UF _{6 is} enriched in ²³⁵ U to its content is 5-6%, the product is defluorinated, and the oxide from this or one of the preceding batches is dissolved in the above reextract to achieve the calculated isotopic composition, after which I mixed the product denitration after or without evaporation of him; fuel pellets are made from the resulting solid solution of a mixture of U and Pu plutonium oxides, and then fuel assemblies (FA). A small part (up to 10%) of enriched natural uranium or regenerated uranium from spent nuclear fuel with increased starting enrichment can be added to enriched regenerated uranium to regulate the energy potential.

It is advisable to irradiate the proposed fuel composition in the maximum burnup mode in specially designed serial reactors with an increased number of overloads so that the uranium from such SNF (or all SNF as a whole) by the amount of ²³⁵ U isotope is no longer of interest in terms of cycling in the nuclear fuel cycle the near term. The amount of such spent fuel from a PWR (VVER) reactor is reduced by 3-5 times compared with the primary spent fuel, depending on its burnout. At the same time, when reprocessing old SNF of the PWR type, the fuel composition may contain additional Pu taken from the warehouse.

In addition, the fuel composition may consist of a mixture of regenerated plutonium and uranium containing 5.25% Pu and 20% (in balance) of regenerated uranium of the above composition, to which regenerated uranium with enrichment of 17% ²³⁵ U added during the processing of highly enriched SNF is added transport or research reactors. The composition also has an equal energy potential with a standard fresh fuel containing 4.34% ²³⁵ U and is used in a similar manner.

The advantage of this type of composition is the use of a single regenerated fuel containing both plutonium and uranium, and its quantity, and consequently, the number of serial reactors used for it, in stationary mode is 4-5 times lower than their number with a standard composition of natural uranium. This creates the preconditions for using a larger number of reactors for this composition, involving in the reprocessing of already accumulated spent nuclear fuel and / or coming from foreign nuclear power plants, which are prohibited from supplying plutonium-containing nuclear fuel to the standards of non-proliferation of weapons-grade nuclear materials.

Thus, the essence of the invention lies in the fact that as a fuel composition for water-cooled thermal neutron reactors, a composition is used that includes a mixture of regenerated plutonium and enriched uranium in the form of oxides and characterized in that enriched regenerated uranium is used as enriched uranium, at a ratio components determined by the energy potential equal to the potential of freshly prepared nuclear fuel from enriched natural uranium, providing 100% loading of ac ivnoy reactor.

Moreover, the composition for the VVER-1000 reactor with a standard SNF burnup of 50 GW * day / t contains regenerated enriched uranium and regenerated plutonium in concentrations from 0.9% to 5.3% plutonium and from 4.2 to 3.5% ²³⁵ U mixed with its other isotopes, respectively, while ensuring equal energy potential with fresh fuel from natural uranium with an enrichment of 4.33% ²³⁵ U. In this case, the fuel composition may contain regenerated plutonium and part of the uranium regenerated together with it without enrichment, after which the rest is added to it Xia part of enriched reprocessed uranium.

To ensure continuous production, the composition may contain uranium and plutonium, regenerated at different times, including from spent nuclear fuel of reactors of various types and / or lots, and mixed by calculation in any combination to achieve the required energy potential. In order to fine-tune the energy potential of the composition, enriched natural uranium with an enrichment of not less than 4.33 ²³⁵ U and not more than 0.5% of the total enrichment can be added to the mixture of regenerated plutonium and enriched regenerated uranium.

For the manufacture of the fuel composition, a mixture of regenerated plutonium and regenerated uranium is used with the addition of regenerated enriched uranium released during the reprocessing of spent nuclear fuel with increased starting enrichment (for example, fuel from transport reactors).

The subject of this invention is the previously unused combination of the ingredients of the fuel composition, since its exact composition depends on the type and mode of operation of the reactor into which it is loaded, and it is somewhat adjusted as a result of mandatory reactor tests and acquired operating experience, which allows some refinement to the working settlement codes available at each nuclear fuel company.

Example 1

The fuel composition consists of regenerated plutonium and enriched uranium dioxides extracted from VVER-1000 spent fuel with a burnup of 50 GW * day / t (initial amount of actinides in the charge) and a 5-year holding, it contains 5.25% of the mass. Pu (3.41% ^{239 + 241} Pu) and uranium with a composition of 3.45% ²³⁵ U, 2.23% ²³⁶ U and 1.3 ∗ 10 ^-6 % ²³² U (the rest is ²³⁸ U), and uranium is obtained by enrichment of the initial regenerated uranium with an isotopic composition of 0.91% ²³⁵ U, 0.59% ²³⁶ U and 3 * 10 ^-7 % ²³² U (the rest is ²³⁸ U). The composition has equal energy potential with a standard fresh fuel containing 4.33% ²³⁵ U, and is suitable for loading 100% of the VVER-1000 reactor zone.

Example 2

The fuel composition consists of regenerated plutonium dioxide and enriched uranium, separated from VVER-1000 spent fuel with a burnup of 50 GW * day / t and aging for 5 years, it contains 5.25% of the mass. Pu (3.41% ^{239 + 241} Pu) and final uranium 3.45% ²³⁵ U, 2.23% ²³⁶ U and 1.3 * 10 ^-6 % ²³² U (the rest ²³⁸ U), and uranium is obtained by enrichment 84 % of the initial regenerated uranium with an isotopic composition of 0.91% ²³⁵ U, 0.59% ²³⁶ U and 3 ∗ 10 ^-7 % ²³² U (the rest ²³⁸ U) up to 1.2 times more enrichment, and the rest is taken as a mixture of regenerated plutonium and uranium obtained directly from the reprocessing of said nuclear fuel. The composition has nuclear energy properties as in example 1.

Example 3

The fuel composition consists of regenerated plutonium dioxide and enriched uranium, separated from VVER-1000 spent fuel with a burnup of 50 GW * day / t and aging for 5 years, contains 4.75% of the mass. Pu (3.10% ^{239 + 241} Pu) and uranium with a composition of 3.12% ²³⁵ U, 2.0% ²³⁶ U and 1.15 ∗ 10 ^-6 % ²³² U (the rest ²³⁸ U), with uranium obtained by enrichment of the initial regenerated Uranium is illogical to Example 1. To them is added 0.50% ²³⁵ U from enriched natural uranium with an enrichment of 4.33% ²³⁵ U (a total of 3.62% ²³⁵ U) to improve the balance of delayed neutrons. The composition also has an equal energy potential with a standard fresh fuel containing 4.33% ²³⁵ U and is used in a similar manner.

Example 4

The fuel composition consists of a mixture of regenerated plutonium and uranium containing 5.25% Pu and 20% (balance) of regenerated uranium of the above composition, to which regenerated uranium with enrichment of 17% ²³⁵ U added during the processing of highly enriched SNF from transport or research reactors is added analogously to example 2. The composition also has an equal energy potential with a standard fresh fuel containing 4.33% ²³⁵ U, and is used in a similar manner.

Claims (6)

1. A fuel composition for water-cooled thermal neutron reactors, comprising a mixture of regenerated plutonium and enriched uranium in the form of oxides, characterized in that enriched regenerated uranium is used as enriched uranium at a ratio of components determined by the energy potential equal to the potential of freshly prepared nuclear fuel from enriched uranium natural uranium, providing 100% loading of the reactor core. 2. The composition according to claim 1, characterized in that for a VVER-1000 reactor with a standard SNF burnup of 50 GW * day / t it contains regenerated enriched uranium and regenerated plutonium in concentrations of up to 5.25% plutonium and from 4.2 to 3 , 5% ²³⁵ U in a mixture with its other isotopes, respectively, while ensuring equal energy potential with fresh fuel from natural uranium with an enrichment of 4.33% ²³⁵ U. 3. The composition according to claim 1, characterized in that it contains regenerated plutonium and part of the uranium regenerated together with it without enrichment. 4. The composition according to claim 1 or 3, characterized in that it contains uranium and plutonium, regenerated at different times, including from SNF reactors of different types and / or lots, and mixed by calculation in any combination to achieve the required energy potential . 5. The composition according to claim 3, characterized in that the mixture of regenerated plutonium and enriched regenerated uranium contains part of the latter in the form of enriched natural uranium with enrichment above 4.33% for fine adjustment of energy potential. 6. The composition according to claim 1 or 4, characterized in that the mixture of regenerated plutonium and enriched uranium from SNF of a nuclear power plant contains regenerated uranium from SNF of reactors with starting enrichment above 4.33%.