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Recycling Weapon-grade U and Pu in U-Th Closed-cycle Reactors


Conceptual Studies for Use of High-Enriched (Weapon Grade) Uranium and Plutonium and Power Plutonium under Conversion of Thermal Reactors to the Closed Uranium-Thorium Cycle

Tech Area / Field

  • FIR-FUC/Fuel Cycle/Fission Reactors

8 Project completed

Registration date

Completion date

Senior Project Manager
Tocheny L V

Leading Institute
VNIIEF, Russia, N. Novgorod reg., Sarov


  • University of Vienna / Institute for Experimental Physics, Austria, Vienna

Project summary

During the last four decades of nuclear energy history the specialists from various countries devoted multiple efforts to studies of uranium-thorium cycle. However for some reasons its adoption together with or instead of uranium-plutonium cycle was not successful.

One of the conversion directions in scientific and engineering activities of RFNC VNIIEF evolving in recent years is the problem for increasing nuclear energy safety. Within this scope, the studies are conducted for the conversion of thermal reactors to closed uranium-thorium cycle. The participating Institutions traditionally performed the studies for fuel cycle.

The project goal is to study the advantages of uranium-thorium cycle over uranium-plutonium cycle for the following applications:

- handling of Irradiated nuclear fuel actinides;
- nonproliferation of fissile materials and nuclear technologies;
- increase In the efficiency of using fuel resources Including weapon-grade materials;
- transmutation of generated long-lived actinides including power plutonium and fission products;
- reactor operation safety increase.

Currently these studies seam to be important because of the urgency of the above problems on one hand and the emergence of sufficient amounts (excessive) of high enriched (weapon-grade) uranium and plutonium and power plutonium required to convert to thorium fuel on the other hand.

Financial support by the ISTC would allow the RFNC VNIIEF specialists involved in the calculations of characteristics of nuclear weapons and initiative basis nuclear energy safety studies to convert to urgent nuclear energy problems within two years of collaboration with Russian and Foreign specialists.

Expected results

The analysis of publications and the studies with the results presented in scientific reports of RFNC VNHEF and participating Institutions allow to conclude the following.

In conventional fuel (235U - 238U) is used the actinides are accumulated both on 238U (239Pu, 240Pu, 241Pu, 242Pu, 241Am, 242Am, 243Cm, 244Cm isotopes) and on 235U, 237Np, 238Pu isotopes) The replacement of this fuel by (233U - Th) naturally leads to the generation of actinides within a single sequence with the primary generation of uranium isotopes (233U, 234U, 232U, 235U, 236U).

The lowest ratio cross sections of radiation-driven capture of 233U neutron and 233U fission in thermal and resonance energy domains as well as the existence of two uranium isotopes (233U, 235U) in the Th sequence decrease the generation of 236U and subsequent isotopes more than by one order of magnitude as compared to uranium-plutonium fuel. In closed fuel cycle the uranium isotopes are extracted from the irradiated fuel and used for production of new fuel. In the remaining radioactive waste the concentration of actinide activity will be primarily determined by their loss in reprocessing of irradiated fuel and production of recycled fuel. The thermal reactor using (233U - Th) fuel operates as an efficient actinide transmuting system.

The generation of 232U in Th of course complicates the fuel recycling. However in early phase after the reprocessing of irradiated fuel the extracted fuel does not actually contain the fission products of radioactive 232U series and hence this simplifies the handling of extracted fuel. The accumulation of decay products linearly increases with time and hence the same is true for increase of radiation of rigid gamma-lines with eg > 2.6 MeV and heat release. The existence of 232U in the fuel conceptually complicates the illegal accumulation of highly enriched uranium, manufacturing (after a long cooling) of explosive devices and handling them because of the increase in the intensity of hard gamma-radiation and heat release. Naturally, the control over such products is simplified. Thus the generation of 235U on Th in a thermal reactor seems to be a key factor in solving the problem of nonproliferation for fissile materials and nuclear technologies. The generation of even-even uranium isotopes (234U and 236U) is a natural denaturation of the generated uranium. Additional denaturation is possible for 238U.

Because of the reprocessing factor of fissile 233U isotope close to unity (233U - Th) fuel in thermal reactors allows to burn almost completely Th. For the traditional fuel (235U - 235U) in thermal reactors the energy release is primarily achieved through the fission of 235U with the concentration of 0.72 % in natural uranium. Thus nuclear energy using thermal neutrons and (233U - Th) fuel demonstrates (with approximately the same amount of natural uranium and thorium) two orders of magnitude higher resources of energy release. The use of weapon-grade uranium and plutonium for the conversion to self-sustained (or minimum feeding) closed uranium thorium cycle seems to be the most reasonable way to use them for conversion. In this case 239Pu, 241Pu or 235U initially burn out on their fission neutrons and then do so on the fission neutrons of generated 233U which provides their deep burnout and makes them unsuitable for the weapon use.

The fissionable 233U isotope in thermal neutrons U-Th cycle Is characterized by the highest effective number of fission neutrons h = 2.28. This allows not only to reproduce it but also to use excessive neutrons for the transmutation of accumulated actinides and fission products. The burning of power plutonium with thorium will result in the deepest burnout of 239Pu and 241Pu and the remaining mixture of plutonium isotopes cannot be used for the manufacturing of explosive devices. Thus the thermal reactor using (233U - Th) fuel has undoubtful advantages over the reactor using the traditional (235U - 238U) fuel.

The high reprocessing level of the fuel allows a stable operation of the thermal reactor for a long time with minimum redundant reactivity. In this view it seems to be possible to increase significantly the nuclear safety of the reactor. In addition, thorium demonstrates a melting temperature 600-degree higher and has a higher resistance to radiation loading as compared to uranium. Thus the replacement of U by Th provides high resources for the Increase of thermal reactor safety.

The following should be noted for the brief discussion of economic motivation for the conversion of thermal power reactors to closed uranium-thorium cycle.

The need for future reprocessing of irradiated fuel is determined by the ecological problems related to the storage of irradiated nuclear fuel. The transmutation of long-lived actinide activity generated on (235U - 238U) fuels seems to be necessary in future too. The future large-scale nuclear energy must be wastefree in terms of high elements and should completely use the accumulated energy. The use of natural capabilities for the increase of nuclear reactor safety using new fuel will remove the cost needed to increase the reactor safety in traditional nuclear energy.

Amount of works, technical approach, methodology, schedule

The work should be reasonably pided in 6 phases and completed in two years.

Phase 1. Analysis and generalization of national and foreign theoretical and experimental studies for the utilization of highly enriched (weapon-grade) plutonium and uranium and power plutonium in existing thermal reactors.

Phase 2. Adaptation of RFNC VNIIEF codes for the calculation of thermal neutrons and their comparison with the codes of the project participating institutions and western partners.

Phase 3. Analysis, estimation and development of special neutron constants for the isotopes of U-Pu and U-Th cycles. Test computations.

Phase 4. Computational studies for the advantages of U-Th cycle relative to U-Pu cycle and data analysis.

Phase 5. Feasibility analysis for the key technologies of U-Th cycle and utilization of weapon-grade uranium and plutonium and power plutonium.

Phase 6. Concept development for the use of highly enriched (weapon-grade) uranium and plutonium and power plutonium when converting reactors to the closed U-Th cycle.

Science and engineering approach and methodology of the project would involve the experience of VNIIEF and participating Institutions In the setup and implementation of large scale science and engineering developments and include the following.

1. Compilation and critical analysis of previous theoretical and experimental studies for the initialization of active 235U, 233U, 239Pu, 241Pu isotopes and other actinides in Th reactors.
2. Development of program complexes, testing on reference integral experiments and coordination of computational results obtained with the codes of participating institutions and foreign partners.
3. Checking and updating of constant support based on the description of experimental data for critical assemblies simulating reactor system units.
4. Optimization studies of the improvement of existing reactors to provide efficient utilization of weapon-grade uranium and plutonium, power plutonium and other actinides during the transition to the closed U-Th cycle.
5. Technical-economic studies of key technologies for U-Th cycle.
6. Generalization of research results in terms of the motivation for the conversion of thermal reactors to the closed U-Th cycle to solve the urgent problems In nuclear energy.

Potential role of foreign collaborators

It is reasonable to discuss the problem setup and results for each project phase with foreign specialists and to tend to the development of consistent approaches for the nonproliferation of issile materials and nuclear technologies, reasonable use of weapon-grade materials and power plutonium, the definition of Th role in nuclear energy.