Physics of Plutonium Recycling

Executive summary

The future of the nuclear industry will depend crucially on considerations of economics, resource utilisation and environmental impact/waste management. For nuclear utilities who opt for reprocessing/ recycling of spent fuel, one of the main issues pertinent to these three areas is that of recycling plutonium. Moreover, with the recent commitment to reduce the nuclear weapons stockpiles, there has been renewed and generalised interest in the capabilities of both thermal and fast reactors to help make weapons grade plutonium less readily accessible for use in weapons. Additionally there is renewed and widespread interest in the possible role of fast reactors in burning minor actinides. For all these reasons the OECD/NEA Nuclear Science Committee decided to convene an international study group, the Working Party on Physics of Plutonium Recycling (WPPR), to review the physics aspects of plutonium recycle.

The remit of the WPPR was broad, covering most of the plutonium recycle systems, including the mainstream thermal reactors, (Pressurized Water Reactors and Boiling Water Reactors), advanced converter thermal reactors and fast reactors. The Study commissioned benchmark studies in areas it considered to be of particular interest.

The report organisation and main findings are as follows:

Chapter 1  sets the scene for the report and introduces each of the subsequent chapters.

Chapter 2  reviews all the aspects of the physics of plutonium recycle in PWRs and BWRs.

Chapter 3  discusses the findings of the first two benchmarks. The agreement between the various solutions is not completely satisfactory: whatever the benchmark considered, the spread in k infinity is not less than 1%. Furthermore, this spread in k infinity could translate in a much larger spread in the plutonium content required to achieve the given reactivity lifetime. This means that there is still a need for improvement in both methods, e.g., self-shielding treatment of Pu 242, and basic data for higher plutonium isotopes and minor actinides. Further experimental validation would also be needed, in particular for integral parameters, e.g., reactivity coefficients, in the case of degraded plutonium isotopic composition. In general, this report underlines that recycle of plutonium in LWRs offers a practical near term option for extracting further energy from LWR spent fuel and reprocessed plutonium. Multirecycling of plutonium in PWRs of current design beyond a second recycling can have intrinsic limitations and the related physics issues have been considered, in particular the plutonium content limitation to avoid positive void effects and the minimisation of minor actinide production during multirecycling. The conclusions reached in this report indicate a good understanding of the physics, although further scenario-type studies (including lattice optimisation) would be needed.

Chapters 4  and 5,  devoted respectively to plutonium recycling and waste radiotoxicity reduction and to the role of fast reactors, indicate that the fast burner reactors / LWR symbiosis offers a potential for significant nuclear waste toxicity reduction by further extraction of energy from the multicycled LWR spent fuel and reprocessed plutonium. However, it has been stressed that for the long term, the best use of plutonium is still in fast breeders. Concerning physics issues, the plutonium-burner fast reactor physics benchmarks display a larger spread in results among participants than has been experienced for more conventional breeder designs. High leakage cores, higher content of minor plutonium isotopes and higher actinide isotopes all need further validation work, including critical experiment performance.

Chapter 6  provides a review of the topic of plutonium fuel without uranium. By using inert carriers for plutonium it is possible to avoid the production of fresh Pu 239 from U 238 captures. This is a topic which has been investigated in the past but is presently of relevance to explore the highest plutonium burnup rates in fast and thermal reactors. New research and development is under way to find potential fuel candidates. The work in this field is closely related to the so-called heterogeneous recycling of minor actinides (i.e. Am) in the form of targets based on inert matrices.

Chapter 7  reviews the possible role of advanced converter reactors as an intermediate step between today’s thermal reactors and future fast breeder systems. Advanced converters are thermal reactors in which the conversion ratio is significantly increased above that which conventional thermal reactors can achieve.

Chapter 8  finally provides a brief review of the physics of recycling uranium recovered from spent fuel reprocessing. Although not directly relevant to the subject of this Study, it was felt important to include this for completeness given the close relationship between uranium and plutonium recycle.

Chapter 9  finally gathers the overall conclusions and gives some recommendations for future co ordinated international work, in particular related to possible limitations in MOX multirecycling in LWRs, nuclear data needs and experimental work.

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Last reviewed: 15 June 2011