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Nuclear fuel cycle

And tomorrow: multiple recycling while generating increasingly less waste

Multi-recycle plutonium, make even better use of uranium resources and, over the longer term, explore the possibility of transmuting the most highly radioactive waste: these are the challenges facing future nuclear systems.

Published on 25 July 2016

The challenges of future nuclear systems with regard to nuclear fuel cycle

The fuel cycle of the future first implies multiple recycling of plutonium and uranium in an installed reactor base that includes FNRs1. Transmutation of the minor actinides and other features will then follow” says Bernard Boullis, director of the programme on the “back-end of the nuclear fuel cycle” in the CEA Nuclear Energy Division. If France decides to deploy this technology, spent fuel reprocessing will have to be adapted. Initially, the issue will be to extract the plutonium from spent MOX currently in storage, and blend it (about 25%) with depleted 238U for the first cycle in FNRs. “Reprocessing MOX fuel does not raise any serious problems: Areva has already reprocessed more than 70 tonnes of spent MOX fuel in its La Hague plants. The main difficulty is due to the higher proportion of plutonium,” says Christophe Poinssot, head of the CEA Nuclear Energy Division Radiochemistry & Processes Department at Marcoule. This requires upgrading the plant to handle the plutonium flow, but the basics of the process remain essentially the same.

Repeated plutonium recycling

When all the plutonium from spent MOX fuel has been exhausted, the FNRs will begin to “burn” the plutonium they have themselves created by irradiating 238U . It will then be necessary to reprocess their spent fuel. “The basic principle is the same: dissolve the fuel in acid and extract the plutonium with very specific molecules” explains Bernard Boullis. However, FNR fuel assemblies have neither the same geometry, nor the same cladding material (they will be made of steel instead of zirconium) as existing fuel assemblies. This will require modifying the plant “head-end” facilities in which the assemblies are cut up.

Enhanced separation tests
Enhanced separation tests are conducted in the Atalante complex at Marcoule © P. Dumas/CEA

1 CEA is now developing a 4th-generation sodium-cooled fast reactor. A demonstrator project, Astrid, is currently at the design phase. CEA is the contracting authority.​​

2 In fact, both types of reactors can coexist for many years.

Christophe Poinssot is nevertheless optimistic: “We have demonstrated that we have the necessary technical expertise, since we have already reprocessed 25 tonnes of spent fuel from Phénix and Rapsodie, at Marcoule and at La Hague. The task is mainly to adapt these processes to industrial scale.” At this point, a series of FNRs with a total power rating equivalent to the current PWRs, operating in a closed cycle, would consume 50 tonnes of depleted 238U each year from the existing stockpile — without requiring any natural uranium — and would produce 50 tonnes of ultimate waste2.

Separation and transmutation:
reducing the activity of future waste

Another problem remains: the minor actinides — radiotoxic elements that are currently confined in the glass with the fission products and are the principal source of responsible of very long-term radioactivity in the ultimate wasteforms. Legislative Act 2006-739 dated June 28, 2006, called on the CEA to “coordinate research on partitioning and transmutation of long-lived radioactive elements.” Fast neutron reactors are capable of consuming these nuclides — provided they can be satisfactorily extracted from the spent fuel and reinjected in the cycle, and their transmutation in the FNR core can be mastered. Teams at Marcoule have been working on these issues for the last two decades. First they had to find a molecule resistant to radiation and capable of specifically extracting these elements from the acidic solution in which the spent fuel was dissolved. “We identified molecules and tested them in the laboratory with 15 kg of actual fuel. We demonstrated that we could recover more than 99% of the minor actinides” says Christophe Poinssot. Transmutation can only be tested in an operating FNR core. Since the shutdown of Phénix in 2009, no more research facilities are available in France. “To move forward in this area we must count on international cooperation until the demonstration capability of the Astrid project becomes available” says Bernard Boullis.