Nuclear Fuels and Sustainability

There are basically four general classes of fuel cycles:

• A once-through cycle without reprocessing;

• A cycle with partial recycle of plutonium;

• A cycle with full plutonium recycling; and

• A closed cycle with full recycling of transuranic elements.

The once-through fuel cycle option is the most uranium resource-intensive and generates the most waste in the form of used nuclear fuel. However, the amounts of waste produced are small compared to other energy technologies. One of the most important conclusions of the Generation IV group is that the limiting factor facing an essential role for nuclear energy with the once-through cycle is the availability of repository space. This becomes an important issue, requiring new repository development in only a few decades. In the longer term, beyond 50 years, uranium resource availability also becomes a limiting factor. One of the important conclu sions of the roadmap proposed by the Generation IV Forum is that five of the six proposed new reactor designs are based on the design of a fast reactor in combination with a closed fuel cycle. Systems that employ a fully closed fuel cycle hold the promise to reduce repository space and performance requirements. Closed fuel cycles permit partitioning the nuclear waste and management of each fraction with the best strategy. Advanced waste management strategies include the transmutation of selected nuc-lides, cost effective decay-heat management, flexible interim storage and customized waste forms for specific geologic repository environments. These strategies hold the promise to reduce the long-lived radiotoxicity of waste destined for geological repositories by at least an order of magnitude. This is accomplished by recovering most of the heavy long-lived radioactive elements. These reductions and the ability to optimally condition the residual wastes and manage their heat loads permit far more efficient use of limited repository capacity and enhance the overall safety of the final disposal of radioactive wastes. The advanced separations technologies for Generation IV systems are designed to avoid the separation of plutonium and incorporate other features to enhance proliferation resistance and incorporate effective safeguards. In particular, to help meet the Generation IV goal for increased proliferation resistance and physical protection, all Generation IV systems employing recycling avoid separation of plutonium from other actinides and incorporate additional features to reduce the accessibility and weapons attractiveness of materials at every stage of the fuel cycle. In the most advanced fuel cycles using fast-spectrum reactors and extensive recycling, it may be possible to reduce the radiotoxicity of all waste such that the isolation requirements can be reduced by several orders of magnitude (e.g., for a time as low as 1000 years) after discharge from the reactor. This would have a beneficial impact on the design of future repositories and disposal facilities worldwide. However, this scenario can only be established through considerable R&D on recycling technology. This is a motivating factor in the roadmap for the emphasis on cross-cutting fuel cycle R&D. The studies also established an understanding of the ability of various reactors to be combined in so-called symbiotic fuel cycles. For example, combinations of thermal reactors and fast reactors are found to work well together. As shown in Figures 3 and 4, they feature the recycling of actinides from the thermal systems into the fast systems, and exhibit the ability to reduce actinide inventories worldwide while using the nuclear fuel in a sustainable way. Improvements in the burn-up capability of gas- or water-cooled thermal reactors may also contribute to actinide management in a symbiotic system.

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Fig. 3. Comparison of waste produced using either a once-through cycle or a combination of LWR and fast reactors, from [8]

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Fig. 3. Comparison of waste produced using either a once-through cycle or a combination of LWR and fast reactors, from [8]

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Fig. 4. Comparison of resources available using either a once-through cycle or a closed cycle from [8]

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Fig. 4. Comparison of resources available using either a once-through cycle or a closed cycle from [8]

Thermal systems also have the flexibility to develop features, such as hydrogen production in high-temperature gas reactors or highly economical light water reactors, which are part of an overall system offering a more sustainable future. This is a motivating factor in the roadmap for having a portfolio of Generation IV systems rather than a single system — realizing that various combinations of a few systems in the portfolio will be able to provide a desirable symbiotic system worldwide.

As a final note, the FCCG observed that nuclear energy is unique in the market since its fuel cycle contributes to only about 20% of its production cost. This provides flexibility in separating the approach for meeting the economics and safety goals from the approach for meeting sustainability and safeguards goals. That is, adopting a fuel cycle that is advanced beyond the once-through cycle may be achievable at a reasonable cost.

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