| FUEL & FUEL CYCLE


Canister integrity 10,000 1,000 Once-through 100 Recycle 10 1 10 100 1,000 10,000 Time (years) after disposal of waste 100,000 1,000,000


Left, Figure 3: Radiotoxic hazard as a function of time


Because of its longevity unresolved waste creates an institutional problem for organisations, which must take the responsibility for the waste in the long term. Few organisations can guarantee to last that long. In the United States, this problem was supposedly resolved by the Federal government taking on the long-term responsibility of spent fuel in return for a fee charged to the electricity utility. So far, the government has required payments from industry but not resolved the problem. By taking the responsibility for the problem the US government has not forced the providers of nuclear power to find other ways to resolve their waste issue, and has sustained a once-through fuel cycle which is tolerated despite its shortcomings. The safest form of long-term management of nuclear


materials is not to have them in storage at all. This can be achieved by separating waste for immediate disposal and recycling the rest through “just-in-time” procedures. In order to demonstrate proper sustainability to the


public, nuclear plants (and supporting activities) should be removed or decommissioned to completion soon after they stop operating. Besides being consistent with principles of intergenerational equity this makes best use of existing knowledge, expertise and resources. Fuel components that cause a long-term hazard should


be recycled into new nuclear fuel. Uranium (lightly enriched or depleted in the U-235 isotope) may be added or removed from the cycle for material balance purposes. The irreducible waste of the nuclear process (principally


fission products rather than minor actinides) is only a small fraction of spent fuel, and this fraction is also responsible for most of the troublesome heat generation. The fission product fraction itself can be allowed to stay where it is without further human intervention when the use is finished. That is because the hazard will decay to essentially harmless levels before the physical containment (eg canister) fails. If the heat-generating fraction is separated, the heat could potentially be used. However efficient a recycling scheme is, it cannot


eliminate all waste. Waste described as ‘low level’ in US terminology can generally be managed through existing shallow-burial facilities. Uranium originating from spent


fuel for material balance purposes can be easily removed, and fed back in.


Nuclear physics requirements The nuclear physics characteristics of fuel recycled in this way need to be carefully addressed, but it is possible in principle to combine recycled fuel with enriched or depleted uranium in schemes with thermal and fast neutron reactors. Meeting nuclear physics and material balance requirements is far easier in fast reactor than thermal reactor systems. A primary requirement is to balance the ratio of fissile


and fertile constituents of the fuel. For thermal reactors the fissile ratio will continuously fall as the reactors operate and therefore must be adjusted. For fast reactors the ratio may increase or decrease. Fast reactors can also be employed for the role of ‘incinerating’ problematic species that can build up in the fuel cycle. It might be thought that the only way of increasing the


fissile to fertile ratio is by adding fissile material, but the same effect can also be achieved by extracting uranium (Figure 2). That is far preferable for security considerations. A key material to support the early stages of recycling schemes is high-assay low enriched uranium (HALEU). Internationally recognised standards place a limit of 20% enrichment of U-235 to protect against illicit use for nuclear weapons purposes. Most reactors are currently supplied with approximately 5% U-235, but HALEU (up to 20% enrichment) is extremely useful to achieve material balance for recycling schemes. The US Department of Energy has recently contracted with Centrus Energy Corporation for production of HALEU.


Separation to recover value Spent nuclear fuel needs to be divided into two streams: principally fission products; and a mixture of uranium, plutonium and minor actinides. The objective is to separate fission products so that the hazard decay of the packages follows the green curve in Figure 3 (instead of the red curve for spent fuel itself). Much progress has already been made in developing separations of the type required. U


www.neimagazine.com | May 2021 | 19


Radiotoxicity normalised to natural uranium ore


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45