It is only recently that reprocessing is being promoted as a “solution” to the problem of mounting quantities of spent fuel. In this context, it is often called “recycling.” It is now explicitly being promoted as a means for greatly increasing the use of the uranium resource contained in the spent fuel. Proponents of nuclear power often state that 95 percent of spent fuel (or used fuel or irradiated fuel) can be “recycled” for recovering the energy in it.
This has become a new mantra of nuclear waste management: spent fuel is a treasuretrove of energy.
A new report ('The Mythology and Messy reality of Nuclear Fuel Reprocessing') by the Institute for Energy and Environmental Research (IEER) looks at France (often called an 'inspirational example for nuclear waste management') and shows that for existing spent fuel the slogan belongs in the same realm of economic claims for nuclear energy that would be “too cheap to meter.”
It is worth noting at the outset that reprocessing and breeder reactors were not proposed as asolution to the problem of nuclear waste, which has so far turned out to be intractable for a host of technical, environmental, and political reasons. Reprocessing was also not proposed as an essential accompaniment to burner reactors, like the light water reactors, to increase the use of the uranium resource because its value in that regard is marginal.
In light water reactor systems, almost all the uranium resource winds up as depleted uranium or in spent fuel. Even with repeated reprocessing and re-enrichment, use of the natural uranium resource cannot be increased to more than one percent in such a system. The use of 90 to 95 percent of the uranium resource in the spent fuel is impossible in a light water reactor system even with reprocessing. These are technical constraints that go with the system.
Reprocessing in France
Reprocessing in France is continuing mainly due to the inertia of primarily-government-owned electricity generation and reprocessing corporations (EDF and AREVA respectively). It continues also due to the political and economic dislocations that closing an established large industrial operation would cause in a largely rural area in Normandy that has scarcely any other industries. After it was clear that the breeder reactor program was not going to fulfill its theoretical promise any time soon, the decision to continue reprocessing in France was not about economics, technical suitability, waste management, or significantly increasing the use of the uranium resource in the fresh fuel.(*1) It was driven mainly by the inertia of a system that was government-owned and had already invested a great deal of money and institutional prestige in the technology.
Reprocessing in the US?
French company Areva has no immediate plans to build a reprocessing and associated MOX fabrication complex in the US. Areva spokesman Jarret Adams said the company has been discussing designs for a US reprocessing and MOX fuel fabrication complex with utilities for many years and that it is 'starting to educate President Barack Obama’s administration' on Areva’s vision for nuclear fuel recycling in the US.But on March 25, Alan Hanson, Areva’s executive vice president of technology and used fuel management, said in Washington, that preliminary designs show that a reprocessing and associated MOX fuel fabrication complex built at one site in the US would be a US$25 billion capital expenditure.
Jacques Besnainou, CEO of Areva North America, said in that Areva would be willing to invest its own money to help develop a reprocessing complex in the US. On March 25, Besnainou said that a US reprocessing complex could be a regional hub capable of reprocessing spent fuel from the Americas and small countries from other regions. “I think we should help the [United Arab Emirates] with their used fuel 20 years from now,” he said.
Nuclear Fuel, 5 April 2010
Light water reactors(*2) and reprocessing
Uranium-238 is almost 99.3 percent of the natural uranium resource. It requires about 7.44 kilograms of natural uranium to produce one kilogram of 4 percent enriched uranium fuel, assuming 0.2 percent U-235 in the tails (depleted uranium).(*3) This means that about 86.6 percent of the natural uranium resource winds up as depleted uranium. Even if the efficiency of enrichment improved so that only 0.1 percent of U-235 remained in the tails, it would still mean that about 84 percent of the natural uranium resource would wind up as depleted uranium when it is first enriched. (For simplicity, the authors ignore losses of uranium during milling and the series of processing steps prior to enrichment. These are small compared to the amount of depleted uranium.)
It should also be noted that the vast majority of the uranium in the fresh fuel is still non-fissile U- 238. In the case of 4 percent enriched uranium, made from natural uranium, the other 96 percent is uranium-238. The fraction of U-238 is a little lower in fuel made from reprocessed and re-enriched uranium due to the buildup of other uranium isotopes, notably U-236. A small fraction (about two percent) of this U-238 gets converted into plutonium.(*4) Some of this is fissioned in the course of reactor operation and therefore provides a portion of the energy output of the reactor. But the vast majority of uranium-238 will remain unused in burner reactors – that is, the type of reactors in use today.
Commercial reprocessing using the PUREX process, the only commercial technology at present, separates the spent fuel into three streams – (i) plutonium, (ii) uranium, and (iii) fission products, plus traces of non-fission radionuclides, like neptunium.
France uses most, but not all (see below), of the separated plutonium as a mixed plutonium dioxide uranium dioxide fuel, called MOX fuel for short. It uses depleted uranium to make MOX fuel. However, of the 6.44 kilograms of depleted uranium created in the process of making fresh fuel from natural uranium, in the used example, just over a tenth of a kilogram is used as a component of MOX fuel; most of that remains unused in spent MOX fuel.
France also uses a part of the uranium recovered from spent fuel as a fuel. But this uranium must be re-enriched to the requisite level. To get the same performance as fresh 4 percent fuel, the reprocessed uranium must (because of the degraded isotopic composition of the uranium) be enriched to about 4.4 percent, which means that about 87 percent of the recovered uranium becomes depleted uranium waste. Further, roughly 93 percent of this re-enriched fuel is also uranium-238. When this recovered and re-enriched uranium is used as fuel only a small amount of it is converted to plutonium, while most remains unused. If repeated reprocessing and re-enrichment are carried out, the pile of depleted uranium mounts rapidly, while the amount of fissile material dwindles. Further, it should be noted that the process of enriching reprocessed uranium also increases the concentration of uranium-236, which is not fissile; this reduces the usefulness of re-enriched uranium as a fuel.
The flow of materials in a light water reactor scheme with reprocessing is shown in diagram in Figure 2. It corresponds to the example the authors have been using: an initial fuel loading of 1 kilogram of fresh (4 percent) low-enriched uranium fuel, 0.2 percent U-235 in the depleted uranium tails at the enrichment plant, and 8 percent plutonium in MOX fuel, and assuming that all the recovered uranium is re-enriched.
Figure 2: Fuel and Waste Streams in a Light Water Reactor System with Reprocessing and Re-Enrichment for One Kilogram of Fresh Fuel (4% Enriched)
Notes:
1. Nat U = natural uranium; DU = depleted uranium tails (0.2 percent U-235 assumed for this chart); EU = enriched uranium; Pu = plutonium from spent fuel; REU = re-enriched uranium; MOX = mixed plutonium dioxide uranium dioxide fuel; FP = fission products; SF = spent fuel; TRU = transuranic radionuclides other than plutonium isotopes; RU = uranium recovered from spent fuel; DRU = depleted recovered uranium. Pu value rounded up to nearest gram.
2. U-235 in the tails at the enrichment plant = 0.2 percent.
3. The amount of matter converted to energy (according to the famous E = MC2) is very small (much less than one gram per kilogram of fuel) and is ignored in the above diagram.
Only one round of reprocessing and re-enrichment is shown in Figure 2. At the end of the use of the MOX fuel and re-enriched uranium fuel, only about 6 percent of the kilogram of original fresh fuel has been used to generate energy. In turn this is only about 0.8 percent of the 7.44 kilograms natural uranium resource used to make the single kilogram of 4 percent enriched uranium fuels.
Repeated reprocessing, MOX fuel use, and re-enriched reprocessing uranium fuel use does not improve the picture much. This is because most of the remaining spent fuel is left behind as depleted uranium in each round. In fact after five rounds, about 99 percent of the original uranium resource is depleted uranium. This means that the fraction of the uranium resource that can be used in a light water reactor-reprocessing-re-enrichment scheme is one percent or less. This can be raised slightly by reducing the amount of U-235 in the tails below 0.2 percent.
This is a conservative calculation, done as a simple illustration. It ignores the isotopic degradation of both the uranium and plutonium in the second and subsequent rounds of use in a reactor. Specifically, uranium-236 and uranium-234, which are not fissile isotopes and which degrade fuel performance, build up in the fuel as the reactor operates; uranium-236 increases in concentration with re-enrichment. Small amounts of uranium-232 also build up.(*5) This isotope has a specific activity (defined as disintegrations per second per unit mass) that is 30 million times greater than natural uranium. Unlike fresh uranium fuel, it quickly generates decay products that emit strong gamma radiation, which creates higher worker radiation risks. Fuel quality requirements limit U-232 to a few parts per billion. As a result, re-enrichment becomes more complex and costly for each round of recycled uranium fuel use in a reactor. The fraction of uranium-236 and uranium-232 must be reduced by blending the enrichment feedstock with natural, un-reprocessed uranium or by blending the enriched uranium derived from reprocessed uranium with enriched uranium originating from fresh ore. Similarly, the isotopic composition of MOX fuel degrades with each round of MOX fuel use and reprocessing; this makes reprocessing even more expensive and the fuel less valuable.
As a result of the above considerations, technical and cost considerations limit the practical ability to reprocessing and re-enrich for more than one round past the first use of fresh fuel made from natural uranium.
Even when the initial depleted uranium is left out of the calculation (though it should not be, since it contains most of the natural uranium resource), reprocessing and repeated re-enrichment and MOX fuel use will utilize only about six percent (rounded) of the fuel originally loaded into the reactor, with about two-thirds of that occurring in the initial irradiation and most of the rest occurring in the first round of MOX fuel use. Repeated reprocessing, re-enrichment, and MOX fuel use just does not increase resource use significantly, because most of the uranium becomes part of the depleted uranium stream at each step. Finally, it should be noted that these numbers ignore uranium losses at the uranium mill (where, typically, several percent of the uranium is discarded into tailing ponds along with almost all the radium-226 and thorium-230 in the ore) and in the processing steps needed to make the uranium hexafluoride feed for the enrichment plant. The actual resource utilization based on the uranium content of the ore at the mill is, in practice significantly less than one percent. Fresh fuel plus one cycle of MOX use and re-enrichment uses only about 0.8 percent of the natural uranium resource. This is reduced to about 0.7 percent when the losses of uranium in the processing at the uranium mill and the conversion to uranium hexafluoride are taken into account.
France currently only re-uses a third of the recovered uranium. This means that France uses less than six percent of the uranium resource in the original fresh fuel; about 80 percent of this is used in the first round of fresh fuel use prior to reprocessing. In other words, the expense, risk, and pollution created by French reprocessing only marginally increases the use of the underlying uranium resource. Further, the re-enrichment is not done in France but in Russia. The depleted uranium from re-enrichment, amounting to roughly 87 percent of the reprocessed uranium by weight, remains behind in Russia.
In sum, the French use only about 0.7 percent of the original uranium resource to create fission energy. The rest is mainly in depleted uranium at various locations, or is piling up as reprocessed uranium that is not being used, or is uranium left in spent fuel of various kinds (including MOX spent fuel). This figure cannot be increased significantly even with repeated reprocessing and re-enrichment so long as the fuel is used in a light water reactor system.
Notes (For full references see the original report):
(*1) All calculations are based on four percent enriched fresh fuel made from natural uranium as the starting point, unless otherwise specified. The results would be similar with any starting enrichment for light water reactors, which are designed to use low enriched fuel (generally less than five percent U-235).
(*2) Light water reactors are a specific example of “burner” reactors, which have a net consumption of fissile materials in the course of energy production from fission. Some new fissile material is created, mainly plutonium-239, but the amount of fissile material used (or burned), mainly a combination of uranium-235 and plutonium-239, is greater than the amount of fissile material residing in the irradiated material at the end of the reactor operation period. This discussion is focused on light water reactors (LWRs), and specifically on pressurized water reactors (PWRs), the design used in France. The arguments are essentially the same for boiling water reactors (BWRs). The U.S. commercial nuclear power reactor system consists entirely of PWRs and BWRs. Unless otherwise stated, the examples and figures used in this report are typical of pressurized water reactors. The exact assumptions, such as the enrichment level of the fresh fuel, make no difference to the overall conclusion about the efficiency of resource use in a light-water-reactor system with reprocessing and re-enrichment.
(*3) Used is 4 percent enrichment as a typical figure. Actual enrichments in pressurized water reactors may range from 3 percent to above 4 percent. During enrichment, natural uranium is separated into two streams – the enriched stream, which is then chemically further processed to make reactor fuel, and the depleted stream, which is also called the “tails.” These tails, which consist of depleted uranium, have been accumulating at enrichment plants in the United States and elsewhere. The authors assume a U-235 content of about 0.2 percent in the tails (i.e., in the depleted uranium). In practice, the U-235 in the tails varies and a typical range generally considered is 0.2 to 0.3 percent. The amount of natural uranium needed to produce a kilogram of fuel will vary according to the enrichment of the fuel used and the percent of U-235 in the tails. The lower the percent of U-235 in the tails, the less natural uranium is needed for a given level of enrichment. Hence the example discussed here is a favorable practical case for maximizing resource use in a light water reactor system.
(*4) The main isotope (over 50 percent) in the separated plutonium is Pu-239, but there are also substantial amounts of higher isotopes, including Pu-240 and Pu-241. The mixture is known as reactor-grade plutonium. Pu-240 is not fissile. When used as part of MOX fuel in light water reactors some of it gets converted into Pu-241, which is fissile. Pu-240 can fission with fast neutrons and generate energy.
(*5) Uranium-233 and -237 are also formed in very small quantities and have very little radiological impact. Uranium-233 is a fissile material which gives a tiny added benefit to the reprocessed uranium. (IAEA-TECDOC-1529 2007 pp. 7-8)
Source: 'The Mythology and Messy reality of Nuclear Fuel reprocessing', Arjun Makhijani, Ph.D., April 8, 2010, available at: http://www.ieer.org/reports/reprocessing2010.pdf
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