Reprocessing SNF

The main methods used for reprocessing of SNF flowsheet were reviewed in a 120 pages report by the Nuclear Energy Agency (NEA 2012). The three main processes are the so-called hydrometallurgy processes (PUREX and UREX), pyromet-allurgy processes and its variations, and the fluoride volatility process (quite like the method used at the uranium conversion facilities discussed in Chapter 1). The report reviewed in detail several of these processes that are deployed in different facilities for various types of spent fuel (NEA 2012). In this section, we shall try to briefly present an overview of the main points and the analytical aspects. 

As mentioned earlier, spent fuel from a reactor using LEU fuel contains many components that have a commercial value. Among these, uranium is the major component with an isotopic composition that depends on the original level of the fuel enrichment and on the extent of bum-up (typically 0.5%-1.0% 98.5% 

The SNF (after a cooling period to allow for decay of short-lived radionuclides) is chopped up and dissolved in nitric acid. The gasses emitted in the process are treated to avoid their release to the environment. The solution is filtered to separate the insoluble residues and sent to the solvent extraction stage in which the uranium and plutonium are extracted into the organic phase (usually TBP in a hydrocarbon solvent) and the fission products and minor actinides remain in the aqueous phase. The radioactive fission products may then be treated as high-level-waste while the uranium and plutonium are then separated from each other by selective back-extraction. 

Undissolved fuel, insoluble residues, and fuel-assembly hardware 

FIGURE 2.11 Generic description of the PUREX process. (Based on NBA, Spent nuclear fuel reprocessing flowsheet, OECD Nuclear Energy Agency, Paris, France, NEA/NSC/ WPEC/DOC(2012)15, 2012.) 

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When reprocessing SNF, it is assumed that the extracted fission products first are vitrified and then, after necessary cooling, are enclosed in special containers providing a multi-barrier shielding and transported to be finally disposed in deep geological formations. Minor actinides (except for curium) are not separated from plutonium and are used in the reactor as a fuel component. Curium is extracted and transported to the temporary repository for 100-150-year cooling. Upon being cooled, all curium isotopes (except for curium-245) are transformed into plutonium isotopes. Then this isotopic mixture is used to produce new fuel for the reactor. 

Yucca Mountain, if it becomes the site for the isolation of SNF, will be laced with tunnels, waste in storage casks and monitoring equipment. A waiting period is planned while better isolation alteniadvcs are sought. IfYucca Mountain is not used, it is to be refilled with the tuff material removed earlier. In the United States the SNF that would be isolated in Yucca Mountain would be waste that has not been reprocessed it would be material that has come out of nuclear reactors and has been cooled at the plant site. 

Since the amount of fissile material in the fuel assemblies is only about 3 percent of the uranium present, it is obvious that there cannot be a large amount of radioactive material in the SNF after fission. The neutron flux produces some newly radioactive material in the form of uranium and plutonium isotopes. The amount of this other newly radioactive material is small compared to the volume of the fuel assembly. These facts prompt some to argue that SNF should be chemically processed and the various components separated into nonradioac-tive material, material that will be radioactive for a long time, and material that could be refabricated into new reactor fuel. Reprocessing the fuel to isolate the plutonium is seen as a reason not to proceed with this technology in the United States. 

If one just concentrates on the radioactive material in SNF, the volume is very small, especially compared to waste from other power production practices. However, one can only discuss the separated radioactive material if it has undergone extensive reprocessing. If SNF is to be isolated, as in a place such as Yucca Mountain, with perhaps 70 miles of tunnels, the volume is that of the interior of this minor mountain. Isolation of up to 100,000 metric tons of SNF in Yucca Mountain means that for the United States, approximately all the SNF made to date and that expected in the operating lifetime of all current reactors can be put there. Approximately 2,000 metric tons of SNF are produced each year in the United States. Waste volume and placement depend on the amount of compaction and consolidation at the sites. The plans for the Yucca Mountain present a realistic and understandable picture of the volume of SNF. 

The United States has the most radioactive nuclear waste and the most complicated array of waste types. Reprocessing of SNF is also practiced in some countries. Although costly, this practice... 

Fig. 1. Schematic illustration of the ideal closed nuclear fuel cycle (NRC 2003). In real practice, the reprocessing capacity does not match the generation rate of the spent nuclear fuel. Thus, the excess SNF must be placed in interim storage or disposed of in a geological repository. Under normal circumstances, the SNF will be in interim storage for just a few years. Also, note that excess material from nuclear weapons, e.g.. highly enriched 235U and 239Pu, can be blended down to lower concentrations and used as a reactor fuel.

As in the USA, most of the HLW (SNF or immobilized HLW fluids) in Russia is destined for geological disposal. Site investigations are under way in the area of the two reprocessing facilities (Mayak and Krasnoyarsk), as well as in the regions of nuclear navy bases in the Far East and Northwest. 

High-level waste (HLW), intermediate-level waste (ILW), and low-level waste (LLW) are produced at all stages of the nuclear fuel cycle as well as in the non-nuclear industry, research institutions, and hospitals. The nuclear fuel cycle produces liquid, solid, and gaseous wastes. Moreover, spent nuclear fuel (SNF) is considered either as a source of U and Pu for re-use or as radioactive waste (Johnson Shoesmith 1988), depending on whether the closed ( reprocessing ) or the open ( once-through ) nuclear fuel cycle is realized, respectively (Ewing, 2004). 

Liquid HLW from reprocessing of SNF may consist of 50-60 elements, including about 90 radionuclides of 35 chemical elements of fission products (FP) and more than 120 radionuclides due to FP decay. The total activity of HLW may achieve 1016 Bq/m3 (Nikiforov el al. 1985). The HLW elements can be divided into four groups ... 

Composition (in wt%) reprocessing of commercial LWR fuel at La Hague Savannah River Site commercial SNF plutonium plutonium reprocessing of SNF from water-water energetic reactors... 

Indications from both microscopic analyses of metallic particles from corrosion tests and evidence from the Oklo natural reactors indicate that performance assessment calculations should not assume 99Tc is easily mobilized. It is entirely inappropriate to use "Tc release as a marker for fuel corrosion because Tc is not located in the fuel matrix. The TEM examinations of corroded e-particles have shown that Mo is preferentially leached from these phases, a behaviour that is similar to the one observed at Oklo. It is interesting to note that laboratory dissolution of billion-year old 4d-metallic particles for a chemical analysis required a corrosive mix of peroxide and acid (Hidaka Holliger 1998) similar to the experience at SNF reprocessing plants. It is doubtful that the oxidation potential at the surface of an aged fuel will be sufficient to move Tc(0) from the e-metal particles. 

Figure 16.15 Schematic representation of the new schemes for reprocessing of SNF. (Figure also appears in color figure section.)...

Although the main incentive of reprocessing is to use uranium resources effectively by recovering and recycling the Pu and U remaining in the SNF, the real feature of the Pu flow in the current world can be described as follows ... 

The following sections review recent findings and progress achieved on liquid-liquid extraction systems dedicated to reprocessing of the SNFs. 

As U is the major component of a SNF see Table 1.2, its initial separation in reprocessing alleviates the mass burden of following steps and is considered preferable. The UREX process developed in the AFCI program of the United States is based on the PUREX process (30 vol % TBP in n-dodecane) and suppression of extractions of Pu and Np by reduction/complexation (175-182). Plutonium and Np are reduced by acetohydroxamic acid (AHA, CH3CONHOH) to Pu(III), Np(V), and Np(IV). U is kept in an extractable U(VI) state. Although Np(IV) is also extractable, AHA forms a complex with Np(IV) that is soluble in the aqueous phase. In the case where reoxidation of Pu(III) occurs, the Pu(IV) also transfers to the aqueous phase by forming a Pu(IV)-AHA complex. Thus, U is exclusively extracted. AHA decomposes to hydroxylamine and acetic acid (176). 

In the course of works each of the decommissioning objects can be a source of SNF, SRW, LRW, noxious chemicals or other non-radioactive waste reprocessible and re-usable in industry. Still, the main feature of the complex decommissioning objects is the presence of SNF and generation of RW during work execution. Considering a special importance of SNF, SRW and LRW for the work to be done and for justification of priorities, at some work phase they can be attributed to the category of independent objects of management. This is a quite natural decision because in the course of the... 

Management of SNF. Based on a political decision and also for financial reasons, reprocessing is no longer an option in Sweden the SNF shall be directly disposed of in the Swedish bedrock. Long-lived waste should be disposed of together with the SNF. 

Techniques for the handling and storage of Spent Nuclear Fuel (SNF) in the UK have been developed over a long period. Storage and reprocessing facilities deal with fuel arising from Nuclear Power Plants (NPPs) in the UK and overseas and the UK nuclear submarine fleet. 

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