M. SALVATORES, SeniorScientificAdvisor, Nuclear Energy Directorate CEA (France) and Idaho National Laboratory, USA

Abstract Partitioning and Transmutation (P&T) is considered as a way of reducing the burden on geological disposal, required to isolate long-lived isotopes (present in the spent fuel) from the biosphere in stable deep geological formations for long periods of time. In this chapter, the physics of transmutation and the type of reactors where TRU transmutation can be performed together with basic feasibility issues will be discussed. Key issues of any transmutation strategy are the impact on the fuel cycle (specific features required for major fuel cycle installations, doses experience by workers, etc.), the development of appropriate fuels and of chemical separation processes will be indicated. Possible fuel cycle implementation scenarios, depending on specific country policies, and the potential benefits of geological repositories will be discussed.

Key words: transuranics, waste management, radiotoxicity, fast neutron spectrum, geological disposal.

17.1 Introduction

I n commercial water-cooled, reactors (LWRs and CANDUs), plutonium (Pu) and the so-called minor actinides (neptunium (Np), americium (Am), curium (Cm), etc.), collectively called the transuranic (TRU) elements, are formed through neutron capture by nuclides in the fuel. The capture of a neutron by U-238 without a fission reaction results, finally, in the creation of Pu-239 (see Fig. 17.1). Neptunium-237 is produced primarily from neutron capture in U-235 and subsequent nuclear reaction and decay. From Pu-239, neutron captures lead to the creation of heavier isotopes that, coupled with subsequent decay, create key gateway isotopes (e. g., Pu-241, Pu-242, Am-243). Neutron interactions with these result in the formation of higher mass actinides (i. e., Cm, Bk, Cf). Thus, neutron absorptions, not leading to fission, transmute Pu-241 into the non-fissile Pu-242, which in turn is the gateway to Am-243. Neutron capture in Am-243 leads to the production of Cm-244, which may decay to Pu-240 by alpha emission, spontaneous fission or transmute to Cm-245 by neutron capture. In general, depending on the relative rates of these three processes, an actinide will either reach a equilibrium concentration or else increase monotonically with burn-up.

Figure 17.1 summarizes the uranium nuclei transmutation chain under neutron irradiation and the associated Bateman equations, where n is the nuclide j density, oaj is the absorption cross section of isotope j, o1 K is the cross section corresponding to the production of isotope K from isotope j, A is the decay constant for isotope j, AK is the decay constant for the decay of isotope j to isotope K and, finally, Ф is the neutron flux.

The management of spent fuel is a major challenge for all countries where nuclear energy has been developed, regardless of the perspective applied to its future utilization, from further development to progressive phase out.

Most of the enduring hazard from spent fuel stems from only a few chemical elements — plutonium, neptunium, americium, curium (see Table 17.1) — and some long-lived fission products such as iodine and technetium, present in the spent fuel at concentration levels of kilograms per ton. These radioactive by-products, although present at relatively low concentrations in the spent fuel, are a hazard to life forms when released into the environment. As such, their disposal requires isolation from the biosphere in stable deep geological formations for long periods of time.

Partitioning and transmutation (P&T) is a way of reducing the burden on geological disposal. Since plutonium and the minor actinides are mainly responsible for the long-term radiotoxicity (as expressed by its effective dose coefficient, see e. g. Ref. 1), when these nuclides are removed from the waste (partitioning) and fissioned (transmutation), the remaining waste loses most of its long-term radiotoxicity. Moreover, the P&T strategy may also allow a reduction in the amount of heat generated by radioactive waste and this can have a significant impact on the repository size (and therefore cost). The potential benefits and impact of P&T on geological disposal will be discussed in Section 17.5.

The first requirement of P&T strategies is the deployment of spent fuel reprocessing to separate the TRU elements. These are then refabricated into fuel

Nuclide Half-life Energies of primary emissions (MeV) Specific activity Dose coefficients (Sv/Bq) а P (Ci/g) (W/g) (Neutron min 1 mg 1) 237 Np 2.14 x 106y 4.78 7.07 x 10“4 2.07 x 10“5 < 7 x 10“6 1.1 x 10“7 238Np 2.10 d 0.25 1.24 2.61 x 105 1.27 x 103 239Np 2.359 d 0.332 0.427 2.32 x 105 5.86 x 102 8.0 x Ю10 238Pu 87.404 у 5.49 17.2 0.570 155 2.3 x 10“7 239Pu 2.4413 x 104y 5.15 6.13 x 10“2 1.913 x 10“3 1.35 x 10“3 2.5 x 10“7 240 Pu 6580 у 5.16 0.227 7.097 x 10“3 53.7 2.5 x 10“7 241 Pu 14.98 у 4.9 0.02 99.1 4.06 x 10“3 4.7 x 10“7 242 Pu 3.869 x 105 у 4.90 3.82 x 10“3 1.13 x 10“4 95.3 2.4 x 10“7 241 Am 432.7 у 5.48 3.43 0.1145 3.55 x 10“2 2.0 x 10“7 242Am 16.01 h 0.63-0.67 8.11 x 105 2.08 x 103 24!mAm 144 у 5.207 Isomeric 10.3 3.08 x 10“2 1.9 x 10“7 Transition 243Am 7370 у 5.27 0.200 6.42 x 10“3 2.0 x 10“7 242Cm 162.7 d 6.11 3.32 x 103 122 1.21 x 106 1.3 x 10“8 243Cm 32 у 5.79 45.9 1.677 2.0 x 10“7 244Cm 18.099 у 5.81 80.94 2.832 6.87 x 105 1.6 x 10“7 245 Cm 8265 у 5.36 0.177 5.89 x 10“3 3.0 x 10“7 246 Cm 4655 у 5.39 0.312 1.01 x 10“2 5.58 x 105 2.9 x 10“7 247Cm 1.56 х 107 у 4.87 9.28 x 10“5 2.94 x 10“6 2.7 x 10“7 248Cm 3.397 х 105 у 5.05 4.24 x 10“3 5.34 x 10“4 2.58 x 106 1.1 x 10“6 249 Cm 64 m 0.9 1.18 x 107 2.06 x 104 250 Cm 1.74 х 104 у 8.20 x 10“2 ~0.1 6.49 x 108 2.9 x 10“7 252Cf 2.64 у -7 x Ю10

and ‘transmuted’ i. e. fissioned (or ‘burned’) in a neutron flux to produce useful energy. How this is done very much depends on national long-term energy goals (e. g. the phase-out of nuclear power versus an indefinite continuation) but, if deployed in conjunction with both thermal and fast reactors, it is possible to take maximum advantage of the energy available from uranium while, at the same time, reducing the amount and the toxicity of the resulting waste2 3. Of course, key requirements are that this must be done safely and at reasonable cost.