Nuclear Wastes: Technologies for Separations and Transmutation

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First, the "capture-to-fission" ratios for thermal neutron spectra show marked differences for odd-versus even-neutron isotopes. Odd-neutron isotopes fission well in a thermal spectrum; in fact, such fission is favored over capture by factors of 2 to 10. In contrast, most even-neutron isotopes are not fissioned by neutrons below several hundred kilovolts in energy, due largely to the extra nuclear stability conferred by neutron pairing. Hence, in a thermal reactor, such nuclei are fissioned only by the fission spectrum neutrons.

Second, the thermal neutron "capture-to-fission" ratios are typically higher than those for fast spectra. This effect is exaggerated for even-neutron isotopes; indeed, ²⁴⁰Pu and ²⁴¹Am have large thermal neutron capture cross sections and are parasitic absorbers in thermal reactors. However, odd-neutron isotopes also show the effect by factors of 1.5 to 3, mainly because capture cross sections are typically higher for thermal spectra. Most importantly, the high thermal capture-to-fission ratio of ²³⁹Pu and its capture products result in relatively large amounts of higher-mass actinides in a thermal-neutron transmuter, as compared to a fast reactor.

Transuranic Production and Transmutation. The main source of the principal TRU, ²³⁹Pu, is neutron capture in ²³⁸U followed by two beta decays (i.e., in the initial spent fuel plus possible additional production during transmutation). The ²³⁹Pu fissions well with neutrons of any energy. Alternatively, it produces ²⁴⁰Pu by neutron capture. Table 4-1 lists a capture-to-fission ratio of 0.55 in a thermal spectrum compared to 0.17 and 0.26 in a fast spectrum with metallic and oxide fuel, respectively. Successive neutron captures, starting with ²³⁹ Pu, produce higher mass isotopes of Pu, Am, and Cm.

The minor actinides and other plutonium isotopes, in particular ²⁴⁰Pu and ²⁴²Pu, fission well in a fast spectrum. That isotope ²³⁷Np could undergo neutron capture and furnish additional ²³⁹Pu. That is,

However, in a very high neutron flux, the intermediate nucleus ²³⁸Np could attain a high probability of capturing a second neutron and fissioning before the beta decay could take place. Thus, in principle, a high-flux transmutation scheme could achieve a higher fissioning rate of various isotopes than a scheme that operates at ordinary thermal flux levels.

Two even-neutron isotopes in the plutonium chain, ²⁴⁰Pu and ²⁴²Pu, are key precursors of higher-mass actinides by neutron capture to ²⁴¹Pu and ²⁴³Pu, which can undergo beta decay to ²⁴¹Am and ²⁴³Am, respectively (see Table 4-1). The two americium isotopes fission well in a fast spectrum, but can produce ²⁴²Cm (163 d) and ²⁴⁴Cm (18.1 yr) by neutron capture in a fast or thermal spectrum. However, the higher capture-to-fission ratio characteristic of a thermal spectrum, discussed above, results in a build-up of higher-mass actinides during thermal neutron transmutation. A chain of long-lived curium isotopesis produced by successive neutron captures—²⁴⁵Cm (8,500 yr), ²⁴⁶Cm (4,820 yr), ²⁴⁷Cm (1.56 × 107 yr), and ²⁴⁸Cm (3.7 × 105 yr), even some ²⁵⁰Cm (9.7 × 103 y). The chain branches at ²⁴⁹Cm with the 64-minute beta decay to ²⁴⁹Bk, which leads in steps to production of californium isotopes, in particular ²⁵²Cf. Even higher-mass actinides can be produced in a thermal flux >1015 neutrons/cm²-s, such as in the high-flux isotope reactor (HFIR). Thus, a range of higher-mass isotopes is produced in either a fast or thermal spectrum, although the relative proportions are quite different. The possible effects of the higher actinides are discussed in the sections on several of the transmutation options. In target waste material, ²⁵²Cf would be a potent source of neutrons by spontaneous fission. In addition to alpha emission, spontaneous fission also occurs in curium isotopes, becoming more probable with increasing curium mass, i.e., ²⁴²Cm (8 × 10-6%), ²⁴⁴Cm (1.3 × 10-4%), ²⁴⁶Cm (0.027%), ²⁴⁸Cm (0.83%), and ²⁵⁰Cm (^~99%) (Hoffman et al., 1992). In addition, unburned ²³⁸Pu can be an important source of neutrons by (α, n) processes. Hence, for thermal neutron transmutation concepts, significant neutron emission could present problems during fuel reprocessing and refabrication, quality assurance, and performance verification. To a lesser extent, the issue could arise with fuel rods for transmutation in a fast reactor.

Transmutation Reactor Approaches

Classes of Reactor Concepts

To take possible advantage of nuclear processes for transmutation, two quite different classes of reactors have been studied for neutron generation. The work to date consists primarily of a conceptual analysis of the effects of the neutrons generated in either approach in order to estimate the benefits and hazards that would result from changing the character of the waste to be disposed. The two classes of reactor concepts are:

Critical nuclear reactors: The nuclear assembly, containing the waste and possibly additional fissile material, operates with a net neutron multiplication factor of unity. This class includes thermal reactors, such as the LWR and PBR, and fast reactors, such as the ALMR.
Accelerator-driven nuclear reactors: The nuclear assembly operates with a neutron multiplication factor less than unity, i.e., subcritical, so that neutrons must be added from a source external to the nuclear assembly. Intense

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