K. L. NASH and J. C. BRALEY, Washington State University, USA

Abstract: From the days of the Manhattan Project, the chemistry of actinides and selected fission products has shaped decisions on the handling of irradiated nuclear fuel. This chemistry is characterized by the diversity of the fission products, the rich redox chemistry of the light actinides, high radiation levels, concentrated nitric acid used to dissolve the fuel and the nuclear chemistry of both actinides and fission product lanthanides. This chapter introduces the actinide and fission product chemistry relevant to the nuclear fuel cycle, from the isolation of uranium from mined ores through reprocessing to management of the byproduct wastes. The important features of historically successful solvent extraction separations and alternative chemical processes are described. Finally, the role of nuclear energy as a source of primary power sans greenhouse gases is discussed.

Key words: nuclear fuel, actinide chemistry, solvent extraction, molten salts.

1.1 Introduction

Prior to the 1940s, the only radioactive elements on planet earth were those long-lived enough to have persisted since the condensation of the solar system (primarily 235, 238U, 232Th, 40K, 87Rb), the short-lived isotopes linking actinides to their stable end-member lead or bismuth isotopes (Ra, Rn, Po…), those created as a result of cosmic radiation in the upper atmosphere (36Cl, 32P, 14C, 3H…) and the very small amounts of man-made isotopes that had been created in scientific research. That research and the activities of the ensuing Manhattan Project introduced to the terrestrial environment considerable amounts of both short — and long-lived new isotopes, including most significantly kilogram quantities of some transuranium elements. These activities also required the mining and processing of uranium ores, which increased the accessibility of uranium and its radioactive daughters to the biosphere. The subsequent activities of the Cold War increased the terrestrial abundance of some of these elements to thousands of kilograms. During the years of atmospheric testing of nuclear weapons, weapons tests released significant amounts of radioactive debris into the environment.

Since the institution of a ban on atmospheric testing of nuclear weapons, the only significant injection of anthropogenic radioactivity into the ter­restrial environment occurred in the Chernobyl accident in 1986.

Though nuclear energy was first exploited for plutonium production for military purposes (and was in fact driven by the defense buildup of the Cold War), the production of electricity through the operation of fission reactors began in the early 1960s. The first nuclear reactors designed for electricity production were of a graphite-moderated, gas-cooled design in the United Kingdom. In the US, the first power reactors were designed for use in US Naval ships. The same design ultimately was adapted to stationary applica­tions in water cooled and moderated reactors fueled with partially enriched uranium. The Canadian design utilized natural uranium or slightly enriched fuel with D2O moderation and cooling. Each design has its advantages and limitations, but all produce a similar array of waste byproducts, hence offer similar constraints on the execution of the fuel cycle.

The use of high purity light water (H2O) as a neutron moderator and coolant offers several advantages, starting with its reasonable price and favorable thermal properties. In the operation of a light water reactor, enriched uranium (3-5% 235U, 97-95% 238U) is partially consumed with the resulting creation of transuranium elements (in order of decreasing amounts, plutonium (Pu), neptunium (Np), americium (Am) and curium (Cm)), and fission products including varying amounts of all elements in the periodic table between zinc and erbium. The fission products include noble gases, halides, calcogenides, pnictides, geranium, tin, indium, second row transition metals, alkali metals, alkaline earths and about two-thirds of the lanthanide series. Post removal of the fuel from the reactor, the total uranium content is about 95.5% of the non-oxide mass. Plutonium isotopes account for about 0.9% of the heavy metal content. The isotopic distributions for uranium and plutonium post-irradiation are shown in Fig. 1.1. Weapons grade plutonium is defined as material containing at least 93% 239Pu. Reactor grade pluto­nium is defined as material composed of more than 18% 240Pu. The high

Plutonium 238

239

240

24% sssss 241 ssss 242

neutron capture cross-section of 240Pu aids in limiting the utility of pluto­nium with significant amounts 240Pu from being used in a weapon. At dis­charge, used fuel also includes about 500 g/ton of 237Np.

Though the mass of used fuel is predominantly uranium, the radioactivity of the actinide component is dominated on discharge by the plutonium and curium isotopes, the former based on mass, the latter on the short half-lives of the isotopes present. Most of these isotopes have longer half-lives than the majority of fission products, hence they tend to dominate the radiotoxic­ity of used fuel beyond about 500 years after discharge. In storage, the isotopic distribution of actinides in used fuel changes primarily from the decay of 242Cm, 244Cm, 241Pu resulting in an increase in the 238Pu, 240Pu, and 241Am content of the fuel, respectively. Though it represents only a minor component of used fuel on discharge, 241Am content of the used fuel increases substantially as a result of 241Pu decay; the dominant curium iso­topes decay away in a relatively short time (resulting in a decrease in the total curies). The total radioactivity arising from the actinide isotopes is about 0.11 MCi/ton at discharge and 0.07 MCi/ton after ten years of decay storage (these figures and all subsequent product information are based on 33,000 MWd/t U burnup at a power density of 30 MW/t U and neutron flux of 2.92*1013 Ncm-2s-1, as described in [1]). As all actinide isotopes are radio­active, ultimately all decrease in concentration of spent fuel in storage, though clearly the long-lived isotopes decrease slowly. A summation of the radioactive properties of the important actinide isotopes present in used fuel is shown in Table 1.1.

Among the fission products, there are many short-lived and stable species. At discharge, the radioactive materials content of one ton of fuel is about 15 MCi, dominated by small amounts of very short-lived fission products. The most important fission product radionuclides in used fuel after about one year are 137Cs (1230 g/ton of 30.1 year half-life in equilibrium with its radioactive 137mBa daughter — 0.21 MCi total), 90Sr (543 g/ton of 28.5 year half life in equilibrium with its radioactive 90Y daughter — 0.15 MCi total), 99Tc (841 g/ton of 2.1 x 105 year half-life — 14 Ci total) and 129I (229 g/ton of 1.57 x 107 year half-life — 0.04 Ci total). The latter two isotopes are of greater concern for their potential environmental mobility, their long half-lives and for their bioaccumulation possibilities than the numbers of curies present or the energetic characteristics of their emissions. The cesium/barium and strontium/yttrium isotopes dominate the dose, produce substantial amounts of heat and are the most radiotoxic materials in used fuel from shortly after discharge through several hundred years.

As noted above, rare earth elements are significant byproducts of fission. They represent about 40% of the mass of fission products and including measurable amounts of all lanthanides from lanthanum (La) through erbium (Er) plus moderate amounts of yttrium (Y), which exhibits chemistry similar

to that of the lanthanides. The lanthanide composition of fuel irradiated as described above (33,000 MWd/t U at power density of 30 MW/t U and neutron flux of 2.92*1013 Ncm-2s-1) is shown in Fig. 1.2. [1] This is an impor­tant factor in the used fuel management equation, as several isotopes of these elements have thermal neutron capture cross-sections greater than those of the actinides, hence must be separated to enable any waste man­agement scenario that includes transmutation of actinides by neutron capture. Figure 1.2 also contains the thermal neutron capture cross-sections and resonance integrals for important lanthanides and americium. In a light water reactor, transmutation (by fission) of Am isotopes will be virtually impossible in the presence of lanthanide isotopes. As will be noted below, the separation of fission product lanthanides from actinides can be a chal­lenging obstacle to effective waste management.

As is true of all electricity production technologies based on fuel con­sumption, nuclear power systems operate within the framework of a fuel cycle that includes mining and preparation of the fuel, “combustion” with the generation of heat to power generators, and waste management. In a nuclear fuel cycle, uranium is mined, converted and isotopically altered before fuel elements are prepared, assembled into a critical mass and allowed to undergo controlled nuclear fission. Ultimately, the fuel must be replaced with a fresh load of fuel. Unlike power generation based on fossil fuels, the large majority of “waste” byproducts in nuclear fuel are retained within the fuel assemblies in a fission reactor.

Materials considered as waste in used fuel may simply be disposed of as waste (an open fuel cycle), or recycled to recover fuel for reuse and to improve waste management (a closed fuel cycle). Both waste management methods require the accepted use of a geological repository engineered to retain the majority of the most radiotoxic elements for an adequate time to protect the surrounding environment. An open fuel cycle focuses on permanent disposal of the fuel without concern about the additional energy

potential of the irradiated fuel. The closed fuel cycle focuses on recovering additional fuel value through either recycle of plutonium or recovery of fuel value and transmutation of troublesome isotopes. Both cost and com­plexity increase as additional processing is imposed, but with the advantage of extending the potential life of fuel supplies and a shorter time require­ment for geologic isolation. The decision is one whose dimensions are defined by the need for power, supplies of resources, and the limits of safety.

In the following discussion, the basic chemistry that supports the deci­sion-making process for these options will be discussed. The emphasis will be on the solution phase chemistry of actinides and important fission prod­ucts, the former including uranium, thorium, neptunium, plutonium, ameri­cium and curium; the latter, cesium, strontium, iodine, technetium and the lanthanides. The interrelated features of the nuclear and radiochemistry, oxidation-reduction chemistry, solution chemistry (i. e., complexation and hydrolysis) media will be discussed.