11.86. The radioactivity of the solid reprocessing wastes, as a function of time after removal from the reactor, can be inferred from Fig. 11.6, the data for which were calculated for LWR spent fuel, assuming 99.5 percent removal of plutonium (and uranium) [12]. Solid wastes would differ in the respect that the volatile (or gaseous) species, namely, iodine, krypton, tritium, and xenon, would not be present, since they are either removed or released at the reprocessing plant or during calcination of liquid wastes. The data in Fig. 11.6 refer specifically to 1000 MW(el)-year of LWR op­eration with a burnup of 2.85 TJ/kg of fuel, initially uranium free from plutonium; 0.5 percent of the plutonium (and all the americium and curium) formed as well as 0.5 percent of the uranium are assumed to remain.

11.87. It is seen that at 1 year after discharge from the reactor, the total activity would be about 108 curies or more than 1018 Bq; it decreases by a factor of roughly ten after 10 years and another factor of about ten after 100 years. Subsequently, the activity decreases more rapidly until some 600 years have elapsed, when the decrease becomes very slow. The radio­activity then arises mainly from the long-lived, alpha-emitting transuranium nuclides, especially isotopes of plutonium and americium. There is also a small contribution from the beta-emitting fission product, technetium-99 (half-life 2.1 x 105 years). Since iodine-129 is volatile, it is not present in the solid wastes.

11.88. Because the curves in Fig. 11.6 are for the total activity of all the isotopes of a given element, they do not show some interesting vari­ations in heavy-element buildup. For example, the amount of plutonium — 238, produced in the wastes by the alpha decay of the short-lived curium — 242, increases to a maximum after some 10 years, and plutonium-240, from the decay of curium-244, attains a maximum after about 100 years. Plu­tonium-239, which is formed by alpha decay of americium-243 to neptun­ium-239 followed by beta decay of the latter, reaches a maximum activity after approximately 20,000 years. The sharp decrease in the total activity

For brevity, this will be referred to as 1000 MW(el)-year of operation.

of plutonium between about 10 and 100 years, as seen in Fig. 11.6, results from the decay of plutonium-238 (half-life 88 years) and plutonium-241 (half-life 14 years) which have made the major contribution in the early stages.

11.89. The rate of heat generation (or thermal power) in the wastes, due to radioactive decay, varies in the same general manner as the total

activity. For the 1000 MW(el)-year operation of an LWR referred to, the thermal power of the solid high-level wastes would be about 350 kW after 1 year and roughly 35 kW after 10 years; at the latter time, almost 90 percent of the heat would arise from strontium-90 (and its short-lived decay product yttrium-90) and cesium-137. After 100 years, the thermal power is approximately 4 kW, and after 600 years (or more) it is less than 0.05 kW.