2, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
The radioactive decay of incorporated fission products and actinides in nuclear waste forms leads to self-radiation effects and self-heating that may affect the stability, structure and performance of the waste form in a closed system. The principal heat-generating radionuclides in high-level waste are the short-lived fission products, 90Sr and 137Cs, and if incorporated in waste forms, these radionuclides cause significant self-heating for about the first 600 years. Ionization from gamma rays and beta particles emitted in beta decay of fission products can cause covalent and ionic bond rupture, valence changes, localized electronic excitations and significant changes in ionic mobility. Recent results indicate that ionization effects from beta decay in nuclear waste glasses may negligible due to a threshold in ionization dose rate for observable effects. In the case of waste forms containing actinides or tailored specifically for actinides, self-heating could be important for 1000 to 2000 years. Over the long time periods of deep geologic storage, self-radiation of nuclear waste forms is primarily due to the alpha decay of the actinides. The effects of self-radiation due to alpha decay on the structure and properties of glass and ceramic waste forms have been studied over the past four decades using both short-lived actinides, primarily 238Pu or 244Cm, and energetic ion beams. A wide range of experimental methods has been employed to characterize the changes in density, stored energy, and local structure as a function of radiation dose and temperature. There are limited studies comparing the response of materials to alpha decay and ion-irradiation damage. Only in a few cases do data from ion beam irradiations correctly predict the behavior in actinide-containing ceramics. In other cases, the dose for amorphization can vary significantly from that predicted by ion beam irradiation, due to ionization-induced recovery either from the ions themselves or from alpha particles emitted in alpha decay, and the temperature dependence of amorphization can shift by several hundred degrees (K), which creates a dilemma in deciding what temperature dependent data can be used, if any, to predict the temperature dependence of actinide-bearing waste forms. Over very long time periods, helium accumulation from alpha decay may lead to the formation of helium bubbles that may cause additional swelling and changes in mechanical properties. The relevance of these results radiation effects in hierarchial nuclear waste forms will be discussed.
This work was supported by the U.S. DOE, BES, MSED.