Dose rates within vitrified high-level waste (HLW) initially being of the order of 10² Gy/s are high enough to cause concern on the role of radiation effects on long-term retention of radionuclides and performance of glasses. A significant part (~ 20 to 40%) of the deposited energy in glass, which is of the order of about 4×10⁹ Gy for commercial HLW, is caused by the beta radiation of decaying radionuclides. Experiments within electron microscopes have revealed effective flow of silicate glasses under the electron irradiation including direct visualisation of quasi-melting and flow of vitreous material which is characteristic to its molten state. These experiments raise the question of such effects within vitrified HLW although the dose rates of experiments reported were much higher compared with those specific to HLW. An analysis of the nature of radiation induced flow of glasses and quantitative assessments of irradiation parameters causing flow and potential thresholds (which must not ever be reached in nuclear waste immobilisation practices) are evidently needed. The report analyses the nature of flow both for non-irradiated and irradiated glasses accounting for generation of flow defects in form of broken chemical bonds both by thermal fluctuations and absorbed radiation which can be in form of particles and/or photons. The activation energy of flow Q_H is typically high and constant in glasses (Arrhenius type flow) below the glass transition temperature T_g, however it starts to diminish above the T_g, further decreasing finally achieving its low value Q_L characteristic for melts at the crossover temperature T_A = kT_m, where k = 1.1 ± 0.15, and T_m is the melting (liquidus) temperature regardless of the type of silicate glass-forming liquid. Depending on the temperature and dose rate of radiation the major source of flow defects can be either thermal fluctuations or ionising radiation. The radiation breaks chemical bonds generating flow defects (termed configurons) and modifies the temperature dependence of flow by shifting the low activation energy regime (Q_L)and crossover temperature (T_A) to lower temperatures. Moreover, at high dose rates of radiation T_g can abruptly decrease, thus effectively transforming the glass into a liquid. The equation of viscosity of glasses in radiation fields derived reveals the critical parameters of radiation and enables parametrical estimation of threshold values which separate the liquid-like (molten state characterised by Q_L) from solid-like (glassy state characterised by Q_H) behaviours. The report presents numerical estimations for threshold dose rates and show that these were of the order of ~ 2 10⁶ Gy/s and higher reaching up to ~ 4 10⁹ Gy/sin the experiments with effective quasi-melting of silicate glasses under electron beam irradiation, whereas currently synthesised HLW glasses are characterised by several orders of magnitude lower dose rates below 10³ Gy/s.

Numerous spent fuels in Japan are cooling for future reprocessing. The cooling time of spent fuel from shutdown to reprocessing is a key factor affecting the heat generation rate of vitrified waste and the relevant waste management. The advantage for extending cooling time is to decrease in decay heat of fission products to mitigate the thermal constraints of storage facility and final repository of vitrified waste. Contrarily, ²⁴¹Am is generated by ²⁴¹Pu decay in spent fuel during the interim storage and is a concern for the long-term heat source in vitrified waste.

The present study numerically investigated the trade-off relationship between the decay in fission products and the ²⁴¹Am buildup from the waste management perspectives. For spent fuel with a typical burnup of 45 GWd/MTU, the decay heat in vitrified waste decreases by half in about 10-year cooling and by a quarter in 40-year cooling. This will shorten the storage period of vitrified waste before final disposal. Alternatively, the surplus heat capacity in repository system up to the bentonite buffer limit temperature of 100 °C allows higher waste loadings in vitrified waste. Thereby the waste volume (i.e., number of canisters) and the repository footprint (m²/MTU) can be reduced by up to 13%. However, the disadvantage of ²⁴¹Am buildup owing to the extended cooling time was revealed that the surface temperature of vitrified waste at the time of glass dissolution after disposal will exceed the 60 °C predicted in the safety case. The temperature-dependent glass dissolution rate and the subsequent influences on post-closure safety assessment will also be discussed along with results for high-burnup vitrified waste.

This work was carried out as a part of the basic research programs of vitrification technology for waste volume reduction supported by the Ministry of Economy, Trade and Industry, Japan (Grant Number: JPJ010599).