Thermal Considerations in a Very Deep Borehole Nuclear Waste Repository for Synroc
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Thermal Considerations in a Very Deep Borehole Nuclear Waste Repository for Synroc G.D. Sizgek Australian Nuclear Science and Technology Organisation, Materials Division, Menai NSW 2234, Australia Email: [email protected] ABSTRACT A study of heat propagation in a very deep borehole radioactive waste repository for Synroc is presented. The deep borehole modeled for the high level wastes is hypothetically sited at approximately 4 km below ground level in a granite host rock. The high level waste containers are placed in the lower section of the borehole to within 1.5 km of the surface and backfilled. A geothermal gradient of 0.03°C/m is assumed. Transient temperature field calculations are carried out for both 20 wt% and 10 wt% high level radioactive waste (HLW) bearing Synroc, for cooling periods between reactor discharge and geological disposal varying from 5 years to 50 years. The temperatures in the borehole repository were found to be very sensitive to the surface cooling period as well as the waste loading. The initial heat input of the waste may be controlled by cooling in an interim storage until the total heat output is reduced to a suitably low level. Therefore, surface cooling of at least 15 years for 20 wt% HLW containing Synroc is necessary for the given deep bore hole configuration to avoid the centerline temperatures that might affect durability of Synroc if ground water obtained access to the waste form. On the other hand the corresponding minimum surface cooling period for 10 wt% HLW containing Synroc is predicted to be about 5 years to produce similar temperature profiles in the borehole to those of 20 wt% HLW bearing Synroc with a 15-year cooling period before disposal. A variation in the borehole diameter has also been performed to elucidate the sensitivity of the temperature field of the repository. INTRODUCTION Disposal in very deep drill holes is one of the several repository concepts identified for the permanent disposal of high level radioactive waste in geological formations. This concept relies on using the great depths (up to 4-6 km) so that the possibility of migration of radionuclides to the biosphere by circulating ground waters can be vastly reduced [1]. However, despite its apparent advantages in terms of simplicity, this disposal method does have certain disadvantages. In particular, no significant work has been carried out in studying the practical engineering problems associated with handling, emplacing the waste into drill holes and in achieving a satisfactory borehole seal. For example, the possibility exists of losing holes through jamming of equipment or canisters, collapse or movement of borehole walls. Although the strategy of deep drill-hole disposal and multi-barrier containment is aimed at minimizing access of water to waste, the possibility of water entry must always be considered as a factor in risk assessment analysis. This type of disposal is particularly suited for use with ceramic waste forms that have high thermal stability and are durable at higher leaching temperatu
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