A CASE STUDY FOR THE PERFORMANCE OF A NUCLEAR GEOLOGICAL REPOSITORY IN THE UNSATURATED ZONE

Nuclear power use is expected to expand in the future to meet the strong growth in global electricity demand, and attenuate greenhouse gas emissions, mainly caused by fossil fuels. As a result, it is estimated that hundreds of thousands of metric tons of spent nuclear fuel (SNF) will accumulate. SNF disposal has major environmental (radiation exposure) and security (nuclear proliferation) concerns. Storage in geological repositories is a reasonable solution for the accumulation of SNF. The U. S. Department of Energy (DOE) made comprehensive studies to assess the performance of the proposed repository at Yucca Mountain (YM). One of the key factors that determine the performance of YM repository, an unsaturated zone geological repository, is the release of radionuclides from the engineered barrier system. Over time, the nuclear waste containers are expected to fail gradually due to general and localized corrosions and the water will have access to the nuclear waste. Once radionuclides released by water transport, they will make their way to the accessible environment. Changes in the environment over the waste container and rock fall will lead to different corrosion rates, causing different times and locations of penetration. Considering the uncertainty of the failure sequence of a waste package, two main failure scenarios are expected: the flow through model (penetrations on the top and bottom of the waste package allow water to flow through it), and the bathtub model (penetrations are on the top with the waste package filling with water).

In this paper a bathtub category failed waste container is considered. We shed some light on chemical and physical processes that take place in the pooled water inside a partially failed waste container (bathtub category), and the effects of these processes on radionuclide release. Our study is independent of DOE analysis and it considers two possibilities: temperature stratification of the pooled water versus mixing process. Our calculations show that temperature stratification of the pooled water is expected when the waste package is half (or less) filled with water. On the other hand, when the waste package is fully filled (or above the half) there will be mixing in the upper part of water. The effect of these cases on oxygen availability and consequently corrosion rate and radionuclide release will be also considered.