In “Conditions for criticality by uranium deposition in water-saturated geological formations“, Xudong Liu; Joonhong Ahn of the Dept of Nuclear Engineering, U. Cal. Berkeley; & Fumio Hirano of Japan Atomic Energy Agency, Geological Isolation Research and Development Directorate, Tokai-mura, (2014), raise the question of risk of a criticality event, an uncontrolled nuclear reaction, in a deep geologic repository for nuclear waste. Despite some seemingly faulty assumptions, which appear to understate risk, they rather bravely, considering their affiliations and funding, conclude that there could be a problem. At the end is the statement: “This study was carried out under a contract with METI (Ministry of Economy, Trade and Industry) of Japanese Government in the fiscal year of 2012 as part of its R&D supporting program for developing geological disposal technology.”
You can read the rest @
A similar study was done over 20 years ago with respect to the proposed US high level nuclear waste facility at Yucca Mountain, Nevada. The "experts" were split: some believed the waste could become critical, others did not. [I read the report back then, but I no longer have a copy].
Complicating the issue was a desire to dispose of the nation's excess plutonium by placing packets of it inside the glass logs of radioactive waste [the "can in a can" approach]. As far as I know, that idea never came to fruition, but some of the plutonium was turned into mixed oxide fuel which was used in Japanese reactors. Bundles of it were in the spent fuel pools at Fukushima, and we all know how that turned out.
Nuclear power was and is an extremely bad idea. DO NOT allow it to be resurrected.