Part One of Two Parts:
I have blogged before about permanent geological repositories for spent nuclear fuel and other nuclear wastes. There have been some failed attempts to build such repositories in the past. Germany built one and then had to close it because of unexpected migration of ground water in a salt formation. The U.S. was going to build one at Yucca Mountain but that project was cancelled in 2009, partly because of concerns over ground water movement in the salt formations. There are around a dozen underground laboratories, mainly in Europe, doing research on geological repositories for nuclear waste. Currently there are four underground nuclear waste repositories in operation, two in Finland, one in Sweden and one in the United States. Three more are under constructions, in Finland, Germany and Korea. Ten more are either under discussion, looking for sites, or applying for licenses.
A recent research study on the behavior of ground water in salt deposits found that the earlier theories of ground water penetration were incorrect and the danger of ground water seeping into underground repositories and carrying out radioactive materials was much higher than previously thought. A paper published in 2014 suggested that there is also another danger concerning ground water and underground nuclear waste dumps.
In a paper titled "Conditions for criticality by uranium deposition in water-saturated geological formations," three researchers explore the danger of a critical nuclear event in a nuclear waste repository. Some reviewers suggest that the researchers make assumptions that actually understate the danger. The paper was written under a contract with the Japanese Ministry of Economy, Trade and Industry following the nuclear disaster at Fukushima in 2011. The purpose of the research was to aid Japan in crafting a policy for the permanent disposal of nuclear wastes in underground repositories.
The research paper was concerned about a "critical" reaction in the uranium buried in an underground repository. A critical reaction occurs when there is a sufficient amount of uranium concentrated in a small volume. Neutrons emitted by the uranium cause other uranium atoms to fission so that a nuclear chain reaction starts and is self-perpetuating. Such a critical reaction will release a large amount of radiation in the local area which would be lethal to anyone near the reaction. There will not be a nuclear explosion such as that resulting from the detonation of a nuclear bomb but there will be a local expansion of nuclear materials which could blast apart waste containers and even breach the repository to release radioactive materials and steam into the atmosphere. With respect to the uranium in underground repository, the following factors affect the possibility of a critical event.
(1) The higher the density of a mass of uranium deposited in an underground repository, the lower the mass needed for a criticality.
(2) Smaller concentrations of neutron-absorbing materials in the host rock will increase the probability of criticality as more of the neutrons emitted by the uranium mass will trigger additional reactions in the uranium.
(3) larger porosity of the host rock will increase the probability of criticality by allowing more uranium to accumulate in the pores of the rock.
(4) The higher the temperature of the ground water the higher the temperature of the uranium which increases the speed of the neutrons emitted. The faster the neutrons, the less likely they are to trigger additional uranium reactions and lead to criticality.
(5)The closer that the shape of a mass of uranium is to a perfect sphere, the greater the risk of criticality. The more heterogeneous the geometry of the mass, the less likely it is that there will be a criticality.
(6) If there is a problem with corrosion or damage to waste containers, the uranium inside the containers could be leached out by groundwater. If the surface around the container(s) contains a depression, sufficient uranium could be deposited in the depression to form a critical mass.
Please read Part Two
Waste Isolation Pilot Plant Diagram:
