Radioactve Waste 166 - Complex Technical Problems with Permanent Geological Repositories - Part Two of Two Parts
Part Two of Two Parts (Please read part One first.)
Critics of the criticality study have said that there appears to be a number of faulty assumption in the study that call some of the conclusions into question. The study may have underestimated the probability of criticality in an underground repository.
(1) It is assumed that transuranics will have been degraded to a lower level of radioactivity BEFORE corrosion of the waste containers. There are thin walled waste containers made by the Holtec company that could easily corrode before degradation. This would release radioactive materials that could be deposited along with uranium in a depression and lead to a criticality.
( 2) There was an assumption that the temperature of an underground storages facility would be roughly 68 Fahrenheit. The temperature may be much higher. There is a research lab in Minnesota that is more than two thousand feet underground and the temperatures of the rock outside the lab may be as high as 86 degrees Fahrenheit .
(3) There was an assumption that container designs are sub-critical. The Holtec company has asked for and received exemptions from the U.S. Nuclear Regulatory Agency for the design and construction of their nuclear waste containers. The walls of the sealed part of the containers are only one half inch thick and the aluminum from which they are constructed is prone to corrosion from ground water. The neutron absorber Metamic that is used in the Holtec containers received exemptions from quality control. There are concerns about the welds in the fuel baskets. They could fail and allow a container to collapse into a geometry that may increase the risk of criticality. The baskets are aluminum and do not contain boron neutron absorbers which would reduce criticality risk.
(4) The report states that if two uranium depositions are more than two feet apart, the neutrons emitted by the two depositions will not influence each other. There is the danger of collapse by accident with respect to the Holtec waste containers that might shorten the separation distance. There are also waste repositories which are designed for the roof to collapse after the waste containers are sealed in. The collapsing roof of the chamber might also separate the distance between two containers of waste. If the distance between two depositions of uranium falls under the two foot limit, it might trigger a criticality.
While it sounded like a good idea to bury nuclear waste deep underground, seal it in and forget about it, it turns out to be much more complex than originally thought. Geological repositories may be a workable solution to the need to dispose of many tons of nuclear waste. However, it appears that there is a lot more research than needs to be done before we can be confident that this disposal method is safe and permanent.
Holtec Hi-Storm Nuclear Waste Storage Container:
