What is Chloride-Induced Stress Corrosion Cracking?
What is CISCC and why does it matter?- Chloride Induced Stress Corrosion Cracking
What is Chloride Induced Stress Corrosion Cracking
One of the main concerns that scientists and activists have with the storage of high-level nuclear waste, especially at an interim, above-ground storage site, is the risk of storage canisters to corrode, leak, or become damaged. Nuclear waste experts across the world agree that deep geologic storage, if reprocessing spent nuclear fuel can't be done, is the most appropriate way to store such radioactive material.
But what Interim Storage Partners and Holtec International plan to do with the steel nuclear waste dry storage containers is to set them in the open air, packed in concrete shielding structures, with vents in the concrete to allow for continued cooling. Anything in the atmosphere will be able to contact the canisters through the concrete ventilation system. You can compare this model to the Waste Isolation Pilot Plant (WIPP) in New Mexico that houses Transuranic, or TRU waste; waste there is buried at 2,150 feet deep.
Here we will explain the biggest corrosion concern that the spent nuclear fuel canisters face. A concern that can potentially leak radioactive materials into our groundwater supply, into land used by ranching families, and to any local resident downwind or downstream of the groundwater of the storage sites: Chloride Induced Stress Corrosion Cracking or CISCC.
Stress Corrosion Cracking (SCC)
Stress Corrosion Cracking or SCC is cracking that starts from combining tensile stress and a corrosive environment. Tensile strength is the amount of tensile (stretching) stress a material can withstand before breaking or failing. In the case of the steel canisters that encase the high level nuclear waste, tensile stresses would be caused by welding the canisters shut.
Chloride Induced Stress Corrosion Cracking (CISCC)
Chloride Induced Stress Corrosion Cracking is a type of SCC that involves chloride ions, oxygen, and high temperatures. The combination of tensile stress and a specific corrosive environment can crack stainless steels. This mode of attack is termed stress corrosion cracking (SCC). The most common environmental exposure condition responsible for SCC of stainless steels is the presence of chlorides, aka salts. No stainless steel is totally immune to CISCC (ssina.com)
The atmosphere of each nuclear waste storage site varies as does the amount of salt in the air. The potential for deposition of chloride-containing salts, at least at some sites, is high. Scientists have studied different nuclear reactor sites where waste is currently being stored nearby. Some nuclear reactor sites are in coastal areas and some are inland. Of course the coastal areas have more salt in the atmosphere than the inland nuclear reactor sites.
What is worrisome for the West Texas and Southeastern New Mexico high-level nuclear waste sites is while they are inland, they are surrounded by salt playas: a fact that was conveniently left out of the NRC environmental report.
Nuclear Waste Canisters and CISCC
Potentially corrosive environments may form on the surface of spent nuclear fuel dry storage canisters by deposited dusts that later become liquid when they break down. The canisters that the U.S. uses for storing high-level nuclear waste is made out of steel. The manufacturer must bend the steel in a cylindrical shape and weld it together. The lids that keep the nuclear waste inside are welded shut. These weld areas are what the scientists look at when they study residual tensile stress. From the 2015 study conducted by OSTI, or U.S. Department of Energy Office of Scientific and Technical Information showed that "Results to date indicate that residual stresses will be large and tensile in both the axial and hoop directions, extending through the thickness of the container wall".
When you combine the residual tensile stress and the salt in the atmosphere of the Permian Basin region, you have the perfect recipe for disaster waiting to strike: CISCC.
Why does the Location of Interim Storage Sites Matter?
As we noted above, salt playas surround the Holtec International site and there are two salt playas near the Interim Storage Partners' site. If you are familiar with West Texas and Southeastern New Mexico, you know that our landscape is shaped by the wind. Trees grow gnarled and stumpy, literally formed by high winds; plants grow low to the ground for the same reason. Tumbleweeds criss-cross the highways and cars are covered with red dust.
In the Permian Basin we have regular dust storms and every few years, haboobs ( the Arabic term for intense dust storms like they have in the Sahara Desert) cause havoc as well. Whether our normal dust storms, with winds reaching 40 mph, or are the more intense haboobs, dust particles are picked up and deposited elsewhere in the area.
Salt particles, those that are the contributor to chloride induced stress corrosion cracking, can be easily carried to either high-level waste storage facility. The steel storage containers, while yes, packed in concrete have ventilation to the outside atmosphere. Any chloride ions that land in the area can be deposited on the nuclear waste canisters. The residual tensile stress from the weld locations combined with the salt molecules is our worst nightmare.
Conclusion
While an environmental report was submitted to the Nuclear Regulatory Commission during the application process for Holtec International and Interim Storage Partners, the lack of complete scientific studies on how the Permian Basin's atmosphere and CISCC is a dangerous oversight. Even OSTI, who has been conducting studies on CISCC and nuclear waste storage canisters use has been identified as a high priority data gap. Chloride stress corrosion is a concern that can't be ignored or forgotten about. The salt playas in the area of the interim storage sites is an obvious geological formation that shouldn't have been overlooked to begin with.