Transfer to dry cask storage begins while concerns about unaddressed issues continue
By Heather M. Lightner
Last month, Entergy began transferring spent fuel from Pilgrim Nuclear Power Station’s overcrowded wet pool to dry cask storage. Entergy needs to create space in its spent fuel pool (SPF) so that spent fuel that is removed from the reactor in the future has a place to cool (in the SFP). Two of the three storage casks were loaded, with each cask containing 68 assemblies; the third cask will be loaded in the coming weeks. The casks will be stored onsite at Pilgrim and are likely to remain in Plymouth for an indefinite period of time, as there is no Federal repository for storage of spent nuclear fuel.
The Union of Concerned Scientists and the Massachusetts and New York Attorney Generals offices believe dry cask storage of spent fuel to be safer than wet pool storage because it is passive and does not require human action to cool the fuel. However, the Nuclear Regulatory Commission (NRC) and numerous government and scientific sources have reported problems with the steel/concrete dry casks Entergy has ordered for spent fuel storage at Pilgrim. Concerns regarding the long-term viability and safety of dry casks have been raised, as well as the potential for stress corrosion cracking due to salt water exposure (with subsequent radioactive release) and vulnerability to terrorist attack.
Dry casks have three components: 1. a metal transfer cask to lift and handle the canister and prevent radioactive shieling of the spent fuel assemblies, 2. a leak-proof metal canister capable of holding 68 boiling water reactor assemblies, and 3. a storage overpack made of steel-encased concrete which provides physical and radiological protection of the metal canister when stored on the dry cask pad. This canister is vented for natural convection to dissipate spent fuel decay heat.
Pilgrim’s dry cask storage facility is located only about 175 feet away from the shoreline of Cape Cod Bay and about 6 feet above FEMA’s flood level. The proximity of the dry casks to the water and the effect of storm surge and sea level rise are worrisome. Pilgrim’s salt water environment may lead to premature stress corrosion cracking of the stainless steel canisters within 30 years – or perhaps sooner – resulting in major radiation releases. The concrete overpacks can also suffer from accelerated aging issues as the result of the coastal factors. Other nuclear power plants, such as San Onofre in Pendleton, CA – also located on the water – have documented component failures in similar materials that have occurred in less than 30 years.
Unfortunately, the technology does not exist to inspect even the outside of the stainless steel canisters for cracks once loaded with spent fuel meaning there is no way to know that a stress corrosion crack has occurred. The NRC has given the nuclear industry five years to develop a method for inspecting the outside of the canisters; however, the NRC only plans on requiring inspection of one canister at each nuclear power plant. Even if a method did exist to detect a canister crack, there is no remediation plan if a canister does fail. The technology that is used to repair other stainless steel components cannot be used to repair canisters containing spent nuclear fuel. Per the NRC, if a canister becomes damaged due to a stress corrosion crack, there is no way to repair or replace the canister. Additionally, a canister cannot be transported in a transfer cask if there is a crack.
One potential fuel-handling solution that is currently being considered is the possibility of bringing a cask, or canister, back into the SFP, where it could be opened and possibly repaired or replaced. However, there is no publicly published documentation that a boiling water reactor dry cask has ever been loaded back into a SFP containing other assemblies. Temperature differences between the fuel in the dry cask and the SFP could disturb the properties of the cask, cladding, fuel, and related hardware if the materials were rewetted and rapidly cooled. Reinsertion of dry casks in the wet pool would thermally shock the irradiated fuel rods and cause a steam flash which would harm workers in the facility. Hence, an empty wet pool specifically designated for the reopening of damaged casks would be needed and is currently not available at any nuclear power plant in the country. Technology known as dry (hot cell) transfer has been discussed as an option for handling damaged casks; however, there is no dry handling facility available that is large enough to handle these canisters. Additionally, there is no mobile facility designed for this purpose and designing one may not be feasible.
There are no monitors installed on each cask to measure heat, helium (detection of helium can provide early warning of a problem) and radiation. A daily surveillance of the dry cask passive heat removal system is required to ensure system operability. This can be achieved by either monitoring the casks’ inlet and outlet vent temperatures or performing a visual inspection daily to ensure that the casks’ vents are not blocked. Pilgrim has chosen to perform daily visual inspections to ensure the air inlet and outlet vents do not become blocked and the passive heat removal system remains operable. The NRC expects that thermoluminscent dosimeters (TLDs) will be placed around the storage pad and will be used to monitor radiation on a quarterly to yearly basis. Unfortunately, the TLDs can only read to a maximum threshold (like a radiation film badge); they cannot provide an immediate reading of radiation.
Though the prospect of storing high-level nuclear waste in Plymouth indefinitely is not a pleasant thought and will never be the right or perfect solution for our town, there are steps that can be taken to “do the job right” and make dry cask storage as safe as it can be. Moving the dry casks to higher ground and enclosing them within a building offers multiple benefits: 1. increased protection against a salt water environment, storm surge, and sea level rise, 2. prevention of blockage of dry cask ventilation due to ice, snow, mud, and birds’ nests, thereby lowering the chance of a canister overheating, and 3. decreased visibility to potential terrorists, hence decreasing the site’s vulnerability to an attack.
While there is no current method to repair damaged canisters or casks, the addition of heat, helium, and radiation monitors for each cask would provide real-time information which would be invaluable in terms of identifying and responding to a problem with a dry cask. On-site storage of additional overpacks may offer temporary protection should a canister/cask corrosion crack occur.
Ultimately the best solution is to use casks that are not susceptible to cracks, that can be inspected and repaired, and that have early warning monitoring systems that alert us before radiation leaks into the environment.
Despite the concerns related to dry casks, dry cask storage has many advantages over wet pool storage: it does not require mechanical parts or offsite electrical power; does not need human intervention to function properly; and, is not as vulnerable to acts of terrorism. Dry cask storage also reduces the amount of spent fuel in the SFP, meaning there will be fewer releases of radioactivity in the event of an accident. Sadly, significant gains in safety can only be realized through expedited transfer dry cask transfer and resultant thinning of the spent fuel pool, which currently Pilgrim does not plan to do.
Ms. Lightner is a registered nurse in Plymouth and president of Concerned Neighbors of Pilgrim, a local, grassroots group focused on safer storage of spent nuclear fuel at Pilgrim Nuclear Power Station. She is also a member of the Town of Plymouth Nuclear Matters Committee.