Orano NUHOMS Inspections Reveal No Degradation in Metal Canisters or Concrete Modules in 20-year-old Used Nuclear Fuel Dry Storage Systems

Aging Management Program at three U.S. used nuclear fuel storage sites verifies Orano’s NUHOMS system performing as designed.

BETHESDA, Md., February 9, 2021 – During regulatory inspections of the NUHOMS^® used nuclear fuel dry storage facilities at three U.S. nuclear reactor sites, Orano NPS found no corrosion concerns or precursor indications of stress corrosion cracking (SCC) on the stored 20-year-old stainless steel canisters.

At each dry storage site, known as Independent Spent Fuel Storage Installations (ISFSIs), Orano inspected the “lead canister”—typically the first, oldest, and coolest canister loaded with used nuclear fuel—which is the canister most likely to exhibit any degradation or corrosion. The canisters at all three sites were loaded into their horizontal, above-ground NUHOMS storage modules more than 20 years ago.

These inspections fulfilled the Nuclear Regulatory Commission’s (NRC) licensing requirements for continued storage of used fuel in dry storage systems based on NRC-approved NUHOMS Aging Management Programs (AMPs) and site environmental evaluation criteria established by the Electric Power Research Institute (EPRI). The Certificate of Compliance (CoC) for the NUHOMS storage system was approved by the NRC (CoC-1004) for an initial storage duration of 20 years. With renewal of the NUHOMS CoC, the NRC extended the system’s licensed storage duration by another 40 years, for a total of 60 years. Orano’s above-ground NUHOMS dry storage systems and procedures are designed to simplify these periodic inspections and to continue operating safely for more than 100 years.

Orano’s inspection team used two robots: the slim delivery robot carried the smaller crawler robot between the large side-by-side concrete block NUHOMS storage modules and into the module’s cavity through a ground-level side inlet ventilation slot. The crawler robot was then raised to the used fuel canister’s side, where it detached and adhered with suction while moving and visually examining the canister’s surface for any indications of SCC. The robots also examined the concrete module’s interior surfaces for degradation.

Extensive EPRI and NRC analyses have shown that dry storage of used nuclear fuel is a secure method of interim storage with an Aging Management Program. The analyses also verified that chloride-induced stress corrosion cracking (CISCC) is the most likely potential degradation that could affect a stainless steel used nuclear fuel canister. Orano’s teams conducting these periodic ISFSI inspections include procedures to address this issue.

“The possibility of CISCC is well understood by Orano and the industry,” said Amir Vexler, president of Orano NPS in the United States. “We know how to find it before it becomes a problem, and we know how to fix it, if it ever does show up.”

For CISCC to develop on a NUHOMS stainless steel used fuel storage canister, three criteria must be present: 1) Corrosive chlorides (salts) are in the air. 2) The canister of used nuclear fuel is cool enough for moist air to condense and evaporate on its surface, causing corrosion leading to pitting. 3) The canister’s surface becomes pitted and under enough external stress for the pits to connect and form a surface crack, which could eventually progress over decades through the canister wall. If any of these three criteria are missing, CISCC cannot occur.

As part of the NUHOMS system’s design and licensing, the NRC-approved Time Limited Aging Analyses and Supplemental Evaluations verified that the likelihood of CISCC on Orano's canisters is a low probability occurrence and would require more than 100 years of unattended degradation to become a concern. At any point in the CISCC degradation, mitigation can successfully restore the canister.

One remediation method for a canister’s degraded metal surface is the cold spray process—the leading mitigation technique recommended by EPRI and already widely deployed in industrial and military applications. Orano is helping refine this technique as applied to dry storage systems. Similar to additive manufacturing, the cold spray process directs metal particles at hypersonic speeds onto the degraded location. The particles’ impact onto the canister causes them to weld to the stainless steel and build up, layer upon layer, a new solid surface that is stronger than the original material.

Along with the three ISFSI inspections last year for CoC-1004, Orano has supported Aging Management Program inspections in prior years for the 20-year-old NUHOMS systems at two other sites —also with no concerns identified.

To ensure shared benefits and lessons learned from Orano’s dry storage operating experience, the company added this recent inspection data as reference information in the Aging Management INPO Database (AMID), hosted for the U.S. nuclear energy industry by the Institute of Nuclear Power Operations.