Hydrogen accidental formation and ignition at a spent fuel cask of a nuclear power plant

A hydrogen fire occurred at an independent storage for spent nuclear fuel. The ignition took place when performing welding works on a spent fuel cask, which was part of the multi-assembly sealed basket (MSB). The hydrogen had been generated by a reaction between the cask internal paint and borated water: the zinc in the paint reacted chemically with the acidic borated water from the spent fuel storage pool to produce hydrogen, zinc oxide and zinc hydroxide.
The MSB, loaded with spent fuel, had been moved to the cask decontamination work area in an auxiliary building, for the welding works. Approximately 114 liters of borated spent fuel pool water had been drained from the MSB to facilitate welding of the shield lid, creating an air space below the lid.
The gas ignition displaced the shield lid (weighing about 2898 kilograms), leaving it in place but tipped at a slight angle, with one edge about 7.6 cm higher than normal.

The INITIATING CAUSE of the flame was accidental hydrogen generation by Carbo Zinc 11 paint in contact with a borated water environment. Hydrogen formed a flammable mixture with the air introduced in the cask to facilitate the welding.
The IGNITION occurred during the initiation of the shield lid welding.
The post-incident inspection found an inadequate procedure for placing the transfer cask into the spent fuel pool. Moreover, the consequences of using of the cask paint in a borated water had not been properly assessed. Everal occasions to identify formation during previous cask loading operations had been missed. The possibility of hydrogen generation was not considered and the required controls to deal with were not provided.

The nuclear authority NRC identified several root causes for these violations, including weaknesses in: (1) questioning attitudes, (2) management expectations, (3) resolution of issues, (4) attention to detail, (5) communications, and (6) organizational design.

spent fuel storage installation

Before this event, during the loading of a cask, unexpected water seepage from the cask drain line onto the top of the shield lid was observed. The presence of water was an indicator of pressure within the cask being caused by hydrogen production, but the event remained undocumented, and no assessment was performed.

The Multi-Assembly Sealed Basket (MSB) canister consists of a steel cylindrical shell sealed at the bottom with a welded steel plate and at the top with welded steel shielding and structural lids. Within the canister is the fuel basket, a welded steel structure that provides structural support for up to 24 PWR fuel assemblies. The MSB provides confinement, criticality control, and structural support during storage, loading, and on-site transfer to the storage pad. [https://www.osti.gov/servlets/purl/1553317]

Carbozinc 11 is used as a primer on the surfaces of nuclear fuel casks and Multi-assembly Sealed Baskets for long-term corrosion protection in harsh, high-moisture environments associated with spent fuel storage.

Based on various inspections in 1996 , the NRC determined that violations of NRC requirements occurred. The violations are cited in the Notice of Violation and Proposed Imposition of Civil Penalties (Notice) and the circumstances surrounding the violations are described in detail in the subject inspection reports.

Specifically, the penalty for the violation associated with the VSC-24 spent fuel cask is $25,000.

The nuclear authority NRC stressed the importance of
(1) properly conducting spent fuel cask loading operations.
(2) paying attention to the indications of hydrogen generation that would have alerted staff to the need for adequate controls during welding operations; and
(3) ensuring that cask design details, such as paint, are thoroughly evaluated for interactions with materials in the plant environment by cask vendors and plant engineering.