Reply #310: Install high density racks or shut down

[divvy]

Many plants in the US, including the plant I worked at, have installed high density storage racks in their spent fuel pools. It was that or shutdown... now fuel casks offer another option. These replacement racks increased the boron concentrations in the racks to allow packing more spent fuel into the pools in a tighter configuration. They did not require upgrading the cooling system. Most of the heat load on the pool comes from recently deposited fuel and the hot fuel removed from the core during refueling. I saw one report that one of the Units in Japan was refueling and the core had been offloaded into the fuel pool. This is sometimes done to expedite shuffling the fuel locations in the core or if repairs were being made to reactor internals. This configuration puts maximum cooling demand on the fuel pool cooling system. Maybe that is the one the US thought was dry. During refueling, we usually ran both loops of fuel pool cooling... only one was normally required during operation. The backup was RHR, which could be configured to cool the pool. About those high density racks... they required good water quality... turns out that they degraded faster when silica was high... can you imagine if the roof of the building is laying in the pool or they are open to the dusty outside environment... and the fuel pool demineralizers/cleanup system has been off since the crisis started with no hope of getting it back soon. There is not a lot of analysis for spent fuel pools in these conditions... they are beyond their design basis. These pools may have done better than some US fuel pools... I don't think that high density storage racks would have helped, but that is just a guess... it is more of a nuclear engineer's call, not my degree. I'm familiar with the equipment, not all of their design basis.