All Things Nuclear • Where Did the Water in the Spent Fuel Pools Go?
One cause of low water levels may have been that water splashed from the pools during the earthquake. I haven’t seen reports suggesting significant water loss by splashing or that the water levels in the pools were low shortly after the earthquake. Also, as noted above, the reported rate of heating of the pools at Units 4, 5, and 6 suggests that the water level was not significantly reduced early on.
Moreover, except in the case of the Unit 4 pool, even if the water levels in the pools had dropped by several meters during the earthquake—corresponding to hundreds of tons of water being spilled—the heating and boiling times to expose the fuel would still be too long to account for very low water levels in the pools.
A second possible cause of the low water levels is that the pools at Units 2, 3, and 4 all developed significant leaks. Some reports said that a leak was suspected in the pool of Unit 4 and possibly in Unit 3. This analysis suggests that all three pools may be developed leaks.
Dave Lochbaum has suggested a common failure mode for leaks in the spent fuel pools. Large doors in the side of the pools are equipped with rubber tubes that are inflated to seal around the door. Even if these seals were not damaged, without power to run the pumps that keep the seals inflated, they can lose air over time and create leaks around the door. Such leaks may not show up immediately since it could take some time for the seals to lose air pressure.
The pumps for these seals currently do not have backup power so leaks of this kind can result from an extended loss of power from the grid. This is a vulnerability of this type of plant design that could happen elsewhere, including at a number of plants in the the US, and needs to be addressed.
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