Yeah, yeah, we ALL know that.
However, (comma), fuel pool levels are controlled:
1) Low level will cause pumps to start to add water to restore the level to a normal operating band.
2) A higher level is self correcting by overflow weirs and surge tanks.
So, in ‘nuclear-speak’ really what is said is that that 400kg component that fell into the pool did not breach the pool liner, resulting in a loss of water (level decreasing, or if you will, ‘significantly changing’)
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