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Mike Gray on: The Design Flaw in the Reactor
Mike Gray Q: Talk about the issue of the pressurizer.

MG: The curious design flaw that caused the operators at Three Mile Island to misread their instruments was a U-shaped pipe that connected the pressurizer with the rest of the reactor. Now this U-shaped junction was necessary. The pipe connecting this tank to the rest of the reactor had to go underneath another pipe. And that U-shape is just like the sink trap in a sink drain. It prevents gas from rising back because there'll be a loop of water in there. And that's what happened in the pressurizer. The pressurizer is a sort of like -- it's a separate tank sitting on top or to one side of the reactor vessel. And it has an air bubble in the top and it's almost full of water, two-thirds full of water generally, and that's how you read the reactor level. That is, in other words, your level gauge is looking at that level in this pressurizer, which is a separate tank. It doesn't really tell you what the water level is down here. It assumes that if this is full, then obviously this would be full. But with this U-shaped connector, it's possible for the water level to be rising here as a steam bubble was being created here. So, in other words, it wasn't water that was forcing the level to go up; it was boiling water down here forcing steam into this U-shaped pipe which was pushing the tank full of water up. The operators were seeing this water to up and they said, "Oh, my God, where is this water coming from? It must be filling from some unknown source. We don't know what the source is. Let's shut off the emergency cooling," They were afraid that if they let too much water get in there, that there would be no air pressure cushion for a shock absorber. And without that shock absorber, any jolt might fracture one of these pipes and they'd be into a major loss of coolant accident. So they thought they were saving the plant by cutting off the emergency water when, in fact, they had just sealed its fate.

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