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In the Three Mile Island nuclear meltdown many things went wrong. Early in the stage of the problem, there was a relief valve (PORV) designed to let excess water out when pressure got too high in the inner loop. It opened properly and got stuck open. A light switch was supposed to come on when it closed, but unbeknownst to the operators, whoever designed it had the light come on when current was applied to the solenoid that closed the valve. The light came on and the operators assumed the valve was closed when it was stuck open. This assumption they were dealing with a closed loop caused much confusion contributing to events spiraling out of control and in hindsight this whole thing could be avoided if the Graphical User Interface was better planned (linking that light to a sensor as they now do).

The reactor was scammed and there was decay heat to deal with.

At one point before the meltdown, the automated system detected excess heat in the core and flooded it with the High Pressure Injection System, designed to quench the core if it heated too much. An assumption was made that the core going above a certain temperature meant the core ran dry, which in this case was true.

The operators thought the relief valve was closed and they were dealing with a closed system. In the past the HPIS activated anomalously 7 times at the TMI unit that was in trouble (when temperatures were normal), forcing operators to shut it off and reset computers. The operators assumed the HPIS activated erroneously again and since it was a "closed" water loop, the added water would increase the pressure until the air bubble in the pressurizer unit (the only part that is supposed to have voids) disappeared, or as they say the core goes solid. Eventually, adding water to a closed solid loop would cause the plumbing to rupture and it would be a very bad day.

Of course the cruel irony is that they were not dealing with a closed system. The light made them think the valve was closed but it was open. If they realized from the low pressure readings that the system was open or if the indicator was designed better, they would have let the HPIS cool the core since it had enough water to deal with the decay heat. So if they did nothing, water would have boiled away in the pressurizer, but water would also come directly onto the core.

I was wondering, suppose the HPIS's rate of water flow was triple the rate water of boiled away thanks to the decay heat. So in other words, supposed it flooded the core. And suppose the relief valve was still stuck open for hours (like OTL) while the HPIS (which in this case can put more water into the system than it can boil) stayed on the entire time. What happens next?
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