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Comment by quanticle | Hacker News Reader

On the other hand, one of the contributing factors to the Three Mile Island nuclear disaster was the fact that the temperature sensors were showing their maximum programmed value of 280 degrees Centigrade. The actual core temperature was far far higher, but because the engineers designing the reactor never thought of the meltdown scenario, they programmed the temperature gauge to cut off at 280C, rejecting higher readings as obviously erroneous. This (along with a water gauge malfunction) led the reactor operators to misdiagnose the fault. The operators thought the reactor was overfull with coolant and was at risk of overpressure, when in reality the coolant was draining away.

I wouldn't be so quick to dismiss "obviously incorrect" sensor data.

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While this is true, the point that it's worth thinking through the nature and consequences of "obviously erroneous" sensor data is still a valuable exercise holds.

If the temperature is past 280, generally speaking the same steps to diminish temperature can be taken (... I'm speaking broadly; this may not actually be true of nuclear reactors and if it's not, additional sensors with larger ranges were definitely warranted if there's discontinuity in the safety and disaster mitigation strategies at temps higher than 280).

There is no amount of automatic airplane wing trim that can arrest a 70-degree AOA. When the sensor's getting 70 degrees, the failsafe operation would have been to back out of the control calculation and defer to pilots.

I agree. In the case of the temperature sensor, my opinion is that the correct behavior should have been to show no reading, or an error value. At least that way the operators would have known that they didn't know the true temperature inside the reactor. As it stood, they saw high, but constant temperature, combined with rising pressure. That indicated that the water level was rising inside the reactor, so they opened drain valves to let water out. That was exactly the wrong thing to do, and it contributed directly to the severity of the meltdown.

Maybe 25 degrees is a little bit close to a real value. But think of this / flying along this line of text. That is what the AoA sensor was telling the FCC was happening to the aircraft. That's wildly out of range.

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