Three Mile Island extra water

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?
 
A basic rule in engineering is to check your assumptions. Another is to know your tools. A friend who worked in a soybean reduction plants saw the same problem in their operations, the technical managers & operating technicians did not always know the equipment and made decisions without cross checking with others. The people in the repair and maintenance section who understood the system the best were usually not consulted until things went badly south.

However he never saw anything as stupid as @ Chernoble, where the operating managers ran a test of the backup systems, during the week critical parts of the back up were disassembled for maintenance.
 
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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?

Worst case is the pump/s of the HPIS fail after that many hours. Otherwise at some point someone will think things through to where the problem is discovered.
 
A basic rule in engineering is to check your assumptions. Another is to know your tools. A friend who worked in a soybean reduction plants saw the same problem in their operations, the technical managers & operating technicians did not always know the equipment and made decisions without cross checking with others. The people in the repair and maintenance section who understood the system the best were usually not consulted until things went badly south.

However he never saw anything as stupid as @ Chernoble, where the operating managers ran a test of the backup systems, during the week critical parts of the back up were disassembled for maintenance.
come on .. you wouldn't run your tests during that time?! how else are you to know if your back up works ;) Chernobyl was a mess on so man different levels of crazy improbable space bats to make even the aliens say.. "dude.. wtf just happened.. " . "seriously, did the anal retentive soviets just melt down a Nuke plant?" .. and then.. the inept silence for 3 days.. ( space bats left ASAP btw )
 
Another is to know your tools.

If they ever conducted a test on their PORV and light indicator at TMI (I don't know if they did, but certainly the manufacturers did when designing the thing in the first place) they would see every time the valve closed, the light went on, unless the valve was stuck in a test. Only if you look in an obscure part of the manual do you see it written the light is hooked to an electric circuit with the solenoid, which if you can infer that the indicator shows when the computer to close the valve instead of the valve's actual position, you are correct.

In the words of John Hilllabin "Unbeknownst to the staff, whoever designed the control panel programmed the light to go off once the computer had sent the signal to close the valve -- which isn't the same as when the valve was actually closed. It's a pretty massive goddamned difference, in fact, when the entire point of that console is to let you know if something has malfunctioned. So, for instance, if a valve got stuck open, it doesn't help for the console to shrug and say, 'Eh, I told it to close. Don't blame me.' So, the operators didn't realize the valve had been stuck in the open position for a dangerously long amount of time"

The only way to challenge the assumption they were dealing with a closed loop was the pressure indicator. Water level in the pressurizer rising with pressure falling indicates the loop is open. Also, during the few minutes the HPIS was on, the pressure should have dramatically risen, but it in fact crept up a bit but never reached normal operating levels (the relief valve's opening made the pressure drop below operating levels) TMI was salvaged by the next shift when the incoming shift concluded from the low pressure readings that the light indicator was bugged (technically it was working but poorly designed).

Worst case is the pump/s of the HPIS fail after that many hours.

The accident lasted 5 hours I think, not counting the time it took for the molton slag to solidify after it was cooled and out of the danger zone. The HPIS for TMI's design can add water for 72 hours at full flow rate. After that, the backup system would be unable to maintain pressure and it would decline. An hour later, the pressure would be below the pressure of the core (meaning it couldn't put any water in). And 7 horus after that, it would be out of water assuming maximum flow rate, which it couldn't perform if trying to inject water into a still-pressurized core anyways.

The HPIS stuck on in a closed loop would rupture the plumbing in 20 minutes unless deactivated or throttled down and raise the pressure outside allowable limits much quicker. HPIS is designed to deal with small loss of coolant accidents or deliberate venting, but its flow rate (OTL and my modified "flood" rate) is inadequate to deal with a major leakage such as ruptured pluming. It is also inadequate to stop a meltdown in progress. In OTL, once the core started melting, the water added evaporated on contact because stopping a meltdown in progress requires a faster flow rate than simply trying to add water to a core that is covered in water. Even the increased flow rate of the OP would be unable to deal with that, although there might be the opposite problem if it is flooding the core faster than water is boiling away.

I don't know what might happen with this setup. There shouldn't be a meltdown, but I have a feeling it is going to create a big mess.

come on .. you wouldn't run your tests during that time?! how else are you to know if your back up works ;) Chernobyl was a mess on so man different levels of crazy improbable space bats to make even the aliens say.. "dude.. wtf just happened.. " . "seriously, did the anal retentive soviets just melt down a Nuke plant?" .. and then.. the inept silence for 3 days.. ( space bats left ASAP btw )

Chernobyl was really stupid. They wanted to do the tests during the day, but they couldn't do it as scheduled. So instead of having the day shift, which was briefed for weeks about the test, they had the 'green" ngiht shift take care of it.
 
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