OT- question about 3 mile island incident
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    Default OT- question about 3 mile island incident

    Nuclear reactor incidents intrigue me, Chernobyl, SL-1, etc. I was reading about the 3 Mile Island incident and it seems the root cause of this was water in the instrument air system cause a shutdown of cooling systems.

    Does anyone know how or why moisture in the instrument supply could cause pumps to shut off? Particularly in such a critical service?

    Just curious.

    Sent from my SM-G973U using Tapatalk

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    Guessing old-school pneumatic PID controls, like Moore Products used to make. I remember seeing those on fossil plants like PECO's Eddystone and PP&L's older Martins Creek units.

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    I seem to remember there was no real problem. the instruments indicated a air bubble on top of the pressure vessel. They controls wanted to vent this air space so liquid cooling water could enter that head space. Problem was this air pocket would have been slightly radioactive and they would have had to file a report to the government about the release..
    So the operators decided the instruments must be wrong and ignored the problem. The air pocket was allowed to get bigger until there was not enough liquid in contact with the hot core and the core overheated and bent so the moderation rods no longer fit all a the way.

    Similar to the intentional overheating of Chernoble to see how hot it could get before the shutdowm machinery no longer worked due to heat casued bending etc. That test was run on a Sunday so the higher supervisors would not be there to realize what was happening and cancel the test..
    Bill D

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    I recall in my Intro to Engineering class, the technical communications portion of the class began with the professor making us read a dry technical document. In class, together so we all read it. When nobody could really understand any of it or what it meant, the professor told us that it was a report from an engineer warning about a potential failure in the safety-critical system on Three-Mile Island shortly before the event. Can't remember if it was water in the air lines, air in the coolant, or something else, but it didn't (doesn't) matter to the lesson:

    If nobody can understand the output of your work, you might as well not have done any of it at all.

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    ...Jimmy Carter showed up.....

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    Quote Originally Posted by Bill D View Post
    I seem to remember there was no real problem. the instruments indicated a air bubble on top of the pressure vessel. They controls wanted to vent this air space so liquid cooling water could enter that head space. Problem was this air pocket would have been slightly radioactive and they would have had to file a report to the government about the release..
    So the operators decided the instruments must be wrong and ignored the problem. The air pocket was allowed to get bigger until there was not enough liquid in contact with the hot core and the core overheated and bent so the moderation rods no longer fit all a the way.
    That was secondary. If the instrument air hadn't tripped off the cooling pumps water would have continued to circulate and the PRV wouldn't have lifted.


    Similar to the intentional overheating of Chernoble to see how hot it could get before the shutdowm machinery no longer worked due to heat casued bending etc. That test was run on a Sunday so the higher supervisors would not be there to realize what was happening and cancel the test..
    Bill D
    Chernoble's biggest F'up was the kooky engineer that took reactor power much lower than required to perform the test. The test was to see if backup diesel generators would come online fast enough to power the cooling pumps if the reactor SCRAM'd. There was a certain threshold of power production required for this and he took the reactor well below that prior to the test.

    It was goofy ass system anyway, when the reactor tripped it cut itself off from the grid, so grid power could no longer be used to power the pumps. Kooky Russians....

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    Quote Originally Posted by rklopp View Post
    Guessing old-school pneumatic PID controls, like Moore Products used to make. I remember seeing those on fossil plants like PECO's Eddystone and PP&L's older Martins Creek units.
    This. And pneumatic controls are still widely used to actuate valves. But back in the day you'd have clean, dry instrument air distributed to all manner of pressure, level, flow, and temperature transmitters. 100 pis supply. Regulated at the transmitter, which would transmit signals using a 3-15psi range. That range would be linear with some field measurement. So a 0-100 inch of water column level transmitter would transmit 3psi for 0 inches, and 15 for 100 inches. But you need clean, dry air. Synthesizing the answers above, it may be that the instruments didn't read the correct level. This could be caused by moisture. Normally instrument air is dried to a dew point of around minus 70°F.

    The PID controllers - really pneumatic computers - would take the signal, compare it with the setpoint, and then take this "error" signal and multiply it (Proportional action, the "P" in PID), integrate it ("I", and this action removed any persistent offset from the setpoint) and took the derivative (slope with time, the "D", which improved control for systems with large inertia, as when you were controlling temperature in a large vessel). So they'd compute a 3-15psi output signal that would transmit a desired position to a valve. The valve actuators are those things that look like mushrooms.

    The mechanisms consisted of bellows, orifii, levers and such. They were fantastic. And the tubing work, to hook it up, was often a work of art.


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    Not "Kooky" Russian engineers.

    "Insane"
    "Inept"
    "Arrogant"

    and most importantly

    "Deadly"

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    Quote Originally Posted by Cole2534 View Post
    That was secondary. If the instrument air hadn't tripped off the cooling pumps water would have continued to circulate and the PRV wouldn't have lifted.



    Chernoble's biggest F'up was the kooky engineer that took reactor power much lower than required to perform the test. The test was to see if backup diesel generators would come online fast enough to power the cooling pumps if the reactor SCRAM'd. There was a certain threshold of power production required for this and he took the reactor well below that prior to the test.

    It was goofy ass system anyway, when the reactor tripped it cut itself off from the grid, so grid power could no longer be used to power the pumps. Kooky Russians....
    Nah, biggest screw up at Chernobyl is the fact that the test was delayed so that it couldn't be completed in a single shift, and they went ahead and did it anyway. Without telling the next shift, who had no idea the test was happening. Why didn't they shut down the reactor when it was going critical? The alarms were temporarily disabled, because they were running a test!

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    Quote Originally Posted by bosleyjr View Post
    They were fantastic. And the tubing work, to hook it up, was often a work of art.

    I'm a sucker for well executed instrument tubing runs.

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    Quote Originally Posted by BoxcarPete View Post
    Nah, biggest screw up at Chernobyl is the fact that the test was delayed so that it couldn't be completed in a single shift, and they went ahead and did it anyway. Without telling the next shift, who had no idea the test was happening. Why didn't they shut down the reactor when it was going critical? The alarms were temporarily disabled, because they were running a test!
    I thought they did, but the graphite tipped control rods caused a spike during insertion? That spike plus the poisoning from the Xenon production led to a power excursion that set it all in motion.

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    Quote Originally Posted by bosleyjr View Post
    That might as well be porn.

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    Quote Originally Posted by Cole2534 View Post
    I thought they did, but the graphite tipped control rods caused a spike during insertion? That spike plus the poisoning from the Xenon production led to a power excursion that set it all in motion.
    Yep, you're right. I thought it was a "too little too late" thing because of timing, but it was because most of the control rods were manually retracted for the test. When all the retracted ones first entered into the reactor, the graphite tips briefly accelerated the reaction, and they were moving slowly enough that things were well and truly FUBAR before they did any good.

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    Have any of you guys watched the mini series on HBO about Chernobyl?

    Considering I was 1 year old at the time of the incident, I found the series fascinating. I believe the technical aspects of the reactor explosion are pretty well based on what really happened. The characters portrayed are based on real people though I'm sure their actual roles were quite different than the series. There were over 600,000 people involved in the clean up, so they have to condense things down to keep it manageable.

    The point the series drives home is not really about the explosion. It's more about the dangers of the government lying about it and hiding critical information about the dangers of the reactor design from the people who were expected to run them. According to the series, the dangers of the graphite tipped control rods was known for many years before Chernobyl, but was classified by the KGB as a state secret.

    Careless as the crew was in carrying out the test, they believed there was a fail safe mechanism that would prevent any kind of disaster.

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    Quote Originally Posted by ewlsey View Post
    Have any of you guys watched the mini series on HBO about Chernobyl?
    I haven't yet, but I certainly will.

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    I have been reading a book written by a Westinghouse exec.

    How they felt their way along blindly those early days.

    Getting a powerhouse reactor to "go critical" was krept up upon
    very slowly.

    I am no expert, but they explained how moving from "dead reactor"
    to pulling out some rods to "tickle things up" could massively
    accelerate power levels up in microseconds.

    Also, how Westinghouse kept with the pressurized water system, to be the safest,
    to G.E. proving a boiling water can be made to work as well.

    I like the Candu reactor, known as a "Slow Poke" reactor, and IIRC can burn up fuel
    more, leaving less radioactivity in the used up rods.

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    I watched a show on PBS about a resurgence in nuclear power. There is a lot of research being done on reactors cooled by liquid sodium. Apparently is a much safer as no pumping is required.

    According to the show, the main reason we use water cooled reactors is due to the massive amount of research funding for the nuclear navy. Obviously, liquid cooling makes sense when you are floating in water. That research then made it's way into general power generation.

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    Quote Originally Posted by ewlsey View Post
    Have any of you guys watched the mini series on HBO about Chernobyl?
    Excellent, at the top of HBO's generally high production standards. At the end there's a quote from Gorbachev about how Chernobyl was responsible for the dissolution of the USSR. Broadly true except that he couldn't bring himself to say that it was the exposure of the institutionalized lying permeating their culture that did it. The individual parts are very well acted.

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    Quote Originally Posted by digger doug View Post
    ...Jimmy Carter showed up.....
    Yeah, and then he cancelled the Rockwell B-1 while I was working on the program. I never got to thank him personally for that.


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