More Thoughts About Nuclear Power Plants

[John Rogers]

I was able to get through the US Navy as a nuclear operator for a bit over 20 years before I retired due to health reasons. Here are some things to think about while worrying about the plants in Japan.
- The design of the fuel area is such that the heated water goes up (natural as heat rises) but then the pipes turn downward to get to the bottom of the steam generators so now the hot water is going down.
- After some of the heat is removed by the steam generators, the colder water now has to climb back to the reactor vessel so that is also against natural flow. This is why they have to move the coolant with pumps as there is NO natural circulation. The US Navy built some reactors that would be able to self cool and they were on submarines but large carriers and civilian power plants are not self cooling.

- The temp inside is less than the pressure so the coolant stays as liquid, I.E. for 500 degree coolant that can make 1000# steam the pressure is kept at 2000# or so. Now if the coolant recirculating pumps stop, generally the reactor scrams (rods drop) but for many hours there is heat built up in the fuel area due to residual radiation so in this case with no circulation the pressure will go up!

- Pressure build up will happen very quickly since the water (coolant) is acting like a solid and expands as it gets hotter. The old (now decommissioned) GE power plants on ships like the CGN25 would be pressure tested to 150% operating pressure or 3000# and then all bets are off if the pipes and welds would hold! So how to keep pressure and temps down, generally by bleeding off some steam which made cold slugs in the coolant pipes or hopefully trying to get the recirculating pumps running and it appears this is not happening in Japan. They did have power, alternate power, backup generators and even batteries but they all are under water I guess so no pumps.

- Generally pump systems such as the Navy used had a set of very high pressure pumps which were used first, then a set of massive low pressure high capacity pumps to try to food the reactor vessel when pressures dropped and they suctioned from the ships fresh water tanks and finally from the sea using the fire pumps and fire main system. This all need power though.......

- So to help keep pressure down, steam is vented from the reactor side but since there are no pumps to keep putting water in as it is vented off, pretty soon there will be steam bubbles in the core are and when temps get so high, water will just bounce off the fuel plates, like on a hot iron, but much worse. Even flooding with fresh or sea water will not cool the fuel and it will probably melt towards the bottom of the vessel.

- If that happens the Uranium and zirconium will actually catch fire and melt through the steel pressure vessel and on into the ground where it will eventually stop when the earth, acting as a large enough heat sink will cool it off. The explosions happen when the Hydrogen gas mixes with some O2 and then things will rupture. Has this ever happened, yes, even BEFORE the Chernobyl disaster when some fuel assemblies overheated and melted through the containment vessel on the Chalk river in Canada and burned for months before it was out out by covering it with sand!

- Will this affect us, yes as the upper atmosphere comes from west to east and back when China was doing nuclear testing above ground, we would get contamination on our feet and have to "decon" before we went into the reactor buildings for work! Will it be monitored, I sure hope so, at least for the kids we have?!

[Grog]

Remember, the Japanese reactors that are having problems were built in the 60's and 70's. The Trident subs, Ohio was the first, construction started in 1976. Natural circulation was new for the navy. I don't know for sure, but I believe they went with natural circulation to reduce noise (no coolant pumps running, they had them to startup, then would shut them down). On commercial power plants they wanted to make steam, and alot of it, so they used pumps to move the coolant, didn't care about noise. Must have figured they had enough ways to cool the core in an emergency. I agree with you, that for safety reasons, that natural circulation would be better. Maybe the newer designs have it, I don't know.

For the record I was on a 640 class. My brother was on 2 different tridents, so I know only what he has told me about them.

[John Rogers]

True info about the noise and I didn't want to get into that but...... I talked to an old retired shipmate of mine that worked in civilian plants until a couple years ago and the ones he worked in were all essentially "single level" and still into the early 2000's they never did any accident analysis with more than one thing failing! The Navy's accident scenarios were the same, only one item failing at a time so an earthquake and tsunami together would have never been calculated for.

[IROC]

Originally Posted by john rogers

- The design of the fuel area is such that the heated water goes up (natural as heat rises) but then the pipes turn downward to get to the bottom of the steam generators so now the hot water is going down.

The Japanese plant with the problem is a BWR. No steam generators.

[John Rogers]

I talked with a friend who worked at B&W before retirement and he said they are in fact boiling water type, but still don't get any natural circulation which can take away the amount of heat that has been generated since the scram when the quake hit. He noted that most all are built so the bottom of the condenser and the feed pump(s) are usually a level below the reactor so the cold water still has to rise, not the way nature really likes to do things I guess?

[IROC]

Originally Posted by Grog

Must have figured they had enough ways to cool the core in an emergency. I agree with you, that for safety reasons, that natural circulation would be better. Maybe the newer designs have it, I don't know.

Basic BWR design has numerous system to remove residual heat following a shutdown. The problem is that all of these require power... When the tsunami took out the backup diesel generators, things got serious.

Nominally (speaking from my basic GE BWR background) the system used to remove heat initially is aptly called the Residual Heat Removal (RHR) system. There are two pumps and two heat exchangers per unit (and each are very large). This system circulates water through the core and then rejects the heat to a closed cooling water system (not seawater or cooling towers). If this fails, there is a system called the Core Spray system that is intended to keep cooling water in the core and above the fuel. Again, two very large pumps with their own heat exchangers.

If the pressures in the reactor are too high to pump water in (that can happen), then there is a system called the High Pressure Coolant Injection (HPCI) system that "injects" water into the reactor. It runs off of residual steam (not electricity). The problem with this system is that it is low volume - you're not going to pump large amounts of water into the reactor this way. There is also a system called the Reactor Core Isolation Cooling (RCIC), that functions to put water in the reactor.

All of the pumps for these systems are located well below the reactor level to maintain net positive suction head. During nominal operation, a BWR bleeds off some steam from the turbines to drive pumps that pump condenser water back into the operating reactor, but without steam, these are useless.

If all of these fail, you throw the hail mary pass and inject borated water into the reactor, which absorbs the neutrons. It ruins the reactor, but at this point that is the least of your worries.

One thing that I do not understand is how the Japanese are pumping seawater into the reactor. I don't understand an easy way to do that...

[John Rogers]

I am curious about how they are getting sea water in also? In the Navy's pressurized water systems when it came time to swing the blanks to allow sea water into the system, it meant that the primary system had failed structurally and that pressure was below fire main pressure. That also meant that the fuel had probably already started melting or had done that. They might have some emergency pumps for that as the last resort but no one is talking specifics and even through CNN had one of their plant's designers in an interview, I am guessing he was not allowed to talk about things in great detail.

As for radiation from a meltdown, during a refueling at one of the reactors in upstate NY in the early 70s, the risk analysis folks did lots of "what if" tests. The worst turned out to be when the reactor vessel top was being lowered into place that the large lifting hook snapped. They assumed the very large chunk of steel dropped 18 inches and the shock caused the fuel rods to jump to approximately 50% of their travel. Since there are no drive mechanisms attached they bounced and the result was the reactor going critical and then very rapidly accelerating in temp so fast that the meters could barely read it. By the time the operators saw the high temps the fuel had started to melt and even though the huge low pressure cooling pumps were started, the water being pumped in just bounced off the metal like a hot iron. The fuel then melted through the steel vessel bottom, through the concrete floor and approximately 300 feet into the ground where the earth stopped it. As for radiation, everyone in the ball died within 10 minutes, everyone on base died in about 2 hours and in the little town outside the base all died in a month. The QA folks checked the hook by x-ray right after that and it was okay!

[turbo6bar]

Why can't the plants generate electricity to power the cooling pumps? Sounds obvious to me, so I must be missing something.
jurgen

[RWebb]

all I've seen re seawater is that it was something they cobbled together quickly or words to that effect

at least some of them are GE reactors

How would the plants generate electricity to power the cooling pumps?

latest news is that the operators say they cannot rule out a meltdown...

[s_wilwerding]

Best article I've seen about the Japanese nuclear situation (it's long - scroll down to the non-italicized text):

https://morgsatlarge.wordpress.com/2011/03/13/why-i-am-not-worried-about-japans-nuclear-reactors/

[IROC]

Originally Posted by turbo6bar

Why can't the plants generate electricity to power the cooling pumps? Sounds obvious to me, so I must be missing something.
jurgen

That might have been an option last Friday, but no longer. Contrary to The China Syndrome, there are actually many ways to cause a plant to shutdown and with as many safety system issues as they are having right now, I would imagine that even getting one of the currently undamaged units started up and producing power is impossible.

I would imagine that there is some fuel melting at this point. IMHO that, in an of itself, isn't really a big deal until the point it results in a failure of the reactor vessel itself. Until then, just keep it cool no matter what.

BWRs are generally designed to keep the core covered under what they call the "Design Basis Accident". In the case of the plant I worked at, that meant essentially one of the recirculation lines that attach directly to the reactor vessel falls off. BWRs have a recirculation system that merely circulates water through the core. The pumps, motors and piping for this system all exist within the secondary containment (the drywell) and the piping is large (24" diameter pipe IIRC). All of the safety systems were intended to keep the core covered and cool with a 24" diameter hole in the side of the vessel. We also had 8 backup diesel generators (locomotive engines connected to generators) to power things in the event the plant shutdown and all incoming power was lost.

[cashflyer]

Originally Posted by john rogers

The Navy's accident scenarios were the same, only one item failing at a time so an earthquake and tsunami together would have never been calculated for.

The Japanese may not have calculated for the "earthquake + tsunami scenario", but I hope they accounted for the "Godzilla Scenario".

[Grog]

Power plants need power to start up. Sounds like they lost their connection to the grid (either quake or big wave) My gas turbine plant still uses almost a megawatt at rest, when we are starting up with all required systems running we use up 10 megawatts. Nuke plants and coal plants use more than a couple hundred megawatts just to operate the systems. The coal plant I worked at had a induced draft fan that was 13.8k volts, 10K hp, had four of these just to pull air through the boilers. I don't know of any nuke plants with black start capabilities. (can start without outside power) There are rules for power companies to have the ability to start one or more plants without the grid. Our old gas turbine plant next to us has a diesel generator that outputs enough to start a small Alstom turbine, 20mw. Then it is used to start one of their 60mw GE units. The emergency back up diesels even if running wouldn't be big enough to start one reactor plant.

[Flieger]

Originally Posted by IROC

The Japanese plant with the problem is a BWR. No steam generators.

How do you generate electricity from boiling water without using a steam generator?

This is a little scary to me. At school, I live within 20 kilometers of Diablo Canyon nuclear power plant in Morro Bay. There are tons of faults here. one thrust fault runs from here to Morro Bay and is just about a mile from where I am now. I do not know if they throttled back Diablo Canyon while the tsunami waves came in. They were only 4 feet but it was like the tide coming in and out at about 10 mph. I think the intakes for cooling water (I believe they use sea water for secondary heat exchanger circuit) would be safe.

[turbo6bar]

Originally Posted by RWebb

How would the plants generate electricity to power the cooling pumps?

Aren't the plants for electricity generation? If there's heat in the reactor, can it not be used to make steam to turn a turbine? Use that electricity to pump coolant, and eventually the system dies due to losses.

Seems like you can't start from a stop, as the posts above suggest. What a nightmare.

[Grog]

A steam generator is a heat exchanger, uses one system to heat or cool another. A steam turbine uses the steam generated from the steam generator to turn a generator.

[cashflyer]

Originally Posted by Flieger

How do you generate electricity from boiling water without using a steam generator?

You still have steam in a BWR, but it's produced differently and at much lower pressures.

[Porsche-O-Phile]

Reading a summary of the TMI situation from 1979 - apparently the standard heat output in the core right after the scram (emergency shutdown) was 160 megawatts, an hour later it was down to 30 megawatts and after three hours was down to 20 megawatts. The point is, yes, the output of heat drops pretty quickly but you're dealing with HUGE amounts of it. Even 20 MW of heat is a LOT if there's not way to cool it or bleed it off.

Also consider that with these water cooled reactors, I believe the cooling water temps in the core flirt regularly with the critical point of H2O which is in the 700-ish degree range. Above that, I doubt you'd get any cooling benefit at all and you'd get a cascading effect until you could get the inlet cooling water temperature below that level...

Kind of a beast to control when things go south, but not impossible.

[Flieger]

Originally Posted by Grog

A steam generator is a heat exchanger, uses one system to heat or cool another. A steam turbine uses the steam generated from the steam generator to turn a generator.

Ah, thank you.

[Grog]

Originally Posted by turbo6bar

Aren't the plants for eletcricity generation? If there's heat in the reactor, can it not be used to make steam to turn a turbine? Use that eletcricity to pump coolant, and eventually the system dies due to losses.

Seems like you can't start from a stop, as the posts above suggest. What a nightmare.

Systems off the top of my head that need power before you start up;

control systems, computers, motor operated valves instruments etc.

Lube oil systems

many different cooling systems for the other systems, oil, hydraulics, air compressors, bearing water cooling systems

cooling tower circ water pumps, tower fans

vacuum pumps for condensers, seal water systems for these pumps

condensate systems, feedwater systems, big pumps ours are 5000hp, some plants use steam driven feed pumps.

service water, cooling tower make up water.

demin water

generator cooling systems, we have one generator cooled by hydrogen and the other is water cooled.

you need all this up and running before you can start the plant. If our plant is already hot it still takes us 3 to 4 hours before we are generating, any residual steam and heat would not last long enough to get on line.