Saturday, March 19, 2011

Nuclear Briefing on Japan Reactor Failure

This is a good background piece on the developing reactor disaster in Japan.  By the time this is read, we hope those reactors will have cooled sufficiently to be under reasonable control.

A lesson can be drawn.  These reactors were built long before much experience existed regarding tsunamis.  The risk is now obvious and a present treat however unlikely.  The fix is also easy.  Merely place critical assets such as backup generators fifty feet above sea level.  This is hardly a design problem.

The other lesson of course, is that no one can truly imagine all failure modes and without a good imagination, all design becomes purely cost driven.

It is an easy read.

A nuclear engineer's briefing on the emergency in Japan 

This Q&A briefing provides a concise overview of much of what you need to know on the nuclear emergency in Japan. Nuclear engineer Mark Mervine gave this interview to his daughter Evelyn Mervine.

March 14, 2011

My name is Mark Mervine. I graduated from the US Naval Academy in 1981, and went into the Navy nuclear power program. I was in submarines, and while I was in the Navy I qualified on two different types of Navy nuclear power plants and served as an instructor in the Navy nuclear power program.

Q: OK, and then after you got out of the Navy?

A: After seven years of active duty, I went into the Reserves, and I stayed in the Reserves and I retired as a commander in the Navy Reserves.  I went to work, initially, for Wisconsin Electric, which at that time had a 2-unit Westinghouse pressurized-water reactor in Turbridge, Wisconsin.  While I was there, I completed my SRO certification, which allowed me to do senior review and oversight, as a member of the plant management staff.  And I also qualified and served as a shift technical advisor, which is a position that was added in the nuclear power industry, after Three Mile Island, that is a degreed engineer position, that’s available to the on-shift crew on a 24-hour basis.  Some plants do it on an 8 hour watch, at that time, Wisconsin Electric did it on a 24 hour watch, so I would actually stay at the plant for 24 hours; we had a place where we could sleep, and my job was to advise the crew whenever they needed advice on what was happening with the plant.

After a few years at Wisconsin Electric, I went to work for Vermont Yankee, where I also completed the SRO certification, Senior Reactor Certification, which allowed me to do senior level reviews as a member of the plant management staff, and I also served on the Outside Review Committee, which is a very high-level committee for the main Yankee nuclear plant, until it closed, and also Vermont Yankee.

Q: Excellent. So, you’re qualified to talk a little bit about nuclear power, it sounds like.

A: I can talk a little about nuclear power, yes.

Q: OK, excellent. So, my first question for you is really basic, since maybe people are not familiar with this, but what, can you just describe quickly, what is a nuclear power plant?

A: Yes I can, maybe what I’ll do first is explain what a power plant is.

Q: Sure.

The vast majority of power plants in the world generate steam, by some method, some by burning oil, some by burning coal, and heat up water, and make steam, that steam then powers a turbine, and attached to the turbine, you have a generator, and that generator generates electricity, and through transformers is connected to the electrical power grid.

So, in that respect, a nuclear power plant is a lot like other power plants, in that you have this turbine, that’s steam driven, with a generator, that’s attached to a transformer and then to the grid. The difference is, what a nuclear power plant does, is it generates, depending on the type of the plant, hot water or steam, by the fissioning of uranium.

Q: Right.  And that’s providing the power, basically.

A: So, there’s two major types of nuclear power plants in the Western world. One is a pressurized water reactor where the water’s kept at high pressure and doesn’t boil; an there’s a heat exchanger, and on the other side of the heat exchanger, that water is allowed to boil which generates the steam. And then you have a boiling water reactor, where the water in the reactor actually boils and generates steam directly, and that steam is used to power the turbine.

Q: So, another question I have for you, is one of the main problems they’re having in Japan is that they’re not able to cool the power plant. So, can you explain why a nuclear power plant needs to be cooled?

A: Absolutely, so what happens in a nuclear power plant is that the atoms fission or split in half and that generates heat.  There’s also other materials that are created (I don’t want to get into too much detail and confuse people) that continue to decay and that also generates heat. So for some period of time after you shut down a nuclear power plant you have to continue to cool the reactor core. Because you’re still, I mean, to begin with, it was very warm because you were generating hot water under a lot of pressure or steam and it needs to be cooled, obviously, down and because of the decay of these materials in the fuel they also continue to generate heat for some period of time until the decay trails off.

Q: So they’ve actually shut down the plant in Japan and they’re just trying to cool it?

A: Okay, well if you’re talking specifics, the plant that we’re aware of that is in the most difficulty right now is the Fukushima plant, Unit One.  That plant is a General Electric boiling water reactor, it first achieved criticality in 1970, it’s similar to a couple of other plants that we have here in New England. It’s very similar to Pilgrim, which is down in Massachusetts, and Vermont Yankee, in Vermont.

And, that plant was automatically shut down, when the earthquake occurred, and for about the first hour, they were running on their diesel generator. Once a plant shuts down, it has two ways to get electricity, one is from the grid, and another is from emergency diesel generators that they have on site. In this case, because of the magnitude of the earthquake, the grid basically went dark, so they were operating on their diesel generators and everything was functioning as it should be. But then, based on news reports, about an hour after the earthquake and the shutdown, the tsunami hit, and flooded the plant, where the diesel generators were, and that caused them to lose their diesel generator power and reduced them to their emergency battery backup power only.

Q: And that wasn’t quite enough to have the cooling capability that they needed?

A: The emergency backup on the batteries gives them, you know, very very limited capabilities, so they were having a very difficult time keeping the plant cool.

Q: Do they sort of have to go to a smaller cooling system, smaller pumps and that sort of thing, that can be run off of their battery?

A: I don’t know the specifics of that plant and what they might have done in Japan. Obviously, Japan being in an earthquake zone probably had additional requirements for the plant that we wouldn’t have to have in other places around the world. But, in any event, based on news reports, they did have some type of cooling capability using their battery power, the problem of course is, the batteries are only good for a few hours.

Q: Yeah, the news reports said that the Japanese military was actually trying to get in replacement batteries to cool the plant, I’m sure they’ve continued that effort but I haven’t heard any update on that in the news.

A: So, the reports that I saw on the news said exactly that, they were trying to supply the plant with additional batteries and a portable diesel generator.

Q: Right, I hope they’re successful soon. So how are nuclear power plants in general built to withstand earthquakes and tsunamis? You may not know about this, since you work on power plants that are in more tectonically stable regions, but are there some specific requirements for natural disasters?

A: There are, and depending on what the worst case scenario would be anticipated for an earthquake, their requirements are different. So probably the best example I could give is, I once participated in an inspection of the Trojan nuclear power plant which was in Oregon.  That plant has been shut down now, but compared to the plants that I had worked in Wisconsin and in Vermont they had a lot more requirements on them for earthquake protection. So the way you do that, there is a lot more supports for all the equipment, all kinds of hydraulic dampeners which allow the equipment to move back and forth without breaking. I know in Japan they have a requirement that all the plants have to be built on bedrock, so, they actually have to go down to bedrock in order to begin to build the supports of the plant. So, yeah, there’s numerous precautions that are taken and, like I said, there were probably additional backup system requirements that were required by the Japanese government, for those plants, being in an earthquake zone.

Q: Yeah but this was just such an enormous earthquake, I mean, I don’t think they’re released the official report yet, but this is probably in top five biggest earthquakes so even if they prepared for the absolute worst, this is something that really stressed all of their systems and backups, I imagine.

A: Well, I think really the key here was not so much the earthquake. By all reports, the plants functioned exactly as they were supposed to do in the earthquake, they shut down automatically, when the grid was lost their diesel generators started, and everything was fine. What really put us in the situation we’re in now is the tsunami as a result of the earthquake, but not the earthquake itself.

Q: So, what happened with the explosion that happened earlier today, do you know anything about that?

A: Well I can only comment on what I’ve read in news reports and a little bit of speculation based on my knowledge of how nuclear power plants work.

So again, in this case, this is a boiling water reactor, so when its operating, normally the reactor is full of water to a certain point, and then above that, steam, so the core is kept covered in water, but above that steam is generated, and that steam goes through pipes, normally, turns the turbine, and then is cooled and returned back to the reactor.

Because they’re on a very limited backup capability, only to get, probably, a small percentage of the water that they would normally be able to pump into the reactor to cool it, they were probably allowing the water to boil, which you wouldn’t do normally, during a shut down. But by allowing the water to boil you’re taking heat away from the reactor and thereby cooling it.

Because of the lack of power, they wouldn’t be able to use their normal and back up systems to remove the steam and cool it and return it to the reactor, because there was no power. So they were probably trying to vent this steam into the buildings at the plant. If they could vent a little bit of steam, add a little bit of cool water, they could keep the reactor cool enough to keep it from melting down.

Q: I see, and I guess the big question that everyone has today is, has the explosion or any of the damage, I guess there hasn’t been a lot of damage to the plant, it’s just overheating, do you think any of this is causing nuclear leakage and if so, is that a big problem?

A: So, I‘ve actually looked at the before and after picture from the explosion that’s available on the news and, in my opinion, they have an extremely serious situation at this nuclear power plant. So, my speculation is they were venting the steam in order to try and cool the reactor, unfortunately, without power they don’t have a lot of their normal instrumentation that they would have.

Q: So they can’t monitor things to the same degree –

A: They don’t even have their backup power, I mean they basically have the bare minimum of instrumentation provided by whatever battery power they have left. My guess is, and it was reported in the news that they had a hydrogen explosion, so they obviously had hydrogen and other gases that were generated, that built up to an explosive level and if you look at the photos the entire building surrounding the reactor, the only thing left of it is the steel frame, the whole building has collapsed. That would normally be called the auxiliary building, and that building actually does house a lot of the emergency systems for the reactor. So I think we have a very very serious situation at this power plant where the entire auxiliary building has been destroyed.

According to reports, the containment is intact, so if there has been any release of radioactivity, it has been very minor, to this point, but they have got to find a way to get some electricity, and cool that reactor. And the last report I saw said that there plan was to use seawater. So obviously, they’re going to get some temporary pumps, they’re going to use seawater, mixed with boron. Boron is a substance that will absorb neutrons, very similar to borax that you could go buy to wash your clothes with, that will keep the reactor from going critical again when they add the cold seawater. Even though the control rods have been fully inserted, when you add cold water, cold water is denser than warm water, and it can cause the neutrons that are still bouncing around the reactor to moderate, to a speed at which, (so moderate means slow down), they could strike the fuel and cause a fission.

We obviously don’t want any more fission because that generates more heat and we certainly don’t want the reactor to go critical because that generates a lot of heat. And, critical is not the bad word that you see in the news, where you say “Oh, reactor’s going critical!”; when it operates, it’s normally critical; all critical means is it has a self sustaining reaction, which is what you need to operate. What we wouldn’t want it to do is to go to a terminology called super-critical, that would be really bad. But in any event, when you add the cold water and you don’t add the boron, then you have the potential of causing the fission level to go up in the reactor and more heat to be generated, which you don’t want to do. This is beyond the last resort, to do this, at a nuclear plant.

Q: To use sea water to cool it –

A: I think they’re basically down to their last option here.

Q: So what do you think is the best case scenario for this plant, and added to that question, what is the worst case scenario?

A: I think the best case is that the military get the generators on-site with some emergency pumps and they’re able to rig up a cooling system to cool that reactor, to keep it cool, and they’ll have to cool it for several days before it gets to the point where the heat is decayed off. Obviously the plant is destroyed, and I’m sure it will have to be decommissioned. The question is how much additional damage is there at the site, because, there’s actually six nuclear reactors at that same site and two more that were planned or are under construction.

Q: I see, so this is just one that’s been failing.

A: This is just one of six reactors at that site that were in commercial operation.

Q: Oh that’s scary, so that there could be trouble with the other ones.

A: The question is, as a result of this explosion – has any damage occurred in any of the other, adjacent, reactors and also what is the situation of the additional reactors?

Q: Right, if they don’t cool them, it seems like this same thing could happen to them.

A: They would have the same problem, so a couple of the plants were shut down for maintenance so they’re probably less problematic because their cores would have cooled down but the ones that were operating at the time that the earthquake occurred, could all be a concern.

Q: So I guess a final question I have for you is, do you think nuclear power plants should be built in an earthquake prone area such as Japan?

A: I think it’s important for the nuclear industry, to be unemotional about what has happened here. So, like I said, it does appear that all of the design features that were required for the earthquake, functioned, and the plant was going through a normal shutdown sequence. Obviously, when the tsunami came, that was something that was not designed for, because it flooded the location where the emergency diesel generators were and caused them to lose all power, and we’re now in a scenario that’s well beyond any design contingencies that were designed for that plant.

So, I think the nuclear industry has to take a serious look at what has occurred in Japan. Although nuclear power is an important source of electricity, I think we have to seriously question any plants that are located next to the ocean and the worst case scenario for this type of event, an earthquake followed by a tsunami, as to the impact it would have on that plant, and the emergency backup system.

Clearly, in this case, this was not taken into account and the net result is, we have a nuclear plant that appears to be very very close to a core meltdown.

Q: And what would a core meltdown lead to? I mean, is this going to be contained? Is there any chance that this is going to be like a Chernobyl type situation? I mean, I know that’s a different scenario, but is there a potential for a large radiation leak here?

A:So, you ask a good question, and probably one that is on the mind of the public. So, the first thing is, this is a different scenario from the one that happened at Chernobyl.  And let me just explain a little bit. Chernobyl reactor was a completely different type of design than those that we typically have in Western society. That was a graphite moderated reactor and probably the big difference between either a pressurized water reactor or a boiling water reactor like we have in the West, is that a water cooled reactor is what we call inherently stable.

In this boiling water reactor, even though, it’s not good that the core would not be cooled, as the water level drops, and you generate steam, the steam is less dense than the water, so that means that there’s less molecules of water to moderate or slow down the neutrons. So, when a steam void forms, it actually causes the power level to drop in that vicinity, or the heat generation to drop in that vicinity.  The problem you have, of course, is you do need to cool the reactor because you have all this residual heat, but a pressurized or boiling water reactor is inherently stable, as opposed to the Chernobyl design which is inherently unstable.

The other big difference is, all Western reactors have to have a containment building. And so, according to the news reports, although the auxiliary building has been destroyed, the containment, or steel liner, has not been destroyed. So that’s still intact, so in theory, as long as they can maintain the pressures in that, and there should be relief valves on that, to maintain the pressure, even if the core was to melt, the vast majority of the radioactivity should be contained within that containment building. At Chernobyl we didn’t have that, so when the core melted and caught on fire, all the radioactivity will spread to the atmosphere and to the countryside. In this case, that should not occur, however, again we’re beyond the worst case scenario here, where the last resort now is to try to rig something up to use seawater to cool the plant and the auxiliary building, with all the safety systems has been destroyed.

Q: Well, we’ll just keep our fingers crossed and I hope that there are a lot of nuclear engineers and military people really working hard to keep this from being an even worse disaster than it is already.  Thank you very much Dad, for — Er, sorry did you want to say something?

A: Yeah, I was going to say, it’s obviously a very grave situation however, the one good thing is that Japan has many many nuclear power plants and they have a lot of nuclear experts in that country, so in addition to the help and expertise that they can get from the US and other folks that have a lot of nuclear experience they have a lot of their own people who have a lot of expertise. And I’m sure that they’re doing everything they can, but again, I do have to emphasize that I think this is an extremely serious situation.

Q: OK, thank you so much, Dad, for all of your insights.  I’m so glad I have a nuclear engineer in my family.

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