This is a good background piece on the developing reactor disasterin Japan
A lesson can be drawn. These reactors were built long before much experience existed regardingtsunamis. The risk is now obvious and apresent treat however unlikely. The fixis also easy. Merely place criticalassets 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 imagineall failure modes and without a good imagination, all design becomes purelycost driven.
It is an easy read.
A nuclearengineer's briefing on the emergency in Japan
ThisQ&A briefing provides a concise overview of much of what you need to knowon the nuclear emergency in Japan
March 14, 2011
This Q&A briefing provides a concise overview of much of what youneed to know on the nuclear emergency in Japan
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My name is Mark Mervine. I graduated from the US Naval Academy
Q: OK, and then after you got out of the Navy?
A: Afterseven years of active duty, I went into the Reserves, and I stayed in theReserves and I retired as a commander in the Navy Reserves. I went towork, initially, for Wisconsin Electric, which at that time had a 2-unitWestinghouse pressurized-water reactor in
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 managementstaff, and I also served on the Outside Review Committee, which is a veryhigh-level committee for the main Yankee nuclear plant, until it closed, andalso Vermont Yankee.
Q: Excellent. So, you’re qualified to talk a little bitabout 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 reallybasic, since maybe people are not familiar with this, but what, can you justdescribe 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 agenerator, and that generator generates electricity, and through transformersis connected to the electrical power grid.
So, in that respect, a nuclear power plant is a lot like other powerplants, 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, whata nuclear power plant does, is it generates, depending on the type of theplant, 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 anddoesn’t boil; an there’s a heat exchanger, and on the other side of the heatexchanger, that water is allowed to boil which generates the steam. And thenyou have a boiling water reactor, where the water in the reactor actually boilsand generates steam directly, and that steam is used to power the turbine.
Q: So, another question I have for you, is one of themain problems they’re having in
A: Absolutely, so what happens in a nuclear power plant is that the atomsfission or split in half and that generates heat. There’s also othermaterials that are created (I don’t want to get into too much detail andconfuse people) that continue to decay and that also generates heat. So forsome period of time after you shut down a nuclear power plant you have tocontinue 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 pressureor steam and it needs to be cooled, obviously, down and because of the decay ofthese materials in the fuel they also continue to generate heat for some periodof time until the decay trails off.
Q: So they’ve actually shut down the plant in
A: Okay, well if you’re talking specifics, the plant that we’re aware of thatis in the most difficulty right now is the
And, that plant was automatically shut down, when the earthquakeoccurred, and for about the first hour, they were running on their dieselgenerator. Once a plant shuts down, it has two ways to get electricity, one isfrom the grid, and another is from emergency diesel generators that they haveon site. In this case, because of the magnitude of the earthquake, the gridbasically went dark, so they were operating on their diesel generators andeverything was functioning as it should be. But then, based on news reports,about an hour after the earthquake and the shutdown, the tsunami hit, andflooded the plant, where the diesel generators were, and that caused them tolose their diesel generator power and reduced them to their emergency batterybackup power only.
Q: And that wasn’t quite enough to have the coolingcapability that they needed?
A: The emergency backup on the batteries gives them, you know, very verylimited capabilities, so they were having a very difficult time keeping theplant cool.
Q: Do they sort of have to go to a smaller coolingsystem, smaller pumps and that sort of thing, that can be run off of theirbattery?
A: I don’t know the specifics of that plant and what they might have done in
Q: Yeah, the news reports said that the Japanese militarywas actually trying to get in replacement batteries to cool the plant, I’m surethey’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 tryingto supply the plant with additional batteries and a portable diesel generator.
Q: Right, I hope they’re successful soon. So how arenuclear 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 moretectonically stable regions, but are there some specific requirements fornatural disasters?
A: There are, and depending on what the worst case scenario would beanticipated for an earthquake, their requirements are different. So probablythe best example I could give is, I once participated in an inspection of theTrojan nuclear power plant which was in
Q: Yeah but this was just such an enormous earthquake, Imean, I don’t think they’re released the official report yet, but this isprobably in top five biggest earthquakes so even if they prepared for theabsolute worst, this is something that really stressed all of their systems andbackups, I imagine.
A: Well, I think really the key here was not so much the earthquake. By allreports, the plants functioned exactly as they were supposed to do in theearthquake, they shut down automatically, when the grid was lost their dieselgenerators started, and everything was fine. What really put us in thesituation we’re in now is the tsunami as a result of the earthquake, but notthe earthquake itself.
Q: So, what happened with the explosion that happenedearlier today, do you know anything about that?
A: Well I can only comment on what I’ve read in news reports and a little bitof speculation based on my knowledge of how nuclear power plants work.
So again, in this case, this is a boiling water reactor, so when itsoperating, normally the reactor is full of water to a certain point, and thenabove that, steam, so the core is kept covered in water, but above that steamis 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 topump into the reactor to cool it, they were probably allowing the water toboil, which you wouldn’t do normally, during a shut down. But by allowing thewater 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 normaland back up systems to remove the steam and cool it and return it to thereactor, because there was no power. So they were probably trying to vent thissteam into the buildings at the plant. If they could vent a little bit ofsteam, add a little bit of cool water, they could keep the reactor cool enoughto keep it from melting down.
Q: I see, and I guess the big question that everyone hastoday is, has the explosion or any of the damage, I guess there hasn’t been alot of damage to the plant, it’s just overheating, do you think any of this iscausing nuclear leakage and if so, is that a big problem?
A: So, I‘ve actually looked at the before and after picture from the explosionthat’s available on the news and, in my opinion, they have an extremely serioussituation at this nuclear power plant. So, my speculation is they were ventingthe steam in order to try and cool the reactor, unfortunately, without powerthey 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 bareminimum of instrumentation provided by whatever battery power they have left.My guess is, and it was reported in the news that they had a hydrogenexplosion, 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 entirebuilding surrounding the reactor, the only thing left of it is the steel frame,the whole building has collapsed. That would normally be called the auxiliarybuilding, and that building actually does house a lot of the emergency systemsfor the reactor. So I think we have a very very serious situation at this powerplant where the entire auxiliary building has been destroyed.
According to reports, the containment is intact, so if there has beenany release of radioactivity, it has been very minor, to this point, but theyhave got to find a way to get some electricity, and cool that reactor. And thelast report I saw said that there plan was to use seawater. So obviously, they’regoing to get some temporary pumps, they’re going to use seawater, mixed withboron. Boron is a substance that will absorb neutrons, very similar to boraxthat you could go buy to wash your clothes with, that will keep the reactorfrom going critical again when they add the cold seawater. Even though thecontrol rods have been fully inserted, when you add cold water, cold water isdenser than warm water, and it can cause the neutrons that are still bouncingaround the reactor to moderate, to a speed at which, (so moderate means slowdown), they could strike the fuel and cause a fission.
We obviously don’t want any more fission because that generates moreheat and we certainly don’t want the reactor to go critical because thatgenerates a lot of heat. And, critical is not the bad word that you see in thenews, where you say “Oh, reactor’s going critical!”; when it operates, it’snormally 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 aterminology 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 thepotential of causing the fission level to go up in the reactor and more heat tobe generated, which you don’t want to do. This is beyond the last resort, to dothis, 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 forthis plant, and added to that question, what is the worst case scenario?
A: I think the best case is that the military get the generatorson-site with some emergency pumps and they’re able to rig up a cooling systemto cool that reactor, to keep it cool, and they’ll have to cool it for severaldays before it gets to the point where the heat is decayed off. Obviously theplant is destroyed, and I’m sure it will have to be decommissioned. Thequestion is how much additional damage is there at the site, because, there’sactually six nuclear reactors at that same site and two more that were plannedor 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 withthe other ones.
A: The question is, as a result of this explosion – has any damage occurred inany of the other, adjacent, reactors and also what is the situation of theadditional reactors?
Q: Right, if they don’t cool them, it seems like thissame thing could happen to them.
A: They would have the same problem, so a couple of the plants were shut downfor maintenance so they’re probably less problematic because their cores wouldhave cooled down but the ones that were operating at the time that theearthquake occurred, could all be a concern.
Q: So I guess a final question I have for you is, do youthink nuclear power plants should be built in an earthquake prone area such as
A: I think it’s important for the nuclear industry, to be unemotional aboutwhat has happened here. So, like I said, it does appear that all of the designfeatures that were required for the earthquake, functioned, and the plant wasgoing through a normal shutdown sequence. Obviously, when the tsunami came,that was something that was not designed for, because it flooded the locationwhere 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 thatwere designed for that plant.
So, I think the nuclear industry has to take a serious look at what hasoccurred in Japan
Clearly, in this case, this was not taken into account and the netresult is, we have a nuclear plant that appears to be very very close to a coremeltdown.
Q: And what would a core meltdown lead to? I mean, isthis going to be contained? Is there any chance that this is going to be like a
A:So, you ask a good question, and probably one that is on the mind of thepublic. So, the first thing is, this is a different scenario from the one thathappened at
In this boiling water reactor, even though, it’s not good that the corewould not be cooled, as the water level drops, and you generate steam, thesteam is less dense than the water, so that means that there’s less moleculesof water to moderate or slow down the neutrons. So, when a steam void forms, itactually causes the power level to drop in that vicinity, or the heatgeneration to drop in that vicinity. The problem you have, of course, isyou do need to cool the reactor because you have all this residual heat, but apressurized or boiling water reactor is inherently stable, as opposed to the Chernobyl
The other big difference is, all Western reactors have to have acontainment building. And so, according to the news reports, although theauxiliary building has been destroyed, the containment, or steel liner, has notbeen destroyed. So that’s still intact, so in theory, as long as they canmaintain the pressures in that, and there should be relief valves on that, tomaintain the pressure, even if the core was to melt, the vast majority of theradioactivity should be contained within that containment building. At Chernobyl
Q: Well, we’ll just keep our fingers crossed and I hopethat there are a lot of nuclear engineers and military people really workinghard 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, theone good thing is that Japan has many many nuclear power plants and they have alot of nuclear experts in that country, so in addition to the help andexpertise that they can get from the US and other folks that have a lot ofnuclear experience they have a lot of their own people who have a lot ofexpertise. And I’m sure that they’re doing everything they can, but again, I dohave to emphasize that I think this is an extremely serious situation.
Q: OK, thank you so much, Dad, for all of yourinsights. I’m so glad I have a nuclear engineer in my family.
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