Why Fukushima Isn’t Like Chernobyl

Why Fukushima Isn’t Like Chernobyl

By The Diplomat

March 29, 2011

Despite media hype about the radiation dangers, the Fukushima nuclear crisis won't end like Chernobyl, Alexander Sich tells The Diplomat.

Photo Credit: Flickr / Daveeza

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Is the kind of massive radiation release that occurred with Chernobyl possible at the Fukushima plant?

No, it can’t have that kind of massive release. It simply can’t do that. The question is to what extent the zirconium alloy, which clads the fuel pellets, is damaged in the core, and how much of the fuel has failed.

And I don’t necessarily mean melted, I mean failed. There’s been an ambiguous use of the word ‘melting’ applied to the core. But when people talk about meltdown, they should be very specific about what they mean by the word.

At Fukushima, there are four primary barriers to releases: the fuel zircalloy cladding, a pressure vessel, an inner containment structure, and a confinement building. To a large extent, the core material seems to be contained.

Apart from, of course—and this is where the speculation runs wild—there’s the question of the source of the radiation they’re detecting in certain areas where water has accumulated. Indications today are that it isn’t the cores. They’ve been dumping or spraying tremendous amounts of water onto and into the damaged buildings, so surely someone is considering this water as a possible source.

But until they go in and see, we have little more than speculation to go on, because they don’t know to what extent—if any—the cores are damaged, and they don’t know to what extent the pressure vessels are damaged, although that’s unlikely.

They also don’t know to what extent the pipes are damaged, and they don’t know to what extent the lower portion of the containment building is damaged. So, on the one hand, I can’t speculate on what is going on inside. But even so, and given what nuclear engineers know in terms of the plant layout, it’s just not true that it’s a Chernobyl situation.

So, you’d say it was unfair to draw parallels between Fukushima and Chernobyl?

They are very, very different and it’s very unfair to draw that parallel. There are two parts to this. One is the myths that currently surround Chernobyl. The other is the sheer difference between the incidents—the causes of the accidents and the structural, engineering and physics differences.

For a start, there are three or four primary and important differences between the two reactor designs. The first is the difference between the Western Light Water Reactors—pressurized water reactors and (like those at Fukushima) boiling water reactors—and the plant at Chernobyl.

Western Light Water Reactors are water cooled and water moderated. The first one is simple—water is used to cool the fuel, to take away the heat, to eventually create steam and then after that make electricity. It’s the water moderation that’s the very important difference.

What moderation means is that the water is used to slow down neutrons in the core and make them accessible for the reaction to take place. In the Chernobyl type reactor, water is a coolant, but it’s not a moderator—the moderator is graphite, and that points to one important design and structural difference.

In the Light Water Reactor core, apart from the fuel itself, it’s virtually all metal. You have the fuel contained in a special kind of zirconium alloy, there’s the stainless steel vessel, and the super structure is metal.

In the Boiling Water Reactor (BWR) that you find at Fukushima, you have a reactor pressure vessel that’s approximately six inches thick steel—it’s basically a big kettle that contains the core. In the Chernobyl reactor, there was no pressure vessel.

So right there, there are two very big differences—the BWR is contained in a very robust pressure vessel, the Chernobyl reactor was not. The BWR reactor is a singular metallic vessel, while the Chernobyl reactor is approximately 1700 individual pressure tubespiercing about 2,000 tonnes of graphite. Western LWRs contain essentially no graphite. Those are very big differences.

The third very big difference is that all Light Water Reactors have some sort of containment structure or containment building. The Chernobyl type reactors, of which there are 11 still operating, have no containment building. The final difference between them is basically an operational one.

The BWR is controlled by both control rods and coolant jets that form a ring around the reactor—inside the reactor pressure vessel but around the core—and by basically turning up or slowing down the speed of those pressure pumps you can adjust the power. It’s a very nice way of doing it.

And also the size of the BWR core is about 2.5 metres in diameter and about 3.7 metres in height. The Chernobyl reactor, in contrast, is 11.8 metres in diameter and 7 metres in height. It’s a very big reactor, and that’s what I mean by the operational differences.

The Chernobyl reactor is a ‘decoupled’ core, which means one side of the core doesn’t always ‘know’ what the other side is doing neutronically. What that means is that the reactor operators need to keep a very close eye on what’s going on in the Chernobyl type reactors. That isn’t the case for Western reactors. In those, one side of the core ‘knows’ fairly well what the other side is doing and it naturally adjusts.

This leads to the final point. In a Western Light Water Reactor, there’s something called a negative feedback principle at work. If a Light Water Reactor heats up for whatever reason, the reaction actually slows down—it’s worse for the reaction if the reactor gets too hot.

In the Chernobyl reactor, it works the opposite way in certain power ranges. In other words, the hotter the reactor gets, the more you boil water. The more you boil water, the more you introduce steam voids, the more you introduce steam voids, the faster the reaction goes.

This has been largely addressed since the Chernobyl accident by the Russians. Still, it doesn’t take a rocket scientist to understand that you don’t want such a situation because of the positive feedback. I’m not saying it happens all the time, but in certain power ranges it’s true. This isn’t possible in a Light Water Reactor.

So all those things together are a rough summary of how different they are in terms of structure. That leads to the second thing I mentioned, which are the causes of the accidents. At Fukushima, the earthquake didn’t cause the accident, but caused the reactors to automatically shutdown. The systems worked—they went subcritical.

The problem was that the ensuing tsunami devastated some equipment on the outside of the plant that was supposed to ensure continued core cooling. Unfortunately, that equipment got damaged and that’s why in the past week the Japanese have been pulling power lines to the reactors and restarting those things.

At Chernobyl, what caused the accident was 1) what I just mentioned before, the kind of inherent flaws in the reactor design and 2) external production pressures to conduct what was ironically a safety experiment. The accident occurred on April 26, 1986, just a few days before May Day, which in the former Soviet Union was a very big national holiday.

And they were being pressured to produce as much electricity as possible so they could get their bonuses, and also this safety experiment so they could get their bonuses. In fact, the dispatcher in Kiev, about a day before the accident, ordered the reactor crew not to go down on power as they needed another nine hours of production.

So there was this external stuff going on. And the final thing that caused the accident was the errors on the part of the operators—they intentionally overrode safety precautions, safety barriers and interlocks. They did that because they were pressured to do this experiment. All this means that the two accidents are vastly different.

At Chernobyl you had a massive, massive release of radioactivity. While we still don’t have the numbers for Fukushima, I would compare it maybe to a matchstick and a stick of dynamite. It’s a crude analogy, but it gives you some insight.

You mentioned some myths surrounding Chernobyl. How have these impacted the view of events surrounding Fukushima?

Unfortunately, certain interested parties have been employing sensationalist rhetoric, inaccuracies, and outright hoaxes regarding Fukushima. I refuse to dignify with comments SMS hoaxes in the Philippines as well as a map, purportedly from Australia, predicting lethal dose rates affecting the western coast of the United States. Nonsense—that’s all these are.

But then there’s Congressman Ed Markey of Massachusetts who warned of ‘another Chernobyl’ and predicted ‘the same thing could happen here (the United States),’ and then proceeded to call for an immediate suspension of licensing procedures for a new generation safer reactor design. I find that repugnant.

Michio Kaku, a theoretical physicist from the City College of New York who studies string theory, is also out of his depth when it comes to nuclear reactors. He’s not a nuclear engineer, and yet that hasn’t stopped him making borderline hysterical statements during interviews. Kaku claimed, for example, that a ‘China Syndrome’ was possible, that the ‘(Chernobyl) vessel and roof blew out simultaneously’—factually incorrect on both counts: Chernobyl-type RBMK reactors have no reactor pressure vessel.

Also, without providing a shred of evidence, Kaku asserted ‘We’re still seeing people dying of that (Chernobyl) reactor accident.’ He’s no doctor nor health physicist. Kaku also claimed the situation ‘had gone from bad to worse…the reactor is in free fall, and you have three simultaneous meltdowns, and a raging spent fuel pond that could explode.’

Most troubling was Kaku’s careless recommendation, ‘If I had the ear of the Japanese prime minister I would recommend the Chernobyl option (dumping materials from helicopters).’ In fact, dropping tons of materials from helicopters high in the air onto debris and inner reactor building structures might well compromise the integrity of structures designed to contain releases in the first place.

Partly what drove this view is the well-known video footage of helicopters flying over Chernobyl dropping material on the core. But the fact is that they never hit the core. What they were bombing was something that was burning off to the side of the reactor.

The second myth about Chernobyl is that the sarcophagus—what they built on top of the reactor—is some kind of monolithic concrete structure that has recently cracked and is releasing radiation. This isn’t true. It’s not a monolithic structure; it was more like a steel tent. This myth about concrete is one of the most pernicious ones about Chernobyl.

But I worked for a team on site that is building a new sarcophagus so that the clean-up and decommissioning efforts of the Soviet-built sarcophagus can take place with a much-reduced risk of spreading contamination. They are taking the old one down because it was built on debris, and so no one knows what the actual robustness of that structure.

What have you made of the media coverage of the situation in Japan?

I think the press needs to be very careful about which talking heads they choose to comment on the accident. Unfortunately, the press doesn’t usually understand the big difference between a theoretical physicist and a nuclear engineer.

But the difference is roughly like comparing a general practitioner to a brain surgeon—a brain surgeon needs to know the brain inside out, while a general practitioner can only make general comments about the brain. So, theoretical physicists can make general comments, but can hardly comment on the specifics of important nuclear reactor design details.

I would therefore urge the media to identify BWR specialists who understand the Three Mile Island incident inside out, and who understand very well the Chernobyl accident in Ukraine and the Windscale accident in Britain so that sober and serious and correct information can be provided to the public.

The second thing I would urge is some caution and not to criticize the Japanese. I’m not saying the media itself is doing this, but it certainly tends to permit ‘air time’ a kind of sensationalist criticism of the Japanese, and I’m not sure they deserve it.

The Japanese are doing the best they can in a very difficult situation. I’m not saying they are spot-on perfect with everything they do. But, I wouldn’t criticize. I would try to help them, I would bring in specialists, I would do what is possible, and of course ask the Japanese to provide as much information—including verifiable numbers—as they can.

I’ve been keeping track of the media, and in the early days, up to a week after the accident, sensationalist headlines abounded. But then you would look through the actual articles trying to look for numbers, and few if any were provided.

For example, you find today’s story about how ‘levels of iodine have been detected in Massachusetts’. But you have to provide a number—provide a comparison before saying something. Radioactive iodine and other contaminants were released in far greater quantities in Windscale in England, for example, or Chernobyl, and certainly by the atmospheric testing of nuclear weapons.

If you’re talking trace amounts, then you’re talking trace amounts. After all, our bodies are radioactive. You get a dose of radioactivity when you sleep next to your spouse. Are we talking about those kinds of levels or something else? So I would like to see some circumspection. Of course, never stop asking questions. But ask for numbers.

Alexander Sich is an associate professor of physics at Franciscan University of Steubenville in Ohio. Sich was the first American researcher to investigate the Chernobyl reactor meltdown on site. The views expressed are his own.

Photo Credit: Flickr / Daveeza

Source: the-diplomat.com

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