Monday, March 28, 2011

Japan and Probabilistic Risk Assessment

The Situation in Japan

The Fukushima reactors have power, but they are still struggling. Yesterday, Areva confirmed that TEPCO had just asked them for help. There is no announced change in the status of the reactors. However, reports of the spread of radiation are not looking good at all. I think Capacity Factor, once again, has the best analysis so far. He concentrates on the cesium fall-out, and does not agree with the optimistic assessment from BBC.

There is no doubt that the Japanese situation is serious. The question for people in Vermont about the safety of Vermont Yankee, however, is not so much about the consequences of a huge problem, but whether our safety precautions will avoid such problems. I think they will and I have reasons to think so. Among other things, we assess risk in a different way.

How the Nuclear Industry Assesses Risk

This morning, the New York Times had an article Japanese Rules For Nuclear Plants Depended on Old Science. It turns out that when the United States went to Probabilistic Risk Assessment (PRA) for nuclear plant evaluation, the Japanese did not. (The original plant designs rarely included PRA, because it was too new at the time. Re-evaluations and uprate licensing in the United States included it.)

Because the Japanese did not use this method, The Japanese estimates of tsunami damage and their defenses against it were inadequate. Here's a quote from the article about PRA:

"The Japanese fell behind," Mr. Hardy said. "Once they made the proclamation that this was the maximum earthquake, they had a hard time re-evaluating that as new data came in."

The Japanese approach, referred to in the field as "deterministic" -- as opposed to "probabilistic," or taking unknowns into account -- somehow stuck, said Noboru Nakao,

According to the article, the NRC uses "some" aspects of PRA in evaluating plants, but could use more. EPRI did considerable research in this area (though I was not one of the researchers) and I am sure that this research is available to the NRC.

If you don't use PRA, you are likely to not assess the situation as far out on the "improbable accident scenario" spectrum as you will if you use PRA. "Deterministic risk assessment" sounds safer, but it is not.

An ounce of prevention is worth a pound of cure. We developed PRA, we use it, and we should probably use it more. It is clear to me that risk analysis in this country has been far more thorough than the analysis in Japan, and our plants are far safer.

Where was the Japanese NRC Equivalent When These Decisions Were Made?

I am going out on a limb here. I have been watching some Teaching Company videos on economics. One of the points made in the course is that the Japanese economic miracle of the 70s and 80s was due to close cooperation of industry and government. It seems to me that the entire situation of government-industry cooperation might have become too close and too cozy.

Nuclear opponents consider that the NRC is too cozy with the nuclear industry. From within the industry, however, the NRC looks much different. It looks adversarial.

Inspectors are rotated in and out of plants so they won't get friendly with the workers. Reviews of new types of plants take years, maybe decades. The NRC is truly a watchdog. This is typical of all regulation in the U.S. Regulators in the United States generally take an adversarial view of their industry. This adds to the safety (and the cost) of most industries in the United States.

The NRC has been doing a good job. I know that nuclear opponents will have a field day with this statement, but this is my opinion, and everything I read confirms it.

Graphic is The Great Wave from 36 Views of Mount Fuji by Hokusai


Charles Bell said...

Good morning Meredith. Very informative blog. I agree with you about NRC being true watchdog based on my own first hand experience with them over 28 years. I came to respect and trust their observations and decisions. They are in fact the best independent set of eyes watching the plants in the U.S. The NRC came a long way from the days of the 1980s when you might find that an inspector had been a former refrigerator repair person. They hire top notch people and treat their employees well. They no longer tolerate the antics of examiners like John Monroe, who was notorious for pulling tricks on operator candidates in the simulator to catch them and flunk them. The Generic Fundamentals exams replaced all the old theory exams which were notorious for having eccentric questions that had no generic written basis. The operating tests improved to watch for critical mistakes and decision making, communication and team work skills.

Tom Murphy said...

"The question for people in Vermont about the safety of Vermont Yankee, however, is not so much about the consequences of a huge problem, but whether our safety precautions will avoid such problems."

I would state the question differently, "Are the risk and consequences of an accident outweighed by the benefit?"

The consequences of an airplane crash are death for those on the plane and the risk is low. Since the risk is low and only local, most people do determine that the benefits outweigh the risks and consequences.

The consequences of a nuclear accident where containment is breached is contamination of the area surrounding the plant for many years. So even though the risk of that type of a nuclear accident is very small, it needs to be virtually eliminated, so that the general public believes that the benefits outweigh the HUGE potential consequences.

Nuclear reactors need to be made "walk away" safe - not just passively safe, not just inherently safety, not just tamper-proof safe.

A design basis of no AC power available, no DC power available, no cooling water available, no operators available.

Howard Shaffer said...

PRA (Probabilistic Risk Assessment)now called PSA (Probablistic Safety Assessment) is a fancy way of saying "calculating the odds."

Using PSA a very severe but very rare- big odds against- event would be calcualted. A judgement can then be made about investigating what is needed to cope with it, and if it is worth it.

We did this in the '70's after the first development of PRA in WASH 1400, known as the Rasmussen Report, after the Professor of the same name at MIT. I was there as a grad student at the time and heard many presentations on it. PRA/PSA looks at every piece in the plant and every interconnection that affects safety. It then assigns the odds of failure. The totals for the plant are calculated. Big computers are needed for the massive amount of data, so PRA/PSA wasn't even possible until computing power developed.

PSA looked at Station Blackout, Core meltdown etc. For meltdown US plants have a "hardened vent" to allow venting under favorable weather conditions. An evacuation will have taken place days before. Note when the first evacuation in Japan took place - March 11- the day of the tsunami.

After the TMI-2 accident Prof. Rasmussen checked his work. It showed that the odds said an accident like that - partial core melt, stopped, with no significant release-should already have happened!! This is a good benchmark for the validity of his work. PSA is being used in other technologies, I believe.

Howard Shaffer said...

Mr. Murphy is asking for almost perfection. Has he considered a plane headed for Keene airport with a dangerous cargo that crashes into Brattleboro High School? There could be deaths and contaminated land.

Plant blackout from loss of AC power has been included in the design of VY and other plants now. After 9-11, long term balckout was looked at and action taken. I don't know what it is since post 9-11 changes are "security information." I can guess its trucked in/helicopterd in diesels.

The events in Japan point out that blackout at VY and core damage with containment venting aren't likely to happen in vacuum. A river flood is almost geographically impossible, but an F-5 tornado that tears up all the power lines, blocks the roads for miles around and blacks out the VY plant is not.

There is a type of reactor that is "walk away safe" and tested to be. It was developed and prototyped in Germany, continued as a project in South Africa but dropped for financial reasons. Now China has just broken ground for the first production model. Look up PBMR.

Tom Murphy said...

"Mr. Murphy is asking for almost perfection. Has he considered a plane headed for Keene airport with a dangerous cargo that crashes into Brattleboro High School? "

I am sure we can list many activities that have similar (or higher) risks and consequences as the release of fission products into the environment, but what about the benefits of those activities? There are two sides to the equation. Also the airplane into Keene is probably the fastest mode of transportation where as there are other ways to generate electricity. [Please, before you goo there I am aware that other methods of power generation have their own risks and consequences.]

Furthermore, I am not the one who needs convincing, it is the General Public needs convincing so there is an educational aspect to this as well that would need to overcome some strong human instincts.

Anonymous said...

Folks, now the water is so hot around the Fukushima reactor it can kill the workers in a matter of minutes, their dosimeters can't even detect the level properly as their badges only detect up to 1000 millisieverts per hour (5000 millisieverts total exposure is the fatal dose). Plutonium detected finally in Fukushima soil, means it must be floating up and around in those radioactive clouds encircling the northern hemisphere as the smoke rises and the fuel pools burn and the fuel rods degenerate and melt down, and the radioactive water leaks into the trenches of water around the reactors that is leaking into the ocean...

You should have no doubt that what is happening in Japan will one day happen in America. It's just a matter of time.

Anonymous said...

By Julian Ryall, in Tokyo 3:54PM BST 30 Mar 2011
Hospitals and temporary refuges are demanding that evacuees provide them with certificates confirming that they have not been exposed to radiation before they are admitted.

The situation at the plant remains critical, with the Nuclear and Industrial Safety Agency yesterday that radioactive iodine-131 at more than 3,350 times permitted levels has been found in a sample of seawater taken from near the facility.

The water is the most highly contaminated sample taken from the sea and indicates that radiation from the core of one or more of the reactors, where fuel rods have partly melted, is leaking into the Pacific Ocean.

Meredith Angwin said...

Anonymous commentators

You might look at the IEAE site about worker injuries:

And you might look at this about changes in nuclear plants in this country. Somewhat too optimistic in my opinion, but a good antidote to frantic "It Will Happen Here" declarations.

In my opinion, we need more than "it WILL happen here" or "it CAN"t happen here." Such unequivocal powers of prediction!