Friday, June 23, 2006

Greenpeace and the Reality of Reactor Safety

Back in April when Chernobyl's 20th anniversary was approaching, Greenpeace published a report titled An American Chernobyl: Nuclear "Near Misses" at U.S. Reactors Since 1986. In Greenpeace's report, they claim that an American Chernobyl could happen due to a "nuclear reactor meltdown and the subsequent failure of containment."

On the 20th anniversary of the Chernobyl disaster, the worst commercial nuclear power accident in history, Greenpeace has documented nearly 200 "near misses" at U.S. nuclear reactors since 1986.
I'm not sure how Greenpeace defines "near miss" but the NRC uses the terms "significant", "important" and "precursor" when categorizing events.

This categorization is based on event probabilities. To give you a general background, the Nuclear Regulatory Commission and the nuclear industry measure and run reactors based on probabilities of the risks for core damage.

As a reactor runs, events occur which have some risk. For instance, if I need to replace a pipe or valve, some risk exists in replacing it depending on the type of pipe or valve that I'm working on. If my action has little impact on the overall operations of the reactor than little risk is involved.

Risk always exists when operating nuclear reactors. After decades of experience an appropriate and accepted level of risk for both the regulator and the industry is 1E-6 reactor years or a one in a million reactor years risk of core damage.

A reactor year is one year a reactor operates. If you have 10 reactors which each have run one year then you have 10 reactor years. If you have 100 reactor years who each have run 10 years than you have 1,000 reactor years of operation. (For ease of calculation, 7,000 critical hours per year are assumed for a reactor year.)

Here's what the NRC says:

For assessing public safety and developing regulations for nuclear reactors and materials, the NRC traditionally used a deterministic approach that asked "What can go wrong?" and "What are the consequences?" Now, new information for assessing risks also allows NRC to ask "How likely is it that something will go wrong?"

New techniques for measuring, analyzing, and ranking public health risks make it possible for the NRC to incorporate risk insights into its regulations. By risk-informing its regulations and regulatory processes, NRC can focus the attention of its licensees on those design and operational issues most important to safety and move away from prescriptive regulations based on conservative engineering judgments toward regulations focused on issues that significantly contribute to safety.

To give you more of an idea, check out this link to a NRC SECY (letter to the commissioners) on this issue (pages 2 & 3):

The Accident Sequence Precursor Program systematically evaluates U.S. nuclear power plant operating experience to identify, document, and rank the operating events that were most likely to have led to inadequate core cooling and severe core damage (precursors), accounting for the likelihood of additional failures.

(snip)

The objective of the Standardized Plant Analysis Risk Model Development Program is to develop standardized risk analysis models and tools that staff analysts use in many regulatory activities, including the ASP Program and Phase 3 of the Significance Determination Process (SDP).

On to Greenpeace (pg. 17):
According to the NRC, accident precursors with a Conditional Core Damage Probability or CCDP or CDP of 1 in 1000 are considered significant, accident precursors with a CCDP of 1 in 10,000 are considered important and those with a CCDP of greater than 1 in a Million are consider precursors.
Other than an incorrect tense at the end of this statement this should give you an idea of the probability categories of running a reactor.
Of the nearly 200 "near misses" to a meltdown cited in US Nuclear Regulatory Commission (NRC) documents, eight "near misses" are considered the most significant. This means that according to the NRC, the risk of a core meltdown is greater than one in 1,000.
Of the 8 significant near misses Greenpeace documents, the greatest risk since 1986 was the Davis Besse vessel head degradation event. This event had a risk of 6E-3 reactor years of potential core damage. In other words it was .6% reactor years away from core damage.

Does less than one percent chance of core damage sound risky? To the nuclear industry and its regulator, it's huge. Because it's huge to us it should give the reader an idea of how conservatively we operate these plants.

As well, the low probability of core damage from the Davis Besse event should tell the reader that many more events needed to happen in order for the reactor core to become damaged.

Why is it huge to us? Well, if we have 100 reactors operating for 10 years and every one of them has a vessel head degradation event, than the probability says that 6 of them will have core damage as a result.

Here's a little snip from our fact sheet on Safety Benefits of Risk Assessment:

The many nuclear plant improvements and new regulations based on risk assessment have dramatically improved nuclear power plant safety. Industry wide, the likelihood of a reactor core-damaging accident declined from an already low level of one in 12,000 per year in the early 1990s to one in 40,000 per year in 2000.

(snip)

Combining the known information on radiation heath effects with the results of risk analysis, the risk of death for a resident near a nuclear plant is less than one-in-a-million per year. To put this in perspective, this is 25 times lower than the risk of being killed by a lightning strike and 12,000 times lower than the risk of dying in a car accident. Other forms of energy production, such as hydroelectric or fossil-fueled power stations, result in risk impacts higher than those of nuclear plants.

I did a random Google search for other risks and found a nice table halfway down this page. According to the link, this year you have a 1 in 100 chance of your car being stolen, 1 in 200 chance of your house catching fire, 1 in 500 chance of dying from cancer and a 1 in a million chance of winning the state lottery.

Now that we have covered risks and probabilities, lets go to containment structures and Greenpeace:
If any of these "near misses" had progressed to a meltdown, the government regulators have little confidence that any of the nuclear reactor containments would survive. In fact, some containment designs used in General Electric and Westinghouse reactors are virtually certain to fail after a meltdown of the radioactive fuel.
According to 10 CFR Part 50 Appendix J, reactors before operation and during are required to test and demonstrate they can withstand the pressures and contain the leaks which may occur during operation of the plant.
One of the conditions of all operating licenses for water-cooled power reactors as specified in § 50.54(o) is that primary reactor containments shall meet the containment leakage test requirements set forth in this appendix. These test requirements provide for preoperational and periodic verification by tests of the leak-tight integrity of the primary reactor containment, and systems and components which penetrate containment of water-cooled power reactors, and establish the acceptance criteria for such tests. The purposes of the tests are to assure that (a) leakage through the primary reactor containment and systems and components penetrating primary containment shall not exceed allowable leakage rate values as specified in the technical specifications or associated bases and (b)periodic surveillance of reactor containment penetrations and isolation valves is performed so that proper maintenance and repairs are made during the service life of the containment, and systems and components penetrating primary containment.
While Greenpeace can pull all the references they want, it appears they are not aware of the federally mandated requirements for the containment structures. These structures are tested before the reactors can become operational and they are tested every 10 years thereafter.

I hope I've shed some light to the reader that nuclear operations and safety go much further than a few claims and spins. With over 3,000 reactor years of operation in the U.S. and more than 12,000 worldwide, the nuclear industry has gained a tremendous amount of experience and is getting better every day.

Technorati tags: , , , , , , , ,

5 comments:

Sinus said...

Why don't you take this to court?
Sue them for libel and bankrupt them.

gillo said...

Frankly, as normal citizens, and not as scientists or experts, do we really care about what exactly "risk" is? If just one of those cases went wrong, the consequences will be terrible anyway.

KenG said...

As normal citizens, we have to consider risk every day whether we realize it or not. The decision to drive a car can have terrible personal consequences for us (and does for about 40,000 people in the US each year).

When we decide not to build a nuclear unit we decide it is acceptable to burn more coal/oil/gas or to limit the availability of electricity. All those things have real risks and consequences.

Deciding not to build nuclear plants (in spite of the clear environmental and public health advantages) because of fear of the "big accident" is just an extension of the logic that causes some people to drive cars instead of taking an airline flight because of the difference in the number of people killed in a single accident.

Paul Primavera said...

Yet, Keng, in spite of the truth you point out, the same anti-nukes who decry nuclear energy as too risky are quite willing to court far greater risk to their personal lives in driving a car on a busy highway. Not even Chernobyl actually killed 40,000; yet from what you pointed out, automobile use in the U.S. does so every year.

The Precautionary Principle which is the cornerstone of this philosophy is simply illogical.

BTW, I agree completely with Sinus. If anti-nukes are openly propagandizing what is blatantly untrue, then the industry should sue them just as they sue the industry for non-real hazards. If the anti-nukes money were all tied up in the court system trying to justify their disinformation, then they would have no time and no money to spread their campaign of fear and hysteria. I don't understand why what they do to the industry cannot be done to them. If they lie, then sue them - period.

neutrino said...

I am a supporter of nuclear power and its continued development but some of the stuff in this post is just ridiculous - suing greenpeace, they're lying, etc. Read the original quote - core melt & subsequent containment failure. Let's see, how does core melt occur? The license is based on analysis proving no core melting during the design basis events. And these events credit operation of the engineered safeguards systems. Core melt, then, implies a beyond design basis event, or one in which the redundant ESF systems fail. With respect to the App J containment testing, it is done at the containment design pressure, where this pressure is defined by the design basis events discussed above (ie, with only a single failure in the ESF systems). Since the mid 1960s the NRC / AEC has licensed plants where the containment design credits operation of the ESF systems. This, I think, is the Greenpeace beef - the containments are not designed for the core melt scenarios. Whether the containment structures are capable of surviving these events is another question (yes, they are). But I don't think the statement is a lie. And calling people liars is not the way to establish the truth, or to promote a valuable technology. Let's all be honest and keep the facts straight.