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:
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):
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.
On to Greenpeace (pg. 17):
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.
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).
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:
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.
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.
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.
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 Â§ 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.
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