Nuclear Reactor Safety

An important theme in Jared Diamond’s Collapse is how often people won’t change behavior that gives status; the Easter Island example is cutting down trees for statues. There are many examples today, such as how and how often we drive and fly.

Another way we can get into trouble is by not changing our views with changing realities. Union of Concerned Scientists has just produced a report on the dangers of nuclear power. The report is new, but the data are mostly old, except for the Davis-Besse plant.

From David Bodansky’s Nuclear Energy (Second Edition), chapter 14:

The historical record of nuclear reactor performance can be interpreted as showing that they are very safe or that they are very dangerous. The former conclusion follows if one limits consideration to plants outside the former Soviet Union (FSU). The latter conclusion follows if one focuses on the Chernobyl accident and takes it as a broadly applicable indicator.

As of 2003, commercial reactors outside the FSU have a cumulative operating experience of more than 10,000 reactor years (by now it’s 11,000 reactor years). No one has died from an accident from radiation exposure, neither worker nor public. [Two people died in a Japanese reprocessing accident.] Military reactors in the West have had problems: in 1961, three army technicians died.

No reactor has a zero chance of accident. Accidents range from release of radionuclides (Chernobyl) to incredibly expensive damage to the reactor core (Three Mile Island, TMI) to near misses to harmless breakdowns. The latter two categories can be expensive if remedial measures and lost time add up.

Potential major accidents are of two types:

Criticality accidents, where the chain reaction becomes uncontrolled. In light water reactors, negative feedbacks (feedbacks which work in opposition to whatever is happening) make this event improbable.

Loss-of-coolant accidents, where the reactor becomes so hot that melting of the fuel cladding and the fuel can occur. Radioactive materials could possibly escape from the reactor vessel and perhaps the outer reactor containment. The TMI accident resulted in substantial core melt, but no large loss of radioactivity from containment.

Criticality accidents are essentially impossible in Western reactors, but loss of coolant remains a concern.

Achieving Reactor Safety—Changes in Design

• Emphasize passive safety systems even more. Active systems depend on proper responses from pumps or valves, for example, while a passive system might depend on gravity or that warm metal expands.

• Improve redundancy, either more of identical units or more than one way to respond in an emergency.

Multiple barriers or defense-in-depth to prevent radionuclide release: zircaloy cladding of fuel, pressure vessel and closed primary cooling loop, and heavy outer reactor containment. The reactor containment was successful for TMI, but no containment system existed in the Chernobyl reactor.

• [One academic told me that current design assumes a malicious operator, because there is no essential difference between a malicious operator and one who puts paper over warning lights. This is a subset of passive systems changes.]

• Do probabilistic risk assessments on each design. While this does not provide an accurate risk assessment, they suggest areas of relatively high risk.

Post-TMI Safety Developments

The accident at Three Mile Island showed that efforts by government and industry up to 1979 were inadequate. The nuclear reactors themselves, operator training, and inter-utility communication needed improvement.

One consequence of improving reactor design, a very expensive delay post-1979 of new construction and retrofit of old reactors, was a general improvement in capacity (90% today for nuclear power plants, 73% for coal); both accidents and refueling take time. Output dropped between 1979 and 1982, began to rise substantially in the mid-1980s, and reached a high in 2002, six years after the last American nuclear power plant went into operation.

The Institute of Nuclear Power Operations was established to improve communication. The Nuclear Regulatory Commission (NRC) intensified its watchdog role.

Their data show a decrease in unplanned scrams, automatic shutdowns of a reactor following failure. The rates were 7.3 per 7,000 hours of reactor operation in 1980, 1.2 in 1990, and under 0.1 in 2001. One result was the improved capacity factor, referred to earlier.

The rate of industrial accidents (this has nothing to do with radiation) per 200,000 worker-hours in 2001 was 0.24 for nuclear reactors, compared to 4.0 for U.S. manufacturing as a whole. [The accident rate is likely higher/worker-hour for the coal industry. Coal requires many more workers to supply the same amount of energy, a coal train every day versus a truckload of fuel every year or two, as 100,000+ times as much fuel is required. So no matter how coal compares per worker-hour, both worker safety and transportation accidents for coal compare unfavorably per kWh.]

There are still problems, in spite of all these improvements. In early 2002, deep corrosion was discovered during refueling in the Davis-Besse reactor vessel head. Boric acid had leaked through cracks in nozzles, producing a cavity 4 in by 5 in, to a depth of 6 in, large enough that it might have been discovered earlier if workers had been sufficiently vigilant. Inspections at other similar reactors produced no evidence of similar leaks.

How serious was the leak? There was no radioactivity release, and no damage except at the location. The NRC analysis was not complete when the book was published, Perhaps there was no likelihood of core damage, even if the corrosion penetrated the wall. While the safety systems appear to have been intact, and would likely have worked in spite of the corrosion,

the failure to detect and correct the corrosion promptly showed serious weaknesses in the monitoring procedures of the reactor operator and the NRC. This single event does not negate the very good and improving record of nuclear reactor performance, but should serve as a reminder against complacency.

The cost to Davis-Besse was hundreds of millions in repair costs, tens of millions in fines, and two former workers, and one former contractor, indicted for providing false information.

If I get a chance, you’ll see a writeup on safety in the coal industry.

Union of Concerned Scientists is also hosting a cartoon survey, choose your favorite cartoon on the attacks on science.

One Response to “Nuclear Reactor Safety”

  1. Sam says:

    Excellent summary of safety in the nuclear industry. However , with rising prices of coal, oil, natural gas and other fossil resources, nuclear energy is seen as an attractive option by many countries. Is it possible to increase safety by mandatory government regulations that allocate capital expenditure on safety systems every year, to continuously make them better and better?
    Also any ideas about the safety of nuclear reactors in France? They are supposed to generate nearly all their electricity from nuclear power only.