Accidents and Transients

, former director, Nuclear Safety Project | October 22, 2013, 6:00 am EST
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Nuclear Energy Activist Toolkit #17

The terms “accidents” and “transients” appear frequently in NRC and industry documents about nuclear plant safety. Systems, structures, and components are designed to prevent accidents and transients, or mitigate them should they occur anyway.

But what are accidents and transients?

They are alike in that nobody wants either to happen.

They are different in that if one has to happen, everybody prefers a transient to an accident.

Transients, abbreviated as AOOs, are discussed in Chapter 15.0 of the NRC’s Standard Review Plan for Light Water Reactors.  This chapter explains that transients—Anticipated Operational Occurrences (AOOs)—are events likely to occur one or more times during a reactor’s operating lifetime. Examples of transients include steam generator tube ruptures like the one that happened on San Onofre Unit 3 in January 2012, loss of normal makeup water flow to the reactor vessel of a boiling water reactor or steam generators of a pressurized water reactor, disconnection between the plant’s switchyard and the offsite electrical transmission grid, and malfunction of a control system.

Accidents, on the other hand, are events postulated but not expected to occur during a reactor’s operating lifetime. Examples of accidents include rupture of the largest-diameter pipe connected to the reactor pressure vessel, ejection of a control rod from a pressurized water reactor or drop of a control rod from a boiling water reactor, and seizure (i.e., immediate stoppage) of the largest pump circulating water through the reactor core.

The NRC established criteria to evaluate whether plant designs included adequate protection against transients and accidents. For example, no transient should result in the pressure inside the reactor vessel and its attached piping from exceeding 110% of their design values. And no transient should erode margins established to protect the nuclear fuel rods from damage.

For design-basis accidents, the criteria include having no accident that should cause the temperature at the center of the nuclear fuel pellets to exceed 2,200°F or result in the release of radioactivity to the environment that causes radiation doses to members of the public exceeding the limits in 10 CFR Part 100.

As anticipated, transients are occurring. The NRC’s Industry Trends Program monitors their occurrence and reports their findings annually. Figure 1 of Enclosure 2 of its 2013 report showed that the average pressurized water reactor (PWR) in the US experienced 0.422 transients in 2012 while Figure 2 showed the average boiling water reactor experienced 0.554 transients that year. (Put another, perhaps more useful way, these statistics indicate that the average US nuclear power reactor experiences one transient every two years.) Subsequent figures parsed out specific causes of these transients, whether due to loss of normal makeup water flow, loss of the offsite electrical grid, or other reasons.

The postulated accidents haven’t occurred in recent years, at least not in the United States.

Bottom Lines

Once the list of transients and accidents is defined, the challenge faced by plant designers and equipment vendors was in satisfying all the criteria. For example, pipes that pass through the containment wall are equipped with devices to close them in case the section of piping outside the wall breaks. These closure devices guard against radioactive material exiting via a large opening and thereby negating the containment barrier. But if the pipe remains intact, a device that is closed when it should be open could block the flow of essential cooling water to the reactor vessel and its nuclear core.

Success produces a design that satisfies all the criteria for all the transients and accidents.

Failure yields a design vulnerable to one or more transient and accident scenarios.

As Fukushima demonstrated, a successful design does not assure a successful outcome when the nature of a transient or accident exceeds that assumed when building in protection against it.


The UCS Nuclear Energy Activist Toolkit (NEAT) is a series of post intended to help citizens understand nuclear technology and the Nuclear Regulatory Commission’s processes for overseeing nuclear plant safety.

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  • So, explain how this is any different from the maintenance of airliners (which kill an order of magnitude more people than nuclear energy)?

    Would the public be terrified to hear from some airplane mechanic blog what heavy maintenance checks find on a typical airliner? You bet they would. Yet like nuclear energy, airlines are the safest at what they do.

    Where is your equivalent to protect us from the evil airline industry instead of the evil nuclear energy industry? We should all be thankful there isn’t one. Your articles do more harm than good because they mislead the public into thinking nuclear energy is dangerous and that your oversight is necessary. An equivalent blog about airliner maintenance and design would have the public terrified to fly.

  • jharragi

    Air-crash deaths are exciting and attributable. Deaths that occur due to radiation exposure are generally obscured by life events in the span of time before they occur. You may believe the industry contention that the only deaths caused by the Chernobyl Disaster were the thirty-some that were obvious and undeniable. But it is quite apparent that there were significantly more deaths. Estimates range from tens of thousands to over a million. But is any one of those deaths conclusively and directly attributable to the accident? No, but they can be determined statistically. Unfortunately the promoters of this industry have a huge financial stake in suppressing this information and distorting the science. Not only that, the day-to-day operation of the plants present some hazard due to continually escaping isotopes and the myriad smaller and larger leaks and spills and discharges. Other that a hand-full of deaths due to gross exposure can any death that occurs years later be attributed to that routine exposure? How many could there be, one? ten? a million?

    It appears that the nuclear industry is actively engaged in suppressing science that would reveal an accurate accounting of just what the risks of atomic energy are. Additionally government agencies participate in the suppression of this information – and have from the inception of the industry. So we have a government and industry acting in an untrustworthy way essentially telling people to trust them …is it really that much of a surprise that people fear this? Now throw in the fact that many of the same entities participated in the weapons industry. Also consider the period of domestic weapons testing that resulted in so many deaths after the exposure of hundreds of thousand of both military and civilians that it became statistically undeniable. When these things are taken together, the fear is not unfounded.

    All of that aside, your point regarding air travel higher death rate may still be on the mark if it turns out that aerosols and soot cause a great amount of polar ice melt. On the other hand, if one of the fuel pools collapse at Fukushima (or elsewhere), nuclear wins!

    Ahh, drama. People thrive on it…