Accidents and Transients

October 22, 2013
Dave Lochbaum
Former contributor

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.