Updated
In the wake of Friday’s earthquake and tsunami, Japan has reported having problems with nuclear reactors at Fukushima Daiichi Nuclear Power Station, located about 150 miles north of Tokyo. This explainer provides an overview of nuclear power generation, summarizes what has happened (effective Saturday March 12, 5 pm PST), and rebuts an Internet meme that Japan is on the verge of another Chernobyl.

boiling water nuclear reactor

Boiling Water Reactor Power Plant
(Asian Nuclear Safety Network)

What is a nuclear power plant?

Most thermal power plants generate steam to power a turbine attached to a generator that makes electricity. A nuclear power plant generates its steam through the fissioning of uranium oxide contained in fuel rods.

How do nuclear power plants work?

In fission, atoms are split in half, which generates heat; some materials that are created during this process will decay for some period of time, and they also generate heat. This heat is used to generate steam. A pressurized water reactor (water does not boil) uses a heat exchanger to generate steam in a separate chamber (a two-stage process). A boiling water reactor is a one-stage process; when it is operating, the core is kept covered in boiling water and the steam is collected above that level.

What happens when you shut down a nuclear power plant?

The self-sustaining (critical) atomic reaction that runs a reactor can be shut off in a matter of seconds; the goal is to keep the reactor sub-critical. The core material, which is radioactive, will generate heat, at a decreasing rate, for some time. Therefore, nuclear engineers need a way to keep this heat from building up in order to protect the radioactive fuel (rods) and the reactor.

In order to cool the reactor, nuclear plants must have power that runs a complex system of motors, valves and instruments that push water through the nuclear core and carry the heat elsewhere; newer plants have been engineered to simplify this system. Should there be a loss of electricity from the power grid, diesel generators provide emergency power. Should the diesel generators fail, the battery-powered systems can keep water over the core for short periods.

The initial cooling process is fairly rapid; within the first hour after shut down, decay heat may decrease to about 2 percent of the pre-shutdown level. This drops to about 1 percent of the pre-shutdown level after the first day.

What about Japan?

The facility under media scrutiny is Fukushima Daiichi Nuclear Power Station, located about 150 miles north of Tokyo; the facility has General Electric boiling water reactors. It is similar to several U.S. facilities, Brown’s Ferry (Alabama, Tennesee Valley Authority), Peach Bottom (Pennsylvania, Philadephia Electric Company, PECO), Pilgrim (Massachusetts, Entergy) and Vermont Yankee (Entergy). The Fukushima Daiichi plant seemed appropriately designed to withstand the earthquake, even though the magnitude of the quake was outside of its design standards. The design weakness was exposed after the tsunami hit.

The earthquake triggered an automatic shut down of the nuclear power plant and caused a loss of off-site (grid) power. A boiling reactor continues to generate steam after shut down; when the steam has enough force to run a turbine, it can power pumps that provide coolant for the core but it will not provide power for controls or instruments.

For about the first hour after the earthquake, diesel generators provided power to Unit 1. However, the tsunami seems to have flooded either the diesel generators or the seawater pump house. This loss required engineers to rely on the battery-powered emergency systems; batteries have a limited life. Without power the engineers cannot/could not monitor the situation. However, Tokyo Electric Power Company (TEMPCO) reported on Saturday that off site power is available, which implies that the batteries are being charged.

Assuming the seawater pump house was not damaged by the tsunami, the still-cooling reactor core could remain covered with cooling water. However, there have been reports of fission product releases which could indicate fuel damage; fuel damage can lead to hydrogen generation and accumulation.

What happened when the plant exploded Saturday?

The apparent reactor building explosion may have resulted from a build-up of hydrogen; moreover, this building houses many of the emergency systems for the reactor. Although the explosion caused the refueling floor siding and roof to blow off, published photos show that the concrete structure below is still there. The primary containment lives inside the lower concrete building. However, the building contains pipes that carry water, other equipment and electrical wiring that support the cooling system. Thus the detonation inside reactor building increases the uncertainties about equipment condition.

According to reports, the reactor is intact, but engineers must continue to keep the core covered and cool the reactor to prevent a meltdown. There are reports that engineers are pumping sea water mixed with boron into the reactor (boron absorbs neutrons) in order to keep the reactor sub-critical. In the context of nuclear power, “critical” is not the bad word that it is when a patient is in the hospital; any operating nuclear reactor is “critical” because the word means you have a self-sustaining nuclear reaction.

Is this really another Chernobyl?

In a word, no. The situation in Japan is different from Chernobyl, which had a completely different nuclear power plant design. Chernobyl had an inherently unstable graphite-moderated reactor, not an inherently stable water-cooled reactor. In addition, all western (modern) reactors must have a containment building; Chernobyl did not have one. Therefore, even if the reactor core in Japan were to melt, the primary containment, a steel liner surrounding the reactor core, should limit the release of radiation. However, we do not know if the primary containment was weakened by the earthquake. Nevertheless, the longer the time between shut-down and collapse the better the outlook because the reactor generates less and less heat with time.

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Science Journalist @BrianDunning

What is a core meltdown?

A core meltdown occurs when the heat inside the reactor grows so great that the fuel (rods) melts; generally, this means temperatures exceeding 1,000 degrees Fahrenheit. A meltdown, in and of itself, does not necessarily mean “disaster” as imaged by Hollywood. The reactor core is designed to contain high levels of heat, pressure and radiation; if it remains intact, the melted fuel is a mess but it does not affect the external environment. If the core breaches (cracks) but the containment facility built around the core remains intact, the melted fuel is typically entombed within specialized concrete.

Could there be a nuclear explosion?

There is no danger of a nuclear explosion. If the reactor fuel melts, it will fall to the floor of the containment vessel. There is a possibility that the fuel could burn through the floor of the containment facility, either because of poor design or because of damage from the earthquake. Should that happen, radiation could spread throughout the local area. Added Sunday 3.30 PDT

What is the risk to the United States west coast?

The U.S. Nuclear Regulatory Commission writes: “we are not expecting to experience any harmful levels of radioactivity here [Hawaii, Alaska, the U.S. territories and the U.S. West Coast].” Added Sunday 5.30 PDT

Sources

KATHY GILL, Technology Policy Analyst
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