THE SCARE, MARCH 16, 1979

A nuclear power plant is undergoing an emergency shutdown procedure known as a “scram” when there is an unusual vibration and the coolant level drops precip­itously. Subsequent investigation by a shift supervisor reveals that X-rays of welds have been falsified and other problems exist with the plant that could potentially cause a core meltdown that would breach the containment building and cause an explosion. However, the results of the investigation are squelched and the plant is brought up to full power. The shift supervisor takes the control room hostage but is then shot by a SWAT team as the reactor is scrammed. A meltdown does not actually occur.

No, this did not really happen, but these events—portrayed in the movie The China Syndrome—evoked a scenario in which a nuclear core meltdown could melt its way to China and contaminate an area the size of Pennsylvania. It also exposed a nuclear power culture that covered up safety issues rather than fixing them. It made for a compelling anti-nuclear story that scared a lot of people.

And then a real core meltdown happened, 12 days later.

THREE MILE ISLAND, MARCH 28, 1979 How the Accident Happened

The worst commercial nuclear power reactor accident in US history1 began on Three Mile Island, an island in the Susquehanna River three miles downstream from Middletown, Pennsylvania (hence its name). Two nuclear reactors were built on this island, but one of them (TMI-1) was shut down for refueling while the other one (TMI-2) was running at full power, rated at 786 MWe. At 4:00 a. m., what should have been a minor glitch in the secondary cooling loop began a series of events that led to a true core meltdown, but no China syndrome occurred and there was little contamination outside the plant. Nevertheless, it caused panic,

roused anti-nuclear sentiment in the country, and shut down the construction of new nuclear power plants in the United States for decades.

The nuclear reactors at Three Mile Island were pressurized water reactors (PWR), the type of reactor that Admiral Rickover had designed for power plants in US Navy nuclear submarines (1). About two-thirds of the 104 nuclear reac­tors in the United States are of this design. The fuel rods containing the enriched uranium are in the 4- to 8-inch thick high-tensile steel reactor vessel. When the control rods are removed, fission begins and the core heats up.

Water circulates through the reactor core and serves both as a moderator to slow down the neutrons and as the heat-transfer medium. The water heats up to about 585°F at a pressure of 2,200 pounds per square inch but it does not boil. This high-pressure, hot water is pumped through a heat exchanger that gener­ates steam in a secondary cooling loop and cools the high-pressure water in the primary loop. The high-pressure steam in the secondary loop then goes through the turbines, turning the generator to produce electricity. A condenser converts the steam back to water that recirculates through the steam generator. The main feedwater pumps are critical to maintaining this flow of water to extract heat from the reactor core. In principle, a coal-fired power plant and a nuclear power plant are similar in that they both make steam to turn turbines and a generator. The big difference is that you can turn off a coal-fired power plant and it cools down right away. With a nuclear power plant, it takes a long time to cool down after you shut it down by inserting the control rods (see Chapters 6 and 9 for more details about fission).

The reactor core and steam generator are all contained within a primary con­tainment vessel that has steel-lined walls of concrete 3 feet thick. The water that circulates through the reactor core never goes outside the primary containment structure, and the water in the secondary loop is not directly exposed to the reac­tor core. The most critical component of a nuclear power plant is the cooling water because if the cooling water stops flowing, the reactor can heat up so much that it can melt the Zircaloy (zirconium alloy) cladding of the fuel rods and cause a meltdown of the fuel.

At 4:00 a. m. the graveyard shift at TMI-2 was monitoring the normal reactor operation when a pump in the secondary cooling system shut down, triggering the turbine to shut down and the reactor core to scram—the control rods are rapidly and automatically inserted into the core and fission halts. “At this point, as has been said many times before, if the operating staff had accidentally locked itself out of the control room, the TMI accident would never have happened” (1). Instead, because of poorly designed controls and warning lights, malfunctioning valves and indicators, and the chaos of clanging alarms and flashing lights, the operators made faulty decisions that led to a partial core meltdown.

As soon as the main feedwater pumps stopped working, the reactor core was no longer being cooled and pressure built up, triggering a relief valve in the pres — surizer to open and relieve the pressure (see Figure 5.1 in Chapter 5). The relief valve should have then automatically closed but it did not, yet the operators were not aware that it was still open. As the pressure dropped, emergency cooling water pumps turned on to flood the core with water. There was no indicator of the actual water level in the core, and the operators had no signal to tell them that the valve was stuck open. Even worse, a light on the control panel falsely indicated that the relief valve was closed when it was actually still open. The operators thought there was too much water in the core and in the pressurizer, which can be a serious problem, so they turned off the emergency cooling pumps—a fatal mistake. Water poured out of the stuck-open valve into the primary containment building floor, and the core began to heat up with the lack of cooling water. The operators were still unaware of the problem. At 6:00 a. m., a new shift arrived and shut off the venting of cooling water through the relief valve, but it was too late and 32,000 gal­lons of radioactive water had already spilled into the primary containment build­ing. Ten minutes later the Zircaloy cladding of the fuel rods ruptured and the fuel began to melt. Eventually, it was determined that about half of the core had melted. Radiation monitors began going off and the operators finally realized they had a loss of coolant accident on their hands. They turned the emergency cooling pumps back on and appeared to have things under control by the end of the day (1-5).

The crisis was not yet over, however. Water on the floor of the primary contain­ment building was inadvertently pumped into the auxiliary building, and on the morning of March 30, about 13 million curies of noble gases, mostly isotopes of xenon, with traces of iodine (17 curies of 131I), were released into the atmosphere (1, 6). The gases rapidly dispersed, but because of this release and confusion about the plant’s condition, Governor Richard Thornburg advised women and children within a 5-mile radius to leave the area, causing panic. Another concern devel­oped during the day. When Zircaloy cladding on the fuel rods gets hot enough, it reacts with water and produces hydrogen gas. Irradiation of water also produces hydrogen gas. The hydrogen gas formed a large bubble in the pressure vessel, and there was concern that it might burn or even explode. This concern was height­ened throughout Saturday, March 31, but ended on Sunday, April 1, when experts determined that it could not explode because of a lack of oxygen. In spite of the operator mistakes and faulty signals and valves, the containment building design was robust enough to contain the core meltdown without contaminating “an area the size of Pennsylvania” as dramatized in the The China Syndrome.

It took another month for the reactor to achieve cold shutdown status, meaning that the reactor core had a temperature of less than 100°C, the boiling tempera­ture of water. The crisis was over, but the consequences were not. The reactor was destroyed and had to be cleaned up and mothballed at a cost of about $975 mil­lion (1). The other reactor on the site, TMI-1, continued to operate and is licensed to run through 2034.