The near-meltdown of a nuclear reactor at Three Mile Island in the US in 1979 remains a major warning of the danger of nuclear power generation. What makes an industrial accident involving a nuclear power plant so much more dangerous than an accident in other power plants, and how was this shown at Three Mile Island?
Electricity in large quantities is produced in turbines, where an energy source, such as steam or flowing water, is converted into electricity. In a nuclear power plant, a nuclear reactor generates heat. This heat is carried away by water or some other means, and is then used to drive a turbine.
In contrast to coal or oil-based electricity generation, where new fuel has to be constantly added to maintain heat, the reaction of nuclear fuel must be constantly slowed down to prevent it from reaching maximum energy output (a nuclear explosion). The reactor design must achieve a balance between the rate of the nuclear reaction and removal of heat from the core of the reactor. If this fails, there is meltdown, a runaway nuclear reaction. Unlike other industrial accidents, this releases huge amounts of extremely poisonous material.
At the time of the Three Mile Island accident, the possibility of a nuclear reactor meltdown was the subject of a mainstream movie, The China Syndrome. The release of unknown quantities of radioactive material and subsequent reports of health problems for local people and farm animals showed that the Three Mile Island accident had taken a toll.
Supporters of nuclear power claim that Three Mile Island was an isolated accident, and hasn't been repeated in the US. This is a spurious argument: after the accident, the US industry ground to a halt as far as new reactors were concerned. So, what happened?
At 4am on March 28, 1979, alarms rang in the Unit 2 control room at Three Mile Island. This wasn't unusual, and operators set about finding out what had gone wrong. The initial failure had occurred in part of the plant's pumping apparatus. Water in the secondary cooling system stopped flowing. The reactor relied on the primary cooling system to cool fuel in the reactor's core, and on the secondary cooling system to remove heat from the primary cooling system.
The backup system designed for such a case didn't work at first because valves that should have been left open had been closed. By the time this was discovered, water in the steam generator had boiled away, so heat was trapped in the core. This opened a primary pressure-relief valve, and should have triggered an automatic shut-down by the reactor.
At this time the plant operator reported, "The reactor is being cooled according to design by the reactor cooling system, and should be cooled by the end of the day". What was actually happening was that the primary relief valve opened, but failed to close when the pressure dropped. A design flaw meant the control room was unaware of this.
Although water was being pumped out of the primary cooling system through the faulty valve, operators thought there was too much water in the system and were taking measures to further reduce the dangerously low amount of water. This resulted in damage to 60% of the reactor's 37,000 fuel rods. In two and a half hours more than 1 million litres of water poured out onto the floor of the containment building, resulting in the release of radioactive gases into a building not designed to contain radioactive material.
More by good luck than by planning the accident was brought under control. Metropolitan Edison estimated that in the course of the accident at least five equipment breakdowns occurred, involving valves, pumps and fuel rods.
In 1979 around 140,000 people lived within 15 kilometres of the reactor, but there was no mass evacuation. Environmental activist Ralph Nader commented at the time: "For political reasons the mass evacuation that should have been carried out was not because it would have shown 150 million people watching on TV a picture of half a million people fleeing from a potential disaster".
Transcripts of US Nuclear Regulatory Commission discussions during the accident have NRC commissioner Gilinsky asking about evacuation: "Do they have places to tell people to go?" To this, evacuation expert Don Collings replied, "I imagine that they do, but you know, those are things that people decide when they do it". The effectiveness of this approach was shown a quarter century later in New Orleans.
The impact of the accident has since been the subject of debate, often based on attempts to minimise the threat. Australian pediatrician Dr Helen Caldicott at the time described the possible impact: "Three thousand people would have died immediately. Ten to a hundred thousand people right now would be going bald. They would be getting ulcers on their skin, severe vomiting and diarrhea, and, as their blood cells died, they would be dying of massive hemorrhage or infection.
"Thousands of men would be rendered sterile from the radiation on their testicles. Thousands of women would stop menstruating permanently. Thousands of babies would be born with small heads, because the radiation affects the developing brain.
"Thousands more babies would be born mentally retarded for life. There would be an epidemic of leukemia five years later, and hundreds of thousands of cases of cancer would appear 15 to 50 years later. It would have killed approximately half a million Americans."
Unit 2 began producing power for consumption on December 30, 1978. By having it in operation by the end of 1978, its owners were able to get approval for a US$49 milllion rise in electricity rates, along with tax concessions worth $37 to $48 million. To achieve this, testing was rushed.
By the time of the accident, the reactor had been operating at full capacity for about five weeks.
From Green Left Weekly, June 28, 2006.
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