Tuesday, January 29, 2008

Nuclear Accidents

There have been two serious accidents involving nuclear power reactors and it's right that they have received very much attention. They are at the heart of the debate over whether or not to expand nuclear energy to minimize global warming.

The reactor at Chernobyl was different from all the other power reactors outside the Soviet Union: it was inherently unstable, meaning that the reactivity in the core went up when it got hotter so that once the operators lost control there was no way to get it back.

The accident happened this way[source]. The night crew was told to perform a test to see if the reactor could sustain a sudden disconnection from the power grid. It happened that the night crew was inexperienced (presumably because of seniority rules), though that probably wouldn't have made any difference. What was supposed to happen was that the flywheel inertia of the turbine blades in the electrical generators would give enough power to run the coolant pump until the diesel-powered generators could start and power up.

The crew didn't know that the reactor was operating at an abnormal condition, having run at full power all day and then being cut back to part load, but that probably wouldn't have made any difference, either.

It's not clear why, but the coolant pumps were run at their maximum flow. Possibly the crew thought they were increasing the safety margin. But the resulting cooler temperatures lowered the steam pressure and water filled more of the reactor's internals. Water absorbs neutrons more than steam does, so the control rods had to be withdrawn to maintain power.

The automatic controls would ordinarily have shut down the reactor under these conditions, so the crew disabled the emergency cooling system and the emergency shutdown rods (usually called SCRAM rods).

The crew disconnected the plant from the power grid. But the pump power from the turbine blades wasn't sufficient so the reactor started heating up. Because of the instability this reactor had, the higher temperature raised the reactivity rate, causing more heating, etc. At that point the reactor was out of control. Steam drove water out of the core, and reactivity increased more. Once the crew realized something was wrong, they inserted the control rods. But the control rods inserted slowly, not quickly as the shutdown rods would have. To make matters worse, the tips of the control rods were made of graphite instead of boron. Graphite raised the reactivity rate instead of lowering it as boron would have done. The rods jammed when they were partly inserted.

The reactor continued to heat up. A steam explosion drove some parts out through the sheet-metal roof that kept rain off the reactor. Finally, the reactor body, which was made of graphite, reached its ignition point. The hole in the roof allowed air to enter and the reactor caught fire.

After the accident, the World Health Organization did an extensive investigation and continual followup; its findings were that actual deaths have numbered about 50 and theoretically there could be as many as 4000 fatal cancers in the future.[source] As tragic as that is, it doesn't approach the death rate due to burning coal.  Even in the US, tens of thousands of people die every year just from the pollution from generating electricity with fossil fuels.[Abt Associates Report, Exhibit 6-4]

What's interesting is that a big part of the region around Chernobyl now is healthier than before the accident. The chemical refineries and coal-burning plants caused terrible health problems. Now that they're shut down, the air is clean. Some people have moved back into the parts which officially are quarantined but where radiation isn't especially high. They eat vegetables from their gardens and drink water from their wells, and take eggs from their bug-eating chickens, and they're doing just fine. Wildlife have flourished in the area, including the hot spots. Wildlife biologists are studying the animals and plants and even after all these years they're not finding any radiation-related health problems. There's a superb book on Chernobyl's aftermath: Wormwood forest : a natural history of chernobyl by Mary Mycio.

So what are the differences between Chernobyl-style Soviet reactors and all the power reactors in the rest of the world? There are too many differences to list here, but we'll tick off the major differences that led to the accident.

1. The reactor was unstable.
2. The reactor had no containment structure.
3. The reactor was made of graphite, protected only with a sheet-metal shed. Outside the Soviet Union, power reactors have multiple layers of steel and concrete protection.
4. The crew hadn't been trained for the test it was performing.
5. The crew was working without supervision and went against plant operating regulations.

To understand why the reactor was built and operated so unsafely, you'd have to understand how the Soviet system worked. I'm not qualified to explain it, but if you read some Solzhenitzyn you'll get the idea. The accident did, however, prove that anti-nukes had vastly overstated the harm such an accident could cause. It turned out that the consequences, serious as they were, were of the same scale as disasters that happen every year.

More important, the accident at Three Mile Island in Pennsylvania in 1979 totally destroyed the reactor but resulted in no adverse health effects, which validated the defense-in-depth designs used in all US power reactors.[source]


Charles Barton said...

There was another major nuclear incident that often is ignored in discussions of nuclear safety. That was the 1957 "Windscale" reactor fire (http://en.wikipedia.org/wiki/Windscale_fire).

Red Craig said...

Charles, thanks for the comment. I've been focusing here on nuclear energy and not on the weapons program at all. Many things were done poorly in the weapons program, but I consider that not to be part of the debate. The energy program has always had to show much more robust safety standards. TMI showed that the defense-in-depth concept worked: even though the reactor's internals were mostly destroyed, the public was kept safe.

But the Windscale accident is significant to the debate, anyway. Although no fatalities have been associated with it, it had a strong effect on UK attitudes toward nuclear energy.

Thanks for bringing it up.


workplace accidents said...

Great post! yesterday i found another great video post about Accidents. Here is the link
workplace accidents

TRAVELS said...

Nuclear safety dont excist!