zondag 3 november 2013

Could Chernobyl Happen Again?

As energy economies based on fossil fuels become more and more unsustainable, nations all around the world are looking for new sources of electricity. Proponents of nuclear power maintain that nuclear fission is the only energy source powerful enough to run the economies of the future, but one major obstacle lies in their way: Chernobyl.

The Disaster at Chernobyl

The Chernobyl nuclear power plant, near the city of Pripyat, was the site of the most notorious industrial disaster of the 20th century. Construction began in 1970 and he plant opened in1977 when the first reactor was brought online. Four reactors were complete by the time of the 1986 disaster, with two more under construction. They were never completed.

In the early morning hours of April 26th, 1986, the crew working reactor number 4 were instructed to begin a test of the backup cooling systems. The test had been delayed through much of the day, and supervisors were anxious to begin despite strange readings from the reactor. They didn't know that time spent running at low output had “poisoned” the reactor with large amounts of xenon-135, artificially lowering the reactor's output. Additional control rods were removed to make up for the deficit.

Even with the reactor acting strangely, the plant engineers continued with the test. The steam flow to generators powering the coolant pumps was cut off, and the backup cooling systems were engaged. The test didn't last long; less than a minute later, the emergency shutdown was activated. The reasons for this shutdown are still unknown, but it had quite the opposite affect. A massive power spike shattered the fuel channels inside the core, causing a steam explosion that threw the 2000 ton reactor lid up into the air and demolished most of the reactor building.. The graphite fire burned for days, spreading radiation through much of Europe, causing a still unknown number of deaths and costing the USSR 18 billion rubles.

Could such a disaster happen again?



Design Flaws and Bad Decisions

To answer this question, it's important to know how the Chernobyl disaster happened in the first place.

One of the biggest problems with the Soviet reactor design used at Chernobyl was something called a positive void coefficient. When water coolant boils, the liquid becomes a gas (steam). In all modern reactors, this reduces the power output of the reactor, but in the Soviet design used at Chernobyl it resulted in a power increase. The increase led to more heat, more steam, and a feedback loop that destroyed the reactor.

The crew was also uninformed about the structure of the control rods used in the design. The control rods had graphite tips, which acted to increase the power output of the reactor for the first few seconds of insertion instead of decreasing it. This is counter-intuitive, and when the engineers' ignorance was combined with poor judgment (deciding to continue the test on a poisoned reactor) the attempted emergency shut down caused the very explosion they were trying to avoid.

These design flaws have all been eliminated from reactors built since the disaster, of course, and remedied in most reactors that still follow the old Soviet design. Human nature, however, cannot be redesigned, and so errors and miscalculations will always present some risk to even the best-designed

Sam Jones writes about energy and environmental issues. He keeps a close eye on the energy suppliers and recommends that people use an energy comparison site to help move to a green or cheap gas and electricity tariff

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