No danger of major Japan radiation leak: UK expert

LONDON Sun Mar 13, 2011 10:10am EDT

LONDON (Reuters) - Frantic efforts in Japan to cool three nuclear reactors may avert a collapse of the radioactive cores and a more costly clean-up, but there is no risk of an extensive radiation leak, a top UK academic said.

Engineers fought on Sunday to avert a meltdown at earthquake-crippled nuclear reactors by pumping in cooling seawater after Tokyo said it was assuming partial damage had already happened.

The risk is that uranium and plutonium fuel may fall in on itself, as their metal sheaths melt, creating a molten deposit at the bottom of the reactor which may be impossible to remove, as happened at Three Mile Island in Pennsylvania in 1979.

"Because of the way the core was destroyed (there) it meant that even after it cooled down they couldn't take the fuel out," said Robin Grimes, director of the Center for Nuclear Engineering at Imperial College London.

"They had a load of rubble at the bottom of a metal pot."

The core of a nuclear reactor has a series of metal pipes or rods, containing pellets of uranium fuel, bundled into what engineers call fuel assemblies. Water is pumped between the pipes to keep them cool.

Japan's biggest recorded earthquake knocked out the back-up cooling systems at stricken reactors in Fukushima prefecture north of Tokyo, causing a build-up of heat and pressure which probably damaged but had not yet destroyed some fuel rods.

"After it's all cooled down it may well still be possible to simply remove the fuel and dispose of it in a relatively normal procedure," said Grimes.

"What's clear because of the incidental radiation being released at the moment, which is significant but not overwhelming, is that the structure of the core is probably still intact. So it's not as bad as Three Mile Island."

DAMAGE TO THE CORE

He was in little doubt that there had been some damage.

Evidence was provided by a blast at one reactor on Saturday, probably caused by a chemical reaction between overheated metal sheaths of the fuel rods and the surrounding water.

"That produces hydrogen and that hydrogen is what was vented and detonated. That tells you, first, that some of the fuel got very hot, and second, that there was a chemical reaction between the cladding of the fuel and the residual water, steam and so forth, and that hydrogen was evolved."

"So you know there's been damage to the core."

That damage meant the 40-year-old reactors were already written off, Grimes said.

Three Mile Island, also rendered useless, was a new reactor. In that case there was no dangerous radiation leak, according to the U.S. Nuclear Regulatory Commission, because successive protective shells shielding the core weren't breached.

Serious radiation was also unlikely at Fukushima, said Grimes, given thick, surrounding walls.

"There's no risk of an extensive radiation leak into the surrounding areas. The worst-case scenario is it's just going to be more difficult to clean up."

Authorities have set up exclusion zones around the plants and around 140,000 people have been moved from the area.

The Japan case has little parallel with the Soviet plant at Chernobyl where fundamental design faults led to explosions ripping through a flimsy shell in 1986, causing hundreds of deaths among emergency workers and contamination across Europe.