OVERHEATING REACTORS:The Fukushima Daiichi plant's reactors cannot be compared to those at the Chernobyl power plant in Ukraine 86, writes DICK AHLSTROMScience Editor
FEARS OF a new Chernobyl disaster are in everyone’s thoughts as workers at the Fukushima Daiichi nuclear power plant continue to struggle with three overheating reactors.
There have already been two major explosions, which are being played and replayed on our television screens. And a reactor core meltdown is constantly being put forward as a possible result of overheating reactors.
Three reactors at the Fukushima Daiichi are currently overheated, according to Japan’s Nuclear and Industrial Safety Agency, as reported to the International Atomic Energy Agency.
Japan as a member reports to the agency and other members, including Ireland, are able to share the information, said Dr Ciara McMahon, director of environmental surveillance and assessment at the Radiological Protection Institute of Ireland.
The institute met over the weekend to consider the latest information coming out of Japan and issued a statement indicating that the Japanese plants represented no threat to Ireland.
The fear all around the world is that the situation might deteriorate and a reactor core meltdown might occur. This in itself would not, however, necessarily pose a major threat or automatically lead to a discharge of radioactive material from the reactor, Dr McMahon said.
“Even if there was a fuel melt this doesn’t mean we would have a catastrophe,” she said. All modern reactors are protected by a containment vessel, a combination of thick concrete and steel strong enough to deal with radiation discharges or even explosions in the reactor.
These reactors cannot be compared to those at the Chernobyl nuclear power plant in Ukraine. The reactor that blew up there in 1986 did not have this kind of containment, Dr McMahon said.
The 2,000-tonne cover used instead was blown apart by an explosion and as the Chernobyl reactor core melted it caught fire. With no protective enclosure the huge fire was able to discharge large amounts of radioactivity into the upper atmosphere where it spread across western Europe, she said.
There have also been two hydrogen explosions at Fukushima reactors 1 and 3 but these have not had any effect on the containment, according to the Japanese authorities.
These explosions are reminiscent of the Chernobyl accident but arise for a completely different reason and will not lead to a core meltdown.
The reactors use uranium fuel rods that are positioned close together. Once side by side they begin to produce large amounts of heat, with this reaction speeded up, slowed or stopped using control rods.
The heat is used to boil water flowing through adjacent channels, and this produces steam that in turn drives turbines to produce electricity. The water runs in a closed circuit so no radiation can escape. Sea water is used in another closed circuit to cool the steam back to water.
This essential cooling system failed first when the earthquake knocked out electricity and then a second time when the tsunami hit, shutting down the backup generators. Batteries used as a third layer of safety could not cope and so the reactors began overheating.
Once temperatures started to rise the reactors automatically shut down with the control rods stopping the nuclear reaction, Dr McMahon said. “They are all shut down, none of them are operating.” At Chernobyl the reactor was actually running at full power when the accident occurred.
Lots of heat remained in the Japanese reactors, however, and despite their efforts, plant workers could not get the original cooling systems to work.
The next move was to connect the cooling sea water directly to the closed loop that formerly carried the reactor water. “It is actually very effective,” Dr McMahon said, but the decision to do this means that the reactors will never run again.
Currently reactor 3 is in “cold shutdown”, at a target temperature of less than 93 degrees, she said. Reactors 1, 2 and 4 remain in full shutdown but are still being cooled using sea water.
The fact that the agency has not upgraded the severity of this incident suggests that the situation is stable, she said. This is currently a level-four incident, as set by the agency. It ranked the Three Mile Island and the Windscale accidents as level five, with the Chernobyl accident set at the maximum level – seven.
The hydrogen explosions likely arose because of the high temperatures in the reactors, said Frank Turvey, a former assistant chief executive at the institute and a nuclear engineer with working experience of reactors.
At about 1,100 degrees the steam is superheated and can begin to dissociate into oxygen and hydrogen. This hydrogen is an explosive risk and will be vented away from the containment vessel so it can’t build up.
“It may be this vented hydrogen from the containment building built up in the turbine building,” he said, thus causing the two explosions seen so far.
The Japanese authorities said yesterday that pressures recorded in the cooling systems remained within the normal ranges after the explosions.
Radiation levels also remained low afterwards indicating that material had not been released by the reactor core.
The venting is also the likely source of the radioactivity detected for some distance around the plant. The main substances are radioactive iodine and caesium, but the mix could also include strontium or other substances, Dr McMahon said.
The iodine can contaminate food and when eaten accumulates in the thyroid, potentially causing cancer. Caesium acts like potassium when in the body and can build up in tissues to deliver a harmful radiation dose.
Levels are low, however, and there has not been a major release, Dr McMahon said. Provided the reactors can be cooled the greatest radiation risk will then be linked to the material left behind in the reactor. “It is a very bad situation for the power plant and the people who will go in to clean it up,” she said.
1 UNIT 1 – On Saturday, a hydrogen blast destroys the housing around the complex's Unit 1 reactor, leaving the shell intact but resulting in the mass evacuation of more than 185,000 people from the area.
2 UNIT 2 – The drop in water levels at Unit 2 yesterday, leaving the uranium fuel rods completely exposed and raising the threat of a meltdown, had officials the most worried. "Units 1 and 3 are at least somewhat stabilised for the time being," said official Ryohei Shiomi. "Unit 2 now requires all our attention."
3 UNIT 3 – Explosion at Unit 3 yesterday actually lessened pressure building inside the troubled reactor, and officials said the all-important containment shell — thick concrete armour around the reactor — had not been damaged. The explosions injured 11 workers at the plant.
4 UNIT 4 - Also in emergency status after damage to the core cooling systems.