What is going on at Fukushima?

There have been reports on the reactor crisis in Japan that range on the John Cleese scale of alerts all the way from “No worries” to the never before used extreme ”The Saturday barbie is cancelled”. So perhaps one should start with some facts and build up to the uncertainties.

Reactor Types:

There are boiling light water reactors, pressure light water reactors and graphite based reactors, as at Chernobyl, that all require enriched uranium. On the other hand, CANDU heavy water reactors use natural uranium as a fuel. The distinction for CANDU is heavy water enriched in deuterium. Graphite power reactors were operated within the Soviet Bloc but not elsewhere as there has always been a concern about neutron damage to graphite moderators, that is, the elements in the reactor that slow down the neutrons. Most power reactors are boiling or pressure water reactors where the water is both moderator and coolant.

The fuels used are either low enriched uranium, about 2.7% U-235, or mixed oxide fuel that uses weapons grade plutonium along with natural or depleted uranium. The uranium, as an oxide is formed into ceramic pellets that are put in a tube of zirconium alloy. Zirconium is a good conductor of heat and transparent to reactor neutrons. However if the fuel rods reach a temperature of around 1,000 ⁰C then water will oxidize the zirconium releasing hydrogen. Normally a boiling water reactor operates with a water temperature of around 300 ⁰C at a pressure of 75 atmospheres.

Radioactive iodine with a half life of 8 days and cesium with a half life of 30 years are products from the fission of U-235. Iodine, if it falls to the ground is taken into cows’ milk and if drunk by us will concentrate in the thyroid. While cesium has a much longer life, it is not assessed as a significant hazard.

Japanese Nuclear Power Plants

Japan has 55 power reactors and Fukushima is one of the top fifteen nuclear power plants in the world.

There were 6 operating nuclear reactors at Fukushima for a total of 4,700 MW of electric power.. This is about 75% of the capacity in the Latrobe valley. There are a further 2 reactors totaling 2,800 MW of electric power under construction. The first reactor commenced commercial operation in 1971 and number 6 reactor came on line in 1979. The reactors are all boiling water designs supplied by General Electric for units 1, 2 and 6, by Toshiba for units 3 and 5 and by Hitachi for unit 4.

The Fukushima reactors are all “Generation II” reactors. The reactor is within a steel pressure vessel contained within a second concrete structure. The reactor sits at the top of the building and the concrete structure includes chambers beneath the steel vessel that can be used for emergency venting of steam from the pressure vessel. In addition, the spent fuel rods are kept in adjacent cooling pools at the same level as the reactor.

Past Nuclear Accidents

Before discussing what has or may have happened it is useful to look at the two major events at nuclear power plants, Three Mile Island and Chernobyl.

On March 28, 1979 a light water reactor at the Three Mile Island nuclear power plant suffered a partial meltdown. Within weeks attorneys filed law suits against Metropolitan Edison Company (a subsidiary of General Public Utilities) on behalf of all manner of businesses and residents within 25 miles of the plant. Over 2,000 personal injury claims were filed, with plaintiffs claiming a variety of health injuries caused by gamma radiation exposure. The Pennsylvania district court quickly consolidated the claims into ten test cases. Over the next 15 years, the cases went to the Supreme Court and back, and through various district and appeals courts. Finally, in June 1996 district court judge Sylvia Rambo dismissed the lawsuit granting summary judgment in favor of the defendants. There was no significant radiation exposure beyond the limits of the power station. An estimate of the likely number of deaths from radiation induced cancer is less than one.