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.

On 26 April 1986 a reactor at the nuclear power station at Chernobyl in the Ukraine exploded. The Chernobyl reactor had a graphite core, a design not followed in the West although the first reactor built at the University of Chicago had a graphite core. The disaster at Chernobyl was the fire from burning graphite that lifted radioactive dust particles into the atmosphere. About 200 plant workers and firemen got radiation doses that could lead to Acute Radiation Illness and 31 died almost immediately with the delayed deaths of a further 21. But no one in the general public got acute radiation sickness. 650,000 “liquidators” who cleaned up Chernobyl and surroundings were certainly exposed to some radiation. Interestingly, a report by WHO in 2006 on Health Effects of the Chernobyl Accident states that of a group of 61,000 liquidators, 4,995 deaths were recorded between 1991 and 1998 and this death rate was lower but not significantly different from that of the whole Russian population. Of these deaths, 216 were estimated as attributable to radiation. However there were some 2000 reported thyroid cancers with 20 deaths. The tragedy of this was that it was avoidable. However the Soviet authorities did not advise families not to drink local milk for a month and a number of antidotes to iodine were available. For cesium, there is an estimate of 4000 cancer deaths over the next 50 years in the former Soviet bloc. That is 80 a year compared to 6,000 deaths from cancer each year in the United States.

State of the Fukushima Plant

It is clear that at the start of the earthquake, all the operating reactors were rapidly shut down. Power would have fallen from full to about 8% almost immediately and within 10 hours it would have been 1% of full power. Full power in each reactor was of the order of 3000 MW so within ten hours it was 30 MW. Unit 4 does not appear to have been operating as its fuel had been withdrawn some 3 months before the earthquake. Explosions were reported from unit 3 that may have been the venting of hydrogen from the pressure vessel with an explosion following. Smoke was reported from unit 4 but the source is not clear. The loss of back up pumps for cooling water and the delay in pumping sea water into the reactors and cooling pools no doubt exacerbated the situation. A possible explanation for the delay was the time necessary to diagnose the problems and a reluctance to use seawater that would lead to a write off of the plant.

There is so little coherence in reports that it is not clear if any pressure vessels have fractured although it does appear that the outer concrete structures of some units may have cracked.

The safety of the operators appears to be in hand and following radiation procedure limits. It is not clear that the sea water used for cooling is being treated for removal of radioactive products but the reports are not encouraging.

It is too early to make any informed judgement except to think that this remarkable incident, given the magnitude of the earthquake and tsunami, is nearer in consequences to Three Mile Island than to Chernobyl.