According to recent media reports an explosion has blown the roof off an unstable reactor north of Tokyo. The reactor is Fukushima Daiichi nuclear power station’s unit no 1. World Nuclear News reports:
Television cameras trained on the plant captured a dramatic explosion surrounding unit 1 at around 6pm. Amid a visible pressure release and a cloud of dust it was not possible to know the extent of the damage. The external building structure does not act as the containment, which is an airtight engineered boundary within. The status of the containment is not yet known.
Here’s some background on Fukushima Daiichi and the events leading up to the incident.
Number of reactors: The Fukushima Daiichi nuclear power station has six reactors. According to the Tokyo Electric Power Company (Tepco), when the quake hit three of the reactors were shut down for periodic inspection (units 4, 5 and 6). The other three (1, 2 and 3) were shut down in response to the quake.
Type of reactor: The reactors at Fukushima Daiichi are boiling water reactors (see diagram below).
In a typical commercial boiling water reactor the reactor core creates heat, a steam-water mixture is produced when very pure water (reactor coolant) moves upward through the core absorbing heat, the steam-water mixture leaves the top of the core and enters the two stages of moisture separation where water droplets are removed before the steam is allowed to enter the steam line, the steam line directs the steam to the main turbine causing it to turn the turbine generator, which produces electricity. The unused steam is exhausted to the condenser where it is condensed into water. The resulting water is pumped out of the condenser with a series of pumps, reheated, and pumped back to the reactor vessel. The reactor’s core contains fuel assemblies which are cooled by water, which is force-circulated by electrically powered pumps. Emergency cooling water is supplied by other pumps which can be powered by onsite diesel generators. Other safety systems, such as the containment cooling system, also need electric power.
What caused the problem?: After a reactor is shut down it needs to be cooled. According to Ron Chesser, director for the Center of Environmental Radiation Studies at Texas Tech University:
Reactors are not like your car that you can turn off and walk away. They’re going to continue generating a great amount of heat until the core is disassembled. Without cooling water, then you stand a real chance of a meltdown of core that could result in a large release of radiation, potentially.
Water for cooling is relies on electric pumps. These are normally powered by the electricity grid but in an emergency most reactors rely on diesel generators. When the reactors at Fukushima Daiichi automatically shut down, power from the grid was lost and the back-up diesel generators supplied powered the pumps. However after an hour the diesel generators suddenly stopped. This triggered an emergency.
According to a report in World Nuclear News a lack of cooling caused pressure within the containment of Fukushima Daiichi 1 to rise:
The company has decided to manage this "for those units that cannot confirm certain levels of water injection" by means of a controlled release of air and water vapour to the atmosphere. Because this water has been through the reactor core, this would inevitably mean a certain release of radiation. The IAEA said this would be filtered to retain radiation within the containment.
Some news agencies are reporting that the explosion was caused by the release of hydrogen gas. According to the BBC:
The BBC’s environment correspondent Roger Harrabin says he understands the blast at the nuclear plant may have been caused by a hydrogen explosion – also one of the possibilities laid out by Walt Patterson of Chatham House. "If nuclear fuel rods overheat and then come into contact with water, this produces a large amount of highly-flammable hydrogen gas which can then ignite," our correspondent says.
According to a recent report by AFP (posted on the BBC’s site) "Government spokesman says the nuclear reactor container at the Fukushima-Daiichi plant has not been damaged, and the level of radiation has dropped following the explosion earlier on Saturday".
Scientific American has posted an interview with Scott Burnell, public affairs officer at the U.S. Nuclear Regulatory Commission (NRC) where Burnell explains how nuclear reactors are cooled during an emergency.
Previous safety problems: In 2002 Tepco was involved in a major scandal when it [was accused of falsifying] data to cover up the discovery of cracks in pipes at Fukushima Daiichi and another nuclear plant, Kashiwazaki Kariwa. According to a report by Steve Connor in the Independent:
Officials from Tepco found that more than half of the 61 pipes it had inspected in its Fukushima plant’s number three reactor showed signs of damage. Corrosion was so severe in six of the stainless steel pipes that they measured less than half their original thickness.
Akira Kawate, vice-governor of the Fukushima prefecture, said Tepco and the national government had covered up details of the inspections since at least 2000.
According to Ikuko Kao at Reuters, the scandal delayed Tepco’s plans to build two more reactors at Fukushima Daiichi.
The MOX controversy : A recent controversy involving the Fukushima Daiichi nuclear plant has been the use of mixed-oxide (MOX) fuel. MOX is a mixture of plutonium and uranium. In 2009 Greenpeace protested against shipments of MOX from France to Japan. According to Reuters and Japan Today, the no. 3 at Fukushima Daiichi was to be loaded with MOX in 2010.
The US Nuclear Regulatory Commission has a FAQ page about MOX.
Fukushima Daiichi No 1 is a
Mark 1 General Electric Boiling Water Reactor. The cut-away diagram immediately below shows the layout of this kind of reactor (from US NRC Reactor Concepts Manual). The diagram below that provides labels for the key features of the containment. The bottom diagram comes from Magdi Ragheb‘s ‘Containment Structures‘. See his paper for an explanation of the diagram (Via The Capacity Factor).
The New York Times has an interactive feature with photographs and a cut-away model of the reactor.
Cooling system fails at No 3 reactor: Reuters reports:
The emergency cooling system is no longer functioning at the No.3 reactor at Tokyo Electric Power Co’s Fukushima Daiichi nuclear power facility, requiring the facility to urgently secure a means to supply water to the reactor, an official of the Japan Nuclear and Industrial Safety Agency told a news conference.
Comparisons to Three Mile Island: A number of reports raise the possibility of a meltdown. In 1979 a meltdown occurred at Three Mile Island Unit 2 nuclear power plant near Middletown, Pa. According to US Nuclear Regulatory Commission "it led to no deaths or injuries to plant workers or members of the nearby community". NBC’s Today show interviewed Richard Lester , head of the department of nuclear science and engineering at MIT about the possibility of a meltdown at Fukushima Daiichi:
Can you give us a sense of what a meltdown would entail? what would happen in that process?
One possibility is that the fuel or some part of the fuel in the core would melt and would gather at the bottom of the reactor vessel and would stay there. that is essentially what happened at the three-mile iceland nuclear power plant in 1979 . there was no significant release of radioactivity outside of the vessel. if a partial meltdown happens in the core, that’s what we would hope to see. of course, let me emphasize that at this point, it’s not at all clear that any melting of the fuel has taken place.
Thoughtful discussion nearly impossible, writes Christine Russell: Writing in the Atlantic, Russell notes that that in the Three Mile Island incident there was "tremendous uncertainty, confusion, and contradictory information about what was actually happening and what might happen." She notes that there are few reporters working today who specialise in covering nuclear energy and technology. She writes:
The current Japanese nuclear energy emergency will undoubtedly have a profound global impact on public views of the safety hazards–and siting–of old and new nuclear power plants. And the speed of communication today–unheard of in the pre-cell phone, pre-Internet Three Mile Island era–means that news and speculation about what is happening, or might happen, in Japan is traveling so far and fast that thoughtful discussion is nearly impossible.