Nuclear reactor incidents in Japan
The Fukushima nuclear power plant
By John Price
The earthquake at 14.46 local time on 11 March 2011 affected at least fourteen nuclear reactors at power stations in Japan. I write this piece based on what I have seen and heard in the media and my general understanding of the safety philosophy of boiling water reactors (BWR). It is difficult to sort out these reports and even the International Atomic Energy Agency (IAEA) has admitted some errors in their reporting.
Most of the reactors have already achieved safe shut down and have established core cooling. However, it is clear that Fukushima Daiichi Units 1-3, are now working within a design level known as “emergency level”. At this level of damage it is intended that a major release of radioactivity is avoided but the reactor is likely never to operate again, at least not without major repairs. The key issue is the extent of damage to the fuel core of the reactor. The avoidance of core damage in any nuclear power station is achieved by a long series of cascading safety measures known in safety philosophy as, “defence in depth”.
The first point of great importance is that all the reactors shut down when the earthquake occurred. At that time the control rods entered the core and shut down the nuclear reaction.
When the reactor shuts down it does not immediately produce zero power. The core has in it large quantities of radioactive products of the reaction, which still take time to decay. After shut down the power levels fall to a small proportion of what the power station was generating. The information I have seen suggests 7 per cent of the energy continues immediately after shut down and this drops to 2 per cent within an hour and 1 per cent within a day.
Even at these lower levels of power production, the reactor core has to be cooled by circulating water through it. In the BWR, the water normally boils as it passes through the core and in doing so the steam carries away heat from the core. After shut down it is necessary to supply a flow of water to the core and to keep cooling water pumps operating. The motive power for the pumps is off-site power, coming from the electricity grid or, if this not available, from emergency diesel generators.
An important feature of the Japanese nuclear incidents seems to be that, in some of the plants, electricity failed. That is, there was a blackout. According to what I have seen, this blackout was caused by the arrival of the effects of the earthquake or perhaps the tsunami. These effects may have caused fires or damaged wiring or otherwise blacked out parts of the stations.
There is a second level of electric supply in the power station which uses batteries. This smaller level of electricity supply is not enough to drive the pumps, but should enable some control of what is happening in the reactor by allowing valves to be operated and conditions to be monitored. As far as I can tell, these systems are operating in the damaged reactors, though there is some debate about whether all the data being collected is correct.
In the power stations affected there seems to have been a range of successes at this point. In some stations, cooling water pumps may have worked after the earthquake but in others there may have been failures.
If the cooling fails at this stage, the plant moves to a more difficult stage in the safety cascade which is called “passive cooling”. Having lost electricity, the cooling of the core must be accomplished by water already in the reactor circuit. The water circuit heats up and increases in temperature, and at some point will blow the safety valves, releasing steam. The steam is slightly radioactive; the radioactivity is in a form called tritium. This isotope is not normally thought to be a safety hazard; it has soft Beta radiation and is dispersed quickly through the atmosphere. If there are solid particles in the discharge, then these could be radioactive, but this depends on their origin and composition.
At Fukushima Daiichi 1 a building exploded on Saturday. There was a larger explosion at Daiichi 3 on Monday and today a report of an explosion in Unit 2. On the video of the Unit 3 explosion I can see flame immediately after the explosion. This indicates a hydrocarbon-air explosion has occurred. The operators are describing this as a hydrogen explosion. The source of hydrogen has not been explained.
My lead possibility is the dissociation of the water, resulting in hydrogen gas. Such a reaction is aided by higher temperatures. Hydrogen is also on the plant because the reactor cooling water is injected with hydrogen to suppress stress corrosion cracking in stainless steel components. Another source of hydrogen is the electrical generators, which are probably cooled with hydrogen gas. These last two could be the cause of the hydrogen build up if their pipework has been damaged, however they are unlikely to have caused multiple explosions.
I saw yesterday a plume of black smoke. If such a plume occurs it contains soot or carbon, indicating that the fire is from a hydro-carbon fuel. Unless diesel fuel has been ignited, I cannot explain this, since there will be only limited quantities of hydrocarbon - mainly electrical insulation - in the plant. I don’t believe such a plume could come from the nuclear circuit.
Even if all these systems were to fail there are still some safety systems left. The systems which may still be intact are the reactor containment building, the reactor pressure vessel and the cladding on the nuclear fuel rods. It is not until all of these are breached that large releases are possible. As the time from shutdown increases, after a few days or weeks, the likelihood of avoiding damage to these systems increases.
If some of the fuel rods are not perfectly sealed then releases can occur in the form of steam. This is not, in my view, defined as a meltdown. A meltdown is an extremely high temperature in the core which could cause the fuel cladding to melt and fuel and cladding to drop to the bottom of the reactor vessel. In the current situation, this material would then be cooled by water and quickly become solid again. Real meltdown, where a hot liquid mass burns through the vessel, probably only occurs if the nuclear reaction shut down is not achieved at the beginning of the incident. As I have said, shut down was achieved on all reactors.
Breakage of the fuel cladding may occur at a much lower temperature due to internal pressurisation and a damage phenomenon known as “creep”. I believe that meltdown has been averted in all the plants, though there are some reports saying there has been “partial meltdown”. World Nuclear News is today reporting that Tepco made a notification at 8.50 pm (Monday 14 March) that some fuel rods were presumed broken based on radiation detected.
There is another issue in the safety cascade which is apparently affecting Daiichi units 1-3. After blowing off the safety valves there is a need to supply make up water to the reactor circuit to continue cooling the core. The fact that sea water cooling has been mentioned indicates that this step is difficult in some of the plants, in particular, Daiichi 2 where there is also some discussion about the correct operation of a water level gauge. I suspect that one of the results of the tsunami is that fresh water supplies to the local area may be short so the operators will pump water from the sea to make up the cooling water supply. This will irrevocably damage the reactor.
There have also been recent mentions of Onagawa where there are three reactors. On Saturday there was a fire there, but probably nowhere near the reactors, and it appears to have been extinguished. However in certain areas of the plant, a fire might pick up small particles and blow them into the air, which may explain recent reports of radioactive releases at this plant.
This is not Chernobyl! Chernobyl was a fire. This reactor contains water that does not burn. There are also numerous other differences. It is more like Three Mile Island 1979 where major core damage did occur due to loss of cooling, but no significant radioactive releases occurred.
The most reliable sites for news I have found are World Nuclear News www.world-nuclear-news.org and the International Atomic Energy Agency web site www.iaea.org.
Dr John Price is a former member of the Safety Policy Unit of the National Nuclear Corporation UK, formerly a Professor at Monash University and now a private consultant.