Australian nuclear experts respond to the damage to several Japanese reactors in the wake of Friday’s 8.9 earthquake in Japan.
Dr John Price is a former member of the Safety Policy Unit of the National Nuclear Corporation UK, a Professor at Monash University and now a private consultant.
“There are incidents affecting at least five 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 as to which of the 14 reactors are involved and there is sometimes conflicting information.
“I think some of the reactors may be shutting down within the design level known as “emergency level”. At this level of damage, major release of radioactivity is avoided but the reactor is likely never to operate again, at least not without major repairs. I think major nuclear core damage has been averted. 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 14:46 local time on 11 March 2011. 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% of the energy continues immediately after shut down and this drops to 2% within an hour and 1% 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 the cooling water pumps operating. The motive power for the pumps is off-site power which comes 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 black out. 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 fire in the diesel generator building, 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 monitoring conditions and operating valves.
“In the power stations affected there seems to have been a range of success at this point. In some stations 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, though slightly radioactive, the radioactivity is in a form which is not threatening to the local population and is soon dispersed in the atmosphere.
“There may be a system in place whereby the operators try to keep the steam inside the containment building to completely avoid release. I am not certain whether this was the case for these stations.
“At Fukushima Daiichi 1 a building next to the reactor exploded. I can see no flash or flame on the video I have seen, so I think the theories attributing this explosion to a fuel-air or hydrogen-air explosion may not be correct. If I am correct about the intention to avoid releases of steam, then the pressure may build up in the buildings from the released steam. At Daiichi 1, in my guess, the steam pressure has exceeded the turbine building’s design limits and caused the explosion.
“Even with all these failures 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 success of avoiding damage to these systems increases.
“If some of the fuel rods are not perfectly sealed then releases can occur in the steam. This is still not defined as a meltdown. A meltdown is an extremely high temperature in the core which could cause the fuel cladding to melt. I believe that meltdown has been averted in all the plants. Overnight the reports indicated that the operators feel that the situation was almost under control at all reactors.
“There is another issue in the safety cascade which is apparently affecting Daiichi 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 3. I suspect that one of the results of the tsunami is that fresh water supplies to the local area may no longer exist, so the operators may be considering pumping 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 also 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, water does not burn. It is more like Three Mile Island but it is not clear yet whether there has been any core damage.
“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.”
Please note that this piece was adapted from an article written for www.news.com.au
Professor George Dracoulis, Department of Nuclear Physics, Australian National University. The following notes were written by Professor Dracoulis on Saturday 12 March:
“When there is a power failure or any other incident, control rods would automatically be inserted into the core of the reactor so that the fission process goes below critical and turns off.
“Reactors usually have a number of sources of electricity, usually independent electricity lines, several backup diesel generators which don’t need electricity, and some level of battery backup.
“Because some of the fission products that are produced in the fuel rods when the reactor is running have half-lives of a few hours or a day or two, as they decay, they generate heat, even though the main fission process has been stopped. That is why you have to continue to provide cooling to the core for a few days.
“There is no question of a nuclear explosion but because of the heat, unless you keep some cooling to the core, you could cause some physical damage to the fuel rods (which are made out of very high temperature zirconium alloy) – very hardy.
“This reactor is one of 6 at a plant on the sea shore and like most Japanese reactors, it draws water from the sea for cooling, and discharges water at a slightly higher temperature, under environmental controls.
“Just like the Newport gas-fired power station on the Yarra River, back of Footscray in Melbourne. That water is not radioactive, it is in a secondary circuit separate from the closed circuit inside the reactor that has water that acts both as a coolant and a moderator.
“A BWR reactor (Fukushima #1 is a fairly small reactor (about 0.5 GWe) built in the early 70s) is a little different to a PWR, the other common design, because the water at the top actually boils, directly producing steam to drive a generator. In a PWR, the core is kept under pressure so that the water doesn’t boil, and heat is transferred through a heat exchanger to another closed water circuit that produces steam to drive the generator.
“If the water continues to boil you might get slightly higher pressure and radiation levels.”
Facts about other nuclear incidents:
(i) The operator bypassed all the procedures that would normally control the reactor shutdown to do some unapproved tests.
(ii) The reactor was flawed (would never get a license in the west) because it used graphite as a moderator and water as a coolant, and water can also act as a moderator. Modern reactors have water as a coolant and a moderator. If you lose the water, there is no moderation of neutrons so the fission process stops.
(iii) The Russian Reactor had no Containment Vessel. When there was a chemical explosion (not a nuclear explosion), this blew off the roof and threw radiaoctive material into the atmosphere.
Three Mile Island (TMI)
The issue there was that they didn’t realise they had lost the water during a shutdown procedure – a valve that was supposed to closed remained open. In the end, the core got so hot that it partially melted. No explosion. No damage outside the containment vessel.
Misinformation to the public resulted in evacuations, but there were no dangerous releases to the atmosphere of radioactive gases.
2007 Earthquake – The radioactive water spills were from drums of very low-level waste water that were knocked over in the yard, not spills from the reactor itself.