Research scientist and writer Michael R James writes: The earthquake offshore from Sendai began at 2.46pm (JST) Friday March 11. To clarify the nuclear incidents in Japan, I have attempted to summarise them. But news is still breaking, the most recent being a fire in reactor #4 this morning.
Japan has 55 nuclear reactors distributed around 17 sites. There are 10 reactors clustered in two nearby sites (six at Daiichi and four at Daini) at Fukushima just south of Sendai and about 240 kilometres north of Tokyo. Only seven of these reactors were operational at the time of the earthquake; three reactors at Daiichi were closed temporarily for maintenance and inspections. The earthquake caused all operating reactors to automatically shut down (control rods are inserted, which stop the nuclear fission reaction by absorbing neutrons).
After that, the precise timing of events is less clear but it has been reported that emergency diesel generators, which had kicked in to run the cooling system after the electrical power grid failed, shut down about an hour after the earthquake. It is assumed that this failure was caused by flooding of the generator and electrical control rooms by the tsunami. Several design flaws are revealed by this: first, even with several back-up systems (two independent diesel generators, some use three) a single event, such as a tsunami or fire, can knock out all back-ups (i.e. the redundant back-up systems are not truly independent); and second, if the electrical control room is low and gets flooded, even bringing in a new generator will not necessarily restore system control. The electrical rooms at these plants are described as basements. Experts have interpreted this as a design flaw due to the assumption that the tsunami seawalls would keep the site dry. This comprehensive failure appears to have occurred in all three operating reactors at Daiichi and less severely at three of the four operating reactors at Daini.
After failure of the diesel generators, another back-up kicked in: a battery system that provides up to eight hours of operation of the cooling pumps. Details are not yet available but presumably differences in efficiency or restoration led to insufficient cooling of, so far, Daiichi Units #1 and #3 and Daini Unit #1. When cooling fails in a fully operational reactor or shortly after shutdown, the water quickly boils off creating increasing steam pressure in the core containment vessel and exposing the dry fuel assembly to increasing temperatures and radiation. The zirconium metal assembly reacts with the steam to give hydrogen and oxygen, an explosive mix.
Daiichi Units #1 and #3 have had explosions in their outer shells where the excess pressure in the inner core had been vented deliberately in an attempt to allow pumping in of water (doped with boric acid to moderate the neutrons and stop further fission). The explosion at No.1 reactor occurred at 340pm (JST) on Saturday, 26.5 hours after the earthquake. The explosion at #3 occurred on Monday.
At 6.14am on Tuesday an explosion occurred at Daiichi Unit #2. Unlike the previous events, the plant operators reported that this one was exacerbated by a valve that was stuck for many hours and that may have left the core dry for the longest of the three reactors. This would explain the larger spike in radiation detected after the explosion but, despite some media claims, it seems unlikely the main containment vessel was breached. The radiation reached 400 millisieverts/h, which is 40 times the allowable dose for a nuclear worker per year. At press time it is very unclear if this high rate was a brief spike and has returned to much lower levels. In any case this is not the level outside the plant, nevertheless the authorities extended the exclusion zone, previously 20 kilometres, to 30 kilometres.
The authorities had earlier warned that an explosion may happen at Daini Unit #1 but it seems that by Monday the normal cooling systems were reinstated for all Daini reactors.
These explosions are not enough to breach the concrete and stainless steel shell of the inner core but the detection of caesium-137 and iodine-131 indicates that at least part of the fuel rods have been exposed and damaged. (Water levels inside the core are unknown because the gauges have failed, itself indicating damage.) If left exposed for too long the fuel pellets melt and fall to the bottom of the core. If it continues, all the fuel melts into a pool at the bottom of the core, continues to overheat — and at some point with loss of neutron moderation in the molten pool it becomes self-accelerating — until nothing can contain it and it burns through the core, i.e. the China Syndrome.
Contact with other material or any liquid, along with accumulated hydrogen will result in explosive dispersal of the highly radioactive material — not as a nuclear bomb — but a far worse kind of dirty “bomb” in terms of radioactive contamination of hundreds or thousands of square kilometres depending on prevailing winds. As time moves on the likelihood of this kind of catastrophic event becomes less and less likely because the energy released by the fuel diminishes quickly. Some experts believe the danger point for such a total meltdown has been passed but Tuesday’s events make any outside assessment very problematic.
On Monday, the Japanese Nuclear and Industrial Safety Agency said that as many as 160 people may have been exposed to radiation around the plants, and Japanese news media said that three workers at the facility were suffering from full-on radiation sickness but only one is showing overt radiation sickness. Consequences of Tuesday’s leaks are not yet available.
Little discussed so far is that all these sites have storage pools of spent nuclear fuel. Spent fuel contains the highest level waste in the entire fuel cycle. The long-lived byproducts of uranium fission including uranium-234, neptunium-237, plutonium-238 and americium-241 are the truly nasty problematic ones because they persist for thousands of years. Caesium-137 and iodine-131 are measurable indicators of fuel pellet breakdown/contamination because they are the most volatile and will be in any steam release, deliberate or core breach.
But there is also a high level of relatively short-lived radionuclides, which release lots of heat via their fast radioactive decay. So the spent fuel is kept on-site in deep pools of water for about six years before they are more amenable to transport for fuel reprocessing or long-term storage. This allows much of the short-lived elements to decay. An added dangerous ingredient in the cocktail is that these reactors use mixed-oxide (MOX) fuels, which means they have plutonium plus uranium in the fuel pellets.
These storage pools need constant cooling similar to the way a shutdown reactor core needs cooling. So two extra potentially unsafe complications arise: contamination by dirty sea water and failure of cooling systems. With the spent fuel encased in ceramic at the bottom of the 15-metre pools, it seems there is unlikely to be an immediate radiation safety issue from the flooding. But not if cooling is not restored. Indeed Japan’s prior worst and most controversial nuclear accident occurred after the 2007 Chūetsu offshore earthquake shut down all reactors at the world’s largest nuclear power facility, the Kashiwazaki-Kariwa site in Niigata. In addition to radiation leakage from reactor buildings, there was also leakage from a spent fuel storage pool and 40 drums of waste. The seven reactors producing about 8GW of power were closed for almost two years.
Later on Tuesday (before 10am) another explosion followed by fire was reported for one of the reactors that was not operational at the time of the earthquake (Daiichi #4). This is considerably more worrisome because the heat output of fuel from a reactor shut down months earlier should require a lot less cooling than recently shut down reactors. So this event indicates that cooling for all six reactors at Daiichi may have failed, and the remaining two reactors (#5, 6) may be at risk. The reactors at Daini might still be at risk, although the management has previously indicated the cooling problems (for Daini #1, 2, 4) were less severe. Apparently the regular cooling pumps were reinstated by Monday and all Daini reactors are considered out of risk. The authorities have not been clear about whether any of these other shut down reactors (Daiichi #4-6) (or their storage pools, see below) were being cooled by standard pumps or by emergency seawater.
This most recent fire is now reported by the New York Times to be in a temporary storage pool for reactor #4 to which the fuel had been transferred while maintenance is performed. This pool is not like the 15-metre deep permanent storage pools discussed above but much smaller ones near the top of each reactor. It explains why the contamination issue is very grave, because these open-air fires release more dangerous radioactivity and the work site becomes an extremely dangerous place for the workers. Most staff have been evacuated leaving a small crew of 50 to fight the fire and keep the sea water cooling of all reactors (and presumably storage pools). The radioactivity is released by such fires and wafted by the fire to high altitudes where it can be distributed over long distances. The precedent of the KK accident and the apparent simplicity of topping up the storage pools suggest that there is likely to be unexplained complications such as leaks in the pools, possibly from the explosions in the adjoining reactors.
This latest development appears to be a replay of the events of the 2007 Kashiwazaki-Kariwa accident.