Original URL: https://www.theregister.com/2011/03/17/fukushima_thursday/
Fukushima on Thursday: Prospects starting to look good
'Worst probably over' says Australian prof
Posted in Science, 17th March 2011 13:48 GMT
The story of the quake- and tsunami-stricken Fukushima Daiichi nuclear powerplant continues to unfold, with reports suggesting that the situation with respect to the three damaged reactors at the plant may soon be stabilised without serious consequences. The focus of attention has now moved to problems at a pool used to keep spent fuel rods cool. There remain no indications that anyone has yet suffered any radiation health effects, and the prospect is growing that this will remain the case.
Picture for layout purposes - ignore the out of date situation
The storage pool now at centre stage in the incident is a deep steel and concrete lined structure situated high up in the building housing the site's No 4 reactor. It is placed there for easy access when fuel rods are taken out of the top of the core. Spent fuel rods continue to generate heat for some time when removed from the reactor, and are placed in such pools to keep them cool, so preventing possible damage from their own internal heat. The water also blocks radiation emitted from the rods.
The No 4 reactor was shut down when the quake hit, and its fuel had been removed and placed in its pool. Storage pools need cooling pumps to operate if the water is not to heat up and eventually evaporate, but the levels of heating are comparatively low: normally, loss of cooling at a pool would not cause any problems for "a few weeks", according to the Japanese Nuclear Energy Institute (pdf).
Despite this, the No 4 building was hit by an explosion two days ago, which appeared likely to have been caused by a hydrogen gas buildup. As the No 4 reactor was un-fuelled, the only plausible source of hydrogen there would be steam reacting with the alloy casings of spent-fuel rods, suggesting that rods or parts of them are exposed above the water level.
Though the heating in spent fuel rods is hugely less powerful than that inside a reactor core, the situation is dangerous because the pools are not inside heavy overhead shielding as reactor cores are. If the spent fuel rods get really hot and start to melt, material from them will have a much easier path into the air than exists from the damaged reactor cores at the site.
Fears have been stoked by statements made by US Nuclear Regulatory Commission (NRC) chairman Gregory Jaczko in Washington yesterday, having spoken to an NRC team in Japan assisting with the incident.
"My understanding is there is no water in the spent fuel pool," Jaczko told reporters. "I hope my information is wrong."
Plant operator TEPCO has continued to suggest that the explosion and subsequent fires at No 4 may have had other causes than hydrogen and that the pools there and at No 3, while likely to have lowered water levels, are not in an urgent condition. Under this theory the high radiation levels seen near units 3 and 4 results mainly from steam venting from the core at No 3.
According to the Japanese Nuclear Energy Institute:
Even if the water level in the pools was to decrease sufficiently so that the fuel were exposed to air, the same level of overheating that can occur in a reactor accident would not occur in the used fuel pool because the used fuel assemblies in the pool are cooler than the assemblies in the reactor. It is highly unlikely that used fuel temperatures could reach the point where melting could occur, although some damage to the cladding cannot be ruled out.
The Japanese authorities and TEPCO nonetheless agree that getting more water into the spent-fuel pools at No 3 and No 4 is now their highest priority. Military helicopters have been used to dump water onto the top of No 3, whose roof has been blown off by previous hydrogen explosions: with the pools situated just below the roof area, it's hoped that some water at least went where it was wanted. High levels of radiation above the buildings had earlier caused the aircraft to be held back.
Helicopter water dumps not having much effect
According to WNN, the helicopter drops "did not appear accurate enough to be effective" and radiation around the Nos 3 and 4 buildings fell only slightly, indicating that "the effect at present seems marginal at best".
An operation is now underway to move fire trucks equipped with high-pressure pumps to the buildings and cool the pools with water pumped from ground level. Police water cannon may also be employed. This effort is being delayed by the need to bulldoze a path for the vehicles through heavy debris and mud deposited by the tsunami and subsequent explosions at the site.
Radiation levels at Nos 3 and 4 have been recorded as running at between 3 and 4 millisieverts/hour. Japanese regulators have now raised the permitted dose that site workers can sustain from the incident before being withdrawn from 100 millsieverts to 250: this would indicate that some personnel who have been involved since the beginning could only work shifts near the two buildings for another 40 hours before being forced to retire from the struggle.
There is a risk of radiation sickness after sustaining a dose of 1,000 millisieverts, climbing to 50 per cent at 2,000. Most sufferers with lower doses recover but at 4,000 millisievert the death rate has climbed to 50 per cent. There is no indication that anyone at Fukushima will be permitted to get even close to 1,000 however.
The primary health concern for the workers there is an increased possibility of developing cancer in the long term, though the fact that protective measures (suits, masks, breathing apparatus, heavily shielded and filtered control rooms etc) prevent radioactive materials actually entering their bodies removes much of this issue. Cancer being a very normal way to die in all wealthy countries, and only small numbers of people being involved, future investigations in decades to come will probably have difficulty assigning any deaths directly to service at Fukushima.
Radiation levels away from the reactor buildings themselves appear to be dropping as of the early hours this morning UK time. Japan's nuclear regulators reported as of the early hours this morning (UK time) that readings at the plant gates had dropped from 0.7 millisievert/hour to 0.3 over the previous 12 hours: workers would be able to endure such levels for months if required, as these hourly doses are equivalent to those sustained by everyone on Earth from background radiation every few weeks.
The situation with respect to the actual reactor cores at Fukushima no longer appears to be causing huge concern. The three damaged reactors (1, 2 and 3) are all being cooled using seawater pumped into their pressure vessels using the site's firefighting systems. Periodically steam is vented off from the pressure vessels via the suppression tanks: it would normally be held here to let short-lived radioactives generated in the pressure vessels decay before venting onward to atmosphere, but it appears that the suppression tank at No 2 (and possibly at No 3) has suffered a breach and is leaking steam directly.
Possibilities for disaster closing off one by one
This situation does constitute a break in primary containment and has led to heightened radiation levels nearby, but there is no sign of massive damage to the cores or release of long-lived radioisotopes in significant quantities. The radiation near the reactors is mainly emitted by fast-decaying isotopes in the steam which decay away within seconds or minutes of being created. TEPCO admits that portions of fuel rod continue to be uncovered at times, but residual heating levels in the fuel are now hugely lower than they were in the days immediately after the quake and the rods' heat-conducting alloy cladding helps transfer heat from the exposed portions to the water.
Provided that cooling can be continued, the pumps will gradually win the battle with the cores as the residual decay reactions in the rods die slowly away. Some redundancy and increased capacity seems set to be restored to the stricken plant soon, with reports indicating that a new power line will be up and running today. This will ease the plant's precarious dependence on mobile diesel generators and pumps brought in after the tsunami destroyed its own diesels.
"If the restoration work is completed, we will be able to activate various electric pumps and pour water into reactors and pools for spent nuclear fuel," a TEPCO spokesman tells AFP.
Health consequences for people who have not been at the site look set to be effectively zero from events so far. Significant emissions of dangerous radio-iodine, the main health hazard which could eventuate, don't look to be on the cards from the cores: and the spent fuel at the No 4 pool - not having been involved in a chain reaction for months - no longer has significant quantities of iodine-131 in it, this isotope having a half-life of 8 days.
The last remaining possibility of serious consequences would appear to be that of severe heating in the spent rods at No 4 melting them down and so perhaps causing significant airborne emissions of longer-lived radioisotopes such as caesium, or even heavy fuel metals. This would be unlikely to affect peoples' health given the evacuation and public protection measures already in place, but it might mean areas having to be abandoned for lengthy periods as occurred after Chernobyl.
However, airborne dispersal of radioactives from Chernobyl was driven by the fact that they were mixed with graphite (carbon) coolant which remained on fire for days, carrying carbon soot mixed with radioactives far from the site. Fires at Fukushima's cooling pool thus far have been doused within hours, and there seems no obvious reason why large amounts of rod material should become airborne even if rods do melt down in defiance of the Japanese industry's assurance that they won't.
"If someone can explain to me how those heavy particles, the heavy metals and even the non-gaseous fission products can be carried over a wide area, I'd like to hear it because I don't know a mechanism where that could happen in these sort of reactors," Professor Barry Brook told Australian media yesterday. Brook is director of Climate Science at the University of Adelaide's Environment Institute.
"To be honest, and I don't want to sound too optimistic, but I think the worst is probably over," added the prof, who is a longstanding advocate of nuclear power as a means to battle climate change.
In summary it is looking more and more probable that the death and injury toll from the Fukushima quake strike will be limited to the one worker killed in a crane accident and others hurt by the quake and subsequent explosions at the site, perhaps with some very minimal long-term radiation effects among site workers. All being well, nobody else will have their health damaged in any way, and prospects are good that most or all of the current evacuation zone will be re-opened in a reasonable amount of time.
There are still plenty of ways for things to go wrong: some have raised the possibility of the spent fuel rods' zirconium alloy cladding catching fire in a fashion that might throw material from them into the air. Barring this or some other disaster, however - given the undisputed fact that both quake and tsunami hugely exceeded the levels the powerplant had been designed to take - the picture at the moment is one of a vindication for Japanese nuclear power safety.
Needless to say this is not the general perception around the world - but hopefully the facts speak for themselves. ®