Fukushima on Thursday: Prospects starting to look good
'Worst probably over' says Australian prof
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. ®