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Short Sharp Science: A New Scientist blog

Quake took out Fukushima cooling system before tsunami

Paul Marks, senior technology correspondent

It wasn't only the massive tsunami that stripped the Fukushima-Daiichi nuclear power plant of its essential cooling capability on 11 March: the magnitude-9.0 megaquake had almost immediately caused a failure of the number 1 reactor's primary emergency backup cooling system, which uses the steam generated by the reactor to push coolant around it.

Japan's public broadcaster NHK says the news was revealed in the nuclear plant's operational records released by the Tokyo Electric Power Company (TEPCO) on 16 May. "An emergency condenser system at the Number 1 reactor functioned for less than 10 minutes after the earthquake. The failure lasted for three hours," NHK World reports today. This early, pre-tsunami overheating may have speeded the reactor's meltdown.

The revelation will be seized on as further evidence that nuclear plants have no place in seismically active zones - even if they have high tsunami walls and backup power sources sited well above any potential high-water line.

Last week, prime minister Naoto Kan asked for - and got - the closure of what is thought to be Japan's worst placed nuclear plant, in Hamaoka, which straddles two subduction zones. It may re-open after a 15-metre tsunami wall is built in three years' time - but how backup cooling systems themselves cope with the severe physical agitation of megaquakes will doubtless now inform the wisdom of that decision.

Meanwhile, it is becoming increasingly clear that the Japanese government has been privately concerned that tsunamis posed strong threats to nuclear plants, while outwardly suggesting that the Fukushima magnitude-9.0 quake and 15-metre tsunami was "beyond expectations". The Mainichi Times unearthed a December 2010 report from the Japan Nuclear Energy Safety Organisation - a government body - that said:

Our analysis shows that a tsunami of a certain height - some 7 metres in the absence of a sea wall and some 15 metres if one were present - or higher would have almost a 100 per cent chance of damaging the reactor core.

The lack of action on such warnings is fuelling the growing notion in Japan that its nuclear power firms and the thicket of regulatory bodies (that often employ the former's retired staff, in a process called amakudari) have too many vested interests to regulate a safety-critical industry effectively. But similar worries are being expressed about the toothless nuclear regulatory situation elsewhere, such as the US.

There was some good news from Fukushima today, however. Nature News is blogging that the explosion at the plant's number 4 reactor building may not been caused by a hydrogen build-up from its own crippled reactor, but because hydrogen from the damaged number 3 reactor seeped through shared gas treatment pipework into reactors 4's building.

This might mean the fuel pile is unharmed and that consequently reactor 4 could take a lot less time, effort and risk to clear up.

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Reactor 4 was shut down for maintenance at the time of the quake. It's spent fuel pool was quite full and needed extra cooling which it lost in the disaster. It had been thought that the hydrogen for the explosion at reactor 4 came from boiling away the water in the spent fuel pool, not from the shut down reactor as stated in the blog post. If the hydrogen came from reactor 3, then the spend fuel pool at 4 may have been less of a problem than thought, especially by the NRC in the US which questioned TEPCO's priorities in applying cooling water early on.


> firms and the thicket of regulatory bodies (that often employ the former's retired staff, in a process called amakudari) that process the other way round (senior civil servants retiring to take posts in private sector)?


Firstly the passive condenser cooling which operated for ten minutes was deliberatly shut down by the operators, not by a failure of the system. The shut down was in response to the rate the pressure in the reactor pressure vessel was dropping. The operating proceedures, which were followed, were written based on assumptions that a fast pressure drop could result in more damage to the core than than the immediate need for cooling at that point. However the tsunami hit while the system was shut down and it was three hours before any further cooling was initiated. Read the actual report - it's quite clear the system was deliberately shut down per the manual.

Secondly Unit 4 reactor was empty of fuel, shut down for routine maintenance prior to the quake so "crippled reactor " in reference to unit 4 is incorrect. However the Unit 4 spent fuel pool on the upper floor of the building had not only previously used spent fuel but all the active fuel bundles temporarily removed from the reactor. It was feared that the hydrogen explosion in unit 4 was caused by hydrogen generated in the spent fuel pool due to overheating and fuel element damage when the water level dropped low enough to stop cooling the fuel bundles. We now know from analysis of the water in the pool that it is very unlikely those fuel bundles have (yet) been damaged, and the current theory is the hydrogen which caused the unit 4 explosion was generated in unit 3. The "yet" refers to the fear that the fuel pool is already structurally damaged and a further aftershock could cause it to collapse or simply leak faster than the rate it can be filled using the current method of spraying from a concrete pumper truck.

The cooling situation for units 1 through 3 is relatively stable though generating on the order of 20 tonnes/hr of contaninated water which needs to be stored and treated. At this point the gravest danger is likely the unit 4 spent fuel pool situation.

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