Fukushima -- possible alternate scenario?

[FusionJim]

Some time has passed since this accident happened but this morning I couldn't sleep and a though occurred. Back in 2011 when the tsunami flooded the diesel generators and knocked out offsite power to the Fukushima power plant, yet the reactors themselves were ok and their ability to work (at least before the meltdowns) wasn't crippled. Now they shut the reactors down according to rules, but this got me wondering. Given they probably quickly realized the dire situation that there is no offsite power and the diesels are dead and gone , couldn't they make a risk assessment and essentially shut down only 5 of the 6 reactors at the plant meanwhile use the sixth reactor (or whichever in number) at some minimal power level to keep generating electricity to be able to power the circulation pumps for the other shut down reactors?

Given the situation they faced, and I'm sure they understood it just as well back during when all of this was still unfolding, wouldn't it have been a an overall safer way to deal with the issue?

Sure I might not know some stuff related to how a nuclear plant must be dealt with during an emergency so please tell me what was there to stop them from doing this, but overall it seems like a legit idea.

 

[Astronuc]

Some time has passed since this accident happened but this morning I couldn't sleep and a though occurred. Back in 2011 when the tsunami flooded the diesel generators and knocked out offsite power to the Fukushima power plant, yet the reactors themselves were ok and their ability to work (at least before the meltdowns) wasn't crippled.

The earthquake knocked out the transmission systems (transmission towers collapsed). The plant site suffered damage from the earthquake, the the tsunami did much more damage to the site, including flooding ot the basement, and not only the diesel generators, but also the circuits. The tsunami also destroyed the fuel supply for the diesel generators.

I'm not sure about a common bus, or distribution system within the Fukushima Daiichi site. However, Units 4, 5 and 6 were shutdown at the time of the accident. Unit 4 had been defueled (all fuel removed from core) pending some inspection and maintenance.

Unit 5 (a sibling 784 MWe unit to Units 2, 3 and 4) and Unit 6 were also shutdown at the time of the accident. The would have had to do a lot of inspection to ensure no damage to the various structuras and safety systems (days or weeks), and then startup would have taken several days.

https://en.wikipedia.org/wiki/Fukushima_Daiichi_units_4,_5_and_6#Units_5_and_6

Both reactors were offline at the time the earthquake struck (Reactor 5 had been shut down on 3 January 2011 and Reactor 6 on 14 August 2010), although they were still fueled,
. . . .
On 17 March, Unit 6 was reported to have a single operational air-cooled diesel-generator as well as dry switchgear (inside the only GE Mark II reactor building) and this was to be used to power pumps in Unit 5 to run the Make-up Water Condensate System (MUWC) to supply more water. Preparations were made to inject water into the reactor pressure vessel once external power could be restored to the plant, as water levels in the reactors were considered to be declining. NISA reported that connections from the grid to all units was complete 20 March through new cables and transformers.

Certainly 9 days was too late.

Core damage is believed to have started in Unit 1 after 4 hours from the earthquake. Units 2 and 3 had their core damage starting about 48 hours to 72 hours later. Unit 4 had the explosion and fire ostensibly from hydrogen leaking from Unit 3 through the shared ventilation system. Unit 4's core had been removed, but the spent fuel was in the spent fuel pool to the side of the reactor.

See - Event sequence following earthquake (timing from it: 14:46, 11 March)
https://world-nuclear.org/informati...y/safety-of-plants/fukushima-daiichi-accident

See also - https://www.iaea.org/bulletin/learning-from-fukushima-daiichi-factors-leading-to-the-accident

 

[gmax137]

The operators didn't decide to shut the operating units down - they were tripped automatically due to the earthquake, as detected by seismic event monitoring equipment.

The IAEA report (start with the director general's report) is also a good source of information. The DG report and the five "technical volumes" are available as pdf files here:

https://www.iaea.org/newscenter/new...generals-report-on-fukushima-daiichi-accident