So, the one year anniversary of the Tohoku earthquake, tsunami and Fukushima Daiichi accident made me look through
this thread the other day, just to see the general reactions from both J-Bore and us others. The reactions and anxiety about the nuclear accident was particularly striking to me, given that we were all reacting to the VERY scarce information to come out of TEPCO and the japanese government. I thought I'd make a thread about this now, since a lot of info was hidden away for many months, and lots of investigations and reconstructions have since been made. Nuke geekery go go.
MeltdownsIn the old thread, while the crisis was going on, everyone hoped and was made to believe that meltdowns in any of the three operating reactors (1, 2 and 3) was very unlikely even with total loss of power, and that the containment structures would prevent a large-scale contamination incident even if meltdowns took place.
What we know now, from the released TEPCO data and various simulations and reconstructions (most notably one done by NHK along with senior experts in material science and nuclear engineering) is that all three reactors suffered total fuel meltdown and melt-through a fairly short time after the tsunami hit. In reactor #1, the water level in the reactor pressure vessel (RPV) sank to the top of the fuel rods in approximately one hour after total loss of power. The same night (March 11:th), the fuel rods were completely uncovered, and reactor #1 suffered a total fuel meltdown at around 1AM on March 12, and the molten fuel breached the RPV and started falling into the bottom of the containment structure at around 2AM on March 12.
Noone had any idea that this was happening, because the only instruments working in the control room after the total loss of power were the RPV water gauges, which were only powered up and working around 5PM (the technicians brought in 12V batteries from vehicles on the site). We also now know that these gauges were showing faulty values, since they are calibrated using a water level inside an instrument to measure pressure. The calibration water had been evaporated by the heat, causing the water gauges to show way too high readings, and values that actually increased as the water boiled away. The same thing happened in the TMI plant in the Harrisburg incident. Regulators and nuclear scientists have been clamoring for a better system, but it is yet to be implemented in japanese reactors.
On the evening of March 11:th, more batteries was brought in and the containment pressure gauge was powered up, showing a significantly elevated pressure. The steam pressure in the RPV had started leaking out into the containment structure, so the steam needed to be vented to prevent a containment breach. Prime minister Kan authorized the venting, but all the valves had to be operated manually, so it took well into the next day for the venting to be initialized. The steam was vented through the filter stack, but radionuclides still escaped to contaminate the surroundings. About an hour after the first venting, the hydrogen explosion at unit #1 took place.
In the following day or two, reactor #2 and #3 also suffered total meltdown and melt-through, with reactor #3 suffering a similar (but more powerful) hydrogen explosion as #1, and reactor #2 suffered an internal explosion and breach of the suppression torus at the bottom of the containment structure. This led to the biggest dispersion of radioactive material of the entire incident.
The Isolation CondensersIn reactors of the type used in Fukushima Daiichi, there is really only one passive cooling system for the reactors: the isolation condenser (RCIC). It's a water-filled tank at a high level in the reactor building. If steam pressure builds up and the water level drops in the RPV, you can open valves to lead steam from the RPV through a piping system into the isolation condenser, where it is cooled by the water in the tank, condenses to water again, and is fed back to the reactor. The problem is, the valves for the RCIC are powered-open, so if a total loss of power incident occurs, the valves have to be opened manually. In the US, this is a well-known fact, and is a routine that is in the NRC-audited manuals. US plant personnel perform drills regularly to be aware how to open the RCIC valves manually.
In Japan, this routine has not been established, and the personnel at Fukushima Daiichi were completely oblivious to the fact that the RCIC valves were shut when power was lost. At around 4PM on March 11:th, two technicians were sent to do a visual inspection of the RCIC in reactor building #1, but the technicians turned back when they encountered elevated radiation readings when entering the reactor building. If the technicians had gone through with the inspection, the RCIC might have been put in use manually, and may have prevented or at least significantly delayed the meltdown. It was not until after 5PM, when the UPS batteries temporarily re-powered the control room bridge, when the valves were opened. Shockingly, the valves were ordered shut around an hour later, because of fear that the RCIC cooling water had evaporated, and that the tank might be breached, allowing unfiltered steam to escape.
Subsequent simulations and reconstructions have confirmed that the RCIC should definitely be in use at all times during a loss of power incident. Even with minimal cooling water in the RCIC tank, it would have contributed in stabilizing, or even raising, the cooling water level in the RPV, thereby delaying or even preventing a fuel meltdown. Also, the researchers commissioned by the NHK unanimously ruled out any risk of piping breaches in the RCIC, there would not be enough steam pressure. The data and reconstructions I've looked at only concerned the happenings at reactor #1, but I assume the same confusion probably happened in the two other reactors as well.
AftermathThe evacuation not only forced around 150000 people to leave their homes, but also prevented tsunami survivors to search for missing family members. Trapped survivors inside the evac zone may very well have been saved, or at least recovered earlier, but the first body searches in areas close to the plant, like Okuma, were not conducted until almost two months after the tsunami hit. The 20km radius exclusion zone remains a no-entry area, with limited exceptions being granted for residents who are thoroughly screened on entry and exit.
A massive decontamination operation costing billions of dollars is now being initiated, with some estimates claiming that 3% of the total land area of Japan need to be sanitized in some way. Some areas that became hotspots after falling snow brought down radionuclides to the ground may stay uninhabitable for decades, due to the long half-life (30 years) of Cesium 137.
Info from JAIF:
"In advance of decontamination work for evacuation area, which is supposed to be conducted by the central government, MOE conducted radiation measurement at every 100 square meters in the area. Detected highest radiation is 89.9μSv/hr."
With 89.9μSv/hr, the yearly dose rate is
787,5 mSv/year. The maximum allowable for civilians is
20mSv/year. MEXT has decided to lower the threshold for cumulative external radiation permitted at schools and kindergartens to a maximum annual exposure of 1 mSv. You can check the full JAIF PDF
here.
Current statusAs of Feb 20:th, only TWO out of Japan's total 54 nuclear reactors are running:
The rest of the reactors are currently halted awaiting results from stress tests being evaluated at Kansai EP Co:s Ohi plant in the "Genpatsu Ginza" area on japan's west coast. The results should be published fairly soon.
