This is an update of the situation as of 10 am JST Wednesday 16 March. (For background on events of 15 March and earlier, start with this post and its included links.) Note that this is a blog, not a news website, and thus the following analysis, like all others on BraveNewClimate, is a mixture of news and opinion — but facts remain paramount.
First, the situation is clearly (but slowly) stabilising. As each day passes, the amount of thermal heat (caused by radioactive decay of the fission products) that remains in the reactor fuel assemblies decreases exponentially. When the reactors SCRAMed on 11 March after the earthquake, and went sub-critical, their power levels dropped by about 95 % of peak output (the nuclear fission process was no longer self-sustaining). Over the past 5 days, the energy in the fuel rods dropped by another ~97 %, such that the heat dissipation situation is getting more and more manageable. But we’re not out of the woods yet, and the reactor cores will need significant cooling for at least another 5 days before stability can be ensured.
Yesterday there appears to have been a fracture in the wetwell torus (see diagram: that circular structure below and to the side of the reactor vessel) in Unit 2, caused by a hydrogen explosion, which led to a rapid venting of highly radioactive fission product gases (mostly noble [chemically unreactive] gases, the majority of which had a half-life of seconds to minutes). It also caused a drop in pressure in the supression pool, which made the cooling process more challenging. However, despite some earlier concerns, it is now clear that containment was not breached. Even under this situation of extreme physical duress, the multiple containment barriers have held firm. This is an issue to be revisited, when the dust finally settles.
Units 1 and 3, the other two operating reactors at Fukushima Daiichi when the earthquake struck, continue to be cooled by sea water. Containment is secure in both units. However, like Unit 2, there is a high probability that the fuel assemblies have likely suffered damage due to temporary exposure (out of water), as the engineers struggled over the last few days to maintain core coolant levels. Whether there has been any melting of the clad or rods remains unclear, and probably will continue to be shrouded in a cloud of uncertainty for some time yet.
The other ongoing serious issue is with managing the heat dissipation in the spent fuel ponds. These contain old fuel rods from previous reactor operation that are cooling down, on site, immersed in water, which also provides radiation shielding. After a few years of pond cooling, these are transferred to dry storage. The heat in these rods is much less than those of the in-core assemblies, but it is still significant enough as to cause concern for maintaining adequate coverage of the stored fuel and to avoid boiling the unpressurised water. There have been two fires in Unit 4, the first tentatively linked to a failed oil pump, and the second, being of (currently) unknown cause, but the likelihood is that it was linked to hydrogen gas bubbling.
There appears to have been some exposure of this spent fuel, and radiation levels around this area remain high — making access in order to maintain water levels particularly troublesome. Note that apart from short-lived fission product gases, these radiation sources are otherwise contained within the rods and not particularised in a way that facilitates dispersion. Again, the problems encountered here can be linked to the critical lack of on-site power, with the mains grid still being out of action. As a further precaution, TEPCO is considering spraying the pool with boric acid to minimise the probability of ‘prompt criticality’ events. This is the news item we should be watching most closely today.
An excellent 2-page fact sheet on the spent fuel pool issues has been produced by the NEI, which can be read here: Used Nuclear Fuel Storage at the Fukushima Daiichi Nuclear Power Plant (this includes an explanation of what might happen under various scenarios).
This figure illustrates the current reported state of the Daiichi and Daini reactors, last updated 1230 on 16 March (click to enlarge):
• Radiation Levels
o At 10:22AM (JST) on March 15, a radiation level of 400 milli sievert per hour was recorded outside secondary containment building of the Unit 3 reactor at Fukushima Daiichi Nuclear Power Station.
o At 3:30PM on March 15, a radiation level of 596 micro sievert per hour was recorded at the main gate of Fukushima Daiichi Nuclear Power Station.
o At 4:30PM on March 15, a radiation level of 489 micro sievert per hour was recorded on the site of the Fukushima Daiichi Nuclear Power Station.
o For comparison, a human receives 2400 micro sievert per year from natural radiation in the form of sunlight, radon, and other sources. One chest CT scan generates 6900 micro sievert per scan.
• Fukushima Daiichi Unit 1 reactor
o As of 10:00PM on March 14, the pressure inside the reactor core was measured at 0.05 MPa. The water level inside the reactor was measured at 1.7 meters below the top of the fuel rods.
• Fukushima Daiichi Unit 2 reactor
o At 6:14AM on March 15, an explosion was heard in the secondary containment building. TEPCO assumes that the suppression chamber, which holds water and stream released from the reactor core, was damaged.
o At 1:00PM on March 15, the pressure inside the reactor core was measured at 0.608 MPa. The water level inside the reactor was measured at 1.7 meters below the top of the fuel rods.
• Fukushima Daiichi Unit 3 reactor
o At 6:14AM on March 15, smoke was discovered emanating from the damaged secondary containment building.
• Fukushima Daiichi Unit 4 reactor
o At 9:38AM on March 15, a fire was discovered on the third floor of the secondary containment building.
o At 12:29PM on March 15, TEPCO confirmed extinguishing of the fire.
• Fukushima Daini Units 1 to 4 reactors: all now in cold shutdown, TEPCO continues to cool each reactor core.
This indicates a peak radiation level of 400 mSv/hr, which has come down to about 0.5 mSv/hr by the afternoon. This ‘spot’ radiation level was measured at a location between Unit 3 and 4. It was attributted to a hydrogen explosion in the spent fuel pool of Unit 4 — but this is still under debate. The radiation level at the site boundary is expected to have been much lower and, to date, there is no risk to the general public.
Two other useful sources of information are from the WNN: Radiation decreasing, fuel ponds warming and Second fire reported at unit 4. ANS Nuclear Cafe continues to be a great collator of key official channels and top news stories.
Finally, this is a useful perspective from an MIT staffer that is well worth reading:
What is known is that this is a situation very different than Chernobyl or Three Mile Island. There was no operator error involved at Fukushima-Daiichi, and each reactor was successfully shut down within moments of detecting the quake. The situation has evolved slowly but in a manner that was not anticipated by designers who had not assumed that electrical power to run emergency pumps would be unavailable for days after the shutdown. They built an impressive array of redundant pumps and power generating equipment to preclude against this problem. Unfortunately, the tsunami destroyed it.
There are some characteristics of a nuclear fission reactor that will be common to every nuclear fission reactor. They will always have to contend with decay heat. They will always have to produce heat at high temperatures to generate electricity. But they do not have to use coolant fluids like water that must operate at high pressures in order to achieve high temperatures. Other fluids like fluoride salts can operate at high temperatures yet at the same pressures as the outside. Fluoride salts are impervious to radiation damage, unlike water, and don’t evolve hydrogen gas which can lead to an explosion. Solid nuclear fuel like that used at Fukushima-Daiichi can melt and release radioactive materials if not cooled consistently during shutdown. Fluoride salts can carry fuel in chemically-stable forms that can be passively cooled without pumps driven by emergency power generation. There are solutions to the extreme situation that was encountered at Fukushima-Daiichi, and it may be in our best interest to pursue them.
More updates as further information comes to hand. Otherwise, for me, it’s back to the mad TV and radio media circus.
UPDATE: From World Nuclear News: Problems for units 3 and 4
Chief Cabinet Secretary Yukio Edano had outlined problems that had occured on the morning of 16 March with Fukushima Daiichi 3 and 4.
At 8:34am local time white smoke was seen billowing out of Fukushima Daiichi 3. Efforts to determine the cause of this development were interrupted as all workers had evacuated to a safe area due to rising radiation readings. Readings from a sensor near the front gate had fluctuated for some time, although Edano said that on the whole there was no health hazard. Earlier in the morning readings had ranged between 600-800 microsieverts per hour, but at 10am readings rose to 1000 microsieverts per hour. Readings began to fall again from around 10:54.
Edano said that one possibility being considered was that the unit 3 reactor had suffered a similar failure to that suffered by unit 2 yesterday, although there had been no reported blast or loud sound, which had been the case for unit 2. The immediate focus, said Edano was on monitoring of levels and checking pumping operations.
Edano also outlined plans for units 4-6. Preparations were being made to inject water into unit 4, however the high levels of radiation from unit 3 were imparing those preparations. When possible, the water injection would be done gradually as there were safety concerns over pouring a large amount of water at once. The water will be pumped into the reactor building from the ground, plans to drop water from a helicopter having been abandoned. Although he said that “all things were possible” Edano did not believe that recriticality at unit 4 was a realistic risk
Second fire at unit 4
Earlier, the Nuclear and Industrial Safety Agency said that a blaze was spotted in the reactor building of Fukushima Daiichi 4 at 5.45am local time this morning.
Attempts to extinguish it were reportedly delayed due to high levels of radiation in the area. A spokesperson for TEPCO said that by around 6:15am there were no flames to be seen.
The incident at unit 4 is believed to be in the region of a used fuel pond in the upper portion of the reactor building.
Tokyo Electric Power Company issued a notice of an explosion at unit 4 at 6am on 15 March. This was followed by the company’s confirmation of damage around the fifth floor rooftop area of the reactor building.
On that day, a fire was discovered but investigations concluded it had died down by around 11am.
At present it is not clear whether today’s fire was a completely new blaze, or if the fire reported yesterday had flared up again.