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[ATTACH]41901[/ATTACH]

the levels are not over the top imho, they basically require people to stay indoors

of course the alarm in peoples minds and the concern is that high levels of radiation could potentially occur if things go wrong from here

Fukushima Nuclear Accident – 16 March update

                   Posted on 16 March 2011 by Barry Brook               

                                        [ATTACH=full]189379[/ATTACH]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.

 [ATTACH=full]79463[/ATTACH]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):

[ATTACH=full]79464[/ATTACH]The status report from the The Federation of Electric Power Companies of Japan (FEPC) is given below:

•    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.