Some remarks on the recent observations regarding plant 2.
These days there had been reports about measurements of highly elevated radiation levels at Fukushimas plant No 2. Enenews has quite a compilation of news reports about that. According to Kyodo:
The radiation level inside the containment vessel of the No. 2 reactor at the crippled Fukushima Daiichi nuclear complex stood at 530 sieverts per hour at a maximum, the highest since the 2011 disaster, the plant operator said Thursday.
…The previously highest radiation level monitored in the interior of the reactor was 73 sieverts per hour.
In this handout issued by Tepco it is written that this 73 sieverts/h level was monitored in march 2012.
The high radiation was apparently recalculated from the damage on the images which were taken by a robotor, which took photos inside the plant, i.e. according to japan times:
Tepco said it calculated the figure by analyzing the electronic noise in the camera images caused by the radiation. This estimation method has a margin of error of plus or minus 30 percent, it said.
Here you can see one of the images, which seems to show on the top a part of the device (31) which drives the moderating rods between the fuel rods, i.e. it shows a part of the control rod drive (CRD) from below.
A detailled explanation of what you can see there can be found in this handout by Tepco.
Apparently the melted fuel was partially splattered but mainly went down and in particular went through a grating platform which is located in the pedestal underneath the control rod drive and thereby left a one-by-one meter hole.
Unfortunately I couldn’t find anything on the elevated radiation measurements on the Tepco site. That is I had a brief look on their twitter and facebook pages and their news releases and I scanned the recent regular updates and likewise the parameter page and found nothing.
The recent usual radiation measurements in unit 2 have a “ND”, which means “the instrument reading is below measurable limit” of 0.15 (System A) and 0.17 (System B) \( Bq/cm^3\). The CSV file basically shows this throughout all of January. If I then look into the archives at the data in 2012, then the summary chart starts only in July and displays also an “ND”, however here the limits are 0.24 and 0.23 respectively. The measurements from march 11 until end of Jun seem to show nothing (I didn’t scan all) and the CAMS radiation monitor finds on march 10:
(as of 11:00 , 3/10)
which I read as D/W (?) measured 6.2 and 2.5 for system A and B and S/C 0.05 and 1.36 Sieverts/h. This seems to have been a temporary measurement. Likewise on the other march days. I.e. in particular I couldn’t find the 73 sieverts, which were supposingly measured in march 2012.
Conclusion: The radiation measurements in the primary containment vessel of plant 2, which I found above neither seem to display the measurements of 73 from 2012 nor the 530 Sieverts/h as of now.
What I found seems to show the radiation measurements of Xe 135. If one would measure elevated and changing levels of Xe 135 then this would indicate that a bigger fission would most probably still be going on. A low or no level in Xenon 135 (that’s what the measurements show) could be due to no or not much fission taking place or be due to a process where every newly produced Xenon 135 atom almost immediately captures a neutron and transmutes into Xe 136.
I only guess but the latter process sounds to me on a first guess the less likely one.
There could though also be a measurement failure.*
Nevertheless it is clear that if you e.g. approach the melted debris then radiation might still get quite high, alone the half-life of Uranium 235 is 703800000 years.
*unfortunately I also didn’t find anything on the type of measurement instruments and their location