- #1,021
Sotan
- 324
- 239
Remember the X-6 penetration to which access is needed for an internal investigation of Unit 2 PCV, but proved difficult to decontaminate to an acceptable level until now?
On Aug 26 this report was published (in Japanese) as part of a new "mid- and long-term roadmap" update:
http://www.tepco.co.jp/nu/fukushima-np/roadmap/images1/images2/d160825_08-j.pdf
Just a few points I picked from it:
-Decontamination work at the entrance of X-6 started in October 2015. They washed and vacuumed and scraped the floor and walls in the area, but the measurements on the floor still indicated values as high as 8 Sv/h (in January). Even if they were to use some form of shielding, it was assumed that doses in the air in the work area would not fall below 100 mSv/h.
- They would like to attain a value of 20 mSv in the air, behind shield - a value considered low enough to allow a worker to spend 5 minutes in there.
- Page 4: initially they were thinking of installing a simple shield based mainly on a 90 mm thick, simple square lead plate. This is called Case 1. As an alternative, they now came up with a more complicated shield design, with multiple lead plates arranged as a container and aiming to provide better blocking of radiation. This is Case 2. These two designs have been analyzed to see what results they can provide.
- Page 5 shows the results of the simulation for Case 1 shielding; the conclusion is "not good enough".
- Page 6 shows that Case 2 shielding could give the desired effect; the higher remaining values (17.3 and 13.2 mSv/h) are due to the poorer shielding in areas that will be occupied by various devices to be used effectively in the later investigation, devices which have less shielding power.
So they are now trying to design and construct that complicated shielding structure, which poses a weight problem (they can only move a 2t weight at a time in the area), so most likely the "shielding container" will have to be made of two separate parts.The advantage is that it can be installed by remote controlled machinery and it can achieve the desired degree of shielding without requiring further decontamination work, which has been a big pain and unsuccessful until now.
- From this report I (think I) learned somethign about the various radioactivity measurement results that appear in Tepco reports. If you look at page 3, you see the highest value given for the floor is 278 mSv/h. On the other hand, Page 10 shows some radioactivity values, measured in June this year, which are rather in the Sv range (exceeding the device maximum scale of 10 Sv in one spot); these are actually measures in June this year. Page 13 explain the mystery: the "lower" values reported on page 3 are measured using a device equiped with a lead "colimator" which has a reduction factor of 1/500... I did then try to read a little about radiation detection and measurement, but it's such a complex subject. Two conclusions I draw, 1) numbers are not everything, their meaning & the method behind them must be well understood, and 2) as you see it is very likely that I make mistakes every so often in my posts, so... I kind of rely on you to correct them when needed.
On Aug 26 this report was published (in Japanese) as part of a new "mid- and long-term roadmap" update:
http://www.tepco.co.jp/nu/fukushima-np/roadmap/images1/images2/d160825_08-j.pdf
Just a few points I picked from it:
-Decontamination work at the entrance of X-6 started in October 2015. They washed and vacuumed and scraped the floor and walls in the area, but the measurements on the floor still indicated values as high as 8 Sv/h (in January). Even if they were to use some form of shielding, it was assumed that doses in the air in the work area would not fall below 100 mSv/h.
- They would like to attain a value of 20 mSv in the air, behind shield - a value considered low enough to allow a worker to spend 5 minutes in there.
- Page 4: initially they were thinking of installing a simple shield based mainly on a 90 mm thick, simple square lead plate. This is called Case 1. As an alternative, they now came up with a more complicated shield design, with multiple lead plates arranged as a container and aiming to provide better blocking of radiation. This is Case 2. These two designs have been analyzed to see what results they can provide.
- Page 5 shows the results of the simulation for Case 1 shielding; the conclusion is "not good enough".
- Page 6 shows that Case 2 shielding could give the desired effect; the higher remaining values (17.3 and 13.2 mSv/h) are due to the poorer shielding in areas that will be occupied by various devices to be used effectively in the later investigation, devices which have less shielding power.
So they are now trying to design and construct that complicated shielding structure, which poses a weight problem (they can only move a 2t weight at a time in the area), so most likely the "shielding container" will have to be made of two separate parts.The advantage is that it can be installed by remote controlled machinery and it can achieve the desired degree of shielding without requiring further decontamination work, which has been a big pain and unsuccessful until now.
- From this report I (think I) learned somethign about the various radioactivity measurement results that appear in Tepco reports. If you look at page 3, you see the highest value given for the floor is 278 mSv/h. On the other hand, Page 10 shows some radioactivity values, measured in June this year, which are rather in the Sv range (exceeding the device maximum scale of 10 Sv in one spot); these are actually measures in June this year. Page 13 explain the mystery: the "lower" values reported on page 3 are measured using a device equiped with a lead "colimator" which has a reduction factor of 1/500... I did then try to read a little about radiation detection and measurement, but it's such a complex subject. Two conclusions I draw, 1) numbers are not everything, their meaning & the method behind them must be well understood, and 2) as you see it is very likely that I make mistakes every so often in my posts, so... I kind of rely on you to correct them when needed.
