Fukushima Japan Earthquake: nuclear plants Fukushima part 2

[turi]

English translation of the PDF with enhanced images:
http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170202_01-e.pdf

 

[Greg Bernhardt]

I'm sorry if this is old news?

Extreme Radiation Levels Detected At Fukushima Reactor, Highest Since 2011
http://www.forbes.com/sites/william...hima-reactor-highest-since-2011/#270fbde33381

 

[Sotan]

Yes, a little old, and not of the well-informed sort.
It's about the value of 530 Sv/h reported on Jan 30. At that time the internet was filled instantly with articles written in that way.
"The level of nuclear radiation detected at the troubled Fukushima nuclear plant in Japan spiked on Thursday to its highest level since the triple core meltdown in 2011." Saying it like that suggests an evolution, an event, something that happened now (I remember my heart jumped too when I read it). Except, it's more like saying that my networth spiked to double its previous value because I found $10 in the pocket of my other coat, which I didn't wear for 6 years. (Although I kind of knew that stash must be there somewhere.)
Other links are even worse. "Time to reconsider that trip to the east coast of Japan", "Radiation levels in the Fukushima reactor are soaring unexpectedly" etc. And such statements are mixed with technically correct information ("radiation that would kill even after momentary exposure" even though "it is contained inside the reactor"), so it must be hard for some people out there to dismiss the incorrect parts.

 

[jim hardy]

Einstein on reporters:

http://einsteinpapers.press.princeton.edu/vol7-trans/262?ajax (2 short pages)

 

[I_P]

OK, so is there a competent, current analysis of the situation? Seems that they still don't know where the fuel is. Is #2 the only vessel that was breached in this fashion? Doesn't this knowledge imply an increased threat to the groundwater and the ice containment wall (and thus continued releases to the sea)? Or is expected that the fuel is still within, and contained by, the concrete basement structure?

 

[etudiant]

How much of a threat can the residual fuel/corium actually be at this point? Does it matter where it is now, other than as a factor determining the eventual retrieval?
The remaining site emissions are many orders of magnitude below the releases during the acute phase of the accident and the ongoing water treatment seems to have any ground water contamination reasonably well in hand. Indeed TEPCO is managing the ground water level within the plant to keep a sustained inflow to the site and thus to prevent any further spreading of the contaminant plume.

 

[Hiddencamper]

OK, so is there a competent, current analysis of the situation? Seems that they still don't know where the fuel is. Is #2 the only vessel that was breached in this fashion? Doesn't this knowledge imply an increased threat to the groundwater and the ice containment wall (and thus continued releases to the sea)? Or is expected that the fuel is still within, and contained by, the concrete basement structure?

The fuel has been cooled and solidified for years now.

There's also still no evidence of a containment melt through at any unit (although unit 1 may have some fuel between the liner and the base mat... we don't know and won't for a while).

Remember this molten fuel is in a containment system. It's not like the reactor vessel was the only line of defense.

 

[Sotan]

Hi I_P.
In addition to what etudiant and Hiddencamper said above, I will add that it is hard to find a "current analysis", short and up-to-date, because of the amount of information that is coming out almost every day, as a result of the tremendous amount of work that is carried out every day. The closest thing that can be considered a current analysis, translated in English, is a little old: "Progress Status and Future Challenges of the Mid-and Long-Term Roadmap toward Decommissioning of TEPCO Holdings’ Fukushima Daiichi Nuclear Power Station Units 1-4 (Outline)", of 22 December 2016. You can read it here and it might help answer some of your questions.

 

[Sotan]

http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images1/handouts_170206_05-j.pdf (in Japanese)
New report - this time regarding Step 6 of the investigation under way, the step in which they will try to wash away / remove some of the sediment/deposit from the lower portion of the CRD rails. Worth taking a look even if Japanese only. The operation will be carried out tomorrow.

Page number 6 also gives a correction regarding the radiation dose rates reported earlier, deduced from the radiation artifact seen in the pictures taken. The 50 Sv/h value reported for point number 1 (just after entering the PCV) was reduced to 30 Sv/h after further study of the images.

On the same page there is a note about the high value of 530 Sv/h reported for point number 2: although a direct comparison is not 100% correct, if one was to measure the radiation right on the surface of a fuel bundle one day after the stopping of the nuclear reaction, the dose rate value would be of the order of several tens of thousands Sievert per hour. (This certainly helps put things in perspective.)

 

[Charles Smalls]

So much great information on this thread. Hi Sotan, thanks for all of your effort, I have been quietly reading your contributions for so long now. Decided to make an account just to thank you and everybody else for your coverage on this.

One question about this temperature thing, before I go back to lurking; I have read that it can take up to 5 years after a reactor shuts down until the decay heat levels in the fuel reduce enough to let it be air cooled. Does anyone know how accurate that is and how it would apply to fuel in a solidified mass rather than the normal 'good configuration' where it is in separated bundles?

Many thanks to all you guys again.

 

[Sotan]

Hi Charles, welcome and thank you. FYI I am no specialist, I just follow the progress in Fukushima plant with a personal interest, and post translations of some stuff from Tepco and other sites, which is released mainly in Japanese language. I am most grateful to all the knowledgeable people who contribute to these pages.

Pages 38-39 in this forum contain several posts (see #756~...) that relate to your question.

Edit:
Tepco has just released this info
http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images1/handouts_170207_04-j.pdf (in Japanese)
As they were checking the "sediment cleaning device" before inserting it and guiding it onto the CRD rails, they discovered that the pump that is supposed to provide the high pressure water to the device was not working. The pump will be replaced with a similar one and they will do that cleaning test when all works fine.

 

[turi]

While we're waiting with bated breath for progress at Unit 2 the preparations for the removal of fuel from the spent fuel pool of unit 3 are making progress:
http://photo.tepco.co.jp/date/2017/201702-j/170207-01j.html

 

[Hiddencamper]

So much great information on this thread. Hi Sotan, thanks for all of your effort, I have been quietly reading your contributions for so long now. Decided to make an account just to thank you and everybody else for your coverage on this.

One question about this temperature thing, before I go back to lurking; I have read that it can take up to 5 years after a reactor shuts down until the decay heat levels in the fuel reduce enough to let it be air cooled. Does anyone know how accurate that is and how it would apply to fuel in a solidified mass rather than the normal 'good configuration' where it is in separated bundles?

Many thanks to all you guys again.

For fuel that still has normal configuration, BWR fuel is air coolable in 4 months and PWR fuel at 11 months according to studies headed by the us nuclear regulatory commission.

Being in a non standard configuration would likely reduce surface area and reduce heat transfer.

 

[Sotan]

Anyone able to edit the Wikipedia page? The last phrase of this paragraph especially...

2017 radiation spike

On 3 February, 2017, several news agencies reported a significant spike in radiation levels from Reactor #2, peaking at 530 sieverts per hour, the highest level recorded since the March 11, 2011 incident. [37]These levels were announced by TEPCO, the utility company that oversees the Fukushima reactors.[36] The previous high was 73 sieverts per hour, in 2011. TEPCO believes the spike may be caused by melted nuclear fuel burning through the bottom of the containment.[35]

 

[jim hardy]

Anyone able to edit the Wikipedia page? The last phrase of this paragraph especially...

i tried

but it didn't take and i got a warning about blanking pages. oh well.

 

[Hiddencamper]

i tried

but it didn't take and i got a warning about blanking pages. oh well.

Tepco officially states that dose rates are not going up

http://www.tepco.co.jp/en/announcements/2017/1374451_10494.html

 

[Hiddencamper]

Anyone able to edit the Wikipedia page? The last phrase of this paragraph especially...

2017 radiation spike

On 3 February, 2017, several news agencies reported a significant spike in radiation levels from Reactor #2, peaking at 530 sieverts per hour, the highest level recorded since the March 11, 2011 incident. [37]These levels were announced by TEPCO, the utility company that oversees the Fukushima reactors.[36] The previous high was 73 sieverts per hour, in 2011. TEPCO believes the spike may be caused by melted nuclear fuel burning through the bottom of the containment.[35]

I took care of it. I'm horrible with Wikipedia though. There probably needs to be a new section about the findings from the Pcv inspection and a screen shot of the spliced images of the grating melt. I'll try to do these later if I get my computer set up again. But if someone else wants to do it that would be great.

Edit: another use keeps changing it. Not sure how to proceed

 

[Charles Smalls]

For fuel that still has normal configuration, BWR fuel is air coolable in 4 months and PWR fuel at 11 months according to studies headed by the us nuclear regulatory commission.

Being in a non standard configuration would likely reduce surface area and reduce heat transfer.

Hi Hiddencamper, thank you too for so much valuable insight.

The fuel cooling subject is an interesting one. My tutor was talking about surface area and heat transfer comparing french fries to a baked potato and it brought me to thinking about the reactor fuel melts. The mass must have different retention properties compared to the bundles. On the other hand, I really had no idea there was such a difference between BWR and PWR fuel heat decays or that either would be so short.

https://www.nrc.gov/waste/spent-fuel-storage/faqs.html

The NRC website on the above page quotes a more typical wet storage time frame of 3-5 years before transferring to dry casks. Is the additional time to allow the fuel to 'cool' radioactively as well as thermally or just good operating procedure?

 

[Hiddencamper]

Hi Hiddencamper, thank you too for so much valuable insight.

The fuel cooling subject is an interesting one. My tutor was talking about surface area and heat transfer comparing french fries to a baked potato and it brought me to thinking about the reactor fuel melts. The mass must have different retention properties compared to the bundles. On the other hand, I really had no idea there was such a difference between BWR and PWR fuel heat decays or that either would be so short.

https://www.nrc.gov/waste/spent-fuel-storage/faqs.html

The NRC website on the above page quotes a more typical wet storage time frame of 3-5 years before transferring to dry casks. Is the additional time to allow the fuel to 'cool' radioactively as well as thermally or just good operating procedure?

The difference between PWR and BWR fuel has to do with power density. Due to lower heat transfer in BWRs because of boiling, a BWR core needs 3-4 times as many fuel assemblies to have the same power output as a PWR.

As for the duration, the 3-5 years is because of cask safety analysis for storage casks. You can store fuel in shorter times, but you would have to put less fuel rods in the cask as a result, and that is wasteful. The casks have both local heat limits and total heat limits based on loading.

 

[jim hardy]

@Hiddencamper

what little i could see of reactor bottom looked in surprisingly good shape to me.

Ours(PWR) of course had only traveling incore tube penetrations , i think not even a drain . So it was much simpler.

Did i miss sight of a lower head melt-through?

With all the seawater that went through in early days sediment is no surprise. Need a few pieces of it to analyze. Surely they've done that ?

old jim

 

[turi]

Need a few pieces of it to analyze. Surely they've done that ?

Not yet. They inserted a camera to check for any obstacles for the planned robot insertion. Now they first want to use a "sediment cleaning device" (don't know about sample taking capabilities) and after that the robot (which will have radiation and temperature sensors and cameras, but again no sample taking capabilities as far as I know).

 

[jim hardy]

Edit: another use keeps changing it. Not sure how to proceed

hmmm looks better now ! was that you ?

Meanwhile , back at the reactor

??
http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170202_01-e.pdf

 

[Rive]

Need a few pieces of it to analyze. Surely they've done that ?

I don't think they should rush with that... To safely sample something from 'there' would require some serious thinking.

Ps.: As it is now I won't even try to retrieve that robot... Unintended 'sampling' would be a bad karma.

 

[turi]

View attachment 112768

Tepco identifies it as the TIP guiding pipe support. This picture is better zoomable:
http://photo.tepco.co.jp/library/170202_01/170202_01.jpg
In the background you can see quite a few details (are those chains from the CRD as in this patent:
https://data.epo.org/publication-se.../19851127/patents/EP0162163NWA1/document.html ?). Interesting will be what all that molten and solidified looking stuff is.

Edit: PIP -> TIP above

 

[jim hardy]

Thanks Turi

that's a much better photo.

But this patent
https://data.epo.org/publication-se.../19851127/patents/EP0162163NWA1/document.html
is for the magnetic Westinghouse style control rods not GE hydraulic rods.

 

[turi]

But this patent (...) is for the magnetic Westinghouse style control rods not GE hydraulic rods.

Thanks! Those things look like chains to me, but I wouldn't know what they are for:

Edit: I think Tepco identifies them as part of the CRD exchanger, I wouldn't know.

 

[Charles Smalls]

Thanks! Those things look like chains to me, but I wouldn't know what they are for:
View attachment 112770

Edit: I think Tepco identifies them as part of the CRD exchanger, I wouldn't know.

The images circulating from Unit 5 give a close idea of the equipment in its original state.

https://s28.postimg.org/v3qlx6esb/zoom.jpg

 

[Sotan]

http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images1/handouts_170209_08-j.pdf (in Japanese)
Results of the first attempt to remove/clear away sediments from the CRD rails.

Page numbered 4 shows two pairs of images taken somewhere at the top of the CRD rails; the left image is taken before clearing away sediment, the right image is taken after. (Left pair is taken with guide-pipe camera, right pair is taken with the camera installed on the device itself.)

Page 4 shows another two pairs of images taken a bit lower on the slope, where the sediment is better stuck to the surface and harder to remove. Each pair contains before/after images. Left side pair is taken using the guide-pipe camera, hence the device is seen; right side pair of images are taken with the device camera.

Page 5 shows two images (one from guide-pipe camera and one from device camera) taken as the device is traveling on the sediment itself.

Page 6 - conclusions:
- they cleaned about 1m of the 5 metres they had planned to clean (not clear if the planned all 5m for one single session);
- as we move towards the pedestal, the sediment sticks more strongly to the surface and takes longer to clear away;
- they stopped the operation when visibility became bad, and retrieved the device, as they don't want to cause extra visibility troubles for the next step (inspection robot);
- the device was able to climb over and drive over the sediment but there were also places where it couldn't advance/travel over the sediment.

They will analyze the data obtained in view of deciding the next step.

Larger photos and a video (which I couldn't see yet) here:
http://photo.tepco.co.jp/date/2017/201702-j/170209-01j.html

Edit: I saw the video now. There are many fragments of the "sediment" which appear to be pieces of aluminum foil or something similar. At 1:37 the water stream peels off and overturns something that looks like a "slab" of material, maybe 1-2 cm thick, maybe 10x10 cm or more in area. Slab may not be the right word, it still looks like a pretty light material. Lots of radiation-induced noise in the video image.

 

[Charles Smalls]

Now I see it:

 

[Sotan]

I watched the first part of the press conference of Tepco where they showed the "sediment removal" video.
- The image getting darker (the part above where I clumsily translated as "visibility getting worse") in fact refers to the camera itself getting tired, most likely die to the damaging effect of the high radiation on the camera sensor. The images were getting darker and darker and they had to pull the device out sooner than planned - while they still had video. The 2h lifetime is in general agreement with the evaluated radiation dose and the 1000 Sv integrated, overall radiation resistance of the camera. The inspection robot to be used in the last step will probably behave similarly, as far as the camera life time is concerned.
- They didn't put this in the PDF report for lack of time, but will be announced later: they analyzed again the radiation-induced noise in the video images and they obtained a new value fo approximately 650 Sv/h, approximately in the same area where they evaluated 530 Sv/h the other day. So if the first time they might have had some doubts about that value, this new one in the same range appears to reinforce the conclusion that there is something, not very sure where, which causes images of that region to show such dose rates. (The speaker mentioned briefly that the 530 Sv/h reported the other day has been wrongly interpreted by certain news sites abroad, as an "increase" in radiation. He underlined again that a day after stopping the nuclear reaction in the vicinity of a used fuel bundle there are probably tens of thousands of Sv/h.)
- I said above that the device was unable to "crawl" in certain areas, the reason is that the tracks allow for approximately 2 cm ground clearance, the robot can easily go over 2 cm obstacles, but the highly irregular in shape sediment sometimes has much more height. It rubs onto the belly of the robot, the tracks don't provide traction any more, so it stalls. The inspection "scorpion" robto has very similar tracks so they will have to think very well what to do next.
- There is no plan to insert a second sediment-cleaning device, they can't easily replace the camera (the device itself is contaminated too), so for now there is no plan of a second attempt.

 

[jim hardy]

The 2h lifetime is in general agreement with the evaluated radiation dose and the 1000 Sv integrated, overall radiation resistance of the camera.

Thanks !

there is something, not very sure where, which causes images of that region to show such dose rates.

Probably something that's been around a lot of neutrons.
I'm not jumping on the Corium bandwagon yet, a local hotspot could be a little fragment of something that got vented in first weeks.

 

[etudiant]

How would any fuel fragment wind up beneath the reactor vessel other than via a melt through?
Is not the vessel set directly on the pedestal, with access openings only from below?

 

[jim hardy]

How would any fuel fragment wind up beneath the reactor vessel other than via a melt through?
Is not the vessel set directly on the pedestal, with access openings only from below?

My grasp is not crystal clear
but i thought the 600 sv-ish reading was not under the vessel inside the pedestal, but in the region between the containment penetration and the pedestal .
So correct me if I'm wrong
And i believe there are pipes routed through there connecting safety valves to suppression pool in torus
and lots of other pipes connected to vessel

a fuel fragment in one of them would shine right through the pipe wall.
So the question becomes what's nearby that high radiation area ?

old jim

 

[F X]

Two questions/thoughts

Isn't the entire containment flooded with water?

If so, why isn't the water shielding the camera from the radiation?

If it is, (both underwater, and shielding is happening), then wouldn't the actual radiation source be much much higher than they are calculating?

Related question, what is the dose rate (Svr/hr) for nuclear fuel that has spent 6 years cooling?

As in, if the fuel rods had been cooling for 6 years in a spent fuel pond, how radioactive would they be?

And is this rate being measured in reactor 2 the same?

 

[Sotan]

The containment is not flooded with water.
Water at the bottom of the PCV of Unit 2 is only about 30 cm deep, as you can see on page 18 of this document.
Also, if you take a look at the videos taken by Tepco on/around the CRD rails, it's pretty clear that nothing is under water there.
As for the radiation values it's a bit of a blur because of insufficient information but "in general" the dose rates measured (indirectly, just from analysis of video images) in the PCV of unit 2 "make sense for unshielded irradiated fuel", as Hiddencamper wrote somewhere 2 pages before.

 

[Rive]

Isn't the entire containment flooded with water?

It can be confusing because in the very first days they tried to flood/fill up the entire containment with seawater, but they could not succeed.
They could not even correctly measure the water level that time, so finally they stopped when the water pumped in were already several times more than the amount needed to fill the containment.
Right now it is as Sotan says: the containments are not flooded, but there are some (different depth for each unit) water at the bottom.

 

[etudiant]

Thank you, jim hardy, that diagram really helps clarify the situation.
It shows mheslep's suggestion is the more plausible, as the source is apparently some lateral distance away from the reactor.

 

[jim hardy]

It shows mheslep's suggestion is the more plausible, as the source is apparently some lateral distance away from the reactor.

From a couple pages back, translating from a press conference

Many reporters were troubled by the numbers reported for radiation level. So (if you look in the figures for Step 4 and Step 5 in this investigaton), we insert a guide pipe through the X-6 penetration, horizontally; as soon as we get inside the PCV, there's about 50 Sv/h. We keep inserting the pipe inside, horizontally, until we reach roughly around the middle of the space between PCV wall and pedestal wall. Here, in the air, image analysis suggests 530 Sv/h (!). Then the guiding pipe bends down and goes all the way to that hole in the pedestal wall; just as we enter the pedestal space, image analysis shows 20 Sv/h.

and from another link posted by Sotan, http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170130_02-e.pdf
(orange ovals mine)

Thanks Sotan !

I'd guess the camera went past something small and "hot" .

 

[Sotan]

You are correct with that drawing Jim.
Tepco has already published in a report (the one given in post #1109) this drawing below, which identifies the spots for which they evaluated the radiation doses.
The blue dots numbered 1, 2, 3 are the spots where they measured (by image analysis only) 30, 530 and 20 Sv/h respectively.
1 is immediately after entering the PCV. 2 is in the air, about 2.3m from the pedestal wall. 3 is immediately after entering the pedestal.

Changing the subject, I came home to watch the second part of the press conference held after the "sediment clearing session"... only to find that it is not available anymore. I should have downloaded it, I guess.

 

[Charles Smalls]

Changing the subject, I came home to watch the second part of the press conference held after the "sediment clearing session"... only to find that it is not available anymore. I should have downloaded it, I guess.

So many of the videos and reports become unavailable so quickly. I find some links referenced to NHK news both English and Japanese don't work after a short time. They doesn't seem to do much archiving.

Was footage of the probe traveling through the high dose range published? I have seen video of the camera entering the X-6 penetration and of it exiting into the pedestal but not the travel through the tube where it encounters the 650Sv/h level of distortion.

 

[Hiddencamper]

Two questions/thoughts

Isn't the entire containment flooded with water?

If so, why isn't the water shielding the camera from the radiation?

If it is, (both underwater, and shielding is happening), then wouldn't the actual radiation source be much much higher than they are calculating?

Related question, what is the dose rate (Svr/hr) for nuclear fuel that has spent 6 years cooling?

As in, if the fuel rods had been cooling for 6 years in a spent fuel pond, how radioactive would they be?

And is this rate being measured in reactor 2 the same?

The severe accident guidelines have you flood containment to backfill the hole in the reactor following a rupture. The immediate concern is to establish minimum debris submergence level (get the core slag under water). Then flood up to try and establish core cooling. Once you've flooded containment you could feasibly open up the drywell head and vessel head and "look down" into the core.

In this case, there is containment system damage causing water to spill out. So until they find and plug those leaks they will be unable to use normal means for shielding the core slag.

Note: post Fukushima severe accident guidelines have changed and do not emphasize immediate debris submergence for mark 1/2 containments. The goal is to establish minimum debris cooling injection rates to prevent a rapid steam pressure rise in the drywell and to preserve the suppression pool as a water scrubbed release path.

 

[turi]

English reports on debris removal work (probably based on Sotan's work ):
http://www.tepco.co.jp/en/press/corp-com/release/2017/1375551_10469.html
http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170209_01-e.pdf
http://photo.tepco.co.jp/en/date/2017/201702-e/170209-01e.html

English summary of newest quarterly report:
http://www.tepco.co.jp/en/press/corp-com/release/2017/1375451_10469.html
Summary of summary:

• SFP fuel removal planned in 2018 for Unit 3 and in 2020 for Unit 1, preparatory work underway.
• Ice wall: Amount of water pumped up daily has declined from 400 cubic meters to 140 cubic meters.

Report in Japanese:
http://www.tepco.co.jp/press/release/2017/pdf1/170210j0102.pdf

 

[Sotan]

Thank you turi.

I just want to refer to the last link you posted above, the 91-page long report in Japanese. The title would sound something like "Progress of the reform plan on nuclear power safety" and aims to show the work Tepco is doing to generaly improve the safety level in its three nuclear facilities (Fukushima Daiichi, Fukushima Daini, Kashiwazaki-Kariwa plants, the last one, world's largest nuclear power plant, still employing 6000 workers and hoping to restart production some day). I only browsed through it enough to see that there's not much new content related to the mainly technical scope of this thread, so... I don't think I will quote anything of it in here.

 

[turi]

IRID released a document showing a robot called "PMORPH" for investigating Unit 1:
http://irid.or.jp/en/topics/pcv内部調査／1号機原子炉格納容器内部調査（ペデ/

 

[F X]

The containment is not flooded with water.

Seriously? So the fuel is just sitting somewhere, in air? For how long?

 

[turi]

Seriously? So the fuel is just sitting somewhere, in air? For how long?

Since 2011. More under constant rain than just air though.

 

[jim hardy]

Seriously? So the fuel is just sitting somewhere, in air? For how long?

Don't get excited until they find it. The tempest might still be in the teapot.

 

[turi]

Newly stitched together images from Unit 2, courtesy of IRID:
http://photo.tepco.co.jp/date/2017/201702-j/170215-01j.html

Edit: To me, if I interpret the pictures correctly, it looks as if the alternative route to the left for the inspection robot isn't feasible as well. Something that could take hold of the middle beam would be necessary, but that would be a much more complex robot (or probably an electro magnet or two could help).

 

[Sotan]

The link posted by turi above leads to a PDF report in which Tepco shows us the options that they are considering these days, regarding the final step of the planned investigation of PCV/pedestal of Unit 2, namely the step which involves sending in the "scorpion" robot equipped with cameras, thermometer and radiation meter.
Pages 1-3 you've seen them before, they show the general concept, object and steps of this investigation. (Page numbers as inscribed at bottom-right corner)
Pages 4-6 compile various information obtained in previous steps - including a few newly stitched and reprocessed images. Page 5 underlines the gap (45 to 140 mm wide) that exists between the end of the CRD rails and the grating platform. White dotted line on Page 6 shows a possible route for inserting the robot (which, as you have seen in a presentation video, is capable of crossing some gaps).
Page 7 mentions that additional information could be obtained by performing this final step, with the robot taking over from where the sediment-cleaning robot stopped. The scorpion robot is shown, with its front and back camera, thermometer and radiation measuring device (which by the way still has a plus/minus 20% error). The latter device, installed in a "corrugated tube" (?) practically tied to the power cable of the robot, will be in contact with the sediment and therefore the reading will not be purely, 100%, air radiation dose.
Pages 8 and 9 show the two investigation options that they are considering, with pros and cons:

Page 8: In case they send the robot only down to the end of the CRD rails:
- they will be able to peek at the pedestal installations from an even lower point of view, getting info about the state of the hardware and the features of the sediment;
- they will measure temperature and radiation dose;
- if the device gets in trouble while crawling over the sediment, they can pull it back using the power cable. The robot might end up unable to proceed, due to: long tracks; relatively large weight; changes in the center of gravity due to manoeuvering the back-end camera.
Even if the robot does not make it to the final destination (end of the CRD rails) it would still contribute useful new data.

Page 9: In case they attempt to send the robot onto the grating platform:
- they would get more images of the holes in the grating, as well as of the bottom of the lower part of the reactor, and be able to assess the state of the CRD housings, of the sediment etc.;
- they will measure temperature and radiation dose;
However:
- the access is difficult and risky; the robot might end up unable to advance or - even worse - fall through the holes;
- while crossing the gap the robot will be in its fully extended configuration, so no images will be available from the back-end camera to help with planning the best route; they will depend only on images from the front camera;
- if the robot falls down through some opening, it might be impossible to retrieve just by pulling the power cables;
- as the investigation is the main priority, if the time needed to retrieve the robot becomes a limiting factor they might simply leave the robot in, at the end.

Finally page 10 gives some general perspective of the grating platform which is the object of this whole 7-step investigation. Red areas show places where grating has fallen, blue is safe grating, grey is unexplored area.

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Same link given by turi above also leads to this second report, also in Japanese only, which is an attempt from Tepco to put the announced radiation doses into some perspective. They explain on page 1 that radiation is stopped/diminished by shielding (lead, iron, concrete...), by distance, and by the passage of time. On page 2 they point towards the place where the 650 Sv/h was estimated the other day, further explaining that the people operating the guiding pipes in this investigation, behind the shield, were subjected to only 6 mSv/h, while the value measured at the monitoring post located at the fence of Tepco's land shows only 2 microSv/h. All these are intended to show that there's no danger for the public, and even for the workers they do all they can to reduce exposure.

 

[turi]

Thank you Sotan! The PDF links weren't present when I've seen the images. Note to self: Recheck TEPCO's photo pages periodically.
Edit: English version link, currently without PDF links:
http://photo.tepco.co.jp/en/date/2017/201702-e/170215-01e.html