Japan Earthquake: nuclear plants Fukushima part 2

[Hiddencamper]

Pressure and level gauges are outside the containment.

Could it have been some type of notching or reference leg boiling?

 

[MadderDoc]

Something was affecting reactor water level indication shortly before the explosion

from http://www.ic.unicamp.br/~stolfi/EXPORT/projects/fukushima/plots/cur/out/plot-un3-t-I-full.png

The water level measuring system at the plant was not very advanced, and could not cope well with elevated temperature, and evaporation from the reference leg under these accident conditions. There was no way of correction for loss of water from the reference leg, meaning the readout values for the water level would likely systematically indicate a significantly higher level of water in the RPV, than the level actually was. The gross unsteadiness of the readings in the hours before the explosion come on the background of a period with limited and intermittent injection of coolant into a water starved RPV. One could say, perhaps it was the reactors way of saying "duck and cover or run"..

They had been injecting sea water by fire engine from the vertical shaft at the backwash valve since 1:12 pm on the 13th. In the period around midnight between the 13th and the 14th, they were running out of sea water from that source, searches for other sources in the area failed. At 01:10 am injection from the emptied vertical shaft had to be suspended, and a firetruck was used to supply more sea water to it. At 03:20 am they repositioned another firetruck, such that it could take water in from a deeper spot in the vertical shaft at the backwash valve, and injection was restarted. At 09:20 am, they had managed to get 2 firetrucks in position on the shallow draft quay at the seaside and arranged a line with a hose for water supply from there and up to the vertical shaft. At 11.01 am the explosion occurred and injection stopped, due to damage to the firetruck and the hose. A new injection line into the reactor had to be constructed, after which the injection of sea water could be restarted, at 16:30 pm.

 

[Hiddencamper]

Just to add to the last comment. When you start boiling in the reference legs due to elevated drywell temperatures, you see notching and level perturbations. Levels can bounce high and low as bubbles work their way out and eventually boiling occurs. Also, if a rapid depressurization occurs to less than 400 PSIG, it's not uncommon to see degassing happen which also causes level notching. We have a step in our procedures if our reference line keep fill system isn't in operation to slow the rate of depressurization around 400 PSIG to monitor for notching or degassing occurring, and to trip our transient data system for engineering evaluation.

The reference legs are subject to boiling (as are the variable legs during severe accidents). Obviously this happens as drywell temperature is high and rpv pressure is low.

At elevated drywell temperatures that do not cause boiling, your minimum usable level degrades. For example, my wide range level indicators can only indicate a minimum of -159" at normal containment temperatures. Above 100 degreesF in the containment and the minimum usable level is -149" (it may not indicate less than that, or be erratic below that point). At 200 degF it's -139". Engineering was nice to us and worked the numbers so the numbers round out at 10" : )

Anyways, it's hard to tell exactly what's going on with level during severe accidents, which is why drywell temperature and pressure is important to know, as they help give you the full picture.

On a loss of all level indicators or indications that instrument lines are boiling, operators are to immediately exit the current emergency operating procedure they are in and enter contingency 4, RPV flooding, to attempt to recover level above the main steam lines and verify water level is above the top of the fuel by indications of water overflow through the relief valves or drain lines. If the site has already transitioned to severe accident guidelines then they will just say in those, as the SAGs place no dependence on RPV water level indications and have you take conservative actions regardless of what level is or whether it is known or not.

 

[Sotan]

Japanese sites report that the removal of spent fuel bundles from (2 of?) the spent fuel pools of Fukushima Daiichi is to be postponed.

The video is in Japanese but there is English translation under it:
"Regarding the spent nuclear fuel of TEPCO's Fukushima Daiichi nuclear power plant, the government abandoned the start of taking out in 2020 and found that the policy of delaying it for several years was strengthened.
Nuclear fuels that have been used up at nuclear power plants are kept cold in large pools in the same building as the reactor. In Fukushima Daiichi Nuclear Power Plant, it is one of the greatest risks that this nuclear fuel remains, and it is considered necessary to extract this for "decommissioning furnace". Under these circumstances, in Units 1 and 2 of Fukushima Daiichi Nuclear Power Plant, it was found that the amount of workers' exposure was higher than the initial expectation due to the damage situation at the site, the government abandoned the start of taking out in 2020, It is said that the policy of delaying is settled. It is planned to be officially decided by the middle of this month.
If the withdrawal of spent nuclear fuel is delayed, there is a possibility that the schedule of the entire decommissioning work that it is supposed to finish in 30 to 40 years may be affected."
On another TV station they mentioned that the 2018 term planned for Unit 3 SFP is likely not to be affected by this decision. Another site reports that the start of the nuclear debris removal might be postponed by (only) a year.

 

[etudiant]

This seems a very significant decision.
The site radioactivity is not going to fall significantly for many years, decades really, so if exposure is too high now, it will still be too high when the Tokyo Olympics roll around and long after that.
Is this a first step towards permanent abandonment/entombment of Daiichi instead of rehabilitation?

 

[Sotan]

One page PDF file in Japanese, showing two samples - one taken from the bottom of the PCV of Unit 1 in April 2017 and another one, a small amount of material from the tip of a cable that has been inserted to check the state of a PIP duct in Unit 2 in 2013. On Sep 20 these samples have been sent to some outside facility for detailed analysis of shape/appearance and composition (chemical elements) of the surface.
http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images2/handouts_170921_08-j.pdf
I wonder why it took so long.

 

[Azby]

<I wonder why it took so long.>

Yes, it's a bit baffling. Also, there's no indication of where the samples are being sent for analysis.

 

[jim hardy]

...bated breath...

 

[Azby]

Here's the English version of the press release about the samples:

http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170921_01-e.pdf

 

[jim hardy]

Here's the English version of the press release about the samples:

ahhh so something did run down the TIP tube. Should be telling.

Can they estimate neutron spectrum from little samples like that ?

 

[Astronuc]

ahhh so something did run down the TIP tube. Should be telling.

Can they estimate neutron spectrum from little samples like that ?

Their may not be much in the way of TU nuclides. They could do inductively coupled plasma (ICP) emission spectroscopy, gamma spectroscopy and neutron activation analysis.

 

[jim hardy]

They could do inductively coupled plasma (ICP) emission spectroscopy, gamma spectroscopy and neutron activation analysis.

I guess you need glass or mica to see fission fragment tracks?

 

[MadderDoc]

ahhh so something did run down the TIP tube. Should be telling.

It is one of 'bonus'- samples that came out of the tests they did way back in July 2013, probing the soundness of the TIP lines of Unit 2. They didn't manage to get any of the probes past the TIP indexers. (That is about 8-10 meters in from the PCV wall). When the probes were withdrawn, some substances were withdrawn with them, adhered to the tips of the probes. The sample depictured in the recent handout, and which appears now to have been sent for external analysis for some reason, is material from inside of the TIP (B) line, collected somewhere between the line entry and the indexer.
http://photo.tepco.co.jp/en/date/2013/201307-e/130708-02e.html

 

[Astronuc]

I guess you need glass or mica to see fission fragment tracks?

The range of fission products is on the order of a few microns, which is less than a typical average grain size of 10 to 15 microns for UO₂.

One could do a mass spectrometer in vacuum, but they would more likely do a gamma spectrometer.

 

[Gary7]

Tokyo newspaper Tokyo Shinbun reporting today that the plan to remove spent fuel rods from Units 1 and 2 of Fukushima Dai-ichi has been pushed back 3 years, and is now expected to start in 2023. The reason given is the high radiation readings around the area where the work is required to take place. They note that for Unit 1, the destruction of the concrete lid of the containment vessel is allowing strong radiation to escape the reactor core. (nb. Italics are mine) The spent fuel from the pool of Unit 3 is expected to be extracted in mid-2018.

This was the first I'd heard anything about the destruction of the concrete lid of Unit 1, but I haven't been following this as closely as I used to.

 

[MadderDoc]

This was the first I'd heard anything about the destruction of the concrete lid of Unit 1, but I haven't been following this as closely as I used to.

Yes the concrete shield plugs in unit 1 are in severe disarray. Here is a link that may be used as an entry to the subject, and to the original handouts about the observations:
http://www.fukuleaks.org/web/?p=16279

 

[Rive]

Just found this link somewhere:
https://www4.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170928_01-e.pdf

 

[jim hardy]

Thanks, Rive ...

I take that to mean "The muon measurements, while interesting, suggest the fuel has left the core area but don't tell us much about where it actually is ."

 

[Azby]

I'm not sure if this has been posted here yet, but it's very informative regarding TEPCO estimates of fuel melt and disposition, including muon scan results. Unit 3 muon scans weren't available yet, I think:

Estimation of current status inside RPV and PCV at Fukushima Daiichi NPS
July 3, 2017
http://ndf-forum.com/ref/d2_mizokami_en.pdf

 

[etudiant]

I'm not sure if this has been posted here yet, but it's very informative regarding TEPCO estimates of fuel melt and disposition, including muon scan results. Unit 3 muon scans weren't available yet, I think:

Estimation of current status inside RPV and PCV at Fukushima Daiichi NPS
July 3, 2017
http://ndf-forum.com/ref/d2_mizokami_en.pdf

No expert, but for me, this document is impressive, a carefully documented and sober summary of the current situation.
TEPCO and the Japanese investigators have worked hard and it shows.
Of course, it also gives some idea of how very long the road will be still to clean up this mess.

 

[FleaFlyFloh]

Yes Azby, we know this doc already, but thanks anyway:
Japan Earthquake: nuclear plants part 2

 

[Sotan]

A new batch of files from the "progress on the mid- and long-term roadmap" series has been published on 26 October on the Tepco site at http://www.tepco.co.jp/decommision/planaction/roadmap/index-j.html

A few quick notes on two of those files (both are only in Japanese at the moment):

http://www.tepco.co.jp/nu/fukushima-np/roadmap/2017/images2/d171026_05-j.pdf
This is the file about decommissioning and management of radioactive water.
Page 2:
- they are installing a wind-protection cover (fence) on Unit 1 building to help prevent the spreading of dust as they keep working on removing debris from the operating floor.
- the fourth section of the dome-shaped building cover is being installed om the operating floor of Unit 3.
- they will be removing the remaining highly contaminated water from the condensers of Units 2 and 3. (The one in Unit 1 has already been emptied in Aug 2017.) When that is done the remaining inventory of radioactive substances in the water still existing in the buildings will be about 20% of the amount in 2014. Video of the training on mockups for this operation can be seen here http://www.tepco.co.jp/tepconews/library/archive-j.html?video_uuid=c3pwuwri&catid=69619
- it hasn't been a subject in this forum but they have announced some time ago that they had made a mistake in the setting of water level meters in 6 subdrain wells. They noticed the mistake on Sep 28 and went back to review the water level measurements. They found what they were fearing - that is, water level in subdrain well 203 temporarily fell below the level of the water in the Rw/B building of Unit 1, at some point in time. However, luckily the nearby subdrain wells still had levels higher than that in the Rw/B building, and therefore they concluded that no additional radioactive water seeped out from the building.

http://www.tepco.co.jp/nu/fukushima-np/roadmap/2017/images2/d171026_07-j.pdf
This is the file about progresses regarding the removal of spent fuel from the pools.
- progress in the dismantling of Unit 1 building cover and plans for the wind-protection fence
- page 8~ : progress in the instalation of the dome-shaped building cover for Unit 3

 

[Sotan]

http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images2/handouts_171030_05-j.pdf
(in Japanese)

-The document, which aims to present the general safety principles to be followed during the removal of the nuclear debris in Units 1-3, starts with a summary on what is known (from the various investigations carried out) regarding the state of each reactor/PCV/pedestal area (pages 1-7). It does show a few progresses in the analysis of the data obtained from muon measurements and robots etc., mentions "melted stuff that has adhered to CRD housings and their supporting structures", shows the approximate locations of various types of "sediments" observed (from sand to rock-size to large formations) and so on.

- The remaining pages give a few general ideas regarding the method to be used for removing the fuel debris and what they are thinking that needs to be done to ensure safety during that operation.

Page numbered 9 has a big table with the main safety issues in the first column: ensuring the stability of the PCVs and Reactor Buildings; avoiding criticality; maintaining cooling; maintaining a tight enclosure; avoiding worker irradiation. Column 2 gives details of that specific issue, columns 3 and 4 give specific items to be considered for each of these issues in the initial stage, and in the large scale operation stage, respectively.

Page 10 gives a sketch of the fuel debris removal operation - in the version where they approach the PCVs from the side using special machines with long extending structures.

Page 11 shows the first step - removing obstacles and bringing in the machines; main points and safety issues are also listed (same on next pages too, with each step that follows)

Page 12, step 2: making the hole in the PCV wall and connecting the chain of machines, while maintaining a tight enclosure

Page 13, step 3: removing the fuel debris

Page 14, step 4: transport and temporary storage

Page 15 - getting back to the safety issues listed first in the table on page 9; additional safety consideration and connecting each risk with the operational steps in which they can become of importance.

Page 16 is a listing of all the technologies and machinery needed in the various stages and operations of the fuel debris removal.

Pages 17 and 18: an illustration of the fuel debris removal operations during their full large scale stage. In case of access from the side and in case of access from the top. (I am not sure how "decided" they are on which of the methods to employ, I haven't kept up with reading.)

 

[etudiant]

http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images2/handouts_171030_05-j.pdf
(in Japanese)

Is this good engineering? The whole thing seems brutally complicated.
It might be helpful to set up a B team, to explore alternative approaches. For instance, would entombment of the reactors be feasible and useful?

 

[turi]

Unit 1 sample results are in: http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images2/handouts_171030_07-j.pdf
Didn't have time to have a closer look at them yet.

 

[jim hardy]

snip from spectral plot on page 10
I'm out of my field here.
Does that say whatever it is it's mostly iron?

 

[Sotan]

@etudiant - They haven't ever spoken much about the possibility of reactor entombment, I doubt they would start now

@turi. Nice find thank you!

I went through it and I have to say there aren't a lot of results in there. It's a sort of intermediary report listing the samples, where they sent them, what they obtained until now (not much) and what they hope to achieve in the end. This "end" is somewhere closer to the end of the fiscal year - which is March 31st 2018.

- Page 2: Sample of turbid water containing a powdery suspension from the water at the bottom of Unit 1 PCV (taken in April 2017). Simple X-ray and gamma-ray analyses were done on site. They detected uranium, but no plutonium; and they detected Cs-134, Cs-137, Co-60, Sb-125 (see the table for their concentrations). Page 3 lists additional considerations. The hope to make the uranium findings quantitative. They hope to get some info about the harder layer of sediment from the area where this floating powder was taken. They hope to find out whether the sediment date back to the early stages of the accident - or they were formed as deposits during the long years of water cooling.

- Page 4: Sample of stuff that was obturating the inside of the TIP guiding pipes in Unit 2. You may remember they found just a tiny bit of such matter on the tip of the cable they pushed through the TIP pipe trying to pierce through or dislocate the obturation. The finding is from 2013. They did a simple X-ray analysis on site and found Mo and Cl, among others. Page 5 gives additional considerations. Because the stainless steel of the TIP pipes would melt sooner (at lower temperatures) than nuclear fuel, they hope to find traces of fission products (FP) from the early stages of the accident. The Mo is of high interest due to its affinity to Cs (it forms Cs2MoO4) which could help form a picture of the spreading of Cs in and out of the PCV. They are interested to see if there's uranium in there too. Also (I didn't understand this very well...) there seem to be some bead-shaped insolluble Cesium particles in the sample which could give clues about the timeline of the accident and the state of the reactor during various phases.

- Page 6: Sample obtained by wiping the surfaces of the robot that went in Unit 2 PCV in Feb 017. It is hoped it contains some of the sediment from the CRD rails where the robot travelled. This sample is now being prepared for transportation.

- Page 7: Sample obtained by wiping the surfaces of the underwater swimming robot that investigated Unit 3 PCV in July 2017. During the swim, the thrusters of the robot stirred light material from the deposits at the bottom of the PCV and it is believed that some of that stuff adhered to the robot surfaces. This sample is also being prepared for transportation.

- Page 9 shows (I think) a sample being prepared for SEM (?) examination; it is stuck to some tape and then its surfaces are treated with Platinum (sorry if this sounds/is wrong, I don;t understand it very well)

- Page 10 shows an early report / preliminary result regarding the SEM analysis of the first sample mentioned (the one from the water at bottom of Unit 1 PCV). Top left is bi-dimensional electronic microscope photo; bottom left says "U distribution", probably uranium distribution; and right side graph which raised Jim's interest is an "EDS spectrum", with energy in keV on horizontal scale and KCnt (?) on the vertical scale. The two lines below say: "uranium particles of micrometer-size have been observed" and "we will aim to obtain detailed information by means of TEM analysis". I can't comment anything on the scientific meaning of that Fe though, sorry Jim.

- Page 11 shows an early report / preliminary result regarding the SEM analysis of the second sample mentioned (the one from the TIP pipes of Unit 2) prepared in the same way as shown above, by treatment with Platinum. The comment on the blue line is interesting, "the cylindrical objects seen here are hairs from a toothbrush". 1 through 5, here and above too, are places selected for detailed observation.

- Finaly page 12 refers to "interesting place #1" from the same sample discussed on page 11, with the red rectangle showing the inspected area and the right side graph giving the corresponding spectrum. They mention below that "in addition to Fe, Cr, Ni, Mn and other elements from the various supporting structures, Zr has been detected, which is usually to be found in the internal structures of the reactor and also in the outer covers of the fuel cells (i know it's a clumsy translation... maybe Zr cladding?)

 

[jim hardy]

Thank You Sotan for your remarkably clear translation !

They mention below that "in addition to Fe, Cr, Ni, Mn and other elements from the various supporting structures, Zr has been detected, which is usually to be found in the internal structures of the reactor and also in the outer covers of the fuel cells (i know it's a clumsy translation... maybe Zr cladding?)

Yep, sounds like stainless steel reactor internals and clad . They use low cobalt alloys wherever there's neutrons.

Speculating ;
If there's not much uranium it could mean the ceramic fuel wasn't melted yet when that sample landed wherever they found it, instead was a crumbly mass still somewhat in place, as suggested in many of those ORNL studies. But that's just one line of thought to contemplate.

Sober analysis will continue to trickle out and the puzzle will look increasingly complete.
I just like to consider possibilities so as to not get totally surprised when analysis leads a different direction than i was expecting...

Thanks again,

old jim

 

[etudiant]

Just excellent stuff, Sotan San!
It feels as if we were looking over the shoulders of the people trying to solve this situation.

That said, it may be a problem to have all the effort under one umbrella. Just looking at the various steps of the projected program, one can see lots of points where things can go wrong, stuff can jam or be obstructed, just as we have already seen several times. A proper evaluation would factor in those delays, because the costs continue even if no work is getting done.
Obviously costs may be irrelevant, as appears to be the case in the Hanford cleanup. Still, it can be argued that the Fukushima community would be better served with a restoration of essentially rural conditions around some concrete sarcophagi than with a fifty year experimental clean up effort.

 

[Astronuc]

KCnt (?)

May mean kilo (thousands) of counts.

Then intensity is related to the proportion of the elements, and perhaps to cross sections. It would appear to be mostly iron (Fe) from steel, with a little from alloying elements from stainless steel (Cr, Ni, Si, Mn). The Zr would be from the fuel cladding and channels, and the U would be from the fuel. The core support structures would be stainless steel while the RPV would be a carbon steel (e.g., SA 508 Class 2) with a thin stainless steel liner.

I found an article which states: "all of the Japanese reactors of BWR and PWR types have A533-B Class 1 (QT) pressure vessels except Tsuruga which uses A302-B (QT) . . . ", where QT = Quenched and Tempered.
http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/30/013/30013703.pdf

 

[Sotan]

"Fukushima Today" video - 2017 version.

 

[Sotan]

http://photo.tepco.co.jp/date/2017/201711-j/171109-01j.html
(in Japanese)

One page PDF report, photos and a video showing that the crane and fuel handling machine for taking out the spent fuel from Unit 3 SFP have been shipped towards Fukushima. The FHM is supposed to be raised into position on Nov 11, followed by the crane on Nov 20.

 

[Sotan]

I just found out this link:
http://www.sciencealert.com/probe-c...il&utm_term=0_fe5632fb09-8c185069a7-365520397
"Melted Nuclear Core at Fukushima Finally Discovered by Robot After 6 Years"

I haven't even looked at it properly. I will add more details if/when I can find them.

Edit: I don't think this is new... Sorry guys. I was confused by some images in the video which I had the feeling I never saw before.

 

[Sotan]

A new monthly report has been added regarding the progress in Medium and Longterm Roadmap at http://www.tepco.co.jp/decommision/planaction/roadmap/index-j.html

From the various files posted I find this one interesting as it announces new investigation in PCV of Units 2, and some new results of the investigation into the PCV of Unit 3:
http://www.tepco.co.jp/nu/fukushima-np/roadmap/2017/images2/d171130_08-j.pdf

Page 5 (as indicated by the PDF reader): Some results of the past investigation in PCV of Unit 1 - Jan-Feb 2017.

Page 6: The newly planned investigation. They will use X-6 penetration and CRD rails, reach the grating again and aim to take a better look below the grating where it is possible to find nuclear fuel debris.

Page 7: Compared to last time they will modify/improve the telescopic arm used to gain access into the PCV. It will be longer and allow the deployment of an "investigation unit" which includes a camera. This unit will be lowered through the spaces created by the missing grating panels. The longer arm will allow a new better look above too, in the direction of the CRD housings.

Page 8 shows the way they will maintain a sealed boundary during the work to prevent any gas from inside the PCV from escaping outside (similar with last time).

Page 9 gives the schedule. After training and preparations the investigation is scheduled for end of Jan - beginning of Feb 2018.

------------
Page 11 (again, as indicated by the PDF reader; otherwise the page numbers start again from 1 in each report): Reminder of the past investigation carried out in July 2017 with the underwater swimming robot

Page 12: Top-left photo: Melted and resolidified material is stuck on the CRD flange. Photo below: the level and distances among neighbouring CRD flanges differs.
When the underwater robot peeked through the space between CRD housing towards the surface of the water, a disturbance of the water surface was observed in Area A1 as if water drops were falling in that place. There were several other areas where this was observed - as explained later in the report on page 14. Bottom-center photo: how the CRD housing area of Unit 3 looked before the accident. Top-right photo: CRD flange and PIP cable.

Page 13: Some more results. Melted and re-solidified material adhering to the support structure of the CRD housing in top-right photo. Top center: a presumed "CR guide pipe", broken, has been filmed near a CRD housing flange. The reasons they believe that is a "CR guide pipe" will be explained below in the report on page 16.

Page 14: Water surface disturbance has been observed in locations 1 to 4. The speculate that this may indicate the possibility that there is damage to the RPV not only on its bottom area, but also somewhere else along its body.

Page 16: Characteristics of the pipe observed: it has some sort of rod-like object in it, which has visible "notches" at regular intervals. At the time of the accident the CR were fully inserted and only the CR index tube was left inside the CR guide pipes. This indicates that the larger broken pipe is a CR guide pipe (diameter 28 cm) and the rod-like object inside, with notches every 15 cm, is a CR index tube.
CR guide pipe is made of SUS304 stainless steel that melts at ~1450 deg C. CRD index tube is made of nytrogen-treated (?) XM-19 stainless steel with similar melting point.

Page 17: careful evaluation'estimation of the distances between notches and sizes of various elements seen in these images support the same conclusion regarding the "identity" of these parts.

Page 18: In area designated B1 they cannot confirm the presence of the expected grating. Structures below grating are visible. So the grating must have fallen from that area.

Page 19: The circular rails used to rotate the platform inside the PCV are preserved. Deposits can be seen on the brackets that hold the rails.

Page 20: The surface of the pedestal walls (on the inside) shows scaling off of the epoxy layer as well as some rougher surfaces but no large-scale damage or deformations.

Page 21: Inside the pedestal. Deposits like small rocks can be observed in area C1. Sand-like deposits and fallen grating in area C2. Boulder-like deposit in C3 and C4.

Page 23: They revisit the "water surface disturbances" issue. The blue areas show the places where these disturbances were observed. The red areas show places where they identified damage of CRD housings and adjacent structures.

Page 24: This diagram shows the approximate location of various objects and deposits observed on the bottom inside the pedestal, yellow circles depicting the location of the big boulders.

Page 25 combines the contents of pages 23 and 24.

Page 26 speculates on the nature of some other fallen objects seen inside the pedestal, showing similarities to parts of a Control Rod. A definitive conclusion could not be drawn though because they couldn't find the most distinctive part, the umbrella-like "speed limitor in case of the falling of the control rod" (sorry I have some trouble translating this one).

Page 27 speculates that another object could be the upper "tie plate" (definitive conclusion was not possible). It could be from a A-type 9x9 fuel unit, or from a MOC unit, or from a so-called "dummy fuel unit".

Page 28 and 29: some more tubular objects that resemble CR guide pipes.

Page 30: at about 270 degrees om the pedestal inside wall some damaged cables are observed, probably destroyed by flowing molten material (some of it adhered and solidified here).

 

[Astronuc]

CRD index tube is made of nytrogen-treated (?) XM-19 stainless steel with similar melting point.

XM-19, aka Nitronic 50®, is a high nitrogen austenitic stainless (UNS 20910) with 22% Cr, 13% Ni, 5% Mn and 2.2% Mo with additions of Nb and V ~0.2%. The addition of N in solid solution to about 0.2% provides strength, such that XM-19 in the annealed state has strength similar to that of 20% CW 316. XM-19 was patented (US 3592634) by Armco (now AK Steel) in 1971. It is also used as bolting/fastener material in nuclear systems (usually ex-core applications) and components in variety of industries including aerospace and oil and gas exploration/development. XM-19 (Nitronic 50) is one of several Nitronic® stainless steels. Nitronic is a trademark of Armco and its successor AK Steel. XM-19 is a more common designation, or 22-13-5 (or 22Cr-13Ni-5Mn). It's a material of interest to me in my research.

http://www.nsalloys.com/products/stainless-steel-bar/austenitic/xm-19.html
https://www.cartech.com/en/product-solutions/cartech-22cr-13ni-5mn-stainless/
https://www.atimetals.com/Products/Pages/ati-xm-19-ati-50.aspx
http://www.aksteel.co.uk/1-products/2-stainless-bar/1-nitronic.html
http://standards.sae.org/ams5764e/

In the context of stainless steels, it is considered 'high nitrogen' if it "contains more nitrogen than can be retained in the material by processing at atmospheric pressure." Processing commonly includes electro-slag refining (ESR) or pressurized ESR.

The left image with (CRDインデックスチューブ) is very interesting and useful.