Japan Earthquake: nuclear plants Fukushima part 2

[Rive]

I can't even begin to imagine the physical processes that took place in those moments in the pedestal area.

I think I have it.
- The core were relocated, broke the RPV and ended in the slump pits within the pedestal. It started to burn the concrete.
- Not so long after that there came a lot of water through the RPV and ended within the pedestal through the holes of the RPV.
- The water started to move the burnt concrete and carried it through the pedestal opening. I think some of it ended in the torus.
- Since there was a strong water stream through the pedestal opening, we can see a sediment pad in front of it and a depression toward it where the stream was strong enough to carry away everything. That slight depression might be seen on that 13:05:38 picture.

If I got it right then the sediment should be rough near the pedestal opening and should has finer particles further away.

Were i looking for Corium i'd turn off the lights and look in the water for telltalle blue glow of Cerenkov .

There was some complaining here as I recall when for the first inside peek they forgot to switch off the light

 

[Cire]

The pictures from under the reactors do not show an environment that was subjected to a corium melt and heat soak. There are way too many small features visible that would have melted and slagged down to the bottom. You can't melt through a 10" steel vessel, transport TONS of steel melting material into a confined space and still have things like flat mild steel grates, Control rod cables, wires visble.

What you guys are describing as a corium melt would look more like this. (Slag dump).

More then likely we have fuel pellets that melted through their cladding, spilled out and piled up in the bottom of the RPV. When the reactors depressurized high velocity steam pulverized those loose ceramic pellets into find dust and this fuel material was transported out of the RPV with the steam and water.

Just my thoughts on the matter.

 

[Charles Smalls]

The pictures from under the reactors do not show an environment that was subjected to a corium melt and heat soak. [...] You can't melt through a 10" steel vessel, transport TONS of steel melting material into a confined space and still have things like flat mild steel grates, Control rod cables, wires visible.

The images from the unit 2 CRD room show almost exactly that same thing can happen; the environment was subjected to corium heat, which had melted through the steel RV directly above, but the nearby steel grates and control rod cables still remained visibly intact.
(image link here: https://s30.postimg.org/9pswq3t5d/post_1.jpg)

I agree there could be a lot of reasons for the sediment, but the main reason I think it is from spalled concrete and or other materials that weren't in the reactor, is because of the relatively low radiation levels the robot probe measured from it. TEPCO must have suspected this sediment could be from fuel fragments and on page 3 of the latest report, you can see they lowered the robots camera and dose apparatus almost directly onto the surface of the sediment but only recorded 6.3 sv/h.
(report link here: http://s16.postimg.org/4718oap51/Screenshot_2017-03-28-10-46-42-1.png)

I don't know enough to calculate the effects of the water shielding or the amount of time the fuel had to decay, but 6.3sv/h seems too low for the sediment to be made up of a fuel based component.

 

[Sotan]

That is an awesome photo Cire and it's been haunting me since last night :) Indeed that's almost how I picture what happened. (Maybe the flow was not quite as massive, but otherwise that's about how I imagine it...
And as Charles said, while in Unit 1 we don't seem to see enough evidence of such a catastrophic event, I too thought that Unit 2 looked close enough.

More then likely we have fuel pellets that melted through their cladding, spilled out and piled up in the bottom of the RPV. When the reactors depressurized high velocity steam pulverized those loose ceramic pellets into find dust and this fuel material was transported out of the RPV with the steam and water.

Does this mean you believe the RPV bottom was not pierced?
Also, as a layman, I find it very hard to explain to myself how water/steam could turn 100 tons of heavy melted stuff into dust and then carry it away.
If it did - what are the implications? Where is the corium? is it even possible to "remove the melted fuel debris"? All approaches until now appear to be based on the assumption that the corium is pretty much compact and mostly in one place.

 

[Rive]

What you guys are describing as a corium melt would look more like this. (Slag dump).

Since the matter relevant in reactor safety there were several experiments and simulations, so the process is more or less known. Compared to a slag dump, the main difference is that once the core material is relocated to the RPV bottom, the molten pool is around 1500-2500C (slag: 1000-1200C). So it's not that warm red, but shinning white.
Another difference is, that if it's under pressure then the first leaks are not like some spilling, but more like some laser from a wrong sci-fi movie. The kind, when you see the 'laser'. Kinda' like a cutting torch.

According to the known simulations, the whole core of U1 were relocated. The muon scanning seems to support this.

For the other two units, the simulations suggests only a partial relocation. I can't recall if they could find any sediment of the same kind there? If the sediment is missing that's another good clue.

 

[Cire]

Does this mean you believe the RPV bottom was not pierced?

I believe there was probably some leakage of water/steam through the penetrations while the reactor was at pressure and overheating.

Also, as a layman, I find it very hard to explain to myself how water/steam could turn 100 tons of heavy melted stuff into dust and then carry it away.

I think only a small portion of the fuel pellets where transported this way. I believe most are down at the base of the RPV.

A good place to go back too is the Three Mile Island accident. Here is a photo of the fuel rods after they failed.

Uranium fuel pellets that are aged and overheating will be highly fractured in their tubes. When the cladding fails you're not going to get a nice solid pellet dropping out, most likely you're going to get a crumbling bunch of fuel material spilling out. That material would be easily transported in a high velocity steam event.

Now here is a small steam boiler exploding.. Notice how much material and debris is transported. The dark colored steam is mostly the scale and junk from inside the vessel being ejected with the steam. You would only need to eject a very small amount of fuel to get the radiation readings we see today. This would also explain the odd readings in some places of the RPV.

If it did - what are the implications? Where is the corium? is it even possible to "remove the melted fuel debris"? All approaches until now appear to be based on the assumption that the corium is pretty much compact and mostly in one place

.

I don't believe the fuel pellets ever melted (in a corium context) at Fukushima. The cladding most certainly did. There will be lower temperature metals that did melt and mix with the pellets. Corium has only ever been seen at Chernobyl. Its a totally different reactor with different materials present.

Personally I think the safest way to clean the reactors is to dissolve the fuel and slurp out the radioactive byproducts then process that waste. I'd start by installing sprayers all around the RPV and inject an acid to dissolve the fuel/material and pump out the RPV as you go.

That's a grossly over simplified explanation. You would of course need to make sure that you where not leaking this soup out of the containment, etc.

 

[Sotan]

Thank you Cire. Very informative and interesting post!

Edit: Just noticed Tepco published some additional images resulted from enhancing the photos taken during the investigation of the PCV of Unit 2. Take a look:
http://photo.tepco.co.jp/date/2017/201703-j/170330-01j.html

Also, there is a short film which presents in summary the robotic investigation of Unit 1 PCV
http://www.tepco.co.jp/tepconews/library/archive-j.html?video_uuid=w870f623&catid=69619
(the animation at 00:47 insists on the "slag dump" supposition though.)

 

[jim hardy]

Thanks Cire for two very common sense posts.

TIP tubes almost certainly melted which makes a path out of vessel to that equipment room, wherever it is in a BWR. They're small tubes, in my PWR less than 1/4 inch, but crumbled up fuel debris could get carried out of the vessel that way.
I don't recall seeing what is level of radiation in area of TIP drives.

 

[jim hardy]

Edit: Just noticed Tepco published some additional images

I was never under a BWR , just my PWR.

This, snipped from last one in Sotan's link, looks to me like small leaks have coated everything with sea salt. Had bottom of vessel melted one would expect those rod drives to be down on the floor ?

 

[Rive]

This, snipped from last one in Sotan's link, looks to me like small leaks have coated everything with sea salt.

After so much time, in that rainy environment?

Had bottom of vessel melted one would expect those rod drives to be down on the floor ?

I could not find any (serious)experiment/simulation where it's about a real control-from-bottom type reactor, but every other one suspects the break on the sides, where the curved bottom meets the cylindrical part.
Since U1-U2-U3 have plenty of additional cooling surface on the bottom (control rod penetrations/drives), I think this scenario is even more likely.

I don't believe the fuel pellets ever melted (in a corium context) at Fukushima. .

I hope it's you who is right, but the simulations so far proved to be pretty accurate.

 

[jim hardy]

After so much time, in that rainy environment?

Yes. Boric Acid too, It makes a white crystalline deposit that looks a lot like this.

Immediately after the accident US military airlifted tons of boric acid to them.

 

[Charles Smalls]

Hey Sotan,

Can you shed any light on what the gentleman in the short film is saying at the 1:10 mark? He seems to be talking about the pedestal doorway and the cube shaped hole in the pedestal floor.

 

[Sotan]

Hey Charles.
He's saying "It is presumed that the fuel from the RPV melted and fell inside this supporting structure (pedestal) and then, through openings such as these, spread into the space outside." It's just their working hypothesis.

- I also watched most of the 2h press conference on March 30th related to progress in the Mid- and Long term Roadmap. There were a few materials shown there some of which are not yet posted on the site (the enhanced photos of Unit 2 were posted but others not yet). They have new data on the operating floor of Unit 1, radiation readings of 100-200 mSv/h in most parts, except in the area of the well plug where the readings rise to 500-600 mSv. They inserted a sensor through the concrete plates that make up the well plug (which as you know have moved in the accident), and readings went up as high as 2 Sv/h.

- Also, they announced that they are ready to proceed today (March 31st) with the sampling of water+floating stuff from Unit 1 PCV, and they are trying to analyze some of the stuff that got stuck onto the tracks of the robot that drove on the gratings of Unit 2.

Edit: The new Mid and Long Term Roadmap Progress materials are posted here (in Japanese only for now)
http://www.tepco.co.jp/decommision/planaction/roadmap/index-j.html
Look for the section at the top that says "New" and "2017年3月30日".

 

[Sotan]

A few selections from those large files in the latest Roadmap progress update:

http://www.tepco.co.jp/nu/fukushima-np/roadmap/2017/images1/d170330_07-j.pdf (in Japanese)

- Page 7 (as given by Adobe Reader): this is hos they figure the operating floor of Unit 1 looks, assuming the rubble from the roof was removed. The FHM and its trolley are on the floor, the overhead crane is bent but hanging in, the well plug slabs look are displaced.

- Page 8-12: the well plug, before the accident and after. The photos are a bit hard to follow (I didn't have too much time). But the colored schemes show the present configurations (with approximation) of the blocks, as seen from various directions. If until now they had confirmed only the displacement of upper and middle layer of concrete slabs, now they can say that the lower layer has been displaced too.

- Pages 13-18: before, they had measured the radiation doses on the operating floor; this time they also inserted the sensor through the small openings among the well plug concrete slabs (the sensor went down as low as the lower layer of slabs, not further). Page 15 shows that the values are 113-130 mSv/h in areas unrelated with such openings, but rise to 443-512 mSv/h in the openings, showing - I think - that there is radiation coming from inside the well. Pages 17-18 show a sort of improvisation: they attached a radiation meter to a (what's the word) endoscope? that could be inserted between upper and middle layer of concrete. You can see on page 18 values rising as the tip of the endoscope was inserted towards the center of the well plug, up to 2.2 Sv/h.

- Pages 19-20 show an evaluation of the particle/grains of the dust on the operating floor (they want to avoid spreading such dust in the surroundings). 92% of those particles are 0.3~0.5 microns in size.http://www.tepco.co.jp/nu/fukushima-np/roadmap/2017/images1/d170330_08-j.pdf (in Japanese)

- From the last page - conclusions regarding the enhanced photos from Unit 2 PCV:
- By enhancing those photos they were able to extract further information regarding the extent of grating affected (fallen) as well as the degree of damage in PIP cables and other structures from the CRD housings.
- They found an additional panel of grating that has fallen, further back than what had been seen in the first evaluation - but also one panel that was confirmed to be in its original place, in an area not seen in the first evaluation.
- Cables or similar fallen objects can be seen traversing the open spaces formed where grating panels have fallen.
- TIP guide pipes appear to lay scattered on the grating.
- Not much damage in the structures located area above the CRD rails.
- In an area center-left of the pedestal (?) it was not possible to confirm the presence of PIP cables and LPRM cables. (They might be gone or could be there but are simply impossible to see in these photos.)

 

[oksuz_]

I am curious about why did the operators in Unit 1 allow the isolation condensers to boil dry?

 

[Sotan]

I can only quote from two official reports as follows:

The IAEA report: http://www-pub.iaea.org/MTCD/Publications/PDF/Pub1710-ReportByTheDG-Web.pdf

P.34 "Just before the tsunami struck, the Unit 1 isolation condenser was stopped by the operators in accordance with established operating procedures to control the reactor cooling rate. This was accomplished by closing the valves (located outside the primary containment vessel and DC operated, as shown in Box 2.2). About 2.5 hours after the loss of indications, at 18:18 on 11 March, some of the status lamps for those valves were found to be functioning, confirming that the control valves were closed. The operators attempted to start the isolation condenser by opening those valves. However, the isolation condenser did not function, indicating that the AC powered isolation valves inside the primary containment vessel were closed.(Footnote 40) Thus, the fundamental safety function of core cooling at Unit 1 was lost when the isolation condenser was stopped by the operators just before the tsunami, and the Unit 1 core heated up from that time."

Footnote 40: The valve positions were not clear to the operators owing to the uncertain timing and sequence of each type of power loss that would determine the status of isolation valves. All the isolation condenser valves would keep their position when the AC power was lost, but the AC powered isolation valves would close, by design, if the control power (i.e. DC power) was lost.

P.56 "The isolation condenser (see Box 2.2) for cooling Unit 1 started automatically as the result of a high reactor vessel pressure signal. It opened the isolation valves in the condensate return lines (other isolation valves in the lines were open during normal operation) when the reactor shut down following the earthquake. As required by the operating procedures, the isolation condenser system was stopped and restarted several times by the operators to prevent the reactor from cooling down too quickly and causing thermal stress exceeding the reactor pressure vessel design values. This was done by opening and closing the isolation valves in the condensate return lines [8]. At the time the tsunami inundated the site and electrical power was lost, the operators had just stopped the isolation condenser system by closing a valve on the return line outside the primary containment vessel. Operators had no information available on the isolation condenser valve positions, and it was not until approximately three hours later that they first attempted to manually restart the isolation condenser. The operators were not fully trained to understand how the valves worked under these conditions. They ultimately made two unsuccessful attempts from the main control room to restart the isolation condenser by opening the outer isolation valves. The operators had no procedures to manually operate the isolation condenser. At the time of writing, the exact location of all of the valves in the isolation condenser system was unknown, but indications are that the isolation condenser did not function following the tsunami [8]."

P.72 "Operators at the Fukushima Daiichi NPP had not been specifically trained on how to manually operate systems such as the Unit 1 isolation condenser and fire trucks as an alternative source for low pressure water injection."

The report made by the Commission of the National Diet of Japan https://www.nirs.org/wp-content/uploads/fukushima/naiic_report.pdf

P.31 "5.The isolation condensers (A and B2 systems) of Unit 1 were shut down automatically at 14:52, but the operator of Unit 1 manually stopped both IC systems 11 minutes lat-er. TEPCO has consistently maintained that the explanation for the manual suspen-sion was that “it was judged that the per-hour reactor coolant temperature excursion rate could not be kept within 55 degrees (Celsius), which is the benchmark provided by the operational manual.” The government-led investigation report, as well as the government’s report to IAEA, states the same reason. However, according to several workers involved in the manual suspension of IC who responded to our investiga-tion, they stopped IC to check whether coolant was leaking from IC and other pipes because the reactor pressure was falling rapidly. While the operator’s explanations are reasonable and appropriate, TEPCO’s explanation is irrational."

 

[nikkkom]

Personally I think the safest way to clean the reactors is to dissolve the fuel and slurp out the radioactive byproducts then process that waste. I'd start by installing sprayers all around the RPV and inject an acid to dissolve the fuel/material and pump out the RPV as you go.

That's a grossly over simplified explanation. You would of course need to make sure that you where not leaking this soup out of the containment, etc.

http://svcf.jp/pdf/Three_Mile_Iland2NP-6931.PDF
"The cleanup of Three Mile Island Unit 2: A technical history, 1979-1990"

If you read the above excellent document, you'd see what sort of problems arise when someone needs to remove damaged fuel from an *intact* RPV, on an *intact* plant. Nothing was easy. Even things which "had to be" relatively easy, weren't.

Removal at Fukushima would be some ten times harder, per each reactor.

As an example, the "dissolve everything with acid" plan would dissolve concrete foundation faster than it dissolves ceramic fuel pellets. Not good.

After reading TMI cleanup document, I'm firmly in the "these ruined reactors do not need cleanup, they need cocooning a-la Hanford" camp.

 

[Cire]

As an example, the "dissolve everything with acid" plan would dissolve concrete foundation faster than it dissolves ceramic fuel pellets. Not good.

We have plenty of concrete there. We can afford to dissolve a bunch of it if needed. The fuel / fragments have much higher surface area per volume then the foundation does. You could also pre-treat the foundation concrete with a rubber sealer like Kalrez from DuPont or a Silicone.

A spray on Silicone works great because you can later come back and peel off that thick layer and take with it any particulate it managed to embed when it was applied.

Here are some patents on the process.. There are many ways to go at it.

https://www.google.ch/patents/US20030092954
https://www.google.ch/patents/US5523513

A good read:

http://www.cresp.org/NuclearChemCou...el Cycle Separations - Final rev 2_3_2_09.pdf

-

 

[nikkkom]

We have plenty of concrete there. We can afford to dissolve a bunch of it if needed. The fuel / fragments have much higher surface area per volume then the foundation does.

However, concrete is noticeably dissolved even by "weak", household-grade acids, whereas many kinds of ceramic are not. Reprocessing uses concentrated nitric acid to dissolve spent fuel - I don't think you propose to pour *that* into the PCV?

You could also pre-treat the foundation concrete with a rubber sealer like Kalrez from DuPont or a Silicone&

You cannot pre-preat foundations inside PCV. It's no-go for humans (rad fields of several Sv/h), and robots won't do, they are far too clumsy.

 

[Cire]

However, concrete is noticeably dissolved even by "weak", household-grade acids, whereas many kinds of ceramic are not. Reprocessing uses concentrated nitric acid to dissolve spent fuel - I don't think you propose to pour *that* into the PCV?

Nitric acid seems completely appropriate for the job. The RPV itself is clad in stainless steel (308/309) and most of the internal plumbing is inconel. Both are resistant to Nitric acid. You simply control the dissolution rate of the fuel debris by varying the concentration of the acid.

I would start by washing the external surfaces in the dry well, pedestal and other area to dissolve and remove as much external material as possible, thus lowering radiation levels around the RPV. I'd then pump out the suppression wet well and treat it next.

Once done I'd switch to injecting directly into the RPV and work via the drains; trap and extract whatever leaks out of the bottom.

The more fuel and actinides you can remove the lower the radiation and the easier it becomes to send people to work in those areas.

There are no engineering problems here that aren't solvable.

 

[nikkkom]

Nitric acid seems completely appropriate for the job. The RPV itself is clad in stainless steel (308/309) and most of the internal plumbing is inconel. Both are resistant to Nitric acid.

Sure, inside surface of the RPV is okay.
Are _other surfaces_ okay? Like, the inner surface of the PCV? The basement of PCV?

I would start by washing the external surfaces in the dry well, pedestal and other area to dissolve and remove as much external material as possible, thus lowering radiation levels around the RPV. I'd then pump out the suppression wet well and treat it next.

For starters, _how_ would you do that? You cannot send people into PCV, you need to do it with robots. And god forbid a robot would break while inside, or just tip over, be caught in the piping, or otherwise unable to exit. Now you need to dissolve the robot too.

As to "removing material", yes, you can achieve quite a bit of that with acid. The material in question would be, foremost, concrete floor and basement walls. Not good.

There are no engineering problems here that aren't solvable.

Correct. As long as cost and schedule are completely ignored.

BTW, looks like you did not read the TMI cleanup document. Please do that.
Especially section "7.4.3 Containment basement" - it's an eye-opener.
They needed to clean up a basement polluted "merely" by the radioactive primary loop water - not a single gram of solid fuel made it out of RPV during TMI accident. Should not be too difficult, right? "There are no engineering problems here that aren't solvable".
So, take a guess. Did TMI cleanup operation succeed in cleaning up TMI-2 containment basement?

 

[Rive]

As to "removing material", yes, you can achieve quite a bit of that with acid. The material in question would be, foremost, concrete floor and basement walls. Not good.

This job is not exclusively for nitric acid.

https://en.wikipedia.org/wiki/In_situ_leach#Uranium

(sodium bicarbonate, ammonium carbonate, or dissolved carbon dioxide)

I dare to say that there might be 'concrete-friendly' choices. The real problem - I think - is that nobody tried this technology with corium and/or heavily contaminated mixed materials and nobody wants to invest in a new technology which will be used only once.

 

[Hiddencamper]

Sure, inside surface of the RPV is okay.
Are _other surfaces_ okay? Like, the inner surface of the PCV? The basement of PCV?
For starters, _how_ would you do that? You cannot send people into PCV, you need to do it with robots. And god forbid a robot would break while inside, or just tip over, be caught in the piping, or otherwise unable to exit. Now you need to dissolve the robot too.

As to "removing material", yes, you can achieve quite a bit of that with acid. The material in question would be, foremost, concrete floor and basement walls. Not good.
Correct. As long as cost and schedule are completely ignored.

BTW, looks like you did not read the TMI cleanup document. Please do that.
Especially section "7.4.3 Containment basement" - it's an eye-opener.
They needed to clean up a basement polluted "merely" by the radioactive primary loop water - not a single gram of solid fuel made it out of RPV during TMI accident. Should not be too difficult, right? "There are no engineering problems here that aren't solvable".
So, take a guess. Did TMI cleanup operation succeed in cleaning up TMI-2 containment basement?

There was fuel material on the containment floor at TMI. Not melted through the vessel, but gaseous and transported fuel pellet fragments.

Remember a relief valve was stuck open, and cladding failure occurred while the relief valve was open. This allowed for transport of pellet fission products out to the pressurizer relief tank, which had a blown rupture disc, and allowed those fission products to transport to the containment itself where containment sprays would have relocated it to the floor.

 

[nikkkom]

There was fuel material on the containment floor at TMI. Not melted through the vessel, but gaseous and transported fuel pellet fragments.

Remember a relief valve was stuck open, and cladding failure occurred while the relief valve was open. This allowed for transport of pellet fission products out to the pressurizer relief tank, which had a blown rupture disc, and allowed those fission products to transport to the containment itself where containment sprays would have relocated it to the floor.

Yes, at TMI-2 fission products were transported outside RPV - in forms such as salts, vapor (iodine) and gases (Kr, Xe). But not macroscopic (say, millimeter-sized) bits of fuel ceramic. PRV on pressurizer is rather far away from the RPV.

Compared to the state of Fukushima reactors, TMI-2 cleanup is very easy. Yet, even it was actually quite difficult in real life, and containment basement was eventually left not cleaned up - after years of work they finally admitted it does not make sense: too much work and and too many $$$ for questionable gains.

 

[Sotan]

http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images1/handouts_170407_05-j.pdf
Just two photos documenting the sampling of the water and sediment at the bottom of Unit 1 PCV, carried out yesterday.

 

[Rive]

Just two photos documenting the sampling of the water and sediment at the bottom of Unit 1 PCV, carried out yesterday.

If that guy really dares to keep the sample in hand, then there is no way it's fuel.

 

[Gary7]

Were i looking for Corium i'd turn off the lights and look in the water for telltalle blue glow of Cerenkov .

Would there still be Cherenkov radiation after 6 years and considering all the impurities in the melt and the deformed geometry, etc...?

 

[jim hardy]

Would there still be Cherenkov radiation after 6 years and considering all the impurities in the melt and the deformed geometry, etc...?

honestly - i don't know whether there'd be enough to see. With the lights out our whole spent fuel pit was aglow. Of course the fresher the element the brighter.

 

[etudiant]

There are several immediately accessible SFPs on the site that could be used to test the idea. Their fuel loadings also date from the accident or prior.
However, the water in the reactor structures is not especially clear according to the images sent back thus far. Would that dim the glow too much?

 

[mheslep]

Would there still be Cherenkov radiation after 6 years and considering all the impurities in the melt and the deformed geometry, etc...?

There are photo multiplier based devices available to pick up the Cherenkov from a single beta decay. As far as what can be seen by eye from fuel in water:

Cherenkov radiation can be used to detect high-energy charged particles (especially beta particles). In nuclear reactors or in a spent nuclear fuel pool, beta particles (high-energy electrons) are released as the fission fragments decay. The glow is visible also after the chain reaction stops (in the reactor). The cherenkov radiation can characterize the remaining radioactivity of spent nuclear fuel, therefore it can be used for measuring of fuel burnup.

http://www.nuclear-power.net/nuclea...-particles/beta-particle/cherenkov-radiation/