Question about spent fuel rod cooling pools in Japàn reactor

GUS

Could anyone who actually works on this kind of thing answer a couple of questions about the current situation in Japans nuclear crisis :

Is it true that cooling pools for spent fuel rods are stored on the rooves of these reactors ? If so we see two of these rooves have been blown off - so were there rods on it and if so what happened to them ?
What are the consequences of the cooling pools for spent fuel rods drying up and the spent fuel rods catching fire ?

Astronuc

In the containment systems at FK, the spent fuel pools are located at the top of the concrete structure, under the metal building part. They are below the floor level (top of concrete structure).

The pool is filled with water to cool the fuel and shield personnel from radiation.

The concern now is that water in the SFPs of Unit 4 and possibly in Units 1,2,3 might evaporate. Then the concern is that those fuel rods will overheat and release their radioactive material - thus compounding the radioactivity release at the plant and out to the environment.

The plant personnel are attempting to prevent the pools from going dry, while trying to minimize the (or prevent further) damage to cores of three units.

GUS

Thanks for the reply.
I still feel a little unclear on the answer though, Ill try and rephrase them .
Were the spent fuel rods that were stored at the top of the concrete structure blown up with the explosion ?
If the pools run dry and the spent fuel rods release their radiocative material is it true that the consequencies globally would/could be far worse than Chenobyl ?

Astronuc

No - as far as we know. The SFPs would have about 30 feet (10 m) of water above the spent fuel - unless they suffered some structural damage during the earthquake, and the damage caused a leak in the pool. That is unknown at this point - or at least has not been reported.

Not necessarily, but that remains to be seen. There are big difference in the reactor design. Chernobyl was a graphite0moderated reactor, which got really hot, and there was basically not containment around the damage core. The Fukushima units are LWRs, specifically BWRs, and we believe there is water in the core/pressure vessel, and the containment (reinforced concrete) is largely intact, but there are some leak paths. However, there are three units with core damage, and one unit with a spent fuel pool at risk. The three other spent fuel pools could also be at risk perhaps. Unfortunately, we do not have the techical details to do a reliable assessment of the status of the cores and SFPs. We can only monitor the activities being released.

Those of us on the outside of the plant lack the critical information and data regarding the stata of the cores, reactor pressure vessels (RPVs), containments, and SFPs - and the fuel in the cores and SFPs.

GUS

Thanks again for the reply - but I really only want an answer ot one question (hypothetically of course) I am not enquiring as to the current state of the reactors, but the potential consequences of the spent fuel pools drying out . The fact that they are not releasing such critical data would indicate that there is quite a lot of spent fuel rods . Im just trying to seperate scaremongering from fact. Ill clarify :

IF the spent fuel pools dry out and their radiocative material is released into the atmosphere could the consequences be far worse than chenobyl (globally) ?

Mech_Engineer

The answer is no, because at Chernobyl a large explosion and graphite fire launched radioactive material high into the atmosphere where it traveled long distances- that won't happen here as long as pressure is properly moderated in the reactors themselves.

GUS

How likely is it that they can "properly moderate the pressure in the reactors" ?
I was reading that the burning spent fuel rods release radioctivity as a gas and so it could get more or less anywhere.

vanesch

My not so educated guess is that there is good hope that this won't have such largely spread consequences, because the driving force behind spreading it (a burning run-away reactor at Chernobyl, while "just" overheating fuel bars here) is smaller. However, the total amount of potential highly radioactive material is probably much larger in this case, and in both cases (Chernobyl and here) there's no, even partial, containment left to hinder this.

So in the hypothetical as of now, but not to be excluded case if the site is totally abandoned,
the potential is there for a very ugly contamination, but one can hope that it won't be spread high up in the atmosphere as in Chernobyl. This will also depend how much "recent" fuel is there, and how hot the stuff is going to become when left to itself.

CAbound

In reactor #3 it looks like the spent fuel rod pool above the reactor is gone! It was located in the roof of the building that was blown off in the explosion on Monday. Why cant they see up close with a satellite to see the damage and confirm? Where are all the spent fuel rods from reactor #3? Did they explode into the air onto the ground and ocean?

CAbound

In reactor #3 it looks like the spent fuel rod pool above the reactor is gone! It was located in the roof of the building that was blown off in the explosion on Monday. Why cant they see up close with a satellite to see the damage and confirm? Where are all the spent fuel rods from reactor #3? Did they explode into the air onto the ground and ocean?

Xylourgos

Dear Astronuc (or others)

Several questions regarding the spent fuel rods;

If water is added to the dried out storage tank, will future radioactive emissions be contained? even if the protective jacket is breached?

Assuming that the outer protective jacket of the spent fuel rods have been damaged/melted, when water is added to the pond in order to cool the reaction, will the water itself be contaminated by the released radioactivity?

If this is the case, how will the contaminated water eventually be collected and contained?

In the case that the water evaporates due to the absorption of the heat, will the evaporated water vapor be radioactive?

What are the decay products in the spent fuel rods? Do they have long half-lifes?

I attach a link that illustrates the location of the storage tank. The tank is not located on the roof. However, it is located in the upper part of the concrete structure. Can anyone explain why the tanks are not designed to be built into the lower ground level? Seems to me to be a more prudent design to have the tank supported on the ground. http://www.npr.org/2011/03/15/134569...-nuclear-plant

It would seem that a relatively small volume of concrete could effectively cover the tank to prevent radioactive leakage - say 2-3,000 m3. Would this be an effective way to contain the radioactive leakage?

Thanks in advance for answering my questions.

I wish good luck to those on the ground containing the problem!

Astronuc

If the fuel rods breach, they release fission product gases, Kr and Xe, and volatiles such as Br and I, and possibly Cs. Cs, like Na or K, is more likely to dissolve in water. Other fission products, which are more or less solid, would be retained in the fuel matrix or dissolve in water. Some UO2 may oxidize, a process occurring mostly along grain boundaries (grain size on the order of microns), and particles of fuel, either single grains or groups, can fall out of the fuel into the water. Single grains or lightest particle may become dust.

Yes, some fission products will dissolve in water, and some particulates will migrate into the water.

Very good question. I would expect that a circulation system would be established, and the particles collected on filters. The challenge will be the volume of water, the surface areas, and the mass of fuel to be collected. Not only is the SFP affected, but also the primary and secondary containments.

Noble gases and some volatiles can be carried with the steam plume.

I'll post a chart of fission products in a bit, maybe in a separate thread. Some decay in seconds, others in the fuel matrix in thousands or millions of years. Short-lived FPs decay away rapidly. In 10 half-lives, activity drops by a factor of 1000, in 20 HL, activity drops by a factor of 1M, and in 30 HL, activity drops by a factor of 1 Billion. Basically the activity drops by 2^N, where N is the number of HL.

The spent fuel pool is located near the reactor cavity so that discharge fuel can be removed under the cover of water from the reactor core to the spent fuel pool. The fuel to be reused/reinserted in the next cycle may also be placed temporarily in the SFP. The SFP is an interim storage to allow cooldown (thermally and radiologically) of the fuel until it is placed in dry storage. In some modern containments, the spent fuel pool is placed in a separate, but well protected area.

Possibly. However, it would complicate longer term plans to decontaminate and decomission the facility. It would also increase the load on the containment below the pool, and possibly increase the risk of structural failure, particularly in case of another significant seismic event.

Experts knock notion of burying Japanese reactors
http://news.yahoo.com/s/ap/20110318/...nuclear_burial

You're welcome.

Me too!


FYI -

SUBJECT: DRAFT FINAL TECHNICAL STUDY OF SPENT FUEL POOL ACCIDENT RISK AT DECOMMISSIONING NUCLEAR POWER PLANTS
http://www.nrc.gov/reading-rm/doc-co...0/4711885.html

Fact Sheet on NRC Review of Paper on Reducing Hazards from Stored Spent Nuclear Fuel
http://www.nrc.gov/reading-rm/doc-co...pent-fuel.html

Safety and Security of Commercial Spent Nuclear Fuel Storage: Public Report (2006)
Board on Radioactive Waste Management (BRWM)
http://www.nap.edu/openbook.php?record_id=11263

"Radiological Terrorism: Sabotage of Spent Fuel Pools"
Journal Article, INESAP: International Network of Engineers and Scientists Against Proliferation, issue 22, pages 75-78
http://belfercenter.ksg.harvard.edu/...terrorism.html


Yay Mothers for Peace!
http://mothersforpeace.org/data/2001..._download/file (it may take a while to download) - 18.4 MB
Alternatively http://mothersforpeace.org/data/2001...eg1738Pdf/view

Xylourgos

Thank you Astronuc for your reply.

Upon reflection, I agree with your assessment of the concrete containment. I believe, as you do, that this would further complicate the issue.

As a final question regarding the spent fuel rod cooling pools; assuming that the tanks are intact i.e. not having significant leaks, that the tanks may be refilled and kept filled, how long in terms of weeks (months) would the rods need to be submersed before they could safely be transported for permanent storage or recycling? Assume that the rods have been recently removed from the reactor.

Based on your knowledge of what's happening inside the double containment reactors, do you believe that the situation is under control? If so, how do you see the situation playing out over the next weeks and months?

As before, thanks in advance for your reply.

Xylourgos

Astronuc

For shipping a fuel rod or small group, a utility would allow perhaps months to a year for cooldown - more so from radiological the thermal. There are dose limits on the surface of the container. They might special containers/casks, if they wish to empth the pool sooner. And they'd need a facility to accept a relatively large quantity of fuel. However, the concern will be damage to the fuel - if the fuel was exposed.

In a BWR fuel assembly, eight of the fuel rods are tie rods. A tie rod has threaded top and bottom endplugs (i.e., the shanks are threaded). The fuel rod is fasted to the bottom cast stainless steel tie plate (or nozzle). The upper end plug shank protrudes above the top of the upper tie plate, through a lock washer, and a nut is fasted over the lock washer.

Here's an illustration of a 9x9 BWR fuel assembly. It is fabricated by NFI and is based on Siemens 9x9-9Q or Atrium-9 Design. It has 72 fuel rods surrounding a centrally located water channel. JNF makes a 9x9 design with two cylindrical water rods displacing 7 fuel rods.
NFI-9x9: http://www.nfi.co.jp/e/product/prod02.html#c
GE8x8 - http://www.world-nuclear.org/uploade...sembly%202.jpg (shows 4 assemblies and control rod arrangement in core)

CFD model of BWR assembly
http://www.ansys.com/Industries/Acad...+Austin,+U.S.A.

Refueling a core - http://www.gereports.com/from-nuclea...iched-uranium/ - what it is supposed to look like.

Moving forward the steps would be:

Decontamination
Assess fuel in SFPs (remote rad-resistant cameras)
Dismantle destroyed upper containments & Debris removal - need to bring in big cranes
Cover/secure SFPs or remove fuel (At unit 4, it would make sense to put the fuel back in the core - without the fresh fuel).
Somehow inspect cores - probably with submersible robots and rad-resistant cameras.
Removing the damaged fuel from core could take several years.
Depending on the contamination - the units might be mothballed for decades.

Xylourgos

Astronuc

I appreciate your response to my questions over the past 2 days. I have learned a lot from your postings and links.

I don't have any tie to the nuclear industry. However, I have been dismayed by the sensational fear mongering exhibited by the msn. I believe the news channels have exhibited far greater meltdown in their hysterical reportage than the situation on the ground in Japan merits. I believe nuclear energy should play a larger role in energy production going forward. Lessons learned from Japan should be applied. Dispense with the hysteria and educate the public. It's going to be a hard sell.

My background is civil engineering. I have a construction company in Greece. We specialize in roads, bridges and tunnels. Should you come to Greece, let me know. I would like to buy you a retsina!

Best regards,

Xylourgos

SredniVashtar

What is the mechanical resistance of these rods?
I see most people here are worried about the water level in the tank, but could these rods resist the impact of large chunks of concrete falling from several meters without buckling and rupturing?

Astronuc

Depends on the temperature, degree of oxidation (metal loss from metal-to-oxide), and degree of hydrding. If there is significant hydriding, e.g., at the end plug or adjacent to sites of oxide spallation, then there would be little resistance - few ksi perhaps.

I suspect rupture in core, and if any occurred in the fuel in the SFP, it's ruptured hydrided areas, or perforations due to excessive oxidation.