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Stored Used Reactor Rods

04/26/2011 7:01 PM

Somewhere near every nuclear reactor is a storage pool full of used, depleted (Mostly, not entirely) rods. The used rods have many raioactive fission products, and not a high enough percentage of U235 to produce satisfactory power to continue their use. Freshly enriched rods would be about 3-4 % U235/U238. Some of the U238 nas absorbed a neutron and ejected an electron to become Pu239. The U235 fisses to become Iodine, /strontium, Xenon, and a whole pile of fission products, and a shower of fast neutrons. The fissile level of U235 is borderline for reaction, and that would require moderating or slowing the neutrons to continue the reaction. The reactable percentage of fast neutrons is low, so a highly enriched material is required to reach a critical mass for a nuclear bomb.

Normally enriched fuel grade Uranium is not sufficiently concentrated in U235 to proceed to criticality, mainly due to the fast nature of the neutrons. Adding water to the reactor slows the neutrons to the point that chain reaction can be sustained. Without the water, the reaction rate diminishes o the point of little or no heat, a la the Okla natural reactors.

So why do we store slightly depleted enriched Uranium fuel rods in water, where the fission can still proceed, and then if the enriched rods are not submersed, why do they overheat, when the Oklo's cooled without water? Couldn't an encasement of the used rods with the depleted U poison and seal the rods individually, and allow storage in the dry? I understand the US has little more than 475,000 tons of depleted U from it's enrichment plants. Wouldn't that be a fitting use for most of it? Something of a recycling?

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#1

Re: Stored used reactor rods.

04/26/2011 7:45 PM

http://en.wikipedia.org/wiki/Nuclear_reprocessing

I'm not real familiar with this stuff, and I probably shouldn't be posting, but from what I understand, the reason we're not reusing spent, (not really), nuclear fuel can be laid at the feet of Jimmy Carter and his fear of dirty bombs.

Personally, I'm all for it. Bullets can prevent it from getting into the wrong hands.

http://www.smartplanet.com/blog/intelligent-energy/should-we-recycle-nuclear-waste/4907

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#2
In reply to #1

Re: Stored used reactor rods.

04/26/2011 9:25 PM

"Personally, I'm all for it. Bullets can prevent it from getting into the wrong hands."

And ironically, those bullets can be made from depleted uranium.

What is an Okla?

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#3
In reply to #2

Re: Stored used reactor rods.

04/27/2011 12:31 AM

Some long time ago, I read an article about an underground concentration of uranium sufficient to form a natural reactor. I think it was in South Africa. Maybe there was a similar instance in Oklahoma? (Just a guess.)

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#4
In reply to #3

Re: Stored used reactor rods.

04/27/2011 7:08 AM

Okla is the name of the uranium deposit area in Gsabon, Africa, where maybe 15 natural reactors, cycled by groundwater seepinmg in, moderating the neutrons, generating sufficient power to evaporate the water, stopping the reaction, until the next fill. The current guess is that these natural reactors cycled about 8 times a day, for millions of years. The natural uranium ore was 3.5% 235, which in the last several billion years, has decayed to the current levels of 0.75% 235.

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#5
In reply to #4

Re: Stored used reactor rods.

04/27/2011 10:24 AM

OOPS. You did it again. It's Oklo, not Okla.

The Oklo Natural Nuclear Reactor

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#18
In reply to #2

Re: Stored used reactor rods.

04/28/2011 4:29 PM

Okla sounds like a derivative of a candy manufactured in Willy Wonka's chocolate factory.

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#6

Re: Stored Used Reactor Rods

04/27/2011 11:33 PM

Trying to understand your post well enough to reply semi-intelligently!

I think the reason for removing used fuel from the reactor to storage is not so much the decrease in uranium as it is the increase in the fission products. The fission products absorb the neutrons, reducing the reactivity of the remaining uranium.

It is also these fission products that require cooling water in the storage pools. The radioactive decay of the fission products produces heat that must be removed.

I think some of the newer reactor designs utilize the fast neutrons, and can even use the used fuel from thermal neutron reactors as fuel. That's confusing! The "waste" from the current thermal reactors can become fuel for the fast neutron reactors. In addition to using the used fuel, it produces less waste that has a far shorter half-life.

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#7

Re: Stored Used Reactor Rods

04/27/2011 11:48 PM

First off, you are starting from an incorrect assumption. The fuel rods that are removed from a reactor are nowhere NEAR "depleted". They still have about 95% of the potential energy they had when they went into the reactor the first time. A significant portion of the U238 has absorbed neutrons and been transmuted to Plutonium as you note (actually some became Pu240) which in of itself is fissile. By the time a fuel bundle is due for replacement, most of the fission is Pu fission, not U fission. The reason a fuel bundle is removed has little to do with the remaining fissile material and instead has everything to do with the amount of neutron poisons that the fuel has built up. Many of the daughter products absorb neutrons and transmute to other elements but do not fission releasing other neutrons. These are neutron poisons. Reprocessing "spent" nuclear fuel bundles is a process by which these neutron poisons are removed, and the remaining fissile material is reconstituted into a mixed oxide fuel bundle. Early processes chemically separated the uranium and the plutonium separately and was developed for the express purpose of separating plutonium for weapons use, not to re-use nuclear reactor fuel. Newer processes do not separate the uranium from the plutonium and are much less likely to result in nuclear proliferation. The resulting waste from these processes are chock full of industrially and medically important radioisotopes. Now to answer your question concerning spent fuel storage pools. The bundles are kept far enough apart in the pool to prevent a chain reaction from continuing, but when the rods are removed from the reactor they are LOADED with short-lived isotopes which were neutron activated and they are FURIOUSLY radioactive, and the decay heat from the neutron activated isotopes is equally brutal. A rod stored in air would (and did in japan) melt from the decay heat from these short lived isotopes, so the fuel is kept in a pool of water (because water has one of the highest specific heats of any coolant.) until these short lived isotopes have decayed to an acceptable level. once the decay heat generation has dropped to a manageable level the fuel could be reprocessed and reused in recycled fuel bundles, or in the case of the US, encased in steel and concrete casks and allowed to stack up in a storage yard adjacent to the reactor uselessly. All the work and energy to mine and enrich the uranium is lost in such a shortsighted policy decision. The waste stream from reprocessing is only dangerously radioactive for a few hundred years, and many of the longer lived isotopes are industrially useful, whereas the very short lived isotopes are usually medically useful. Contrasted with an unreprocessed fuel bundle which is dangerous for tens of thousands of years.

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#8
In reply to #7

Re: Stored Used Reactor Rods

04/28/2011 12:24 AM

And just yesterday, the gurus in Washington assured us that reprocessing the fuel was not even a remote possibility in the US...

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#11
In reply to #7

Re: Stored Used Reactor Rods

04/28/2011 7:26 AM

Thanks for that excellent answer. It did make me wonder, though, why we can't use that "furious" radioactivity in those depleted rods that are "loaded" with short-lived isotopes to boil water to drive a steam turbine. That is after all, what a reactor does. Is the magnitude of the two processes (fresh rods vs. spent rods) so different as to make it out of the question, and does not the acceleration of this isotope release also decrease half life?

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#16
In reply to #11

Re: Stored Used Reactor Rods

04/28/2011 8:54 AM

Part of the problem is that the thermal mass ratio of the coolant pool to the rods is intentionally kept very lopsided to prevent the possibility of the coolant in the pool boiling away and exposing the rods to the air (as happened in japan when the walls of the pool cracked and the water leaked out.). as a result the actual temperature rise in the spent fuel pool is generally pretty low. I've spoken to divers that have worked in these pools to inspect and repair the pools and was told that the water is a nice and toasty 80 degrees F. nowhere near high enough to drive a turbine.

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#19
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Re: Stored Used Reactor Rods

04/28/2011 4:38 PM

Just a thought. Remove enough water so that is was at 250 degrees, boil water, make steam, etc. I'm trying to figure out how this is anything more challenging than using the premium fuel in the reactor next to this one.

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#20
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Re: Stored Used Reactor Rods

04/28/2011 4:50 PM

Because you don't WANT the water to boil. steam bubbles are thermal insulators and won't carry away the decay heat properly and you'll get hot spots on the surface of the rod and if it gets hot enough it the high temp zirconium cladding of the rod will react with the water and make hydrogen which will collect in the building and go boom, just like it did in Fukushima. An instantaneously formed steam bubble was what caused the runaway of SL-1 in Idaho Falls.

http://en.wikipedia.org/wiki/SL-1

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#21
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Re: Stored Used Reactor Rods

04/28/2011 5:36 PM

You are not listening. I DO want the water to boil. You must make steam to drive a turbine. Just not uncontrollably like in Japan. Was it not spent fuel that caused the hydrogen bubble in Japan? You are telling me that controlling this process is more difficult than controlling U235 fusion. It's a lower energy phase, isn't it? Is it that the energy available for generating heat is less palpable than that created by fresh rods, so lets keep replacing them. The risk is high, I understand. But it is much like debt, in that you may wake up tomorrow with a giant bill, and find that the party wasn't worth it. It seems that the consensus is that we have to keep creating more spent fuel, enlarging the risk scenario. I'm just asking why not consume more of the fuel, before replacing it, with the likely outcome being reduced production, and less spent nuclear refuse.

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#22
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Re: Stored Used Reactor Rods

04/28/2011 5:43 PM

I'm telling you that allowing the water to boil is entirely too dangerous for the amount of energy you'd get out of it. yes i know YOU specifically wan it to boil, but I'm telling you you and everyone in a 50 mile radius of the plant won't like the outcome.

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#26
In reply to #21

Re: Stored Used Reactor Rods

04/28/2011 11:05 PM

Me thinks we need to consider the newer reactor designs that can use the current "waste" as fuel, thus can use it much more completely.

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#23
In reply to #16

Re: Stored Used Reactor Rods

04/28/2011 6:22 PM

But isn't PFR really asking why not adjust the ratio you are describing such that it could boil water and therefore drive a steam turbine? Wouldn't a close loop system, essentially the reverse of a refrigeration system, work in that scenario?

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#24
In reply to #23

Re: Stored Used Reactor Rods

04/28/2011 6:24 PM

Sorry to all. I didn't expand all of the OT comments. Explanation is there.

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#25
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Re: Stored Used Reactor Rods

04/28/2011 8:06 PM

Great answer Rorschach.

Monolithic blocks of text are hard to read on the screen, breaking it into easily readible paragraphs would help.

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#28
In reply to #7

Re: Stored Used Reactor Rods

04/29/2011 5:13 PM

OK, so the U235 is greatly depleted, and the fast responsive Pu is active. The moderation of the neutrons should help reduce the fission rate of Pu, cross-rod. Then water is good for minimizing the rod heat. Isn't the Pu just as exothermal as the U? Pu or U, cabon rods control the rate by absorbing the neutrons. So, if you debundled the rods, embedded them individually in carbon sleeves in steel pipe, packaged them hot, would you need to cool them in the pool? You could still pack the same number of total rods in the cask, but with reduced neutron flux. Would that drop the ongoing reaction to the point of thermal limitation?

How are depleted rod assemblies removed? Are they exposed however briefly, without water? What about fresh fuel. Are the new assemblies exposed during installation?

Just curious that you need water to operate the reactor, and for heat transfer, and then you put them in water to store them. Maybe sulfer, or bismuth, lead, tin could be used to transfer heat in the pools, rather than assisting the U to continue, even wth a lower flux only because of separation. And, after you remove the assemblies from the pool, you have fast neutrons and Pu. Do they again warm up, without the neutron moderation of the water?

Just curious.

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#29
In reply to #28

Re: Stored Used Reactor Rods

04/29/2011 5:27 PM

um, no the U235 is not that greatly depleted either. what has happened is that the u238 in the remainder of the rod has transmuted to plutonium 239 and 240 thereby shifting the ratios. I don't think you grasp just how hot these things can get. Zirconium has a melting point of around 4000 degrees F. they can and will get hot enough to melt the zirconium if they are not kept cool. they are not getting hot due to a continued reaction, this is decay heat ONLY. water is used because it does not transmute to anything nasty when bombarded with neutrons AND it has a very high specific heat which means it can absorb a LOT of energy before it will change phase into steam. the fact that it is a neutron moderator as well is irrelevant (and by the way, graphite is a very good neutron moderator too which is why the Russians used it in their abortion of a reactor design.). water is also a pretty decent radiation shield whereas graphite isn't all that great.

the bundles are pulled from the reactor and promptly carried by a remotely controlled crane over and then into the cooling pool. they are out of the water for no more than a couple of minutes. yes they heat up quite a bit in the interval.

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#30
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Re: Stored Used Reactor Rods

04/29/2011 9:36 PM

In order to help me to understand this issue, could you contrast the potential temperature of the fresh rods to the potential temperature of the depleted rods, and what the primary difficulty is (or would be) in trying to regulate heat output in each closed loop heat exchange to make steam to drive a turbine. Is your statement about the potential melting of zirconium relative to only fresh, only depleted, or both conditions of fuel?

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#32
In reply to #29

Re: Stored Used Reactor Rods

04/30/2011 2:47 AM

I agree, and one more argument on neutron moderators: graphite can burn and water not. That's just one more reason why Chernobyl radioactive impact was so high.

If you put some tons of carbon to burn, the smoke will spread fission products easier.

Kind regards

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#34
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Re: Stored Used Reactor Rods

04/30/2011 9:23 PM

Then water is good for minimizing the rod heat. Yes, water minimizes heat in a sense, because it transfers the heat out of the fuel rods. cabon [I think you mean carbon.] rods control the rate by absorbing the neutrons [No--carbon moderates, but is not primarily an absorber.] So, if you debundled the rods, embedded them individually in carbon sleeves in steel pipe, packaged them hot, would you need to cool them in the pool? [Yes, because the heat we need to get rid of is from decay of the fission products, NOT from the nuclear reaction which has stopped. Besides, your packaging would insulate the heat-producing used fuel rods from the cooling medium.]

How are depleted rod assemblies removed? Are they exposed however briefly, without water? [As explained an another post, they are not in cooling water for a very short time, and do heat up in this time, but not dangerously.] What about fresh fuel. Are the new assemblies exposed during installation? [It doesn't matter whether they are exposed or not, because they do not heat from the decay of fission products--because there are no fission products while it is fresh fuel.]

Just curious that you need water to operate the reactor, and for heat transfer, and then you put them in water to store them. Maybe sulfer, or bismuth, lead, tin could be used to transfer heat in the pools [As explained elsewhere, water is good all-around; but some reactor designs are cooled with liquid metal--sodium I believe.]

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#37
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Re: Stored Used Reactor Rods

05/01/2011 9:39 AM

The problem with liquid sodium is it's propensity to burn in contact with water....

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#9

Re: Stored Used Reactor Rods

04/28/2011 12:40 AM

Spent fuel is often packed in some kind shielding to prevent high radiation leak in normal condition and when these packed containers / drums are stocked, some care is taken to separate them and place them at distance where build up of radiation level and build up of temperature level will be under safe limits. Another critical area is the corrosion of the package to sustain for the desired time.

When some disaster that may happen and earth may shake water may enter the area affecting the normal condition and containers may come together in one corner of storage and then corrosion may leaks the radioactivity, then lot may go wrong as such conditions are often ignored in planning for spent fuel storage. In real sense the so called shielded container may also not be used in hope that fuel one day may be useful for reprocessing and it may be stored as easy to reuse type storage.

All these are policy decisions and at times they look all right and at times they are ending into something very miserable. It is easy to blame people involved but they do have some good thinking of what they are doing except that they have put completely unaware people at very high risk. Are these people allowed to take risk? well, some Government agency is always told about the risk and then also told that nothing will happen and it is all safe and if something happens then we will handle it easily as we have all highly trained people and sophisticated systems to handle it in time. Then comes this big mesh for all to cry and world to look at very big trouble. In fact these disaster makers have planned it and taken risk deliberately for the poor good people around who trusted them for the so called clean nuclear environment. I will say that never trust them and be as far as possible. It will kill you and your next three generations as well.

Nuclear people never tell real truth to the common people. Common people with least nuclear knowledge are at much greater risk as they do not know what is happening and what may happen to them. If it is possible to live away from nuclear plants then one should simply do it and one should keep away himself/herself and family at minimum 50km from nuclear facilities.

There are small disaster in the mid of cities caused by radioactivity handlers like hospitals and education and research institutions and in poor countries by scrap stockings and scrap reprocessing plants.

People should agitate and must through such agencies out from their cities. These people can cause problem unexpectedly. This did happen in mid of Delhi which is capital of India and i happens again and again as if Government loves it that way. Never believe these people when they say everything is safe now. They will say same thing each time it will happen again and again.

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#10

Re: Stored Used Reactor Rods

04/28/2011 4:52 AM

Fresh removed fuel rods assemblies are stored in cooling pools to remove the well explained heat production.

Upon some time they are dry stored in specially designed spent fuel casks. I know at least a couple of licensed models of such casks. Try to google a little bit. Here's a link:

http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/dry-cask-storage.html

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#12
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Re: Stored Used Reactor Rods

04/28/2011 7:35 AM

and then this cooling pool also becomes a serious radiation hazard. perhaps chain reaction here is at low level and below four factor formula criticality level.

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#13
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Re: Stored Used Reactor Rods

04/28/2011 8:11 AM

thanks. All I can say is that jargon and cannibalism are equally useless and anti-social.

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#14
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Re: Stored Used Reactor Rods

04/28/2011 8:17 AM

they are reality and cause of nuclear contamination and in worst case nuclear explosion. perhaps there are not many options for nuclear power business. either it is clean or it is very ugly. it has only those two faces. you can toss if you like.

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#27
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Re: Stored Used Reactor Rods

04/29/2011 11:49 AM

One consideration usually missed in these discussions is the fate of Uranium ore, if we just leave the weird yellow dirt where it is as daughter products are slowly formed and dispersed by (mostly) wind erosion. When we dig it up and burn it for fuel instead of coal, we concentrate the evil fission products where potentially (if we do our job) they can be kept out of the biosphere. Reprocessing "spent" fuel and dust control at the exposed surface deposits would increase this positive contribution.

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#33
In reply to #27

Re: Stored Used Reactor Rods

04/30/2011 9:04 PM

Please explain more of what you are trying to say. I don't follow your logic.

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#35
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Re: Stored Used Reactor Rods

05/01/2011 12:34 AM

I am making a comparison of the natural course of events in which surface ore bodies of soft sedimentary rock are eroded, and uranium and its daughter products are released directly into the air, with a situation in which these ore bodies are refined, enriched, and used to displace coal in the generation of electric power.

What I hadn't said yet is that many coal deposits contain radioactive elements, which are released as flue gasses and fly ash.

In short, the natural course of things for dangerous radioactive elements is that they are formed and leak or blow into the atmosphere unimpeded. When we operate nuclear programs, we have the opportunity to confine these troublesome products in slugs of high lead glass and stash them away in deep boreholes in stable rock.

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#15
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Re: Stored Used Reactor Rods

04/28/2011 8:30 AM

I've had the oppt'y to tour the facilities where these "dry fuel" storage vessels are produced.

"Most impressive" to observe. World class fabrication / weld / verification processes in place. I'm not a nuclear physicist and can't comment on how these vessels perform their intended function....but the processes deployed are the "best in the world" as far as I could see / tell.

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#42
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Re: Stored Used Reactor Rods

05/01/2011 4:22 PM

So what will happen when they bury the pools and reactors in concrete, the Russian sarcophagas? If the rods are still generating heat, then, just like Chernobyl, the concrete will crack from the internal heat build=up. Thje Russians now want aid to replace their crumbling casket.

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#44
In reply to #42

Re: Stored Used Reactor Rods

05/02/2011 6:54 AM

Sorry, I was talking about the OP "Stored used reactor rods". I didn't talk about inherently unstable (at low power levels) reactors in which someone playing disconnect the reactor protection system, with no containment building....

The main inherent risk to human life are just human beings...

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#17

Re: Stored Used Reactor Rods

04/28/2011 9:25 AM

All the spent fuel you need...

Big four toxic dumps - The legacy of the Soviet Union's chemical and nuclear programme

Andreeva Bay - Russia's nuclear dustbin, home to some 22,000 spent fuel assemblies, 35 metric tonnes of radioactive materials, and 12,000m3 of solid and liquid waste taken from decommissioned vessels of the Northern Fleet

Atomflot - Final resting place of the Lepse (the floating Chernobyl), an abandoned supply vessel crammed with 650 damaged nuclear fuel rods

Chernobyl - Engineers are racing to complete the construction of a new shield that will cover the leaking, 20-year-old sarcophagus

Chemical weapons - Russia has the largest declared stockpile of chemical weapons - more than 40,000 tonnes. Much of this is stored in artillery shells and rocket warheads

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#31

Re: Stored Used Reactor Rods

04/29/2011 9:50 PM

Boiling water has excellent heat exchange properties (high latent heat, thin film coefficient, etc.) However, if localized heat flux boils the water too rapidly, and large steam bubbles coalesce, then some portions of hot surface may be exposed to gas only. There the heat exchange is poorer, resulting in hot spots, maybe even hot enough to melt cladding.

I'm not a reactor operator/designer, so this is only a qualitative estimate.

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#36

Re: Stored Used Reactor Rods

05/01/2011 12:46 AM

Since we are in the Sustainable Engineering forum why don't we use some well known technologies to capture the low grade heat from the pool. Economics aside it would be a simple matter to place a heat exchanger into the pool and use that nice toasty(?) 80 degree water to heat the cool side of a heat pump used to heat the some space inside the office complex at the plant. I can see the PR material now, "Nuclear plant office space heated by free waste heat recovered from spent fuel rod pool!!!" Much easier than putting solar panels on the roof. One can only dream.

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#38
In reply to #36

Re: Stored Used Reactor Rods

05/01/2011 9:41 AM

now THAT idea has merit.

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#39
In reply to #36

Re: Stored Used Reactor Rods

05/01/2011 10:04 AM

Nuclear fuel is not removed just for small power change but also removed due to deformation of rods and rupture of cladding or radiation leakage problems. It is not easy to use that hot water for bathing in a swimming pool. It has all nuclear hurdles and world class news generating consequences.

Indian nuclear policy now badly affected by what did happen in Japan. All sensible countries will think twice. In India uneducated villagers agitated and changed the Government policies. I am not sure what happens elsewhere but it is something very interesting that happens in my country. Politicians can not ignore people's view in this country. Scientists have very little say as they do not decide national policies.

Gamma garden and food irradiation are the technology that use some of the nuclear technology in normal life. Even these are not finding place in India.

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#40
In reply to #39

Re: Stored Used Reactor Rods

05/01/2011 12:21 PM

No, fuel bundles that deform or have cladding failures are NOT typical of why fuel bundles are removed. If that is the case you have a criminally negligent and incompetent idiot running the reactor. Anyone who would do that has no business anywhere NEAR a reactor, much less running one.

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#41
In reply to #40

Re: Stored Used Reactor Rods

05/01/2011 1:20 PM

it happens and fuel rods swell and sometime they can't be pulled out nd they somehow cut them or do whatever feasible. It is all there in this business. It is not made public.

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#43
In reply to #41

Re: Stored Used Reactor Rods

05/02/2011 6:40 AM

I don't know about practices in refueling outages in India, but what you say is hard to believe. After 35 years in the nuclear field in several countries and having many outages at my back I never heard about a fuel rod that be cut to extract it from reactor. If someone intended to do such a thing, the radiation levels in the reactor building must raise a lot tripping all alarms. It sure would be on the next news edition and Utility surely get the operation permit revoked by the corresponding Regulatory Body.

Kind regards

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#45
In reply to #43

Re: Stored Used Reactor Rods

05/02/2011 8:52 AM

See my #40...

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#46
In reply to #45

Re: Stored Used Reactor Rods

05/04/2011 7:00 AM

I've read it, but seemed no effects (see #41).

I can agree that sometimes there are rods leaks and they produce an unscheduled outage to change that element. I know a couple of cases here in Spain that were detected by the radiation monitoring system in the primary system (not in the air).

But to say it's a common practice to cut the used fuel rods....

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#47
In reply to #46

Re: Stored Used Reactor Rods

05/04/2011 8:54 AM

my point was that if it is a common occurrence there, then they are incompetent to be running them.

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