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Spent Fuel for Heating

by nrg
Tags: fuel, heating, spent
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nrg
#1
Oct3-05, 03:38 PM
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Spent reactor fuel continues to produce heat for a very long time and I am curious as to why this heat cannot be used for something, such as district heating. Obviously there are some barriers out there or it would be done. I'm wondering what these barriers are. Any discussion surrounding this topic would be appreciated. Thanks.
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mathman
#2
Oct3-05, 04:16 PM
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The major difficulties are practical. Since the spent fuel is highly radioactive, you would need a shielded facility with some heat transfer mechanism (circulating water?) which would have shielding penetrations. I suspect there are further engineering problems.
Astronuc
#3
Oct3-05, 09:26 PM
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The decay heat of a nuclear fuel rods is on average about 0.6% of full power operation, and then it drops off very rapidly as time progresses from shutdown. The most radioactive nuclides decay in seconds, minutes or days, and the result is very low heat as compared to what one would want for process steam or district heating.

As mathman mentioned, spentfuel is highly radioactive. It must be shielded, and it must still be segregated from the environment. Hence, when moved out the spent fuel pool, which is under about 10 m or so of water, into dry storage, the fuel is placed in hermetically sealed containers and those are loaded into steel or re-inforced concrete casks. There is some allowance for air circulation to cool the containers, and they do get quite hot, but they have a relatively low heat flux.

Grogs
#4
Oct3-05, 09:36 PM
P: 226
Spent Fuel for Heating

Quote Quote by nrg
Spent reactor fuel continues to produce heat for a very long time and I am curious as to why this heat cannot be used for something, such as district heating. Obviously there are some barriers out there or it would be done. I'm wondering what these barriers are. Any discussion surrounding this topic would be appreciated. Thanks.
Well, as mathman said, spent fuel tends to be quite radioactive. New fuel rods contain just uranium, plus the structural materials fo the fuel pellets/rods/tampers/etc. Since most of commercial fuel is U-238 with a half-life of 4.6 Billion years, it's specific activity is very low. Low activity = low dose. You could walk into a room full of unused fuel rods and receive very little radiation.

Spent fuel rods, on the other hand, contain all the radioactive decay products produced from the fission. Many of these have a relatively short half-life, so you would get a much, much larger dose of radiation standing near a spent fuel rod than an unused one. It's also not all that uncommon for a fuel pellet to rupture during it's life cycle and if that happens the decay products can escape.

If you simply put the spent fuel rods into a pool of water and pumped the water through your house, the escaped decay products would begin to coat the pipes and before too long the pipes would become radioactive too. Also rememer that you'd need electricity to power the pump and you'd probably spend more in electricity for the pump than it would take to heat your home.

What you *don't* want to do is let the water boil off the spent fuel rods. If you have enough spent fuel rods close enough together, the decay heat may be great enough to melt the rods, releasing *all* of the radioactive decay products rather than just a small portion. Without the water, you also have no shielding to stop the radioactivity.

The goals of the cooling pools at nuclear power plants are (as I understand them) twofold. First, the water provides a shield that stops most of the radiation from the fission products. Secondly, you want the fuel rods placed far enough apart so that there is no danger of the water boiling. The idea is to keep them there until a significant amount of the short-lived fission products have decayed away and the rods are safer to handle and produce significantly less heat.

I guess my overall answer to your question would be safety. In order to *safely* extract useful energy from the spent rods you would probably need a set up that resembled the reactor itself: pressure vessel, containment building, primary and secondary coolant loops, radiation scrubbers for the primary loop, etc. Reactors aren't cheap and this setup would produce *significantly* less power than a reactor. You'd probably come out better financially if you just took all that money to the bank, converted it to $1 bills, and burned it.
Paulanddiw
#5
Oct4-05, 11:35 PM
P: 96
Don't forget that these spent fuel bundles contain SIGNIFICANT amounts of fissile material that could diverted to clandestine purposes. Getting a license to do anything with them would be a real challenge--as we have seen.
Astronuc
#6
Oct5-05, 09:26 AM
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The goals of the cooling pools at nuclear power plants are (as I understand them) twofold. First, the water provides a shield that stops most of the radiation from the fission products. Secondly, you want the fuel rods placed far enough apart so that there is no danger of the water boiling.
The spent fuel pool provides a shield of water protecting personnel from gamma and beta radiation from fission products, activated structural materials, and transuranics which are produced from successive n-capture in U-238. The water also provides for cooling, and a heat removal system is necessary. The intact assemblies are stored in the spent fuel pool, in the as-manufactured or as-use geomentry. Rod consolidation - removal of fuel rods from an assembly for denser storage - has been attempted but proved too problematic to be useful or economical.

As spent fuel storage capacity disappears, utilities are opting for dry storage, where assemblies are stored in a dry (He) gas environment in sealed casks.

One issue with nuclear fuel is the presence of failed fuel rods in which the cladding has ruptured, i.e. the fuel rod has lost its hermeticity. The failure rates are roughly 1 or 2 per hundred thousand on average, but at some plants the failure rate can be on the order of 1 per 10 thousand. Separating the failed fuel rod from its assembly and subsequent storage can be costly, and is only economically worthwhile if the fuel assembly has low burnup.

Water flowing in a spent fuel pool will not become radioactive by activation, but it will pick up radioactive elements (corrosion products) which have been deposited on the fuel during operation, or from corrosion of the metal surfaces of the fuel.
Morbius
#7
Oct5-05, 09:31 AM
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Quote Quote by Paulanddiw
Don't forget that these spent fuel bundles contain SIGNIFICANT amounts of fissile material that could diverted to clandestine purposes. Getting a license to do anything with them would be a real challenge--as we have seen.
Paulanddiw,

Actually - NO!!!

If a reactor is operated in the manner of a power reactor - that is the fuel is run to fairly
high burnups - 30,000 - 40,000 Megawatt-Days per metric tonne or more; then virtually
ALL the original fissile U-235 is burned up.

There is a significant amount of Pu-239, but because the fuel has been left in for so long -
there is also an appreciable amount of Pu-240, Pu-242... The spontaneous fission by
the heavier isotopes of Plutonium make high burnup power reactor fuel not the best
weapons fuel.

The U.S. DOE acknowledges that it is possible to use so called "reactor-grade" fuel as
a weapon fuel - but it requires design expertise that one would find only in the design
groups of the most experienced weapons programs in the acknowledged weapons States.

The real "weapons-grade" Plutonium is produced in reactors in which the fuel is not
subjected to the high burnup of a power reactor.

So the fissile content is not really the problem - the problem is the radioactivity of the
fission products which have half-lifes of less than 30 years.

Dr. Gregory Greenman
Physicist


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