The Elephant in the Control Room: The Dilemma of Spent Reactor Fuel

In the last post I dangled a lure in the water (thus far ignored by the fishes...) about large scale nuclear (nü-klē-ər) power plant construction as one possible component of our long-term energy policies. This is of course far different than the issue of Nukeyouler power that may have be discussed over the last 8 or so years. One of the many emotional issues rightly conjured up by the nuclear power option is the problem of the long term storage and management of nuclear waste materials. For now I'm going to forgo any conversation about the safety of the plants themselves and concentrate on a high level review of the storage challenge. Nor am I going to talk about the pollution problems associated with the alternatives which aren't any bed of roses either.

Very Basics

No doubt everyone knows the basics. The traditional nuclear power plant generates electricity in essentially the same fashion as coal or oil fired plants. Most power plants use a heat source to boil water that is used to generate steam. The steam is usually processed into superheated dry steam which is then used to drive turbines as a separate circuit. The turbines drive generators and electricity is produced. This is the typical set up of the majority of reactors in the US (Pressurized water reactors). Coal and oil fired systems use combustion to heat the boilers while nuclear plants generate the heat from fission chain reactions in partially enriched (U) uranium-235 or (Pu) Plutonium-239 containing fuel rods. In the US most reactors are 'light water' types as opposed to 'heavy water' (deuterium) types favored in places like Canada. There are advantages and disadvantages to each type. The light water reactors require enriched uranium (2.5-3.5% U-235) to operate while the heavy water ones can use uranium as mined from the ground (0.7% U-235) because of the superior modulation of heavy water. This slows down fission-generated neutrons sufficiently to help support the chain reaction in the fuel. If they aren't slowed they tend not to be captured by U-235 atoms and thus fission doesn't take place.

Of course there are other types of reactors such as the fast breeder variety that causes all the international concerns. In the breeder reactor much higher grade uranium is required (15-30% U-235) because neutrons will be used not just to sustain the chain reaction in the U-235 fuel but also to bombard a jacket of U-238 surrounding the core of the reactor. Here fast neutrons are preferred as U-238 absorbs these fast neurons in a series of reactions that end up with Pu-239 in significant quantities. These reactors are usually liquid metal types like sodium rather than water so as to not slow the neutrons down before they can react with the U-238. In this way a breeder reactor in 10 years for example can produce enough new fuel to power another reactor an additional 10 years. Pretty impressive but it also happens to be how larger quantities of Plutonium are produced for making bombs - hence the concerns. Only a tiny fraction of the energy contained within the nuclear fuel rods actually is used over the life cycle of the rods.

Nuclear Waste

Pretty much anything that comes into contact with the fission reactions becomes irradiated but the overwhelming majority of nuclear power plant waste comes from the fuel rods used to generate the fission reactions in the first place. This is called High Level Waste or HLW. Obviously you can't just sweep ashes out of the nuclear furnace and spread it over your flower beds (well you can until your gums bleed and hair falls out but...) The process of decommissioning spent nuclear fuel (SNF) involves a number of steps. First the hot rod must be removed from the reactor and cooled (careful...). This may take months to years before the fuel is cooled to a point where further processing is possible. This is why most US reactors have large associated cooling pools on site. When cooled you face a dilemma. Store the rod or reprocess it. Before considering the question of which option may be best it helps to consider what's in the SNF.

What is in the rod at this point (assuming a standard Uranium fuel rod to begin with)? Well it is a satanic mixed bag of nasties that cannot be allowed to mingle with the public for quite a spell - how quite a spell is something we'll come back to in a minute. The SNF includes unused U-235, 238 (~95%); Pu-239 (~1%); and bits of (Sr) Strontium-90; (Cs) Cesium-137; (I) Iodine-129, and some ultra short-lived isotopes. In short somewhere around 97% of the spent rod is Uranium or Plutonium. That is important and we'll come back for that factoid in a minute.

Half'-Life

So how long do you have to store SNF? Well, it all comes down to the question of half-life. Of course you know that half-life is the time needed for a radioisotope to decay to a point where only 50% of the original material is left. What you may or may not know is that for purposes of safety it is generally considered wise to store an isotope for a period encompassing 10 half-lives before declaring it safe (a point where only 1/10 of one percent of the original isotope remains). How long is that? For Sr and Cs the half-life is 29 and 30 years respectively. That means we need to store it for between 290 - 300 years just to be safe. For context, imagine that a fuel rod stored in 1708, about the time that Peter the Great was winning a battle against the Swedes, would just now have safe levels of Cs and Sr. These two (and the Pu of course) are the really nasty ones from a human perspective because Cs can replace potassium and Sr replace Calcium in tissues and bones. Both potassium and Calcium are highly present in animal bodies like ours and therefore the Cs and Sr can be concentrated in animal tissues. This is considered bad. Another problematic one can be (Co) Cobalt-60 with a half-life of about 14 years. It too can be absorbed into tissues and is known to cause cancer in humans.

Plutonium takes a bit longer... The half-life of Pu-239 is around 24,000 years. So technically you need to store it for 240,000 years. A fuel rod used about 10,000 years after the initial appearance of Homo neanderthalensis and Homo sapiens would have safe levels of Pu-239. Pu is one of the most toxic substances on earth. A single grain ingested by a human will result in cancer formation.

Iodine (some isotopes are much shorter), a bit longer. Its half-life is a bit less than 16,000,000 years. So a rod stored by an unfortunate Allosaur in the mid-to-late Jurassic period would have only trace I-129 by now.

But these are mere pretenders to the Uranium twins. U-235 has a half-life of ~ 713,000,000 years. Which of course means that a rod would have had to have been hanging around about 2.5 billion years in space waiting to be incorporated into the protoearth... U-238? Well, U-238 created at the time of the Big Bang (I know it couldn't happen - this is a metaphor) would need another 31 billion years to completely cool off... That of course is a good thing if you like nuclear reactors or living on the good Earth. If Uranium had a short half-life then it would have long since decayed. No nuclear reactors could have ever been constructed and the core of the Earth, still heated in part by nuclear reactions might have cooled, lacking a molten core we would have no magnetic fields and we would have been baked long ago by the solar winds. So it isn't entirely crazy to say that radiation sometimes is our friend.

The storage issue please...

Ah yes, that brings us back to the problem of storage. You need to store the spent fuel a long time. According to German engineers (1) the highest threat dissipates within about 10,000 years. How can that be? Well remember that all radiation is not generated equal. Highly radioactive elements like the Sr and Cs for example are the ones that decay faster. That's why they are so radioactive. They also tend to generate more gamma rays while the slow burners generate more alpha and beta particles (2). Alpha and beta radiation is is less energetic than gamma radiation and consequently much easier to shield against. As we said before, the Sr and Cs are of particular concern so that first 300 years is a biggie. If the Pu is removed then much of the long term danger is mitigated. I'm not saying that 10,000 years would be trivial but it would be a darn sight easier than a few billion in my humble opinion. But that 10,000 year time frame kind of makes you wonder about this NRC regulation though.
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"NRC guidelines: § 72.236 Specific requirements for spent fuel storage cask approval and fabrication.

........(g) The spent fuel storage cask must be designed to store the spent fuel safely for a minimum of 20 years and permit maintenance as required........"

Oh well... Gotta love the gov!

Of course radiation isn't the only storage issue that has to be considered. A great deal of heat is generated by all this decay and has to be managed. All this together means that we have to be careful to store this gorgon's blood somewhere were it won't be disturbed for eons. Real easy to do on a planet with tectonic plates in motion, convection cycles, ground water migration, erosion, volcanism, etc. As all those fossil beds demonstrate (if not the diabolical work of Satan...) simply burying the stuff anywhere probably isn't a good idea. Yucca Mountain in Nevada continues to be debated as our chosen 'valley of the kings' for burying spent fuel for now because of its allegedly stable geology. At our current rate of dumping we are expected to fill what it is licensed to handle in pretty short order unless fuel rods are reclaimed. This site isn't the only one that will eventually be needed if the nation embarks on a more aggressive plan to build reactors but the issues raised in its creation and the time it has taken to move toward activation is instructive of why the issues need to be raised now. (I recommend that you check some sources on the Yucca mountain debates. Interesting politics. The Dept of Energy site is referenced below)

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Mitigating the storage challenge

Are there any other options? Actually yes. Recall near the beginning that 97% of the SNF is either Uranium or Plutonium. This can all be recycled! Wouldn't recommend leaving at the curb for pick up but it can be reprocessed into new fuel rods (or some can be used to make 'silver bullets' - those depleted uranium penetrators that we use against tanks. Ignoring the fact that they very substance scattered about when one hits a tank is the same stuff we say has to be stored for generations here at home. Consistency is a great thing). Using reprocessing we are left with around 3% of the fuel rod to be stored as waste. Still needs to be stored away but it means that the storage sites fill up a lot slower.

Only one little problem. On April 7, 1977 , President Carter banned the reprocessing of commercial reactor spent nuclear fuel. Here in this country (alone as usual) only the military can reclaim spent fuel. The rationale at the time was the concern about diverting of the plutonium through theft. One might claim that leaving the old Soviet arsenal unattended with the keys in the door for 15 years makes the diversion of trace amounts of plutonium in spent fuel rods less of an issue, but you never know. So if we as a nation decide to move forward with new nuclear power plants then we are going to have to readdress this issue of reclaimed fuel. In many ways it doesn't make sense to proceed without it.

So there you have it. A little start in the discussion of whether nuclear power is going to take a bigger place in our futures. There are a lot of issues that go along with this that can't be covered all at once but one thing is certain; nuclear power can produce a lot of energy and we are going to need something pretty soon. Is this the right approach? I have no clue as usual... There ED - more science ;)

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Here a few potentially interesting resources on the issue for anyone interested. Though googling such topics will no doubt land you on some watch list somewhere. Though I cannot imagine the few degenerate souls who on occasion read this stuff, not already being on several....

1) Long Term Storage and Disposal of Spent Fuel http://www.iaea.org/Publications/Magazines/Bulletin/Bull281/28104681520.pdf
2) Waste Management in the Nuclear Fuel Cycle (http://www.world-nuclear.org/info/inf04.html)
3) Spent Nuclear Fuel Reprocessing (interesting Jordanian presentation/symposium)
http://ctaps.yu.edu.jo/STPNE/Symposium-Presentationns/Xounbi-The%20politics%
20science%20Environment%20and%20common%20sense%20of%20SNF%20Reprocessing.pdf
4) US Dept of Energy Site on Yucca Mountain
http://www.ocrwm.doe.gov/ym_repository/index.shtml

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