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LostCosmonaut

Let's Talk About Spent Fuel Disposal!

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One of the most commons objections to the use of nuclear power is the waste produced. While nuclear energy produces much less waste by volume than other forms of energy such as coal, the unique properties of the waste mean that there are special challenges associated with disposing of it.

 

In this topic, I'll be talking about high level waste, low level waste has far less challenges associated with its disposal.

 

As not all fissile materials within a fuel element will be used up in a reactor, "spent fuel" (especially that which hasn't been reprocessed) still contains significant quantities of U-235. More importantly, large amounts of fission products with varying half-lives are contained in the fuel. The decay of these fission products generates significant waste heat, which provides the main short-term challenge associated with spent fuel disposal.

 

This site; http://large.stanford.edu/courses/2012/ph241/tilghman1/

 

gives the following equation for the waste heat produced by spent fuel; P/P0 = 0.066 × [ (t-ts)-0.2 - t-0.2 ]

 

By messing about with various numbers in that equation, you can see that a year after shutdown, waste heat will be about 1/1000 of reactor output. This seems trivial, but for a 1000 MW reactor, that is still 1 MW of waste heat, more than enough to melt a fuel element with no cooling mechanism. (Just as a simple calculation, a fuel element producing 50 kW of waste heat and a .5 m2 surface area in a vacuum would have an equilibrium temperature of about 1150 K). For this reason, spent fuel is usually stored underwater after removal from the reactor; this allows for convective heat transfer and for the mass of water in the pools to act as a heat sink. The water also provides the additional benefit of being a very effective radiation shield.

 

 

To do list:

 

 

  • Long term storage (WIPP, Yucca Mountain)
  • Medium term storage (NRF, dry storage)
  • Safety concerns associated with storage
  • Answer any other questions as much as possible

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Yeah, specifically they take U-238 and turn it into Pu-239, and also get rid of transuranic wastes (the various isotopes of plutonium and heavier stuff that gets produced in an operating reactor), by converting them to more fission productions.

 

In order to get the fission products out of your spent fuel, you have to chemically separate them somehow. INTEC used to have a facility where they would dissolve spent fuel into solution and separate out the fission products that way, but had the problem of leaving a whole bunch of nasty liquids laying around.

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The biggest issue to date is the nature of our Republic, the power of individual states and NIMBYism. As an example, we have metric shit tons of nuclear waste buried here in Hanford in poorly designed and leaking containers that are seeping into the Columbia River. This should have been moved to Yucca Mountain, Utah decades ago except Senators and Congressmen in Oregon and Utah combined with special interest groups have blocked the transport and removal of this waste.

 

In fact, the only reason why Hanford was built in the first place was because the United States was in a World War and the federal government was able to use its war powers to say "Fuck you, Hanford is being built in the desert and you don't need to know why."

 

It will take another similar catastrophe - perhaps a major earthquake centered in Eastern Washington - which happens every couple hundred years - for the Federal government to be able to work with the states tell Utah Nevada to fuck off, no, you're getting this nuclear waste. And you'll like it.

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*Yucca Mountain, Nevada, but yes, it should have been sent there years ago. My understanding from people I talk to at work is that Harry Reid had a tantrum about it right before it became operational, maybe now that he's on the way out it can open again. This also affects the people I work with in Idaho, since they have an agreement with the state to get a certain amount of fuel out of the state by 2023 and 2035. That agreement was negotiated back when Yucca Mountain was planned to be operational around 2005. Oops. So at the moment we're stuck with WIPP.

 

Yucca Mountain as a site is pretty cool though, the tunnels (already excavated) are 1,000 feet above the water table and 1,000 feet underground.

 

2-YuccaMountain-Fig1.jpg

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Oh shit. It is Nevada, not Utah. Sorry about that. Brain fart on my part there.

 

But yeah, Harry Reid has been the main monkey-wrench here coupled with the fact that my state's two Democrat senators (Murray and Cantwell) are incompetent ciphers who are too afraid to buck Reid on this issue.

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What is up with breeder reactors? Aren't they capable of using spent fuel or turning spent fuel back into fuel grade material? 

 

Most breeder reactors are fast-fission types, and fast neutrons can be used to fast-fission the long-lived actinides.

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When disposing of nuclear fuel, there's three main types of radioactive material you have to worry about. The first is leftover fuel (U235/238/Pu239 or whatever else) and other heavy stuff like actinides and transuranics. The heaviest transuranic elements are a relatively small contributor; they can only be formed in the reactor through repetitive neutron capture and have short half lives. Uranium 238, 235, and other stuff in their decay chains have long half lives, and so put out little heat. However, due to their long half lives (on the order of hundreds of millions of years) they will remain radioactive virtually indefinitely. They are a concern for long term storage.

 

The second, and arguably most important, is fission products. Dozens of different isotopes of medium-mass elements are produced by fission of uranium or plutonium (along with trace amounts of lighter stuff like helium, lithium, and beryllium).

 

800px-ThermalFissionYield.svg.png

 

Many of these isotopes are only produced in trace amounts, and are relatively unimportant.

 

Some of the important ones are;

 

  • Krypton-85; one of the few gaseous fission products, with a half life of about 10 years. Decently radioactive, and poses special challenges in the event of fuel damage or a criticality accident. The krypton remains within the fuel after fission, but could be released if the structure is damaged or if the fuel is heated up enough.
  • Strontium-90;  29 year half life, and one of the more common fission products (roughly 4.5% yield). Acts similarly to calcium in the human body, and lasts long enough to make it into bones. Also outputs a decent amount of heat.
  • Zirconium-93; 1.5 million year half life, roughly 5% fission yield. One of the more common long-lived isotopes.
  • Technetium-99; 211,000 year half life, and roughly 6% fission yield. It is one of the most common long-lived isotopes, and therefore is a concern for long term storage.
  • Cesium-134; 2 year half life, a major source of gamma radiation and heat in the near-medium term. As spent fuel is stored underwater, gamma radiation is the primary concern (unless fuel is damaged and fission products are released).
  • Cesium-135; 2.3 million year half life, and the most common long-lived fission product, with 7% fission yield.
  • Cesium-137; 30 year half life, and 6% fission yield. A major gamma emitter and heat source.

Other well known fission products (Iodine-131, Xenon-135, Samarium-149) have very short half lives or are stable, and are therefore of less consequence for fuel storage.

 

The third major source of radioactivity for spent fuel is crud (not an acronym). Radioactive crud is most composed of activated iron, nickel, and cobalt (including the infamous Cobalt-60). As it is deposited on the outside of fuel, shock or mechanical stresses can cause the crud to separate from the fuel and become dispersed (either airborne or in water). However, much of the crud is not radioactive, and is of small enough quantity that it is less concerning than fission products. Cleaning can remove most crud from the fuel before storage.

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Crud will be a major concern with the first fusion plants though.  D-T fusion has extremely high neutron flux, and all sorts of bits of the reactor itself will slowly be ground into radioactive dust.

Isn't this why plasma-facing wall materials are supposed to be things like carbon and boron? So that you don't get too many horrible activation products?

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Isn't this why plasma-facing wall materials are supposed to be things like carbon and boron? So that you don't get too many horrible activation products?

 

Yes; that should help contain the activation of materials to mostly just the side walls.  D-T fusion puts out way more neutron radiation per unit of energy yield than does fission, and D-T fusion neutrons have about seven times more mean energy than neutrons from fission.

 

Also, about 80% of the energy from D-T fusion is the KE of those neutrons.  I'm not entirely sure how you're supposed to get that to spin turbines.

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I'm not sure what kind of crud would show up in a fusion reactor, unless they make the power generation stage a PWR with the fusion reactor as the neutron source.

 

Crud's honestly more of a concern for an operating reactor than during storage, since it tends to reduce flow through orifices and impede heat transfer.

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I like the idea of basically turning radioactive waste (specifically Plutonium products) into glass with the help of blast furnace slag as mentioned:

 

here... http://www.sciencedirect.com/science/article/pii/S0022311513010313

 

and here: http://www.sheffield.ac.uk/news/nr/nuclear-research-sheffield-university-fukushima-1.324913

 

Dramatically reduces the waste's volume and its radioactivity while being reasonably affordable. 

 

Outside of this, breeders (as some have brought out) are a massive help in disposal and could be much more so in the future. 

 

IMHO MSR burner designs are also supposed to burn through large amounts of waste.

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