Criticality Unlikely at Yucca Mountain

I recently found myself in a discussion about the likelihood of a criticality (BOOM!) in the proposed repository at Yucca Mountain.

The argument for its being a concern went roughly as follows: Each giant steel canister of spent fuel would contain a large amount of U-235 and Pu-239. The reactor in Oklo contained less fissile (BOOM!able) material than that. Therefore, criticality is a real concern. The presence of water or hydrogen (e.g., associated with any steel corrosion products), or collapse of the shelves holding up the spent fuel would increase that possibility.

I’m not convinced a nuclear reaction is so easy to start. For one thing, we’re talking about spent nuclear fuel (SNF). That means it’s already undergone about as many fission reactions as our nuclear engineers could get out of it in the power plant. If it were so easy to cause the stuff to fission more, wouldn’t we still be using it for fuel? The fissile isotopes may be in there, but in order to for them to undergo fission again the fuel would need to be reprocessed, so that the fissile isotopes are separated from the non-fissile material. Even if some of the SNF is suitable for going critical a second time, there are neutron absorbers (I think mostly boron, but I’m not sure) in the package that should prevent this.

The license application for the proposed Yucca Mountain repository specifically addresses this issue in a 2042-page mega-document called “Features, Events, and Processes for the Total System Performance Assessment: Analyses”. According to this analysis, “for a configuration to have potential for criticality, all of the following conditions must occur: (1) sufficient mechanical or corrosive damage to the waste package outer corrosion barrier to cause a breach, (2) presence of a moderator (i.e., water), (3) separation of fissionable material from the neutron absorber material or an absorber material selection error during the canister fabrication process, and (4) the accumulation (external) or presence of a critical mass of fissionable material in a critical geometric configuration.” After analyzing the likelihood of each of these four factors, the report concludes that “the probability of criticality for the in-package location is much less than 1 chance in 10,000 of occurrence within 10,000 years after disposal.” There are a couple of revisions in later documents, but the ultimate conclusion is that a criticality event is so unlikely that there's no point in worrying about it.

Pros and Cons of Yucca Mountain

Ah, Yucca – the juggernaut of U.S. nuclear waste policy issues for the past several decades. Should we bury our nasties there or not?

Even after 5 years of scientific research related to nuclear waste, I still haven’t formed a strong yes or no opinion. In fact, I think the complicated nature of the problem is a big part of why I find it so fascinating. The answer I usually go with is that I think I could maybe support the site itself, but I can’t support the project the way the U.S. government has been planning it. Here are some pros and cons, each of which will eventually get its own blog post.

Pros
1. On-site interim storage can only last so long, and that stuff has to go somewhere.
2. It is in a fairly remote, sparsely populated area.
3. The government already owns the land.
4. The mountain is located in a large basin (the southern Great Basin), so if nasties do get into the water, they will hopefully be contained in just that one area of the country.
5. Supposedly with the planned engineered barriers (a big steel can, basically) the site is good enough that everything will be a-ok for thousands of years. This is according to a computer program that tries to take everything we know about the site into account. This Discover article spends some time describing the program.
6. Sunk cost: The U.S. has already spent over $10 billion prepping for Yucca. Do we really want to start over?

Cons
1. It is unfair to the state of Nevada, which doesn’t have a single nuclear power plant and is resolutely against accepting the waste. There is some discussion about this on Alas! A blog.
2. It is only about 100 miles from Las Vegas.
3. Transporting all the waste out there is not straightforward.
4. It’s in the Basin and Range Province. That means earthquakes and maybe even volcanoes. The main concern in technical circles is actually not a big fat whopper of a shake-down so much as increased fracture formation, leading to more water dribbling in, meaning more corrosion and dissolution. Still.
5. There is an enormous amount of uncertainty in the models for what will happen, especially if you really have to try to predict out to, say, 1 million years. There is so much uncertainty that it is unclear how meaningful any assurance of long-term safety based on these models actually is.
6. I’ve heard the dryness of the area sometimes mentioned as a pro. Well, it’s not actually that dry, and I’m not actually convinced this isn’t a con, because it means you have an oxidizing environment.