As I discussed in a past posting on uranium enrichment, the uranium we dig out of the ground is unable to sustain a nuclear chain reaction without a lot of coaxing – it has to be immersed in heavy water or surrounded by graphite as opposed to natural (light) water. This is why we enrich uranium – boost the amount of fissionable U-235 from the natural abundance of 0.72% to a richer 3% or higher and we can make a nuclear reactor; raise it to the point where 90% of the atoms are U-235 and we can make a bomb. This is why most nations stick with enriching uranium to 20% – high enough to produce a useful number of neutrons in the core (more on this in a moment) but not enough to explode. And this is one reason why the United States and Russia are both working to replace highly enriched fuel in research reactors (there are 82 around the world at the moment) with less dangerous stuff. As an aside – under the Non-Proliferation Treaty, non-nuclear weapons powers are not permitted to enrich uranium beyond 20% U-235 for weapons production, but a loophole in the treaty permits high-enrichment uranium for military reactors and for civilian purposes.
So (you might wonder), if a reactor can sustain criticality with only 3% U-235 then why would we even want to go to the extra work to make reactor fuel potent enough to explode? And it’s not only research reactors, by the way, that rely on such highly enriched uranium – the reactors on American nuclear-powered naval vessels are fueled with weapons-grade uranium (although other nations have been moving away from HEU fuel).
There are a few reasons, actually, depending on the use to which the reactor will be put. On a military vessel, for example, the reactor has to be compact enough to fit inside a submarine or ship hull. Since each fissioning uranium atom releases the same amount of energy, cramming more fissioning atoms into the same volume means that the reactor produces more power. If my submarine reactor had used commercial-grade fuel then the reactor would have been far too large (or far too wimpy). So in our case the reason for such high-powered fuel was space considerations.
A side benefit was that our core was longer-lived than would otherwise have been the case. As the U-235 atoms fission they are lost to the core; as they are used up the U-235 enrichment necessarily drops. When it drops too far the reactor becomes less-suited for combat operations for a number of reasons – packing more U-235 into the core means that the reactor will last longer before it needs to be refueled. So running weapons-grade uranium meant that our core lasted over a dozen years before it was replaced – compare this to the typical 18 months or so between refuelings at the typical commercial reactor.
Both of these are good reasons, but neither really applies to a university-based research reactor (or an industrial isotope production reactor). Universities are not as space-conscious as submarines and a reactor that operates only intermittently doesn’t really have the longevity demands of a military plant – so why run on weapons-grade uranium?
The main reason comes down to neutron flux – if each fission produces 2-3 neutrons then packing more fissionable atoms into the same volume means that the number of neutrons in each volume of the core (the neutron density) will be higher than with lower-enriched fuel. And if the purpose of the reactor is to produce, say, radionuclides for medical or for research purposes (cobalt-60 is produced when a stable cobalt-59 atoms captures a neutron) then a higher neutron density means a higher rate of isotope production. A reactor fueled with weapons-grade uranium produces more radionuclides at a faster rate than one with lesser concentrations. This is why we once made civilian reactor fueled with HEU.
In the 1950s and 1960s – before the Non-Proliferation Treaty – the United States and Soviet Union both built research reactors for a number of nations, hoping to win followers during the Cold War, and many of these reactors contained weapons-grade uranium. Today this seems sort of silly – spreading highly enriched uranium around the world – but at the time, in a world that was more or less stabilized by the Cold War and in which fears of terrorism did not include weapons of mass destruction, it sort-of made sense. The problem is that today, with the Cold War over and terrorism (not to mention wanna-be nuclear states) on the rise, we’ve got to address the problem.
There are a few potential snags. One is that reactors fueled with less-enriched uranium have a much lower neutron flux and it simply takes a bit longer to get the same amount of neutron activation than with an HEU-fueled reactor. This is not a show-stopper so much as an inconvenience, but it must be acknowledged. Another is that, for scientists running experiments that require a long-duration irradiation at very high neutron flux, there may be few (if any) viable alternatives. But there are few experiments that really have both of these requirements so this affects few (if any) active research programs.
The other snag is that refueling reactors is neither simple nor cheap. The irradiated fuel is chock full of radioactive fission products and must be handled carefully to avoid any harm to the staff engaged in the work. It also has to be kept shielded and cooled (not to mention secure) during transport, and then it has to be placed into storage or recycled at the end of its journey. Not to mention refueling the reactor with fresh low-enriched uranium if the reactor is still to be used. And that doesn’t even get into the details of modifying the reactor’s operating license, re-training staff, and so forth! There’s not a step in there that’s cheap or easy – but it’s certainly better than the alternative, which is why the US and Russia have been engaged in refueling or defueling this part of their Cold War legacy. As with my sons’ room, cleaning up this particular mess is neither exciting nor easy – but it needs to be done.