Every so often we hear something in the news about nuclear reactors fueled with highly enriched uranium (HEU); usually with regards to nuclear weapons proliferation. Back in the good old days both the US and USSR constructed over a hundred small HEU-fueled reactors and shipped them all over the world – Uruguay had one, there were some in the Balkans, the Ohio State University reactor was fueled with weapons-grade uranium, and there were plenty more. Given today’s concerns about locking up and accounting for every gram of weapons-grade uranium it’s only natural to wonder “What were they thinking?”
Even today there are a number of reactors fueled with uranium that could be turned into nuclear weapons. One category is military reactors – the nuclear reactor on my submarine was fueled with HEU. But outside of the military, the biggest reason to use high enrichments is for research and to produce radioactive materials for research and medicine. Here’s why.
First a little bit of background. Two kinds of radionuclides are produced in nuclear reactors – in one, stable atoms that capture a neutron can become radioactive by a process called neutron activation and the products are called neutron activation products (activation products for short). Cobalt-60 is a neutron activation product, formed when stable cobalt-59 captures a neutron to become radioactive cobalt-60. In the other process, a uranium atoms splits (fissions) and the fission products are radioactive; these include the nuclides we saw in Fukushima (radioactive isotopes of iodine and cesium mainly) as well as molydebenum-99 (the parent nuclide of technetium-99 that is the workhorse of nuclear medicine) and others.
So – to create a neutron activation product you need two things – target atoms (such as cobalt-59) and neutrons; the neutrons come from uranium fission. All things being equal, a larger number of neutrons means a larger amount of radioactive product; a higher flux of neutrons means more rapid production. The way to get a lot of neutrons is to have a lot of fissions – the more fissionable atoms that are crammed into a volume, the more neutrons. A higher uranium enrichment is the best way to cram the highest number of fissionable atoms into a volume and, thus, to increase the neutron flux.
With fission products this rule works double – a larger number of fissionable atoms not only boosts the neutron flux but it also gives a larger number of target atoms to fission. So, again, using more highly enriched uranium produces a higher yield of the desired nuclides. And with nuclides that have a low probability of being produced the only way to make useful quantities is to use more-enriched uranium.
In both cases we end up with the same choice – do we choose the greater profits from HEU or the lower risks of LEU? An LEU-fueled reactor can do everything that an HEU-fueled one can – just more slowly.
At the moment it looks as though the nation is moving towards security rather than production. Unfortunately for the nuclear medicine industry this coincides with the shutdown of some Canadian reactors that produced medical isotopes, causing some shortages in our supplies of medical nuclides.
With medical science using radionuclides in ever-increasing amounts, this places a strain on our nuclear medicine system (with the exception of PET nuclides, which are produced on-site in a type of particle accelerator called a cyclotron). Our only real options are to cut back on nuclear medicine procedures or to build more isotope production reactors.
There is more to HEU-fueled reactors than producing medical nuclides – they’re also used to produce nuclides for industry, for basic research (bombarding rocks with neutrons, for example, can tell us what the rocks are made of), developing and testing nuclear instruments, and more. All of these things go more quickly with a higher neutron flux, but they can also be done in a less neutron-rich environment. When we put it all together we pretty much have to conclude that HEU-fueled reactors are nice, but they’re not essential. If our priority is to make the largest amounts of nuclides possible then we need the HEU-fueled reactors; if security is more important then we have to shut them all down and replace them with the slower (but more proliferation-resistant) LEU-fueled devices.
There is one development that might help to resolve this issue to some degree – using high-density LEU fuel. In this case, the uranium is kept at 20% enrichment (which is unsuitable for making a bomb), but it’s packed into the fuel more tightly than in a standard reactor. The higher density of U-235 atoms can produce both a higher neutron flux and the dense packing of fission products (more or less) that will help to produce both activation and fission products. Although this type of fuel has been under development for over 30 years it never really caught on. It could be that it’s time has finally arrived.