Hi Doc! I saw your piece about fusion energy and it made me wonder what sort of radiation safety program that sort of thing calls for. Would it be easier for a fusion reactor than for nuclear fission?
You know, this is one of those things that doesn’t get discussed very often – at least, not outside of the scientific or technical literature. I think that most people understand that fusion energy doesn’t produce spent reactor fuel, can’t melt down, and doesn’t create radioactive fission products – that it’s “clean” – and assume that there’s no radiation, and no radiation safety required. And that’s a reasonable assumption…but it’s not exactly the case. So let’s take a look!
First – what happens in in a fusion reactor is that hydrogen is heated to extraordinarily high temperatures; a recent paper reports reaching temperatures of over 100 million degrees. Atoms and molecules move more quickly and have more kinetic energy as they heat up and at such high temperatures the hydrogen atoms have a huge amount of energy. Normally the positive electrical charge of the two protons will slow them down and keeps them from approaching closely enough for the strong nuclear force to hold them together; at these temperatures, though, the atoms stick together; when they do so the reaction releases energy and creates a heavier atom. Most of the hydrogen in the universe is simply a single proton with a single electron and, although this is what powers most stars, our fusion reactors are not yet able to produce power this way; instead, we use heavier versions of hydrogen that have one or two neutrons attached – deuterium (1 neutron) and tritium (2 neutrons). These heavier hydrogen atoms fuse more readily; they also make for more interesting radiation safety.
The reason for this is that some of these neutrons are emitted during the fusion reaction and they can be absorbed by stable atoms within the reactor and its associated machinery, in the radiation shielding surrounding the reactor chamber, and wherever else they come to rest. Adding a neutron to an atomic nucleus changes its nuclear properties and can cause the atom to become radioactive. Thus, any fusion reaction that produces neutrons will create radioactivity.
Fusion also produces gamma radiation; the gamma rays plus the neutron radiation can be dangerously high – some fusion research facilities have reported producing potentially fatal doses of radiation from very short-duration fusion reactions; the continuous fusion required to produce useful amounts of power will produce much higher doses of radiation that must be shielded to keep the people operating and maintaining the reactor and power plant safe.
There’s also the issue of radioactive contamination. Hydrogen and deuterium are both stable atoms, but tritium is radioactive and can cause contamination if any leaks from the locations where it’s stored or handled. Not only that, but not every atom of tritium will fuse and unfused tritium can settle out on the inside of the reactor chamber, where it can spread to anyone entering the chamber or reaching into it to make adjustments, perform maintenance, or any other work. But that’s not the only possible source of contamination – remember the atoms made radioactive by neutrons? They’ll be distributed throughout the entire thickness of the walls of the reactor chamber, the radiation shielding, and the materials of the building in which the reactor is operated. This radioactivity is contained within the material in which it forms – but cutting into, scraping, welding, grinding, or sanding those materials can create radioactive dust that can settle out as contamination that will need to be contained and cleaned up.
The other thing about this contamination is that it’s got the potential to be inhaled or ingested – this means that there will be the need to have a program to make sure that workers don’t have any internal radioactivity by developing and maintaining a bioassay program. The bioassay program I once ran required workers to submit urine samples for bioassay measurements if they’d used volatile radioactive chemicals outside of a fume hood, it they’d had skin contamination or were in a room with airborne radioactivity, or any of several other conditions. We would then put the samples into a sample counter to see if there was any radioactivity present. The few times we got positive counts I needed to investigate to make sure there was an actual intake, to find out how it happened, calculate radiation dose to the affected worker, and then try to come up with a way to keep it from happening again.
So…here’s what we’ve got to consider when we’re looking at radiation safety for a fusion power plant:
- Gamma and neutron radiation at levels that are potentially lethal
- The production of radioactivity in structural, reactor, and other materials
- The potential for radioactive contamination, and possible intake of that contamination
- The need for a bioassay program
This is starting to sound like a lot of radiation safety, and you might be wondering about how “clean” fusion power really is. And make no mistake – running the radiation safety program at a fusion reactor is going to be more challenging than managing radiation safety for, say, a simple soil gauge. But that’s not saying much – soil gauges, along with any number of other simple devices produce low levels of radiation, no contamination unless the source is ruptured, don’t create radioactivity, and don’t put anyone at risk; they don’t call for a complex radiation safety program. But think about the differences between a fusion reactor and a fission plant and the benefits of fusion become pretty clear.
For example, as a fission reactor operates it creates more and more radioactivity in the fuel, creating spent fuel that is dangerously radioactive. Fusion, in comparison, consumes its tritium as it operates (and tritium is much less dangerous than the radioactivity in spent reactor fuel). Both types of reactors produce neutron activation products, so that area is a wash.
The most significant difference, though, is what happens when things don’t go well. If a nuclear reactor loses power there’s the potential for the fuel to lose cooling and to heat up to the point of melting, at which point those fission products can be released into the environment, or the melted fuel can accumulate, creating lethal radiation levels. If a fusion reactor loses power, though, the hot plasma comes in contact with the cool walls of the reactor chamber and the reaction stops, likely with no release of radioactivity at all, and certainly nothing that could hurt people. And with less radioactivity that’s produced during operation, we can expect fusion reactors to have less contamination, less opportunity for workers to have an intake, and to require a less complex bioassay program. So – yes – there will be the need for a radiation safety program at any fusion reactors that might be built, but they should be much less complex than those at fission reactors.
There’s a lot more than radiation safety when it comes to choosing an energy source and when you look at those, fusion power comes out even further ahead of fission and other forms of energy. To learn more about it, here are a few good articles:
https://www.iter.org/sci/Fusion
https://www.scientificamerican.com/article/what-is-the-future-of-fusion-energy/