The sustainability of the nuclear fuel cycle will be an eventual threat to the expansion of nuclear power, even if Uranium is cheap at the moment. As mentors of mine have argued, if we can't move to a more advanced fuel cycle, nuclear power will likely have "no future". The fungibility of fuels used in commercial power plants is critical to this discussion, but it's often not well understood. I'll be offering some perspective on a recent string of papers describing the potential use of Thorium in fast reactors, high temperature gas cooled reactors, and combined fuel cycles with molten salt reactors. I'll also be sharing my feelings on two commercial ventures that hope to put Depleted Uranium and Thorium to work as a power source.
Traveling Wave Reactor
Not long ago there was an article in Technology Review that showcased some new design features of Terrapower's traveling wave proposed reactor. This includes changes said to make the design more "buildable", and possibly more similar to conventional integral fast reactor (IFR) designs. Many experts were never convinced of the design advantages offered by the traveling wave design and 60 year core lifetime and were already asking "why not build an IFR?" I have plenty of my own thoughts on this issue and had considered putting together a full post about the engineering tradeoffs involved with designing a core to breed new fuel and the innovation offered in a traveling wave reactor. In short, building new fuel completely with Uranium-238, breeding it, and burning it without reprocessing is very appealing but there is an engineering conflict with the burnup limits that the fuel materials can handle, and accomplishing this task could be difficult without a long and expensive program to improve fuel performance, and assure performance for a 60 year service life (yikes!). The other challenge is building a core that has fuel for 60 years with a moving active heat producing region. That means that you'll only be getting heat out of 5% of the core but will still be pumping coolant through the entire thing. An old article by the inventors behind the traveling wave reactor reveals their intended solution, which is to regulate flow over different regions of the core using thermocouples, a practice that has not been used in the nuclear industry but would have an obvious economic benefit in any kind of reactor.
Fuel reshuffle without reprocessing may be a compromise between innovation and proven designs that Terrapower goes with. Wikipedia is hosting a fun image illustrating the dream of a 60 year life traveling wave nuclear reactor core. The conventional (Uranium-235) fuel lies in the center, and the wave propagates out from there into the Depleted Uranium fuel (although Thorium could work as well), with the green representing the heat producing region. This kind of burn pattern is the alternative to fuel reshuffle and/or reprocessing and is the essence of a "traveling wave".
Thorium in Light Water Reactors
Out of 440 total, the IAEA counts 359 LWR reactors operating and in shutdown. In other words, that's the majority. Fortunately, there are steps that can be taken to reduce Uranium usage in these rectors by partially replacing it with another fuel. I consider this to be a very important fact to bear in mind as the Uranium supplied from weapons stockpiles runs out, which will be sooner or later. Lightbridge Corporation is a company currently poised to capitalize on the use of Thorium in the most common types of reactors in the world.
Use of Plutonium from LWRs in Advanced Thorium Burning Reactors
Paper: Evaluation of implementation of thorium fuel cycle with LWR and MSR
Researchers from an Australian and Japanese university argued for a comprehensive view of sustainability and a fuel cycle that recycles Plutonium from current LWRs into Molten Salt Reactors (MSR). The MSR concept is a very advanced, very sexy, reactor design with a long history and even has an impressive community following behind it. The Plutonium is only needed to start such reactors, by the way, and from then on they can produce enough new fuel from Thorium to fuel itself and even a little extra.
One major head-turner for me was the focus on electric vehicles (EVs). At first I was doubtful of the connection, but apparently Thoirum is produced as a byproduct from mining for rare-earth minerals such as neodymium and dysprosium, which are precious commodities used in the manufacture of strong permanent magnets in electric motors. It is interesting to note that mining for the materials to make EVs can also produce an energy source that powers it. The concept of a thorium energy bank, or "THE Bank" is argued for. If I understand correctly, surplus Uranium-233 would be sent back to the bank as "interest" on the use of the Thorium. It's a neat idea, but I question if the value of Thorium would justify any such measure, since its abundance and lack of current use would leave the cost at bargain basement prices.
Thorium-Uranium Fast Wave Reactor Concept
Paper: Nuclear burning wave in fast reactor with mixed Th-U Fuel
Several researchers from the Ukraine argue for a type of traveling wave reactor different from the Terrapower idea in this paper. If you refer to the above animation of the Terrapower reactor, imagine the direction coming out of the page, the vertical direction (or z-axis here), that is the direction the below illustration is showing. This idea calls for a traveling wave going up the reactor, instead of expanding out from the middle.
Aside from the difference in geometry, the rest of the idea is very similar. The active fuel region starts in the "ignition zone" where you begin with fissile material and propagates out into the region containing newly bred fuel. This particular design uses an ignition zone at the bottom of the core, which would probably require a neutron reflector to be placed below it. A good argument against this arrangement is that no neutron reflector is perfect, and some neutron economy is lost. The 2nd major problem I mentioned with Terrapower is still there, at any given time only a small fraction of this core will be producing heat and significant pumping is still required for circulating coolant through the rest of the core.
Thorium in Gas Cooled Reactors
Paper: Reactor physics ideas for large scale utilization of thorium in gas cooled reactors
Researches from an Indian research center and a Japanese university lay down some practical ideas for the use of Thorium in gas cooled reactors, noting specifically that Helium coolant and graphite moderators work well for this due to their good neutron economy. Japan has a long history with advanced research reactors and references their High Temperature Test Reactor (HTTR) design here. They propose some modifications to use Thorium in the core and denote this new design HTTR-M. The new design involves two types of Thorium assemblies, one with Thorium rods aligned next to Uranium rods, and one with only Thorium rods, they call "seedless".
The results they publish are very representative of what we should be prepared to expect when mixing fissile and fertile fuel in new ways. In this first graph, they show that the HTTR-M, with Thorium has a much smaller swing in reactivity. That is to say, the neutron balance changes less over the life of that core. This is very good for safety, because the flatter that line is, the less danger there is of accidentally having too much reactivity, which can complicate an accident (see "recriticality" concerns from Fukushima Daiichi).
The next graph shows how the peaking factor changes over the life of the core. The peaking factor is a measure of the highest rate of heat production to the average. This shows that management of the reactivity through the use of Boron as a neutron absorber can basically achieve better results, meaning a more uniform core. Using Boron, however, can be equated to "throwing away" neutrons, neutrons that could be used create more fuel.
There are inroads to the use of the abundant fertile isotopes of Thorium and Uranium-238 in just about every reactor design you can think of, and this series of journal articles articulates these specific cases. There will be an art to balancing the use and arrangement of fissile (the "seed") and fertile isotopes in reactors of the future. Nuclear fuel managers technically already do this with Uranium-235 and Uranium-238, but the imperative to stretch the world's Uranium-235 resource will certainly intensify in the future, and much more radical uses of fertile isotopes should be planned for.