For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled.
– Richard P. Feynman
This post is an old-fashioned myth busting, and the topic is thorium, the fuel that could replace uranium in our nuclear reactors. A movement has grown around it as a solution to the energy problem—in other words, as an affordable, sustainable, and clean source of energy. The myths in question are about the nuclear waste and weapons proliferation resistance characteristics of thorium. Unless you're already internet-informed about thorium, I recommend watching this TEDx video for context.
First, a disclaimer is in order: I believe we should be using closed thorium (and uranium) fuel cycles to replace existing nuclear fuel cycles and fossil fuels. My objective here is to clarify a couple of points, reducing thorium from a be-all end-all solution to a bridge solution until society is powered by some ideal mix of renewables and nuclear1.
Two ideas popularized by thorium advocates is that closed thorium fuel cycles (1) would eliminate the nuclear waste issue and (2) are highly proliferation resistant. These ideas are an oversimplification to the point where it is morally questionable for individuals who know more to spead them. First, I will explain why it is an oversimplification to portray closed thorium fuel cycles as a conclusive solution to nuclear waste, then I will debunk claims of their proliferation resistance.
Spent nuclear fuel will be stored in deep geological repositories, so the nuclear waste issue revolves around these repositories. In the eyes of the public, the main problem with spent nuclear fuel stored in deep geological repositories is health risks for future generations. Here is where the matter goes beyond general public knowledge2. There are two wholly different sources of health risks associated with these repositories: (1) ground water transport of radioactive material into the biosphere, and (2) direct human contact with nuclear waste in the event of catastrophies. To evaluate closed thorium fuel cycles as a solution to nuclear waste, one must to evaluate how it addresses each of these two sources of risk. A Nuclear Energy Agency (NEA) report titled Trends in the Nuclear Fuel Cycle does this for the more general concept of partitioning and transmutation (P&T)3:
It is generally expected that lower actinide inventories would lead to a significant reduction of the consequences of low probability accidents (i.e. increase of actinide mobility in certain geochemical situations; radiological impact of human intrusion) and might diminish the impact of uncertainties about repository performances. As P&T of actinides reduces the hazard (radiotoxicity) of the emplaced materials, it lessens the consequences of strongly disruptive scenarios that can bring man in direct contact with the disposed waste [...]. No significant advantages would, however, derive from the adoption of P&T strategies in relation to doses to the biosphere from the normal evolution scenarios of geological repositories. This is due to the dominating influence of long-lived fission and activation products on total doses.In other words, P&T (and closed thorium fuel cycles) alleviates the health risk from direct exposure to radiotoxic waste, but not the health risk from leakage to the biosphere. Therefore, the problem of nuclear waste does not disappear for closed thorium fuel cycles, unlike the implicit message that is spread by many advocates.
Then, there is the topic of proliferation resistance. Closed thorium fuel cycles are frequently misrepresented as highly proliferation resistant. The same NEA report mentions the proliferation risks associated with closed thorium fuel cycles: “The thorium fuel cycle is claimed to be advantageous in generating very low quantities of transuranic materials, in particular plutonium. This enhances proliferation resistance and decreases the long-term radiotoxicity burden after fission products have decayed. However, reduction of transuranics calls for recycling highly uranium-233 enriched uranium which may cause risks of proliferation.” Despite the production and isolation of bomb material (uranium-233), thorium advocates typically claim proliferation resistance based on uranium-232 contamination in the uranium-233. Uranium-232 has decay products which are strong gamma ray emitters, making the handling of the uranium-233 for weapons production difficult. However, the extent to which the presence of uranium-232 self-protects uranium-233 is often exagerrated, and there are fairly simple ways of modifying a thorium-fuelled molten salt reactor for pure uranium-233 production with no uranium-232 contamination (source).
To summarize, thorium is not a silver bullet. The reality is that the nuclear waste and proliferation aspects of P&T and closed thorium fuel cycles are a tradeoff: partly solved nuclear waste issue in exchange for proliferation risks. This is far from what many thorium advocates have been telling the public.
It's important for individuals in the nuclear community to avoid misguiding the public and governments, not only for the sake of moral righteousness but for nuclear energy experts to maintain credibility. If there is something that could improve the public's perception of nuclear power, any associated disadvantages should also be communicated.
1 Renewables and nuclear have complementary pros and cons, and the cons of nuclear are significantly alleviated if all actinides are burned or if we use fusion instead of fission.↩
2 I had to take a graduate course on nuclear fuel cycles in order to learn this.↩
3 In this context, partitioning and transmutation means separation and destruction of actinides from nuclear waste. Ideal closed thorium and uranium fuel cycles involve partitioning and transmutation.↩