John P. Millis, Ph.D. for redOrbit.com — Your Universe Online
This article is the first in a new series where redOrbit´s in-house experts will answer questions submitted by you, the reader. Got a science or space question that´s stumping you? Each week we´ll select a handful of the wiliest questions you can whip up to tease the brains of our resident gurus (we call them ‘geeks’).
I’ve been reading about thorium and how it can be used in liquid sodium reactors to produce electricity; in the process it would also be able to consume spent nuclear waste from our nuclear plants. I have come to find that India and China may be also researching this technology but the US is not. Why is that? I see it as a more stable output of electricity versus solar and wind which have fluctuating output reliant on weather.
As the world stands at the edge of an energy crisis the search for alternative fuels is accelerating. It seems that nuclear power plants, specifically fission reactors, are the best immediate solution to our energy needs. (Fusion reactors are still decades away from viability while solar, wind, and other solutions do not have the needed efficiency to replace the power needs of high-consuming countries like the US.)
However, Uranium — the most popular constituent fuel for fission reactors — is an exhaustible resource. So, as we seek the next evolution in reactor design it seems prudent to explore other fuel possibilities that are more abundant.
What is Thorium Power?
It has long been assumed that Thorium would supplant Uranium as the primary fission fuel. It is far more plentiful than Uranium and can be used in its natural isotope 232Th.
However, the Thorium itself does not contain high quantities of fissile material, therefore it needs to be transmuted into the artificial Uranium isotope 233U using additional fissile material — such as Uranium — to ignite the reaction.
In general Thorium fission would proceed in a similar manner to Uranium based reactions. However, there are differences in how the material is handled and “ignited” that require unique considerations.
Is Thorium Power Viable?
Thorium has been pushed as a nuclear fuel because, in addition to being more plentiful than other fissile elements, there are substantially fewer radioactive byproducts (waste) than in typical Uranium reactors.
Additionally, during the reaction process 232U and 233U are inevitably interspersed and cannot be chemically separated. As a result the enrichment of the uranium to weapon´s grade levels is not possible. (However, 233U has been tested in at least one nuclear warhead, though the yield was significantly lower. It is unclear how the contamination from 232U would affect this.)
But, while it seems that the use of Thorium is a foregone conclusion there are roadblocks that stand in the way. One problem is that some reactor designs would create significant 233U, a long-lived radioactive waste product. Also, while the reactions work well in lab tests, achieving high fuel utilization — a measure of the amount of energy extracted from the fuel source — would present a challenge in the majority of reactors currently in operation and would therefore require significant modification.
Also, the various reaction chains that could be pursued have inherent challenges. Namely, they either produce intermediate products that can corrupt the yield of 233U, or they require uranium recycling technology that is unproven and in its infancy.
Is Thorium The Future?
Maybe. There are clear advantages to increased use of Thorium in nuclear reactor designs, however the technology is still unproven and there are certainly disadvantages as well.
Proponents of Thorium power argue for Molten-Salt reactors, like those pioneered in the 1960s. On paper such reactors have all the benefits of traditional Thorium driven reactions with none of the downside. However the technology is underdeveloped and the specifics of up-scaling — taking a proof of concept to a full scale reactor — are unknown, so these claims are unverified. And in this age of tight regulatory control in the United States, Molten-Salt reactors face a long, expensive road to viability.
In light of this, firms in the United States are partnering with our colleagues in Russia and China to explore the use of Thorium as a primary nuclear fuel source. But a 2011 study argued that Thorium reactors, because of the need for significant up-front capital investment, are not expected to enter the mainstream in the near-term, despite the potential benefits.
However, the use of Thorium in mixed oxide (MOX) fuels is a possibility right now. As an alternative to enriched Uranium, the addition of Thorium to the fuel mix would lower the radioactive waste and maximize the destruction of hazardous Plutonium.
The likelihood of Thorium reactors completely replacing Uranium designs is very low, but the expanded use of Thorium in fuel mixtures is a reasonable stop-gap until Fusion reactors finally come online 20 — 30 years down the road.