The question of what role nuclear power will play in this country's energy future is filled with uncertainty. On one hand, it is the only low-carbon source of gigawatt scale power that even environmentalists like James Lovelock and Stewart Brand have endorsed. On the other hand, post-Fukushima safety concerns, rising cost projections, and the steadily decreasing cost and rapid growth of renewables have led authorities like Amory Lovins to declare it an unnecessary risk in the decades to come.
What is certain is the fact that we already have a large stockpile of highly radioactive nuclear waste that we don't know what to do with. In this country alone, there is roughly 65,000 tons of the stuff, some of which will remain radioactive for tens of thousands of years. The reversal of support for a long-term storage facility at Yucca Mountain in Nevada exemplifies the challenges and the potential long term costs of dealing with this waste pile, which continues to grow.
Dr. Peter McIntyre, professor of physics at Texas A&M University has come up with a new process called accelerator-driven subcritical fission in a molten salt core (ADSMS) that he claims can destroy the long-lived transuranic elements in spent nuclear fuel, rendering it safe enough to be easily manageable, while producing clean energy in the process. He proposes a two-step process that can extract the dangerous transuranic components (including plutonium) from spent nuclear fuel rods and then destroy them in a sub-critical fission that also produces a substantial amount of energy along the way. The process also recovers pure uranium from the fuel rods so that it can be used to generate more energy.
McIntyre is a long-time innovator of accelerator technologies: he was the first to propose proton-antiproton colliding beams that were used to discover the top quark and the Higgs boson, and he is developing high-field superconducting magnets to triple the collision energy in CERN's Large Hadron Collider.
I spoke with Dr. McIntyre about his breakthrough and his hopes for how it might be used most beneficially. Essentially, he is taking advantage of a pyro-processing operation to extract the contents of spent fuel rods and recover the transuranic elements from their contents. This process, which has roots that run back to Argonne National Lab, Idaho National Lab, and the Republic of South Korea, involves a molten salt electro-chemical bath into which spent fuel rods can be inserted and broken down. The resulting three components are pure uranium (about 95 percent of the mass), fission products, and transuranics. It is the last group, the transuranics, that represent a waste storage nightmare, since not only are these materials highly radioactive, but some of them have half-lives that are greater than 100,000 years.
McIntyre said, "In my opinion, the only way to properly deal with transuranics is to destroy them. They are an unthinkable hazard if they ever get into the biosphere. There has long been discussion that we could find a site like Yucca Mountain that's so isolated from groundwater and so stable geologically that we could say with confidence it will be the same 100,000 years from now as it is today, and that burying fuel there, closing the door and forgetting it. Keep in mind that there has only been human civilization for about 5,000 years. How can we lock a Pandora's box and be sure it will stay locked for a thousand centuries?"
Credit: Idea Go.
Once the transuranics have been isolated they can then be subjected to a subcritical fission reaction in which a high-energy proton beam generates fast neutrons that drive fission in the molten salt fuel. The fission literally destroys the transuranics over time, breaking them down into fission products that are still radioactive, but with half-lives that are a thousand times shorter than before. The projected economics of this process is such that the power produced (on the order of 10 billion kWh) will more than pay for the cost of setting up and running the operation. When compared to the potential cost of constructing, protecting, and maintaining a deep storage facility for thousands of years, this could offer a monumental improvement. McIntyre estimates that an additional one-third the amount of energy originally produced can be squeezed out of the spent fuel rods. The sub-critical reaction is, by definition, inherently safer than today's reactors in that it is not a chain reaction. As soon as the particle beams are turned off, the reaction will stop, and the core cannot undergo meltdown since it is already molten salt.
But McIntyre insisted there is an even greater opportunity to operate the ADSMS core not only as a hazardous waste incinerator, but rather as an ongoing waste treatment process used to augment a continuing nuclear power industry and make its economics even more attractive. That's because ADSMS does more than just destroy the transuranics and convert them into far more manageable short-term fission products with half-lives of a hundred years or less. It also recycles the depleted uranium so that it can be fissioned to generate 10 times more energy than was produced in a conventional reactor.
The use of this system would eliminate the spent fuel storage pools which were a major source of problems at Fukushima.
If, at this point, McIntyre's ideas seem almost too good to be true, it's appropriate to ask what the downsides to this are.
First of all, most of this is theoretical. No such process has yet been demonstrated or tested.
Second, this approach is not consistent with or embraced by the Department of Energy's current nuclear fuel strategy. What that means is that a great deal of review and analysis by government agencies would be required before a process like this could be adopted or approved.
Third, the proton beam required to drive this process would have to be 10 times more powerful than anything that has been produced to this date. This last challenge led Dr. McIntyre to invent a new way to accelerate proton beams, called a strong focusing cyclotron, that is capable of producing the beam power needed for ADSMS. In fact, the core ideas of subcritical fission have been around since the 1950's, but only with the advent of the strong focusing cyclotron has it become feasible to make it practical.
Finally, on the question of nuclear proliferation, there is a double-edged sword here. On the one hand, by destroying the transuranic components (e.g. plutonium), the process makes these materials unavailable for purposes of making weapons. However, as part of the process of destroying them, they must first be extracted from the fuel rods and isolated, at which point they would be prime targets for nefarious agents.
The destruction process is also not immediate. These elements can only be destroyed at the rate they were produced, so depending on the size of the facility and how much waste is being processed at once, it could take decades or more to burn through the current stockpile.
Professor McIntyre is currently preparing a proposal to the U.S. Dept. of Energy, requesting funding to take his concepts to the next step. If awarded, he will be able to construct the first strong focusing cyclotron that could be used to power the ADSMS system.
CAD simulation of an ADSMS core. Credit: Dr. Peter McIntyre.