Re-Engineering the Nuclear Future

Principal Investigators

At a Glance

The technical characteristics of leading small nuclear reactor designs do not allow them to simultaneously address the major challenges confronting the expansion of nuclear power.

Research Highlight

Nuclear power continues to be an important component for the planned energy infrastructure of several countries; one motivation for this choice is its potential for climate mitigation because of the low level of carbon emissions as compared to fossil fuels. During the past year, the Re-engineering the Nuclear Future project led by Alexander Glaser and M.V. Ramana has assessed the technology of various small modular reactors (SMRs)—with power outputs of 10 to 300 megawatts—currently proposed as a means to facilitate the expansion of nuclear power. A particular focus of this assessment is evaluating the risk of nuclear weapons proliferation that might come with the adoption of these different reactor designs.

Along with Zia Mian, Ramana examined the potential for SMRs that are being developed to overcome various specific challenges confronting nuclear power, in particular (1) economic competitiveness, (2) potential for catastrophic accidents, (3) production of radioactive waste, and (4) linkage to nuclear weapon proliferation¹. Mian and Ramana analyzed the technical characteristics of different kinds of SMRs and argued that all four of the problems cannot be simultaneously solved. The leading SMR designs under development involve choices and trade-offs between desired features. For example, one way that nuclear engineers have tried to reduce the quantity of radioactive waste generated has been to design reactors that operate with fast neutrons (i.e., neutrons that haven’t been slowed down by a moderator). This feature results in the production of about twice as much plutonium per unit of electricity produced and at nearly six times the concentration (475% more) in the spent fuel, compared to standard light water reactors. This implies a higher risk of proliferation because a much smaller quantity of spent fuel is needed to separate enough plutonium to make one or more nuclear weapons. Historically, the production of plutonium through reprocessing of spent fuel has been the proliferation pathway of greatest concern. Although the initial build up of plutonium stockpiles globally was to manufacture weapons, since the end of the Cold War, the stockpile of plutonium from the reprocessing of civilian spent fuel has been fast growing (see Figure 3.3).

Figure 3.3. Evolution of the global plutonium stockpile from 1945 to the present. Figures for plutonium in mass units (kg) are converted into weapon equivalents by assuming that 3 kg of plutonium are used to make a weapon in case of plutonium explicitly produced for weapon purposes and that 5 kg of plutonium are needed to make a weapon in case of plutonium separated from spent fuel generated by civilian nuclear power reactors. Although the nuclear weapon stockpile has declined since the end of the Cold War, the plutonium content of weapons that have been dismantled is still part of the stockpile since there is so far no widely accepted method for disposing the plutonium. (Graph courtesy of the Glaser group.)

Glaser also supervised a student study of molten salt reactors, SMRs that use nuclear fuel dissolved in a liquid carrier salt². Molten fuel is continuously cycled in and out of such a reactor; outside the reactor, unwanted fission products are removed and makeup fuel is added. This form of continuous fuel processing prevents build-up of various isotopes within the reactor that would otherwise slow down the fission process and impede a sustained chain reaction. Not all isotopes need to be removed, however, and different MSR designs do involve different levels of chemical processing. This continuous processing of fuel creates a proliferation risk, facilitating the extraction of weaponsusable materials (e.g. plutonium) from the fuel. Postdoctoral associate Ali Ahmad and Glaser’s computer simulations showed these reactors offer significant advantages in uranium requirement (when compared to conventional light-water reactors), and specific design choices could increase or decrease associated proliferation risks.

Over the last few years, there has been much hope invested in small modular reactors helping with a revival of nuclear reactor construction in countries with many existing nuclear plants as well as with allowing smaller countries with no nuclear plants currently to set up their first reactors. Several governments around the world are supporting the development and deployment of SMRs in a variety of ways. But if the construction of SMRs is to not lead to increased nuclear weapon proliferation, proliferation resistance must be adopted as an explicit criterion at the outset.

Glaser and Ramana propose to examine in detail some of the characteristics of SMRs that have been held out as distinctive, including the possibility of constructing them underground and their potential for relatively rapid changes in power output as a way to meet fluctuating electric demand, and study the impact of these deployment on the economic competitiveness of SMRs.

References

Ramana, M.V., and Z. Mian, 2014. One Size Doesn’t Fit All: Social Priorities and Technical Conflicts for Small Modular Reactors. Energy Res. & Soc. Sci., 2: 115-124. doi:10.1016/j.erss.2014.04.015.
Ahmad, A., E.B. McClamrock, and A. Glaser, 2015. Neutronics Calculations for Denatured Molten Salt Reactors: Assessing Resource Requirements and Proliferation-Risk Attributes. Ann. Nucl. Energy, 75: 261-267. doi:10.1016/j.anucene.2014.08.014.