Is the Fusion-Fission Hybrid Necessary?

Introduction

 Fig. 1: Engineers prepare a mockup of the Experimental Breeder Reactor II's internal parts for a test run. (Courtesy of the U.S. Department of Energy)

This decade, sustainable design has become that one outstanding item on every designer's agenda. Mankind has an increasing need to use CO2, and this need is intensifying every day. If this need persists, studies have calculated that by 2050 we will need an extra 10-30 terawatts of carbon free power. [1] There is an increasing demand for environmentally friendly products, so designers are always looking for newer and more advanced ideas. The fusion-fission hybrid reactor is one of these ideas. Manheimer suggests that the fusion hybrid "is one of the rather few possibilities for generating this power economically, in an environmentally acceptable way, and with little proliferation danger". [1]

This reactor produces two types of energy: fissile material and electricity. The fusion reaction is what merges two atoms into one producing excess neutrons. These excess neutrons then yield two more neutrons off a uranium or thorium nucleus, or instead they yield four neutrons in a uranium or thorium fission reaction. [2] This method of producing energy is beneficial says Bethe because the energy released in each fission is eleven times great than in a fusion reaction alone, it is also a breeding reactor because each fast neutron generates several more smaller ones. [3] For a more detailed description of the fusion, fission, and fusion-fission processes, please refer to Kates-Harbeck. [2]

What Lies Behind the Name?

The fusion-fission hybrid sounds like a revolutionary discovery, in theory. However, we must question exactly what the hype is about: is it actually the hybrid between nuclear fusion and nuclear fission or is it something else?

Blanket U + Th Th
Produced U-233 (kg/yr) 1200 1600
Produced Pu-239 (kg/yr) 500 0
Fusion power, MW(th) 510 1150
Net electric power, MW, Q = 1 220 -750
Net electric power, MW, Q = 1 600 120
Table 1: The quantities shown are for a 3000 MW (thermal) "standard" fusion-fission reactor. [3]

Table 1 shows two designs: one that includes Uranium and one that does not. It is obvious that the design with Uranium develops five times as much fission power as the one without it. Bethe states that "In a 'standard' fission reactor of 3000 MW(th), operating at 70% of capacity, about 1000 kg of fissile material are consumed per year, by fission or by radiative capture of neutrons in fissile nuclei. But a large fraction of this is reproduced, by other neutrons being captured in fertile material that is also in the fission reactor." [3]

Nuclear fission, Barrett argues has been accessible for twenty years, but is not sustainable enough to take the form as a commercial product. Subsequently, nuclear fusion, in its current state, consumes too much energy for what it generates. Thus, the fusion-fission hybrid has simply provided a short cut for fusion to become a commercialized product. Barrett explains, "In this concept, the energetic neutrons from the fission reaction are captured in a blanket of fertile material, thereby breeding fissile fuel, which can be reprocessed for use in conventional fission reactors." [4] He goes on to argue, "Although the fusion-fission hybrid is an outgrowth of fusion research, it is essentially a fission energy system. Almost all of the energy produced is due to fission reactions, either in the hybrid blanket or in the fission reactors it supports. Furthermore, the hybrid requires the same type of fuel cycle facilities, as does the fast breeder reactor. Thus, the hybrid should be viewed as a direct competitor to the FBR as a long-term source of fission energy." [4] FBR is a fast-breeder reactor, a product that has been established and commercialized. It is argued that there are not enough differences between the FBR and the hybrid to justify further investment in the hybrid.

Fast-Breeder Reactor vs. Fusion-Fission Hybrid

The hybrid is an undeveloped design that is yet to hit the shelves; the FBR, on the other hand, is a secure and tested product that has already made ground in the commercial world. Barrett argues that utility companies would be more likely to back the FBR as there is a greater capital investment required for the hybrid, additionally, there is increased complexity of maintenance and operation, and its susceptibility to frequent unscheduled outages. Subsequently, both the FBR and the hybrid use the same fuel cycle operation, so they have the same likelihood of chemical pollution and produce the same radiological hazards. [4] Given their similarity we must consider why the hybrid was really designed when the FBR sits there, under-utilised.

Barret thinks the decision to invest in the fusion-fission hybrid was for political gain. He says, "if the next generation of reactors is to be manufactured by private industry and operated by utilities, the fast breeder reactor cycle would be preferred. If, on the other hand, the federal government becomes the manufacturer and operator of fissile breeders, the hybrid would have the advantage." His argument comes from the fact that the FBR "will probably be a commercial technology in the near future, the fusion-fission hybrid has yet to be proven scientifically feasible. A decision to commit federal funds for the demonstration and commercialization of the hybrid would have to be based on a conviction that the hybrid is vastly superior to the LMFBR as a breeder of fissile fuel." Thus, before scientists continue spending lots of money on the advancement of the fusion-fission hybrid they should go back and reconsider why they did not continue with the advancement of the FBR, if it was not just for government gain.

© Fran Tew. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.

References

[1] W. Manheimer, "The Fusion Hybrid as a Key to Sustainable Development," J. Fusion Energy 23, 223, (2004).

[2] J. Kates-Harbeck, "The Fusion-Fission Hybrid," Physics 241, Stanford University, Winter 2011.

[3] H. A. Bethe, "The Fusion Hybrid," Physics Today 32, No. 5, (May 1979).

[4] R. J. Barrett and R. W. Hardie, "Fusion-Fission Hybrid As an Alternative to the Fast Breeder Reactor," Los Alamos Scientific Laboratory, LA-8503-MS, September 1980.