April 15, 2008

Developing the next generation of reactors

By James M. Hylko

The pluses of particle management

Actinide management, common to all the Gen IV alternatives, would reduce the volume of nuclear waste in the mid-term and provide assurance of nuclear fuel availability in the long term. This mission overlaps a national responsibility addressed in the Nuclear Waste Policy Act, namely, the disposition of spent nuclear fuel and high-level waste. The mid-term (30 to 50 years) actinide management mission consists primarily of limiting or reversing the buildup of the inventory of spent nuclear fuel from current and near-term nuclear plants.

Actinides may be a waste product for an LWR, but they are fissionable in a fast reactor. As mentioned earlier, a transuranic is a very heavy element with a higher atomic number than uranium (92); it is formed artificially by neutron capture and possibly by subsequent beta decays. Extracting these long-lived radionuclides from spent fuel and irradiating them in a closed fuel cycle using fast reactors does more than generate electricity. It also transmutes the long-lived radionuclides that would otherwise require isolation in a geologic repository such as Yucca Mountain into shorter-lived radionuclides. Transmutation changes atoms of one element into those of another by neutron bombardment that causes neutron capture and/or fission. In the longer term, the actinide management mission can beneficially produce excess fissionable material, currently supplied through mining and the enrichment of natural uranium, for use in systems optimized for other energy missions.

Making the most of uranium

Fast reactors play a unique role in the actinide management mission because they operate with higher-energy neutrons than LWRs and thus are more effective in fissioning the actinides and transuranics recovered from an LWR’s spent fuel.

Theoretically, a fast reactor can recycle all of the uranium and transuranic radionuclides. In contrast, thermal reactors, such as LWRs, use lower-energy neutrons and extract energy primarily from fissile isotopes. The only naturally occurring fissile isotope is U-235, which has only 0.7% natural uranium; enrichment increases this natural concentration of U-235 to about 3% to 5%, which is enough to enable operation of an LWR. But because LWRs cannot be used for complete recycling, over 99% of the uranium initially mined ends up in their spent fuel and in the residue from the enrichment process. Fast reactors maximize the use of uranium because they support multiple fuel recycles that make all of the fuel’s heat content usable.

Kick-starting the hydrogen economy

Another feature of many of Gen IV reactors is their ability to produce hydrogen as a by-product. Realizing this potential could make the use of fuel cells for transportation and power generation more economic and environmentally benign while reducing America’s dependence on imported oil.

Sufficient quantities of hydrogen for commercial use would be produced during off-peak periods, improving the operating economics of nuclear baseload plants. A long-term objective would require dedicated Gen IV nuclear plants, operating at higher temperatures, to produce hydrogen at a steady rate for storage and subsequent use by large (>1,000-MW) banks of fuel cells to address daily peak demand.

—James M. Hylko (james.hylko@prs-llc.net) is an integrated safety management specialist for Paducah Remediation Services LLC and a POWER contributing editor.