Advanced Ceramics: Nuclear Ceramics: A Renaissance?
Pros and ConsNuclear power is considered a zero “greenhouse emissions” technology and currently constitutes 16% of the world’s electrical power generation (~20% in the U.S. and ~75% in France). Fifty percent of U.S. electricity comes from burning coal—a known contributor to greenhouse gases and acid rain. Coal combustion also releases a significant amount of radiation into the biosphere. Typical levels of radioactive elements in U.S. coal are ~1 ppm uranium; ~3 ppm thorium; and smaller traces of radium, radon and polonium. Even with precipitators capturing most particulates, it has been estimated that the effective radiation exposure dose is 100 times greater for burning coal compared to nuclear. Additionally, in 1999 the generating costs per KWh in the U.S. was less for nuclear than for coal.
The cons focus on reactor safety and waste disposal, which are serious issues for concern and vigilant monitoring. Yet the existing U.S. civil nuclear power industry has not caused any fatalities or releases of radiation that have threatened public health. Standardized reactor designs and new technology, such as the pebble bed modular reactor, promise improved safety margins. Deep geological disposal of radioactive waste, using multiple natural and engineered barriers to migration of hazardous isotopes, has been approved by international regulatory agencies. The risk-benefit analysis of modern nuclear power weighs heavily in favor of increasing the percentage of nuclear in our energy mix, and these new nuclear power plants will provide opportunities for advanced ceramics. The major opportunities are in two areas—nuclear fuel and waste immobilization.
Nuclear FuelUranium dioxide is the preeminent fuel for civil reactors in the U.S. After leaching uranium ore, U3O8 is precipitated out. This material is then sent to government controlled processing plants, where it is converted to UF6 gas. Using either diffusion or centrifugal processing, U235 F6 is separated from U238 F6. The U235 enriched gas is reacted to form UO2 powder, which is pressed and sintered into pellets. These are loaded into zirconium alloy tubes making fuel rods.
In Europe, spent fuel is frequently reprocessed. Since plutonium is created in the fission process, reprocessed fuel contains both radioactive U and Pu, and is referred to as mixed oxide fuel. A 1000 MW reactor will “burn” 25 tons of UO2 per year, and a pound of fuel costs about $450. Thus a 1000 MW reactor will consume ~$22.5 million of UO2 annually.
Waste ImmobilizationSo-called high level waste (HLW) accounts for over 95% of all radioactivity resulting from nuclear power generation. The long-lived radioisotopes in HLW must be contained for tens of thousands of years. The major requirements for waste immobilization material are:
1. The radioactive elements must become immobilized as part of the crystal or glass structure of the material,
2. The leaching rate of radioactive elements must be acceptably low, and
3. The cost must be acceptable.
Deep burial in stable geological structures is the preferred final repository for HLW. HLW not already contained in spent fuel rods, such as HLW from fuel reprocessing, is vitrified into borosilicate glass and cast into stainless steel cans for ultimate burial. Burials are scheduled to begin in 2010. A year’s worth of HLW from a 1000 MW reactor can be stored in ~26 m3. A second generation immobilization material, synroc, is in advanced development. This synthetic rock, based on mixed titanate phases such as zirconolite, hollandite or perovskite, incorporates the elements in the HLW into its crystal structure, yielding excellent chemical stability. Synroc features leach rates better than an order of magnitude lower than borosilicate glass.
Other ApplicationsBoron10 has an unusually large neutron capture cross section. Natural boron contains ~20% B10, and this percentage can be “enriched” if desired. Boron compounds are widely used as neutron absorbers in reactors, i.e., boron carbide in control rods. Other applications for ceramics will be in radiation detection instrumentation and sensors.
While there will be many business opportunities for advanced ceramics if nuclear power experiences a revival, the real payoff is the opportunity to reduce greenhouse and radioactive emissions into the biosphere.