John P. Kopasz

Tritium release from lithium titanate, a low-activation tritium breeding material

Abstract The goals for fusion power are to produce energy in as safe, economical, and environmentally benign a manner as possible. To ensure environmentally sound operation low-activation materials should be used where feasible. The ARIES Tokamak Reactor Study has based reactor designs on the concept of using low-activation materials throughout the fusion reactor. For the tritium breeding blanket, the choices for low activation tritium breeding materials are limited. Lithium titanate is an alternative low-activation ceramic material for use in the tritium breeding blanket. To date, very little work has been done on characterizing the tritium release for lithium titanate. We have thus performed laboratory studies of tritium release from irradiated lithium titanate. The results indicate that tritium is easily removed from lithium titanate at temperatures as low as 600 K. The method of titanate preparation was found to affect the tritium release, and the addition of 0.1% H 2 to the helium purge gas did not improve tritium recovery.

Water balance in a polymer electrolyte fuel cell system

Polymer electrolyte fuel cell (PEFC) systems operating on carbonaceous fuels require water for fuel processing. Such systems can find wider applications if they do not require a supply of water in addition to the supply of fuel, that is, if they can be self-sustaining based on the water produced at the fuel cell stack. This paper considers a generic PEFC system and identifies the parameters that affect, and the extent of their contribution to, the net water balance in the system. These parameters include the steam-to-carbon and the oxygen-to-carbon ratios in the fuel processor, the electrochemical fuel and oxygen utilizations in the fuel cell stack, the ambient pressure and temperature, and the composition of the fuel used. The analysis shows that the amount of water lost from the system as water vapor in the exhaust is very sensitive to the system pressure and ambient temperature, while the amount of water produced in the system is a function of the composition of the fuel. Fuels with a high H/C (hydrogen to carbon atomic ratio) allow the system to be operated as a net water producer under a wider range of operating conditions.

Modeling of tritium behavior in ceramic breeder materials

Computer models are being developed to predict tritium release from candidate ceramic breeder materials for fusion reactors. Early models regarded the complex process of tritium release as being rate limited by a single slow step, usually taken to be tritium diffusion. These models were unable to explain much of the experimental data. We have developed a more comprehensive model which considers diffusion and desorption from the grain surface. In developing this model we found that it was necessary to include the details of the surface phenomena in order to explain the results from recent tritium release experiments. A diffusion-desorption model with a desorption activation energy which is dependent on the surface coverage was developed. This model provided excellent agreement with the results from the CRITIC tritium release experiment. Since evidence suggests that other ceramic breeder materials have desorption activation energies which are dependent on surface coverage, it is important that these variations in activation energy be included in a model for tritium release. 17 refs., 12 figs.

Advanced understanding of the tritium recovery process from the ceramic breeder blanket

Abstract The key to successful operation of a tritium breeder blanket is to understand the tritium transport and release characteristics and the role that hydrogen plays in this process. Indications are that grain size (surface-to-volume ratio) largely determine whether tritium release is limited by diffusion or desorption. That is, the larger the grain size the higher the probability that bulk diffusion will determine the release rate. For smaller grain size, the actions taking place on the grain surface become extremely important especially as regards the role that hydrogen plays in the overall process. Experimental studies have indicated that the presence of 0.1% H 2 in the helium purge gas enhances the release of tritium from the lithium ceramic. The tritium released has been found in the form of both HT and HTO. The ab initio calculations on the dissociative hydrogen chemisorption on lithium oxide surfaces provide one component of the quantitative basis for an understanding of the role of hydrogen in affecting the release of tritium from lithium ceramic breeders. These calculations suggest heterolytic adsorption of hydrogen onto the ceramic surface.

Spatial tritium transport in single-crystal lithium aluminate

Abstract Isothermal anneal tests have been performed on large single-crystal samples (approximately 1.5 mm diameter) of LiAlO 2 in an attempt to determine the diffusivity of tritium in lithium aluminate. To understand the effects of impurities on the tritium transport, single crystals of Mg-doped LiAlO 2 were also studied. The crystals were sectioned after annealing to determine the tritium concentration profiles within the crystals. From these profiles the tritium diffusivity, D , and the desorption rate constant, K , were calculated. The tritium diffusivities determined for the doped and undoped materials were the same within experimental error. The measured diffusivity is given by ln D( 2 /s) = −11.8 - (1.50 × 10 4 )/ T . Tritium release for these samples was generally in the mixed diffusion-desorption regime, as determined from the ratio aK D , where a is the grain radius. Our data indicate that for grain radii less than 100 μm, the tritium transport will be in the desorption-controlled regime.

Tritium release from lithium aluminate: can it be improved?

Lithium aluminate is an attractive material (in terms of its chemical, mechanical and irradiation properties) for breeding tritium in fusion reactors; however, its tritium release characteristics are not as good as those of other candidate materials. To investigate whether tritium release from lithium aluminate can be improved, we have studied the tritium release from irradiated samples of pure lithium aluminate, lithium aluminate doped with Mg, and lithium aluminate with a surface deposit of platinum. The release was studied by the temperature programmed desorption (TPD) method. Both the platinum coating and magnesium doping were found to improve the tritium release characteristics, as determined by TPD. Tritium release shifted to states with lower activation energies for the altered materials.

Enhanced tritium transport and release by solids modification

In order to improve the tritium release characteristics of lithium ceramics, we are investigating the effects of dopants on tritium transport and release. Prior work has suggested a correlation between tritium and lithium diffusion in lithium-containing ceramics. This correlation has led us to propose a mechanism for tritium diffusion in which the tritium diffuses in the form of a lithium-vacancy/tritium complex. If this is the case, one should be able to increase the tritium diffusivity by increasing the number of lithium vacancies and thereby increasing the number of lithium-vacancy/tritium complexes. The size of the increase in the diffusivity, however, will be dependent upon several parameters, including the binding energy of the lithium-vacancy/tritium complex. Our calculations indicate that, under conditions comparable to those in some in-pile irradiation experiments, a binding energy of around 84 kJ/mol should increase the diffusivity and lead to a decrease in the steady-state tritium inventory by about a factor of six.

Research Strategies for Development of an Efficient and Effective Electrocatalyst for Polymer Electrolyte Membrane Fuel Cells and Progress Summary

The current electrocatalyst formulation for the polymer electrolyte membrane fuel cell (PEMFC), platinum supported on carbon (Pt/C), is known to be an effective promoter of redox reactions in fuel cells. However, the cost of Pt (currently ~

Reforming petroleum-based fuels for fuel cell vehicles : composition-performance relationships.

2,000/troy ounce) hinders its use as a practical catalyst in commercial fuel cell-powered vehicles at current platinum loading. Another issue with respect to adoption of any electrocatalyst for vehicle applications is durability, especially in light of transportation drive cycle operation with start/stop, start-up/shut-down, and transient requirements. Thus, a robust alternative to current Pt/C technology is needed as the PEMFC electrocatalyst for the oxygen reduction reaction (ORR) on the cathode. The U.S. Department of Energy is funding cathode catalyst research on low-platinum group metal (PGM) catalysts, including alloys and core-shell systems, and on non-PGM catalysts. This paper provides an overview of the issues, approaches, and status of the research.

Performance of ceramic breeder materials in the SIBELIUS experiment

Onboard reforming of petroleum-based fuels, such as gasoline, may help ease the introduction of fuel cell vehicles to the marketplace. Although gasoline can be reformed, it is optimized to meet the demands of ICEs. This optimization includes blending to increase the octane number and addition of oxygenates and detergents to control emissions. The requirements for a fuel for onboard reforming to hydrogen are quite different than those for combustion. Factors such as octane number and flame speed are not important; however, factors such as hydrogen density, catalyst-fuel interactions, and possible catalyst poisoning become paramount. In order to identify what factors are important in a hydrocarbon fuel for reforming to hydrogen and what factors are detrimental, we have begun a program to test various components of gasoline and blends of components under autothermal reforming conditions. The results indicate that fuel composition can have a large effect on reforming behavior. Components which may be beneficial for ICEs for their octane enhancing value were detrimental to reforming. Fuels with high aromatic and naphthenic content were more difficult to reform. Aromatics were also found to have an impact on the kinetics for reforming of paraffins. The effects of sulfur impurities were dependent on the catalyst. Sulfur was detrimental for Ni, Co, and Ru catalysts. Sulfur was beneficial for reforming with Pt catalysts, however, the effect was dependent on the sulfur concentration.