A. René Raffray

Thermal conductivity of a beryllium gas packed bed

Abstract An unsintered packed bed has been suggested as a material form for the solid breeder and multiplier in fusion reactor blankets. Study of the effective bed thermal conductivity can provide tools for analysis of the blanket performance under different operating conditions, and for analysis of how to control actively the thermal behavior of the blanket. Issues of particular interest are the ability to predict and to control the thermal conductivity. The UCLA 2-D model is used to study the effects of the particle diameter, solid-to-gas conductivity ratio, bed porosity, contact area and surface roughness characteristics on the bed thermal conductivity. The study shows that all the parameters except the bed porosity play important roles in determining the bed thermal controllability. The effect of the bed porosity is minimal. Four models (the UCLA 2-D model, the modified Hall-Martin model, the SZB model, and the Kunii-Smith model) were compared with the recent UCLA single-size beryllium packed bed experimental data. The sensitivity of each model to uncertainties in the input parameters, such as the surface roughness characteristics and particle-to-particle contact area, are examined. The UCLA 2-D model gives the most reliable prediction of the Be-He packed bed effective thermal conductivity, using reasonable parameters. The modified Hall-Martin model predictions agree well with the experimental data, using a larger empirical particle-to-particle contact area. The SZB model works well for BeN 2 or Be-air particle beds. The Kunii-Smith model is not suitable for a packed bed with a high solid-to-gas conductivity ratio.

Thermal, Fluid Flow, and Tritium Release Problems in Fusion Blankets

AbstractThe thermal, fluid flow, and tritium release problems represent a major part of the research and development issues for liquid-metal and solid breeder blankets. The issues are characterized, and recent progress in model development and experiments is described. The issues for liquid-metal blankets are dominated by magnetohydrodynamic (MHD) effects on fluid flow and heat transfer. New design solutions have been proposed, new facilities have been constructed, and models are being developed to address MHD effects. The problems of solid breeder blankets are dominated by tritium release, containment, and inventory. There has been remarkable progress in understanding tritium transport in solid breeders because of successful irradiation experiments of solid breeders in fusion reactors. New models on tritium transport have provided key tools for the analysis and interpretation of the experimental results, A new issue related to thermal control in solid breeder is the subject of active experimental and mod...

Modeling of tritium transport in ceramic breeder single crystal

Abstract Simple existing models were found not to adequately reproduce recent tritium release data for LiAlO 2 single crystal, indicating the need for a more comprehensive model. To help understand and interpret the data, a new model, MISTRAL-SC, was developed, incorporating bulk diffusion as well as the four major surface processes, and allowing for the variation of surface activation energies with coverage and for the presence of H 2 in the purge. The model is described in this paper and an analysis of the single crystal data presented. Based on the analysis a bulk diffusion coefficient for tritium diffusing in LiAlO 2 is estimated and compared to previous estimated values from past experiments. Discrepancies are discussed and recommendations are proposed for values of the diffusion coefficient to be used in LiAlO 2 experiment and blanket analyses, and for future work.

Thermal Control of Ceramic Breeder Blankets

AbstractThermal control is an important issue for ceramic breeder blankets since the breeder needs to operate within its temperature window for the tritium release and inventory to be acceptable. A thermal control region is applicable not only to situations where the coolant can be run at low temperature, such as for the International Thermonuclear Experimental Reactor (ITER) base blanket, but also to ITER test module and power reactor situations, where it would allow for ceramic breeder operation over a wide range of power densities in space and time. Four thermal control mechanisms applicable to ceramic breeder blanket designs are described: a helium gap, a beryllium sintered block region, a beryllium sintered block region with a metallic felt at the beryllium-cladding interface, and a beryllium packed-bed region. Key advantages and issues associated with each of these mechanisms are discussed. Experimental and modeling studies focusing on beryllium packed-bed thermal conductivity and wall conductance, ...

A helium-cooled solid breeder concept for the tritium-producing blanket of the international thermonuclear experimental reactor

The usefulness of the tritium-producing blanket in the International Thermonuclear Experimentall Reactor (ITER) to the fusion research and development program can be maximized by selecting design parameters, features, and options that are reactor relevant without significantly increasing the risk in key areas such as device safety and operational reliability. For that reason, a helium-cooled solid breeder (SB) blanket is proposed since it combines the operation of the SB at high reactor-relevant temperatures with the operation of helium at moderate temperature and pressure to minimize risk. Results of the analysis done for this blanket concept indicate that it is very attractive. It can achieve a high tritium breeding ratio without breeding in the space-limited inboard region, It offers important safety features, including the use of inert gas with no chemical reaction or corrosion, low activation SB, and multiple containment of tritium. the concept provides great operational flexibility to accommodate changes in ITER operating parameters, such as power level, and to optimize the operating temperature of the structure. A novel and practical concept is proposed for the thermal resistance gap between the coolant and SB to allow their operating temperatures to be optimized.

Benefits of natural convection in solid breeder blankets with poloidal coolant channels under LOFA conditions

Abstract This paper analyzes natural circulation flow in solid breeder blanket designs with poloidal channels under a LOFA condition. Analyses couple the flow transient behaviors with transient heat transport models. While two-phase natural circulation exists in the system, a homogenous mixture model is used. The results of example calculations performed for an ITER solid breeder design concept indicate that in a 3-loop system with an elevation head ( ΔZ ) of 20 m, the removal of the afterheat (energy stored in the blanket elements and decay heat) depends on two-phase natural circulation flow with a quasi-equilibrium flow quality of 26% at the outlet of the blanket segment. However, if the available hydrostatic head is about 15 m or less, the amount of natural circulation flow is reduced due to significant increases of friction and acceleration losses in a two-phase system. To rely on natural circulation flow as an afterheat removal mechanism, the design of coolant system with its flow channels should be addressed analytically and experimentally to permit stable steady-state operation under conditions of the presence of vapor in the coolant channels.

Impact of neutronics considerations on the selection of solid breeder and multiplier materials and configurations

A large number of solid breeder materials have been proposed for fusion blankets. A study was performed to quantitatively compare the effect of solid breeder (SB) material choice on blanket attractiveness and to recommend priorities for focusing the solid breeder experiments. This paper summarizes the results of the extensive neutronics analysis which evaluated the achievable tritium breeding ratio (TBR) and the power multiplication factor (M) as key parameters. The study considered ten solid breeder materials (e.g., Li2O, Li2SiO3, LiAlO2), six neutron multipliers (Be, BeO, Pb, PbO, PbBi and and Zr5Pb3) and two coolants (helium and water). These breeder and multiplier materials were considered in six different design configurations which included homogenized breeder/multiplier mixtures and a separate multiplier zones. The general conclusions which can be made from the TBR results are that: (1) Be exhibits the most superior performance among neutron multiplier (M) candidates; (2) Li2O is the most attractive solid breeder candidate, showing the highest TBR for the unmultiplied, separate multiplier and homogeneous SB/M cases; (3) Li2O is the only solid breeder with a plausible chance of performing satisfactorily without a multiplier; (4) the homogeneous SB/Be cases show exceptional TBR performance and should be considered in the test program, (5) multi-region solid breeder multiplier (SB/M) cases show still excellent TBR performance; and (6) the TBR with water coolant in all cases is 5–10% higher than with helium coolant. The power multiplication results tend to vary in accordance with the TBR results and thus reinforce the above conclusions. There are two notable exceptions; the first is the case of Li7Pb2, which, although showing a high effective TBR of 1.22, exhibits a rather low M of 1.18. The other exception is that changes in 6Li enrichment affect M only slightly in all cases. The overall M is actually decreased due to the decrease of the radiative capture reaction rate in the structure material used in the blanket utilizing highly enriched lithium.

High-Heat-Flux Removal by Phase-Change Fluid and Particulate Flow

A new concept based on particulate flow in which either or both the particulates and the fluid could undergo phase changes is proposed. The presence of particulates provides not only a mechanism for additional heat removal through phase change but also the potential for increasing the rate of heat transfer by enhancing convection through surface region/bulk [open quotes]mixing[close quotes], by enhancing radiation, particularly for high-temperature cases; and for the case of multiphase fluid, by enhancing the boiling process. One particularly interesting coolant system based on this concept is [open quotes]subcooled boiling water-ice particulate[close quotes] flow. A preliminary analysis of this coolant system is presented, the results of which indicate that such a coolant system is better applied for cooling of relatively small surface areas with high local heat fluxes, where a conventional cooling system would come short of providing the required heat removal at acceptable coolant pressure levels. 14 refs., 8 figs.

Analysis of Tritium Transport Mechanisms at the Surface of Lithium Ceramics

AbstractAn analysis was made of the surface fluxes of hydrogen species in order to determine the activation energies which affect the bulk and surface inventories in a lithium ceramic. It was found that in the absence of protium, Sievert Law is obeyed and the concentration depends on However, in the presence of protium, the concentration depends on PHT. The bulk and surface concentrations were found to depend on a combination of all four surface energies in the activation energy of solution and the heat of adsorption.

CHARACTERISTICS OF HELIUM PURGE FLOW IN SOLID BREEDER PEBBLE BED AND ITS IMPACT ON DESIGN CONSIDERATIONS

One of the design objectives of helium purge in the solid breeder is to keep the HT partial pressure in the purge system low to minimize the tritium inventory and permeation through the clad. The helium purge flow rate is set at the highest possible value to achieve this criterion. A distinguishable feature of the helium purge flow in a pebble bed design is its velocity jet due to high local porosity near the wall. Thermalhydraulic calculations based on 2-D non-Darcian momentum equations showed that the peak velocity is about 8 times or more higher than the bulk velocity at Reynolds number relevant to solid breeder blanket application. This velocity profile results in reducing the tritium permeation rate through the cald based on the low tritium partial pressure at the wall associated with this peak velocity by a factor of 2 or more.