Thermal hydraulic design and two-phase flow problems of solid target for high energy accelerator beam in spallation neutron source

Thermal hydraulics design and two-phase flow problems of the water cooled solid target system in the spallation neutron source for N-arena of Japan Hadron Facility (JHF) are reported. Targets for the proton beams of 3 GeV, 0.6 and 1.2 MW and having parabolic distributions in an ellipsoidal shape 40 mm and 140 mm in diameters are examined. Target assemblies made of tungsten or tantalum plates 60 mm x 167 mm in area and in various thicknesses placed parallel in a target vessel were studied. A water loop 2 m3/min in the maximum flow rate, 1 MPa in the maximum pressure, and about 1 meter in width, 6 meters in length and 3 meters in height, the similar conditions and sizes to those of the designed target cooling system in N-arena, was used to examine the distributor shape. The tested target system made of transparent acrylic resin had 19 simulated target plates 5 mm in thickness and placed parallel with 1.5 mm gaps. Experimental studies by flow visualization up to 10 m/s in the channel velocity have been conducted for the distributors with various shapes of the corner. Uniform velocity distribution in the channels was obtained in each corner shape. Numerical simulation was carried out to determine the corner shape to minimize the vortices at the corner. The heat deposition rate in tungsten and tantalum targets calculated by the Monte-Carlo method was used. Its distribution had the maximum value near the beam window and it decreased near a exponential curve. Heat conduction and thermal stress in the target plates at each heat deposition rate was calculated to optimize the target thicknesses. The target surface temperature should be below a certain temperature to avoid the coolant boiling. The temperature in the target center which determined the thermal stress should be also below a certain value. The water fraction in the target should be minimized to increase the efficiency of the target and to decrease radioactivation of the water. A practical method for the optimization of the target thickness was proposed and discussed for the various beam conditions. The material strength of tungsten and tantalum under the sevier radioactive damage over 10 dpa has not been well studied. Thermal stress in the operating targets in ATP, tungsten, and in ISIS, tantalum was calculated. The calculated values were applied to the limit of the design in N-arena. The designed results indicated that the target thickness was determined not by the surface temperature but by the thermal stress. Therefore, the heat flux at each target plate was not uniform. Two-phase flow problems of the designed target were evaluated as accidental subcooled boiling in parallel narrow rectangular channels with uniform velocity and non-uniform heat flux. Flow excursion due to one channel boiling with many parallel non-boiling channels should be considered if the flow velocity was decreased due to the pump trip. Preliminary experimental study on forced convective subcool boiling in a channel was carried out to estimate the minimum velocity for cooling the target safely. (author)