The Study And Design Of The Temperature Control System For The Spallation Target

Accelerator driven sub-critical?ADS?systems may be employed to address several missions,including transmuting long-lived radionuclides present in nuclear waste into short-lived or stable elements;generating electricity from fission energy;producing fissile materials for subsequent use in critical or sub-critical systems by irradiating fertile elements.An accelerator driven subcritical system consists of a high-power proton accelerator,a heavy metal spallation target,and a subcritical reactor core.The spallation target is irradiated by the high-power proton beam to produce spallation neutrons for driving the sub-critical reactor.The spallation target will produce a huge amount of energy deposition while producing spallation neutrons,and the accumulated energy will cause the temperature of the spallation target to rise sharply which will change the properties of the target material and damage the target.Therefore,it is necessary to remove the deposited energy in the target and keep the temperature of the target in an effective and reasonable range.This paper studies several techniques of temperature control for the spallation targets in ADS systems.Prior to development of the control techniques for the target's temperature,we have performed the simulations with a heat transfer model to analyze the temperature profile of the target as a function of the beam intensity and the velocity of coolant.The heat conduction,the thermal radiation,fluid convection and the heat source term have been considered in the heat transfer model,where the heat source term comes from the energy deposition of the proton beam into the target.The simulations have been performed for a cylindrical target with a radius of 120mm.The target is located vertically at the centre of a sub-critical core and is bombarded vertically by protons with an energy of 250 MeV.Two types of beam shapes have been considered in the simulation.First,the target's temperature by the proton beam with a shape of Gaussian distribution has been studied.Then,in order to avoid formation of a very hot spot in the target center,the proton beam scanned on the target surface has been used to maintain the uniformity of target temperature during the irradiation.The temperatures of the target as a function of the velocity of coolant and the beam intensity have been studied in both the Gaussian beam and the beam scanning.The simulation results indicate that the temperature curve becomes flat when using beam scanning,in comparison with that of the Gaussian beam.The method of beam scanning reduces dramatically the highest temperature inside the target.The simulation results also indicate clearly that the highest temperature inside the target decreases with the increasing velocity of coolant for a given beam intensity and the highest temperature increases with the increasing beam intensity for a given velocity of coolant.When the highest temperature becomes very high,we can decrease the temperature of the target by either increasing the velocity of coolant or decreasing the beam intensity.Thus,we can set a maximum temperature value T_max and a maximum velocity of coolant,V_max,to control the temperature and protect the target.When the highest temperature inside the target is higher than the setting T_max,the velocity of coolant increases automatically.When the velocity of coolant is higher than the setting V_max,the beam intensity decreases automatically.The heat transfer model has also been employed to investigate the spatio-temporal variations of the target temperatures when coolants stay static in the target.The simulation results indicate that the highest temperature inside the target increases dramatically with the irradiation time.When coolants stay static in the target,it takes only 4s for the highest temperature to attain 1000K for the beam scanning with an intensity of 1 mA,respectively.Thus,in order to protect the target against from abnormal irradiation by the accelerator's beam,the protective function of target will be initiated by stopping immediately the beam,when the velocity of coolants decreases to zero.Based on the simulation results,we have developed a control system of temperature for the target.The developed control system is divided into two subsystems,one for controlling the velocity of coolants,and the another one for controlling the beam intensity.The control subsystem of velocity receives the monitoring signals of both the temperatures and velocities of coolants at the entrance to the target and at the exit from the target.Then,the measured temperature is compared with the setting maximum value to decide whether the velocity of coolants should be increased,and the measured velocity is compared with the setting maximum value to decide whether the beam intensity should be decreased.On the other hand,three inputs are sent to the control subsystem of beam intensity,i.e.the measured value of the beam intensity at the entrance of the beam to the target,a command from the control subsystem of velocity for automatically decreasing the beam intensity,and the value of the beam intensity set manually in the main control room.If the measured intensity is different from the setting value,the control subsystem of beam intensity adjusts the beam intensity.