Physical Design And Research Of A Small Modular Fluoride Salt Cooled High Temperature Reactor

Fluoride salt cooled high temperature reactors as the fourth generation reactors combine several advantages of other advanced reactors: using fluoride salt as the coolant and coated-particles as the fuel particle,employing more passive safety technologies and adopting mature conventional island design and energy conversion system.It has been appraised that fluoride salt cooled high temperature reactors have excellent performance in safety,economy,stainability and non-proliferation,and high commercial feasibility.Small modular reactors have received great global attention due to their ability to meet the need of a wider range of end users and applications for flexible power generation.They also show excellent safety performance through inherent and passive safety characteristics and provide better upfront capital bearing capacity.Small modular reactors are suitable for cogeneration and non-electric applications.A small modular fluoride salt cooled high temperature reactor(SM-FHR)is proposed in this research based on the advantages of fluoride salt cooled high temperature reactors and small modular reactors.SM-FHR uses Flibe as the coolant of the reactor and the fuel particle is TRISO particle.The fuel element is prismatic,the power is 150 MWth and the expected core life is 2 years.Firstly,the effects of carbon/heavy metal ratio and fuel kernel diameter on the attainable burnup,life and reactivity temperature coefficients were analyzed on the single assembly level,thus the parameters of the assembly were determined.The results show that in order to meet the core life expectation of 2 years and make sure that the coolant temperature coefficient and the overall temperature coefficient are negative,the carbon/heavy metal ratio should be less than 500 and the fuel kernel diameter is 350 ~750?m.The carbon/heavy metal ratio and fuel kernel diameter of the reference SMFHR core are 260 and 425?m.The core life of the reference core can reach 927 days and the burnup is 99MWd/kg U.The temperature coefficients are negative.The initial reactivity of SM-FHR reaches 34000 pcm.In order to reduce the complexity of use of control rods,burnable poisons were employed in the assembly to minimize the reactivity swing during the core life.The reactivity swing and refueling cycle were optimized for different burnable poison loadings,particle sizes and spatial distribution and the consumption pattern of burnable poison was analyzed.The results show that the combination of loading volume ratio of fuel to burnable poison of 52,particle size of 200?m and decreasing loading in the periphery assemblies makes the best reactivity repression.The reactivity is reduced to 2000 pcm.Though the core life is reduced to 776 days,it still can meet the design expectation of 2 years.Moreover,the burnup and power peaking factor through the assemblies are flattened,which is beneficial to improve the core safety.Furthermore,control rods are employed to adjust the core criticality and the power up and down operation of SM-FHR on the premise of minimizing their influence on the core physics parameters.Two schemes are considered for the arrangement of control rods: they are employed in the reflector and in the core center.It is found that the reactivity value of the control rods in the reflector is low,so it is not suitable for the control rods employed in the reflector.However,six controls rods employed at the corners of the central assembly can meet the requirements of reactivity control under various conditions.The distribution of flow and temperature of SM-FHR was obtained by modelling 1/12 core using CFD,and further the failure probability of TRISO particles was analyzed.The analysis shows that the fuel temperature in the core does not exceed the operating limit;the maximum local failure probability of TRISO particle is 6.5×10-5 and the average failure probability is 2.0×10-7,proving that the reactor has good safety characteristics under current design;the distribution of burnup and TRISO failure in the core are still uneven,which is mainly due to the uneven axial power distribution caused by control rods.The feasibility of driving the primary circuit only by natural circulation was analyzed by building single channel model of SM-FHR.The relationship between natural circulation height and reactor power,coolant temperature difference,coolant channel size and heat exchanger pressure loss is established.In order to establish a natural circulation primary circuit at 150 MW,it is necessary to increase the coolant temperature difference and coolant channel diameter,and reduce the pressure loss of the heat exchanger as more as possible on the premise of ensuring heat exchange.It can be considered to reduce the power to 30 MW to realize the primary natural circulation and the refueling cycle can reach 10 years.