Study On The Key Thermal-hydraulic Process Of Loss-of-flow Accident Occurred In Deep Pool-type Nuclear Heating Reactor

Loss of flow accident(LOFA)is a kind of design basis accident that may occur during the operation of deep pool-type nuclear heating reactor(DPNR),which will endanger the core safety.Therefore,this problem needs to be fully considered in reactor design.In order to improve the safety and economy of DPNR,inertia tank is used to replace flywheel installed in primary pump.When LOFA occurs,in the early stage of LOFA,the system relies on the potential energy stored by difference of liquid level between pool and inertia tank to slow down core flow,ensure the safe core heat export,Meanwhile,inertia tank cooperate with reactor emergency shutdown system to make reactor safely transition from initial full-power condition to low-power condition.After that,the system relies on the natural circulation established between core and pool to cool the core for a long time.Thus,inertia tank and natural circulation established in pool play a key role on core safety during LOFA in DPNR.In this paper,theoretical analysis,experiment and numerical calculation are used to study the action characteristics of the inertia tank in the early stage of LOFA,the flow and heat transfer characteristics of the natural circulation in the middle and late stage of LOFA and the transient thermo-hydraulic characteristics of LOFA.Firstly,taking the inertia tank in DPNR as the research object,the scaling analysis work is carried out,and the similarity criterion of inertia tank is obtained.Using RELAP5 program,the similarity criterion of inertia tank and scaled model are preliminarily evaluated.And then,the inertia tank experimental setup is designed and built.Based on inertia tank experimental setup and RELAP5,the work mechanism of inertia tank after pump trip is studied,and the effects of inertia tank size and flow resistance in loop are revealed.The research results show that inertia tank can replace flywheel of main pump and slow down the decreasing speed of core flow after main pump trips.And the hydraulic process of inertia tank can be divided into inertia action stage,potential action stage and oscillation stage.In addition,it is found that the inertial tank diameter,the flow resistance in inertia tank upstream,and flow resistance in connection pipe are the key factors affecting the performance of inertia tank.Then,taking natural circulation inside DPNR pool as research object,scaling analysis is carried out based on H2 TS method to obtain the similarity criterion for natural circulation inside pool.And the similarity criterion and scaled model are preliminarily evaluated by comparing the RELAP5 simulation results between scaled model and prototype.And then,the natural circulation experimental setup was designed and built.Based on the natural circulation experimental setup,the maximum natural circulation capacity and ultimate heat transfer capacity of system are determined by experiment.Furthermore,it is observed that with increase the heating power,the system exhibits two types of instability: continuous high-frequency oscillations and intermittent large-amplitude oscillations.In addition,the natural circulation in experimental loop is simulated using RELAP5,and it is found that RELAP5 can predict flow and heat transfer characteristics in experimental loop.Moreover,the T.W.Ha model significantly improves the prediction accuracy of RELAP5 for the maximum natural circulation capacity and ultimate heat transfer capacity.Based on this,the effects of system’s parameters on the maximum natural circulation capacity and ultimate heat transfer capacity of the system are analyzed.Finally,based on RELAP5 simulation model of DPNR,the transient thermo-hydraulic characteristics of LOFA are studied,and the coupling characteristics between inertia tank and natural circulation inside pool is revealed.It is found that the LOFA process of DPNR can be divided into(Ⅰ)inertia tank action stage,(Ⅱ)oscillation stage and(Ⅲ)natural circulation stage.The coolant temperature at core outlet and max clad temperature will appear three peaks,and the first peak is much larger than the second and third peaks.In addition,the effects of connection pipe flow resistance,flap valve opening criterion and flap valve flow resistance on LOFA in DPNR are also revealed.It is found that the first peak of max clad temperature can be significantly reduced by reducing the flow resistance in connection pipe,and the performance of inertia tank is weakened when flap valve opens during inertia tank action stage.What’s more,the max clad temperature can be significantly decreased by reducing the flow resistance in flap valve.