Research On Thermohydraulic Design Of Residual Heat Removal System Based On Heat Pipe Technology In Nuclear Reactor

The large volume water tank is generally used as the final heat sink in passive residual heat removal system（PRHRS）of nuclear reactor plants,which transfers the core decay heat to the final heat trap through multiple natural circulations.Although good results have been achieved,the effective running time of PRHRS is limited by the water volume of the tank.When the water in the tank can’t cover the heat exchanger,the function of the PRHRS will be weakened or even lost.Therefore,in order to ensure the safety of the reactor,the design of the water tank volume has a high degree of redundancy.On the other hand,in order to ensure the power of natural circulation,the water tank needs to be placed at high position in the containment.Since the weight and size are huge,this scheme puts forward the extremely high request to the structure strength of the water tank.It means strict seismic measures must be adopted which greatly increases the investment cost and the difficulty of the layout of the water tank.In this paper,in order to improve system’s safety and reduce equipment size,a new passive heat removal system based on the separate heat pipe（PRHRS＿HP）is designed for the modular reactor IP200.In this scheme,the reactor pressure vessel is immersed in an open storage water pool and the residual heat of the core is transferred to the pool through the residual heat exchanger connected to the steam generator,then the heat is transferred by the separate heat pipe system placed in the pool to the outside seawater.After completing the preliminary design of the scheme,a coupled boundary modeling method is proposed,based on the operating characteristics of the heat pipe,to simulate the overall heat transfer characteristics of the heat pipe.Then a preliminary analysis of the feasibility of the design scheme is carried out.The results indicate that the system scheme can well remove the core decay heat,and the coupled boundary modeling method can be well applied to the simplified design calculation for the heat pipe system.The operating pressure of the heat pipe in the system is always sub-atmospheric pressure.At present,the calculation range of most reactor system analysis programs has not been extended to this condition.In order to make the analysis more accurate,based on a detailed discussion of the mechanism of boiling in the sub-atmospheric pressure conditions,the applicability of the system analysis program RELAP5/MOD3.3 is studied firstly,then the model of transient analysis which can be used for separation is developed based on relevant experiments and theories.At last,the upgrading of RELAP5 is finished.The analysis and verification results show that the modified RELAP5＿ME program has better calculation results for the heat transfer characteristics of the separate heat pipe,and can capture the significant heat transfer phenomenon of the heat pipe.In order to obtain appropriate quantitative evaluation indicators,this paper proposes the effective heat transfer coefficient（EHTE）to measure the pool heat balance performance.The meaning of this index is the ratio of the heat power transferred by the research object to the heat power of the heat source.The evaluation of the system by EHTE shows that this indicator can quantitatively describe the optimization goal,visually display the establishment process of each heat transfer link,and also show the delay degree and convergence speed of each heat transfer channel.The thermal system which includes buffer can be evaluated by the EHTE.By the RELAP5＿ME program,this paper has carried out the optimization technology research on the design scheme based on the evaluation index EHTE.The heat transfer characteristics of a single group of heat pipes and heat pipe systems are compared and analysed with three different volume filling rates 45%,57%and 68%.The analysis recommended that 68%can be used as initial volume filling rate of the heat pipe system as it gives better results.What’s more,an optimization strategy with the goal of improving the slope and curve inflection time of the first stage of the EHTE_u/d curve is proposed by evaluating the change law of index EHTE_u/d.These two parameters characterize the heat transfer equilibrium speed in the pool and the time for the heat pipe system to reach the maximum heat transfer power.The influence rules of the pool water quantity and the number of heat pipes on these two parameters are determined through the sensitivity analysis.Then the design scheme is optimized to reduce the water capacity of the pool by 50%and to maintain the initial water level in the process of SBO accident.The calculation results show that the optimization strategy has achieved good results and the optimization design has obtained a more reasonable system design.In this paper,the concept design,mechanism analysis,model development and design optimization of the new PRHRS for IP200 are carried out.A relatively complete conceptual design process for the new RHRS is established.The obtained design scheme can effectively improve the system’s safety and economic;at the same time,it provides a simplified calculation model of the heat transfer characteristics of the separate heat pipe and expands the calculation range of the reactor system analysis program.So,it has important reference value and practical significance for broadening the application area of the heat pipe and developing the innovative design of the reactor system equipment.