Effect Of SGTR On Thermal Safety Of Lead-Based Reactor

Lead-based reactor,due to its good neutronic,thermal-hydraulic and chemical safety characteristics,has become a research focus in the field of advanced reactors in recent years.The lead-based reactors are usually designed as pool-type configuration.The secondary loop coolant water will enter into the steam generator(i.e.the primary heat exchanger)tube at high pressure and sub-cooled conditions and make heat exchange with primary liquid metal at higher temperature.Since there is a large pressure and temperature difference on both sides of the tube,the probability of the Steam Generator Tube Rupture(SGTR)accident in these working conditions could not be ignored.In the case of a SGTR accident,the thermal hydraulic problem such as the steam migration,pressure wave and temperature transient may seriously threaten the safety of the reactor.In the present study,based on the thermal hydraulic problem brought by SGTR accident of lead-based reactor,the experimental studies on mechanism of hydraulic separate effect(steam migration)and the coupling effect between hydraulic and thermal(temperature transient)under water-Lead Bismuth Eutectic(LBE)interaction were firstly investigated.The physical law of water-LBE interaction was presented and the computer code used in safety analysis was validated by the experimental data.On this basis,the full-scale simulation of integral effect on a postulated SGTR accident of lead-based reactor was performed.The main contents of the thesis are summarized as follows:(1)Mechanism study on separate effect of steam migration depth:A gas injecting two-phase flow experimental apparatus was set up and corresponding steam migration hydraulic similarity experiments were conducted by jetting gas into water with the gas velocity from 20 to 100 m/sec and the nozzle diameter from 4 to 10 mm.The results showed that the bubble cavity was formed by submerged vertical gas jet in liquid ambient and the maximum penetration depth was about 127 mm.The penetration depth increased linearly with the initial velocity of gas and was proportional to the nozzle diameter of tube.A physical model of penetration depth with Froude number in the range from 4.99×103 to 2.53×105 and the density ratio between the jet and liquid coolant was proposed based on dimensional analysis.(2)Mechanism study on coupling effect of pressure wave and temperature transient:The experiment of water-LBE interaction was carried out by injecting high pressure and sub-cooled water(pressure equal to 2 MPa and temperature equal to 200?)into low pressure LBE at 408?.The results indicated that the pressure rapidly raised up to 0.40 MPa in the reaction vessel during the test process for about 3 sec and the LBE temperature quickly dropped to 378? during the transient of about 0.8 sec.In order to verify the code,the experiments were simulated by the Neutronics-Thermohydraulics Coupled Simulation Program(NTC)and the calculated results showed a good agreement with a large number of experimental data.(3)Numerical simulation on integral effect of thermal safety in a postulated SGTR accident of lead-based reactor:A hypothetical SGTR accident of lead-based reactor was simulated by NTC code with a full-scale geometrical model and the influence factors including the failure location(from the inlet of heat exchanger to outlet),pressure of secondary loop(from 4 to 18 MPa),primary vessel size(from 1 to 2 times)on the transient thermal safety characteristics were investigated.The results were given as,1)Steam migration:During the secondary loop operating at 10 MPa,the tube failure at the outlet of heat exchanger might be the worst case because the amount of steam entering the active region of the core was the most with the maximum volume fraction of about 9.16%.However,the volume fraction of steam was less than 0.51%when the failure was located in the inlet and middle part of heat exchanger.The volume fraction of steam in active region increased with the increasing of the pressure of secondary loop.When it was operated under saturation pressure,the volume fraction of steam in active section was about 2.37%.Otherwise,when it was operated above saturation pressure,liquid water vaporized in the core and produced more steam which was several times as the under saturation pressure condition.The primary vessel size had little effect on quantity of steam entrapped to the core.2)Pressure wave:The sharp peak and growth rate of pressure increased with the increasing of the pressure of secondary loop.When the tube ruptured at 4,10,18 MPa,the maximum transient pressure peak near the vessel wall was about 1.59,1.74,2.12 MPa and the growth rate of pressure was about 0.104,0.161,0.197 MPa/sec,respectively.The vessel size had little effect on sharp peak of pressure in the LBE area.However,the sharp peak and growth rate of pressure in cover gas decreased with the increasing of the vessel size.When the vessel size was about 1,1.5,2 times,the sharp peak in cover gas was about 0.54,0.08,0.06 MPa and the growth rate was about 0.161,0.035,0.012 MPa/sec,respectively.3)Temperature transient:The calculated results showed that the local minimum temperature of LBE dropped to 140? for only several seconds.The minimum temperature of LBE in the primary pool was above the freezing point.It was concluded that the blockage resulting from LBE solidification could not occur during the SGTR accident.This study will provide not only theoretical and experimental supports to further understand the physical mechanism and safety assessment about SGTR accident of lead-based reactor,but also further reference and recommendation for the safety design of heat exchanger.It has referential significance on the prevention and mitigation of SGTR accident.