| Lead bismuth eutectic(LBE)has been a primary candidate material for coolant in advanced nuclear reactors and high-power spallation neutron targets in accelerator driven systems(ADS).However,it is well known that steels are severely corroded by LBE if they are exposed to LBE directly at medium to high temperature.Corrosion of structural materials presents a critical challenge in the use of LBE in ADS and advanced nuclear reactors.Previous studies showed that vary kinds of metal elements could be dissolved in liquid LBE or liquid lead,and their dissolution rate would increase with temperature.Therefore the corrosion rate would decrease as the temperature decreases.However,decreasing the operating temperature may reduce to decrease the thermal efficiency of the plant,which would have a direct impact on the efficiency of electric power system.From this point of view,low temperature operation is not optimal for nuclear power plant.Previous studies also show that oxygen concentration in liquid LBE is a key factor affecting the corrosion rate of steel.By carefully controlling the oxygen concentration in LBE,it is possible to maintain a protective oxide film on the structural surfaces,and the corrosion as well as dissolution would become negligible so that the safety of the p lant could be assured.At present,there are two methods for controlling oxygen concentration in the liquid metal: gas-phase control and solid-phase control.Most of the studies focus on gas-phase control technology while solid-phase control technology is under development.In our country,there is few researches focusing on the solid-phase control technology.Therefore,it is important to carry out the theoretical and experimental study on solid-phase oxygen control technology,and promote the development of the applications to practical advanced fast reactors and ADS.Firstly,the LBE tank type system was designed and built based on previous researches.Lead oxide ceramics were developed as it would compensate certain oxygen concentration in LBE.Then,the experimental studies focusing on solid-phase oxygen control technology were carried out.In solid-phase control technology,the dissolution rate of solid lead oxide is the key factor of the regulation.By modeling the mass transfer processes between LBE and lead oxide in MATLAB,the dissolution rate of lead oxide in bypass mass exchangers was calculated.The simulation agrees with the previous experimental data.In the simulation,it had been found that lead oxide dissolution rate would increase linearly with flow rate and increase exponentially with temperature.In LBE tank,experimental studies focusing on dissolution rate of solid lead oxide in LBE,response speed to oxygen concentration,lead oxide poisoning,surface properties of structural materials in oxygen control were carried out.The conclusions can be made as follows:(1)oxygen concentration can be adjusted quickly and efficiently by solid-phase control.In static LBE at 435?,the regulation efficiency of gas-phase control and solid-phase control were compared andcan be found that oxygen concentration has faster response speed by solid-phase control without excessive oxidation generated.It indicates that solid-phase control has better performance in adjusting oxygen concentration without polluting the coolant system.(2)Lead oxide pelletsfabricated by us has good properties.Dissolution of the pellet was not consistent due to uneven distribution of flow field,but after the experiments,we found that lead oxide pellets could maintain their integrity.In condition of low flow rate and high oxygen concentration,our lead oxide pellets dissolved faster compared with the data found in other references.(3)Lead oxide poisoning were experimentally studied in static LBE with high oxygen concentrations.After the experiments,ferric oxide layer was not found on surfaces of lead oxide samples by using SEM and EDX analyses,which implies that the poisoning problems would not occur by using solid lead oxide in LBE with high oxygen concentrations.During the operation of pool-type fast reactors,low oxygen concentration would not likely happen so this study also demonstrates its inherent safety of solid oxygen control system in pool-type fast reactor.(4)Poor wettability of stainless steel was found after oxidation.In this work,319 L,T91 and CLAM stainless steels were studied.Corrosion experiments were carried out by immersing the samples in LBE with saturated oxygen concentration a t 450 ? for 400 h,800h and 1200 h.We found that the contact angle were increased by using sessile drop method for LBE droplets.Those researches were performed under support of NSFC.Our studies on related issues would provide valid experimental data for the application to solid oxygen control technology in LBE. |
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