Study On Heterogeneous Diffusion Behavior Of Boric Acid Solution

Boric acid control is an important means to control reactivity of general PWR.Under normal operating conditions,boric acid solution can be used to compensate for reactivity changes during reactor startup,shutdown and refueling;Under accident conditions,the core emergency cooling system injects the high concentration boron containing water in the safety injection water tank and refueling water tank into the pressure vessel to submerge the core,and introduces negative reactivity to ensure the safety of the core.When the core cooling is insufficient due to the break accident or the shutdown accident of the main pump,a dilution water mass with low boron concentration will be formed in the cold pipe section of the primary circuit and enter the core after the main pump resumes operation,resulting in increased reactivity in the local area of the core and threatening the safety of the reactor.Aiming at the transient accident of Boron Dilution,the non-uniform mixing diffusion between deborated slug and safety injection boric acid solution in pressure vessel is measured based on planar laser-induced fluorescence technology.The experimental model device takes HPR1000 pressure vessel as the prototype,is made of plexiglass,and the optical path simulation analysis software is used to correct the imaging aberration of the model device.The high-speed camera was used to photograph the T-shaped safety injection tube area,downcomer area,lower head area and core inlet area of pressure vessel.By splicing the measurement results of each experimental area,the Reynolds numbers of different experimental loops,different cold tube inlets,different safety injection inlets and different dilution water mass volume were obtained,Mixed diffusion results and concentration distribution characteristics of deborated slug and safe injection boric acid solution in the whole experimental area.The experimental results show that under the single loop condition,after the mixed fluid of dilution water mass and safety injection solution enters the pressure vessel from the cold pipe section,it will flow down the head area in an envelope around the annular downcomer,form a vortex in the left area of the lower head,and form a concentration retention area in the lower area of the downcomer,resulting in uneven distribution of boric acid concentration in the cross section of the core..Increasing the inlet Reynolds number will enhance the mixing degree of deborated slug and boric acid solution entering the core area,and reduce the share of boric acid safety injection solution in the cold pipe inlet fluid.Therefore,the concentration of deborated slug does not necessarily increase before entering the core.The Reynolds number at the safety injection inlet will affect the distribution of deborated slug and boric acid solution in the cold pipe and the proportion of boric acid solution in the mixed fluid,and then affect the concentration change in each experimental area.Due to the limited mixing effect of boric acid solution,when the volume of deborated slug increases,the influence on the concentration change in each experimental area increases;Under the condition of double loop,the fluid at the inlet of each loop still maintains the flow characteristics of single loop.After the two loop fluids meet at the adjacent circumferential position,the vertical flow will be enhanced,and after the two loop fluids meet at the opposite circumferential position,the horizontal flow mixing effect will be enhanced.After the two loop inlet fluids enter the core,they meet in the experimental area to form an obvious concentration boundary,and the fluid mixing is more intense at the interface.When the inlet Reynolds number increases,the circumferential flow characteristics are further strengthened,resulting in the enhancement of the vertical flow characteristics at the intersection of the adjacent circumferential positions of the two inlet fluids.The effect of safety injection Reynolds number on the minimum concentration of deborated slug is the same as that of single loop experiment.For the deborated slug with the same volume,due to the concentration boundary at the lower head in the double loop working condition,the two loop fluids cannot overcome the resistance to enter each other’s action area,resulting in poor mixing effect between the deborated slug and safety injection solution compared with the single loop working condition.In conclusion,in this paper,the full field measurement of fluid mixing experiment is carried out by using the plane laser-induced fluorescence method,which comprehensively reveals the mixing and diffusion characteristics of deborated slug and boric acid safety injection solution in pressure vessel,which can provide corresponding reference for numerical simulation research and reactor reactivity control.