| During a loss-of-coolant accident(LOCA)in pressurized water reactor(PWR),the high temperature and pressure coolant with vapour released form the pipe break could dislodge the materials neaby the break and cause massive physical debris,such as thermal insulation,surface coating and concrete due to jet impingement.Some components and materials within containment are exposed to corrosive environment,it is possible to form chemical precipitates due to chemical reactions between the released metal ions and other chemicals in solution.The coolant could bring these debris to containment sump,potentially blocking the sump strainer and some debris may pass through the strainer and enter into the reactor.The accumulation of debris in fuel assembly can result in head loss,impeding the reactor-long-term cooling.Furthermore,when the fuel rods were covered by debris,the resistance of heat transfer on surface of fuel rods would increase,which may cause locally dryout in severe case,thereby threatening the nuclear safety.Accordingly,investigating the flow characteristics of LOCA-generated debris in fuel assembly is significant for nuclear power plant safety.It can also provide important input parameters when verifying the availability of reactor-long-term cooling after LOCA.According to the different operations after LOCA in various PWRs,we analyzed the accumulation of debris and corresponding pressure drop in three fuel assemblies:Qinshan 300 MW,AP1000 and CAP 1400,the experimental study on the source of chemical debris was also peroformed,we obtained the following main results:(1)The design of gap between the fuel assemblies has a significant influence on debris accumulation and head loss in fuel assembly.The pressure drop under wide gap condition was relatively lower,but the more debris was transported into upper location of fuel assembly.The debris was effectively captured by the bottom nozzle(BN)and pretective grid(P-grid)under narrow gap condition,while the corrosponding head loss was much higher,which would challenge the operation of pump in Emergency Core Cooling System(ECCS);(2)The behavior of debris accumulation and pressure drop were various under same experimental conditions,the difference among these debris bed could reach 100 times,which suggests the randomicity of debris accumulation in fuel assemblies;(3)The size and amount of fiberglass have significant influence on the blockage of fuel assembly.The pressure drop of fuel assembly using short fiberglass(<1 mm)was lower,and the more debris passed through the flow channel of BN.The fiberglass with longer size(>1 mm)was mostly captured by the BN and formed a fibrous bed.A uniform and stable debris bed could form with sufficient fiberglass,once formed,the chemical debris could cause "compact effect" on debris bed and even a samll faction of chemical debris could result in significant head loss;(4)The blockage caused by debris in fuel assembly were significant when the recirculation flow rate was low.Because most of debris accumulated in BN and P-grid under this condition and formed a single-layer debris bed,which had a much higher flow resistance compared to the multi-layers formed in high flow rate condition;(5)The more debris captured by the BN and less debris found in upper location in CAP 1400 fuel assembly compared to the AP1000 and Qinshan fuel assemblies.Combining with the comparison result among the three different BN structures.It is conclude that the BN of CAP 1400 fuel assembly present higher filtration efficiency;(6)The AlOOH was the main source of chemical sediments among the materials exposed in post-LOCA environment.The higher pH condition could induce more chemical sediments,and the Zn and Si could effectively prohibit the formation of chemical sediments. |
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