Investigation On Effect Of PH Control Reagent On Flow Accelerated Corrosion Of Carbon Steels For PWR Secondary Piping

The carbon steel piping of secondary loop is prone to flow accelerated corrosion(FAC) under secondary loop water chemistry in pressurized water reactor (PWR) plants,which leads to pipelines’ breaks and flow components’ failings, and therefore causeseconomy losses and casualties, brings bad society influence, and threatens the safetyand reliability of plants. In China, the PWR secondary loop water systems are mainlytreated by reductive all volatile treatment (AVT) using ammonia and hydrazine. Due tothe high evaporation rate of ammonia, in the vapor-liquid two-phase region, theammonia concentration and the pH value in the liquid phase are significantly lower thanthat in vapor phase, which is inclined to FAC of carbon steel. Thus, in the design ofnew PWR plants, pH control reagents with lower volatile factor are considered toreplace ammonia, among which ethanolamine is a good choice.This thesis investigated the effect of ethanolamine and ammonia, as pH controlreagents, on FAC of A106GrB and A672GrB60carbon steels, which are in commonuse in PWR plants secondary loop, with self-designed FAC experimental device, underPWR secondary loop water chemistry condition (150oC,3MPa, pH=9.5, velocity=9m/s). In the meantime, this thesis tested the feasibility of an on-line testing method ofwall thinning rate for carbon steel piping based on change of direct current potential.The experiment results show that, FAC rate of both A106GrB and A672GrB60decreased remarkably since ETA replaced ammonia, from0.103mm/a and0.107mm/ato0.013mm/a and0.015mm/a, respectively. Also, if we analyzed FAC rate of A106GrBand A672GrB60from their chemical compositions, A672GrB60would have lower FAC rate, due to the higher concentration of Cr in A672GrB60, which forms morecompact oxide film on the surface to protect the basic metal, thus A672GrB60shouldhave stronger resistance against FAC. However, due to the higher concentration of non-metallic inclusions in A672GrB60used in the experiments, the experiment results don’tshow much difference of corrosion rate between A106GrB and A672GrB60. Theexperiment results also verified the on-line testing method of wall thinning rate, yetmore improvement is needed.This thesis also did some numerical simulations on FAC with FLUENT. FAC ratewas calculated by obtaining mass transfer coefficient and solubility of Fe3O4based onseveral hypothesis. First, FAC under different temperatures, flow velocities and pHvalues (controlled by ammonia) were simulated. The results show that, FAC rateincreases as the temperature and velocity increase, and decreases as the pH valueincreases while temperature is under150oC. Meantime, simulation under experimentconditions with ammonia was done, and the FAC rate turns to be0.0672mm/a. Then, amodificatory factor k was proposed by comparing the simulated result with experimentresult, to make the simulated result more close to the actual value. Finally, simulationson the flow field downstream an orifice with different orifice diameters(d) were done,show that the peak value of both wall shear stress and turbulent kinetic energy appearswithin the2pipe inner diameters(D) downstream the orifice, which represent the FACdangerous zone, where special attention needs to be aroused in refueling outageinspection. Also, these peak values decrease as d/D increases, and the distance betweenorifice and peak values decreases as d/D increases.