| Corrosion will occur in the pressure boundary materials of pressurized water reactors(PWRs)when they are exposed to coolant(high-temperature pressurized water).Part of corrosion products will release into circulating coolant with the forms of oxide particles and metal ions.The corrosion products will deposit on the surface of fuel cladding under the condition of subcooled nuclear boiling(SNB)when they flow through cladding with circulating coolant.Crud deposition could induce a series of problems such as axial offset anomaly(AOA)of power,crud induced localized corrosion(CILC)and increase of radiation dose rate,severely affecting the service safety and economic benefit of PWRs.In order to understand crud deposition mechanisms in PWRs and propose effective methods for mitigating crud deposition,the facility for efficiently simulating crud deposition in laboratory was developed at the present work.The corrosion behavior and crud deposition behavior on the surface of domestic zirconium alloy fuel cladding and accident tolerant fuel(ATF)cladding FeCrAl alloy were studied using the facility.The laws and mechanisms for the effects of Zn water chemistry(ZWC),Ni/Fe concentration ratio,concentration of dissolved oxygen(DO)on corrosion behavior and crud deposition behavior were studied specially.The facility and relevant experimental technologies were developed for simulating crud deposition on fuel cladding surface in laboratory.The technical challenges including simulating SNB condition on fuel cladding surface,controlling stable of solutes concentrations in circulating water,and avoiding dissolution of B-containing species in crud were resolved.Thus,crud deposition could be simulated efficiently.The characterizations of the crud simulated in laboratory are similar to the real crud in PWRs.Typical chimney structure characterization could be observed on the surface of simulated crud.The size and density of the chimney as well as the size distribution of the crud particles of the simulated crud are similar to the real crud in PWRs.The crud thickness could be controlled within the range from several microns to larger than one hundred microns during a short period.Crud deposition behavior on the surface of zirconium alloy fuel cladding in hightemperature pressurized oxygenated water were studied.The study found the morphology characterizations and phase compositions of the simulated crud are similar to the real crud in PWRs.The characterizations of the crud under different growth stage were analyzed,and the mechanisms of crud deposition and evolution process were proposed.Under SNB condition,the initial bubbles are more easily to appear at the grinding traces and flaws on cladding surface.The formation of bubble could impede the flow of surrounding solution,and a circular solution stagnation zone around the bubble is formed.Water within the stagnation zone evaporates into bubble and solutes precipitate as crud.The continuous deposition of crud improves the temperature on cladding surface,which induces gradually nucleation of bubbles on the zones that have not been covered with crud.The crud thickness increases gradually in the wake of the continuous formation and detachment of bubbles,resulting in that the surrounding solution dose not contact with bubbles directly.But the solution still could be pumped into the capillary channels within the crud under the action of pressure imbalance,maintaining the concentration and precipitation process of solutes in the capillary channels.H3BO3 and LiOH are pumped into the capillary channels and react with each other,precipitating with the formation of LiBO2.The effects of ZWC on corrosion behavior and crud deposition behavior on the surface of zirconium alloy fuel cladding in high-temperature pressurized water with controlling DO concentration less than 5 ppb were studied.The study found the corrosion mass gain of zirconium alloy fuel cladding and the thickness of the oxide film on cladding surface are not affected by ZWC.Corrosion Nodules are observed on cladding surface,and crud is more easily to deposit on the sites of corrosion nodules.The oxide film in corrosion nodules is more easily to spall,and part of spalled oxide is retained in crud.ZWC does not affect the areal density and structure of surface corrosion nodules,along with the surface roughness and wettability of oxide film.Experimental detection and thermodynamic calculation both verify Zn could not be incorporated into the ZrO2 oxide film.Thus,ZWC can not affect crud deposition on the surface of zirconium alloy fuel cladding by affecting dissolution rate and morphology characterizations of oxide film.The effects of ZWC on corrosion behavior and crud deposition behavior on the surface of FeCrAl fuel cladding in high-temperature pressurized water with controlling DO concentration less than 5 ppb were studied.The study found the oxide film under crud layer presents bilayer structure,consisting of inner chromite-based and outer ferrite-based spinel phases.Crud deposit on the surface of oxide film gradually during the process of oxide film growth.The transformation of Zn from solution to the surface of oxide film is partially impeded by crud layer,but small amount of Zn also could decrease the thickness of inner and outer oxide films and reduce the size of outer oxide particles.ZWC reduces the release of Fe from cladding matrix,improving the Ni/Fe concentration ratio of the solution around the cladding,which further improves the Ni/Fe concentration ratio in the outer oxide film.Zn injection reduces the size and amount of outer oxide particles,leading to the decrease of the surface roughness and the improvement of the wettability of oxide film,which impedes crud deposition.Zn could be incorporated into the crud particles with spinel structure,decreasing the size of crud particles.Fe,Cr,Al and Mo elements that released from cladding matrix also could be incorporated into the crud particles,and concentrations of these elements decrease gradually along the direction of crud growth.The effects of ZWC and Ni/Fe concentration ratio on corrosion behavior and crud deposition behavior on the surface of FeCrAl fuel cladding in high-temperature pressurized oxygenated water were studied.Ni/Fe concentration ratio could affect the stability regions of the species in potential-pH diagram,and thus affect the species of outer oxide particles and crud particles.Fe element released from cladding matrix could affect the Ni/Fe concentration ratio of localized solution around cladding.Outer oxide particles are closer to the cladding surface than crud particles,thus the localized solution around outer oxide particles is affected more obviously.No matter whether Zn was injected or not,the inner oxide film is composed of Fe2O3 and Fe3O4(or(Ni,Zn)Fe2O4)and the obvious enrichment of Cr in not observed.The inner oxide film exhibits porous structure and the thickness of the inner oxide film is not affected by Zn injection.According to these obtained results,the effective methods for mitigating crud deposition on the surface of fuel cladding are proposed,which includes controlling surface states(such as surface flaws,roughness)of cladding,improving nodular corrosion-resistant ability of cladding,employing ZWC technology,and avoiding high DO water chemistry condition. |
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