Impact of the startup temperature and primary chemistry program on the oxidation and release of corrosion products from alloy 690 steam generator

摘要

The radioactivity of the Reactor Cooling Systemmainly comes from the release of corrosion products ofsteam generator tubes made of Ni-base alloys. In order toreduce this activity and thus the radiation exposure inpressurized water reactors, it is necessary to minimize therelease. That requires an understanding of the variousmechanisms. EDF R&D constructed a loop, BOREAL, tospecifically measure rates of release in various primaryenvironment. Usually, tubes were tested under conditionsof nominal primary chemistry at constant hightemperature. So, it is necessary to carefully investigatethe impact of the start-up conditions. The release testswere carried out with a temperature and chemistryprogram close to an actual reactor start-up.Characterizations by Scanning Electron Microscopy(SEM) were done on corroded specimens of steamgenerator tubes. These observations will be correlatedwith release kinetics.The kinetic results show that temperature has aneffect on the corrosion product release. At varyingtemperature, a slow rise of temperature has demonstratedthat the release phenomenon appears from 170°C,increases up to the temperature plateau which stabilizesat 325°C. The data seems also indicate that the majorreactor start-up conditions do not have an impact on theoxidation and the release at the end of the tests. Therelease is around 11 nm at 325°C regardless of firststages conditions. SEM observations show that there is athin oxide layer covering the surface. Very smallcrystallites, sticks and needles shaped elements areobserved. TEM observations, GDOES and Ramanspectroscopy must be performed in order to determine apotential difference and to better understand themechanisms. For a better understanding of the releasephenomenon and the results, future tests are planned.