Corrosion particles in the primary coolant of VVER-440 reactors

Corrosion and activity build-up processes are of major concern in ageing and life-extension of nuclear power reactors. Researches to study the migration of radioactive corrosion particles have been initiated at Paks Nuclear Power Plant (NPP), Hungary in order to better understand the corrosion of the primary circuit surfaces, the transport and activation of the particles of corrosion origin and their deposition on in-core and out-of-core surfaces. Radioactive corrosion particles were collected from the primary coolant and the steam generator surfaces of the 4 reactor units and subjected to detailed microanalytical and radioanalytical investigations. Scanning electron microscopy and energy dispersive X-ray microanalysis (SEM-EDX) were used to study the morphology and the composition of the matrix elements in the particles and the deposited corrosion layers. Particles identified by SEM-EDX were re-located under optical microscope by means of a coordinate transformation algorithm and were separated with a micromanipulator for further studies. Activities of γ emitting radionuclides were determined by high resolution γ spectrometry, and those of β decaying isotopes were measured by liquid scintillation (LS) spectrometry after radiochemical processing. High sensitivity of the nuclear measuring techniques allowed us to determine the low activity concentrations of the long-lived radionuclides, i.e. ⁶⁰Co, ⁵⁴Mn, ⁶³Ni, ⁵⁵Fe in the individual particles. Finally, high resolution sector-field inductively coupled plasma mass spectrometry (SF-ICP-MS) was applied to determine the ultralow concentrations of Co, Fe, Ni in the same particles. Specific activities of ⁶⁰Co/Co, ⁵⁴Mn/Fe, ⁵⁵Fe/Fe and ⁶³Ni/Ni were derived from the measured activity and concentration data. Specific activities of the radioactive corrosion products reveal the history of activity buildup processes in the particle. Typically, Fe-Cr-Ni oxide particles formed as a result of corrosion of the steel surfaces are released into the coolant and activated during the residence time (τ) in the reactor core that varies in a wide range. Smallest τ values are obtained if the particle circulates without deposition in the core and high values reflect a long deposition time in the core, probably on the fuel cladding surfaces. Residence times (τ) were calculated from the measured specific activities under various migration-activation conditions. Conclusions about particle transport-activation processes in case of the 4 reactor units have been drawn. The residence time of the migrating corrosion particles is usually small, less than 50 days. Periods of stable operation, shutdown and startup were comparatively evaluated. The 4 reactor units of the plant which have different operation history (including steam generator decontamination in certain cases) were also compared with each other. Significant differences in the Fe/Cr/Ni ratio of the oxide layers of the steam generators were observed. Correlation between the behavior of the migrating particles and particles deposited on the steam generator surfaces were also studied. (author)