Corrosion induced long-term effects during loss-of-coolant accidents in pressurized water reactors
Measures to ensure the long-term core coolability after design-basis accidents are integral part of the safety concept of nuclear power stations in the whole world. In case of loss-of-coolant accidents (LOCA) in pressurized water reactors (PWR), the cooling water spilling out of the leak in the primary cooling circuit is collected in the reactor sump and recirculated into the primary cooling circuit by emergency cooling pumps during sump recirculation operation. During the LOCA, coolant impurities may affect both, the water chemistry and the clogging of retaining components within the emergency cooling circuit. Especially, the long-term contact of the water jet from the leak with hot-dip galvanized steel internals (e.g. grating treads, support grids of sump strainers) installed in the containment may cause corrosion of the corresponding materials forming soluble and insoluble corrosion products. While insoluble corrosion products are transported in the coolant flow and deposited at retaining components (e.g. sump strainers), soluble corrosion products influence the coolant chemistry. In case of zinc corrosion, it cannot be ruled out that corrosion products precipitate as zinc borates at hot zones inside the cooling circuit (e.g. hot-spots inside the core) and subsequently deposit at spacers and/or on cladding tubes of fuel rods. Such processes may eventually influence the discharge of the post-decay heat during the late stage of a LOCA.
The influence of corrosion at hot-dip galvanized containment internals on the water chemistry as well as on the formation and deposition of particulate products are investigated in co-operation with the TU Dresden and the Hochschule Zittau/Görlitz. The studies are aimed to support the development of methodologies for improved analytical evaluation of the operation of emergency cooling systems during the late stage of a postulated LOCA as well as for evaluation of the fuel rod integrity.
Due to the complex corrosion chemistry, separate investigations of influencing factors are necessary, which are realized by single-effect experiments where basic experiments regarding the corrosion chemistry are carried out in a stirred tank reactor. The results are the basis for long-term corrosion and precipitation studies at LOCA-specific process conditions. The experimental conditions are defined on the basis of thermal-hydraulic simulations of different LOCA scenarios using the system code ATHLET, which are carried out by the Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH.
The retrograde solubility of zinc corrosion products in boric acid containing coolants with increasing temperature was identified as potential source for deposition of solids at hot zones within the cooling circuit (e.g. hot-spots inside the core) by means of batch experiments in the stirred tank reactor. Based on these results, a cycle of zinc corrosion in the sump and precipitation of zinc corrosion products in the core cannot be ruled out during sump recirculation operation.
The corrosion products were identified as zinc borates having the general formula x ZnO • y B2O3 • z H2O by means of chemical analysis. Different zinc borate species are formed depending on the precipitation temperature.
The processes of zinc corrosion and subsequent zinc borate deposition at hot surfaces during specific LOCA scenarios were investigated using a lab-scale corrosion test facility. This facility represents the operation principle of the emergency cooling circuit during the sump recirculation phase in a simplified manner. Hot surfaces inside the core are simulated by heating elements consisting of electrically heated PWR fuel rods made of zircaloy.
To estimate the effects of zinc borate deposits on the thermal-hydraulics in the core, selected LOCA scenarios are validated at technical scale by experiments of the project partner HS Zittau/Görlitz.
Federal Ministry for Economic Affairs and Energy:
- Partikelentstehung und -transport im Kern von Druckwasserreaktoren; Physikochemische Mechanismen (FKZ: 150 1430, 150 1467)
- Lokale Effekte im DWR-Kern infolge von Zinkborat-Ablagerungen nach KMV (FKZ: 150 1496)
- TU Dresden, Institute of Power Engineering, AREVA Endowed Chair of Imaging Techniques in Energy and Process Engineering
- Hochschule Zittau/Görlitz, Institute of Process Technology, Process Automation and Measuring Technology (IPM)
- Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH
- HZDR, Institute of Resource Ecology
- Kryk, H.; Harm, U.; Hampel, U.
Corrosion of hot-dip galvanized containment installations - A potential cause for thermal-hydraulic effects after LOCA in PWR?
ICONE 24 - Conference proceedings; Volume 3: Thermal-Hydraulics: ASME - Digital Collection, 978-0-7918-5003-9 (2016)
- Seeliger, A.; Alt, S.; Kästner, W.; Renger, S.; Kryk, H.; Harm, U.
Zinc corrosion after loss-of-coolant accidents in pressurized water reactors - Thermo- and Fluid-dynamic effects
Nuclear Engineering and Design 305(2016), 489-502
- Kryk, H.; Hoffmann, W.; Kästner, W.; Alt, S.; Seeliger, A.; Renger, S.
Zinc corrosion after loss-of-coolant accidents in pressurized water reactors - physicochemical effects
Nuclear Engineering and Design 280(2014), 570-578
- Kryk, H.; Hoffmann, W.; Waas, U.
Influence of corrosion processes on the head loss across ECCS sump strainers
Kerntechnik 76(2011)1, 46-53