Investigations About Released Debris and Corrosion Products and Their Impact to Head Loss Build Up and Decay Heat Removal Under Water-Chemical PWR-LOCA Conditions

Abstract

Investigations about the release, transportation and deposit of fibrous insulation material (FIM), corrosion products as well as resulting compounds and debris mixtures become more important to reactor safety research, when considering long-term behavior of emergency core cooling systems (ECCS) during loss of coolant accidents (LOCA). Debris released by a leakage jet leads to head loss buildups at the sump strainers, the debris filters and the spacers of fuel assemblies. However, these complex processes may influence the decay heat transfer out of the reactor core. A similar but newer scenario implies that the boric acid coolant in pressurized water reactors (PWR) can support corrosion processes at hot-dip galvanized installations, leading to a significant release of ionic zinc into the coolant during the sump recirculation phase. A long-term change of chemical properties of the coolant (e.g. pH, zinc ion concentration) has to be considered in safety analyses. Chemical analyses showed that the solubility of zinc in boric acid coolant is inversely proportional to the coolant temperature. Consequently, zinc ions can be dissolved at lower temperatures in the containment sump. Precipitations of zinc borate (ZBP) are possible at hot spots in the reactor core. The ZBP can be formed as a flocculent disperse phase in the coolant or as solidified layers at hot fuel rod surfaces. Layer spalling could lead to the release of further solid particles into coolant flow inside the reactor core. In several joint research projects between the Zittau/Görlitz University of Applied Sciences (HSZG) and the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) investigations on the impact of these processes to the head loss buildup and the heat transfer from core were done at laboratory and semi-technical scaled test facilities. Generic experiments showed the formation of ZBP in heating rod configurations. The ZBP may remain in the core structures or can be transported on debris filter cakes in upstream and downstream components of ECCS and influence the head loss. After this, research addressed the systematical clarification of physico-chemical mechanisms and their influence on thermal-hydraulic-dynamic processes occurring as a consequence of flow induced corrosion effects during LOCA. This paper includes a description of the most important involved test facilities, applied measuring techniques, an overview of boundary conditions considered experimentally and selected results.