98/00370 Power production and the price of electricity: an analysis of a phase-out of Swedish nuclear power

Abstract

Stabilized austenitic steels are susceptible to intergranular stress corrosion cracking (IGSCC) under boiling water reactor (BWR) conditions. This important finding for the German nuclear power station industry arises from the detection of cracks during the last 3 years in reactor hot water pipes made from titanium-stabilized steel AISI 321 in six BWRs and in reactor core components made from the niobium-stabilized steel AISI 347 in one BWR. All the observed cracks had a common feature: they had their origin in the chromium carbide precipitates at the grain boundaries and in the associated chromium-depleted region near the grain boundary. These microstructural features in the heat-affected zones of the hot water pipe weldments were caused by the heat input during deposition of the root bead. The TiC partially dissolved in the region near the fusion line and the released carbon reacted to form chromium-rich M23C6. Regarding the cracks found in the core shroud and the core grid plates, it was shown that a sensitizing heat treatment of rings taken from the same heat of steel could give rise to a microstructure susceptible to IGSCC in the region of a weldment. High carbon contents coupled with low stabilization ratios led to sensitization. Residual stresses developed during welding provided the significant contributions to the tensile stress necessary for IGSCC. With regard to the service medium, the influence of the electrochemical corrosion potential (ECP) was recognized as a dominant factor, together with the conductivity. The corrosion potential was mainly determined by the radiolytic formation of H2O2; with increasing distance from the core, the H2O2 content decreased owing to catalytic decomposition. For the pipes the problem of IGSCC could be resolved by the use of optimized steel (lower carbon content with maximum allowable stabilization ratio), by production modifications (lowest possible heat input during welding with favourable residual stress development) and by improvements in water chemistry (allowing the very low limits specified for normal operation to be exceeded for only short periods). Hydrogen control is seen as an optimal measure, whose introduction in Germany BWR plants must be considered in terms of the benefits and disadvantages. The outage times required for the replacement were dependent on the technical conditions prevailing in the individual plants and were generally justifiable. However in one plant the outage time was 1 year and in another it was more than 2 years. The intended replacement of the core shroud components has been cancelled owing to the decision to decommission the reactor for commercial reasons not exclusively related to the core shroud damage.