THE FLOW-ACCELERATED CORROSION AND HYDROGEN ABSORPTION OF MARTENSITIC STAINLESS STEEL UNDER CANDU REACTOR PRIMARY HEAT TRANSPORT SYSTEM CONDITIONS

摘要

The core of a CANDU reactor contains an array of horizontal fuel channels (FCs) in which nuclear fuel is contained. In currently operating designs, part of the FC contains a Zr-2.5Nb pressure tube (PT) roll expanded into an AISI Type 403 martensitic stainless steel (403SS) end fitting (EF). During reactor operation, atomic deuterium (D) produced by corrosion processes on the EF can undergo solid-state diffusion to the PT through regions of intimate metal-to-metal contact in the rolled joint. This route of D entry into the PT effectively bypasses the ZrO_2 film, which is known to provide the PT considerable protection against direct absorption of D. The primary concern regarding the ingress of D to the PT is the possibility for delayed hydride cracking in regions containing precipitated hydrides/deuterides and high stress. Recent out-reactor, light water loop tests have monitored the in-situ rates of corrosion and hydrogen absorption on 403SS under simulated CANDU reactor PHTS conditions. The results have suggested that 403SS undergoes flow-accelerated corrosion (FAC) at significant rates and that the hydrogen produced by FAC is absorbed at near 100% efficiency. Furthermore, the effects of varying the concentration of dissolved hydrogen and oxygen gases within the loop electrolyte were investigated. In theory, the FAC of 403SS may produce a considerable flux of D in the EF and, thus, provide a significant contribution to ingress within the rolled joint region of the PT.