Abstract
It is well known that Intergranular Stress Corrosion Crack (IGSCC) mitigation of nuclear reactor structural materials can be achieved by injecting hydrogen into the feedwater. However, to mitigate SCC in some vessel internals requires the use of high levels of feedwater hydrogen, which results in main steam radiation dose rate increases. Recent studies have shown that the presence of noble metals on nuclear reactor structural materials, by alloying or surface deposition by plating, by various thermal spray coating techniques and by noble metal chemical addition (NMCA) significantly reduce the hydrogen demand necessary to achieve the IGSCC protection potential of -0.230 V(SHE). Of these techniques, NMCA has shown considerable promise and uniqueness as a method of distributing noble metal on reactor internal surfaces including the lower plenum. In this process, a noble metal chemical is introduced into the reactor water which deposits on all reactor internal surfaces that come in to contact with water. The principle behind NMCA is identical to other noble metal technologies that result in lowering of the feedwater hydrogen requirements necessary to achieve the IGSCC protection potential of -0.230 V(SHE).
This paper describes the long-term durability of NMCA treated surfaces in retaining their catalytic activity in the presence of hydrogen over a long period of time. The structural materials tested include Type 304 SS, Type 316 SS, Inconel 600 and alloy 182 weld metal. After NMCA treatment, these materials have been tested for durability of the catalytic activity under high (low conditions (i.e. up to 14 ft/s of linear fluid velocity), in HWC over an extended period of time and by long-term exposure to crud containing high temperature water. The NMCA treated specimens have shown stable catalytic activity following these durability tests, some of which have already lasted close to two years.
