T91 steel
In fact, Chromium-bearing steels with low Carbon content were developed around the turn of last century. Research to establish the hardenability and workability of high Cr (12 to 30 %) steels was largely completed before 1920. Because of the relatively high cost and periodic interruptions of international supply of Cr, there has been incesitive to minimise Cr-content. The underlying reason for interest in steels containing at least 9 to 10 % Cr is their relatively high passivity or resistance to rusting and corrosion in industrial air environment. But the key consideration motivating the development of such steels was their creep and tensile strength at elevated temperatures. Several major steelmakers and equipment manufacturers have attempted to develop improved high temperature (550 °C-650 °C) with ~9 % Cr-Mo base since the late 1950’s. Thus, several version of this class of steels have been developed and standarised in ASME 1983 for non-nuclear application and in ASME1985 for nuclear ones.
The grade containing 9 % Cr, 1 % Mo microalloyed with V and Nb with controlled nitrogen content the so-called T91 presents valuable advantages related to their excellent elevated temperature strength and creep behavior, improved resistance to thermal fatigue, and good transfer and low expansion coefficient compared to austenitic steels. This type of material has been identified as candidate material for fast breeder reactor in the past and is the ultimate candidate for a DEMO in the frame of the international fusion program.
At this point of time, despite very limited lack data within MYRRHA operating conditions on neither the effect of irradiation, liquid metal and their synergy, T91 is designed as the prime candidate materials to be in contact with the LBE and to contain the fuel in Myhrra. These materials have been selected as the best compromise between fabricability and resistance to severe irradiation conditions in the operating temperature range from 200 to 450 ºC.