I’m not sure which low alloy steel is used in fabricating the reactor but it is not stainless steel. Yes, there are safety factors involved in the design and the operation of a nuclear power plant. For one thing the quality control requires mill certifications on the structural materials used in the construction. As on example, some of the steel may be an A36 spec. That means it will have a minimum tensile strength of 36,000 psi and ultimate tensile strength of 58,000–80,000 psi. The mill certification which documents the strength determined by the tests conducted on samples taken from the melt, is higher than 36,000. But 36,000 is what the safety factor is based on.
I don’t recall the ultimate but many of the mill certs we had for A36 was greater than 40,000 psi.
A36 is 80,000 or greater.