One unexpected finding was shallow, inter-granular cracking in all metal surfaces exposed to the fuel salt. The cause of the embrittlement was tellurium – a fission product generated in the fuel. This was first noted in the specimens that were removed from the core at intervals during the reactor operation. Post-operation examination of pieces of a control-rod thimble, heat-exchanger tubes and pump bowl parts revealed the ubiquity of the cracking and emphasized its importance to the MSR concept. The crack growth was rapid enough to become a problem over the planned thirty-year life of a follow-on thorium breeder reactor. This cracking could be reduced by adding small amounts of niobium to the Hastelloy-N.