SUMMARY AND CONCLUSIONS, PAGE 87
The heats of Hastelloy N used in fabricating the MSRE have shown a systematic deterioration of mechanical properties with increasing neutron fluence. The material exposed for the longest period of time in the core has reached a thermal fluence of 1.5 x 10^21 neutrons/cm2 and a fast fluence (> 50 kev) of 1.1 X 10^21 neutrons/cm2. These values are quite close to those anticipated for future reactors with a 30-year design life.
The ductility of the material was too low, but the microstructure was free of irradiation-induced voids and defects other than helium bubbles. Several heats of the modified alloys have been exposed to the MSRE and these have better postirradiation properties. They also seem to have good corrosion resistance.
The standard Hastelloy N removed from the core shows some evidence of corrosion. The corrosion seems generally to be due to the selective removal of chromium, as predicted by prenuclear tests.
Some observations that have not been explained adequately are
(1) the presence of grain-boundary cracks in the straps that held parts of the surveillance assembly together,
(2) the modified microstructure near the surface, and
(3) the formation of intergranular cracks originating from the surface when irradiated materials are strained.
One of the modified alloys, heat 67-504, was exposed to the cell environment. The fluence was higher in the core, but the postirradiation properties were superior to those of the material exposed to the cell environment.
We presently have no explanation for the observed behavior.
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