Both the stress limits imposed by fatigue considerations and
the allowable static stresses used in this study follow the
guidelines of the ASME Boiler and Pressure Vessel Code [1].
For fatigue, the Code recommends a safety factor of two in the
design stress or 20 in the number of cycles, whichever gives
the lowest design stress. In all cases considered for PULSAR,
the safety factor of two on stress is the limiting case.
The peak primary stresses (due to the coolant pressure) are limited
to the allowable stress intensity (S). For the vanadium
alloy design, the peak secondary stresses (thermal + pressure
stresses) are limited to 3S
, although fatigue restrictions
superseded this limit under all the conditions that were studied.
Criteria for choosing secondary stress limits in the SiC composite
are not well defined. The ARIES-I study [2] chose
of the longitudinal ultimate strength as a limit for the combined
primary and secondary stress based on thermal-shock testing of
this material; we adopt that limit here.
Off-normal loads were not considered in this study. In particular, disruptions have not been accounted for in the analysis.
Radiation damage in the form of swelling, transmutation, etc. limit both the first wall and divertor to 7.5 and 8.4 full power years for the SiC composite and V alloy designs respectively. We anticipate a 7400 s cycle (7200 s burn time and 200 s dwell time), thus the first wall and divertor would undergo 32,000 (SiC) and 36,000 (V) cycles in this time. For this study, we used 40,000 cycles as the goal for both designs.