Radiation Tolerant Startup Controller for Snap 10A

A transistorized Startup Controller for the SNAP 10A space reactor was designed to be capable of surviving one year in orbit while accumulating a radiation dose of 1 x 1013 nvt fast neutrons and 2 × 107 R gammas. The challenging aspect of this design was to achieve long term reliability in the presence of moderately high radiation using semiconductors. Redundancy techniques, often used for reliability enhancement, were found ineffective in reducing the failure probability induced by radiation damage. Indiscriminate application of large derating factors, also in the interest of improving reliability, could have resulted in premature failure in a radiation environment due to the resulting misapplication of components. Additionally, the lack of appropriate radiation effects and failure rate data on many electronic components further hampered design efforts. Solutions to these and related problems were achieved through using Weibull statistical analysis and plotting techniques to predict the influence of radiation on component failure rates. The resultant controller design implements startup and control functions with transistor-diode NOR logic, which has an inherently superior tolerance to component parameter variations. Clock pulses at a 1.7 cps rate are generated directly by means of a unique magnetic oscillator designed to maintain ±5 percent frequency stability for one year under the influence of -20°C to +60°C temperature variations plus radiation. Switching load currents of 2 amps reliably in the radiation environment required the use of relays rather than semiconductors to drive the control drum actuators.