Radiation can cause spacecraft electronic hardware to lose functionality via a non-recoverable destructive event or a recoverable Single Event Upset (SEU). The risk due to this radiation induced loss of functionality needs to be accounted for in PRA because it can be a significant contributor to risk. Hardware failure due to non-recoverable destructive events can be modeled the usual way in the PRA, using functional failure rates. The failure rate associated with non-recoverable destructive events can simply be added to the functional failure rate. In many cases, if the hardware is designed for the operating environment the impact on the functional failure rate should be negligible.

Modeling recoverable SEUs is more challenging since the occurrence time of the SEU, and its corresponding recovery time, needs to be accounted for in the model. Event-tree/fault-tree PRA models calculate the probability of failure in a given amount of time, but they do not calculate the time when a failure occurs. This paper will describe how to model these events, accounting for component upset rates for nominal background radiation and worst-case Solar Particle Events (SPEs) along with the corresponding recovery time, to determine the probability of simultaneous loss of multiple components.