SINGLE EVENT UPSET AS A FAILURE MECHANISM IN SAFEGUARDS EQUIPMENT

The theoretical possibility that Neutron-induced Single Event Upset (SEU) could affect modern microelectronics at ground level has been verified, as a widely used Safeguards surveillance system was found to be vulnerable to SEU. Although in this particular instrument it was possible to circumvent the impact of the vulnerability, the phenomenon has significant implications on the design, development, testing, operation, and installation of all new Safeguards instruments that use modern, fine-structure microelectronics. The central issue is that SEU is a purely statistical phenomenon and can therefore strike at any component at any time. It is further recognized that some electronic structures are more resistant than others. Older circuits with their larger structures and higher operating voltages are much more difficult to upset than modern circuits. For example, flash memories and EPROMs are essentially immune to upset since their retention does not depend on the active state of a transistor. Because SEU is statistical, the components of every modern safeguards instrument have a susceptibility distribution in which some components are more vulnerable than others. In this paper, we describe the symptoms, the SEU mechanism, and the tests that verified SEU as the root cause. Finally, we discuss the design restrictions and circumvention features that developers must incorporate in new instruments to mitigate this vulnerability.