Evaluation of the effects of radiation-induced conductivity on charge separation in nuclear weapons during radiographic inspection

摘要

Radiography is routinely used for non-destructive inspection of nuclear weapons at Pantex. For example, X radiography can be used to observe the positions of valves, to verify that a stronglink is in the safe position, or to inspect internal mechanical assembly details. Because of the presence of heavy metals in warheads, such operations are carried out with high energy x-rays produced by linear accelerators (Linacs), and substantial doses can be accumulated, especially if images from more than one direction are required. In December 1996, the basis for safety assurance of Linac operations at Pantex was called into question. Questions concerned the level of electrical charge separation in the high explosive (HE) dielectric and possible consequences of high electrical fields. Linac operations, which affect other critical missions at Pantex, were suspended and the Weapons Labs were asked to perform a critical analysis to determine what controls were required to assure safety. The postulated mechanism by which fields could build-up involved creation of charge (primarily Compton Electrons) by incident X-ray photons, and their accumulation in the high explosive. Building on a model originally developed by Mike George at Los Alamos for somewhat different conditions, Livermore developed a model which predicted the voltages which would occur in the vicinity of the detonator cables, and showed how these voltages depend on bulk resistivity of the HE and on the dose and dose rate. The authors proposed that the effects of radiation induced conductivity (RIC) would dominate, and showed that at steady state, neither dose nor dose rate would affect the voltage. They also proposed a series of experiments on HE assemblies to measure the RIC and to confirm the level of voltages attained. The experiments were conducted in March and April. These efforts were successful and showed that voltages were insignificant, and did not depend on dose or dose rate.