Following the 1966 Palomares B-52 crash and the 1968 Thule Air Base B-52 crash, accident investigators concluded that the conventional explosives used at the time in nuclear weapons were not stable enough to withstand the forces involved in an aircraft accident. The finding triggered research by scientists in the United States into safer conventional explosives that could be used in nuclear weapons. The Lawrence Livermore National Laboratory developed the "Susan Test" — a standard test that uses a special projectile whose design simulates an aircraft accident by squeezing and nipping explosive material between metal surfaces. The test projectile is fired under controlled conditions at a hard surface to measure the reactions and thresholds of different explosives to an impact.