Following the 1966 Palomares B-52 crash and the 1968 Thule Air Base B-52 crash, concerns were raised by accident investigators about the high explosive used in the nuclear devices, which had detonated on impact. Efforts were started to find an explosive that was stable enough to withstand the forces involved in an aircraft accident. The Lawrence Livermore National Laboratory developed the "Susan Test" – a standard test designed to simulate an aircraft accident by squeezing and nipping explosive material between metal surfaces of a test projectile. Following experiments with this device, the Los Alamos National Laboratory developed a new safer type of explosive, called insensitive high explosive (IHE), for use in U.S. nuclear weapons.