Godiva device

The Lady Godiva device was an unshielded, pulsed nuclear reactor originally situated at the Los Alamos National Laboratory (LANL), near Santa Fe, New Mexico. It was one of a number of criticality devices within Technical Area 18 (TA-18). Specifically, it was used to produce bursts of neutrons and gamma rays for irradiating test samples, and inspired development of Godiva-like reactors.
The radiation source within the Godiva device was a fissile metallic mass (usually highly enriched 235U), about in diameter. This was located at the top of a high metal tower. The burst of radiation was produced when a piston of fissile material was quickly inserted into and extracted from a cavity within the larger fissile mass. During the time these two masses were combined, they formed a critical mass and a nuclear chain reaction was briefly sustained.
Godiva's design was inspired by a self terminating property discovered when incorrectly experimenting with the Jemima device in 1952. Jemima operated by remotely lifting one stack of enriched uranium-235 disks up towards another, fixed, stack. On 18 April 1952, due to a miscalculation, Jemima was assembled with too many disks; this caused an excursion of 1.5 x 1016 fissions—an automatic scram—but no damage.
On 3 February 1954 and 12 February 1957, accidental criticality excursions occurred causing damage to the device, but only insignificant exposures to personnel. This original Godiva device, known as "Lady Godiva", was irreparable after the second accident and was replaced by the "Godiva II".
Godiva II.
Godiva II was constructed inside a concrete building with walls and roof in a canyon a quarter-mile (400 m) away from the control room.
In 1959, Los Alamos agreed to make Godiva II available to DOD contractors free of charge for 2 days each month, acknowledging its unique facility for radiation tests.
Godiva's success in creating intense bursts spurred development of similar pulsed reactors, which also suffered accidental excursions, for example: 28 May 1965 at the White Sands Missile Range (parts were thrown ); and 6 September 1968 at the Aberdeen Proving Ground (middle melted, disks warped and bolts stretched).
In December 2002, the U.S. Department of Energy announced it was to move its TA-18 testing equipment including the Godiva burst machine from the LANL to the Device Assembly Facility (DAF) at the Nevada Test Site (NTS).