Clementine (nuclear reactor)

Shielding and support structure

The reactor core was wrapped in a series of neutron reflectors and shielding structures, starting with a 15 cm (6 in.) thick cylindrical blanket of natural uranium immediately surrounding the core. This blanket was open at the top and bottom and could be moved up and down. Next was a 15.2 cm (6 in.) thick steel reflector and 10 cm (4 in.) of lead. Finally, most of the reactor was surrounded by multiple laminations of steel and boron plastic. This entire assembly was surrounded and supported by a thick concrete shell that provided additional shielding. A number of holes ran through the shielding to provide fast neutrons to the various physics experiments.

Reactor control

The reactor was the first reactor to demonstrate reaction control via control of delayed neutrons, this was more of a function of being one of the very early reactors, rather than a special design feature. Control was accomplished by several means. The uranium blanket described above could be raised and lowered. ²³⁸U is a good neutron reflector, so the position of the blanket controlled the number of neutrons available to the reaction. When the blanket was raised, more neutrons were reflected back into the core, causing a greater number of fissions and, consequently, greater power output.

Additionally, there were two shutdown/control rods composed of natural uranium and boron that was enriched in the boron-10 isotope. ¹⁰B is a very effective neutron poison that could be inserted to control and shut down the reaction.

Shutdown of the reactor involved simultaneously dropping the uranium blanket and inserting the two control rods into the center which absorbed neutrons and poisoned the reaction. Up to 20 other holes were available in the core for experimental configurations or additional control or fuel rods.

Use and shutdown

Clementine operated successfully from 1946 until 1950 when the reactor was shut down to correct a problem with the control and shim rods. During this shutdown it was noted that one of the natural uranium rods had ruptured. It was replaced and the reactor was restarted.

It was again operated successfully until 1952 when the cladding on one of the fuel rods ruptured. This caused contamination of the primary cooling loop with plutonium and other fission products. At this time it was decided that all the primary objectives of Clementine had been achieved and the reactor was permanently shut down and dismantled.

Results of the Clementine experiment

The experience and data provided by operating the Clementine reactor were very useful for both military and civilian applications. One of the notable achievements of the Clementine project included measurements for the total neutron cross sections of 41 elements to a 10% accuracy. Additionally, Clementine provided invaluable experience in the control and design of fast neutron reactors. It was also determined that mercury was not an ideal cooling medium for this type of reactor due to its poor heat transfer characteristics.

Specifications

• Type: fast neutron reactor
• Fuel: plutonium-239
• Coolant: mercury at 2 kilograms per second (260 lb/min) maximum.
• Moderator: none
• Shielding: multiple layers of uranium-238, steel, lead and boron-impregnated plastic.
• Power: 25 kW maximum.
• Core temperature: inlet 38 °C (100 °F), outlet 121 °C (250 °F), max core 135 °C (275 °F)