Pulse Reactor

a nuclear reactor that operates in the pulse mode. In contrast to a stationary nuclear reactor, whose power level is constant with respect to time, in a pulse reactor short power pulses and, correspondingly, neutron flux pulses are generated. The duration of the pulses ranges from a few microseconds to a few seconds. A pulse reactor makes possible the production of high power and an intensive neutron flux in short time intervals. This mode of operation is advantageous for some research purposes, such as experiments involving the measurement of the speed of neutrons during their traversal of a known distance. The occurrence of the power pulse in a pulse reactor is due to the vigorous development of the nuclear chain reaction. For this purpose an excess quantity of nuclear fuel is rapidly introduced into the pulse reactor, or the neutron absorbers are removed. To “quench” the pulse the “excess” nuclear fuel is often removed.

A distinction is made between single-pulse and periodic-pulse reactors. In single-pulse reactors, the shutdown of the chain reaction takes place because as the temperature rises (as a result of the liberation of energy during the chain reaction), the neutron fission factor decreases, leading to cessation of the chain reaction. A repeated power pulse can be produced only dozens of minutes or longer after complete cooling of the system. The Lady Godiva fast-neutron single-pulse reactor, developed in 1951 at the Los Alamos laboratory in the United States, was one of the first pulse reactors. The pulse power of such reactors is about 100 million kilowatts (kW), with a pulse duration of about 50 microsec. Such a pulse increases the reactor temperature by 400°C. Longer pulses (up to several seconds) are generated in single-pulse thermal-neutron reactors.

In periodic-pulse reactors the power pulses are repeated at intervals of several seconds, and the power in each pulse is less than in a single-pulse reactor. Since the time interval between pulses is short, the pulses in a periodic-pulse reactor prove to be coupled because of the so-called delayed neutrons, which are emitted several seconds after fission.

The first periodic-pulse reactor (a fast-pulse reactor) was developed in the USSR in 1960 and has been used successfully for more than ten years at the United Institute of Nuclear Research in Dubna to study the structure of atomic nuclei, solids, and fluids. In this pulse reactor the power pulse (lasting 60 microseconds) occurs upon the brief introduction into the plutonium-rod assembly of a uranium slug that is secured to the rim of a rapidly rotating disk. A power pulse corresponds to each rotation of the disk. The maximum pulse power of the fast-pulse reactor reaches 500,000 kW, with an average power of about 20 kW. Several similar high-power units are being built in the USSR and abroad.