As a result, prompt supercriticality causes a much more rapid growth in the rate of energy release than other forms of criticality.
These are called prompt neutrons, and strike other nuclei and cause additional fissions within nanoseconds (an average time interval used by scientists in the Manhattan Project was one shake, or 10 ns).
A steady-state (constant power) reactor is operated so that it is critical due to the delayed neutrons, but would not be so without their contribution.
The exponential increase of reactor activity is slow enough to make it possible to control the criticality factor, k, by inserting or withdrawing rods of neutron absorbing material.
Using careful control rod movements, it is thus possible to achieve a supercritical reactor core without reaching an unsafe prompt-critical state.
Nuclear reactors can be susceptible to prompt-criticality accidents if a large increase in reactivity (or k-effective) occurs, e.g., following failure of their control and safety systems.
In all these examples the uncontrolled surge in power was sufficient to cause an explosion that destroyed each reactor and released radioactive fission products into the atmosphere.
In the other two incidents, the reactor plants failed due to errors during a maintenance shutdown that was caused by the rapid and uncontrolled removal of at least one control rod.
At the SL-1 plant in 1961, the reactor was brought from shutdown to prompt critical state by manually extracting the central control rod too far.
As the water in the core quickly converted to steam and expanded (in just a few milliseconds), the 26,000-pound (12,000 kg) reactor vessel jumped 9 feet 1 inch (2.77 m), leaving impressions in the ceiling above.
Conclusive evidence revealed that the SL-1 excursion was caused by the partial withdrawal of the central control rod.
Indeed, one of the design problems to overcome in constructing a bomb is to compress the fissile materials enough to achieve prompt criticality before the chain reaction has a chance to produce enough energy to cause the core to expand too much.
This generally means that nuclear bombs need special attention paid to the way the core is assembled, such as the implosion method invented by Richard C. Tolman, Robert Serber, and other scientists at the University of California, Berkeley in 1942.