A tertiary shutdown system operates by injecting boron balls into the reactor.

It is currently undergoing an upgrade to its safe shutdown system which already met NRC standards.

There are two independent shutdown systems installed, designed to shut the reactor down effectively within a second.

He said he was confident that the crucial automatic shutdown system had worked.

Liquid neutron absorbers are also used in rapid shutdown systems for light water reactors.

The automatic shutdown system, as intended, shut down the reactor without incident immediately after the fire broke out, he said.

Transocean had been warned that the human element - the need for crew members to act quickly and correctly under stress - made the shutdown system vulnerable.

Calibration tests to assure shutdown systems will activate on over-power or over-temperature conditions.

According to Rostekhnadzor, the automatic shutdown system of the water intake pipes' gates failed after failure of the turbine 2.

By not allowing the normal shutdown systems to interfere, the reactor power dropped to near zero within about 300 seconds.