Chernobyl’s SCRAM and ‘neutron poison’
Stories from Physics for 11-14 14-16 16-19
The failure of a SCRAM procedure (see above) contributed to the Chernobyl disaster. When a technician pressed the emergency shutdown button, the control rods started moving at 0.4 m/s into the 7 m-high core. The rods had graphite tips below a neutron-absorbing boron carbide section. As the non-neutron-absorbing graphite tips entered the reactor, they displaced neutron-absorbing water and caused a power surge in the reactor, overheating the core. The pressure of the heated fuel and fission product gases caused the fuel rods to rupture and to mix with the cooling water leading to a steam explosion. During this incident, the power of the reactor jumped to ten times its normal operating output, reaching over 30,000 MW. The pressure of steam was sufficient to lift the 2,000 tonne upper biological shield and a refuelling machine from the top of the reactor.
In addition to control rods, some reactors may have an emergency ‘neutron poison fuse’. This is a thin metal wire which melts if the reactor gets too hot, releasing a spring-powered injector that injects a neutron-absorbing material (a ‘neutron poison’) into the reactor.