Reactivity Coeffcients and the Chernobyl Incident

The reactor involved in the Chernobyl nuclear accident was a 3,200-MWt RBMK (High Power Channel-type Reactor), a boiling-water pressure-tube, graphite moderated power reactor that was developed and operated in the former Soviet Union.

The Chernobyl Nuclear Power Plant.

Because of the pressure-tube design using water as coolant within channels in the graphite moderator, RBMK reactors have a significant positive void coefficient of reactivity in which a reduction in the coolant density results in an increase in the system reactivity due to a reduction in neutron absorption by the coolant. This reactor also has a positive moderator coefficient of reactivity in which the reactivity increases as the temperature of the moderator increases. Both of these operating characteristics are compensated by the negative temperature coefficient of the fuel which loses reactivity as the fuel temperature increases.

Schematic diagram of an RBMK reactor.

At 00:28 on the day of the accident, the monitoring systems were adjusted to the lower power levels, but the operators failed to reprogram the computer to maintain power in the 700 to 1000 MWt range. The power fell to 30 MWt. The majority of the control rods were withdrawn to counteract the negative reactivity effect of xenon (fission product) poison which built up during the delay in power reduction. The power climbed and stabilized briefly at 200 MWt. At 01:03 All eight pumps were activated to ensure adequate cooling after the test.

The control room inside reactor 4 at Chernobyl

This violated two rules, one on high flow rate, the other protecting against pump cavitation. The resulting high flow rate increased heat transfer and thereby maximized coolant (neutron) absorption to require still more (prohibited) control rod withdrawal. It also maximized the reactivity increment available from the change in neutron absorption associated with coolant voiding. The combination of low power and high flos produced instability and required many manual adjustments. The operators turned off other emergency shutdown signals.

Engineers test a reactor's control panel at the Chernobyl nuclear power plant control room in Chernobyl

At 01:22 The computer indicated excess reactivity. Under pressure to complete the test, the operators reserved the possibility of rerunning the test by blocking the last remaining trip signal just before it would have shut down the reactor. "01:23 The test began. As power started to rise, coolant voiding increased and, through the positive reactivity feedback mechanism, led to accelerated power increase. Recognizing the potential consequences, the operators began insertion of all control rods.

Inside Chernobyl number 3 reactor unit

The power surged to 100 times the reactor's normal capacity in the next four seconds. A second pulse may have reached nearly 500 times full power and caused the fuel to disintegrate, breach the cladding and enter the water coolant. A steam explosion was caused by contact of the fragmented fuel with the water-steam coolant mixture.

The resulting force lifted the massive top shield, penetrated the concrete walls of the reactor building, and dispersed burning graphite and fuel. Oxidation of zirconium and graphite produced combustible hydrogen and carbon-monoxide gases that may have contributed to additional explosions. The initial excursion by itself was well beyond the containment design basis. It blew off the building roof and sent a plume of radioactive gases and particulates high into the atmosphere.

Aerial view of the damaged core. Roof of the turbine hall is damaged

Short documentary on the Chernobyl Accident

reference: http://en.wikipedia.org/wiki/RBMK
reference: https://hps.org/publicinformation/ate/q1743.html
reference: http://en.wikipedia.org/wiki/Chernobyl_disaster