Answer to Question #1743 Submitted to "Ask the Experts"

Q

At the Chernobyl Reactor, how long did it take for the control rods to stop the chain reaction?

A

The short answer to your question is that the chain reaction was never actually stopped by the control rods. Instead, criticality ceased when a violent steam explosion caused by an uncontrollable power excursion disassembled the reactor core, disrupting the geometry of the reactor. The power excursion resulted from a combination of the design and operating characteristics of the reactor and human error.

The reactor involved in the Chernobyl nuclear accident was a 1,000-MWe (3,200-MWt) RBMK, a boiling-water pressure-tube, graphite moderated power reactor that was developed and operated in the former Soviet Union. 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. However, this effect dominates only when the reactor is operated at higher power levels. As a result, restrictions are imposed for operations at less than 20% of the licensed power level.

Without going into too much detail on the reactor design and the events leading to the accident, the following description of the principal events that occurred at the time of the accident (26 April 1986) is provided from "Nuclear Engineering: Theory and Technology of Commercial Nuclear Power," 2nd ed., by Ronald Knief, published in 1992 by Hemisphere Publishing Corp.: "00:28 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 The majority of the control rods were withdrawn to counteract the negative reactivity effect of xenon (fission product) poison which built up during the delay in power reduction.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. "01:03 All eight pumps were activated to ensure adequate cooling after the test. This violated two rules, one on high flow rate, the other protecting against pump cavitation. The resulting high flow rate increased heat transfer, essentially eliminated voiding in the coolant, 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. "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. However, the control rods' graphite followers preceded the poison, introduced additional moderation as they displaced water, added reactivity, and accelerated the power increase further. 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. Energy deposition averaging over 300 cal/gUO2—and reaching 400 to 600 cal/g in some locations—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 mixxture. The resulting force lifted the massive (1,000-ton steel and concrete) top shield, penetrated the concrete walls of the reactor building, and dispersed burning graphite and fuel. With movement of the shield, all of the coolant outer pipes were sheared and the control rodds were pulled out. 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 . . ."

For more information, see Hewitt and Collier's Introduction to Nuclear Power, 2nd ed., published in 2000 by Taylor and Francis (click on "catalogue" and type in the book title).

Dr. John Bennion, PhD, CHP