What are the key design features and shortfalls of RBMK-1000 reactor?

RBMK-1000 reactor is a heterogeneous channel reactor using thermal neutrons of 1000 MWt capacity with graphite as moderator and water as heater.

The concept of the channel-type power reactor with the graphite moderation and boiling water was developed in early 60s of the XX century. By that time, the USSR already had an experience in design and operation of water coolant Uranium-graphite reactors (Obninsk NPP, Beloyarsk NPP, Bilibino NPP, and Siberian NPP).

The principal feature of the channel-type reactor construction is absence of any special form shell typical for the reactors of WWER type (water moderated energy reactors). Furthermore, refueling of RBMK reactors can be done while the reactor is operating. This allows an increase in the capacity factor.

During a range of preliminary investigations held after the Chornobyl NPP accident, it was found out that RBMK-1000 reactors have some design shortfalls. To enhance reactor’s safety and reliability, the void coefficient of reactivity should be decreased, and a fast-acting emergency protection system should be installed. It was emphasized that when the reactor was shutdown and the control rods were inserted into the core, while moving down for 5 seconds they brought positive radioactivity instead of a negative one (so called positive shutdown effect). This vividly demonstrates an inadequacy in the control rods design, i.e. a potential accident factor. Most of the shortfalls were eliminated after the Chornobyl accident. In particular, all RBMK-1000 reactors operating in the USSR increased their initial uranium enrichment up to 2.4%. The response time of the control system decreased by a factor of approximately 10 times. This improved the reactor’s neutron and physical characteristics, raised its stability at all power levels. However, according to estimations of some nuclear experts, the control and protection systems of RBMK-type reactors still do not ensure safe operation of the plants.

(From the book “Chornobyl Catastrophe” – edited by V.G. Baryakhtar, Academician of National Academy of Sciences of Ukraine. – K.: Naukova Dumka, 1995. – pp. 19-20)