|Fig. 1: A simplified schematic of an RBMK reactor core. (Source: Wikimedia Commons)|
The RBMK nuclear reactor is a soviet-designed reactor dating back a few decades in design. There were almost twenty of these reactors completed, and 11 of these reactors are still in use in Russia. This reactor type is rather infamous because of the Chernobyl accident, the Chernobyl-4 reactor which melted down was of the RBMK design. All of the RBMK reactors in current use are retrofitted from the original specifications.
The RBMK reactor is graphite-moderated, so a core of solid graphite is responsible for slowing down fast neutrons in the reactor core. In fact, the name RBMK is a Russian acronym for "High-powered channel-type reactor". This is a relatively uncommon reactor design, with most reactors in use recently using water as their moderator. This graphite-moderated design allows the reactor to use relatively unenriched uranium as its fuel source.  A simplified diagram of the internal workings of an RBMK reactor is shown in Fig. 1.
RBMK reactors look like one would expect a nuclear reactor to look (see Fig. 2. But one notable design aspect of the RBMK reactor type not visible at first glance is its void coefficient. The void coefficient measures how the reactor's reactivity changes as voids are introduced in the moderator or coolant of a reactor. One way these voids can be introduced is by steam bubbles forming in water that is used as a coolant or moderator. As these voids form, a reactor with a negative void coefficient will become less reactive, and a reactor with a positive void coefficient will become even more reactive. The RBMK reactor type had an extremely positive void coefficient, which means that without careful proper oversight, the reactor can rapidly become unsafe. 
|Fig. 2: The reactor hall of an RBMK plant. (Source: Wikimedia Commons)|
At Chernobyl, the dangerous qualities of the reactor type came to a head, and the reactor Chernobyl-4 melted down in 1986. This catastrophe prompted the retrofitting of all existing RBMK reactors with several additional safety measures--many of these safety measures are directed at the reactor core in an effort to lower the void coefficient. For instance, every RBMK reactor was loaded with more than 80 additional absorbers, and the fuel enrichment for the reactor was increased from 2.0% to 2.4%. Additionally, controls were put in place to keep the operational reactivity margin under tighter control.  The drives on the reactor's control rods were also modified, reducing the time for control rods to fully enter the reactor to 12 seconds, down from 19. 
Today, 11 RBMK reactors are currently in use, all of them in Russia. The design is much improved from the original due to the additional safety constraints placed on reactor design. However, there are some detractors of these reactors, as many of them were given extensions to their lifetimes, so many of them are still in use despite being originally scheduled for decommission years ago. 
© Alex Stephanus. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 L. Lederman, "Safety of RBMK Reactors: Setting the Technical Framework," IAEA Bull. 38, No. 1, 10 (1996).
 "INSAG-7: The Chernobyl Accident: Updating of INSAG-1," Internation Atomic Energy Agency, Safety Series No. 75-INSAG-7, November 1992.
 J. Haverkamp, "Lifetime Extension of Ageing Nuclear Power Plants: Entering a New Era of Risk," Greenpeace, March 2014.