Chernobyl - Appendices: Post Accident Changes to the RBMK

 March 2001 

Immediate Safety Changes 

After the accident at Chernobyl unit 4, the primary concern was to reduce the positive void coefficient. All operating RBMK reactors, in the former Soviet Union therefore, had the following changes implemented to improve operating safety: 

• To improve the operational reactivity margin the effective number of manual control rods was increased from 30 to 45.
• The installation of 80 additional absorbers in the core to inhibit operation at low power.
• An increase in fuel enrichment from 2% to 2.4% to maintain fuel burnup with the increase in neutron absorption.
 

• Scram (shut down) rod insertion time cut from 18 to 12 seconds.
• The redesign of control rods.
• The installation of a fast scram system (-2ß/2.5s).
• Precautions against unauthorised access to emergency safety systems.
 

RBMK Modifications

In addition to the safety changes, it has been recommended that RBMKs are modified, a procedure which is currently on going at the Leningrad site. Chernobyl unit 1 was licensed for operation in October 1995, following extensive maintainance which included the removal of some fuel channels to evaluate the metal and some backfitting. The modification process is commonly referred to as backfitting and consists of: 

• Replacement of the fuel channels at all units except Smolensk-3.
• Replacement of the group distribution headers and addition of check valves.
• Improvements to the emergency core cooling systems.
• Improvements of the reactor cavity over-pressure protection systems.
• Replacement of the process computer, SKALA.

Control Rod redesign

One of the post-accident changes to the RBMK was the redesign of the control rods.  

In the RBMK, 179 of 211 control rods are inserted into the core from the top. To improve their effectiveness, they are equipped with "riders" fixed to their bottom end but with a gap between the rider and the bottom tip of the control rod. Approximately 1.0m water columns remained under and above it. When the control rod is in its uppermost position, the rider is in the control rod cooling tube within the fuelled region of the core. The rider being made substantially of graphite, is almost transparent to neutrons, while water, which would occupy the tube otherwise, plays as an absorber. When the reactor is "poisoned" with Xenon and with partially inserted control rods, the major part of the power is produced within the lower region of the core. This means that when the rod started to move down from its uppermost position, the rider removed water from the lower part, causing an increase in reactivity and hence in power.

It was calculated that a surge of reactivity after the emergency shut down button had been pressed could reach +2ßeff. To counter this problem the control rod design has been changed with the rider tie length increased to prevent water columns in the lower part of the core.