RBMK reactor

Design

The RBMK (Реактор Большой Мощности Канальный, "high-power channel-type reactor") is a graphite-moderated, light-water-cooled, pressure-tube boiling-water reactor. The core consists of a cylindrical graphite stack 7 m high and 11.8 m in diameter, penetrated vertically by 1,693 fuel channels and 170 control-rod channels. Each fuel channel is a zircaloy pressure tube (8.4 cm inner diameter) housing two fuel assemblies, one above the other, each containing 18 fuel rods.

Because the moderator (graphite) and coolant (light water) are physically separate, moderation is sustained even as the coolant boils. This separation from Western boiling water reactors (BWRs) is the fundamental reason for the RBMK's positive void coefficient.

The chief designer was Nikolai Dollezhal (NIKIET) and the scientific supervisor was Anatoly Aleksandrov (Kurchatov Institute). Initial fuel enrichment was 1.8%, later raised to 2.0%, 2.4%, and eventually 2.8%.

Generations

• Generation 1: Leningrad 1–2, Kursk 1–2, Chernobyl 1–2 (commissioned 1973–1979). Designed before OPB-82 safety regulations.
• Generation 2: Leningrad 3–4, Kursk 3–4, Chernobyl 3–4, Smolensk 1–2 (1980s). Conforming to OPB-82 standards.
• Generation 3: Smolensk 3 (commissioned 1990). The only RBMK built to post-Chernobyl OPB-88 standards.

Two uprated RBMK-1500 units (1,500 MWe each) were built at the Ignalina NPP in Lithuania.

Design Flaws and Chernobyl

INSAG-7 (1992) identified the following design characteristics as primary causes of the accident:

1. Positive void coefficient: As coolant boils and the void fraction increases, neutron absorption decreases, raising reactivity. At the time of the accident, Chernobyl Unit 4 had both a positive void coefficient and a positive power coefficient.
2. Positive scram effect: Each control rod carried a 4.5 m graphite displacer at its tip. As a rod descended, the displacer pushed water out of the lower region of the core, causing a local reactivity insertion before the boron carbide absorber section arrived. This effect was discovered at Ignalina NPP in 1983 but was not rectified before the accident.
3. Slow scram insertion: Total rod insertion time was 18 seconds, caused by hydraulic damping in the narrow control-rod channels.

Post-Accident Modifications

• 80–90 additional fixed absorbers installed in the core
• Operating reactivity margin (ORM) raised from 26–30 rods to 43–48
• Fuel enrichment increased from 2.0% to 2.4%
• Control rods redesigned to eliminate the positive scram effect
• Scram rod insertion time reduced to 12 seconds
• Fast-acting emergency protection (FAEP) system installed, inserting at least 2β of negative reactivity in under 2.5 seconds

Current Status

As of 2024, seven RBMK units remain operational in Russia at Leningrad, Kursk, and Smolensk NPPs. Chernobyl Unit 4 was destroyed in 1986; Units 1–3 were sequentially decommissioned. Both Ignalina units were shut down as a condition of Lithuania's EU accession. Kursk Unit 5 was cancelled at roughly 85% completion.