Glossary

Emergency shutdown button (SCRAM). Inserts all control rods simultaneously. At Chernobyl, pressing AZ-5 initially caused a power increase due to the graphite displacer effect.

A neutron-absorbing rod (boron carbide) used to regulate the fission rate. RBMK-1000 had 211 control rods. Their slow insertion speed (18 seconds for full travel) was a contributing factor to the accident.

Neutrons emitted by fission product decay, seconds after fission. About 0.65% of all neutrons in uranium fission are delayed. This small fraction is what makes nuclear reactors controllable on human timescales.

Departure from Nucleate Boiling, the transition from efficient bubble-based cooling to a less effective film of steam on the fuel surface. Beyond DNB, fuel temperatures rise rapidly.

Temperature-dependent widening of neutron absorption resonances in U-238 fuel. As fuel heats up, more neutrons are captured, providing negative (stabilizing) feedback. The primary self-regulating mechanism in any reactor.

Emergency Core Cooling System, safety system designed to flood the core with water in case of a loss-of-coolant accident. Disconnected before the Chernobyl test to prevent interference.

A vertical pressure tube containing the fuel assembly, through which coolant flows. RBMK-1000 has 1,661 fuel channels arranged in a graphite matrix.

A graphite section at the bottom of each RBMK control rod. When rods are fully withdrawn and then inserted, the graphite tip enters the core before the absorber, displacing water and briefly adding positive reactivity.

A fission product that decays into Xenon-135 with a half-life of 6.57 hours. The buildup of I-135 after a power reduction leads to subsequent xenon poisoning as it decays.

Operating Reactivity Margin, the equivalent number of fully inserted control rods available for shutdown. Minimum safe value is 15 rods. At the time of the accident, ORM was only 6-8 rods.

A simplified model of reactor behavior that treats the entire core as a single point with average properties. Used for time-dependent calculations of neutron population and power.

The paradoxical effect where emergency rod insertion initially increases power. Caused by graphite displacers on RBMK control rods displacing water absorber before the boron absorber enters the core.

State where the fission chain reaction is sustained by prompt neutrons alone (reactivity > 1$). Power increases exponentially on a millisecond timescale, making the reactor uncontrollable.

Reaktor Bolshoy Moshchnosti Kanalnyy, High Power Channel-type Reactor. A Soviet-designed graphite-moderated, water-cooled reactor. Unique among power reactors for its positive void coefficient.

A measure of departure from criticality, often expressed in dollars ($). Positive reactivity means power is increasing; negative means decreasing. One dollar equals the delayed neutron fraction (β ≈ 0.0065).

Emergency shutdown of a nuclear reactor by rapidly inserting all control rods. The term may derive from "Safety Control Rod Axe Man" from the earliest reactor experiments.

Separator vessels above the reactor that separate steam from water. RBMK has two steam drums per reactor, connected to the fuel channels by riser pipes.

Total thermal energy produced by the reactor core, measured in megawatts thermal. RBMK-1000 nominal power is 3,200 MWt. At the moment of the explosion, power briefly reached an estimated 30,000+ MWt.

The change in reactivity caused by steam void (bubble) formation in the coolant. Positive in RBMK (+5$ at full void, dangerously destabilizing), negative in PWRs (inherently safe). The defining design flaw of the RBMK.

A fission product with one of the largest known neutron absorption cross-sections (2.65 million barns). Builds up after power reduction, causing "xenon poisoning" that makes restart difficult and forces excessive rod withdrawal.