How an RBMK-1000 works
The RBMK is a big block of graphite with about 1,600 vertical channels. Water is pumped up through the fuel channels and part of it boils, and the steam goes straight to the turbines. The graphite slows the neutrons down so the uranium keeps splitting. Control rods full of boron carbide soak up neutrons to hold power steady.
The positive void coefficient
Water in an RBMK absorbs more neutrons than it slows. When it turns to steam, fewer are absorbed, so power goes up, which makes more steam. At full power the fuel's own heating holds this in check. At low power with low flow it doesn't, and the reactor becomes unstable. In the game you will feel this when the pumps slow.
Xenon poisoning
Fission makes iodine 135, which decays over hours into xenon 135, the strongest neutron absorber there is. Running at power burns xenon off. Cut the power and xenon keeps building for hours, so you have to pull rods out to hold on. That's the xenon pit.
Pumps, drums, feedwater and the turbine
Eight main circulation pumps push water up through the core, and normally six run with two on standby. The steam and water mix rises to four big steam drums, where the steam goes off to two turbines and feedwater tops the drums back up. If the reactor makes more steam than the turbine takes, pressure rises, and higher pressure means less boiling and less power. Cut the turbine load too far and pressure climbs to the safety valves. Let the feedwater fall behind the steam and the drum level drops until the pumps start drawing steam. Feed too much and the drums flood and water is carried into the turbine. Warm feedwater matters too: water arriving close to boiling turns to steam sooner, which in an RBMK adds power.
Control rods, graphite tips and AZ-5
Below the absorber, each RBMK rod carried a graphite displacer. When a fully withdrawn rod started to go in, the displacer pushed water out of the bottom of its channel before any absorber arrived, which added reactivity for the first few seconds. With plenty of rods partly in, this barely mattered. With almost all of them pulled out, pressing AZ-5 could push power up instead of down. The rules required at least 15 rods' worth in the core for this reason.
What happened on 26 April 1986
Unit 4 at Chernobyl was due to test whether the turbine, coasting down after the steam was cut, could power the pumps until the diesel generators started. The test was delayed by a day and run at night by a shift that hadn't prepared for it. Power dropped far too low, the core filled with xenon, and the operators pulled out nearly every rod to get it back up. At 01:23:40 AZ-5 was pressed. Within seconds power surged, the fuel shattered, and two explosions destroyed the reactor and its building.
Two plant workers died that night. Twenty eight firefighters and staff died of acute radiation sickness in the weeks after. More than 100,000 people were evacuated, and many thousands more cleared the site and built the shelter. I've tried to write this with respect for all of them. The best accounts I know are the INSAG-7 report and the books by the people who were there.
I'm Arthur. I'm not a nuclear engineer. This is a simplified game: one number for the whole core, made up constants that move the right way, and time sped up so a xenon pit fits in a few minutes. The drums and turbine are a few lines of arithmetic, not a plant model. It is not a training tool and not a model of any real plant. If you know better, email me. For another machine from history, try my Enigma machine simulator or the Lorenz cipher game.
Questions
- What is an RBMK-1000?
- A Soviet graphite moderated, water cooled reactor with 3,200 MW of thermal power.
- What is the positive void coefficient?
- More steam means fewer neutrons absorbed, so power rises.
- Why did AZ-5 make things worse?
- The graphite tips on fully withdrawn rods added reactivity for the first few seconds.
- What is a xenon pit?
- Xenon building up for hours after power is cut, pulling reactivity down.
- Is it accurate?
- It's honest in direction but simplified. It's a game, not a training tool.