During tests on low power, with low coolant flux through the core, the supervisor noticed several control rods being pulled from the core, and found an operator in the basement opening pneumatic valves. Wrongly opened valves were immediately closed, but some of the control rods did not reenter the core and stuck in almost withdrawn positions, but still low enough for their status lights to indicate them as lowered. Due to a miscommunication between the supervisor and the control room operator, wrong buttons were pressed when the supervisor asked for lowering the control rods into the core. Instead of sealing the withdrawn control rods to the pneumatic system, the safeguard bank of four control rods was withdrawn from the core. The operator noticed that the power level was exponentially increasing, doubling each 2 seconds, and tripped the reactor. Three of the safeguard control rods however were not inserted into the core and the fourth took an abnormally long time, about 90 seconds, to slide back, while the power kept rising. After just 10 seconds 17 MW were reached. The cooling water boiled in the tubes connected to the temporary cooling system, and some of them ruptured; the positive void coefficient of the reactor led to yet higher power increase rate. About 14 seconds later valves were opened to drain the heavy water from the calandria. As this took some time, power increased for 5 more seconds, peaked at 80 MW, then went down as the moderator level decreased and was at zero 25 seconds later. Meanwhile, some fuel elements melted and the calandria was pierced at several places; helium leaked and air was aspirated inside. Hydrogen and other gases evolved by high-temperature reaction of metals with cooling water, and 3–4 minutes later oxyhydrogen exploded in the calandria. During the incident, some gaseous fission products were vented to the atmosphere, and heavy water in the calandria was contaminated with the cooling water and the fission products.