Chernobyl Disaster

Explosion and destruction of the reactor

To gain a better understanding of how the explosion occurred, we shall study the principles of the reactor’s operation. The RBMK (abbreviation for the Chernobyl nuclear reactor) had two independent cooling circuits. So, under the pressure, water was supplied through the pipelines by the circulation pumps into the lower part of the reactor and then extruded into the upper part of the reactor, washing on the way the assemblies with fuel rods.

In the fuel rods under the influence of neutrons, there goes a nuclear chain reaction triggering a large release of energy, due to what the water is heated and boils up. The mixture of steam and water via the pipelines entered the separators, where water was separated from the steam. The inlet steam was supplied to the turbine and then turned into the water again, coming to the separator where it was mixed with hot water provided by the reactor, so the water temperature decreased. This water through the pipeline re-entered the pumps and completed the cycle.

The reactor output power, namely, the number of free neutrons released during a nuclear reaction, was managed by the control rods acting as a neutron absorber. Extension of rods contributed to the increase in output power, and their deepening into the active zone of the reactor core promoted the personnel.

The problem was that the RBMK had a number of design weaknesses that led to the fact that at one point it came out of control. Two of these defects were directly related to the causes of the accident. These were a positive feedback loop between power and reactivity arising in certain modes of operation of the reactor, and the presence of the so-called “end effect” – a phenomenon meaning the short-term increase in reactivity of the nuclear reactor (instead of the expected decrease) observed during lowering the control rods from the upper end position. These defects were not properly reflected in the operational documentation, so that mostly contributed to the wrong actions of the

At 1:23:04 a.m. on April 26th, 1986, the experiment began and the rundown of the turbine generator was started. When the turbine generator stopped, the water flow rate lowered, which led to a rapid increase in the amount of steam voids within the reactor core. Under such conditions, the ability of the liquid water coolant to absorb the neutrons had decreased, which in turn, caused the rise in the reactor's power. That was certainly the positive feedback loop mentioned earlier as the increase in power caused those processes in the core that led to an even higher increase in power. Though, it should be noted that the automatic control system successfully stood against this positive feedback, continuously inserting control rods into the reactor core to decrease the power rise.

At 1:23:40, as recorded by the centralized control system, the reactor was shut down by pressing EPS-5 button. After that, the insertion of control rods started, but there appeared the “end effect”

A few seconds after the start of the SCRAM, a massive power spike occurred, the core overheated, and seconds later, turned into the initial explosion. Some of the fuel rods were broken, which stopped the further insertion of the control rods, and within three seconds the reactor output rose

Presumably, the reactor jumped to around 30 GW thermal, ten times the normal operational output. That provoked a steam explosion, which led to the destruction of the upper plate of the reactor. A few second after the first one, the second explosion occurred, emerging from the core itself. The graphite fire hadn’t been totally extinguished and it still smolders even under the sarcophagus.