[Catfish]

edit found a better description here: https://en.wikipedia.org/wiki/Chernobyl_disaster

as a summary, the reactor had bneen held in an in-betwen state for too long (hours, literally), which rendered several security measures impossible. Also some moderators (graphite cylinder bundles to subdue the reaction) had been retracted manually. Maybe this is not a problem but .. why did they do that.

Imho this describes best what lead to the catastrophe (excerpt from above link):

" [...] At 1:23:04 a.m., the test began. Four of the main circulating pumps (MCP) were active (of the eight total, six are normally active under regular operation). The steam to the turbines was shut off, beginning a run-down of the turbine generator. The diesel generators started and sequentially picked up loads; the generators were to have completely picked up the MCPs' power needs by 01:23:43. In the interim, the power for the MCPs was to be supplied by the turbine generator as it coasted down. As the momentum of the turbine generator decreased, so did the power it produced for the pumps. The water flow rate decreased, leading to increased formation of steam voids (bubbles) in the core.

Unlike western light-water reactors, the RBMK had a positive void coefficient of reactivity at low power levels, meaning that when water began to boil and produce voids in the coolant, the nuclear chain reaction increased instead of decreasing. Given this characteristic, the No. 4 RBMK reactor operation was now at risk of spiraling into a positive feedback loop, in which the formation of steam voids would reduce the ability of the liquid water coolant to absorb neutrons, increasing the reactor's power output, causing yet more water to flash into steam, and yielding a further power increase. Throughout most of the experiment the automatic control system successfully counteracted this positive feedback, inserting control rods into the reactor core to limit the power rise. However, this system had control of only 12 rods, as nearly all the others had been manually retracted.
At 1:23:40, as recorded by the SKALA centralized control system, a SCRAM (emergency shutdown) of the reactor was initiated. The SCRAM was started when the EPS-5 button (also known as the AZ-5 button) of the reactor emergency protection system was pressed: this engaged the drive mechanism on all control rods to fully insert them, including the manual control rods that had been withdrawn earlier. The reason why the EPS-5 button was pressed is not known, whether it was done as an emergency measure in response to rising temperatures, or simply as a routine method of shutting down the reactor upon completion of the experiment.

One view is that the SCRAM may have been ordered as a response to the unexpected rapid power increase, although there is no recorded data showing this. Some have suggested that the button was not manually pressed, that the SCRAM signal was automatically produced by the emergency protection system, but the SKALA registered a manual SCRAM signal. Despite this, the question as to when or even whether the EPS-5 button was pressed has been the subject of debate. There have been assertions that the manual SCRAM was initiated due to the initial rapid power acceleration. Others have suggested that the button was not pressed until the reactor began to self-destruct, while others believe that it happened earlier and under calm conditions.[48]:578[49]

When the EPS-5 button was pressed, the insertion of control rods into the reactor core began. The control rod insertion mechanism moved the rods at 0.4 metres per second (1.3 ft/s), so that the rods took 18 to 20 seconds to travel the full height of the core, about 7 metres (23 ft). A bigger problem was the design of the RBMK control rods, each of which had a graphite neutron moderator section attached to its end to boost reactor output by displacing water when the control rod section had been fully withdrawn from the reactor. That is, when a control rod was at maximum extraction, a neutron-moderating graphite extension was centered in the core with 1.25 metres (4.1 ft) columns of water above and below it. Consequently, injecting a control rod downward into the reactor in a SCRAM initially displaced (neutron-absorbing) water in the lower portion of the reactor with (neutron-moderating) graphite. Thus, an emergency SCRAM initially increased the reaction rate in the lower part of the core as the graphite extensions of rods moving down in the reactor displaced water coolant. This behaviour was discovered when the initial insertion of control rods in another RBMK reactor at Ignalina Nuclear Power Plant in 1983 induced a power spike, but as the subsequent SCRAM of that reactor was successful, the subsequently disseminated information had been deemed of little importance. [...]"

We should not forget that those reactors were initially also built for high performance, and to produce plutonium for nuclear weapons.


What i do not undrestand.. in one of the videos posted (from the HBO film) the graphite moderators are popping up and down.. as if there was no electrical control for them? Where are the electric engines to retract or lower the moderators located? I take it there has to be at least one for every bundle of 12 rods?