According to the INSAG-7 Report, the main reason for the accident lies in the design flaws of the reactor:
• The RBMK reactor had a large positive void coefficient. The void coefficient is a measurement of how a reactor responds to increased steam formation in the water coolant. The other reactor designs of the west have a negative void coefficient, i.e. the nuclear reaction rate slows when steam bubbles form reducing the coolant as a whole. This results in faster neutrons which are less likely to split uranium atoms, so the reactor produces less power (a negative feed-back). Chernobyl’s RBMK reactor, used solid graphite as a neutron moderator to slow down the neutrons, and the water in it, on the contrary, acts like a harmful neutron absorber. Thus, neutrons are slowed down even before steam bubbles form in the water. Furthermore, because steam absorbs neutrons less readily than water, increasing the intensity of vaporization which means that more neutrons are able to split uranium atoms, increasing the reactor’s power output. This makes the RBMK design very unstable at low power levels and low coolant levels, and is prone to sudden increase in energy production to a dangerous level.
• Another significant flaw was in the design of the control rods that are inserted into the reactor to slow down the reaction. In the RBMK reactor design, the lower part of each control rod was made of graphite and was 1.3 meters shorter than necessary, and in the space beneath the rods were hollow channels filled with water. The upper part of the rod, the truly functional part that absorbs the neutrons and thereby halts the reaction, was made of boron carbide. With this design, when the rods are inserted into the reactor from the uppermost position, the graphite parts initially displace some water (which absorbs neutrons, as mentioned above), effectively causing fewer neutrons to be absorbed initially. Therefore for the first few seconds of control rod activation, reactor power output is increased, rather than reduced as desired.
• These behaviours were counter-intuitive and were not known to the reactor operators as the managaement had concealed these facts from the operators.