Unfortunately, you can blame engineers here. While it most likely wasn't the immediate cause of the explosion, the way the reactor was built was highly unsafe. Check out the reactor's design flaws:
-The reactor had a dangerously large positive void coefficient. The void coefficient is a measurement of how the reactor responds to increased steam formation in the water coolant. Most other reactor designs produce less energy as they get hotter, because if the coolant contains steam bubbles, fewer neutrons are slowed down. Faster neutrons are less likely to split uranium atoms, so the reactor produces less power. Chernobyl's RBMK reactor, however, used solid graphite as a neutron moderator to slow down the neutrons, and neutron-absorbing light water to cool the core. Thus neutrons are slowed down even if steam bubbles form in the water. Furthermore, because steam absorbs neutrons much less readily than water, increasing an RBMK reactor's temperature 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 prone to suddenly increasing energy production to dangerous level if the temperature rises. This was counter-intuitive and unknown to the crew.
-A more 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 control rod end tips were made of graphite and the extenders (the end areas of the control rods above the end tips, measuring 1-metre (3 ft) in length) were hollow and filled with water, while the rest of the rod â" the truly functional part which absorbs the neutrons and thereby halts the reaction â" was made of boron carbide. With this design, when the rods are initially inserted into the reactor, the graphite ends displace some coolant. This greatly increases the rate of the fission reaction, since graphite is a more potent neutron moderator (a material that enables a nuclear reaction) and also absorbs far fewer neutrons than the boiling light water. Thus for the first few seconds of control rod activation, reactor power output is increased, rather than reduced as desired. This behavior is counter-intuitive and was not known to the reactor operators.
-The water channels run through the core vertically, meaning that the water's temperature increases as it moves up and thus creates a temperature gradient in the core. This effect is exacerbated if the top portion turns completely to steam, since the topmost part of the core is no longer being properly cooled and reactivity greatly increases. (By contrast, the CANDU reactor's water channels run through the core horizontally, with water flowing in opposite directions among adjacent channels. Hence, the core has a much more even temperature distribution.)
-To reduce costs, and because of its large size, the reactor had been constructed without any secure containment. This allowed the radioactive contaminants to freely escape into the atmosphere after the steam explosion burst the primary pressure vessel.
-The reactor also had been running for over one year, and was storing fission byproducts; these byproducts pushed the reactor towards disaster.
As the reactor heated up, design flaws caused the reactor vessel to warp and break up, making further insertion of control rods impossible as the heat deformed them.