Unlike other light-water reactor designs, the RBMK design at that time had a positive void coefficient of reactivity at low power levels. This meant that the formation of steam bubbles (voids) from boiling cooling water intensified the nuclear chain reaction owing to voids having lower neutron absorption than water. The consequent power increase then produced more voids which further intensified the chain reaction, and so on. Given this characteristic, reactor No. 4 was now at risk of a runaway increase in its core power with nothing to restrain it.