Chernobyl's RBMK reactor, however, used solid graphite as a neutron moderator to slow down the neutrons, and the cooling water acted as a neutron absorber. Thus neutrons are moderated by the graphite even if steam bubbles form in the water. Furthermore, because steam absorbs neutrons much less readily than water, increasing the voids means that more moderated neutrons are able to split uranium atoms, increasing the reactor's power output. This was a positive feedback regenerative process which makes the RBMK design very unstable at low power levels, and prone to sudden energy surges to a dangerous level. Not only was this behaviour counter-intuitive, this property of the reactor under certain extreme conditions was unknown to the crew.