The core fire in Chernobyl was believed to have been extinguished by helicopters dropping more than 5000 MT of sand , lead , clay , and neutron-absorbing boron onto it .
It was thought by some that the core fire was extinguished by a combined effort of helicopters dropping over 5000 MT of sand , lead , clay , and neutron-absorbing boron onto the burning reactor .
Chernobyl disaster