Nuclear reactor accident (8)
I believe the answer is:
meltdown
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">nuclear reactor accident</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for meltdown that I've seen before include "Disastrous overheating of core of nuclear reactor" , "Sudden disastrous failure (of nuclear reactor)" , "Total disaster" , "Nuclear reactor accident" , "Disastrous event like overheating of nuclear reactor as in Chernobyl" .)