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I may have to stand corrected Chris, I am now thinking it's to do with the flow of electrodes through the process of nuclear fission, which now certainly sounds far too tricky for a crusader clue, or any other clue really

There was a 19th century controversy over "Neptunian" (water deposited) and "Plutonian" (solidified) origins of rocks - both explanations are of course correct, in the right context.

Many thanks for all your learned replies!! Perhaps the Crusader compiler is a retired physics expert!
Next to neptunium in my dictionary is Neptunian, which means produced by water action. This would be a better fit to the 'flower' reference in the clue.

Anyway, I'll go for neptunium as that's all it can be. Many thanks again for your help.