Chaps repeatedly separating atomic isotopes of hydrogen for a change (9)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

MEN + MEN (chaps repeatedly), inserted into (separating) A (abbreviation for atomic) + D + T (used as chemical symbols for deuterium and tritium, two isotopes of hydrogen, although the recommended symbols are now 2 H and 3 H).