It's between two isotopes of hydrogen, two kinds of hydrogen: deuterium, which is heavy hydrogen, which you can get from seawater, and tritium which is super-heavy hydrogen.
die zwischen zwei Wasserstoff-Isotopen, zweier Arten des Wasserstoffs, Deuterium, also schwerer Wasserstoff, dem man aus Meerwasser gewinnen kann, und Tritium, also superschwerer Wasserstoff.