CLNE pours DEUT into a single pixel of PIPE which drops it into some PROT. The PROT is set to sit still and not disappear, so I don't have to constantly generate more and lose heat.
I have an RTG which is about 5×7, but you could just put URAN+EXOT in a TTAN casing with some TSNS and use that (with ambient heat so it does not melt)