D-T is the easiest, D-D the second easiest and produces He3. So there&#x27;s really no reason to mine He3 unless you happen to be orbiting a gas giant with lots of it handy.<p>Helion, a fusion startup funded by YCombinator, is working on a D-D&#x2F;D-He3 reactor, breeding their He3. They say only 6% of the reactor&#x27;s energy output would be neutron radiation.