You may be thinking about He-3. It is extracted during maintenance from tritium in nuclear bombs (tritium is a neutron source but He-3 a most efficient neutron

You may be thinking about He-3. It is extracted during maintenance from tritium in nuclear bombs (tritium is a neutron source but He-3 a most efficient neutron absorber). I think now there is also a spallation plant at Oak Ridge. Yearly production is a few kilograms. Interesting stuff.