Reply #8: The mass of a mole of neutrons, roughly 1 gram, is the energy equivalent [View All]

of 90 trillion joules or the equivalent - in familiar terms - of about 700,000 gallons of gasoline.

In nuclear terms, the mass of a neutron is equivalent to about 940 MeV. A typical fission event is about 200 MeV per fission (involving much larger masses since a uranium-235 nucleus is roughly 235 as massive as a neutron), of which about 190 MeV may be considered recoverable as heat that is convertible to exergy, with 10 MeV being neutrinos and high energy gamma rays that are not recoverable. (One could argue about gamma rays, but not neutrinos). So yes, a fission consumes less than the mass of a neutron as mass.

Free neutrons have a short half life, and unless they are captured in a nucleus, will quickly decay to give protons. As a practical matter, neutrons are seldom around long enough to decay. A "fast" neutron at 1 MeV is traveling about 13 km/sec, but they go only a very short distance before colliding with a nucleus. The mean free path of a neutron in matter is a function the composition of the matter and, oddly enough, the speed of the neutron.

In the table of nuclides, this sort of thing is described as a "cross section" and there are many different types of "cross section" (whimsically measured the unit "barns" which is a function of the apparent cross sectional area of a nucleus's silouette.) Types of cross section are scattering, capture, fission, and specific cross sections for esoteric types of nuclear reactions.