Why can nuclear fission occur spontaneously while fusion needs a ton of energy?

Question

Also, what makes the products form nuclear fission radioactive and how does one know???

Answer 1

Nuclear fission occurs spontaneously because the fissile material is always undergoing fission, once in awhile, with the emission of neutrons. Capture of any neutrons by other fissile atoms results in more fission events. If you put enough of it together in one place, called the critical mass, a random fission will release neutrons such that they can't miss hitting other atoms, thus setting off an explosive chain reaction among all the atoms in the mass.

Products of fission follow a (bell?) curved distribution in weight. Most of the atoms fall apart 50-50, a lot 49-51, tailing off to some very light products and very heavy ones. Also, a lot blow into half a dozen pieces. Each daughter (product) atom is born with a certain distribution of neutrons and protons. If the relative number of neutrons and protons is different from a stable isotope, then it will be a radioactive one. So some fission products can be stable isotopes. Fissile isotopes of U and Pu have larger ratios of neutrons to protons than lighter nuclei. So the probability is that lighter daughter nuclei will be born with "too many" neutrons.

By contrast, it takes a lot of energy to force positively charged, mutually repelling protons, deuterons, and/or tritons together so that they get close enough together suck one another in to form helium.

Answer 2

Fission is an exothermic process where the goal is a more stable state for the atoms involved. Fusion requires the particles to be moving fast enough to fuse; that requires energy.

The products from nuclear fission are not definite - there are probable products (fission fragments) that usually occur. There is a handy chart called The Chart of the Nuclides - it will show most probable fission products and whether or not those products are, themselves, radioactive. The fission fragments may consist of isotopes of elements that are naturally radioactive and that will decay by various modes (alpha decay, beta decay, electron capture, gamma decay etc.). It is the radiation from these fragments that produce the "radioactivity" plus any neutrons that may be a product of the fission.

Answer 3

Fusion does NOT require a lot of energy, only high temperature and pressure to get H atoms close enough together to fuse into He atoms. Happens on the sun all day long with NO input of energy.

Answer 4

The short answer is binding energy. when two atoms join some energy is given out making the molecule more stable.

Answer 5

maybe not