I thank B. Alan Guthrie for this additional information.
A fission gives off roughly 200 MeV. The fission fragments carry off 82% of the energy in the form of kinetic energy. Prompt neutrons carry off another 2.5%, prompt gammas carry off 3.5%, beta decay accounts for 4%, delayed gammas (from radioactive decay) account for 3%, and neutrinos carry off the remaining 5%. The neutrinos and their energy are lost, since the probability of interaction with neutrinos is so small.
Some fissions also occur when a fast neutron (not yet slowed down by the moderator) strikes a U-238 atom. Also, as the fuel is burned, plutonium is produced, and by the end of a fuel cycle (18 months of operation), 35% of the energy is actually coming from the fission of Pu-239 atoms.
The power level is controlled by control rods, temperature, and chemical shim. By chemical shim, we mean that boric acid is dissolved in the coolant/moderator. Boron-10 has a high neutron aborption cross-section and can be used to poison the chain reaction. If it is desired to increase power, then the boric acid concentration can be diluted, removing B-10 from the reactor core and decreasing its poisoning effect. Chemical shim is used in PWRs and, to a small extent, in CANDUs. Boric acid is not used in BWRs except if all other shutdown methods fail when the reactor must be shutdown.
The permitted power level is a determined by many safety considerations, including that of avoiding letting fuel rods get too hot.
One last nit-pick - about 80% of the neutron absorptions in U-235 result in fission. The other 20% are just (n,gamma) reactions, resulting in just another gamma flying about.
Still another last nit-pick - some of the neutrons will be lost to leakage out of the reactor.
Only about a third of the core is replaced upon re-fueling. A typical US fuel cycle is about 18 months, although utilities are expressing interest in 24-month fuel cycles (I believe that some BWRs are already on 24-month cycles).
On the other hand, in Japan, regulatory concerns require that the plants perform equipment inspection at intervals not to exceed 13 months, so most Japanese fuel cycles last 13-months.
The enrichment required for new fuel is dictated by the length of the fuel cycle (and how long the particular fuel assembly will be burned).
ADDRESS>John McCarthy jmc@cs.stanford.edu