Power reactors Nuclear fission product

in nuclear power reactor, main sources of radioactivity fission products, actinides , activation products. fission products largest source of radioactivity first several hundred years, while actinides dominant 10 10 years after fuel use.

fission occurs in nuclear fuel, , fission products retained within fuel close produced. these fission products important operation of reactor because fission products contribute delayed neutrons useful reactor control while others neutron poisons tend inhibit nuclear reaction. buildup of fission product poisons key factor in determining maximum duration given fuel element can kept within reactor. decay of short-lived fission products provide source of heat within fuel continues after reactor has been shut down , fission reactions stopped. decay heat sets requirements cooling of reactor after shutdown.

if fuel cladding around fuel develops holes, fission products can leak primary coolant. depending on fission product chemistry, may settle within reactor core or travel through coolant system. coolant systems include chemistry control systems tend remove such fission products. in well-designed power reactor running under normal conditions, radioactivity of coolant low.

it known isotope responsible majority of gamma exposure in fuel reprocessing plants (and chernobyl site in 2005) cs-137. iodine-129 1 of major radioactive elements released reprocessing plants. in nuclear reactors both cs-137 , strontium-90 found in locations remote fuel. because these isotopes formed beta decay of noble gases (xenon-137 {halflife of 3.8 minutes} , krypton-90 {halflife 32 seconds}) enable these isotopes deposited in locations remote fuel (e.g. on control rods).

nuclear reactor poisons

some fission products decay release of neutron. since there may short delay in time between original fission event (which releases own prompt neutrons immediately) , release of these neutrons, latter termed delayed neutrons . these delayed neutrons important nuclear reactor control.

some of fission products, such xenon-135 , samarium-149, have high neutron absorption cross section. since nuclear reactor depends on balance in neutron production , absorption rates, fission products remove neutrons reaction tend shut reactor down or poison reactor. nuclear fuels , reactors designed address phenomenon through such features burnable poisons , control rods. build-up of xenon-135 during shutdown or low-power operation may poison reactor enough impede restart or interfere normal control of reaction during restart or restoration of full power, possibly causing or contributing accident scenario.