How do uranium and plutonium provide heat

The fuel assemblies are stored onsite in fresh fuel storage bins until the reactor operators need them. At this stage, the uranium is only mildly radioactive, and essentially all radiation is contained within the metal tubes. Typically, reactor operators change out about one-third of the reactor core 40 to 90 fuel assemblies every 12 to 24 months.

The reactor core is a cylindrical arrangement of the fuel bundles that is about 12 feet in diameter and 14 feet tall and encased in a steel pressure vessel with walls that are several inches thick. The reactor core has essentially no moving parts except for a small number of control rods that are inserted to regulate the nuclear fission reaction. Placing the fuel assemblies next to each other and adding water initiates the nuclear reaction.

After use in the reactor, fuel assemblies become highly radioactive and must be removed and stored under water at the reactor site in a spent fuel pool for several years. Even though the fission reaction has stopped, the spent fuel continues to give off heat from the decay of the radioactive elements that were created when the uranium atoms were split apart.

The water in the pool serves to both cool the fuel and block the release of radiation. From through December 31, , a total of , fuel assemblies had been discharged and stored at the sites of closed and operating commercial nuclear reactors in the United States. Within a few years, the spent fuel cools in the pool and may be moved to a dry cask storage container at the power plant site.

Many reactor operators store their older spent fuel in these special outdoor concrete or steel containers with air cooling. Learn more about spent fuel storage. The final step in the nuclear fuel cycle is the collection of spent fuel assemblies from the interim storage sites for final disposition in a permanent underground repository.

The United States currently has no permanent underground repository for high-level nuclear waste. Nuclear explained The nuclear fuel cycle. What is energy? Units and calculators. Use of energy. Energy and the environment.

Also in What is energy? Forms of energy Sources of energy Laws of energy. Also in Units and calculators explained Units and calculators Energy conversion calculators British thermal units Btu Degree days. Also in U. It is the fission fragments from a nuclear chain reaction and not fissionable itself.

Nuclear power The main nuclear fuels are uranium and plutonium. This means they use normal water as both a coolant and neutron moderator. These reactors pump water into the reactor core under high pressure to prevent the water from boiling. The water in the core is heated by nuclear fission and then pumped into tubes inside a heat exchanger.

Those tubes heat a separate water source to create steam. The steam then turns an electric generator to produce electricity. BWRs heat water and produce steam directly inside the reactor vessel.

Water is pumped up through the reactor core and heated by fission. Pipes then feed the steam directly to a turbine to produce electricity. See below. This nucleus is relatively unstable, and it is likely to break into two fragments of around half the mass.

These fragments are nuclei found around the middle of the Periodic Table and the probabilistic nature of the break-up leads to several hundred possible combinations. Creation of the fission fragments is followed almost instantaneously by emission of a number of neutrons typically 2 or 3, average 2.

Alpha particles from the decay cause a release of neutrons from the beryllium as it turns to carbon However, in solid fuel they can only travel a microscopic distance, so their energy becomes converted into heat. The balance of the energy comes from gamma rays emitted during or immediately following the fission process and from the kinetic energy of the neutrons.

Some of the latter are immediate so-called prompt neutrons , but a small proportion 0. The longest delayed neutron group has a half-life of about 56 seconds. The delayed neutron release is the crucial factor enabling a chain reacting system or reactor to be controllable and to be able to be held precisely critical. At criticality the chain reacting system is exactly in balance, such that the number of neutrons produced in fissions remains constant.

This number of neutrons may be completely accounted for by the sum of those causing further fissions, those otherwise absorbed, and those leaking out of the system. Under these circumstances the power generated by the system remains constant. To raise or lower the power, the balance must be changed using the control system so that the number of neutrons present and hence the rate of power generation is either reduced or increased. The control system is used to restore the balance when the desired new power level is attained.

The number of neutrons and the specific fission products from any fission event are governed by statistical probability, in that the precise break up of a single nucleus cannot be predicted. However, conservation laws require the total number of nucleons and the total energy to be conserved. The fission reaction in U produces fission products such as Ba, Kr, Sr, Cs, I and Xe with atomic masses distributed around 95 and Examples may be given of typical reaction products, such as:.

Both the barium and krypton isotopes subsequently decay and form more stable isotopes of neodymium and yttrium, with the emission of several electrons from the nucleus beta decays. It is the beta decays, with some associated gamma rays, which make the fission products highly radioactive. This radioactivity by definition! This contrasts with 4 eV or 6. This must be allowed for when the reactor is shut down, since heat generation continues after fission stops.

It is this decay which makes used fuel initially generate heat and hence need cooling, as very publicly demonstrated in the Fukushima accident when cooling was lost an hour after shutdown and the fuel was still producing about 1.

Neutrons may be captured by non-fissile nuclei, and some energy is produced by this mechanism in the form of gamma rays as the compound nucleus de-excites.

The resultant new nucleus may become more stable by emitting alpha or beta particles. Neutron capture by one of the uranium isotopes will form what are called transuranic elements, actinides beyond uranium in the periodic table. Since U is the major proportion of the fuel element material in a thermal reactor, capture of neutrons by U and the creation of U is an important process. As already noted, Pu is fissile in the same way as U, i. It is the other main source of energy in any nuclear reactor.

If fuel is left in the reactor for a typical three years, about two-thirds of the Pu is fissioned with the U, and it typically contributes about one-third of the energy output.

The masses of its fission products are distributed around and atomic mass units. One difference is that Pu fission in a thermal reactor results in 2. In a fast reactor, Pu produces more neutrons per fission e.

The main transuranic constituents of used fuel are isotopes of plutonium, curium, neptunium and americium, the last three being 'minor actinides'. These are alpha-emitters and have long half-lives, decaying on a similar time scale to the uranium isotopes.