Nuclear fission product

Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release of heat energy, and gamma rays. The two smaller nuclei are the fission products..
About 0.2% to 0.4% of fissions are ternary fissions, producing a third light nucleus such as helium-4 or tritium.
The fission products themselves are usually unstable and therefore radioactive. Due to being relatively neutron-rich for their atomic number, many of them quickly undergo beta decay. This releases additional energy in the form of beta particles, antineutrinos, and gamma rays. Thus, fission events normally result in beta and additional gamma radiation that begins immediately after, even though this radiation is not produced directly by the fission event itself.
The produced radionuclides have varying half-lives, and therefore vary in radioactivity. For instance, strontium-89 and strontium-90 are produced in similar quantities in fission, and each nucleus decays by beta emission. But ⁹⁰Sr has a 30-year half-life, and ⁸⁹Sr a 50.5-day half-life. Thus in the 50.5 days it takes half the ⁸⁹Sr atoms to decay, emitting the same number of beta particles as there were decays, less than 0.4% of the ⁹⁰Sr atoms have decayed, emitting only 0.4% of the betas. The radioactive emission rate is highest for the shortest lived radionuclides, although they also decay the fastest. Additionally, less stable fission products are less likely to decay to stable nuclides, instead decaying to other radionuclides, which undergo further decay and radiation emission, adding to the radiation output. It is these short lived fission products that are the immediate hazard of spent fuel, and the energy output of the radiation also generates significant heat which must be considered when storing spent fuel. As there are hundreds of different radionuclides created, the initial radioactivity level fades quickly as short lived radionuclides decay, but never ceases completely as longer lived radionuclides make up more and more of the remaining unstable atoms. In fact the short lived products are so predominant that 87 percent decay to stable isotopes within the first month after removal from the reactor core.

Formation and decay

The sum of the atomic mass of the two atoms produced by the fission of one fissile atom is always less than the atomic mass of the original atom. This is because some of the mass is lost as free neutrons, and once kinetic energy of the fission products has been removed, then the mass associated with this energy is lost to the system also, and thus appears to be "missing" from the cooled fission products.
Since the nuclei that can readily undergo fission are particularly neutron-rich, the initial fission products are often more neutron-rich than stable nuclei of the same mass as the fission product. The initial fission products therefore may be unstable and typically undergo beta decay to move towards a stable configuration, converting a neutron to a proton with each beta emission.
A few neutron-rich and short-lived initial fission products decay by ordinary beta decay, followed by immediate emission of a neutron by the excited daughter-product. This process is the source of so-called delayed neutrons, which play an important role in control of a nuclear reactor.
The first beta decays are rapid and may release high energy beta particles or gamma radiation. However, as the fission products approach stable nuclear conditions, the last one or two decays may have a long half-life and release less energy.

Radioactivity over time

Fission products have half-lives of 90 years or less, except for seven long-lived fission products that have half-lives of 211,100 years or more. Therefore, the total radioactivity of a mixture of pure fission products decreases rapidly for the first several hundred years before stabilizing at a low level that changes little for hundreds of thousands of years.
This behavior of pure fission products with actinides removed, contrasts with the decay of fuel that still contains actinides. This fuel is produced in the so-called "open" nuclear fuel cycle. A number of these actinides have half-lives in the missing range of about 100 to 200,000 years, causing some difficulty with storage plans in this time-range for open-cycle non-reprocessed fuels.
Proponents of nuclear fuel cycles which aim to consume all their actinides by fission, such as the Integral Fast Reactor and molten salt reactor, use this fact to claim that within 200 years, their fuel wastes are no more radioactive than the original uranium ore.
Fission products primarily emit beta radiation, while actinides primarily emit alpha radiation. Many of each also emit gamma radiation.

Yield

Each fission of a parent atom produces a different set of fission product atoms. However, while an individual fission is not predictable, the fission products are statistically predictable. The amount of any particular isotope produced per fission is called its yield, typically expressed as percent per parent fission; therefore, yields total to 200%, not 100%.
While fission products include every element from zinc through the lanthanides, the majority of the fission products occur in two peaks. One peak occurs at about strontium to ruthenium while the other peak is at about tellurium to neodymium. The yield is somewhat dependent on the parent atom and also on the energy of the initiating neutron.
In general the higher the energy of the state that undergoes nuclear fission, the more likely that the two fission products have similar mass. Hence, as the neutron energy increases and/or the energy of the fissile atom increases, the valley between the two peaks becomes more shallow.
For instance, the curve of yield against mass for ²³⁹Pu has a more shallow valley than that observed for ²³⁵U when the neutrons are thermal neutrons. The curves for the fission of the later actinides tend to make even more shallow valleys. In extreme cases such as ²⁵⁹Fm, only one peak is seen; this is a consequence of symmetric fission becoming dominant due to shell effects.
The adjacent figure shows a typical fission product distribution from the fission of uranium. Note that in the calculations used to make this graph, the activation of fission products was ignored and the fission was assumed to occur in a single moment rather than a length of time. In this bar chart results are shown for different cooling times.
Because of the stability of nuclei with even numbers of protons and/or neutrons, the curve of yield against element is not a smooth curve but tends to alternate. Note that the curve against mass number is smooth.

Production

Small amounts of fission products are naturally formed as the result of either spontaneous fission of natural uranium, which occurs at a low rate, or as a result of neutrons from radioactive decay or reactions with cosmic ray particles. The microscopic tracks left by these fission products in some natural minerals are used in fission track dating to provide the cooling ages of natural rocks. The technique has an effective dating range of 0.1 Ma to >1.0 Ga depending on the mineral used and the concentration of uranium in that mineral.
About 1.5 billion years ago in a uranium ore body in Africa, a natural nuclear fission reactor operated for a few hundred thousand years and produced approximately 5 tonnes of fission products. These fission products were important in providing proof that the natural reactor had occurred.
Fission products are produced in nuclear weapon explosions, with the amount depending on the type of weapon.
The largest source of fission products is from nuclear reactors. In current nuclear power reactors, about 3% of the uranium in the fuel is converted into fission products as a by-product of energy generation. Most of these fission products remain in the fuel unless there is fuel element failure or a nuclear accident, or the fuel is reprocessed.

Power reactors

Commercial nuclear fission reactors are operated in the otherwise self-extinguishing prompt subcritical state. Certain fission products decay over seconds to minutes, producing additional delayed neutrons crucial to sustaining criticality. An example is bromine-87 with a half-life of about a minute. Operating in this delayed critical state, power changes slowly enough to permit human and automatic control. Analogous to fire dampers varying the movement of wood embers towards new fuel, control rods are moved as the nuclear fuel burns up over time.
In a nuclear power reactor, the main sources of radioactivity are fission products along with actinides and activation products. Fission products are most of the radioactivity for the first several hundred years, while actinides dominate roughly 10³ to 10⁵ years after fuel use.
Most fission products are retained near their points of production. They are important to reactor operation not only because some contribute delayed neutrons useful for reactor control, but some are neutron poisons that inhibit the nuclear reaction. Buildup of neutron poisons is a key to how long a given fuel element can be kept in the reactor. Fission product decay also generates heat that continues even after the reactor has been shut down and fission stopped. This decay heat requires removal after shutdown; loss of this cooling damaged the reactors at Three Mile Island and Fukushima.
If the fuel cladding around the fuel develops holes, fission products can leak into the primary coolant. Depending on the chemistry, they may settle within the reactor core or travel through the coolant system and chemistry control systems are provided to remove them. In a well-designed power reactor running under normal conditions, coolant radioactivity is very low.
The isotope responsible for most of the gamma exposure in fuel reprocessing plants is caesium-137. Iodine-129 is a major radioactive isotope released from reprocessing plants. In nuclear reactors both caesium-137 and strontium-90 are found in locations away from the fuel because they're formed by the beta decay of noble gases which enable them to be deposited away from the fuel, e.g. on control rods.