Nuclear fission produces two smaller atoms (called fission fragments), two or three neutrons, and a burst of energy totaling about 200 MeV per split atom. These are the immediate products, but the story doesn’t end there. The fission fragments are unstable and continue decaying for days, years, or even decades, generating a cascade of secondary products including radiation, heat, and new isotopes.
The Two Fission Fragments
When a uranium-235 nucleus absorbs a neutron and splits, it doesn’t break into two equal halves. Instead, it produces one fragment with a mass around 90 to 100 (lighter group) and another around 130 to 145 (heavier group). This asymmetric split is consistent across millions of fission events, producing a characteristic double-humped distribution of fragment masses.
The most significant fragments, from a practical standpoint, include cesium-137 and strontium-90 in the lighter and heavier groups. Cesium-137 has a cumulative fission yield of about 6.3% from uranium-235, meaning roughly 6 out of every 100 fission events produce it. Strontium-90 has a yield of about 5.7%. These two isotopes matter most for long-term radioactive waste because both have half-lives around 30 years, long enough to remain hazardous for centuries but short enough to produce intense radiation while they last.
Xenon-131, a noble gas, appears with a yield of about 2.9%. Other common fragments include barium, zirconium, molybdenum, and iodine isotopes. In total, fission of uranium-235 can produce over 80 different elements and hundreds of individual isotopes.
Free Neutrons
Each fission event releases an average of 2.44 neutrons. These neutrons are what make a sustained chain reaction possible: one neutron triggers a fission, which releases two or three more neutrons, each capable of triggering another fission. In a nuclear reactor, this process is carefully controlled so that exactly one neutron per fission goes on to cause another split. In a nuclear weapon, the goal is the opposite: let the chain reaction multiply as fast as possible.
The neutrons emerge at high speed, carrying about 8 MeV of kinetic energy collectively. Reactors use a moderator (usually water or graphite) to slow these fast neutrons down, because slow “thermal” neutrons are far more likely to be captured by uranium-235 and trigger another fission.
Energy and Radiation
The total energy released per fission event is roughly 200 MeV. Most of it, about 167 MeV, appears as the kinetic energy of the two fission fragments flying apart at high speed. This is the energy that ultimately heats the reactor coolant and generates electricity. The remaining energy is distributed among several forms of radiation.
Prompt gamma rays carry about 8 MeV, released within a millionth of a second of the split. The free neutrons carry another 8 MeV as kinetic energy. About 7 MeV escapes the reactor entirely in the form of antineutrinos, particles produced during beta decay that pass through matter without interacting. That 7 MeV is essentially lost and can never be captured for useful work.
The fission fragments themselves continue releasing energy as they undergo radioactive decay. This “decay heat” is a defining safety challenge for nuclear reactors. One second after a reactor shuts down, the fuel still generates about 7% of its full operating power from decay heat alone. After one minute, that drops to roughly 4%. Even an hour later, decay heat still amounts to about 1.5 to 2% of full power. For a large power reactor, 2% can mean tens of megawatts of heat that must be actively removed. The inability to remove decay heat was the core problem at Fukushima in 2011.
Radioactive Decay Chains
Fission fragments are not the final products. They’re neutron-rich and unstable, so they undergo a series of radioactive decays before reaching a stable form. Each step in the chain produces a different isotope along with beta particles and gamma rays.
A classic example starts with krypton-90, a direct fission fragment. Krypton-90 decays within seconds into rubidium-90, which also disappears quickly. This produces strontium-90, which persists for decades (half-life of 29 years). Strontium-90 slowly decays into yttrium-90, which itself decays within a few days into stable zirconium-90. The entire chain, from the moment of fission to a stable end product, can take over a century.
These chains explain why spent nuclear fuel contains a complex mixture of isotopes that weren’t directly produced by fission. Some are short-lived and intensely radioactive. Others, like cesium-137 and strontium-90, occupy a middle ground that makes them the primary concern for waste storage. A smaller fraction of fission products are essentially stable within human timescales.
Noble Gases and Volatile Products
A substantial portion of fission products are noble gases, primarily isotopes of xenon and krypton. Because these gases don’t bond chemically with the fuel, they migrate out of the fuel pellets and collect in the gap between the fuel and its metal cladding. In normal reactor operation, the cladding contains them. If the cladding fails, these gases are the first fission products to escape.
Iodine isotopes are another volatile group. Iodine-131, with a half-life of about 8 days, is a major concern during nuclear accidents because it can travel through the atmosphere, settle on grass, concentrate in milk, and accumulate in the human thyroid gland. This is why potassium iodide tablets are distributed near nuclear plants: they saturate the thyroid with stable iodine so it doesn’t absorb the radioactive form.
Xenon-135 as a Reactor Poison
One fission product deserves special mention for its outsized effect on reactor operation. Xenon-135 has the highest neutron absorption cross-section of any known substance: about 2.78 million barns at thermal neutron energies. For comparison, uranium-235’s absorption cross-section is about 680 barns. This means xenon-135 is roughly 4,000 times more likely to capture a neutron than uranium-235 is.
In a running reactor, xenon-135 builds up in the fuel and absorbs neutrons that would otherwise sustain the chain reaction. Operators compensate by withdrawing control rods. When a reactor shuts down, xenon-135 continues to accumulate for several hours (produced by the decay of iodine-135) before it begins to diminish. This creates a period where restarting the reactor can be difficult or impossible because the xenon is absorbing too many neutrons. This phenomenon, called a “xenon pit,” played a role in the Chernobyl disaster when operators tried to raise power levels while xenon-135 concentrations were high.
Transuranic Byproducts
Not every neutron causes fission. Some neutrons are captured by uranium-238 without splitting the nucleus, instead converting it into heavier elements that don’t exist in nature. Uranium-238 absorbs a neutron to become uranium-239, which decays into neptunium-239, which decays into plutonium-239. Further neutron captures produce plutonium-240, plutonium-241, and eventually americium and curium isotopes.
These transuranic elements aren’t fission products in the strict sense, but they’re an inevitable byproduct of operating a fission reactor. They matter because their half-lives range from thousands to millions of years, making them the dominant concern for long-term geological waste storage. Plutonium-239 alone has a half-life of 24,100 years. This is the primary reason spent fuel repositories like Finland’s Onkalo facility are designed to isolate waste for hundreds of thousands of years.