Fission Energy Calculator

Pu-239 releases about 207 MeV per fission compared to 200 MeV for U-235, due to differences in nuclear binding energy and the mass distribution of fission fragments. The heavier nucleus also tends to produce slightly more neutrons per fission (2.87 vs 2.43 for U-235), making Pu-239 useful in both reactors and weapons.

Typical light water reactors achieve a burnup of 40,000-60,000 MWd/tU (megawatt-days per metric tonne of uranium), representing about 4-6% of the theoretical total fission energy. Advanced reactor designs aim for higher burnup to reduce fuel costs and waste volume.

The critical mass for a bare sphere of weapons-grade (93%) U-235 is about 52 kg (the Godiva assembly). With a good reflector (e.g., natural uranium), this can be reduced to about 15 kg. For a weapon with implosion design, effective critical masses can be as low as a few kilograms.

Fusion of deuterium-tritium releases 17.59 MeV from 5 u of reactants (~3.52 MeV/u), while U-235 fission releases ~200 MeV from 236 u (~0.85 MeV/u). So D-T fusion releases about 4 times more energy per unit mass than fission, explaining the greater yield-to-weight ratio of thermonuclear weapons.

When U-235 fissions, it splits into two fragments (fission products) typically in the mass range 90-140 u. Common pairs include Kr-92 + Ba-141, Xe-140 + Sr-93, and many others. These radioactive fission products are the primary source of nuclear waste and decay heat in spent reactor fuel.

Even after a reactor shuts down, fission products continue to decay, generating decay heat. Immediately after shutdown, decay heat is about 7% of full power, dropping to about 1% after an hour and 0.1% after a day. This is why reactor cooling must continue after shutdown — failure of this cooling caused the Fukushima meltdowns.

Thorium-232 is fertile (not fissile): it cannot sustain a chain reaction itself, but absorbs neutrons to produce U-233, which is fissile with Q ≈ 197 MeV. Thorium fuel cycles are of significant interest because thorium is 3-4 times more abundant than uranium and produces less long-lived transuranics.

In most reactors, fission energy heats water (directly or via a coolant loop) to produce steam, which drives a turbine connected to a generator. Typical thermal efficiency is 33-38% for light water reactors and up to 45% for advanced gas-cooled reactors. The rest is rejected as waste heat.

Natural uranium is 99.28% U-238 and only 0.72% U-235. U-238 absorbs neutrons without fissioning (parasitic absorption). Most reactors require enrichment to 3-5% U-235 to sustain a chain reaction. Heavy water reactors (CANDU) can use natural uranium due to lower neutron absorption by deuterium moderator.