Fission Energy Output

One uranium fuel pellet, about the size of a pencil eraser (roughly 7 grams), releases as much energy as burning 1,000 kg of coal or 480 cubic meters of natural gas. That single pellet can generate roughly 2,500 kWh of electricity, enough to power an average U.S. household for about three months.

Each fission of a U-235 atom releases about 200 million electron-volts (MeV) of energy, distributed roughly as: 170 MeV in kinetic energy of the fission fragments, 5 MeV in prompt gamma rays, 5 MeV in kinetic energy of released neutrons, and 20 MeV in beta and gamma radiation from fission product decay (delayed heat). This delayed heat is why reactors continue producing heat after shutdown and why spent fuel requires active cooling.

The energy per kilogram of natural uranium in a standard once-through fuel cycle is about 440,000 MJ/kg (accounting for the fact that only a fraction of the 0.7% U-235 content actually fissions before the fuel is discharged). That is roughly 18,000 times the energy density of coal (24 MJ/kg). Advanced reactor designs that can fission U-238 or recycle spent fuel could theoretically increase this further.

Worked Example

A typical 1 GW nuclear reactor operates at 92% capacity factor for one year.

• Calculate annual energy output. 1,000 MW x 8,760 hours x 0.92 = 8,059,200 MWh = ~8 TWh.
• Calculate fuel consumption. At roughly 440,000 MJ/kg of natural uranium and ~33% thermal efficiency, that is about 200 tonnes of natural uranium per year.

How does this compare to a coal plant producing the same electricity?

A 1 GW coal plant at 85% capacity factor burns about 3 million tonnes of coal per year. The nuclear plant uses 200 tonnes of uranium. That is a 15,000:1 ratio in fuel mass, which translates directly to fewer mines, fewer trains, fewer storage yards, and less waste volume.

Every argument about nuclear's cost, safety, or waste eventually traces back to this ratio: a fuel so dense that mining, transport, and storage all shrink by four orders of magnitude.

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