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The idea is: they absorb most of the neutrons generated by the splitting of atoms before they can hit other atoms.

Uranium is found in mines but most of it is uranium-238 which is not useful for fission. Only 0.72 percent of the mined uranium is uranium-235 that can take part in fission. To use it as a fuel in a reactor, the mined uranium has to be enriched, in the case of Rooppur power plant, to the level of just under 5 percent.

The uranium fuel is made into pellets and then put into rods.

“The nuclear fuel pellet is stacked inside corrosion-resistant metal tubes [Zirconium alloy tubes], called fuel rods… A fuel rod of unit-1 contains approximately 350 to 380 pellets, with a total fuel column height of roughly 3.73 metres within a fuel rod that is roughly 3.93 to 4.0 meters long with a 9.1mm outer diameter,” said Mohammad Shawkat Akbar, former chairman of Bangladesh Atomic Energy Commission.

The fuel mass inside the reactor alone can weigh over 80 tonnes.

Once all the fuel is loaded inside the reactor core, the control rods, the ones that absorb the neutrons, are partially withdrawn so that the chain reaction can proceed and heat is generated.

This energy is used to heat up water. In the case of the Rooppur plant, the energy is used to heat water in a pressurised condition. Water in our kitchen pot boils at 100 degrees Celsius. But since the water inside the reactor is pressurised, it does not boil, even when its temperature is over 300 degrees Celsius.

This hot water is then used for boiling water in another chamber and creating steam, which turns a turbine connected to an electricity generator.

A generator is just a fancy electric motor. The basic principle of a generator is the same as the electric motor used in most households to pump water to the roof-top tank. The generator just works in reverse. Instead of using electricity to turn a shaft, in a generator the shaft turns to produce electricity.