ALTHOUGH many words have been written in the past to justify nuclear power as the magic bullet that would replace fossil fuels, USA and rest of the industrialised nations realised after the 2011 Fukushima accident that nuclear technology has failed to offer a safe way of generating clean power. Yet, a politically unstable country like Bangladesh is going ahead with the potentially risky venture of building a nuclear power plant at Rooppur. Moreover, it is not clear whether the government has the necessary infrastructure (transportation, security, township for staff, availability of cooling water, nuclear waste repository, etc.) and a strong cadre of nuclear engineers in place for the safe operation of a nuclear power plant. There is also lack of information about the safety aspects related to the local topography, geology, and seismology.
In the absence of transparency, a good way of judging whether nuclear option is right for Bangladesh is to understand how a power reactor works. One can then weigh the risks against the benefits of nuclear energy.
A nuclear reactor generates energy by controlled fission reaction. A typical pressurised light-water reactor that will be built at Rooppur consists of four essential components: Fuel to provide energy; a moderator to slow down the neutrons to make it more likely that they will cause fission; coolant to transfer thermal energy away from the fuel; and neutron-absorbing control rods that can be moved in or out to slow down, speed up, or stop nuclear fission.
Because of its abundance in nature, most nuclear reactors use uranium as fuel. Natural uranium contains 0.7% of the fissile uranium-235; the rest is non-fissile uranium-238. When uranium-235 is bombarded with a slow neutron, it captures the neutron to form uranium-236, which undergoes fission producing two lighter fragments and releases energy together with two or three neutrons. These neutrons in turn cause more fission resulting in a self-sustaining chain reaction.
The fuel is loaded into long, thin rods which are submerged into a pool of water. The core of the reactor, housed inside a pressurised containment vessel, contains the fuel rods. Nuclear fission occurs in the rods generating heat in the core. To prevent leakage of the radioactive fission products, the rods are encased in a solid cladding made of an alloy of zirconium.
The core is maintained at a sufficiently high pressure so that water remains liquid even at very high temperatures. The super-hot water is pumped to a heat exchanger to produce steam. The steam drives the turbine, which spins a generator that produces electrical energy.
When the fuel is depleted, the rods are removed and replaced by new ones. Disposal of the spent fuel rods contaminated with highly radioactive material is a major headache for the industry.
The neutrons produced during each fission event move very fast and will pass right through the uranium-235 nuclei without causing them to fission. They must be slowed down to be captured by uranium-235 and induce more fission. To slow down the neutrons, a moderator material is used. The most efficient moderators are light materials such as water. Neutrons slow down when they collide with the hydrogen (proton) in water.
Although protons are very good moderators, they also efficiently capture neutrons to form deuterons. Reactors using water for the moderator compensate for neutron capture by using fuel enriched to about 3% uranium-235. Enrichment is a very dangerous and expensive process, requiring sophisticated technology.
Most power reactors use water as both moderator and primary coolant. The coolant circulates throughout the core to prevent it from overheating and melting down due to the heat generated by the fission process.
Nuclear plants have to routinely remove from the reactor heated water contaminated with radioactive material. The water is filtered and then some of it is either recycled back into the core or released as vapour into the environment through the cooling towers. The rest is usually discharged into the reservoir from where water was drawn to keep the reactor cool. Improper discharge of water will adversely impact the entire ecosystem of the region.
If all the neutrons from each fission event are allowed to produce additional fission, the chain reaction will grow exponentially and result in a nuclear meltdown in a very short time. A reactor is considered safe when a self-sustained chain reaction is maintained with exactly one neutron from each fission inducing yet another fission reaction. This is achieved by using control rods. These are rods made of cadmium or boron which absorbs neutron without fissioning. They are raised if there aren't enough neutrons to sustain a chain reaction and are lowered when there is an excess build-up of neutrons. If at any time during operation there is a runaway chain reaction, the control rods will fall into the core automatically, absorb all the neutrons, and shut down the reactor.
A major cause of concern is that, even though the control rods will almost certainly stop the chain reaction in the event of an accident, the reactor will still be extremely hot because the uranium atoms that have already fissioned produce radioactive by-products that themselves give off a great deal of heat. Thus, the reactor core will continue to produce heat in the absence of fission causing the core's temperature to rise. It will eventually result in a meltdown, unless the emergency core-cooling system starts operating immediately.
However, it is by no means certain that the safety systems are one hundred percent foolproof and will work as designed. The Fukushima disaster has shattered the "zero risk" myth of power reactors.
Finally, the Russian VVER-1000 model reactors that will be built at Rooppur are a bit like the DC-10 aircrafts that Bangladesh Biman still flies. Both are technological artifacts of yesteryear.

The writer is a Professor of Physics at Fordham University, New York.