India’s PHWR technology and kakrapar-3 plant | Controlling Criticality
July 23, 2020
India’s PHWR technology and kakrapar-3 plant:
- PHWR technology started in India in the late 1960s with the construction of the first 220 MWe reactor, Rajasthan Atomic Power Station, RAPS-1 under the joint Indo-Canadian nuclear co-operation.
- Canada supplied all the main equipment for this first unit, while India retained responsibility for construction, installation, and commissioning.
- For the second unit (RAPS-2), import content was reduced considerably, and indigenization was undertaken for major equipment.
- Following the withdrawal of Canadian support in 1974 after Pokhran-1, Indian nuclear engineers completed the construction, and the plant was made operational with a majority of components being made in India.
What is ‘Criticality’ in Atomic/Nuclear Power Plants?
- Reactors are the heart of an atomic power plant, where a controlled nuclear fission reaction takes place that produces heat, which is used to generate steam that then spins a turbine to create electricity.
- Fission is a process in which the nucleus of an atom splits into two or smaller nuclei, and usually some by-product particles.
- When the nucleus splits, the kinetic energy of the fission fragments is transferred to other atoms in the fuel as heat energy, which is eventually used to produce steam to drive the turbines.
- For every fission event, if at least one of the emitted neutrons on average causes fission, a self-sustaining chain reaction will take place.
- A nuclear reactor achieves criticality when each fission event releases a sufficient number of neutrons to sustain an ongoing series of reactions.
- When a reactor is starting up, the number of neutrons is increased slowly in a controlled manner. Neutron-absorbing control rods in the reactor core are used to calibrate neutron production.
- The control rods are made from neutron-absorbing elements such as cadmium, boron, or hafnium.
- The deeper the rods are lowered into the reactor core, the more neutrons the rods absorb and the less fission occurs.
- Technicians pull up or lower down the control rods into the reactor core depending on whether more or less fission, neutron production, and power are desired.
- If a malfunction occurs, technicians can remotely plunge control rods into the reactor core to quickly soak up neutrons and shut down the nuclear reaction.
Why is this achievement significant?
- It is the biggest indigenously developed variant of the Pressurized Heavy Water Reactor (PHWR).
- The PHWRs, which use natural uranium as fuel and heavy water as moderator, is the mainstay of India’s nuclear reactor fleet.
- Until now, the biggest reactor size of the indigenous design was the 540 MWe PHWR, two of which have been deployed in Tarapur, Maharashtra.
- India works to ramp up its existing nuclear power capacity of 6,780 MWe to 22,480 MWe by 2031.
- The 700MWe capacity constitutes the biggest component of the expansion plan.