"Based on the satisfactory Safety and Security reviews … permission is hereby issued for First Approach to Criticality (FAC) and Low-Power Physics Experiments (LPPE) or Kakrapar Atomic Power Project Unit 4," the AERB said in an announcement dated 11 December. The permission is valid until 30 April 2024.

The AERB gave permission for the start of fuel loading at Kakrapar 4 - also referred to as KAPP-4 - in October.

First criticality is when sustained nuclear fission is achieved in the nuclear fuel for the first time. Once first criticality is reached, a series of experiments and tests will be carried out as the reactor's unit is gradually increased, before the unit is connected to the power grid. Kakrapar 3 - the first Indian 700 MWe PHWR unit -  underwent its First Approach to Criticality in July 2020 and was connected to the grid in January 2021. It was declared to be in commercial operation in July this year - when it was operating at a reported 90% of its full power. Operator Nuclear Power Corporation of India Ltd received permission from the AERB to take it to the full rated capacity in the following month.

Construction of Kakrapar 4 began in November 2010. The site is also home to two 202 MWe PWHRs: Kakrapar 1 was connected to the grid in 1992 and Kakrapar 2 in 1995.