Over two years after the “commencement of core loading” at the 500 MWe Prototype Fast Breeder Reactor (PFBR) at Kalpakkam in the presence of Prime Minister Narendra Modi on March 4, 2024 – which marked the start of the process of the long-awaited commissioning of the reactor – the breeder attained ‘first criticality’ at on April 6, 2026. A reactor is termed to have gone critical when the chain of nuclear fission reactions is sustained in the reactor core.This is a very significant milestone in India’s 3-stage nuclear power programme as it signals the beginning of the second stage, making India the second country after Russia to have an operational fast breeder reactor. As The Wire had noted recently, this achievement was imminent. A fast breeder reactor (FBR) breeds more fuel than it burns to generate power. The PFBR uses a mixed oxide (MOX) fuel of plutonium (Pu-239) and depleted uranium (DU) obtained by reprocessing the spent nuclear fuel (SNF) discharged from the fleet of Indian Pressurised Heavy Water Reactors (PHWRs) that use natural uranium (containing 0.7% U-235, a fissile material, and the rest U-238, a fertile material) as fuel.The first stage of the Indian programme comprises PHWRs (and light water reactors, or LWRs, which use low enriched uranium), called thermal reactors, where the speed or energy of fission causing neutrons is reduced by using heavy water (and light water respectively) as moderator. On the other hand, in fast breeders, which do not use any moderator, the neutron energy remains high and hence the term ‘fast’. The efficiency of a fast reactor in converting fertile material into fissile material is much higher than a thermal reactor, and this principle underlies the idea of using fast reactors to breed more fissile material than that is burnt to generate power. Besides converting fertile U-238 in the reactor core into fissile Pu-239, neutrons from the core can also convert a blanket of any fertile material around the fuel core into fissile material. While a blanket of natural uranium or DU (which contains only 0.25% U-235) will get transmuted into Pu-239 – which will enable building further FBRs – a blanket of thorium (Th-232) will get converted into fissile U-233. The latter will form the basis for a self-sustaining chain of U-233 fuelled thermal/fast breeders with Th-232 blankets that will exploit the vast thorium resources of the country in a U-233/Th-232 cycle.This ‘closed cycle’ approach maximizes energy extraction from its limited natural uranium reserves and enables large-scale use of thorium in the future. This essentially defines the underlying strategy of the Indian 3-stage nuclear programme. The significance of PFBR attaining ‘first criticality’ stems from the fact that now the stage is set for steering the programme in that direction with FBRs forming the bridging technology between the PHWRs and the future deployment of U-233/Th-232 cycle-based reactors. Bharatiya Nabhikiya Vidyut Nigam Ltd. (BHAVINI) is the public limited company set up under the Department of Atomic Energy (DAE) as the operator for the fleet of reactors that will constitute the second stage (the PFBR and the subsequent FBRs) just as the Nuclear Power Corporation of India Ltd. (NPCIL) is the operator for the first stage.Though the March 2024 event, with the Modi in attendance, was in some sense a non-event, all the prominence and publicity that it was given was clearly with an eye on the upcoming general elections two weeks later.Just ten days before the event, on February 23, 2024, the Atomic Energy Regulatory Board (AERB) had accorded permission to PFBR only for “loading one Control and Safety Rod (CSR) towards initiation of core loading” (emphasis added). That is, no loading of fuel or blanket sub-assemblies could have taken place then. That gradual process of complete core loading, to be followed by approach to criticality, was a long drawn one, lasting over a two-year period, because of the complexities of technologies involved, and the problems encountered, as will be described presently.Likewise, given the upcoming Tamil Nadu and Puducherry assembly elections, one might be tempted to think that this too was probably a façade. Thankfully, this was not the case and the achievement is real, as The Wire has learnt from scientists associated with the PFBR. “The approach to criticality was very smooth and we can safely say that, after the many challenges it faced, the PFBR team has now fully mastered the technologies and is now confident of taking the system to full power slowly in a phased manner,” said the source who wished not to be named. “At practically every step we faced issues, but solving each challenge has been a great learning experience, which will stand us in good stead” the scientist added.As is the practice, in the ‘first criticality’ operation, the reactor was brought to criticality and sustained very briefly at low power (about 80 kWt) and then shut down. The reactor parameters (about 20) at criticality, which were recorded, will now be analysed in the coming days to understand the system behaviour in detail, which will facilitate bringing the reactor back to criticality when required. The power output will be increased gradually, studying the system stability at each stage, till full power of 1250 MWt is reached. And this will take time. As per the regulatory clearances sought by BHAVINI from the AERB in 2023, the initial fuel loading and the first approach criticality was planned to be achieved in three phases. Phase-1 involved loading of CSRs (9 control and 3 safety rods); Phase-2 included loading of radial blanket sub-assemblies (114 in all) and, finally the main Phase-3 that included loading of “instrumented central sub-assembly (chiefly for monitoring neutron flux), diluent sub-assembly (to manage excess reactivity during the initial core loading operation), fuel sub-assemblies (181 in all with each containing 217 fuel pins), three fission initiating neutron source sub-assemblies (containing antimony-beryllium) and approach to first criticality and low power physics experiments”.After the ceremonial loading of the first CSR on March 4, 2024, following the regulatory clearance of February 23, 2024, requisite AERB clearances had to be obtained at each step for the different operations involved. After completion of Phase-1, approval for Phase-2 was granted on April 10, 2024. Following a multi-tier regulatory review, the AERB gave clearance for Phase-3 on July 29, 2024, with a validity period of one year. However, PFBR engineers faced a serious problem during the trial operations before actual fuel loading into the core could begin. As the then BHAVINI CMD, Mr. K. V. Suresh Kumar, had stated “[during the trial operations of the fuel handling system] it was observed that the Transfer pot of Inclined Fuel Transfer Machine (IFTM) on primary side was not getting lowered fully into the reactor and got stopped at a particular elevation above the intended location. Since liquid sodium [the coolant used in the PFBR] is opaque and direct inspection was not possible, innovative methods [including ultrasonic inspection imaging tool] were used to identify the root cause for the restricted movement. This was systematically resolved…by designing and developing an alternate direct fuel handling scheme for loading fresh fuel sub-assemblies through In-vessel Transfer Port. A new direct handling flask was designed and fabricated in-house in a record time of 4 to 5 months with the available resources in IGCAR, BHAVINI and other units of DAE at Kalpakkam. The flask components were successfully validated in a full scale mock-up facility erected at PFBR.”Based on a revised application from BHAVINI for implementing Phase-3 with the alternate fuel handling system on August 11, 2025, the AERB, following a detailed review, granted permission for core loading with the alternate fuel handling system on October 16, 2025, which will be valid till December 31, 2026. It is indeed creditable that the entire fuel loading operations could be completed within six months after AERB approval. “Lowering the 181 fuel sub-assemblies was a really complex operation and it took almost a day for each,” the scientist remarked. Much of the two-year delay in core loading is attributable to the above problem with the fuel handling system, though there were other small problems as well, the occasional liquid sodium leaks in the Intermediate Heat Exchangers (IHXs) in particular, which have also now been resolved.As of now, following the August 19, 2025, decision of the Atomic Energy Commission (AEC), the revised capital cost of the PFBR stands at Rs 8,181.00 crore, with December 2026 as the targeted deadline for project completion. The approved funding pattern for this outlay is as follows: the total equity funded till September 2025 was Rs 4541.60 crore, shared between the DAE (Rs. 4314.52 crore) and the NPCIL (Rs 227.08 crore), and this entire amount has already been spent. To meet the balance project cost, BHAVINI has secured a 29-year loan of Rs 3,481 crore from the government.The PFBR will now be followed by two 500 MWe units FBR 1 & 2, which are planned to be constructed near the PFBR itself, and the pre-project activities for these have already begun. Besides, introduction of thorium in the blanket for experimental purposes is a possibility. Based on the experiences of construction and commissioning of PFBR, revised AERB codes will be taken into consideration in the ongoing design reviews of FBR 1 & 2.R. Ramachandran is a science writer.