6 min readNew DelhiUpdated: May 10, 2026 06:48 PM IST
It is heartening to see indigenously developed pressurised heavy water reactors (PHWR) finally getting into the mainstream of India’s energy push towards a net-zero compliant Viksit Bharat. The 100 GWe (gigawatt electric) nuclear energy mission launched by the Government is to be realised by 2047.
The high rate of capacity addition necessary for such a deployment would need a credible implementation framework as well as standardised technologies that are mature and commercially competitive. PHWRs are a fit case for the purpose.
The attendant steep increase in uranium imports is, however, a matter of concern. Given the current uranium demand-supply mismatch, expected rise in uranium demand consequent to global nuclear generation growing three to four times as is being projected, and complexities of nuclear-related geopolitics, nuclear fuel supply security could be at risk in about 10-15 years.
A much-needed shift
So, a quicker shift to thorium — that India has in abundance — remains critical to India’s energy security or, in fact, energy independence. Large reactor capacity that can produce uranium-233 (U-233) from thorium at scale while also producing electricity is a prerequisite for this purpose. Thus, the first criticality of the 500 MWe (megawatt electric) prototype fast breeder reactor (PFBR) that would open the path to growing such a capacity through fuel breeding marks a key milestone for India. Thorium can also enable India’s transition to a significant energy exporter from being a major energy importer.
A number of new developments beyond PFBR would also be necessary for the growth of such thorium irradiation platforms. These include a set of reactor and nuclear fuel cycle technologies based on mixed oxide, metallic, and thorium fuels.
The start of thorium utilisation at scale is thus three to four decades away. Pursuing this development is nevertheless critical to our long term energy future. Fast reactors breed new fuel. They are necessary to expand energy generating systems that would run as long as thorium is available.
Nuclear generation capacity through uranium-fuelled reactors is soon expected to cross 10 GWe, the target set for the first stage of the three-stage nuclear power programme. The spent fuel from these plants already constitutes the required inventory waiting to feed the second stage as planned. The experience with PFBR so far suggests that in addition to moving closer to large-scale thorium utilisation, fast reactors can provide economically competitive electricity without dependence on fuel imports. Nuclear generation capacity of a few hundred GWe can be established in this mode. Moving fast on leveraging these possibilities should thus continue to be a matter of high priority.
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Timeline for transition
But what about the timeline for transition from uranium to thorium? The intention behind opening international civil-nuclear cooperation was to remove the hurdles in accessing uranium — priority for thorium was never to be compromised but rather speeded up. Access to imported uranium is now expected to make the first stage 10 times bigger, that is 100 GWe. PHWRs could constitute around 60 GWe.
Apart from electricity supply, such a substantive PHWR capacity coming into existence between now and 2047 (approximately 20 years) would also enable a sizeable capacity for production of U-233 at scale through thorium irradiation in PHWRs. This would happen much earlier than build-up of similar capacity with fast reactors. One can see at least a three-decade time difference between the two.
Use of thorium-HALEU (high-assay low-enriched uranium) fuel in place of natural uranium in PHWRs would enable thorium irradiation at scale while leading to several advantages in terms of safety, economy, waste minimisation, proliferation resistance, and some saving in the mined uranium. One can thus start building inventory of spent fuel that can produce fuel material directly for the third-stage thorium reactors.
In my view, the thorium molten salt reactor (TMSR) operating in the near thermal neutron spectrum offers an ideal choice for the third stage. This would enable significant electricity generation capacity sustainably fuelled by thorium. Rough estimates suggest that PHWRs running on thorium-HALEU fuel could support new TMSR capacity comparable with the mother PHWR annually.
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The availability of thorium-HALEU fuel recycle technology for feeding TMSRs and development of TMSRs would need to be ensured in about 10-15 years from now to start producing electricity from thorium. These technologies are anyway a part of the technology development agenda for the three-stage nuclear power programme. Advancing their deployment will enable us to get on with thorium utilisation at scale without having to wait for completion of second-stage development and large-scale thorium irradiation therein. With fast reactor capacity scaling up to the required level, one can assure U-233 supply and required energy availability at least till fusion energy arrives on the scene.
The world has largely restricted the use of uranium only in once-through mode. This is due to the fear of potential for malevolent diversion. Utilisable energy potential in uranium thus remains two orders of magnitude lower for such countries. On the other hand, proliferation resistance of the thorium-U-233 fuel cycle should enable realisation of fuller energy potential of thorium. Thorium systems, once available, are thus destined to play a greater role in coping up with base load energy demand of the world.
This is a unique opportunity for India. Developing requisite technologies and policies related to use of thorium is the need of the hour. It is time India transitions its nuclear energy programme from uranium to thorium.