In the past year or so there has been a spate of articles in the Indian media, some of them quite tendentious, decrying the Indian nuclear establishment for having failed to establish any technology for thorium utilisation and extolling the virtues of the as-yet-unproven concept of using a foreign-origin thorium fuel in Pressurised Heavy Water Reactors (PHWRs), the mainstay of the Indian 3-stage nuclear power programme. Some milestones achieved in irradiation test runs with this fuel – a blend of thorium (Th) with High-Assay Low-Enriched Uranium (HALEU) – at the Advanced Test Reactor of the Idaho National Laboratory (INL) of the US Department of Energy have also been hailed in the media equally enthusiastically.Named ANEEL (Advanced Nuclear Energy for Enriched Life), the fuel has been developed under a collaboration between Clean Core Thorium Energy (CCTE), a nine year-old Chicago-based based company founded by Indian-origin Mehul Shah, and Centrus Energy Corporation, a Maryland-based enriched uranium supplier. The sample fuel pellets for the tests were fabricated by Texas A&M University under INL’s supervision. PHWRs, also known as CANDU (Canada Deuterium Uranium) reactors, use natural uranium (NU) as fuel, and heavy water (deuterium oxide D2O) as both coolant and moderator. While the content of fissile uranium-235 (U-235) in NU is only 0.7% (the rest being non-fissile but fertile U-238), the standard commercial fuel used in Light Water Reactors (LWRs) has 3-5% U-235 enrichment. On the other hand, HALEU, an advanced fuel that has come into active discourse in recent times in the context of modern reactor concepts – the Small Modular Reactors (SMRs) in particular – has a higher enrichment of 5-20%; hence the term “High-Assay”. It should, however, be emphasized that no operational power reactor that uses HALEU as fuel currently exists anywhere in the world. India’s 3-stage nuclear power programmeThere are 47 operational PHWRs around the world of which 20 are in India. The present fleet of PHWRs – primarily designed to use NU as fuel – comprises sixteen of 220 MWe, two of 540 MWe and two of 700 MWe capacity. These form the major part of the first stage of the Indian atomic energy programme and have given Indian nuclear scientists and engineers enormous experience in understanding, designing, operating and simulating PHWRs. This has resulted in a fair degree of maturity in all aspects of the associated nuclear fuel cycle, including reprocessing of spent nuclear fuel (SNF). India’s 3-stage nuclear power strategy relies on a ‘closed’ fuel cycle to optimally use the country’s limited uranium resources and exploit its vast reserves of thorium (Th-232) – a fertile nuclear material that readily becomes fissile U-233 fuel by absorbing a neutron – for long-term sustainable energy security. Recovery and recycling of fissile and fertile material by reprocessing spent fuel, rather than disposing it off as nuclear waste (as in an ‘open’ or once-through fuel cycle) is a key part of this strategy.Deployment of the aforesaid thorium fuel cycle – a self-sustaining and low-waste nuclear power generating process – constitutes the third and the final stage of the programme. Creating a sufficient stockpile of U-233 by breeding it in a fleet of plutonium (Pu-239) driven fast breeder reactors (FBRs) constitutes the second stage, which for various reasons is yet to take off. Recent reports, however, suggest that the upcoming 500 MWe Prototype FBR at Kalpakkam, should soon go on stream with the completion of full core loading by next month.Based on the apparently superior characteristics of HALEU in comparison to NU in terms of (1) its greater energy density arising from the higher enrichment of fissile U-235 (and the resultant higher burn-up), (2) much lower SNF inventory and (3) reduced fuel fabrication, ANEEL has been touted by CCTE as a “drop-in” fuel for the existing CANDU reactors around the world – Indian PHWRs in particular – for immediate introduction of the thorium fuel cycle in the nuclear power programme. Some commentaries in the Indian media have suggested that the HALEU-Th route should be adopted in parallel to introduce the thorium fuel cycle even as the second stage progresses apace rather than wait till the third stage, which is at least about a couple of decades away. It must, however, be noted here that this claim by CCTE, as also in other commentaries, is not based on any detailed physics analysis for the Indian PHWR design that has either been done by the company or by any other foreign research group. The narrative has also not been situated within the context of the 3-stage Indian programme. BARC scientists’ study questions HALEU claimsPerhaps in response to the publicity blitz by CCTE and the media hype over the past one year and more, detailed performance evaluation of HALEU-Th fuel in Indian PHWRs has now been carried out by K.P. Singh and associates, scientists at the Reactor Research Division of the Bhabha Atomic Research Centre (BARC). The results of this important exercise have been recently published in the journal Current Science.The paper notes that most of the fuel cycle studies towards utilization of thorium in PHWRs were hitherto limited to (fuel) cluster-level (as against the complete reactor core) analysis. In India too, a wide range of fuel bundle designs, with varying uranium enrichment levels mixed with thorium, has been studied over the years to assess their performance characteristics in PHWRs. This recent work has taken the earlier studies to the next level “by comprehensive cluster-level optimization, along with inclusive core-level analysis” with regard to HALEU-Th (ANEEL) fuel, and it is the first-ever full core-level performance analysis of the fuel in PHWRs. The multi-dimensional assessment includes considerations of uranium resource utilisation, sustainability, applicability to the current PHWR design, operational constraints, safety aspects, SNF generation etc., and, most importantly, its relevance to the 3-stage programme. The most significant takeaway from the work’s findings is, to quote the paper’s last para, “HALEU-Th fuel is far from a ‘drop-in option for the present generation of PHWRs. Evolving a modified PHWR design for PHWR-HALEU-Th cycle will impede India’s 3-stage programme aimed at thorium utilisation and, therefore, is not desirable. Utilisation of U-233 generated in PHWR-HALEU-Th cycle is also found to be unsustainable. (Emphasis added)”As will transpire from the discussion that follows, in terms of resource utilization, one actually ends up using more mined-NU per unit of energy by using HALEU-Th in PHWRs than NU. (Or equivalently, one ends up deriving less energy from a tonne of mined-NU with HALEU-Th.) While in situ burning of U-233 (resulting from the transmutation of Th-232 in the core matrix) will contribute to the total energy output, it occurs at the cost of higher NU consumption, which is undesirable in the context of the 3-stage programme. By also significantly reducing the quantity of discharged spent nuclear fuel, the important second stage gets bypassed. It also does not contribute to the third stage because the nuclear composition of SNF from HALEU-Th is such that it makes reprocessing and fabricating U-233 fuel very difficult. So the Indian 3-stage programme stands to gain nothing at all from introducing HALEU-Th in PHWRs. Comparing reactor performance of three different fuel cycles One of the main objectives of using HALEU-Th as fuel in PHWRs is to achieve high burn-up (about 50 gigawatt-days/ton, or GWd/t) – about the same as LWRs with 4-5% U-235 enriched LEU – as compared to about 7 GWd/t with NU. (‘Burn-up’ refers to the amount of energy produced per unit of nuclear fuel burnt in the reactor.) In the present analysis, the BARC scientists have compared the performance of 220 MWe PHWR against different parameters in three different fuel cycles: (1) with natural uranium (NU); (2) with HALEU-Th; and, (3) slightly enriched uranium (SEU), with 1.1% U-235 enrichment, under both open and closed fuel cycle scenarios.For comparing the performance of the three fuel cycles, the study considered HALEU with U-235 enrichment of 19.75% having the following composition of the total HALEU-Th fuel so that utilization of thorium is maximised: U-235: 3.2%, U-238: 12.9% and Th-232: 83.9%. The content of U-235 in the composition has been arrived at by optimizing the saving on mined-NU for one unit of energy produced on the one hand and yielding the target burn-up of 50 GWd/t on the other. In the emergent context of suggestions to use HALEU-Th fuel in Indian PHWRs, comparison with the third option of using SEU – though not very relevant for the Indian programme – is useful to the extent that the analysis has shown that SEU (with 1.1% enrichment) achieves better performance in Indian PHWRs than HALEU-Th in terms of uranium utlisation.Also, both open and closed fuel cycles have been considered in the paper because HALEU-Th (or ANEEL) is viable only in a once-through cycle scenario. As mentioned earlier, reprocessing the SNF in this case, and recovering U-233 for recycling, pose serious challenges. Though the U-233 content in the discharged fuel is relatively high, it contains 250 ppm of U-232 that is formed as a byproduct in the cycle, which decays into bismuth and thallium that are strong gamma emitters. This renders fuel fabrication both technically challenging and uneconomical. Thus the fuel has no specific advantage when used in a closed cycle. In NU-fired PHWRs, since the average discharge burn-up achieved is only about a seventh of that from LWRs, fuel fabrication and discharge spent fuel are much higher in comparison. It must be emphasized that the latter is a problem only in a once-through fuel cycle. While, in principle, it is possible to increase burn-ups in PHWRs to the level achieved in LWRs (by increasing the fissile content in the fuel – using HALEU-Th is one such option) and reduce the amount of spent nuclear fuel, from the standpoint of a closed fuel cycle of the Indian 3-stage programme, reprocessing SNF yields significant high quality Pu-239 for use in the FBRs in the second stage, the study has pointed out. For 1 tonne of NU feed, the HALEU-Th cycle yields only 0.22 kg of Pu (with 0.13 kg of fissile isotopes) as against 3.7 kg (with 2.7 kg of fissile component) in the NU cycle. Further, the study has also drawn attention to the fact that, though SNF from HALEU-Th is only a seventh of that from NU, given the high radioactivity and decay heat of the former, one has to contend with the problem of interim and long-term storage or disposal of SNF – an issue absent in the NU-based closed cycle strategy for thorium utilization, where SNF is reprocessed and recycled. More importantly, according to the BARC scientists, analysis of design and operational aspects – including fuel bundle design and configuration, feasibility of on-power refueling scheme with the present fuel handling system in PHWRs, existing reactivity control and shut-down systems for safe operation, fuel loading patterns and in-core fuel management – shows that only a moderate increase in burn-up from the NU fuel cycle with SEU fuels with discharge burn-up of about 20 GWd/t can be accommodated. “The transition to a much higher burn-up of the order of 50 GWd/t in existing PHWRs poses severe challenges with no tangible gains,” says the paper. Even assuming that the challenges to transition to higher burn-up of 50 GWd/t in PHWR can be met by investing in appropriate design modifications, the economics of the two fuel cycles provides a very interesting comparison. It must be remembered that, in contrast to the NU cycle, the HALEU-Th cycle involves additional process of enrichment up front, and this front-end cost for HALEU with enrichment of 19.75% will be maximum notwithstanding the fact that the quantity of fuel to be produced is significantly lower.It turns out that 1 tonne of mined natural uranium yields only 26 kg of HALEU (with 19.75% U-235 enrichment). Now this quantity of HALEU-Th fuel (obtained from 1 tonne of mined-NU) produces 8.1 GWd of energy, which is only marginally better than 7 GWd produced by burning 1 tonne of NU in a NU-fired PHWR. Based on the reported cost per kg of HALEU than that of NU (100 times or more), simple arithmetic tells us the much higher cost of producing 1 GWd of energy with HALEU-Th as compared to NU. Even though the fuel-fabrication load is lower than that for NU by a factor of 7, additional safety precautions in handling HALEU and in its fabrication will offset this apparent benefit, says the study. To achieve the targeted 50 MWd/t burn-up with optimized use of mined-NU, modifications in the fuel composition and bundle geometry also become necessary. To suppress large excess reactivity and local flux peaking, burnable absorbers and a radial gradation of enrichment need to be incorporated in the fuel, thus increasing the complexity of fuel fabrication significantly. Due to the uniform blending of Th-232 with HALEU, minor actinides, such as neptunium, americium, and curium, will also be produced in this fuel cycle. Though these constitute only a small fraction of high-level waste, given their long half-lives, they overwhelm long-term radiotoxicity. These long-lived actinides are not produced in a pure U-233/Th-232 fuel cycle, and the many advantages of this cycle are lost in the HALEU-Th cycle due to the presence of U-238 in the fuel, the authors point out. While this problem can be circumvented if Th-232 is not uniformly mixed but separated from HALEU at the fuel pin or bundle level, these modifications will give rise to their own complexities in manufacturing, operation and spent fuel management. In a closed cycle scenario, with NU as fuel, one tonne gives rise to 988 kg of depleted uranium (DU) containing 0.25% fissile U-235 and the rest U-238. The accumulated Pu (from the 3.7 kg from every tonne of NU used in PHWRs of the first stage) together with DU serves as the fuel for FBRs in the second stage. While the U-238 in DU breeds more Pu-239 for further FBRs, the Th-232 in the blanket breeds U-233 for the self-sustaining third stage. Since U-233 is bred in the blanket around the core, and not in the core fuel matrix as in the case of HALEU-Th fuel, concentration of U-232 in it can be significantly lowered by introducing some low atomic number material between the core and the blanket to soften the neutron energy. This renders U-233 fuel fabrication feasible, the authors clarified. In the case of HALEU-Th a closed cycle is unviable because separation of U-233 from the spent fuel, which is a mix of 86% of U-238 (86%), 9.5% of fissile uranium (unburnt U-235+U-233) and miniscule Pu and Th-232, requires new reprocessing techniques. While U-233 fuel fabrication from this, as mentioned earlier, is hazardous and greatly challenging, the discharged fuel is not even good to burn in fast reactors because fissile uranium content in it is only 9.5%, which is too low to attain criticality with fast neutrons. Because molten salt reactors (MSRs), where the fuel is dissolved in a fluidized salt bed, do not require fuel fabrication, the study also explored the possibility of using U-233 from the reprocessed spent HALEU-Th fuel in an MSR of a particular design. But the study found that even that option was unsustainable. HALEU-Th fuel push a means of limiting nuclear weapon programme?In sum, the technical design of the present Indian PHWRs is not suitable for high burn-ups, the key feature of HALEU-Th (ANEEL) fuel. Even if they are modified to handle high burn-ups, BARC’s comprehensive evaluation of the use of the fuel in the existing Indian PHWR design has shown that there is no significant difference between NU cycle and HALEU-Th cycle in the utilization of fissile uranium (U-235) per tonne of mined-NU for power generation.In fact, as the authors point out, neutron absorption by thorium in HALEU-Th negatively impacts on the full utilization of U-235 even as thorium itself does not contribute in any way to energy production. Thorium in the fuel only serves to blunt the reactivity in the reactor resulting from high U-235 enrichment and consequent high burn-up. The U-233 resulting from the transmutation of thorium is also rendered useless because of the technical challenges posed in separating U-233 from the spent fuel through reprocessing and its fuel fabrication.So, where is the push for using HALEU-Th in the Indian PHWRs coming from? One is the simple and straightforward commercial angle with both CCTE and Centrus Energy trying to sell it in the world market with much lower SNF as its sole USP. This is already evident from Indian companies jumping into business tie-ups with CCTE. Given the key feature of HALEU-Th as an open cycle fuel that is not amenable to reprocessing and separation of weapons-usable plutonium, the other push would seem to be coming from a non-proliferation angle. In fact, besides higher burn-up and low SNF inventory, proliferation-resistance is another feature with which HALEU-Th is being positioned in the ongoing nuclear discourse around it. It would be recalled that following the first successful test run with the fuel in 2024, which basically demonstrated the physical integrity of the HALEU-Th fuel pins at a high burn-up value of 25 GWd/t, Larsen & Toubro and the National Thermal Power Corporation (NTPC), who are seeking to enter the nuclear power generation sector, have signed MoUs with CCTE, the former in October 2024 and the latter in December 2024. The second milestone of 45 GWd/t burn-up in August 2025 would have helped to further bolster these ill-conceived tie-ups. While L&T aims to collaborate and set up a supply chain for the fuel in India, NTPC has similar plans including indigenization and manufacture of ANEEL as well as supplying enriched uranium to India.In the wake of the present evaluation of the utility of HALEU-Th fuel ANEEL fuel in Indian PHWRs by BARC scientists, it is hoped that the two companies, who have sought to collaborate with CCTE without doing a proper technical reality check with the Indian nuclear establishment, have a rethink on their plans of using it in their nuclear ventures. R. Ramachandran is a science writer.