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| Fig. 1: Th-232 and U-238 decay chains. (Source: Wikimedia Commons) |
Approximately 30% of the world's thorium deposits can be found in the beach sands of Southern India. Thorium has enormous potential for nuclear energy production, which is not immediately obvious. This is because thorium is not itself fissile, or able to be split in an energy-producing nuclear fission reaction through a chain reaction of neutron absorption and re-emission. However, when reacted with a fissile isotope such as Pu-239, which can be created from natural uranium, the thorium can be effectively converted into fissile U-233. In the 1950s, renowned Indian physicist Dr. Homi Bhabha formulated a three-stage nuclear program with the central aim of utilizing India's abundant thorium reserves to meet its growing energy demands and secure energy independence. This three-stage approach remains the guiding framework behind the Indian nuclear power program today. [1-3]
In the first stage, natural uranium oxide (UO2) is used to fuel pressurized heavy-water reactors (PHWRs). Natural uranium is about 99.7% U-238, and only 0.3% is fissile U-235. [4] However, along with electricity production, PHWRs produce fissile Pu-239 which can be used by reactors in the next stage. Heavy water (D2O) is used as a moderator and coolant, slowing fission neutrons which are emitted in the reaction but not absorbing them as H2O does in light water reactors (LWRs). This allows these surplus neutrons to be absorbed by U-238, creating Pu-239 (after two β decays take place, as in Fig. 1). Owing to India's low uranium reserves (~1% of global uranium), Bhabha opted to import UO2 fuel for use in these reactors. In addition to PHWRs, stage 1 also included foreign designed LWRs, for the purpose of training personnel and establishing a nuclear culture. [1] Additional LWR technology has been imported, largely from Russia, in tandem with the three-stage approach. This was in order to accelerate the development of India's nuclear program, after several countries halted fuel shipments to India following its 1974 nuclear weapons test. India currently has 18 PHWRs operational, contributing a total of 4,460 MW of India's total 6,780 MW supplied by nuclear reactors, or 65.78%. This represents 0.92% of India's total 482,232 MW electricity production. [1,5]
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| Fig. 2: Loop and pool type fast breeder reactor schematics. (Source: Wikimedia Commons) |
Stage 2 has the objective of putting to use the Pu-239 and depleted uranium produced from the previous stage, closing the fuel cycle by recycling spent fuel. This is necessary because fissile U-235 is entirely depleted in stage 1. Stage 2 fast breeder reactors (FBRs) operate by breeding fuel in the reactor itself, such that more fissile material is produced than is consumed in the reactor. These reactors breed both U-233 and additional plutonium, to be used in stage 3 reactors as well as other stage 2 reactors, respectively. Thorium is partially put to use in these reactors in the form of blankets surrounding the reactor core, which is the source of the U-233, after it absorbs a neutron and subsequently undergoes two β-decays (Fig. 1). Currently, there is only one operational stage 2 reactor in India. This is the Fast Breeder Test Reactor (FBTR) in Kalpakkam. This reactor is a sodium-cooled, loop-type (see Fig. 2) fast reactor. This reactor serves as a testing ground for developing electrical power generation technology, as well as general operation of sodium cooling and fast breeder type reactors, which will be integral in future stage 2 and 3 reactors. [1,2]
Stage 3 is where India's vast thorium reserves fully come into use inside Advanced Heavy Water Reactors (AHWRs). AHWRs are designed to use a combination of the Pu-239 and U-233 bred in FBRs and Th-232 to produce energy. Specifically, the proposed fuel to be put into use is thorium oxide and plutonium oxide mixed fuels, which will then be converted into a thorium oxide and U-233 oxide mixed fuel after U-233 is initially produced from the reaction of Pu-239 with Th-232. These reactors are cooled by light water but moderated by heavy water. This reactor is also designed to desalinate sea water. As the Prototype Fast Breeder Reactor (PFBR) is still under construction, the development of AHWRs in India is still in the theoretical stage. [1-3] There is currently 1 research reactor in India, known as KAMINI (Kalpakkam Mini reactor), which utilizes U-233 alloy fuel produced by the nearby FBTR. It is a light water, low power research reactor. Its main purpose is to study the properties of U-233 fuel and carry out irradiation experiments. It produces 30 kWt of power and has been operational at full power since 1997. [6]
This three-stage approach has run into several problems, particularly with its stage 2 reactors. The FBTR only in March 2022 reached its rated 40 MWt power level, after starting at 10.2 MWt upon achieving criticality in 1985. [1,7] Though it was initially built by 1977, France did not supply the enriched uranium oxide fuel for the first core design, after sanctions due to India's nuclear weapon test in 1974. They had to use a new core design with mixed carbide fuel instead, which caused the 8-year delay in reaching criticality and significantly decreased the possible power output. Mixed carbide fuels are quite susceptible to oxidation as well as hydrolysis making large scale production difficult. Reprocessing this fuel is also challenging. [1]
Some inherent difficulties with the thorium fuel cycle include the hazardous nature of decay products of separated thorium, with high-energy γ radiation making it far more hazardous than separated uranium. Another problem is in the differential between half-lives in the Th-232 decay chain. Another issue is that the half-life of Th-233 (after 1 neutron absorption) is around 22 minutes while the half-life of Pa-233 (after Th-233 undergoes β decay) is 27 days. This means that there must be a lengthy delay of at least 12 months between the irradiation of Th-232 and reprocessing the spent fuel to maximize U-233 (Pa-233 after a β decay). U-232 is another risk, as it produces hazardous high γ activity in its decay chain. It can be produced by several different decay chains in addition. U-232 present in the U-233 fuel means heavy shielding is needed during processing. [1,3,4]
The first prototype FBR, the PFBR, has run into several challenges and delays. It was originally planned for completion from 2003-2010, yet construction did not finish until 2014. Additionally, commissioning of the reactor has been ongoing since 2015. A major issue faced early on was that revision of the initial project report submitted to the Indian government in 1990 took until 2002 to be re-submitted after revisions. These revisions helped with cost reduction, however, by 2002, most of the personnel who had worked on the development and commissioning of the FBTR had retired. This loss of senior personnel was a precursor to a slew of delays that would be faced during the construction and commissioning processes. One particularly challenging construction delay was considerable difficulty in aligning the reactor roof (supporting all primary circuit components) to the reactor vessel before welding had been completed. Commissioning sodium loops in the cooling system has also run into challenges largely stemming from entrapped argon gas causing components to fail and disrupting the flow of sodium coolant. Another long delay was encountered with the process of preheating the reactor vessel, with an initial 1-month estimate time being extended to over a year due to issues with the amount of hot nitrogen gas being fed into the vessel. [1,8]
Despite challenges and delays, tapping into its vast thorium reserves remains the long-term goal of India's nuclear power program. The PHWRs and LWRs in the first stage continue to be built and supply India's power grid, while breeder reactors seem to be delayed but progressing in their development, nonetheless. For the time being, the completion of the PFBR is planned for 2024, and its performance will provide additional insight into the feasibility of India's current path to energy self-reliance.
© Aanand Joshi. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. G. Bucher, "India's Baseline Plan for Nuclear Energy Self-Sufficiency," Argonne National Laboratory ANL/NE-09/03, January 2009.
[2] S. Parekh, "India's Three Stage Nuclear Program," Physics 241, Stanford University, Winter 2014.
[3] "Thorium Fuel Utilization: Options and Trends," International Atomic Energy Agency, IAEA-TECDOC-1319, November 2002.
[4] "Thorium Fuel Cycle - Potential Benefits and Challenges," International Atomic Energy Agency, IAEA-TECDOC-1450, May 2005.
[5] "Energy Statistics 2023," National Statistical Office, Government of India, March 2023.
[6] S. Usha et al., "Research Reactor KAMINI," Nucl. Eng. Des. 236, 872 (2006).
[7] T. Sathiyasheelaet al., "A Revised Analysis Towards Accurate Estimation of Isothermal Temperature Coefficients in Fast Reactors Having Different Power Levels and Core Sizes," Nucl. Eng. Des. 415, 112728 (2023).
[8] R. D. Kale, "India's Fast Reactor Programme - A Review and Critical Assessment," Prog. Nucl. Energy 122, 103265 (2020).
