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| Fig. 1: HIFAR at Lucas Heights, 1958. From National Archives of Australia, series A1200. (Source: Wikimedia Commons) |
The Open Pool Australian Lightwater (OPAL) reactor is a 20 MW research reactor located at Australia's Nuclear Science and Technology Organisation (ANSTO) facility in Lucas Heights, New South Wales. Commissioned in 2006 and opening officially in April 2007, OPAL replaced its predecessor, the High Flux Australian Reactor (HIFAR, pictured in Fig. 1 in 1958), as the nation's sole operating reactor. OPAL is a research reactor: not intended to generate electricity, it functions as a neutron factory for scientific research, isotope production, and silicon doping. [1] In fact, although it is not at issue here, nuclear power generation is currently prohibited in Australia under two pieces of federal legislation: the Australian Radiation Protection and Nuclear Safety Act 1998 (ARPANS Act), and the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act).
The contrast between OPAL and a commercial power reactor is made clear by comparing their core specifications. The AP1000 is a Generation III+ pressurised-water reactor (PWR) engineered to extract the maximum thermal energy from fission at an industrial scale. [2] Its core must therefore sustain a high, steady neutron flux over a large fuel mass to generate electricity continuously. Meanwhile, a research reactor like OPAL is designed to maximise the useful neutron flux per unit of power. This is achieved through a compact core and the use of a heavy-water (D2O) reflector vessel to scatter neutrons back into the core. [1] To understand the impact of these design choices, we can look more closely at the specifications of each reactor:
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| Table 1. Key numerical comparison between OPAL and the AP1000. [1-4,8] |
The AP1000 has a fuel mass that is more than 2,000 times larger, used to raise its light water coolant to 303°C at 15.5 MPa, in order to drive steam turbines. [3] Yet despite this large difference in fuel mass, OPAL is able to produce a peak neutron flux that is comparable to the AP1000's, on the order of 1014 n cm-2 s-1. [1,4] Even with this neutron production, OPAL's thermal output remains 170 times smaller than the AP1000's - not enough to produce any electric output.
This can be explained by the relationship between neutron flux, fission rate, and core volume, where the neutron flux scales approximately with the neutron production rate per unit volume: φ ∝ Rf/V. As seen in Table 1, OPAL is significantly smaller that the AP1000 - the core is more compact and has a higher power density. This concentrates the fission reactions within a small volume to increase the neutron density.
The key takeaway to emphasise is this: neutron flux is proportional to power density, not total power. The design of OPAL takes advantage of this fact in order to prioritise neutron economy. It should be noted, however, that the 19.70% fuel enrichment level, with its greater power density, can present a larger safety risk. Standard safety limits are typically set at around 5%, since at enrichment above 6%, the fuel can sustain a fast chain reaction, increasing the risk of accidental nuclear criticality. [5]
With such a small power output, OPAL is not used to generate electricity. Rather, the research reactor supports three principal activities: radioisotope production, silicon doping, and neutron beam research. The radioisotopes that OPAL produces are used for nuclear medicine. To create them, uranium alloy targets are lowered into the reactor, where fission of 235U yields Mo-99, the parent isotope of Tc-99m, the most widely used diagnostic radioisotope in the world. Operating at maximum capacity, the facility can produce 15% of global demand for Mo-99 (though it should be noted that reactors mostly operate below their maximum capacity). The facility also produces I-131 by irradiating tellurium dioxide targets, alongside brachytherapy sources like I-125 seeds. [1,6]
OPAL also performs Neutron Transmutation Doping (NTD) of silicon. For this crucial work, large single-crystal silicon ingots are irradiated inside the reflector vessel, and thermal neutrons convert one Si-30 atom in every billion into P-31. The phosphorus introduces free electrons, allowing for precise and homogeneous electrical conductivity in the silicon. ANSTO produced 27 tonnes of NTD silicon in the 2009-10 financial year, meeting approximately 15% of global demand at that time. Finally, OPAL is essential for neutron beam science in Australia. The facility has room for up to 18 instruments, from WOMBAT, a high-intensity powder diffractometer, to QUOKKA, the instrument enabling small-angle neutron scattering. [7]
It is clear that nuclear reactors are not a monolithic technology: the physics of neutron-induced fission can be optimised to meet different goals. At 20 MW, OPAL is built for the neutron flux that contributes to medical isotope production, semiconductor doping, and fundamental science research.
© Natascha Barac. 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. Cameron and K. Horlock, "The Replacement Research Reactor," in 3rd Conference on Nuclear Science and Engineering in Australia, October 1999, p. 24.
[2] T. L. Schulz, "Westinghouse AP1000 Advanced Passive Plant", Nuclear Engineering and Design 236 1547 (2006).
[3] D. Lioce et al., "AP1000 Passive Core Cooling System Pre-Operational Tests Procedure Definition and Simulation By Means of Relap5 Mod. 3.3 Computer Code," Nucl. Eng. Des. 250, 538 (2012).
[4] M. Askarieh and M. White, "Generic Design Assessment: Disposability Assessment for Wastes and Spent Fuel Arising From Operation of the Westinghouse Advanced Passive Pressurised Water Reactor (AP1000) Part 1: Main Report", UK Nuclear Decommissioning Authority, NDA Document LL/10897959, January 2010.
[5] R. S. Kemp et al., "The Weapons Potential of High-Assay Low-Enriched Uranium," Science 384, 1071 (2024).
[6] Nuclear Energy Agency, "The Supply of Medical Radioisotopes: 2017 Medical Isotope Supply Review: 99Mo/99mTc Market Demand and Production Capacity Projection 2017-2022," Nuclear Energy Agency, NEA/SEN/HLGMR(2017)2, April 2017.
[7] S. J. Kennedy, "Construction of the Neutron Beam Facility at Australia's OPAL Research Reactor," Physica B 385 - 386, 949 (2006).
[8] W.E. Cummins, "The Advanced Passive AP1000 Nuclear Plant," Westinghouse Electric Company, August 2021.
