Possible Applications of Modern Aqueous Homogeneous Reactors

Abstract

This chapter describes the potential of the aqueous homogeneous reactor, briefing readers on the physics and history of the subject, whilst providing both current and possible future applications for this reactor technology. These reactors were some of the first nuclear reactors ever constructed, and provided valuable information on critical mass and other nuclear physical properties on fissile solutions. The compact nature of these reactors, combined with their inherent safety characteristics, have made them attractive for the generation of medical radioisotopes and neutrons for experimentation. However, material corrosion issues and advanced development of solid-fuelled light water reactors would curtail much interest in the technology in the 50’s. Although operating temperatures of this type of reactor are usually low, even such low temperature heat is useful in process and industry; such a reactor can be used for environmentally-friendly district heating or the supply of process heat in industry, and could even be used to produce hydrogen. With modern advances in physics and chemistry, and disruptions in conventional energy sources; such reactors in their modern form may serve an important role: supplying various energy demands that could be derived from nuclear power, but may not require more advanced and costly reactor technologies.

1. Introduction

As we face an increasing issue of energy scarcity in light of a global energy crisis [1], nuclear energy is once again gaining attention as a way to reduce our reliance on fossil fuels and the often questionable sources that supply them [2]. Interest in nuclear energy is some of the highest it has ever been, and has seen many nations such as Bangladesh and Egypt build nuclear power plants for the first time, or others like Japan, return to the construction of new reactors after a long hiatus on construction [3].

However, research and development in nuclear energy can be said to be progressing at a slower pace relative to other technologies. Many kinds of fission reactors have been developed since the first was assembled under university football bleachers in 1942, but only a few types are in commercial use today; most using solid fuel with some form of water as coolant and moderator. These solid-fueled reactors have performed well in their service as power generators, proving themselves through tens of thousands of reactor-hours of operational experience, but are often large, complex machines that have become increasingly difficult to finance and construct and are rather inflexible in their siting and energy output [4].

It is important to note that not every application of nuclear energy should be handled by a select few or even a singular reactor type(s), nor is it possible to do so in many cases. As the uses of nuclear energy expand, the flexibility in design of nuclear reactors should also increase. For example, high-temperature reactors are being developed to replace coal-fired boilers in-situ [5], and fast-spectrum reactors are being explored as a way to burn up plutonium and other minor actinides as part of closed-cycle nuclear waste management programs [6].

Amongst the flurry of hypothetical fission reactor designs that were being considered for development in the backdrop of the Manhattan Project, there is one that holds great promise for modern applications — but has been side-lined and underdeveloped. In 1944, the brilliant Enrico Fermi decided to construct the first of a set of reactors at Los Alamos that would use liquid fuel mixed homogeneously with water as the moderator (wartime secrecy stipulated that they be called “water boilers”) [7]; it was a little spherical reactor, contained in a stainless steel vessel no more than a foot across, filled with a solution of uranyl sulfate and light water, reflected by beryllium and graphite, and called LOPO (for low power) as it produced almost no energy in operation.

LOPO reached criticality in May of 1944 and was instrumental in determining the critical mass of uranium solutions. As experiments with LOPO concluded, it was disassembled later that year to make way for a higher-power solution reactor: HYPO [7]. HYPO was a larger reactor with cooling provisions that allowed it to operate at a higher power of 5.5 kW and provide a stronger neutron flux for experiments. HYPO was brought critical in December of 1944 and would be the key for the neutronics measurements needed to design the nuclear fission assemblies for the first atomic weapons.

HYPO itself would be, in turn, upgraded to SUPO (Figure 1) in March of 1951 with improved cooling and an increased neutron flux in sustained operations up to 35 kW of power [7]. SUPO would go on to operate until 1974, and the whole water boiler program would provide critical information that would be useful for the construction of other aqueous homogeneous reactors to come.

Figure 1.

SUPO reactor vessel without its graphite reflector in place. Public domain.

After the success at Los Alamos, engineers at Oak Ridge set out in 1952 to construct a far more powerful solution reactor under the auspices of the Homogeneous Reactor Experiment (HRE) [8]. This reactor, the HRE-1 (Figure 2), was much larger than previously built, with a power level of 1 MW thermal. The HRE-1 had an output temperature of 250 degrees, and was also coupled to a steam turbine, demonstrating an ability to produce 140 kW of electric power from its heat (Table 1). The HRE-1 used a novel control system utilizing magnetically coupled neutron absorbing plates between its core and reflector, dispensing the need for traditional control rods.

Figure 2.

HRE-1 internal details. ORNL drawing D-9065A. Public domain.

Thermal power	1000 kW (1600 kW max)
Fuel	93% enriched UO2SO4 in H2O
Fuel inlet temperature	210°C
Fuel outlet temperature	250 °C
Core working pressure	6.9 MPa
Reflector	254 mm D2O in pressure vessel, ⌀ 1067 mm, forged steel
Core vessel	⌀ 457 mm, type 347 stainless

Table 1.

HRE-1 design parameters.Adapted from Murray [8].

The HRE-1 was the first solution reactor that was built with the goal of extracting power in mind and would go on to conduct experiments regarding control and power output. During such experiments, it was further confirmed that AHRs possessed a very strong negative thermal reactivity coefficient [8], owing to the effect of voiding in the solution at higher-than-designed power levels and subsequently reduced moderation; something originally discovered during an accidental reactivity incursion with SUPO.

These aqueous homogeneous reactors (AHRs) usually have a simple construction relative to solid-fuelled reactors, often consisting of simple tank-like structures, and requiring far less fuel fabrication (a very expensive and complex task in itself) as the fuel is not held in rods, but kept in either a solution or a slurry [9]. This design makes the possibility of meltdown with reactors of this type highly unlikely, and reduces the volume of active coolant, possibly making containment structures more compact and economical.

They have found themselves today as convenient sources of neutrons and fission products, and are often used today to produce radioisotopes for medical and industrial uses [9], as processing of the liquid fuel solution can be done on-line to continuously extract target isotopes before they decay, or done after batches of short reactor runs. However, there are other characteristics of these reactors that could also make them attractive for a wider range of applications.

The thermal energy of the AHR may come in useful in applications where the complexity and cost of high-temperature reactors is not needed, such as for district heating and seawater desalination. With a high fuel burn-up and simple control stratagem, such reactors could be used for long periods of time, almost — if not entirely unattended, as clean and sustainable industrial and commercial heat sources, much like a traditional fuel-fired hot water heater or boiler. All the while, the aforementioned problem of hydrogen arising from radiolysis may actually be a blessing in an era where we are starting to use hydrogen as a fuel itself but find it difficult to produce without fossil fuels.

2. AHRs for the supply of heat

While the limited output temperature of the AHR makes it unsuitable for power generation, this low-grade heat is still usable for other industrial and commercial uses. A plethora of applications that currently utilize process steam could possibly be made nuclear using the AHR at a low capital cost, and replace combustion-fired boilers and heaters at many industrial sites.

The output temperatures of AHRs are limited partly by the high amounts of radiolysis encountered in operation; this, combined with heat, causes dissociation and precipitation of the solution at higher temperatures. These effects limit the output temperatures of AHRs to around 300°C, lower than the output temperature of modern PWRs and far lower than that of gas cooled designs, such as the 1200°C of the output of the HTR-PM [5].

However, with higher temperatures come higher pressures and stresses, requiring stronger, heavier, and more expensive structures to deal with such heat, especially those that hold water under pressure. As most AHR designs are not designed for two-phase flows (boiling) in the main reactor, this would necessitate a pressure vessel strong enough to keep the water from boiling at the reactor’s operating temperature.

Such a pressure vessel could be selected for specific operating conditions and built accordingly, with lower temperature reactors requiring thinner vessel walls and lighter ancillaries compared to higher temperature reactors. This could make lower temperature reactors simpler and more economical to design and construct. Reactors could even be designed for near-atmospheric pressures if temperatures under 100°C are needed.

Low-complexity reactors that have been designed for such low-temperature applications are not a novel idea. For example, the SLOWPOKE Energy System was conceived by the AECL in Canada, and was a pool-type reactor with an output temperature of no more than 100°C. The low temperature allowed for non-pressurized construction and an increased margin of safety, obviating the need for operators; extensive automation was a goal of the design [10].

In the Czech Republic, the TEPLATOR project proposes to use spent PWR fuel bundles in a tank for the express supply of district heat at 98°C, and although it is not a homogenous design, and runs on spent fuel, the low capital cost of constructing such simple reactors may allow TEPLATOR to deliver heat at prices significantly lower than with fossil fuels [11]. A purpose-built AHR for such a purpose could, in theory, be able to do the same, if not more effectively.

Such heat from a low-cost and low-complexity source could enable greater use of nuclear power in seawater desalination and district heating. Contemporary multi-stage flash distillation (MSFD) processes can usually only work with temperatures under 120°C, while multiple effect distillation (MED) processes can only work with temperatures up to 70°C before scale formation becomes an issue [12]. Although nuclear desalination is a tried and tested concept, most implementations have used steam or hot water extracted from the turbines or condensers of power-producing reactors, increasing implementation costs. An AHR could be used solely as a heat source, and be designed for lower temperatures, potentially lowering implementation costs and making desalination a more accessible endeavor.

District heating has historically used network temperatures upwards of 200°C, but modern “fourth generation” systems are trying to bring this down to as low as 70°C [13] in a bid to capture more waste heat from various sources. As a compact and inherently safe design, AHRs could be sited very close to the built-up areas where district heating networks exist, lowering transmission losses and decreasing the need for expensive network piping. The heat could also be used to facilitate district cooling with the use of absorption chillers, allowing for such a plant to operate all-year round in temperate climates by supplying customers with chilled water.

Although direct production of steam is possible within the AHR itself (much like a BWR) [8] it is impractical due to the presence of fission products in the steam. Because of this, most schemes to extract heat from AHRs would have to employ the use of intermediate heat exchangers and/or steam generators. Such a heat exchange loop would probably be of the hi-lo-hi type with high isolation loop pressure that is seen in nuclear desalination applications to preclude the possibility of contaminating network fluids with reactor solution [14].

3. AHRs for hydrogen production

The rapidly developing hydrogen economy currently relies on natural gas reforming to produce the vast majority of the substance, but has been transitioning to use sustainable sources of energy to lessen its reliance on fossil fuels. The use of hydrogen from nuclear sources would greatly help this effort. Most other schemes to produce hydrogen with nuclear energy involve either high temperature reactors using thermochemical cycles, or the use of electrolysis from nuclear electricity [15]. Such schemes involve equipment ancillary to the reactor itself, and require diversion of energy, reducing the power output of the associated plant in cogeneration. Such capital and running costs make nuclear hydrogen production difficult to viably implement at present.

However, the AHR is able to produce it directly as a by-product, potentially at a lower cost than other nuclear-based options, and would provide heat that could be useful for other parts of the hydrogen production process. As the very ions of uranium fuel in the AHR are in direct contact with the water as fission occurs, the intense energy from the fission, fission fragments and other energetic particles cause the water to disassociate to its constituents: hydrogen and oxygen [16]. This radiation-driven process is called radiolysis, and is responsible for the large amounts of hydrogen produced by the AHR. However, combined with the oxygen produced, this potentially explosive atmosphere would constitute a hazard inside the reactor and necessitates its constant recombination and removal.

Soluble ionic catalysts were proposed and tested to reduce this production, with copper shown to be effective in virtually stopping all production with the HRT [8]. But the hydrogen created by the reaction may be a far more valuable commodity going forward. World hydrogen demand is projected to grow 5 to 7 times larger than it was in 2021 by 2050, with hydrogen making up 15–20% of all energy demands [17].

As hydrogen is known to cause embrittlement issues in many materials, and creates an explosive atmosphere, the construction of such a reactor would require the use of novel techniques for extraction of the hydrogen and mitigation of the associated corrosion. Engineers at Los Alamos used mechanical gas separators that would use swirl-vanes to centrifuge out the gas in the HRT and send it to recombiners [18]. It should be noted, however, that hydrogen production in the AHR requires moderately high temperatures, and it would be inefficient to use AHRs to solely produce hydrogen [19]. Such hydrogen producing AHRs would inevitably have to be heating or power reactors, where the production of hydrogen could be seen as a bonus product or be used as a process gas alongside its associated nuclear process heat in industrial settings.

4. AHRs for the supply of power

As demonstrated with the HRE, production of power using steam turbines is possible with the AHR. By running the core solution through a steam generator, the HRE-1 was able to produce almost 1 MW of steam at 250°, giving its 140 kW power plant an efficiency of 14%. Later, the HRT increased this temperature to 300°, but had issues with the solution dissociating; creating hot spots and power fluctuations. These issues were caused by phase instabilities of U2SO4 in water at temperatures above 340° [8], and limit the efficiency of such a setup. Increasing the temperature beyond this also increases the corrosion of reactor internals greatly. Other experiments with different acid-based chemistries were able to increase the operating temperature to 450° [16], but high rates of corrosion make such exotic chemistries impractical even today.

Due to these corrosion issues limiting the output temperature, an AHR would not be the best choice of reactor for power production, but nonetheless, could be a simple way to implement nuclear power in mechanical or electrical applications. The design and construction advantages could be useful in applications where the increased fuel costs from the inefficiencies of the reactor plant do not warrant the need for a more efficient, yet, more capital intensive type of reactor. Such applications could include powering remote towns and facilities, or powering coastal and riverine vessels as part of a nuclear propulsion scheme.

An AHR could also be coupled to a stirling engine, eliminating the need for a traditional steam plant and associated steam generators, potentially reducing implementation costs in certain scenarios. Another option would involve using an organic rankine cycle (ORC), as temperatures below 350° make the use of steam rankine cycles particularly inefficient. ORC systems use an organic working fluid, such as pentane or R134a to achieve higher efficiencies at lower temperatures, and have seen use for capturing waste or low-temperature heat from industrial processes and renewable energies [20]. Such alternative cycles would make the power production from AHRs a more efficient undertaking in the present day.

Given its placid control characteristics, an AHR power plant could be operated autonomously, only needing personnel on-site for periodic maintenance and refueling. This characteristic, combined with the potential for cogeneration with heat, potentially makes such plants particularly attractive for arctic areas, where keeping personnel around is a challenge in itself.

If further drops in implementation costs are desired, the AHR could be used as a two-phase reactor, and include boiling in the core [16]. This would eliminate the need for steam generators, but would also expose the steam plant to fission products, contaminating it in the process. Such a design would also be problematic for the supply of heat, as it would have to be extracted from either the condenser, or off of the turbine stages, and then sent through a heat exchanger to render it safe for use.

5. AHRs as radioisotope sources

Owing to the compact size and intense neutron flux provided by the AHR, it makes it an excellent producer of radioisotopes from various sources [use the IAEA book on Mo-99 production for this section]. The solution chemistry of the AHR allows it to burn-up uranium almost completely and simultaneously remove fission products — both poisons and potential commodities, such as Xe-135 or Mo-99, for use as industrial and commercial sources of radiation.

The extraction of Mo-99 in particular from AHRs is of great interest. Mo-99 is an essential radioisotope for the medical industry, as it is used to produce Tc-99m for use in medical imaging as a tracer [21]. Mo-99 however, has a very short half-life of 66 h, which complicates its production and transportation. The use of an AHR would allow for the use of on-line processing to continuously produce Mo-99 whenever needed. By use of chemical extraction methods, the Mo-99 condensate can be collected from the reactor loop without removing the fissile fuel. This method would also eliminate the amount of waste that would be generated when using a solid uranium target, of which the uranium is only 0.4% spent, and the processing of which releases fission products and unused uranium into waste streams [22].

Other isotopes like Sr-89 are often also difficult to produce using solid target-based systems. With the AHR, it is possible to create Sr-89 without the associated Sr-90 impurity by using extraction of the precursor gasses. At the ARGUS reactor at the Kurchatov Institute, Sr-89 of high purity is extracted by running the reactor for a few minutes at a time to generate the needed krypton precursors, then waiting for the shorter-lived Sr-90 precursor, Kr-90 (half-life of 33 s), to decay out faster than the relatively long-lived Sr-89 precursor, Kr-89 [22].

This Kr-89 is then pushed out of the reactor into a sorbent bed with the help of inert gasses, where it decays to produce Sr-89 and awaits further processing to remove other fission products. After purification, the Sr-90 content of the Sr. recovered is insignificant. Although running such a reactor for the sole purpose of extracting Sr-89 would be impractical, it is something that can be done alongside the production of Mo-99 and other radioisotopes.

The production of other useful isotopes such as Xe-133 — another useful medical tracer, I-131 — a radiotherapy agent, and Cs-137 — a potent gamma emitter with many industrial uses, is also possible, and was demonstrated with ARGUS [22]. If other radioisotopes needed to be produced in solid form, an AHR could be designed with provisions for the insertion of targets. If, for example, Co-60 needed to be produced in rod form, the reactor could be designed to accommodate for irradiation of such rods within its core.

Other radioisotopes of interest may be the Pu-239 or U-233 generated during fission by transmutation in uranium and thorium powered reactors. By using a breeder design with a “blanket” solution around the core, lost neutrons could be absorbed by fertile material such as U-238 or Th-232, rendering it fissile [16]. Such a system could also be adapted, in theory, to produce H-3 (tritium) from either H-2 or Li-6 for use in many applications.

6. Conclusions

The unique design features of the aqueous homogeneous reactor make them a valuable tool in increasing the uses of nuclear power. The simple design and control characteristics of AHRs could make them very useful wherever heat and/or steam is required, but the complexities of solid-fuelled reactors are unwarranted and unwanted. Moreso, if hydrogen needs to be produced, the AHR is capable of doing so directly. Various radioisotopes or fertile fuels could be extracted from such reactors as a value-added product during their normal operations, or be produced by reactors exclusively designed to do so.

Industrial and commercial users could benefit from a heat source that has the potential to be far cheaper than fossil fuels, yet cause far less environmental harm. This will be especially important in an era where governments worldwide are trying to wean themselves off of fossil fuels, which supply over 50% of the world’s heating needs [23]. AHRs can supply heating to entire district heating networks, or be made small enough to supply individual buildings. Cooling can also be provided with the use of absorption chillers, and if done so while generating electricity, could comprise an economical CHP (combined heat and power) or CCHP (combined cooling, heat and power) scheme.

Thermal energy from the AHR could also be used for nuclear desalination purposes, where the energy costs of the process heavily determine the price of the purified water produced. Low-cost nuclear desalination could help arid areas of the world gain access to clean water without the associated fossil fuel needs or pollution. An AHR built expressly for this purpose has the potential to be far more economical than using power reactors or combustion boilers, as its material, logistic and safety requirements are greatly relaxed.

The production of hydrogen in the AHR is something that would usually comprise a problem, but with clever extraction techniques, this annoyance may turn out to be a lucrative opportunity in light of the growth in the hydrogen economy. Of course, if the complexities arising from hydrogen production in such a reactor are found to be a concern, catalysts may be added to the solution to facilitate complete recombination of these radiolysis products. But, as very little extra equipment is required for its extraction, the AHR could easily produce hydrogen as a value-added product for sale, or for use in industrial processes as feedstock or fuel. As the use of hydrogen as a fuel grows worldwide, the AHR finds itself uniquely suitable for the production of the gas without emissions or extra costs.

The production of radioisotopes would be another value-added product that could be produced in heating reactors, or produced by reactors purposely built for such a purpose. The solution chemistry of AHRs allow for the extraction of a wide range of radioisotopes without the production of large volumes of waste as seen with traditional target-based systems, and permits the extraction of certain radioisotopes that would otherwise be impractical to do with traditional systems, such as Sr-89. This same chemistry allows AHRs to achieve a high fuel burn-up, as well as a high breeding ratio, potentially allowing AHRs to operate as breeder reactors.

Although the output temperatures of AHRs are limited, they can still be used to generate power as demonstrated before (albeit, with lower efficiency than higher-temperature reactors). An AHR could provide just enough electricity through a Rankine or Stirling cycle to power itself and associated facilities, or generate enough to power other loads, such as industrial, commercial, and residential consumers of electricity. If done alongside thermal and hydrogen production, this could make the AHR a very flexible reactor type in cogeneration. This energy could also be used for propulsion at sea and inland waterways, where such reactors may enable economically-built nuclear-powered vessels that would require little justification for a more complex reactor type, such as tug boats or coastal bulk freighters.

As the world increasingly becomes more conscious of the deleterious effects of fossil fuel consumption and strives to move away from fossil fuels altogether, nuclear energy will be a vital tool in facilitating its replacement. In doing so, many different designs for nuclear reactors will be needed for the different applications they will be optimal for. The AHR has the potential to economically decarbonize industries and sectors that rely on low-temperature heat, or supply clean electric power. Hydrogen created from the regular operation of such reactors may be harnessed and utilized as a fuel, or be used for other industrial processes. These reactors are also optimal for the production of radioisotopes — especially those for medical uses, and could be made compact enough to be installed near or at the facilities that need them on a regular basis. Using modern technologies, it is not beyond the realm of possibility that the AHR can be deployed for commercial use in the near future. With its inherently safe and simple features bolstered by the use of our advanced tools and knowledge, the AHR is a very promising design looking forward.

Acknowledgments

Special thanks to Bianca, whose hospitality was instrumental in the creation of this chapter. I owe her my utmost gratitude.