Perhaps one of the simplest and most interesting styles of nuclear reactor is also one of the least used and most forgotten. I am speaking of the Aqueous Homogenous Reactor (AHR), in which uranium salts dissolved in water creates a solution which is both the fuel and the moderator. They are so named because the fuel and the moderator constitute a single phase, as opposed to more traditional heterogeneous reactors where the fuel and moderator are in distinct phases. These are different than Molten Salt Reactors (MSRs), because while those use fuel in a liquid phase, the moderator is still distinct from the fuel, meaning the reaction is still considered heterogeneous.
The AHR had tumultuous beginning. Early experiments before 1944 were met with many problems, including a shortage of heavy water (D2O) with which to moderate natural uranium, erroneous values for the neutron cross-section of D2O, and no enriched uranium available.  When it became clear that enriched uranium would soon be available, Los Alamos and Oak Ridge National Laboratories began AHR program in 1943 and 1944, respectively. In 1944, the Los Alamos reactor, named LOPO, achieved criticality.  It was operational, in modified forms, up until 1974, mainly as a high flux neutron source.  Oak Ridge cancelled their reactor plans in 1945, due to several unresolved issues, chief among them the water dissociation into hydrogen and oxygen.  However, plans resumed in 1949, and in 1950 they began construction on "Homogeneous Reactor Experiment 1," which successfully reached criticality in April of 1952.  They followed that successful experiment with Homogenous Reactor Experiment 2, which was confronted with severe corrosion issues.  Construction on a third reactor was completed in 1956, and cancelled in 1961 after several corrosion and fuel stability issues. 
In a comprehensive compilation of aqueous homogeneous reactor research by Oak Ridge scientists in 1958, one of the last of such documents, several benefits and unresolved issues of AHRs were stated. Here we will go into detail regarding certain of these stated issues, especially concerning how certain issues translate from 1958 to 2012. For the full list of issues and benefits, see source. 
High fuel burnout: A main limiting factor in the extent of burnout for current nuclear reactors is neutron poisoning, by Xenon-135 (a decay product of iodine-135, a fission product) as well other fission products. In practice, nuclear fuel must leave typical reactors due to poisoning long before the fissionable material is gone. In an AHR, the dissolved fission products can be separated out more easily, as part of the fuel loop, keeping it below a desired level. Furthermore, the complications of xenon poisoning being improperly identified and addressed contributed in part to the Chernobyl disaster, so this feature may serve to assuage public fears. 
Simple fuel preparation and reprocessing; Continuous plutonium recovery: What were touted as benefits in 1958 might not be considered benefits in 2012, at least not in all parts of the world. Waste from AHRs is cleaner, not in the sense of less fission products or less radioactive material, but in the sense that they are dissolved in water and are more easily separated than in solid nuclear waste.  For a large scale power plant, this benefit might turn into a proliferation fear. It may result in too much waste for which fissile material is easily separable. However, if the separation is easy enough, it may make sense to separate the waste as a preventative measure, resulting in a smaller amount that must be guarded. It may also be done onsite, continually, as part of the fuel loop. This may allow there to never be a large amount of high-risk fuel in one location
Simple Control: Perhaps one of the most attractive benefits of the AHR is the fact that density changes in the solution make the system self-stabilizing. Regulating rods are not needed, reducing the amount of mechanical parts that may fail.  It is worth noting that issues related to control rods contributed to the incidents at Chernobyl.  Therefore, this benefit may indeed help to assuage public fears. Additional control can be obtained by altering the fuel concentration in the aqueous solution.
External circulation of fuel solution: Purely homogeneous reactors remove heat from the reactor core by circulating the fuel/moderator solution itself through heat exchangers, as opposed to just coolant. This leads to radiation shielding and extra corrosion protection needed on a larger amount of machinery, increasing costs. It also means that fissionable fuel will be travelling larger distances, and will be circulated outside of the reactor core, which may create larger terrorist threats. This threat is more salient today than it was in 1958. Some of these problems could be ameliorated by a proposed two-phase homogeneous reactor, which loops aqueous fuel/moderator mixture in one loop and additional water in another for additional cooling and extra moderation. However, this will not change the issue of fissionable fuel travelling outside the main reactor walls.
Explosive decomposition product: In AHRs, energetic fission products can dissociate the water into hydrogen and oxygen. To me, this is the most interesting of the issues stated in the Oak Ridge report. One could make the argument that this is either more or less of a drawback now than it was in 1958. Since then, hydrogen generation was part of the problem at the Three Mile Island incident in 1979, and is suspected to have been part of the problem at the Fukushima incident in 2010. [5,6] However, hydrogen production is inherently different in AHRs than in traditional heterogeneous reactors. In such reactors, hydrogen production is thought to be caused by water reacting with reactor walls.  This reaction only occurs at extremely high temperatures that are already indicative of an emergency. Furthermore, hydrogen production is not intended or expected, and therefore these reactors may be less prepared to deal with it. In contrast, AHRs produce hydrogen under normal operating conditions, and therefore will already have robust means of accounting for it. One such method is the inclusion of catalysts which recombine the dissociation products back into water, such as cupric ion.  However, this brings us to an additional point, which is that AHRs produce not only hydrogen, but also the other dissociation product, oxygen. This means that this gas produced is worse than pure hydrogen, because it is already explosive.
Despite the fact that recent accidents may render this a damning feature, it can also be thought of as a benefit. Though talk of the hydrogen economy has slowed lately, hydrogen is still a very desirable, clean fuel. Since most hydrogen is generated from coal reformation, in effect largely reducing its cleanliness, cleaner methods of hydrogen generation are continually being sought. Indeed, AHRs have been researched as a method of hydrogen generation, however information on such research is scant, and it appears to have largely ceased since 1976. [7,8] Another option for decomposition products is to separate them (which is easy since they are in gas form) and then combust them to generate more heat. This would increase the overall efficiency of the plant because it harnesses some of the energy of the energetic fission products. Combustion so close to a nuclear reactor might generate more fears, however, even though it may well be the safest solution for preventing too much explosive mixture from building up.
It has been said that nothing is as powerful as an idea whose time has come. The Aqueous Homogeneous Reactor is a wonderful idea, but the 1940s and 50s were not its time. With public fear about nuclear reactor safety as well as nuclear proliferation at all time highs, the 2010s and 20s may not be its time, either. However, the AHR has very salient benefits, and its merit should be reinvestigated if the nuclear climate changes.
© Matt Tilghman. 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.
 J. A. Lane, H. G. MacPherson and F. Maslan, eds., Fluid Fuel Reactors (Addison-Welsey, 1958).
 M. E. Bunker, "Early Fuel Reactors: From Fermi's Water Boiler to Novel Power Prototypes," Los Alamos Science, Winter/Spring 1983, p. 121.
 M. Rosenthal, "An Account of Oak Ridge National Laboratory's Thirteen Nuclear Reactors," Oak Ridge National Laboratory, ORNL/TM-2009/181, August 2009.
 J. F. Ahearne, "Nuclear Power After Chernobyl," Science 236, 673 (1987).
 S. Gordon, K. H. Schmidt and J. R. Honekamp, "Generation of Hydrogen During the First Three Hours of the Three Mile Island Accident," Radiat. Phys. Chem. 21 247 (1983).
 R. Stone, "Fukushima Cleanup Will Be Drawn Out and Costly," Science 331, 1507 (2011).
 W. Kerr and D. P. Majumdar, "Aqueous Homogeneous Reactor for Hydrogen Production," in Hydrogen Energy: Proceedings of the Hydrogen Economy Miami Energy (THEME) conference, ed. by T. N. Veziroglu (Plenum Press, 1975), p. 167.
 D. P. Majumdar, H. Reyes and W. Kerr, "The Aqueous Homogeneous Reactor as a Source of Hydrogen and of Process Heat," in 1st World Hydrogen Energy Conference, Conference Proceedings (U. of Miami, 1976), p. 1A-81.