|Fig. 1: Lead-Cooled Fast Reactor. (Source: Wikimedia Commons)|
The future of nuclear power plants is promising, despite the number of blemishes that exist on the historical timeline of reactor operation. In 2000, ten countries began collaborating in a united effort to address the research and development required to move nuclear energy in a safe and sustainable direction. This includes Argentina, Brazil, Canada, France, Japan, the Republic of Korea, the Republic of South Africa, Switzerland, the United Kingdom, and the United States. This organization is called the Generation IV International Forum (GIF).  The group has outlined a list of key facets that include efficient use of nuclear fuel and limited high-energy waste production, safety of plant operation, and protection against weapons production. In order to address these issues, as well as providing a high quality product of electric power and/or process heat, a number of strategies are being considered. These strategies have culminated in a selection of six different plant designs, each having its own set of advantages and challenges. There are a number of research and development areas that require understanding in order to progress towards the manufacture and demonstration of such architectures. The deployment of these systems is projected to be around 2030. 
In order to digest the information that the GIF organization has put forth, the various distinct nuclear reactor design aspects are discussed in the following sections. These general features are common for all nuclear reactors, including those in operation today. Specific mention is made of features that will be unique to next generation nuclear plants.
It is the sustainment of fission reactions that convert stored nucleon bond energy within a molecule to a form that may be transferred and ultimately drive a power cycle. When a fission reaction is initiated by a colliding neutron, this will produce daughter particles as well as multiple high energy neutrons. These daughter particles may in turn be fissionable material, or not. The number of neutrons released per fission reaction is greater than one. Hence, nuclear chain reactions are exponential in growth. 
Initially emitted neutrons from uranium 235 (the enriched, readily fissionable isotope of uranium) have an amount of kinetic energy on the order of a few MeV.  These neutrons are referred to as being "fast." As they move about within the reactor they may collide with anything, including nearby fuel material, coolant species, moderator species, control rod material or eventually reactor shielding material. If they collide and scatter (i.e. are not absorbed) with enough of the correct type of partners, then the neutrons are capable of becoming "thermalized," where their energy is on the order of a few meV (nine orders of magnitude smaller than fast neutrons).  Clearly there is a range of neutron energies that can exist in between. However, the design specifications for next generation reactors fall into two ranges, either being fast or thermal.
The nominal energy of the neutron population is specifically related to how they interact with the remaining nuclear fuel itself. Fast neutrons lose kinetic energy slowly if they primarily scatter off of heavy fuel particles (e.g. uranium and plutonium). A cross section is a way of quantifying the probability of a particular type of event to happen when a neutron collides with another atom. It has been observed that the cross section for neutron interactions vary considerably with neutron speed, typically with an inverse relationship.  As the neutron slows down in small increments, it becomes increasingly probable that it will be captured and consumed by certain fuel species, for instance the more abundant and fertile U-238 isotope. This will in turn create a fissionable Pu-239 species. This plutonium is now a fuel particle that has been created, a new fissionable target for the neutrons that will contribute to the overall "fast" neutron population.  This is distinctly different from the process that occurs with a thermal neutron spectrum.
A thermal neutron spectrum is brought about by means of collisions with partners that are capable of exchanging significant amounts of momentum. More specifically, the neutron kinetic energy becomes on the order of kBT, where kB is Boltzmann's constant (8.62 × 10-5 eV/K) and T is room temperature (~300K), which gives a neutron energy of ~2.6 meV.  This is why water (having two proton nuclei in the hydrogen, which are essentially the same mass as the neutron) is a very suitable moderator species. Scattering collisions of neutrons with the protons decrease the neutron speeds rather quickly. In this case, the range of speeds where the neutron is likely to be captured by the plentiful U-238 atoms is averted. Once within the slow energy range, the probability for these neutrons to cause fission reactions of the U-235 species increases significantly.  This is the process designed into typical existing water reactors. However, water is not the only fluid that can be used to moderate the speed of the neutron population.
|Fig. 2: Gas-Cooled Fast Reactor Diagram. (Source: Wikimedia Commons)|
Given the discussion above regarding neutron spectrums, it is clear that a moderator species is only used in the case where a thermal neutron spectrum is desired. A common species is water because of its ability to effectively exchange momentum. Existing Pressure Water Reactors (PWR) are designed to operate such that the water remains in the liquid phase, albeit under high pressure.  A slight modification to this is the Boiling Water Rector (BWR) which is designed to operate by allowing the liquid water to turn into steam. This can have a particular set of challenges with regards to the two phase issue, where now there exist low density steam pockets (i.e. bubbles) within the dense liquid water.  If the neutrons are expected to thermalize by colliding with water molecules, then these density variations must be understood well. Another option exists for the state of the water, and that is to allow it to completely vaporize. And if the temperature and pressure are sufficient, the water can become supercritical. This phase of water has some attractive qualities that are related to its ability to drive a power cycle efficiently. However, it is also extremely corrosive, and care must be taken with material choices with which it interacts.
Another moderator species that is being included in the future reactor design portfolio is carbon graphite.  This is a solid material, and thus has a spatial density that is well determined, and it has a decent capability of thermalizing neutrons. As a comparison with light water: given the mass of a neutron and that of a proton, it requires about 27 collisions to thermalize a neutron in water. Carbon is 12 times more massive than hydrogen, and requires roughly 119 collisions to thermalize a neutron.  However, graphite is about twice as dense as water. It also has a neutron absorption cross section (i.e. probability that the neutron will be eliminated from the participating population) that is two orders of magnitude smaller than that of a hydrogen atom.  Thus, it is understandable that graphite is a good moderator choice.
The coolant fluid is that which carries thermal energy away from the nuclear reactor core and transfers it to a power or chemical process cycle. In existing nuclear power plants, this is typically a Rankine steam cycle. 
Water is a common coolant fluid in currently operating nuclear power plants. In other words, in addition to acting as a neutron energy moderator, it also transfers thermal energy to a power cycle. In fact, these two properties are coupled: it is the thermalizing collisions with the neutrons that heat up the water, making it an attractive energy resource. The availability of this resource, in a thermodynamic sense, increases with the enthalpy of that water. Thus it is clear that creating hot water vapor or using supercritical water has important energy efficiency consequences.
Another coolant species that is being considered in future nuclear reactors is helium gas.  This has some nice features: for one, the helium-4 isotope (the most abundant form) has a negligible neutron absorption cross section.  This means that the gas will not become appreciably radioactive. Thus, it may be used within the power cycle itself, as opposed to heat exchanging with another working fluid, as is the case with water reactors. Secondly, the fluid remains in the gas phase so there are no sharp density changes to worry about like that associated with boiling water. The challenge in using helium will be with regards to maintaining a positive seal about the reactor vessel and through its flow paths.
Lastly, another type of coolant may be used that can be classified as dense, high boiling point liquids. Examples being discussed for future reactors include liquid metals as well as molten salts. The benefit is again in having a single phase of fluid due to the fact that these liquids have very high boiling points (e.g. typically greater than 1000 degrees C). 
In addition to the radioactive fuel, coolant fluid, and a possible moderator, there is also an additional material used for neutron population control. Control rods are made of solid materials, typically metals such as Boron or Cadmium, which have large neutron absorption cross sections.  They are used as an active control mechanism for keeping the neutron population at just the right amount, and otherwise handling load control for the power output of the plant itself. Without further mention, they are incorporated into the design of all next generation reactors.
Nuclear fuel cycles fall into two categories: open and closed. In an open cycle, the provided fuel has some level of U-235 enrichment. Once this amount decays through the control of a specific neutron population and sustained fission reactions, at some point the fuel is deemed "spent" and it is treated as nuclear waste. It is the handling of this nuclear waste, and finding appropriate storage sites in particular, that continue to be both politically and socially controversial. 
The closed fuel cycle, on the other hand, attempts to make better use of the nuclear fuel. After some level of depletion of the fissile material, the fuel is reprocessed whereby the existing fissile material (namely uranium and plutonium) is separated out from the neutron capturing material (that which works against sustaining a neutron population within the reactor). This can then be sent back into the reactor core for further operation. This process can be efficient in terms of making more use out of the nuclear fuel resource. However, there are issues with regards to weapons production and nuclear proliferation due to the separation of plutonium. 
Now that the basic features of nuclear reactors have been discussed, a brief overview of the proposed future generation of plants will be presented.
This design is based around having the fuel homogeneously mixed within a molten fluoride salt liquid. The molten salt will have a high boiling temperature and thus the reactor coolant will remain in the liquid phase.  The fuel cycle proposed is closed. This will be supported by a flexible design that can accommodate both fast and thermal neutrons in the reactor.  This is very unique, manifested in the fact that the radioactive material is in solution. It is worth noting that fluorine is only slightly heavier than carbon, and has a small neutron absorption cross section (compared to hydrogen).  Thus in a relatively dense liquid form, it is conceivable that either a fast or a thermal neutron spectrum could exist. The heated salt will then be cycled through a heat exchanger and transfer thermal energy to another working fluid for a power cycle. An additional benefit of this plant design is that it only needs to operate at near ambient pressure conditions. 
This reactor design uses flowing liquid metal lead, which heat exchanges thermal energy to another working fluid to drive the power cycle. Again, a benefit is the single phase fluid with a high boiling temperature, which does not require high pressure operation. A closed fuel cycle is proposed, and as such it features a fast neutron spectrum with no moderator present. 
As the name implies, this design uses sodium, here in liquid form, as the coolant. It will be important to seal the reactor vessel well as sodium reacts readily with oxygen and water. The fuel core will be in a typical solid form (as opposed to being dissolved in the liquid coolant with the Molten Salt Reactor). The design is to accommodate a fast neutron spectrum, which again is aimed at utilizing the fuel more completely with a closed fuel cycle. The primary sodium coolant will be heat exchanged to another fluid that will link to a power cycle. There is no moderator material in this reactor design, being a fast neutron population. 
This reactor design operates nominally with water in he supercritical state, which is attained at a pressure above 22 MPa and a temperature of 647 K.  Since its density is lower than that of water, it has the option of being operated with either a fast or thermal neutron spectrum. A closed fuel cycle is proposed in order to facilitate a more complete burn of the common, fertile uranium isotope, U-238. The supercritical water would then heat exchange to run a Rankine steam cycle. Due to the high enthalpy of supercritical water, with reactor outlet temperatures around 600 degrees C, high thermodynamic efficiencies are achievable.
Helium gas would act as the coolant species for this design. One of the goals here is to provide a very high exit temperature which could be used flexibly, either driving a power cycle for electricity generation or to be used as process heat in a chemical process, namely for the production of hydrogen gas.  The fuel cycle proposed is open (i.e. once-through), although the GIF specifies a desire for using uranium enriched with relatively low amounts of the fissionable isotope, U-235. A thermal neutron spectrum is achieved with a graphite structure moderator. The shape of the fuel itself is described as either a pebble bed (stacks of spherical fuel units), or prismatic blocks. In order to attain the desired high temperatures, the reactor vessel will have to be built to withstand very high gas pressures. 
As described by the name, this design calls for a fast neutron spectrum with a gas coolant. The gas of choice is helium, and the fuel cycle will be closed. Since the coolant is a gas, a high pressure vessel will have to be made to support operation. However, since helium does not become radioactive, it can be used as a working fluid within the turbo machinery itself. Additionally, because of the coolant's achievable high temperatures, it offers the benefit of higher thermodynamic thermal efficiency in the power cycle, as well as the capability to deliver process heat for other chemical processes, namely hydrogen production. 
A number of the aforementioned reactor designs allow for either a modular or scalable approach. Consequently, a broad range of power requirements may be met. According to the GIF, the broadest range of outputs will be achievable by the Sodium-Cooled Fast Reactor providing anywhere from 30 MW to 2 GW of electric power. Most other plant designs expect a range of a few hundred to a thousand MW of power. The efficiency of such plants will vary, mostly as a function of the outlet temperature achievable by the coolant fluid. 
The Generation IV International Forum indicates that they are making strong efforts towards being safe with protection against weapons material being generated. In so much as most of the proposed designs include fast-neutron, high burn-up reactors, it is clear that trying to be efficient with the nuclear resource is also of high importance. In doing so, however, there are challenges with unsafe plutonium production and separation. The GIF remarks,
"The advanced separations technologies for Generation IV systems are designed to avoid the separation of plutonium and incorporate other features to enhance proliferation resistance and provide effective safeguards. In particular, all Generation IV systems employing recycle avoid separation of plutonium from other actinides and incorporate additional features to reduce the accessibility and weapons attractiveness of materials at every stage of the fuel cycle." 
© Greg Roberts. 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.
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