|Fig. 1: Schematic of a Reaktor Bolshoy Moshchnosti Kanalnyy (Реактор Большой Мощности Канальный), or "High Power Channel-type Reactor," a class of graphite-moderated nuclear reactor designed in the Soviet Union. The RBMK is the oldest commercial reactor design still in operation. (Source: Wikimedia Commons)|
The neutron moderator is one of the core elements of a nuclear power reactor, responsible for slowing the neutrons that are ejected from the nuclear fuel rods during fission. When the nuclear fuel, such as Uranium-235, undergoes fission, it ejects fast-moving neutrons. However, a nucleus must be hit by a slow-moving free neutron, or thermal neutron, in order to be incited to fission. Thus in order to create a nuclear chain reaction, whereby the neutrons ejected from one nucleus incite fusion in other nuclei, the fast-moving neutrons ejected during fusion must be slowed down by a moderator.
As shown in Fig. 1, nuclear power reactors that use graphite moderators, such as those based on the RBMK design, encase the nuclear fuel rods in chambers with graphite walls so that the fast-moving neutrons emitted from one rod are slowed by the graphite moderator before reaching other rods, allowing a chain reaction to spread throughout all the rods. The heat generated from fission then is used to heat water either directly (such as in the RBMK design) or indirectly, by heating gas that travels elsewhere to heat water.
Three different types of materials are commonly used as moderators in nuclear reactors today today: light (regular) water, heavy water (deuterium oxide), and solid graphite. Only around 20% of today's nuclear reactors use graphite as a moderator.  The popularity of graphite as a moderator has declined significantly in the late 20th and 21st centuries, so it is predicted that the percentage of graphite-moderated nuclear reactors will further decline in the next fifty years.  There are several reasons for this decline in popularity. One is residual public fear of graphite-moderated nuclear reactors due to the fact that the Chernobyl nuclear explosion occurred in an RBMK.  This fear is grounded in a key distinction between graphite-moderated reactors and light-water-moderated reactors. If a water-moderated reactor has a loss-of-coolant event, the reactor stops functioning because the water moderator evaporates away, thus ceasing the nuclear chain reaction. In graphite-moderated reactors, however, the moderator has an extremely high heat of sublimation and thus it remains in place through loss-of-coolant events, allowing the nuclear reaction to continue in potentially catastrophic circumstances. For instance, the Three- Mile Island and Fukushima nuclear disasters both occurred in light water-moderated nuclear reactors, so their moderators evaporated quickly after their coolant systems failed. Conversely, the Chernobyl nuclear disaster occurred in a RBMK graphite-moderated nuclear reactor and thus it was able to progress all the way to a full nuclear runaway explosion. 
Another important factor contributing to the decline of graphite-moderated reactors is the fact that in nuclear reactors with graphite moderators, the graphite moderator is almost always the life-limiting component of the reactor.  Irradiation-induced crack propagation (irradiation creep) and stress from extreme heat fluctuation lead to significant loss of structural integrity in the graphite moderator over the lifetime of the nuclear reactor.  In comparison, a water moderator appears more promising because liquid water is not susceptible to the same structural failures as solid graphite.
Another unique problem associated with graphite moderators is the fact that spent fuel contains a large quantity of graphite, which makes these reactors have different disposal needs than water-moderated reactors.  This requires countries to diversify an already complicated and controversial waste disposal process. In order to avoid this complication, many energy providers prefer to use a single kind of moderator. 
Though graphite moderators currently pose more challenges for plant operators than water moderators, this does not render graphite moderators obsolete. Since a nuclear moderator is supposed to only slow neutrons but not absorb them, a nuclear moderator material's effectiveness increases with its ability to scatter neutrons and decreases with its ability to absorb neutrons. Mathematically, this can be expressed as the moderating efficiency, which is given by the material's neutron scattering cross section divided by its neutron absorption cross section.  Graphite's moderating efficiency (1343) is less than heavy water's (8154), but two orders of magnitude greater than light water's (74.24), so graphite theoretically has a high potential to be a much better moderator than light water.  (The majority of today's nuclear reactors use light water as a moderator. ) Solving the problems of graphite moderator degradation would therefore once again make graphite an appealing moderator material that could make reactors much more efficient than light water reactors.
Several opportunities exist to improve graphite's utility as a moderator in modern nuclear reactors. One is increasing oxidation resistance by either diffusing interstitials throughout the graphite structure or treating the outside surface of each section of graphite in the moderator with oxidation-resistant coatings. [4,5] Another is making the graphite sections more structurally resilient to halt dislocations caused by thermal stress and irradiation creep.  Finally, additional areas for fruitful research can be found in creating a simpler system for handling graphite-moderated reactor waste and testing more dependable ways to prevent nuclear meltdowns in graphite-moderated nuclear reactors.  Thus nuclear graphite would benefit from both highly focused materials science research and also broad environmental health and safety research.
Recently, graphite has fallen out of favor as a moderator material for nuclear power reactors. Although graphite is in theory a much better moderator than light water, light water is currently more appealing because graphite moderators are susceptible to degradation, are capable of causing nuclear runaway explosions in loss-of-coolant events, and require more complicated waste management than light water reactors. These challenges present a variety of research opportunities that could allow modern nuclear reactors to utilize graphite's excellent moderating efficiency without suffering the same problems as graphite-moderated nuclear reactors in the past.
© Dylan Sarkisian. 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.
 G. T. Miller and S. Spoonman, Living in the Environment: Concepts, Connections, and Solutions, 16th Ed. (Brooks Cole, 2009).
 B. Raj, et al, "Challenges in Materials Research for Sustainable Nuclear Energy," MRS Bull. 33, 327 (2008).
 A. Strupczewski, "Accident Risks in Nuclear-Power Plants," Appl. Energy 75, 79 (2003).
 C. Tang and J. Guan, "Improvement in Oxidation Resistance of the Nuclear Graphite By Reaction-Coated SiC Coating," J. Nucl. Mater. 224, 103 (1995).
 D. R. De Halas, "Radiation Effects in Graphite," in Nuclear Graphite, ed. by R. E. Nightengale 7: Theory of Radiation Effects in Graphite," (Academic Press, 1962).
 "Technology Roadmap Update for Generation IV Nuclear Energy Systems," Gen IV International Forum, January 2014.
 J. C. Bryan, Introduction to Nuclear Science (CRC Press, 2008), pp. 150-170.