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Energy view. Renew your contract. Carbon Reduction Commitment. Business FAQ. Talk Power. Talk Power blogs. Talk Power webinars. Subscribe to Talk Power. About Us. One is residual public fear of graphite-moderated nuclear reactors due to the fact that the Chernobyl nuclear explosion occurred in an RBMK. 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. 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.
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.
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.
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.
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