3 Matching Annotations
  1. Jun 2019
    1. In the Chernobyl disaster, the moderator was not responsible for the primary event. Instead, a massive power excursion during a mishandled test caused the catastrophic failure of the reactor vessel and a near-total loss of coolant supply. The result was that the fuel rods rapidly melted and flowed together while in an extremely-high-power state, causing a small portion of the core to reach a state of runaway prompt criticality and leading to a massive energy release,[22] resulting in the explosion of the reactor core and the destruction of the reactor building. The massive energy release during the primary event superheated the graphite moderator, and the disruption of the reactor vessel and building allowed the superheated graphite to come into contact with atmospheric oxygen. As a result, the graphite moderator caught fire, sending a plume of highly radioactive fallout into the atmosphere and over a very widespread area.[
    2. Nuclear graphite for the UK Magnox reactors was manufactured from petroleum coke mixed with coal-based binder pitch heated and extruded into billets, and then baked at 1,000 °C for several days. To reduce porosity and increase density, the billets were impregnated with coal tar at high temperature and pressure before a final bake at 2,800 °C. Individual billets were then machined into the final required shapes.[17] The manufacturing process is designed to ensure uniformity in material properties. Despite this care, recent research using stochastic finite element analysis[18] has shown that tiny spatial variations in material properties may play a significant role in how a graphite component ages.[19] A study carried out in 2016 provides data for the spatial variation of properties such as density and Young's modulus within a typical billet.[14] This information has been used to calibrate random fields for probabilistic simulation.[15]
    3. Nuclear graphite is any grade of graphite, usually synthetic graphite, specifically manufactured for use as a moderator or reflector within a nuclear reactor. Graphite is an important material for the construction of both historical and modern nuclear reactors, due to its extreme purity and its ability to withstand extremely high temperatures.