Recriticality energetics of a hypothetical water reflood accident in a damaged light water reactor

摘要

The Three Mile Island (TM-2) accident in 1979 resulted in approximately 45% of the fuel collapsing into an irregularly-shaped debris bed near the center of the core, while some of the molten material flowed into the lower dome of the reactor vessel where it solidified. The immediate cause of this severely degraded geometry was loss of coolant and subsequent heatup of the fhel from decay heat. The neutron chain reaction had shut down prior to fuel relocation (below 5 wt% U(sup 235) enrichment, fast-spectrum criticality is impossible). After the system had cooled sufficiently to allow for reintroduction of water, the core was reflooded to remove remaining decay heat; to ensure stable shutdown, the reflood water was heavily borated (in excess of 3000 ppm).