Using Ex-Core Neutron Detectors to Estimate Fuel Quantities in the Reactor Vessel Lower Head

During the accident at Three Mile Island Unit 2, a significant mass of core debris relocated to the bottom head of the reactor vessel. Subsequently, defueling activities in the core area redistributed the fuel and caused further core debris to relocate to the lower head region. An analytical approach is presented for determining the relative increase in lower plenum debris bed mass from observed changes in ex-core neutron detector readings. The neutron source magnitude of the fuel in the lower reactor vessel plenum and the degree of subcritical multiplication of neutrons in the lower head are investigated. Based on the mathematical relation formulated, it is estimated that during lower core area defueling (September 1986 to November 1987), between 12 and 23.5 additional tonnes of core materials relocated to the lower head.