TMI-2 Reactor Vessel Fuel Assay: Measurement & Equipment Phase

In 1979 the TMI-2 reactor suffered a loss of coolant accident that raised the core temperature to melting and removed most of the cladding by melting and oxidation. Introduction of cold water further degraded the core by thermal-mechanical stresses. The combined effects of the accident and defueling, distributed sand like core debris into many locations that could not be easily removed. Under water video inspections confirmed residual fuel was present. A more complete assay method was required since fuel mass uncertainties were indeterminate by video means. The object of this paper is to outline the method and equipment used to assay residual fuel in the TMI-2 Reactor Vessel, RV. A companion paper, Ref. 1, covers fuel results from the measurements. Active neutron interrogation, and passive methods were tested with practical assays of smaller systems such as fuel encrusted end fittings in drums and fuel deposits on the TMI-2 \"B\" Steam Generator tube sheet. After reviewing the abilities of the two methods to minimize uncertainty, the passive neutron method was adopted. The RV measurement system consisted of three He-4 counters and pre-amps shielded by 15 cm of lead. The 1740 kg assembly was lowered into the RV. The counter electronics was located in the contaminated Reactor Building, RB, 30 meters from the RV. The electronics were in a 4 m sheet metal box designed to dissipate electrical heat and block fission products. Signals from the detector pre-amps were amplified and discriminated. Logic signals went to sealer modules in the 286-AT computer. A second 286 computer, located outside of the RB, was connected to the first by a dedicated serial link. The assembled counting equipment was able to detect background neutrons and the more intense passive neutrons from residual fuel. The measurement extended over three months. There were no failures and the set points were stable.