Evaluation of radionuclide penetration of structural concrete surfaces in the Three Mile Island Unit 2 reactor building

The March 28, 1979 loss-of-coolant accident at Three Mile Island Unit 2 (TMI-2) resulted in the exposure of ∼3000 m2 of reactor building internal concrete surfaces to both liquid-and vapor-borne contaminants. The period of contact between the major structural concrete surfaces and the aqueous solutions of mixed fission products ranged from a few days to several years. Exclusive of the reactor building basement impingement walls above a height of 1.6 m, all concrete surfaces were protected with an epoxy-based coating. This coating provides a tough, easily decontaminated surface for the concrete during normal operation and maintenance cycles. At the completion of the gross decontamination of the accessible reactor building elevations in 1982, exposure rates remained elevated above expected levels as indicated by early decontamination factors. Exposure rate measurements and small-scale scarification samples of the reactor building surfaces demonstrated that the protective coatings and concrete in the reactor building had absorbed radionuclides, thereby creating a large fixed source. In September 1983, a concrete core sampling program was conducted in the TMI-2 reactor building to assess the depth of contaminant penetration into the coatings and concrete on elevations 93m and 106 m. Sampling of the reactor building basement concrete surfaces [elevation 86 m] was deferred until 1985 and 1986 to provide lead time for remote systems development