Airborne Recontamination of the Three Mile Island Unit 2 Reactor Building

The dose reduction objectives for the Three Mile Island Unit 2 reactor building (RB) were designed to lower the dose rates in working areas so that the total collective dose to workers would be as low as reasonably achievable. As part of these objectives, a large-scale effort was devoted to the decontamination of RB surfaces. The presence of very high removable surface contamination levels, sometimes in excess of 1.7 × 10³ Bq/cm² (4.6 µCi/100cm²), contributed to high airborne radioactivity conditions, which necessitated the extensive use of respiratory protection. It became an objective of the decontamination program, therefore, to reduce the removable contamination levels to such an extent that the use of respirators could be reduced or even eliminated. The progress of the decontamination program was hampered when it was discovered that large areas of the RB were becoming recontaminated. Recontamination rates were measured to be ∼1.5 Bq/cm²·day⁻¹ (4.1 × 10⁻³ µCi/100cm²·day⁻¹). After a series of tests, it was determined that the air handling systems in the RB were distributing radioactivity from highly contaminated surfaces. Cascade impactor studies of the aerosols indicated a bimodal distribution of particle sizes. Particles >20-µm activity median aerodynamic diameter (AMAD) accounted for 30% of the collected activity and particles <5-µm AMAD were associated with 60% of the activity. Examinations by optical and electron microscopy and Raman spectroscopy helped determine that the larger particles were organic dusts associated with the air handling systems and the smaller particles were associated with the boric acid dissolved in decontamination water. Reducing the airflow through the air cooler fans and restricting the airflow to the highly contaminated D-rings helped to reduce the recontamination to 4 × 10⁻² Bq/cm²·day⁻¹ (1.1 × 10⁻⁴ µCi/100cm²·day−1). Subsequently, the recontamination of surfaces due to airborne vectors ceased to be an operational concern. Further decontamination of the floors enabled a significant reduction in the use of respiratory protection equipment.