Development of a multi-element thermoluminescent dosimeter at the Three Mile Island Nuclear Station

Following the TMI-II accident, the thermoluminescent dosimeter (TLD) used to measure personnel exposure was tested to evaluate its beta performance characteristics. The TLD in use was a commonly used 2-chip lithium fluoride dosimeter with absorbers of 30 and 280 mg/cm2. The results of this testing indicated that the device could not adequately discriminate between the beta and gamma fields. Specifically, it was found that (1) the absorber over the penetrating chip was not sufficient to shield the high energy beta radiation (Sr/Y-90) resulting from the high levels of fission product contamination deposited in the Auxiliary and Containment Buildings of Unit II, and (2) the badge design provided insufficient data to allow for an assessment of the existing beta spectrum. Thus it was found that in mixed beta/gamma fields, the penetrating dose was overestimated and the uncorrected non-penetrating dose was underestimated. This information prompted the search for a replacement dosimeter for the 2-chip TLD which would more accurately measure mixed beta/gamma fields. Testing of available TLD systems showed that a multi-element badge with thin phosphors would most accurately monitor the doses from the mixed radiation fields to which personnel would be exposed in the TMI-II recovery operation. The specific badge chosen was a modified Panasonic 802 type. This dosimeter design has been extensively tested, including participation in ANSI N13.11 Test No. 3. The results of this testing segment are given. The design and performance of the TMINS multi-element dosimetry system are presented