Managing metabolic model deficiencies for routine radiation protection purposes at a thorium dioxide facility

At a metallurgical facility using thorium dioxide feed materials, a number of conventional direct- and indirect-bioassay methods were potentially suitable monitoring methodologies when industry-standard dose assessment methodologies were based upon ICRP recommendations issued in 1959. When the ICRP recommended a different dose assessment methodology in 1977, the intake limits for thorium dropped significantly, and conventional bioassay methods for routine monitoring were rendered ineffective. While determining the technical basis for its air monitoring program, the facility noted that the radioactive materials used as feed to the process were highly insoluble. Therefore, their solubility in simulated lung fluids was determined. The results of this determination indicated that the facility's materials, if inhaled, dissolve in the lung at a half-time that is significantly longer than that contained in international consensus standards for Y-class compounds. They also indicated that the secondary dose limits, if adjusted to eliminate the dose contribution from translocated radionuclides, should be higher, and that a derived air concentration based upon more recent international recommendations is more appropriate for use in the facility's routine monitoring program.