Worker protection implications of the solubility and human metabolism of modern uranium mill products in the U.S.

This paper presents an analysis of the implications of some recent studies performed to characterize uranium products from modern uranium recovery facilities important for worker protection. Assumptions about the solubility (related to the molecular species being produced) of these materials in humans are critical to properly assess radiation dose from intakes, understand chemotoxic implications, and establish protective exposure standards (airborne concentrations, limits on intake, etc.). Recent studies, as well as information in the historical professional literature, were reviewed that address the issue of solubility and related characteristics. These data are important for the design of programs for assessment of both chemical and radiological aspects of worker exposure to the products of modern uranium recovery plants (conventional uranium mills and in situ recovery plants; i.e., ISRs). The data suggest strongly that the oxide form produced by these facilities (and therefore, product solubility) is related to precipitation chemistry and thermal exposure (dryer temperature). Given the peroxide precipitation and low temperature drying methods being used at many modern uranium recovery facilities in the U.S. today, very soluble products are being produced. The dosimetric impacts of these products to the pulmonary system (except perhaps in case of an extreme acute insult) would be small, and any residual pulmonary retention beyond a month or two would most likely be too small to measure by traditional urinalysis sampling or the current state-of-the-art of natural uranium in vivo lung counting techniques. Uranium recovery plants should revisit the adequacy of current bioassay programs in the context of their process and product specifics. Workers potentially exposed to these very soluble yellowcake concentrates should have urine specimens submitted for uranium analysis on an approximately weekly basis, including analysis for the biomarkers associated with potential renal injury [e.g., glucose, lactate dehydrogenase (LDH) and protein albumen]. Additionally, implications for compliance with current U.S. Nuclear Regulatory Commission (NRC) regulations (e.g., 10 CFR20) are discussed. NRC, the applicable Agreement State agencies, and licensees need to recognize the importance of the uranium chemotoxicity versus dose relationship in the interest of worker protection.