Associated with the use of 238Pu in thermoelectric power sources for space probes is the potential for human exposure, primarily by inhalation and most likely as 238PuO2. Several models have been developed for assessing the level of intake and predicting the resulting radiation dose following human exposure to 239Pu. However, there are indications that existing models do not adequately describe the disposition and dosimetry of 238Pu following human exposure. In this study, a canine model that accounts for these differences has been adapted for use with human excretion data. The model is based on existing knowledge about organ retention of plutonium. An analysis of the sensitivity of the model to changes in aerosol-associated properties indicated that predictions of urinary excretion are most sensitive to changes in particle solubility and diameter and in the ratio of fragment:particle surface area. Application of the model to urinary excretion data from seven workers exposed to a 238Pu ceramic aerosol gave estimated intakes of 390-8,200 Bq and associated initial pulmonary burdens of 80-1,700 Bq. The resulting 50-y dose commitments to critical organs per Bq of 238Pu intake were estimated to be 0.5 mSv for the thoracic region, 0.2 mSv for the liver, and 1 mSv for the bone surfaces.

Curium isotopes are major by-products in irradiated nuclear reactor fuel and comprise a significant fraction of the alpha-emitting radionuclide inventory. Although little use is currently being made of purified Cm sources, such usage is possible if reprocessing of spent fuel becomes feasible. Because little information is available on the biokinetics and dosimetry of inhaled Cm compounds, a study was conducted in which adult beagle dogs received a single inhalation exposure to either a monodisperse aerosol of 244Cm2O3 (1.4 micron activity median aerodynamic diameter [AMAD]; sigma g = 1.16) or a polydisperse aerosol of 244Cm (NO3)3 (1.1 micron AMAD; sigma g = 1.74). At times ranging from 4 h to 2 y after exposure, animals were sacrificed and their tissues analyzed for Cm content. The data describing the uptake and retention of 244Cm in the different organs and tissues and the measured rates of excretion of these dogs formed the basis on which a biokinetic model of Cm metabolism was constructed. This Cm model was based on a previously published model of the biokinetics of 241Am that was shown to be applicable to data from human cases of inhalation exposure to 241Am aerosols. This Cm model was found to be adequate to describe the biological distribution of Cm in dogs and was also applied to the sparse data from humans. Reasonable agreement was found between the model predictions for lung retention of Cm and for urinary excretion patterns in humans.

The lung retention of uranium was determined in rats that inhaled aerosols of commercial yellowcake powders obtained from two mills (Mill A and Mill D) and whose chemical composition and solubilities in vitro were significantly different. Analysis by IR absorption indicated Mill A yellowcake contained 82% ammonium diuranate (ADU) and 18% U3O8. The Mill D powder contained 25% ADU and 75% U3O8. In vitro dissolution studies indicated for the Mill A sample, approximately 85% of the uranium had a dissolution half-time (T 1/2) of less than one day, with the remainder dissolving with a half-time of 500 days. For the Mill D sample, 25% had T 1/2 less than one day and 75% had T 1/2 of 300 days. Groups of 50 rats were exposed by nose-only inhalation to aerosols of either the Mill A or the Mill D yellowcake. Rats were sacrificed in groups of five at intervals through six months after exposure. Selected tissues and excreta samples were assayed by fluorometry to determine their uranium contents. For the Mill A yellowcake, 78% initial lung (broncho-alveolar) burden cleared with T 1/2 of 0.5 days, and 22% with T 1/2 of 240 days. For the Mill D yellowcake, 25% initial lung burden cleared with T 1/2 of 3.5 days and 75% with T 1/2 of 110 days. Thus, the lung clearance of uranium in the rat mimicked the in vitro dissolution data and supported the contention that ADU should be considered as a Class D compound (T 1/2 = 0.5 days) and U3O8 behaves in the lung as a Class Y (T 1/2 greater than 100 days) material.

A study was conducted in rats to determine solubility and subsequent metabolism of an inhaled aerosol of curium treated at high temperatures. Young adult Fischer-344 rats received a single inhalation exposure to one of three monodisperse aerosols of 244Cm2O3 (0.70, 1.3, or 2.6 micron activity median aerodynamic diameter) heat-treated at 1150 degrees C. Animals were maintained individually in metabolism cages for excreta collection and serially sacrificed in groups of two male and two female rats from 2 to 33 days after inhalation exposure. Additionally an injection study with curium citrate was done to define the systemic behavior of Cm in this rat model. The in vivo solubility was inversely related to the aerosol particle size. The relationship of the results of this study to results from other experimental inhalation studies with curium oxide aerosols is discussed, as is the relevance to bioassay interpretation and risk assessment in man.

A new approach to the estimation of plutonium (Pu) levels in the skeleton based on measurements of Pu excretion in the feces following treatment with diethylenetriaminepentaacetic acid (DTPA) is presented. The estimation method was tested in groups of mice receiving either 0.5 or 5 pCi/kg of *'*u(IV) citrate, and treated with Na3[CaDTPA] starting either at 1 hr or 24 hr after Pu administration. In the case of DTPA treatment begun at 1 hr, the ratio of skeletal Pu content at 1 hr, S, to DTPA-induced fecal excretion of Pu, F, was 1.08+0.03. When DTPA treatment was begun at 24 hr, the corresponding S/F ratio was 1.68 + 0.08 and was independent of Pu injection level. In the mouse, therefore, a reasonable estimate of skeletal Pu content just prior to treatment can be determined from the amount of Pu measured in the feces following DTPA therapy. Application of this method to species other than the mouse is examined, with anticipation that the method might be applicable to cases of human exposure.