The effective atomic number revisited in the light of modern photon-interaction cross-section databases.

The effective atomic number, Z(eff), has been calculated for fatty acids and cysteine. It is shown that Z(eff) is a useful parameter for low-Z materials at any energy above 1 keV. Absorption edges of medium-Z elements may complicate the energy dependence of Z(eff) below 10 keV. The notion of Z(eff) is perhaps most useful at energies where Compton scattering is dominating, and where Z(eff) is equal to the mean atomic number, Z, over a wide energy range around 1 MeV.

Monte Carlo simulations: a useful tool to extend in vivo calibrations and explore alternative approaches.

Designing and testing new equipment can be an expensive and time consuming process or the desired performance characteristics may preclude its construction due to technological shortcomings. Cost may also prevent other types of scenario being tested. An alternative is to use Monte Carlo simulations to make the investigations. This paper exemplifies how Monte Carlo code calculations can be used to fill the gap by describing two investigations: (1) the possible self-attenuation of homogeneously distributed natural uranium in a lung phantom; and (2) the effect of activity deposited in the ribs on the activity estimate from a lung count.

Monitoring methods and dose assessment for internal exposures involving mixed fission and activation products containing actinides.

Internal dose assessment for intakes of radionuclide mixtures is a difficult task. When the radionuclide mixture contains both the easy to detect gamma emitters, e.g., 60Co and 95Zr, and difficult to detect alpha emitters such as 239Pu and 241Am, a single monitoring method, such as in-vivo counting, is inadequate for detection and dose assessment. Recent experience with task related monitoring for such radionuclide mixtures at Ontario Power Generation CANDU nuclear power plants has offered an opportunity to review this topic and suggest a strategy for monitoring that involves a combination of in-vivo and in-vitro methods. Using the radionuclide composition data in a mixture from an actual case as an example, this paper describes a monitoring strategy for mixed fission and activation products, including the advantages and pitfalls of reliance on surrogate radionuclides for signaling the presence of actinides in the mixture. The described monitoring strategy is consistent with the recommendations of ICRP Publication 78, which advocates a "combination of techniques so as to make the best possible evaluation of an unusual situation, for example, a programme of both body activity and excreta measurements." The use of experience and professional judgement for interpreting the combined in-vivo and in-vitro data for interim and ultimate intake and dose assessment is discussed and emphasized.

Retention and excretion of 95Zr-95Nb in humans.

This note describes the retention and excretion of 95Zr-95Nb in humans based on a recent CANDU experience and a literature survey of reported cases. Two data bases, QUEST and INIS were used for the survey. Three reported cases were discovered: two for occupational exposures and one for public exposure from nuclear weapons fallout. Human lung retention from these three cases, plus whole body retention and some limited fecal excretion data from a recently occurred exposure at a CANDU station, were reviewed and tested against predictions based on ICRP Publication 30 model. Based on the fits of this model to the reported data it seems that the three occupational exposures exhibit class Y behavior while the public exposure exhibits class W behavior. For only one case is the chemical compound known with certainty: ZrO2. Zirconium oxides are currently classified as class W in ICRP Publications 30 and 54. This work confirms a suggestion that oxides of zirconium be classified as class Y and should be taken into account by the ICRP in its future publications.

A critical review of 55Fe dosimetric models.

The available literature on 55Fe dosimetry has been devoted to environmental exposures and medical iron kinetic studies. For occupational dosimetry, ICRP (1979, 1988) published a non-recycling dosimetric model for iron. These ICRP publications do not provide information on iron excretion. Johnson and Dunford (1985) published dose conversion factors and urinary excretion curves based on the ICRP (1980) and MIRD (1983) iron metabolic models. A critical review of these models was undertaken to select a model for occupational dose assignment. The review indicated that the information and recommendations in ICRP (1988) and Johnson and Dunford (1985) are dependent on unrealistic assumptions that do not agree with known iron metabolism. Therefore, an alternative model is proposed for dosimetric application. Calculations of dose conversion factors and urinary excretion curve for class W55Fe inhalation exposure (1 micron AMAD) using the proposed model are compared with predictions based on ICRP (1980) and Johnson and Dunford (1985) models. The difference in the practical outcome (i.e., dose assignment) is examined by applying the proposed and reviewed models to a realistic bioassay case. The Johnson and Dunford (1985) model yields a dose estimate which is roughly a factor of ten higher than values predicted by ICRP (1980) and the proposed model. Some of the disagreement is due to uncertainty in the fraction of radio-iron excretion via urine. Further research on this subject is recommended. In the interim, the proposed model is recommended for occupational dose assignment.

Occupational exposure of electrical utility linemen to pentachlorophenol.

Occupational exposure to pentachlorophenol (PCP) for a crew of electrical utility linemen was monitored over a 6-month period by using total PCP in urine per gram of creatinine as a biological monitoring parameter. Urine samples were collected from three groups: A, B, and control, at a 4-week frequency during 1989. Group A was required to use new gloves after each 4-week work period; Group B changed gloves on a need basis as per normal operating procedure. The control group consisted of members of the administrative office staff who were not occupationally exposed. The used gloves returned by Group A were monitored for contamination. On the basis of analysis of the collected data the following conclusions were noted. (1) The linemen experienced a seasonal exposure pattern with exposures peaking in July and August. This seasonal effect was also observed with glove contamination data. (2) The glove contamination levels were significantly associated with urine PCP concentrations when both these variables were expressed as geometric means for the individuals in Group A. Inclusion of work experience as an additional variable enhances this association. Less experienced linemen tended to perform more activities with higher current exposure and had higher urine and glove PCP measurements and higher correlations between these variables than more experienced linemen. (3) Over the study period, the difference in long-term exposures of Group A and Group B linemen was not statistically significant. (4) The long-term individual exposures, calculated as the geometric mean of each individual's sequential sample readings, were all below the biological monitoring guideline value of 1000 micrograms PCP/g creatinine.

A comparison of ICRP Publication 30 lung model-based predictions with measured bioassay data for airborne natural UO2 exposure.

In this paper a comparison is made between the build-up of U thorax burdens and the predicted total lung (lung and lymph) burden, based on the lung model provided in ICRP Publication 30 for a group of 29 atomic radiation workers at a Canadian fuel fabrication facility. A similar comparison is made between the predicted ratio of the total lung burden to urinary excretion and the ratio obtained from bioassay data. The study period for the comparison is 5 y. The inhalation input for the lung model calculations was derived from air-sampling data and the choice of particle size activity median aerodynamic diameter (AMAD) was guided by particle size measurements made at representative work locations. The pulmonary clearance half-times studied were 100, 250 and 500 d. For the purpose of this comparison, averaged exposure and averaged bioassay data for the group were used. This comparison indicates that for the conditions of this facility, the assumption of a 500-d pulmonary clearance half-time and a particle size of 1 micron (AMAD) may be too conservative. It is suggested that measurements of air concentrations and particle size used as input parameters for the ICRP Publication 30 lung model may be used to calculate bioassay parameters which may then be tested against bioassay data obtained as part of an operational health physics program, thereby giving a useful step towards defining a derived air concentration value for U in the workplace.

Extremity dose: its definition, standards and regulatory limits, radiobiological significance, measurement and practical considerations.

This paper reviews several inter-related aspects of partial-body exposures to ionizing radiation, with particular emphasis on hands. The topics featured in this review are: the definition of extremity, the radiobiological significance of extremity dose and its influence on dose standards, current and future dose standards and regulatory limits, monitoring requirements along with the availability and selection of suitable dosimeters, the question of dose averaging and dosimeter placement and practical considerations for the implementation of an extremity dosimetry program. The review is purposely kept general in nature and readers with a specific interest in a particular aspect are encouraged to read the original papers. This review is written from the perspective of a practicing health physicist who may wish to read the available literature on this topic and incorporate some of the information into an existing or planned extremity dosimetry program.

Determination of particle size for airborne UO2 dust at a fuel fabrication work station and its implication on the derivation and use of ICRP Publication 30 derived air concentration values.

When internal radiation exposure occurs via the inhalation route, the activity deposition in the respiratory tract is dependent on the particle size distribution of the radioactive aerosol. This fact has been recognized by ICRP Publication 30 in which annual limit of intake (ALI) and derived air concentration (DAC) values have been calculated for a variety of radionuclides based upon an activity median aerodynamic diameter (AMAD) of 1 micron. When radioactive aerosols have AMADs not equal to 1 micron, ICRP Publication 30 suggests the use of correction factors to calculate the appropriate ALI and DAC values. This work describes the measurement of particle size distribution for UO2 dust at a fuel fabrication work station by means of a cascade impactor. Measurements were made on 39 d spread over a 10-mo period to determine if the particle size distribution was stable over a year. The results of this study indicate that for this work station the measured AMAD values are not stable over a year. A discussion is provided on the treatment of such particle-size distribution data with the objective of extracting a representative particle-size distribution value, upon which an ALI and a DAC value can be determined. The observations made in this study have implications on the application of ICRP Publication 30 recommendations to operational health physics practice.

Measurement of gamma radiation exposure rates from natural UO2 fuel pellets.

An attempt has been made to measure gamma radiation exposure rates from natural UO2 fuel pellets at near contact distances by means of a GM survey meter and LiF thermoluminescent dosimeters. An experimental device was fabricated which facilitated the holding of both the fuel pellets and the necessary measuring devices. A semi-empirical method for predicting exposure rates from a fuel pellet at near contact distances is also described. It is suggested that the experimental set up has the potential for providing a calibration device for gamma radiation instrumentation used to assess exposures in facilities handling U.