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S C Sheppard

Publications and source records attributed to S C Sheppard.

At least 19 recordsLinked to original sources
PubMed2006

Revision and meta-analysis of selected biosphere parameter values for chlorine, iodine, neptunium, radium, radon and uranium.

There is a continual supply of new experimental data that are relevant to the assessment of the potential impacts of nuclear fuel waste disposal. In the biosphere, the traditional assessment models are data intensive, and values are needed for several thousand parameters. This is augmented further when measures of central tendency, statistical dispersion, correlations and truncations are required for each parameter to allow probabilistic risk assessment. Recent reviews proposed values for 10-15 key element-specific parameters relevant to (36)Cl, (129)I, (222)Rn, (226)Ra, (237)Np and (238)U, and some highlights from this data update are summarized here. Several parameters for Np are revised downward by more than 10-fold, as is the fish/water concentration ratio for U. Soil solid/liquid partition coefficients, Kd, are revised downward by 10-770-fold for Ra. Specific parameters are discussed in detail, including degassing of I from soil; sorption of Cl in soil; categorization of plant/soil concentration ratios for U, Ra and Np; Rn transfer from soil to indoor air; Rn degassing from surface water; and the Ca dependence of Ra transfers.

Animals↗
PubMed2005

Parameterization of a dynamic specific activity model of 14C transfer from surface water-to-humans.

Carbon-14 is a particularly interesting radionuclide from the perspective of dose estimation because it mixes readily with stable CO2, and hence enters the food-chain as fundamental biomolecules. A model was developed for the situation of 14C releases to surface waters, where there are distinct changes in the water 14C activity concentrations throughout the year. The model computes the specific activity in water, phytoplankton, fish, crops, meat, milk and air, following a typical irrigation-based food-chain scenario. This paper describes the derivation of the required 14C-specific parameter values. Many of the key parameters are not commonly measured, at least not in the context of dose assessment. Thus, inference from other sources of data was required, and this is the scientific contribution described in this paper. The best estimates and appropriate measures of statistical dispersion are provided. This required consideration of both the temporal and spatial averaging domains to ensure they were correct for parameters as defined in the model. The model coupled with these parameter values represents several new developments for modelling 14C transfers.

Air Pollution, Radioactive↗
PubMed2005

Conceptual approaches for the development of dynamic specific activity models of 14C transfer from surface water to humans.

Carbon-14 is a particularly interesting radionuclide from the perspective of dose estimation. Many nuclear facilities, including power reactors, release 14C into the environment, and much of this is as 14CO2. This mixes readily with stable CO2, and hence enters the food chain as fundamental biomolecules. This isotopic mixing is often used as the basis for dose assessment models. The present model was developed for the situation of 14C releases to surface waters, where there are distinct changes in the water 14C activity concentrations throughout the year. Complete isotopic mixing (equilibrium) cannot be assumed. The model computes the specific activity (activity of 14C per mass of total C) in water, phytoplankton, fish, crops, meat, milk and air, following a typical irrigation-based food-chain scenario. For most of the biotic compartments, the specific activity is a function of the specific activity in the previous time step, the specific activity of the substrate media, and the C turnover rate in the tissue. The turnover rate is taken to include biochemical turnover, growth dilution and mortality, recognizing that it is turnover of C in the population, not a tissue or an individual, that is relevant. Attention is paid to the incorporation of 14C into the surface water biota and the loss of any remaining 14CO2 from the surface water-air interface under its own activity concentration gradient. For certain pathways, variants in the conceptual model are presented, in order to fully discuss the possibilities. As an example, a new model of the soil-to-plant specific activity relationship is proposed, where the degassing of both 14C and stable C from the soil is considered. Selection of parameter values to represent the turnover rates as modeled is important, and is dealt with in a companion paper.

Air Pollution, Radioactive↗
PubMed2003

An index of radioecology, what has been important?

The Journal of Environmental Radioactivity (JER) has been published for the last 20 years, an important time frame for the discipline of radioecology. Certainly the study of radioecology accelerated following the Chernobyl incident. As the discipline moves forward, there will be less emphasis on that incident, and scientists will find a new focus. Issues related to waste management and decommissioning may come to the fore. There is also a large effort world-wide to deal more quantitatively with the estimation of dose and consequences to non-human biota. All researchers in radioecology will have opinions on what topics are important now and will be important in the future. Indeed, Policy Board members and Associate Editors of JER have provided a number of Editorials in the Millennium Editorial Series with their perspectives. This paper explores the trends among papers published in JER since 1995, using the ScienceDirect on-line search capability to objectively categorize the papers and quantify the relative numbers of contributions. Obviously, this is somewhat imperfect, but it does provide an index of what has been important in radioecology, and provides some quantification to recent statements about the future of radioecology.

Animals↗
PubMed2003

Interpolation of solid/liquid partition coefficients, K(d), for iodine in soils.

The measurement of K(d) is difficult for most radionuclides: a different value is expected for every different soil. This study explored a modification of the constituent-K(d) approach used to estimate K(d) in geological materials. Here we selected five soils of very different compositions, four were field soils and one was an artificial potting soil. The soils were blended together in ratios of 1:1, 1:3 and 1:1:2 for all possible (60) 2- and 3-soil combinations. The K(d) was measured for each soil and each of the combined soils using additions of stable iodine. Our hypothesis was that the weighted average of the K(d)s of the original, unblended soils, weighted by the blending ratios, would be a reasonable estimate of the measured K(d)s of the blended soils. The ratios of expected/measured K(d) values did not deviate significantly from unity (a geometric mean of 0.91) for the four field soils. This result suggests that K(d) in the combined field soils could be estimated by the weighted average K(d) for the constituent soils. The resulting variation is consistent with other estimation methods. The practical implication of this finding is that, with K(d) data for a few benchmark soils in a region, one could estimate K(d) for other soils. The potting soil did not conform, and there are several possible explanations for this.

Environmental Monitoring↗
PubMed1997

Variation in transfer factors for stochastic models: soil-to-plant transfer.

The scientific literature is a good source of information about soil-to-plant transfer, usually documented as mean concentration ratios. It does not provide, however, good information about variation in concentration ratio values, because most studies are done with limited numbers of crops and soils. Fortunately, a suitably large and diverse database is available from the International Union of Radioecologists that can be used to investigate variation. It has a total of over 7,000 concentration ratio values for over 22 elements. This paper reports on analysis of the geometric standard deviations obtained from the database. We assert that geometric standard deviation values are comparable and transferable among elements, so that geometric standard deviations calculated from large databases can be applied to other elements when there is a paucity of data. For a fully generic situation, where neither crops nor soil types are known, the geometric standard deviations is typically about 6. This implies a 1,300-fold range to encompass 95% of the data. For concentration ratios obtained for crops of interest on a specific site, the geometric standard deviation values are much lower, about 1.5. This still implies a fivefold range to encompass 95% of the data. Other values of geometric standard deviations for intermediate levels of information are provided. These estimates of geometric standard deviation are important to and appropriate for stochastic simulations of the impacts of soil contamination on crops and subsequent food chains. They are also important to the inclusivity of concentration ratios value distributions for unusual but perhaps important crops and settings.

Agriculture↗
PubMed1996

Burning radionuclide question: what happens to iodine, cesium and chlorine in biomass fires?

Fires can mobilize radionuclides from contaminated biomass through suspension of gases and particles in the atmosphere or solubilization and enrichment of the ash. Field and laboratory burns were conducted to determine the fate of I, Cs and Cl in biomass fires. Straw, wood, peat, dulse (seaweed) and radish plants were combusted with temperatures varying from 160 to 1000 degrees C, representing the normal range of field fire temperatures. Loss to the atmosphere increased with fire temperature and during a typical field fire, 80-90% of the I and Cl, and 40-70% of the Cs was lost to the atmosphere. The remainder was left behind in the ash and was soluble. Typically, the ash was enriched in I by a factor of two to three, with higher enrichments of Cs and lower enrichments of Cl, when compared to the initial fuel concentration during field burns. Most of the I was lost to the atmosphere as a gas. If the elements were radioactive isotopes, such as 129I, 137Cs and 36Cl, fires could cause an increased radiological dose to people through inhalation, exposure to ash, or ingestion of plants because of increased uptake of ash leachate.

Air Pollutants, Radioactive↗
PubMed1992

Germination of seeds from an irradiated forest: implications for waste disposal.

Jack pine (Pinus banksiana Lamb.) retain their seeds from year to year, so that in an irradiated forest, each tree receives a specific dose rate but has seeds that have accumulated a range of total doses. The Field Irradiator-Gamma facility in Pinawa, Manitoba, contains jack pine that have been irradiated longer and at lower dose rates than previously reported. Seed germination and germination rate were examined on seeds irradiated on the parent tree for up to 5 years. Germination rate was most sensitive and showed deleterious effects at 1.1 mGy hr-1. This is not much lower than results reported by others in shorter-term studies. Effects were related to dose rate rather than total dose. Hormesis, indicated by statistically significant increased germination rate, was evident at 0.6 mGy hr-1. To put these results into context, the concentrations of selected radionuclides that, through internal contamination, would deliver 1.1 mGy hr-1 to plants were estimated. For 99Tc and 129I, these concentrations are far above the chemical toxicity thresholds for plants. Clearly, as assessments of waste repositories begin to consider effects on organisms other than humans, such as plants, chemical toxicity will be an important feature.

Dose-Response Relationship, Radiation↗
PubMed1991

Food-chain and dose model, CALDOS, for assessing Canada's Nuclear Fuel Waste Management concept.

The food-chain and dose model, CALculation of DOSe (CALDOS), was developed for assessing Canada's concept for nuclear fuel waste disposal in a vault deep in crystalline rock of the Canadian Shield. The model is very general and based on the Shield as a whole. The critical group is totally self-sufficient and represented by ICRP (1975) Reference Man for dose prediction. CALDOS assumes steady-state conditions and deals with variation and uncertainty through Monte Carlo simulation techniques. Ingrowth of some radioactive daughters is considered during food-chain transfer. A limit is set on root uptake to avoid unrealistic plant concentrations. Integrated ingestion and inhalation rates of man are calculated in a unique way, based on energy needs. Soil ingestion by man and external exposure from building material are unique pathways considered. Tritium, 129I, and 222Rn are treated through special models, and 14C and 129I involve unique geosphere dose limits. All transfer coefficients are lognormally distributed, and the plant/soil concentration ratio is correlated with the soil partition coefficient. Animals' ingestion rates are normally distributed and correlated with each other. Comprehensive sets of internal and external dose conversion factors were calculated for CALDOS. Sample calculations show that dose distributions tend to be strongly right-skewed. Many features of CALDOS are relevant for environmental assessment in general.

Animals↗
PubMed1991

Mobility and plant uptake of inorganic 14C and 14C-labelled PCB in soils of high and low retention.

Quantifying and understanding the mobility of 14C and organic pollutants in soils is important, especially in the context of underground waste disposal. We studied migration of 14C applied as NaHCO3 (14C-CO3) and as 2,2',5,5' tetrachlorobiphenyl (14C-PCB) in carbonated, high-organic-matter-content and acidic, low-organic-matter-content undisturbed soil cores. The mobility of 14C-PCB depends on the profile distribution and amount of soil organic matter, whereas the mobility of 14C-CO3 depends primarily on the soil carbonate content. The solid/liquid partition coefficients (Kd) for 14C-CO3 were 6.7 and 1.2 mL g-1 for the two soils, respectively. For the 14C-PCB, the corresponding Kd values were 49 and 22 mL g-1. Plant/soil concentration ratios (CR) for inorganic 14C have previously been derived using overly conservative assumptions. Using plants grown in outdoor lysimeters, CR values for 14C-CP3 of 0.7 and 1.3, on a dry-weight basis, were measured for the two soils. These values are about 25-fold lower than the currently used values. The corresponding CR values for 14C-PCB were 0.014 and 0.088. For both 14C sources, there was evidence of atmospheric transfer from the soil to the plants. This was especially important for 14C-CO3, where it may have been dominant. Detailed modelling of 14C transport from underground waste disposal should include volatilization as a loss process from soil as well as a source for plants.

Carbon Radioisotopes↗
PubMed1991

Relative importance of atmospheric and root uptake pathways for 14CO2 transfer from contaminated soil to plants.

Plants growing on soil contaminated with 14C can receive the radionuclide from both direct uptake through the roots or indirect uptake from the atmosphere. The contribution from the atmosphere depends on the rate of volatilization from the soil, the size of the source area, and the meteorological dispersion conditions. An atmospheric dispersion relationship was used to calculate the air concentration for a given area source term. The air concentration was then used to estimate the flux to foliage, which was compared with direct plant uptake through the roots. The ratio of atmospheric to root uptake pathways depended on plant height and source area, exceeding unity when the fetch was greater than a few meters for bean plants, but much greater for maize. A very large fetch, greater than 1000 m, is required before a uniform specific activity throughout the local biosphere can be assumed for a soil source. This quantitative analysis describes the limitations for calculations and experiments used to define simplified transfer coefficients for 14C from soil to vegetation.

Carbon Dioxide↗
PubMed1988

Modeling estimates of the effect of acid rain on background radiation dose.

Acid rain causes accelerated mobilization of many materials in soils. Natural and anthropogenic radionuclides, especially 226Ra and 137Cs, are among these materials. Okamoto is apparently the only researcher to date who has attempted to quantify the effect of acid rain on the "background" radiation dose to man. He estimated an increase in dose by a factor of 1.3 following a decrease in soil pH of 1 unit. We reviewed literature that described the effects of changes in pH on mobility and plant uptake of Ra and Cs. Generally, a decrease in soil pH by 1 unit will increase mobility and plant uptake by factors of 2 to 7. Thus, Okamoto's dose estimate may be too low. We applied several simulation models to confirm Okamoto's ideas, with most emphasis on an atmospherically driven soil model that predicts water and nuclide flow through a soil profile. We modeled a typical, acid-rain sensitive soil using meteorological data from Geraldton, Ontario. The results, within the range of effects on the soil expected from acidification, showed essentially direct proportionality between the mobility of the nuclides and dose. This supports some of the assumptions invoked by Okamoto. We conclude that a decrease in pH of 1 unit may increase the mobility of Ra and Cs by a factor of 2 or more. Our models predict that this will lead to similar increases in plant uptake and radiological dose to man. Although health effects following such a small increase in dose have not been statistically demonstrated, any increase in dose is probably undesirable.

Background Radiation↗
PubMed1987

Factors controlling the hormesis response in irradiated seed.

Ionizing radiation at very low doses frequently has a stimulating or hormetic effect on the growth of organisms. Irradiation of seed before planting can stimulate early plant growth, leading to advanced maturity and increased yield. The unreliability of this response has limited its application. However, the technique has been extensively studied and now is practiced on a large scale on some farms in certain countries. Our research has addressed various factors that may determine the stimulation response with the goal of developing an ability to predict the occurrence of increased economic yield. In this paper, we discuss several of these factors in light of our data and data from other field studies in Canada. A hormesis response was observed for most species studied, but varied among cultivars and among seed lots within a cultivar. Seed condition may be the underlying factor in these effects. The response was most frequently evident at very early stages of growth and was often masked in subsequent growth. This suggests that the hormesis phenomenon in seeds may be quite common and is not reliably manifest in advanced maturity or yield increases because of environmental effects. Storage time after irradiation must be minimized to gain the greatest response. There is little evidence to suggest that the photon energy or dose rate of the radiation source is an important factor.

Dose-Response Relationship, Radiation↗