ANSTO :: Browsing by Author "Dyer, LL"

Browsing by Author "Dyer, LL"

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Atmospheric stability effects on potential radiological releases at a nuclear research facility in Romania: characterising the atmospheric mixing state
(Elsevier, 2016-04-01) Chambers, SD; Galeriu, D; Williams, AG; Melintescu, A; Griffiths, AD; Crawford, J; Dyer, LL; Duma, M; Zorila, B
A radon-based nocturnal stability classification scheme is developed for a flat inland site near Bucharest, Romania, characterised by significant local surface roughness heterogeneity, and compared with traditional meteorologically-based techniques. Eight months of hourly meteorological and atmospheric radon observations from a 60 m tower at the IFIN-HH nuclear research facility are analysed. Heterogeneous surface roughness conditions in the 1 km radius exclusion zone around the site hinder accurate characterisation of nocturnal atmospheric mixing conditions using conventional meteorological techniques, so a radon-based scheme is trialled. When the nocturnal boundary layer is very stable, the Pasquill–Gifford “radiation” scheme overestimates the atmosphere's capacity to dilute pollutants with near-surface sources (such as tritiated water vapour) by 20% compared to the radon-based scheme. Under these conditions, near-surface wind speeds drop well below 1 m s−1 and nocturnal mixing depths vary from ∼25 m to less than 10 m above ground level (a.g.l.). Combining nocturnal radon with daytime ceilometer data, we were able to reconstruct the full diurnal cycle of mixing depths. Average daytime mixing depths at this flat inland site range from 1200 to 1800 m a.g.l. in summer, and 500–900 m a.g.l. in winter. Using tower observations to constrain the nocturnal radon-derived effective mixing depth, we were able to estimate the seasonal range in the Bucharest regional radon flux as: 12 mBq m−2 s−1 in winter to 14 mBq m−2 s−1 in summer. © 2016, Elsevier Ltd.
Biotic, temporal and spatial variability of tritium concentrations in transpirate samples collected in the vicinity of a near-surface low-level nuclear waste disposal site and nearby research reactor
(Elsevier Science Ltd, 2011-06-01) Twining, JR; Hughes, CE; Harrison, JJ; Hankin, SI; Crawford, J; Johansen, MP; Dyer, LL
The results of a 21 month sampling program measuring tritium in tree transpirate with respect to local sources are reported. The aim was to assess the potential of tree transpirate to indicate the presence of sub-surface seepage plumes. Transpirate gathered from trees near low-level nuclear waste disposal trenches contained activity concentrations of (3)H that were significantly higher (up to similar to 700 Bq L(-1)) than local background levels (0-10 Bq L(-1)). The effects of the waste source declined rapidly with distance to be at background levels within 10s of metres. A research reactor 1.6 km south of the site contributed significant (p < 0.01) local fallout (3)H but its influence did not reach as far as the disposal trenches. The elevated (3)H levels in transpirate were, however, substantially lower than groundwater concentrations measured across the site (ranging from 0 to 91% with a median of 2%). Temporal patterns of tree transpirate (3)H, together with local meteorological observations, indicate that soil water within the active root zones comprised a mixture of seepage and rainfall infiltration. The degree of mixing was variable given that the soil water activity concentrations were heterogeneous at a scale equivalent to the effective rooting volume of the trees. In addition, water taken up by roots was not well mixed within the trees. Based on correlation modelling, net rainfall less evaporation (a surrogate for infiltration) over a period of from 2 to 3 weeks prior to sampling seems to be the optimum predictor of transpirate (3)H variability for any sampled tree at this site. The results demonstrate successful use of (3)H in transpirate from trees to indicate the presence and general extent of sub-surface contamination at a low-level nuclear waste site. Crown Copyright © 2011, Elsevier Ltd.
Evaluating emergency response models of radiological dispersion in complex terrain
(HARMO, 2008-10) Dyer, LL; Pascoe, JH
Operational airborne releases of trace quantities of the radioactive noble gas Ar-41 from the HIFAR Nuclear Research Reactor located in Sydney, Australia are valuable for evaluating emergency response models incorporating radiological dispersion. The Australian Nuclear Science and Technology Organisation (ANSTO), where the reactor is located, has a network of meteorological stations and GR-150 environmental gamma dose detectors placed in complex terrain within a 5km radius of the site. The current version of ANSTO’s Emergency Response System feeds real-time meteorological data from this network into the wind field model NUATMOS, which is then subsequently used, together with known source emissions of Ar-41 from HIFAR, to drive the Lagrangian mesoscale atmospheric dispersion puff model RIMPUFF. Output from RIMPUFF is compared with data from the gamma dose monitoring network in order to assess the overall performance of the system. An updated model combination LSMC/RIMPUFF has recently been evaluated using Ar-41 peak observations from the detector network during 2002-03 under a range of atmospheric stability conditions. Sensitivity tests of the new model version were performed by perturbing the gridded meteorological data within the model domain, which incorporates complex terrain. Results from statistical analyses of the model output are reported, assessing the suitability of LSMC/RIMPUFF to replace the current models in the Emergency Response System. The LSMC/RIMPUFF evaluation results are compared with two previous evaluations carried out in 2004, of LSMC’s predecessor LINCOM/RIMPUFF and the NUATMOS/RIMPUFF combination currently used at ANSTO. Results from these tests have been favourable for LSMC/RIMPUFF, which is consequently now likely to replace the older models within ANSTO’s Emergency Response System. The decision support system ARGOS, which also incorporates the LSMC/RIMPUFF model, is currently being evaluated for potential use by a number of Organisations within Australia including ANSTO. Some results of this initial evaluation will also be included here.
Evaluation of dispersion models DIPCOT and RIMPUFF used in Decision Support Systems for nuclear and radiological emergency response
(HARMO, 2010-06-01) Andronopoulos, S; Dyer, LL; Davakis, E; Bartzis, JG
This paper presents evaluation of the atmospheric dispersion models DIPCOT and RIMPUFF which are incorporated for operational use in Decision Support Systems for nuclear emergencies. The evaluation is performed through comparisons of model results with real-scale measurements of gamma radiation dose rates in air obtained during the routine operation of the HIFAR Research Reactor located in Sydney, Australia. The area surrounding the reactor is characterized by moderately complicated topography and varying land cover. A total of 16 days have been computationally simulated, covering all atmospheric stability conditions. Qualitative and quantitative model evaluation is carried out, using comparisons of paired in space and time calculated and measured gamma dose rates, statistical indices, scatter plots, and contour plots. The models performance is satisfactory for a number of cases, while for others the performance is poor. This can be attributed to a number of factors, mainly uncertainties in the prediction of meteorological conditions.
Impact of meteorology on fine aerosols at Lucas Heights, Australia
(Elsevier, 2016-11-01) Crawford, J; Chambers, SD; Cohen, DD; Williams, AG; Griffiths, AD; Stelcer, E; Dyer, LL
Ion Beam Analysis (IBA) techniques were used to assign nine years of PM2.5 observations to seven source types, at Lucas Heights, a topographically complex urban fringe site of Sydney. The highest contributions to total PM2.5 were from motor vehicles (Autos, 26.3%), secondary sulfur (2ndryS, 23.7%), a mixture of industry and aged sea air (IndSaged, 20.6%), and smoke (Smoke, 13.7%). The Autos contribution was highest in winter, whereas 2ndryS was highest in summer, indicating that mitigation measures targeting SO2 release in summer and vehicle exhaust in winter would be most effective in reducing the PM2.5 concentrations at this site. Since concentrations of particulate matter can be significantly affected by local meteorology, generalised additive model (GAM) techniques were employed to investigate relationships between PM2.5 source types and meteorological conditions. The GAM predictors used included: time (seasonal to inter-annual variations), mixing layer depth, temperature, relative humidity, wind speed, wind direction, and atmospheric pressure. Meteorological influences on PM2.5 variability were found to be 58% for soil dust, 46% for Autos, 41% for total PM2.5, and 35% for 2ndryS. Effects were much smaller for other source types. Temperature was found to be an important variable for the determination of total PM2.5, 2ndryS, IndSaged, Soil and Smoke, indicating that future changes in temperature are likely to have an associated change in aerosol concentrations. However, the impact on different source types varied. Temperature had the highest impact on 2ndryS (sometimes more than a factor of 4 increase for temperatures above 25 °C compared to temperatures under 10 °C) and IndSaged, being predominantly secondary aerosols formed in the atmosphere from precursors, whereas wind speed and wind direction were more important for the determination of vehicle exhaust and fresh sea salt concentrations. The marginal effect of relative humidity on 2ndryS increased up to relative humidity of 70–80% and then plateaued, confirming previous findings that (NH4)2SO4 is present in the solid phase below relative humidity of about 80%. © 2016, Elsevier Ltd.
Model evaluation of RIMPUFF within complex terrain using an Ar-41 radiological dataset
(Inderscience Enterprises Ltd., 2012-01-01) Dyer, LL; Astrup, P
The newly updated atmospheric dispersion model RIMPUFF is evaluated using routine releases of Ar-41 from the former HIFAR research reactor located in Sydney, Australia. A large number of Ar-41 measurements from a network of environmental gamma detectors are used to evaluate the model under a range of atmospheric stability conditions within the complex terrain area. Model sensitivity of input data is analysed including meteorological station data, land use maps, surface roughness and wind interpolation schemes. Various model evaluation tools are used such as gamma dose rate plots, exploratory data analyses and relevant statistical performance measures. © 2012, Inderscience Enterprises Ltd.
Model evaluation of RIMPUFF within complex terrain using an Ar-41 radiological dataset
(HARMO, 2010-06-01) Dyer, LL; Astrup, P
The newly updated atmospheric dispersion model RIMPUFF is evaluated using routine releases of 41Ar from the former HIFAR research reactor located at the Australian Nuclear Science and Technology Organisation (ANSTO) in Sydney, Australia. Predicting radiological dispersion for emergency response at this site proves challenging due to complex topographical conditions including a steep-sided river valley located between the reactor and the nearest residents. A large number of 41Ar measurements from a network of environmental gamma detectors are used to evaluate the model under a range of atmospheric stability conditions. Topographic and meteorological influences that potentially affect a released plume, such as channelling, wind shear, local terrain slope flows and strong inversions are explored. A sensitivity analysis using various combinations of meteorological station data for model input, including vertical wind and temperature profiles, also identifies model strengths and weaknesses within the complex terrain. Various model evaluation tools, such as relevant statistical indices and gamma dose contour plots, are used to evaluate this new version of RIMPUFF for emergency response purposes at ANSTO and for inclusion in the ARGOS Decision Support System.
Nuclear tools for characterising radiological dispersion in complex terrain: evaluation of regulatory and emergency response models
(Inderscience Enterprises Ltd., 2005-07-01) Williams, AG; Clark, GH; Dyer, LL; Barton, R
Routine operations of a nuclear research reactor and its facilities offer opportunities for collection of rare environmental tracer datasets which can be used for atmospheric dispersion model evaluation studies. The HIFAR reactor near Sydney, Australia, routinely emits the radioactive noble gas Ar-41, and other radionuclides such as Xe-133 and Xe-135 are also emitted from nearby radiopharmaceutical production facilities. Despite extremely low emission levels of these gases, they are nevertheless detectable using state-of-the-art technology, and sensitive detectors have been placed at four locations in the surrounding region which features complex terrain. The high research potential of this unique dataset is illustrated in the current study, in which predictions from two atmospheric dispersion models used for emergency response are compared with Ar-41 peak observations from the detector network under a range of stability conditions, and long-term integrated data is also compared with a routine impact assessment model. © 2005, Inderscience Enterprises Ltd.
Radiation source rate estimation through data assimilation of gamma dose rate measurements for operational nuclear emergency response systems
(Inderscience Enterprises Ltd, 2012-01-01) Tsiouri, V; Andronopoulos, S; Kovalets, I; Dyer, LL; Bartzis, JG
This paper presents an evaluation of an innovative data assimilation method that has been recently developed in NCSR Demokritos for estimating an unknown emission rate of radionuclides in the atmosphere, with real-scale experimental data. The efficient algorithm is based on the assimilation of gamma dose rate measured data in the Lagrangian atmospheric dispersion model DIPCOT and uses variational principles. The DIPCOT model is used in the framework of the nuclear emergency response system (ERS) RODOS. The evaluation is performed by computational simulations of dispersion of Ar-41 that was emitted routinely by the Australian Nuclear Science and Technology Organisation’s (ANSTO) previous research reactor, HIFAR, located in Sydney, Australia. In this paper the algorithm is evaluated against a more complicated Radiation source rate estimation through data assimilation 387 case than the others used in previous studies: There was only one monitoring station available each day and the site topography is characterised as moderately complex. Overall the estimated release rate approaches the real one to a very satisfactory degree as revealed by the statistical indicators of errors. © 2012 Inderscience Enterprises Ltd.
Receptor modelling using positive matrix factorisation, back trajectories and radon-222
(Elsevier, 2007-10) Crawford, J; Chambers, SD; Cohen, DD; Dyer, LL; Wang, T; Zahorowski, W
PM2.5 aerosols were sampled and atmospheric Rn-222 (radon) was measured, at Hong Kong, China, over 3 years 2001-2003. The aerosol samples were analysed using accelerator-based Ion Beam Analysis (IBA) techniques to provide quantitative information on 21 of their major and minor elemental contributions. The radon concentration on aerosol sampling days was then used to classify the degree of land contact (high or low) experienced by air masses en route to the receptor site. It was found that elements known to originate from anthropogenic sources (e.g. Zn, K, Br, Pb and Black Carbon) were positively correlated with observed radon concentration. An eight-factor Positive Matrix Factorisation (PMF) analysis was performed on the data set, which resulted in elemental profiles ("fingerprints") for eight potential sources and we identified source factors that were correlated with radon. The Potential Source Contribution Function technique was then used to identify the geographic regions most likely to have significantly contributed to the aerosol samples collected at the receptor site. © 2007, Elsevier Ltd.
Receptor modelling with PMF2 and ME2 using aerosol data from Hong Kong
(Australian Nuclear Science and Technology Organisation, 2005-05) Crawford, J; Cohen, DD; Dyer, LL; Zahorowski, W
A number of techniques, such as principal component analysis and factor analysis, have been used in receptor modelling where measured aerosol composition at the sampling site are analysed in order to determine the likely source contributions. In this study factor analysis with nonnegative factor elements has been carried out using two techniques as implemented in the PMF2 and ME2 computer codes. The various analysis techniques provided by the two programs are illustrated using measured data at Hong Kong as a case study, which covers a period of three years (2001 to 2003). Both analysis techniques resulted in similar results which are presented in this report. Data bootstrapping was also carried out as an additional check on the quality of the results.

Browsing by Author "Dyer, LL"

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