In-situ Field Gamma Spectrometry in a Radionuclide Air Sampler.

ABSTRACT: The work being presented is on the development of a system to measure the speciation of airborne radionuclide emissions from the environment during a nuclear emergency. On-site air sampling measurements that were conducted during the Fukushima Daiichi accident were limited because field teams had to be sent out to run the sampling systems and retrieve the filters for gamma spectrometry analysis in a separate laboratory. The start of air sampling was delayed, and it was impossible for emergency responders to use the information about the airborne radionuclide composition in a timely way. The goal of the current study is to develop a system that could provide live, near real-time information about the concentrations of different radionuclides in the air without having to rely on human intervention. The development of the prototype in the current work is largely being enabled by Cd-Zn-Te spectrometers, which provide reasonably high-resolution spectrometry given that it is a room temperature sensor, and allow the measurements to be conducted in the field. A custom filter cartridge has been designed to hold a pair of aerosol and iodine filters in place while keeping the gamma spectrometers as close as possible in order to obtain high count rate efficiencies. A single cartridge holds both filters and has an internal flow channel directing the air flow between them. The cartridge design also facilitates replacing the filters as the accumulated radioactivity on the filters becomes too high. An automation system can move a filter cartridge from the fresh cartridge storage bank to the sampling location (filtration and gamma spectrometry) and return the used filter cartridge to the used cartridge storage bank. The radionuclide air sampling system prototype has been designed and constructed. It has been tested with fixed sources located on the respective aerosol and iodine filters. The real-time data capture aspects of the system were also demonstrated with a live 131I capture experiment. The projected performance of the system during a reactor accident was also simulated, emulating the characteristic detector efficiencies and projecting how the airborne concentrations could be reconstructed. The study has designed and constructed a radionuclide air sampler that could be used for measuring airborne radioactivity in emissions from a nuclear accident. Because the gamma spectrometry measurements are done in situ with good resolution and the system is automated, it would allow data to be transmitted back to an emergency operations center immediately rather than having to wait for additional laboratory analysis.

Dispersion Simulations of Radon Discharges between Neighboring Buildings and Their Sensitivity to Meteorology, Discharge Rate, and Building Geometry.

ABSTRACT: In northern climates, it is common to install the discharges of radon sub-slab depressurization systems near ground level. However, this also elevates the ground level outdoor radon concentrations and raises the possibility of radon re-entrainment into homes. The study aims to assess outdoor radon concentrations near above-ground-level discharges along the surfaces of an emitting building and its close neighbor and identify parameters that most influence the dilution. This study employs a series of computational fluid dynamics calculations to assess concentrations along the exhaust-facing and non-exhaust-facing surfaces of the buildings. Different meteorological, venting, and building geometry parameters are explored. Boundary conditions for the CFD calculations are based on field measurements of the ground-level wind speeds and seasonal air temperatures and atmospheric stabilities. Outdoor concentrations can be as high as 7% of the discharge gas, although these become smaller at greater distances from the vent. The direction of the prevailing wind is a particularly important parameter, as it influences the formation of circulating building cavities and building wakes where radon could accumulate. The wind speed, atmospheric stability, and season (plume buoyancy) also have important influences on the outdoor radon concentrations, as do the velocity of the vent system and the size of the buildings. The study has assessed the dilution of the radon-laden exhaust gas and determined the outdoor concentrations that can be expected under a variety of conditions. These results can be used to inform regulators about the potential for radon re-entrainment into homes.

A MCREXS modelling approach for the simulation of a radiological dispersal device.

Assessing the risks of radioactive dose in a radiological dispersal device (RDD) attack requires knowledge of how the radiological materials will be spread through the air surrounding the site of the detonation. Two essential parts of the accurate prediction of the behaviour of this dispersion are a characterization of the initial cloud size, directly after the blast, and detailed modelling of the behaviour of different size particulates. Capturing the transport of contaminants from the initial blast wave is integral to achieving accurate predictions, especially for regions where the blast dynamics dominates, but performing such calculations over a wide range of particle sizes and spatial scales is computationally challenging. Formulation of efficient computational techniques for such advanced models is required to provide predictive tools useful to first responders and emergency planners. In this work, a Multi-Cloud Radiological EXplosive Source (MCREXS) modelling approach for RDD is investigated. This approach combines a stochastic, particle-based, mechanistic model with a standard atmospheric dispersion model. The former is used to characterize the distribution of radioactive material near the source of the explosion, where the blast wind effects are important, while the latter is used to model the transport of the contaminant in the environment over large areas. The particle transport in the near-field of the explosion site is computed based on a Lagrangian description of the particle phase and a reconstructed-Eulerian field for the carrier phase. The information inferred from this physics-based model is then used as a starting point for a subsequent standard Gaussian puff model to calculate the dispersion of the radioactive contaminant. The predictive capabilities of the MCREXS model are assessed against the 2012 DRDC Suffield full-scale RDD experiments. The results demonstrate improved predictions relative to those performed using only a Gaussian puff calculation from an empirical initial cloud distribution.

Overview of the Full-scale Radiological Dispersal Device Field Trials.

In 2012, Defence Research and Development Canada, in partnership with a number of other Canadian and International organizations, led a series of three field trials designed to simulate a Radiological Dispersal Device (RDD). These trials, known as the Full-Scale RDD (FSRDD) Field Trials, involved the explosive dispersal of a short-lived radioactive tracer ((140)La, t1/2 = 40.293 h). The FSRDD Field Trials required a significant effort in their planning, preparation, and execution to ensure that they were carried out in a safe, efficient manner and that the scientific goals of the trials were met. The discussion presented here details the planning and execution of the trials, outlines the relevant radiation safety aspects, provides a summary of the source term and atmospheric conditions for the three dispersal events, and provides an overview of the measurements that were made to track the plumes and deposition patterns.

Real Time in Situ Gamma Radiation Measurements of the Plume Evolution from the Full-Scale Radiological Dispersal Device Field Trials.

During the Full-Scale Radiological Dispersal Device Field Trials carried out in Suffield in 2012, several suites of detection and sampling equipment were used to measure and characterize the explosive dispersal of the short half-life radioactive tracer Lanthanum-140 ((140)La). The equipment deployed included networks of in situ real-time radiation monitoring detectors providing measurements of different sensitivities and characteristics. A dense array of lower sensitivity detectors was established near field, ranging from 10 to 450 m from the detonation location. A sparser array of more sensitive detectors was established in the far field (150 m to 3.5 km from the detonation location). Each was used to collect and report the dose rate data from the radioactive plume passage with a sample time resolution of 1 s. The two systems went through independent calibrations and were compared and shown to be consistent with each other. The in situ gamma radiation measurements have allowed the movement and evolution of the plume to be described and to identify deposition rates and non-uniformities in the temporal shape of the plume. This knowledge could be applied for emergency planning guidance for the case of release of radioactive material by a radiological dispersive device.

Deposition Measurements From the Full-Scale Radiological Dispersal Device Field Trials.

In 2012, Defence Research and Development Canada led a series of experiments, titled the Full-Scale Radiological Dispersal Device Field Trials, in which short-lived radioactive material was explosively dispersed and the resulting plume and deposition were characterized through a variety of methods. Presented here are the results of a number of measurements that were taken to characterize the radioactive ground deposition. These included in situ gamma measurements, deposition filter samples, and witness plate measurements that were taken in situ with handheld beta survey meters. The results from the different measurement techniques are compared to each other and to a simple deposition model. Results showed that approximately 3% of the original source activity was deposited in the immediate vicinity of ground zero, and an additional 15-30% of the original activity was deposited within 450 m of ground zero. Implications of these results for emergency response are discussed.

The Sensitivity of Atmospheric Dispersion Calculations in Near-field Applications: Modeling of the Full Scale RDD Experiments with Operational Models in Canada, Part I.

Three radiological dispersal devices were detonated in 2012 under controlled conditions at Defence Research and Development Canada's Experimental Proving Grounds in Suffield, Alberta. Each device comprised a 35-GBq source of (140)La. The dataset obtained is used in this study to assess the MLCD, ADDAM, and RIMPUFF atmospheric dispersion models. As part one of a two-part study, this paper focuses on examining the capabilities of the above three models and evaluating how well their predictions of air concentration and ground deposition match observations from the full-scale RDD experiments.

Radiation Modeling and Finite Cloud Effects for Atmospheric Dispersion Calculations in Near-field Applications: Modeling of the Full Scale RDD Experiments with Operational Models in Canada, Part II.

Three radiological dispersal devices were detonated in 2012 under controlled conditions at Defence Research and Development Canada's Experimental Proving Grounds in Suffield, Alberta. Each device comprised a 35-GBq source of (140)La. The dataset obtained is used in this study to assess the MLCD, ADDAM, and RIMPUFF atmospheric dispersion models. As a continuation of Lebel et al. (2016), this paper examines different methodologies for making dose estimates with atmospheric dispersion models.

Radioiodine in the atmosphere after the Fukushima Dai-ichi nuclear accident.

About 160 PBq of (131)I was released into the atmosphere during the accident at the Fukushima Dai-ichi Nuclear Power Plant. The chemistry of radioiodine is complicated, and it can be released in several different forms. In addition, the different physical forms, like molecular iodine, aerosol-form iodine, or organic iodine, would have all behaved differently once in the atmosphere, and would have been removed at different rates. These releases were detected by monitoring stations throughout Japan, and from these measurements, key insights can be made about the different chemical forms that were released, as well as the persistence of each in the environment.