research interests

Estimation of adult and child whole body dose conversion factors for external plume exposure using Monte Carlo techniques

The external radiation exposure from an overhead plume containing gamma emitting radionuclides can contribute substantial dose to the ground receptor during normal operations as well as accidental release conditions of nuclear facilities. In order to estimate the effective dose conversion coefficients (DCCs) directly, a finite plume Monte Carlo model along with the reference phantom at the ground receptor location needs to be implemented. In the present study, a comprehensive simulation of radiation transport from the Gaussian plume source to the ICRP reference adult voxel phantoms (receptor) is carried out using the FLUKA Monte Carlo code. The organ absorbed doses as well as the effective DCCs of the adult reference phantom are computed for different meteorological parameters and downwind distances. To illustrate the application of this model, an overhead Gaussian plume containing two different gamma emitting radionuclides, ¹³⁵Xe and ⁴¹Ar are considered. From these simulations, the ratio of the effective dose rate to the kerma rate are estimated as 0.6 Sv Gy^-1 and 0.65 Sv Gy^-1 for the exposure from ¹³⁵Xe and ⁴¹Ar, respectively. This ratio is constant irrespective of the meteorological conditions and cloud models. Further results show that the effective DCCs as a function of the downwind distance vary by an order of magnitude for an unstable weather category; however, the variations are very small in the case of a stable category. This study demonstrates an accurate method for calculating the effective dose to the ground receptor from an external plume which can be further applied for any radionuclide under any meteorological condition.

Development of deterministic model for estimation of radiological dose in public domain due to concealed radioactive sources

Modelling of dose distribution of randomly moving population around a radioactive source is a complex problem. The objective is to develop a model and solution techniques to estimate radiation absorbed dose to the population randomly moving around a radioactive source. The problem is formulated using a second order partial differential equation; different moments of the dose distribution function are defined related to physically realizable quantities, and solutions are obtained using standard moments methods. Alternatively, numerical simulations are performed to estimate the radiation doses using Monte Carlo approach for individual positions and random motions of the people around the source. A good agreement is found between average doses obtained from moments method and numerical simulations. A typical application of this model to different exposure conditions shows that the average dose is highly dependent on the population density. The study results show that average dose decreases with increase in the population density and movement area of random walker. This mathematical model can be used as a rapid assessment tool by the emergency planners in resource optimization by providing quick estimates of likely exposures for triage and emergency response.

Average absorbed dose (Sv) to a random walker exposed to 37 GBq of (a) ⁶⁰Co and (b) ¹³⁷Cs source at the center of 2-D circular finite plane - Monte Carlo simulation results as a function of exposure time, population density and outer radius of the circle (bounded area)

Development of a fast and accurate radioactive waste assay system based on multipoint dose rate measurement

Dose-to-Curie (DTC) conversion is a fast and simple method for quantification of radionuclide content in solid waste packages with a prior knowledge of waste matrix and radionuclide composition of the waste stream. A dose to curie conversion factor generated based on an assumed radioactivity distribution in the package is used for conversion of the measured dose rate to activity. The difference between the radionuclide distribution for drum from field and the assumed distribution is a major source of error in activity estimation using this technique. In this work, the systematic error of DTC method, due to the spatial variation of a single hot-spot in 200 L solid waste drum is subjected to systematic analysis using Monte-Carlo simulation. Data analysis was carried out with 1920 source locations within the drum and up to sixteen measurement points for dose rates around the drum. The span of error obtained for different configurations of detectors were compared to optimise the waste drum assay system. The general trends observed in simulation were found to be in good agreement with the experimental measurements done using a ¹³⁷Cs (318.2 MBq) standard source placed at selected locations. The results presented here clearly establish the advantage of multipoint dose rate measurement to improve the accuracy in activity estimation using DTC method.