Source term assessment using inverse modeling and environmental radiation measurements for nuclear emergency response

Abstract

A simple model 'Assessment of Source Term for Emergency Response (ASTER)' for estimation of time-dependent atmospheric source term based on real-time environmental gamma dose rate measurements is developed for application in Online Nuclear Emergency Response System (ONERS). The model inverts the measured dose rates to the airborne radioactive release rate using the dilution factors predicted by a random-walk particle atmospheric dispersion model called System for Prediction of Environmental Emergency Dose Information (SPEEDI) taking into account the real-time meteorological observations and location coordinates of the gamma radiation detectors. A point-kernel method for estimating the cloud-gamma dose rates at receptor locations is implemented in SPEEDI for this purpose. ASTER is validated against Ar-41 routine release data from a pressurized heavy-water reactor (PHWR) at Kalpakkam for over 100 days covering different seasons during 2014–2015. Comparison of ASTER-simulated overhead plume gamma dose rates with the measured dose rates at detector points gives a correlation of R = 0.59, bias of 89 nGy/h, and RMSE of 104.09 nGy/h indicating a slight overestimation of gamma dose rates at monitor points. Comparison of ASTER-predicted source term with Ar-41 release rates indicated that about 90% of the calculations fall within a factor of 5 of the actual source term. A correlation of 0.584, bias of 0.152 TBq/day, and RMSE of 0.192 TBq/day are found between the computed and actual daily source term values indicating reasonable agreement. The slight underestimation in the computed source term is due to the overestimation of the dose rate by the dispersion model, location of few monitors near buildings and tree canopies, uneven distribution of detectors in various wind direction sectors, and the use of daily source term values for comparison.

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