Source Term Estimation Research Articles

Nuclear and Radiological Preparedness: The Achievements of the European Research Project PREPARE.

The PREPARE project aimed closing gaps identified in nuclear and radiological preparedness in Europe following the first evaluation of the Fukushima disaster. With 46 partners from Europe and Japan, it collected the key players in the area of emergency management and rehabilitation preparedness. Starting from February 2013, the project ended in January 2016. Among others, the project reviewed existing operational procedures for long-lasting releases, cross-border problems in radiation monitoring and food safety and further developed missing functionalities in decision support systems ranging from improved source term estimation and dispersion modelling to the inclusion of hydrological pathways for European water bodies. In addition, a so-called Analytical Platform has been developed to explore the scientific and operational means to improve information collection, information exchange and the evaluation of such types of disasters. The tools developed within the project will be partly integrated into the decision support systems ARGOS and JRODOS.

A review of source term estimation methods for atmospheric dispersion events using static or mobile sensors

Understanding atmospheric transport and dispersal events has an important role in a range of scenarios. Of particular importance is aiding in emergency response after an intentional or accidental chemical, biological or radiological (CBR) release. In the event of a CBR release, it is desirable to know the current and future spatial extent of the contaminant as well as its location in order to aid decision makers in emergency response. Many dispersion phenomena may be opaque or clear, thus monitoring them using visual methods will be difficult or impossible. In these scenarios, relevant concentration sensors are required to detect the substance where they can form a static network on the ground or be placed upon mobile platforms. This paper presents a review of techniques used to gain information about atmospheric dispersion events using static or mobile sensors. The review is concluded with a discussion on the current limitations of the state of the art and recommendations for future research.

Species and temperature predictions in a semi-industrial MILD furnace using a non-adiabatic conditional source-term estimation formulation

A Reynolds-Averaged Navier–Stokes (RANS) simulation of the semi-industrial International Flame Research Foundation (IFRF) furnace is performed using a non-adiabatic Conditional Source-term Estimation (CSE) formulation. This represents the first time that a CSE formulation, which accounts for the effect of radiation on the conditional reaction rates, has been applied to a large scale semi-industrial furnace. The objective of the current study is to assess the capabilities of CSE to accurately reproduce the velocity field, temperature, species concentration and nitrogen oxides (NOx) emission for the IFRF furnace. The flow field is solved using the standard k–ε turbulence model and detailed chemistry is included. NOx emissions are calculated using two different methods. Predicted velocity profiles are in good agreement with the experimental data. The predicted peak temperature occurs closer to the centreline, as compared to the experimental observations, suggesting that the mixing between the fuel jet and vitiated air jet may be overestimated. Good agreement between the species concentrations, including NOx, and the experimental data is observed near the burner exit. Farther downstream, the centreline oxygen concentration is found to be underpredicted. Predicted NOx concentrations are in good agreement with experimental data when calculated using the method of Peters and Weber. The current study indicates that RANS-CSE can accurately predict the main characteristics seen in a semi-industrial IFRF furnace.

A novel integrated approach for the hazardous radioactive dust source terms estimation in future nuclear fusion power plants.

An open issue still under investigation by several international entities working on the safety and security field for the foreseen nuclear fusion reactors is the estimation of source terms that are a hazard for the operators and public, and for the machine itself in terms of efficiency and integrity in case of severe accident scenarios. Source term estimation is a crucial key safety issue to be addressed in the future reactors safety assessments, and the estimates available at the time are not sufficiently satisfactory. The lack of neutronic data along with the insufficiently accurate methodologies used until now, calls for an integrated methodology for source term estimation that can provide predictions with an adequate accuracy. This work proposes a complete methodology to estimate dust source terms starting from a broad information gathering. The wide number of parameters that can influence dust source term production is reduced with statistical tools using a combination of screening, sensitivity analysis, and uncertainty analysis. Finally, a preliminary and simplified methodology for dust source term production prediction for future devices is presented.

A CFD based model to predict film boiling heat transfer of cryogenic liquids

Following an accidental spill of cryogenic liquid (e.g., LNG) on a solid substrate (e.g., concrete), the vapor generation corresponds to different boiling regimes i.e., film boiling, transition boiling, and nucleate boiling. As film boiling phenomena dictate the vapor generation in the early stage of the spill, it is considered as the most important boiling regime in the context of cryogenic (e.g., LNG) source-term estimations. This paper presents CFD simulations of cryogenic film boiling for liquid nitrogen (LN2) and LNG as pure methane. Different aspects of CFD modeling such as vapor-liquid interface morphology, the behavior of heat flux at the heated surface, the effect of wall superheats on bubbles generation frequency and bubbles departure diameter are presented. Based on the results of CFD simulations, a first principle model is applied to correlate the wall heat flux in the film boiling regime. This model can be used to enable a faster estimation of wall heat flux when CFD simulations and use of empirical correlations are not feasible.

An investigation of ionizing radiation dose in a manufacturing enterprise of ion-absorbing type rare earth ore

Objective: To investigate radioactive source term dose monitoring and estimation results in a manufacturing enterprise of ion-absorbing type rare earth ore and the possible ionizing radiation dose received by its workers. Methods: Ionizing radiation monitoring data of the posts in the control area and supervised area of workplace were collected, and the annual average effective dose directly estimated or estimated using formulas was evaluated and analyzed. Results: In the control area and supervised area of the workplace for this rare earth ore, α surface contamination activity had a maximum value of 0.35 Bq/cm2 and a minimum value of 0.01 Bq/cm2; β radioactive surface contamination activity had a maximum value of 18.8 Bq/cm2 and a minimum value of 0.22 Bq/cm2. In 14 monitoring points in the workplace, the maximum value of the annual average effective dose of occupational exposure was 1.641 mSv/a, which did not exceed the authorized limit for workers (5 mSv/a) , but exceeded the authorized limit for general personnel (0.25 mSv/a) . The radionuclide specific activity of ionic mixed rare earth oxides was determined to be 0.9. Conclusion: The annual average effective dose of occupational exposure in this enterprise does not exceed the authorized limit for workers, but it exceeds the authorized limit for general personnel. We should pay attention to the focus of the radiation process, especially for public works radiation.

Utilization of (134)Cs/(137)Cs in the environment to identify the reactor units that caused atmospheric releases during the Fukushima Daiichi accident.

The Fukushima Daiichi nuclear power reactor units that generated large amounts of airborne discharges during the period of March 12–21, 2011 were identified individually by analyzing the combination of measured 134Cs/137Cs depositions on ground surfaces and atmospheric transport and deposition simulations. Because the values of 134Cs/137Cs are different in reactor units owing to fuel burnup differences, the 134Cs/137Cs ratio measured in the environment was used to determine which reactor unit ultimately contaminated a specific area. Atmospheric dispersion model simulations were used for predicting specific areas contaminated by each dominant release. Finally, by comparing the results from both sources, the specific reactor units that yielded the most dominant atmospheric release quantities could be determined. The major source reactor units were Unit 1 in the afternoon of March 12, 2011, Unit 2 during the period from the late night of March 14 to the morning of March 15, 2011. These results corresponded to those assumed in our previous source term estimation studies. Furthermore, new findings suggested that the major source reactors from the evening of March 15, 2011 were Units 2 and 3 and that the dominant source reactor on March 20, 2011 temporally changed from Unit 3 to Unit 2.

Doubly Conditional Source-term Estimation (DCSE) for the modelling of turbulent stratified V-shaped flame

A partially premixed formulation of Conditional Source-term Estimation including doubly conditioned averages, namely DCSE, is applied to several turbulent stratified V-shaped flames using a Reynolds Averaged Navier Stokes (RANS) approach. Three different stratification levels are considered with different turbulence conditions. In contrast to the authors’ initial work (Dovizio et al., 2016), a more elaborate closure of the scalar dissipation rate is implemented in the progress variable variance equation, where a dependence on the local value of the equivalence ratio is considered. Detailed chemistry is included. Good predictions are obtained for the mean axial and transverse velocity components, and progress variable compared to experimental data for the stoichiometric cases, in particular for the lean and rich cases, where the predictions are significantly improved compared to previous numerical studies. The flame thickness is found to be reduced when the stratification increases, consistent with experimental findings. Future work will examine the closure of cross-dissipation terms in the transport equations and the DCSE implementation in LES.

Source term estimation using air concentration measurements during nuclear accident

The focus of this paper is locating and quantifying the diffusion source of 137Cs by using observation data collected from monitoring stations. The estimation method is firstly tested in synthetic experiments and then verified with real 137Cs concentration data of Fukushima accident. An atmospheric dispersion simulation system is used to support particle diffusion model. Besides, ridge regression is applied to calculate release rate. In terms of location estimation, posterior function of the source location can be deduced according to Bayesian inference. particle swarm optimization (PSO) method is implemented to locate diffusion source.

Application of Computational Fluid Dynamics in Simulating Film Boiling of Cryogens

This paper presents a computational fluid dynamic (CFD) model, simulating film boiling based on Rayleigh–Taylor (R-T) instability, using the volume of fluid (VOF) method to track the liquid/vapor interface. Film boiling of cryogenic liquids (e.g., LNG and liquid nitrogen) is simulated to estimate the vapor generation rate during an accidental spill. The simulated heat fluxes were compared with heat fluxes obtained from Berenson and Klimenko correlations. The effects of wall superheats on the bubble generation frequency were studied. This study helps researchers to understand the physics of film boiling that are useful during the risk assessment of a cryogenic spill scenario. For example, it was found that the bubble released from the node and the antinode points between the consecutive bubble generations cycles do not follow the alternating nature under the realistic film boiling conditions. Therefore, empirical expressions assuming alternating bubble generation might be unsuitable for cryogenic vaporization source term estimation.

Conditional Source-term Estimation using dynamic ensemble selection and parallel iterative solution

A modified version of the Least-Square QR-factorisation (LSQR) algorithm has been implemented in conjunction with Conditional Source-term Estimation (CSE) for lean, turbulent premixed methane–air combustion via Large Eddy Simulation (LES). The iterative solver can reduce computational times by an order of magnitude during the inversion phase of CSE in comparison with the conventional LU-decomposition method. The advantages of iterative and parallel iterative solvers become more prominent as the size of the system increases. The ensemble selection procedure for computing averages within localised regions of the simulation domain has also been updated to a dynamic routine. This allows for more flexible and efficient allocation of computational resources along with reduced input from the user, especially for complex geometries. Preliminary LES calculations have shown that the implementation of an iterative solver and a dynamic ensemble selection algorithm will reduce computational times significantly with negligible error contribution for one-condition CSE, which is applicable to purely premixed or non-premixed turbulent combustion problems. In addition, these algorithms provide the foundation for exceptional computational cost savings for the inversion in two-condition CSE, or Doubly Conditional Source-term Estimation (DCSE), which has shown promise for predicting partially-premixed combustion. Parallel computation of the inverse solution is particularly beneficial to DCSE as the computational cost of the inversion process is considerably larger than in one-condition CSE.

Application of Conditional Source-term Estimation to two turbulent non-premixed methanol flames

Conditional Source-term Estimation (CSE) is a turbulent combustion model that uses conditional averages to close the chemical source term. Previous CSE studies have shown that the model is able to predict the flame characteristics successfully; however, these studies have only focused on simple hydrocarbon fuels mostly composed of methane. The objective of the present paper is to evaluate the capabilities of CSE applied to turbulent non-premixed methanol flames, which has never been done previously. The current study investigates two different types of methanol flames: piloted and bluff-body flames. For the piloted flame, the standard k–ε model is used for turbulence modelling, while the Shear Stress Transport (SST) k–ω model is applied to the bluff-body case. Different values of empirical constants within the turbulence models were tested, and it was found that Cε1 = 1.7 for the piloted flame and γ2 = 0.66 for the bluff-body flame provided the best agreement with experimental measurements for the mixing field. Detailed chemistry is included in tabulated form using the Trajectory Generated Low-Dimensional Manifold (TGLDM) method. The predictions including both the Favre-averaged and conditional mass fraction of reactive species and temperature are compared with available experimental data and previous numerical results. Overall, the CSE predictions of conditional and unconditional quantities are in good agreement with the experimental data except for hydrogen. Sources of discrepancies are identified such as the chemical kinetics and neglect of differential diffusion. Large eddy simulations may also help to improve the velocity and mixing field predictions.

The magnitude and relevance of the February 2014 radiation release from the Waste Isolation Pilot Plant repository in New Mexico, USA

After almost fifteen years of successful waste disposal operations, the first unambiguous airborne radiation release from the Waste Isolation Pilot Plant (WIPP) was detected beyond the site boundary on February 14, 2014. It was the first accident of its kind in the 15-year operating history of the WIPP. The accident released moderate levels of radioactivity into the underground air. A small but measurable amount of radioactivity also escaped to the surface through the ventilation system and was detected above ground. The dominant radionuclides released were americium and plutonium, in a ratio consistent with the known content of a breached drum. The radiation release was caused by a runaway chemical reaction inside a transuranic (TRU) waste drum which experienced a seal and lid failure, spewing radioactive materials into the repository.According to source-term estimation, approximately 2 to 10Ci of radioactivity was released from the breached drum into the underground, and an undetermined fraction of that source term became airborne, setting off an alarm and triggering the closure of seals designed to force exhausting air through a system of filters including high-efficiency-particulate-air (HEPA) filters. Air monitoring across the WIPP site intensified following the first reports of radiation detection underground to determine the extent of impact to WIPP personnel, the public, and the environment, if any. This article attempts to compile and interpret analytical data collected by an independent monitoring program conducted by the Carlsbad Environmental Monitoring & Research Center (CEMRC) and by a compliance-monitoring program conducted by the WIPP's management and operating contractor, the Nuclear Waste Partnership (NWP), LLC., in response to the accident. Both the independent and the WIPP monitoring efforts concluded that the levels detected were very low and localized, and no radiation-related health effects among local workers or the public would be expected.

Sparse Approximation-Based Maximum Likelihood Approach for Estimation of Radiological Source Terms

A computationally efficient and accurate method is presented for identifying the number, intensity and location of stationary multiple radiological sources. The proposed method uniformly grids the region of interest resulting in a finite set of solutions for the source locations. The resulting problem is a sparse convex optimization problem based on ${{\cal L}_1}$ -norm minimization. The solution of this convex optimization encapsulates all information needed for the estimation of source terms; the values of the nonzero elements of the solution vector approximates the source intensity, the grid points corresponding to the nonzero elements approximates the source locations, and the number of nonzero elements is the number of sources. The accuracy limited by the resolution of the grid is further improved by making use of the maximum likelihood estimation approach. The performance of sparse approximation based maximum likelihood estimation is verified using real experimental data acquired from radiological field trials in the presence of up to three point sources of gamma radiation. The numerical results show that the proposed approach efficiently and accurately identifies the source terms simultaneously, and it outperforms existing methods which have been used for stationary multiple radiological source terms estimation.

Np(V) solubility, speciation and solid phase formation in alkaline CaCl2 solutions. Part II: Thermodynamics and implications for source term estimations of nuclear waste disposal

Abstract The results of comprehensive solubility experiments with Np(V) in dilute to concentrated CaCl2 solutions which included the spectroscopic investigation of the predominant aqueous Np(V) species and the thorough analysis of the solubility controlling Np(V) solid phases [1](Fellhauer, D., Rothe, J., Altmaier, M., Neck, V., Runke, J., Wiss, T., Fanghänel, Th., Np(V) solubility, speciation and solid phase formation in alkaline CaCl2 solutions. Part I: Experimental solubility (Radiochim. Acta, DOI 10.1515/ract-2015-2489), in the following referred to as “Part I”), showed that the concentration of Np(V) in these systems is limited by equilibrium reactions between previously unknown (qua)ternary solid compounds, CaNpO2(OH)2.6Cl0.4 · 2H2O(s) (I), Ca0.5NpO2(OH)2 · 1.3H2O(s) (II) and Ca0.5NpO2(OH)2(s) (III), and ternary aqueous complexes of Np(V) with the general formulae Ca x [NpO2(OH)2]2x–1, Ca y [NpO2(OH)5]2y–4 with y ≈ 2.4 ± 1.5 (EXAFS result) and innersphere Np(V) chloro complexes, Ca z [NpO2Cl]2z+ with z = 0 and 1 (EXAFS result). A systematic thermodynamic evaluation of the experimental solubility data based on the specific ion interaction theory (SIT) and the Pitzer approach is performed. The stoichiometries of the ternary hydrolysis complexes are assessed as Ca[NpO2(OH)2]+ and Ca3[NpO2(OH)5]2+. The corresponding thermodynamic solubility and complex formation constants at I = 0 as well as the ion interaction parameters (SIT and Pitzer) were derived yielding a comprehensive geochemical model for the Np(V) solubility behavior in pH neutral and alkaline CaCl2 solutions over a large range of ionic strengths. This significantly improves source term estimations for scenarios with calcium dominated aquatic systems. The principally different behavior of Np(V) in NaCl and CaCl2 solutions is highlighted, and the impact of the new findings on geochemical modelling of systems relevant for nuclear waste disposal is discussed.

Np(V) solubility, speciation and solid phase formation in alkaline CaCl2 solutions. Part I: Experimental results

Abstract The aqueous chemistry of Np(V) in alkaline 0.01 to 5.5 M CaCl2 solutions at T = 23 ± 2 ℃ was thoroughly studied by long-term batch solubility experiments from both over- and undersaturation. Applying a comprehensive set of experimental and spectroscopic techniques including Vis/NIR and Np L3-edge EXAFS, the solubility controlling Np(V) solid phases and the predominant aqueous Np(V) species were identified. The results demonstrate that the solubility behavior of Np(V) in alkaline CaCl2 solutions differs completely from the one reported for alkaline NaCl and NaClO4 solutions: as solubility limiting solid phases, three so far not considered Ca-Np(V)-OH compounds were identified and their solubility and stability in CaCl2 solutions analyzed: CaNpO2(OH)2.6Cl0.4 · 2H2O(s) (I) (metastable), Ca0.5NpO2(OH)2 · 1.3H2O(s) (II) (long-term metastable) and Ca0.5NpO2(OH)2(s) (III) (stable). The considerably higher stability of these (qua)ternary phases which readily form in alkaline CaCl2 solutions from oversaturation (addition of NpO2 +) and undersaturation (addition of binary NpO2OH(am)) limit the Np(V) equilibrium concentrations to values that are up to 3 log-units lower compared to those for NpO2OH(am) in NaCl and NaClO4 systems. Based on systematic evaluation of the pH dependence (solubility curve slopes), Vis/NIR and EXAFS spectroscopic information, unhydrolysed NpO2 + and innersphere chloro complexes, NpO2Cl(aq) and Ca z [NpO2Cl]2z+ with z = 1 (EXAFS, Vis/NIR), were identified as prevailing Np(V) species in less alkaline solutions with pHm < 10.5 for [CaCl2] ≤ 2.0 M and [CaCl2] > 2.0 M, respectively. The steep increase of the Np(V) solubility for pHm > 11 with slopes of approximately + 2.5 (for solid phase (I)) and + 3 (for solids (II) and (III)) as well as the Np L3-edge EXAFS results for the aqueous Np(V) species in 4.5 M CaCl2/pHm ≈ 12 confirm the presence of ternary Ca-Np(V)-OH complexes Ca x [NpO2(OH)2]2x–1 (x ≈ 1 is estimated within the thermodynamic evaluation in a subsequent paper (Fellhauer, D., Altmaier, M., Gaona, X., Lützenkirchen, J., Fanghänel, Th., Np(V) solubility, speciation and solid phase formation in alkaline CaCl2 solutions. Part II: Thermodynamics and implications for source term estimations of nuclear waste disposal (Radiochim. Acta, DOI 10.1515/ract-2015-2490), in the following referred to as “Part II” [1]) and Ca y [NpO2(OH)5]2y–4 with y ≈ 2.4 ± 1.5 (EXAFS result) as predominant Np(V) hydrolysis species in solution for [CaCl2] ≥ 0.25 and pHm > 10.5. The thermodynamic evaluation and implications of the new findings with respect to source term estimations for nuclear waste disposal scenarios are discussed in “Part II” [1]).

RANS Simulations of a Series of Turbulent V-Shaped Flames using Conditional Source-term Estimation

In the present study, Reynolds Averaged Navier Stokes (RANS) simulations are applied to a series of turbulent V-shaped flames. Two formulations of Conditional Source-term Estimation (CSE) are developed using singly and doubly conditioned averages for turbulent premixed and partially premixed flames, respectively. Detailed chemistry is included. Conditionally averaged chemical source terms are closed by conditional averaged scalars which are obtained by inverting an integral equation. The objectives are to study a turbulent premixed V-shaped flame using the premixed CSE approach and apply the Doubly Conditional CSE (DCSE) combustion model to a case of stratified combustion. The partially premixed implementation involves double conditioning on two variables, mixture fraction and progress variable. The present study represents the first application of DCSE for a series of turbulent stratified flames. First, CSE is analysed for fully premixed conditions. A sensitivity analysis on the number of CSE ensembles and different scalar dissipation model closures is performed. Good results are obtained in terms of velocity and progress variable profiles. Finally, the partially premixed formulation is applied to the stratified case at three different conditions, corresponding to two different turbulence grids and three different profiles of the equivalence ratio, providing promising results.

Overview of WP4: extension of atmospheric dispersion and consequence modelling in Decision Support Systems

The activities that have been carried out within the frame of Working Package 4 of PREPARE project, entitled “Extension of atmospheric dispersion and consequence modelling in Decision Support Systems (DSSs)” can be grouped under the following themes: (1) Source term (quantity and time variation of release rate of radionuclides from a nuclear accident) estimation based on optimal combination of atmospheric dispersion modelling with measurements through simple and more advanced computational methods integrated within operational DSSs. (2) Implementation of physicochemical properties of radionuclides emitted as particulate matter in computational modules of DSSs that simulate their environmental transport from deposition to mobility in soil and estimate the resulting radiation doses. (3) Extension/update of existing DSSs in the area of atmospheric dispersion on the basis of recent experiences, technological advances and users' requirements. In conclusion, the new features that have been developed regarding atmospheric dispersion and consequence modelling in the frame of PREPARE project, have substantially increased the capabilities of operational DSSs, and have also revealed directions for future research and development.

Fast and simple method with NPP fence measurements for source term estimation and its integration in JRODOS

A fast and simple method using Nuclear Power Plant fence measurements for source term estimation is presented in this work. The method is suitable for usage in a decision support system for off-site emergency management such as JRODOS. The article provides short introduction to basic principles and model application, input data, methodology, uncertainty handling, outputs and integration of software tool in JRODOS.

Innovative integrative tools and platforms. Key results of the PREPARE European Project

The European project PREPARE (Innovative integrated tools and platforms for radiological emergency preparedness and post-accident response in Europe) aims at closing gaps that have been identified in nuclear and radiological preparedness following the first evaluation of the Fukushima disaster. It started February 2013 and ended January 2016. Fourty-six partners from Europe and Japan, among them universities, national research centres, operational emergency management centres and non-governmental organisations, performed research to close gaps identified following the Fukushima disaster. Among others, the project addressed the review of existing operational procedures for dealing with long lasting releases, cross border problems in radiation monitoring and food safety and further development of missing functionalities in decision support systems ranging from improved source term estimation and dispersion modelling to the inclusion of hydrological pathways for European water bodies. In addition, a so called Analytical Platform was developed exploring the scientific and operational means to improve information collection, information exchange and the evaluation of such types of disasters. The tools developed within the project will be integrated into the two decision support systems ARGOS and RODOS.