Source Term Estimation Research Articles

Reconstructing height of an unknown point release using least‐squares data assimilation

Reckoning the height of a release in the source term estimation is important since a ground‐level approximation of the release leads to errors in capturing the actual extent of a plume. A least‐squares inversion technique, free from initial guess, is adapted here for the reconstruction of an elevated point release in a discretized space. Primarily, this involves estimation of the effective height of the release above the ground along with its location and strength from a limited set of noisy concentration measurements. The methodology is evaluated here with the nine runs from the Idaho diffusion experiment (1974) corresponding to low wind, stable conditions. Both real and model‐generated synthetic data are used to test the method. With synthetic data, the methodology can exactly reproduce the input source terms. With real data, the average release height is estimated as 3.1 m, which is very close to the effective release height (3 m) reported in the Idaho data. The release location is retrieved with an average error of 30 m, whereas the minimum distance between source and detectors is 100 m; strength is retrieved within a factor of two in all the runs. The deviations in the source parameters from their prescribed values are explained in the context of model representativeness, wind variability and available monitoring network. The sensitivity of the source term estimation is evaluated against several parameters: (i) receptors' height either neglected or duly taken as 0.76 m, (ii) measurement noises and (iii) number of receptors utilized in the inversion. In addition, the limitations and meteorological issues related to the inversion of an elevated release are highlighted.

PREPARE: innovative integrated tools and platforms for radiological emergency preparedness and post-accident response in Europe.

The PREPARE project that started in February 2013 and will end at the beginning of 2016 aims to close gaps that have been identified in nuclear and radiological preparedness in Europe following the first evaluation of the Fukushima disaster. Among others, the project will address the review of existing operational procedures for dealing with long-lasting releases and cross-border problems in radiation monitoring and food safety and further develop missing functionalities in decision support systems (DSS) 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 will be 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 partly integrated into the two DSS ARGOS and RODOS.

Numerical simulation of the Delft-Jet-in-Hot-Coflow (DJHC) flame using Conditional Source-term Estimation

RANS simulations of the Delft-Jet-in-Hot-Coflow (DJHC) flame operating in the Moderate and Intense Low Oxygen Dilution (MILD) combustion mode, are performed using the Conditional Source-term Estimation (CSE) approach. The non-premixed CSE formulation is extended, by introducing a second mixture fraction. Two flames are simulated, DJHC-I 4100 and DJHC-I 8800. Detailed chemistry tabulations and radiation are included. The objective of the current study is to assess the capabilities of CSE to accurately reproduce the velocity field, temperatures and lift-off height using the DJHC experimental conditions. The present RANS-CSE predictions for the axial velocity and turbulent kinetic energy agree well with the experimental data for both flames. For DJHC-I 4100, the radial CSE temperature predictions are in very good agreement with the experiments. The current CSE performs better compared to previous numerical simulations. This is explained by differences in model formulation and mesh configuration. For the second flame, a good agreement is achieved, in particular in the second half of the flame in the radial direction. A temperature overprediction is observed near the centreline. At the last axial location, CSE overpredicts the temperatures for most parts of the flame. These discrepancies are explained by some inaccuracies in the turbulent mixing due to RANS. The trend of lift-off height decreasing with increasing Reynolds number is well reproduced. However, the lift-off height appears to be lower than the experimental value. The present predictions are shown to be independent of the number of CSE ensembles. Further improvement is expected with LES for the turbulent flow and mixing fields. A third conditioning variable may also be considered to improve the lift-off height predictions.

Performance of conditional source-term estimation model for LES of turbulent premixed flames in thin reaction zones regime

Conditional source-term estimation (CSE) is a subfilter-scale (SFS) model for representing turbulence-chemistry interactions in turbulent reactive flows. In the present study, the CSE model is coupled with a two-dimensional flame-generated manifold (FGM) chemistry reduction technique and modified laminar-flame-based probability density function (PDF) and applied to large eddy simulation (LES) of laboratory-scale turbulent premixed flames. CSE estimates conditional moments by solving an inverse problem and uses a conditional moment closure to determine the influence of the unresolved turbulence on the reaction rates. While the CSE model has been used extensively in simulating turbulent non-premixed flames, the study represents a first application of the combustion model to LES of premixed flames. Two axisymmetric Bunsen-type premixed turbulent methane–air flames are examined corresponding to lean and stoichiometric conditions with both lying within the thin reaction zones regime. The LES predictions are compared to available experimental data for the two flames. The predictions of the CSE model are also compared to those of a more traditional flamelet-based approach: the presumed conditional moment (PCM) with flame prolongation of intrinsic low-dimensional manifolds (FPI) tabulated chemistry, or so-called PCM-FPI model. The comparisons of the CSE and PCM-FPI models allow the relative importance of deviations from the standard flamelet assumption to be assessed for flames lying outside the flamelet premixed combustion regime. The performance of CSE for LES of turbulent premixed combustion is evaluated. The CSE combustion model is found to be stable and converges to physically meaningful values. It also yields LES results that agree very well with experiment, both qualitatively and quantitatively, and would seem able to incorporate correctly the influence of turbulent strain on conditionally-filtered quantities and hence burning rate as flame behaviour deviates from that of a flamelet in the thin reaction zones regime.

Performance and first results of fission product release and transport provided by the VERDON facility

One of the most important areas of research concerning a hypothetical severe accident in a light water reactor (LWR) is determining the source term, i.e. quantifying the nature, release kinetics and global released fraction of the fission products (FPs) and other radioactive materials. In line with the former VERCORS programme to improve source term estimates, the new VERDON laboratory has recently been implemented at the CEA Cadarache Centre in the LECA-STAR facility. The present paper deals with the evaluation of the experimental equipment of this new VERDON laboratory (furnace, release and transport loops) and demonstrates its capability to perform experimental sequences representative of LWR severe accidents and to supply the databases necessary for source term assessments and FP behaviour modelling.

Screening sensitivity analysis of a radionuclides atmospheric dispersion model applied to the Fukushima disaster

Numerical models used to forecast the atmospheric dispersion of radionuclides following nuclear accidents are subject to substantial uncertainties. Input data, such as meteorological forecasts or source term estimations, as well as poorly known model parameters contribute for a large part to this uncertainty.A sensitivity analysis with the method of Morris was carried out in the case of the Fukushima disaster as a first step towards the uncertainty analysis of the Polyphemus/Polair3D model. The main difficulties stemmed from the high dimension of the model's input and output. Simple perturbations whose magnitudes were devised from a thorough literature review were applied to 19 uncertain inputs. Several outputs related to atmospheric activity and ground deposition were aggregated, revealing different inputs rankings. Other inputs based on gamma dose rates measurements were used to question the possibility of calibrating the inputs uncertainties.Some inputs, such as the cloud layer thickness, were found to have little influence on most considered outputs and could therefore be safely discarded from further studies. On the contrary, wind perturbations and emission factors for iodine and caesium are predominant. The performance indicators derived from dose rates observations displayed strong sensitivities. This emphasises the share of the overall uncertainty due to input uncertainties and asserts the relevance of the simple perturbation scheme that was employed in this work.

GEANT4, MCNPX, and SHIELD code comparison concerning relativistic heavy ion interaction with matter

1 During the operation of NICA (the Nuclotron� Based Ion Collider FAcility at JINR), secondary radia� tion will be generated along the collider rings, especially at the maximum beam loss locations. The ensuring of radiation protection measures at the collider requires solving a number of crucial problems: the estimation of the main source terms; prognostication of neutron flu� ence and the effective dose directly behind the shielding and in the environment around the facility; projection of the levels and evolution of induced radioactivity; assurance of all technological elements' radiation hard� ness; and so on. The reliable collider radiation forecast can be based on the simulation of different effects by a Monte Carlo (MC) radiation transport codes, where the projectiles are heavy ions. Many verifications of a few such multipurpose MC codes (FLUKA, GEATN4, MARS15, MCNPX, PHITS, and SHIELD) were carried out with bench� mark experimental data on A–A and A–target inter� actions for light and intermediate atomic weight pro� jectiles with energies of up to several hundred MeV. At the same time, experimental data on neutron produc� tion for primary very heavy ions with an energy of sev� eral GeV/n are practically lacking. Therefore, the pre� liminary choice of a proper code was done by inter� comparing FLUKA, GEANT4, and SHIELD simulations with unique experimental data on neutron production in a 1 GeV/n

Separation of ash and sulfur dioxide during the 2011 Grímsvötn eruption

Modeling the transport of volcanic ash and gases released during volcanic eruptions is crucially dependent on knowledge of the source term of the eruption, that is, the source strength as a function of altitude and time. For the first time, an inversion method is used to estimate the source terms of both volcanic sulfur dioxide (SO2) and ash. It was applied to the explosive volcanic eruption of Grimsvotn, Iceland, in May 2011. The method uses input from the particle dispersion model, FLEXPART (flexible particle dispersion model), a priori source estimates, and satellite observations of SO2 or ash total columns from Infrared Atmospheric Sounding Interferometer to separately obtain the source terms for volcanic SO2 and fine ash. The estimated source terms show that SO2 was emitted mostly to high altitudes (5 to 13 km) during about 18 h (22 May, 00-18 UTC) while fine ash was emitted mostly to low altitudes (below 4 km) during roughly 24 h (22 May 06 UTC to 23 May 06 UTC). FLEXPART simulations using the estimated source terms show a clear separation of SO2 (transported mostly northwestward) and the fine ash (transported mostly southeastward). This corresponds well with independent satellite observations and measured aerosol mass concentrations and lidar measurements at surface stations in Scandinavia. Aircraft measurements above Iceland and Germany confirmed that the ash was located in the lower atmosphere. This demonstrates that the inversion method, in this case, is able to distinguish between emission heights of SO2 and ash and can capture resulting differences in transport patterns. Key Points Ash and SO2 source terms estimated using inverse techniques and satellite data The transport and separation of ash and SO2 are modeled Model simulations correspond well with a range of independent observations ©2014. The Authors.

Derivation of the source term, dose results and associated radiological consequences for the Greek Research Reactor – 1

The estimated source term, dose results and radiological consequences of selected accident sequences in the Greek Research Reactor – 1 are presented and discussed. A systematic approach has been adopted to perform the necessary calculations in accordance with the latest computational developments and IAEA recommendations. Loss-of-coolant, reactivity insertion and fuel channel blockage accident sequences have been selected to derive the associated source terms under three distinct containment ventilation scenarios. Core damage has been conservatively assessed for each accident sequence while the ventilation has been assumed to function within the efficiency limits defined at the Safety Analysis Report. In case of lack of ventilation a parametric analysis is also performed to examine the dependency of the source term on the containment leakage rate. A typical as well as an adverse meteorological scenario have been defined in the JRODOS computational platform in order to predict the effective, lung and thyroid doses within a region defined by a 15km radius downwind from the reactor building. The radiological consequences of the eighteen scenarios associated with the accident sequences are presented and discussed.

Inverse analysis and regularisation in conditional source-term estimation modelling

Conditional Source-term Estimation (CSE) obtains the conditional species mass fractions by inverting a Fredholm integral equation of the first kind. In the present work, a Bayesian framework is used to compare two different regularisation methods: zeroth-order temporal Tikhonov regulatisation and first-order spatial Tikhonov regularisation. The objectives of the current study are: (i) to elucidate the ill-posedness of the inverse problem; (ii) to understand the origin of the perturbations in the data and quantify their magnitude; (iii) to quantify the uncertainty in the solution using different priors; and (iv) to determine the regularisation method best suited to this problem. A singular value decomposition shows that the current inverse problem is ill-posed. Perturbations to the data may be caused by the use of a discrete mixture fraction grid for calculating the mixture fraction PDF. The magnitude of the perturbations is estimated using a box filter and the uncertainty in the solution is determined based on the width of the credible intervals. The width of the credible intervals is significantly reduced with the inclusion of a smoothing prior and the recovered solution is in better agreement with the exact solution. The credible intervals for temporal and spatial smoothing are shown to be similar. Credible intervals for temporal smoothing depend on the solution from the previous time step and a smooth solution is not guaranteed. For spatial smoothing, the credible intervals are not dependent upon a previous solution and better predict characteristics for higher mixture fraction values. These characteristics make spatial smoothing a promising alternative method for recovering a solution from the CSE inversion process.

Assessment of atmospheric dispersion and radiological impact from the Fukushima accident in a 40-km range using a simulation approach

In this study, radiation doses and lifetime attributable health risk to the members of public in a 40-km impact zone around the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) were assessed for the airborne releases that occurred during 11–31 March 2011 from the loss of coolant accident associated with the east Japan earthquake and tsunami event. High-resolution simulations with FLEXPART-WRF Lagrangian particle dispersion model were made using available source term estimates for four significant isotopes (Cs134, Cs137, I131, Xe133). Radiation risk models were adopted to estimate the health risk for leukemia, breast, thyroid, and all solid cancers using simulated distributed lifetime organ doses. The simulations indicate occurrence of hotspots in the spatial activity deposition and radiation dose distribution with high values in the northwest and south-southwest land sectors in a 40-km radius. Large activity depositions (106 to 108 Bq/m2 of Cs137 between 12 and 31 March 2011) and external air doses (10 to 100 μSv/h) are simulated in the above sectors. The simulated air dose rates are found to match with observed doses at 85 % of the monitor stations within a factor of 5. It is estimated that the groundshine and ingestion dose are the principal contributors of the effective dose. Simulated average effective dose during the first year of exposure varied as ~150 mSv in the first few kilometers to 2 mSv at 40 km. The risk incidence was estimated to be high for infants compared to the children and adult age group for all types of cancers. The first 0–20-km range of the FDNPP is characterized with high risk for all types of cancers, for example, 10 to 20 in 10,000 adults for leukemia. The analysis shows that the immediate implementation of countermeasures of evacuation in the 0–20-km zone, sheltering in the 20–40-km zone, and food restrictions by Japan actually reduced significant health risks to the population living near FDNPP. Simulated yearly distribution of total dose indicates that people of evacuated zone can be rehabilitated in about 16 years in the 5–10-km area and 7 years in the 10–20-km area with minimum risk, whereas the near reactor zone of 0–5 km and areas along the plume footprint up to 20 km in the northwest sector of FDNPP require special attention and reclamation measures for rehabilitation. This study demonstrates a total simulation-based approach for estimating the radiation risk for the Fukushima case and helps to assess the time attainment of low risk for inhabitation of the people in the affected areas.

Graphite Particle Adhesion to Hastelloy X: Measurements of the Adhesive Force with an Atomic Force Microscope

In high-temperature gas reactors, graphite particle adhesion and resuspension from structural surfaces play a role in source term estimations. This paper describes measurements of the adhesion force between an irregular graphite cluster (henceforth called a graphite particle) and Hastelloy X samples having different surface conditions. An atomic force microscope (AFM) was used. The graphite particle was attached to the AFM probe and then brought directly into and out of contact with the surface in air; the adhesion force was obtained from the resultant force curve. The adhesion forces of the graphite particle with Hastelloy X (as received, polished, and different oxidations) and mica surfaces were determined. From the resulting adhesion forces, the work of adhesion W12 (energy per unit area) was calculated. Although the values of the measured pull-off (adhesion) forces were found to be of the same order of magnitude, they differed by surface condition depending where on the sample the adhesion force was measured. The theoretical value of the adhesion force was calculated using the theory of Johnson, Kendall, and Roberts. When compared to the values calcluated from this theory, the measured values were lower by a factor of 100 in some cases and 1000 in others. This difference may be due to the approximation of the irregular graphite cluster probe as a perfect graphite particle sphere and to not taking into consideration asperities on the surface of the particle probe. Additionally, covalent bonds may form between the surface elements and the graphite particle because of the applied load. In this paper, the effects of oxidation on the adhesion of graphite particles to the mica and Hastelloy X surfaces are also discussed.

An inverse modeling method to assess the source term of the Fukushima Nuclear Power Plant accident using gamma dose rate observations

Abstract. The Chernobyl nuclear accident, and more recently the Fukushima accident, highlighted that the largest source of error on consequences assessment is the source term, including the time evolution of the release rate and its distribution between radioisotopes. Inverse modeling methods, which combine environmental measurements and atmospheric dispersion models, have proven efficient in assessing source term due to an accidental situation (Gudiksen, 1989; Krysta and Bocquet, 2007; Stohl et al., 2012a; Winiarek et al., 2012). Most existing approaches are designed to use air sampling measurements (Winiarek et al., 2012) and some of them also use deposition measurements (Stohl et al., 2012a; Winiarek et al., 2014). Some studies have been performed to use dose rate measurements (Duranova et al., 1999; Astrup et al., 2004; Drews et al., 2004; Tsiouri et al., 2012) but none of the developed methods were carried out to assess the complex source term of a real accident situation like the Fukushima accident. However, dose rate measurements are generated by the most widespread measurement system, and in the event of a nuclear accident, these data constitute the main source of measurements of the plume and radioactive fallout during releases. This paper proposes a method to use dose rate measurements as part of an inverse modeling approach to assess source terms. The method is proven efficient and reliable when applied to the accident at the Fukushima Daiichi Nuclear Power Plant (FD-NPP). The emissions for the eight main isotopes 133Xe, 134Cs, 136Cs, 137Cs, 137mBa, 131I, 132I and 132Te have been assessed. Accordingly, 105.9 PBq of 131I, 35.8 PBq of 132I, 15.5 PBq of 137Cs and 12 134 PBq of noble gases were released. The events at FD-NPP (such as venting, explosions, etc.) known to have caused atmospheric releases are well identified in the retrieved source term. The estimated source term is validated by comparing simulations of atmospheric dispersion and deposition with environmental observations. In total, it was found that for 80% of the measurements, simulated and observed dose rates agreed within a factor of 2. Changes in dose rates over time have been overall properly reconstructed, especially in the most contaminated areas to the northwest and south of the FD-NPP. A comparison with observed atmospheric activity concentration and surface deposition shows that the emissions of caesiums and 131I are realistic but that 132I and 132Te are probably underestimated and noble gases are likely overestimated. Finally, an important outcome of this study is that the method proved to be perfectly suited to emergency management and could contribute to improve emergency response in the event of a nuclear accident.

Source term estimation of radioxenon released from the Fukushima Dai-ichi nuclear reactors using measured air concentrations and atmospheric transport modeling

Systems designed to monitor airborne radionuclides released from underground nuclear explosions detected radioactive fallout across the northern hemisphere resulting from the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011. Sampling data from multiple International Modeling System locations are combined with atmospheric transport modeling to estimate the magnitude and time sequence of releases of 133Xe. Modeled dilution factors at five different detection locations were combined with 57 atmospheric concentration measurements of 133Xe taken from March 18 to March 23 to estimate the source term. This analysis suggests that 92% of the 1.24 × 1019 Bq of 133Xe present in the three operating reactors at the time of the earthquake was released to the atmosphere over a 3 d period. An uncertainty analysis bounds the release estimates to 54–129% of available 133Xe inventory.

Estimation of the caesium-137 source term from the Fukushima Daiichi nuclear power plant using a consistent joint assimilation of air concentration and deposition observations

Inverse modelling techniques can be used to estimate the amount of radionuclides and the temporal profile of the source term released in the atmosphere during the accident of the Fukushima Daiichi nuclear power plant in March 2011. In Winiarek et al. (2012b), the lower bounds of the caesium-137 and iodine-131 source terms were estimated with such techniques, using activity concentration measurements. The importance of an objective assessment of prior errors (the observation errors and the background errors) was emphasised for a reliable inversion. In such critical context where the meteorological conditions can make the source term partly unobservable and where only a few observations are available, such prior estimation techniques are mandatory, the retrieved source term being very sensitive to this estimation.We propose to extend the use of these techniques to the estimation of prior errors when assimilating observations from several data sets. The aim is to compute an estimate of the caesium-137 source term jointly using all available data about this radionuclide, such as activity concentrations in the air, but also daily fallout measurements and total cumulated fallout measurements. It is crucial to properly and simultaneously estimate the background errors and the prior errors relative to each data set. A proper estimation of prior errors is also a necessary condition to reliably estimate the a posteriori uncertainty of the estimated source term. Using such techniques, we retrieve a total released quantity of caesium-137 in the interval 11.6−19.3 PBq with an estimated standard deviation range of 15−20% depending on the method and the data sets. The “blind” time intervals of the source term have also been strongly mitigated compared to the first estimations with only activity concentration data.

RANS simulation of a turbulent premixed bluff body flame using conditional source-term estimation

In the present work, simulations are performed in order to study a turbulent premixed flame stabilised behind a bluff body burner. Conditional Source-term Estimation (CSE) is the combustion model adopted for this purpose and it is coupled with a trajectory gene- rated low-dimensional manifold method for chemistry reduction: conditional-averaged chemical source terms are closed by conditional-averaged scalars, which are obtained by inverting an integral equation. Two regularisation methods are implemented. The optimal regularisation parameter is determined and a sensitivity analysis is performed related to the effect of the value of the regularisation parameter. In the present study, only small differences in the conditional and unconditional averages are noticed for values of the regularisation parameter located in the optimal range determined by the L-curve. Two-equation k–ε and Shear Stress Transport (SST) turbulence models are used to solve the flow field in either the reactive or non-reactive case. Different values of the constant in the standard k–ε model and of γ2 in the SST k–ω model are tested. Mean axial velocity and progress variable profiles are compared with experimental data. It is found that SST k–ω gives better results for the velocity profile in both the reactive and non-reactive cases. A value of γ2 equal to 0.65 provides best predictions in the reactive case. The progress variable prediction exhibits similar profiles in the case of CSE and the flamelet combustion model approach. Very good agreement is achieved between the CSE predictions and experimental data.

Investigation of conditional source-term estimation applied to a non-premixed turbulent flame

A turbulent combustion model, Conditional Source-term Estimation (CSE) is applied to a non-premixed turbulent jet methane flame. The conditional chemical source terms are determined on the basis of first order closure and the conditional averaged species concentrations are obtained by inverting an integral equation. The Tikhonov method is implemented for regularisation. Detailed chemistry is tabulated using the trajectory generated low-dimensional manifold method. Radiation due to the gaseous species is included. Reynolds Averaged Navier–Stokes calculations are performed using two different turbulence models. The objectives of the paper are (i) assessment of the impact of the main numerical parameters in CSE and (ii) comparison of the CSE numerical predictions with available experimental data and results from previous simulations for the selected flame. The number of CSE domains and the number of points in each CSE domain are shown to have a significant impact on the results if not selected appropriately. The present CSE calculations always converge to unique and stable predictions. The corrected k–ε model yields mixture fraction profiles in good agreement with the experimental data values for axial locations in the first half of the flame. Farther downstream, the RNG k–ε model performs better. Overall, the current predictions for the mixture fraction are in good agreement with the experimental data. The predicted temperatures using CSE and the k–ε turbulence model with a modified value of Cε1 = 1.47 are found to be in very good agreement with the experimental data. Further, the current CSE results are of comparable quality with previous simulations using the flamelet model and conditional moment closure. Future work may include further investigation on optimal determination of the regularisation parameter and alternative regularisation techniques, soot modelling within the CSE formulation, and improved formulation of radiation.

Significance of Phébus-FP results for plant safety in Switzerland

Switzerland, in common with other countries, uses source term estimates to formulate emergency plans in the unlikely event of an accident with release of activity to the environment. In the past estimates have typically been based on conservative treatments of the accident sequence, necessitated by the limited validation status of models used for the phenomena that control the reactor accident evolution. Although analyses using best-estimate tools frequently indicated substantially smaller releases – knowledge and data were insufficient to place reliance on the methods or the calculated results.The Phébus programme is unique in providing a source of integral transient data on fission product release, transport and chemical behaviour under prototypic conditions, capturing the entire portfolio of processes – transport, material and chemical – and their causal interaction. To this day, no other source of such data is available. These data provide a means to assess, improve and validate methods for source term evaluation and establish the needs for establishing new processes to mitigate the source term. There are strong synergies and complementarities between the Phébus project, the International Source Term Project, the iodine-related studies at PSI and the aerosol retention projects at PSI, and current moves toward improved management/mitigation of severe accidents.The Federal Nuclear Safety Inspectorate (ENSI), Paul Scherrer Institute (PSI) and the Swiss utilities running five nuclear power plants share in the findings. The need to strengthen the foundation on which we perform best-estimate evaluations of reactor accidents, identified in the early start of the Phébus programme and which motivated Switzerland’s participation, still remains at this point at the closure of Phébus. Likewise, there is a clear need to enable retention of volatile iodine in the venting filters so as to make venting a feasible accident management measure. Indeed the events at Fukushima Daiichi in 2011 make this need particularly pressing in the context of present challenges to continuation and further growth in nuclear electricity generation.

Verification of source term estimation method against measured data for spent fuel hardware characterization

The Republic of Korea has developed an advanced source term analysis tool, called ASOURCE, to support R&D action plans for the achievement of an advanced fuel cycle employing a pyroprocess in connection with a sodium-cooled fast reactor. ASOURCE has the following functions: (a) generation of inflow and outflow source terms of mixed spent fuel (SF) in each process for the design of the pyroprocess facility; (b) overall inventory estimation for Transuranics (TRUs) and long-lived nuclides in SFs stored at each or all reactor sites for the design of the SFR; and (c) grand source terms of a batch of SFs with different irradiation and cooling profiles for the practical design of a temporary or interim storage facility of SFs. ASOURCE is comprised of three analysis sequences, a Fuel Waste Characterization Sequence, Metal Waste Characterization Sequence, and Grand Source Term Characterization Sequence. In this study, the Metal Waste Characterization Sequence was verified by comparing the nuclide inventory estimated by ASOURCE with the measured nuclide inventory in an irradiated grid plate. It was found that the values calculated by ASOURCE agreed with the measured data within 35%, indicating that the developed program supplies viable source term data.

Source term estimation of atmospheric release due to the Fukushima Dai-ichi Nuclear Power Plant accident by atmospheric and oceanic dispersion simulations

The source term of the atmospheric release of 131I and 137Cs due to the Fukushima Dai-ichi Nuclear Power Plant accident estimated by previous studies was validated and refined by coupling atmospheric and oceanic dispersion simulations with observed 134Cs in seawater collected from the Pacific Ocean. By assuming the same release rate for 134Cs and 137Cs, the sea surface concentration of 134Cs was calculated using the previously estimated source term and was compared with measurement data. The release rate of 137Cs was refined to reduce underestimation of measurements, which resulted in a larger value than that previously estimated. In addition, the release rate of 131I was refined to follow the radioactivity ratio of 137Cs. As a result, the total amounts of 131I and 137Cs discharged into the atmosphere from 5 JST on March 12 to 0 JST on March 20 were estimated to be approximately 2.0 × 1017 and 1.3 × 1016 Bq, respectively.