Hypothetical Accident Scenarios Research Articles

A new perspective on density and strength loss profiles at the surface of thermally oxidized nuclear graphite

Oxidation of graphite components could influence their designed life in a high-temperature nuclear reactor. The oxidized regions could potentially lower the allowed stress capacity. The American Society of Mechanical Engineers rules for the design and construction of graphite-moderated reactors recommend that subsurface regions that might become excessively damaged by oxidation during reactor operation be identified and excluded from geometry and stress calculations. Identification of oxidation-affected regions is possible, in principle, through complex modeling exercises of reactor behavior during hypothetical accident scenarios coupled with graphite oxidation models, but this procedure may not have the precision needed for informed decisions. This paper proposes an alternate method, based on interpretation of a series of well-designed oxidation experiments, which could augment the designer's tools. The procedure is illustrated by data on oxidation by air of several graphite grades (NBG-18, PCEA, IG-110, R4-650) that are corroborated with independent literature information, when available. The Wichner model for graphite oxidation used for this analysis provides conservative results that could be quickly implemented in the design process.

Oxidation behavior of Zr-2.5Nb alloy exposed to steam in the temperature range of 600–1200 °C

Zr-2.5Nb pressure tube material was subjected to isothermal steam exposures in the range 600–1200 °C for different durations to predict its oxidation behavior during hypothetical accident scenarios. The oxidation exponent obtained from power law fitting of the thermogravimetry (TG) curves deviated from parabolic regime. Nevertheless, oxidation kinetics was described by determining the parabolic rate constants (Kp) in two temperature ranges: Kp = 0.029 exp(-111075/RT) (kg/m2)2/sec for 600–900 °C whereas Kp = 77652.6 exp(-254824/RT) (kg/m2)2/sec for 900–1200 °C. In the tested durations, only the 1000 °C sample has shown breakaway transition, after approximately 105 min and has absorbed the highest amount of hydrogen amongst all samples. The trend in hydrogen pickup fraction (HPUF) was verified by two complementary hydrogen determination methods. Clear differences were identified in the α-Zr(O) layer formed on the samples below and above 900 °C. In the α + β range of 600–900 °C, α-Zr(O) consisted of tiny β precipitates. At and above 900 °C in the fully β range, columnar α-Zr(O) grains were observed.

Two-way Eulerian-Lagrangian coupling approach in GASFLOW code for containment spray modelling

Spray droplets are considered to have significant effects on the gas mixing and depressurization in the containment of nuclear power plants during hypothetical accident scenarios. The Eulerian-Lagrangian approach, which enables the two-way mass, momentum, and energy coupling of the continuous gas and the dispersed spray droplets, is promising to model the behaviours of spray droplets in 3D simulations of the full-scale containment. Two-way heat and mass transfer models have been developed in the 3D CFD code GASFLOW. The particle group method is applied in the Lagrangian approach to track the droplets. The ordinary differential equations which couple the local heat and mass exchanges between each droplet group and the surrounding gas mixtures have been solved using the Runge-Kutta method. The GASFLOW coupling this new approach is validated by TOSQAN water spray experiments, which demonstrate the good feasibility of implementing the approach. The analysis of the impact of the spray on containment atmosphere indicates that: 1) the droplet swarm entrains and mixes the surrounding gas, breaking up the gas stratification, with turbulent diffusion dominating momentum mixing in dead zones; 2) the thermal and mass exchanges between the spray droplet swarm and gas jointly determine the atmosphere depressurization, with the evaporation of the spray-formed film/sump on heat structures of great importance for the containment pressure development; and 3) these spray thermodynamic and dynamic effects highly depend on the droplet size distribution and the spray shape.

Recently revisited TH issues and associated R&D A French perspective

This paper presents an overview of some recent evolutions in the field of thermal-hydraulics (TH) within nuclear safety studies in France, therefore restricted to Pressurized Water Reactors (PWR) design, from the perspective of the French Technical Support Organization IRSN (Institut de Radioprotection et de Sûreté Nucléaire). These evolutions can concern either the hypothetical accident scenario (recently raised questions, modified rulemaking, new methodology) or the development of recent sources of knowledge (experimental programs, advanced numerical simulation tools).

Radiological Impact Assessment for hypothetical accident scenarios of a proposed pressurized water reactor using HotSpot code

The radiological risks following two severe hypothetical accidents at a proposed pressurized water reactor (PWR) in Umm Huwayd in the Eastern Coast of Saudi Arabia, have been determined using the health physics code HotSpot 3.1.2, developed by Lawrence Livermore National Laboratory, USA. In this paper the general plume dispersion code was used, the Total Effective Dose Equivalent (TEDE) were estimated and the direction of propagation was included the two accident scenarios for various stability classes A, B, C, D, E and F. At a closer distance to the release point the TEDE peaked faster for class A in both accident scenarios. The maximum TEDE values received by the NPP site workers, member of public and the permanent resident were estimated for both accident scenarios at a distance of 0.12 km, 4 km and 80 km from the released source point, respectively. For the two accident scenarios at two distances (0.10 km and 80 km) from the release point, the TEDE values were evaluated for different pathways (inhalation, submersion, ground shine and resuspension). The outcomes showed that for both scenarios, at 0,10 km, the submersion pathway is the dominant intake followed by inhalation and at 80 km distance, the most dominant pathway was found to be (inhalation) followed by (submersion). The effective release height of 10 m, 20, 30 m and 40 m was simulated and found to have inverse relation with TEDE and the ground deposition. Inversion layer height also was evaluated.

Acute and chronic health risk assessment for inhalation and ingestion exposure in acrylic acid leak accidents

We established a hypothetical acrylic acid leak accident scenario, conducted a health risk assessment of local residents, and compared an actual accident case to the hypothetical scenario. The exposed subjects were divided into four age groups, and a noncarcinogenic health risk assessment was conducted for inhalation and soil ingestion. In the hypothetical scenario, 40 tons of acrylic acid was leaked in Ulsan for 1 h from midnight on January 1, 2017. In the actual accident case, 3 L of acrylic acid was leaked in Hwaseong, Gyeonggi Province, for 1 h from 11:00 am on March 5, 2020. The environmental concentration of acrylic acid was calculated using the dynamic multimedia environmental model. Noncarcinogenic assessment of the hypothetical scenario showed the hazard quotient exceeded 1 across all age groups, suggesting that a health risk is likely to occur due to inhalation exposure to acrylic acid resulting from a chemical accident. In addition, Hazardacute exceeded 1 until 2 h after the accident under the hypothetical scenario, indicating the likelihood of a health risk. Thus, we propose a methodology that can assess changing concentrations in a hazardous chemical leak from a chemical accident based on the time, place, the chemical’s behaviors in different environmental media, and the health risk posed by the exposure of the chemical to local residents in the area affected by the accident.

Environmental vulnerability to oil spills in Itapuã State Park, Rio Grande do Sul, Brazil: An approach using two-dimensional numerical simulation

The growing use of coastal areas for different economic purposes is responsible for increasing pollution by hydrocarbons in marine environments. As a consequence of these activities, accidents during fuel extraction, transport, and storage can occur, causing intense environmental degradation. Numerical modeling of the trajectory of oil stains becomes an important tool with low operational costs, providing powerful support to the government agencies in charge of risk management associated with possible oil accidents, by helping to generate scenarios and strategies for containment and cleaning of affected environments. In this sense, the aim of this study is to estimate environmental vulnerability to oil at beaches located in the Itapuã State Park (PEI), a Protection Conservation Unit. This work focused on describing a methodology to estimate the vulnerability of coastal areas, with emphasis on the fact that the study was carried out in a closed environment. For that, an approach was used based on the integration of: (1) an intrinsic variable to the environment; (2) a dynamic variable determined through diesel oil surface dispersion scenarios. Four hypothetical accident scenarios with 20 m³ of diesel oil were simulated in 2018, during five days of simulations with instant dumping in the navigation channel of the local waterway near the PEI. The results suggest the forcing of the field of intensity and direction of the local winds as preponderant for the dynamics of movement and structure of the spots, with the zonal and meridional components of the fields of superficial currents acting in this process as a secondary factor. The study showed that all beaches in the park are susceptible to contact with oil throughout the simulated year, with Pombas Beach, Pedreira Beach, and Onça Beach being affected in all simulated scenarios, which classifies them as very high vulnerability and defines them as priority protection areas.

Influence of the Fundamental Attribution Error on Perceptions of Blame and Negligence.

Automobile accidents are a frequent occurrence in the United States and commonly result in legal ramifications. Through a fundamental attribution error (FAE) framework (Ross, 1977), the current research examined how individuals perceive blame and negligence in these cases. In Study 1 (N = 360), we manipulated the driver (you vs. stranger) of a hypothetical accident scenario and the situational circumstances surrounding the accident (favorable vs. unfavorable). Supporting the FAE, individuals' situational blame attributions only varied as a function of situational circumstances when they themselves were hypothetically driving. However, neither the driver nor the situation significantly predicted dispositional blame attributions. Yet, Study 1 provided initial support for the importance of an individual's trait tendency to neglect situational constraints when making dispositional blame attributions. In Study 2 (N = 212), we again manipulated situational circumstances surrounding the hypothetical accident, but within the context of a mock civil trial. Results provided additional support for the importance of this trait tendency and expanded our findings of dispositional blame attributions to perceptions of negligence. Implications include the importance of considering trait individual differences in the likelihood to ignore situational demands when individuals are making legally relevant judgments about automobile accidents.

Evaluation of Radiation Doses for Hypothetical Accident Scenarios of Egyptian Second Research Reactor

Inshas site is very important site, it is considered as a first nuclear site in Egypt. It contains the first and the second research reactors and other important facilities such as the Egyptian Fuel Manufacturing Pilot Plant (FMPP) which submits the fuel to the second research reactors. The goals of this paper are to assess different emergency plan scenarios, determine the effects of metrological conditions on the dispersion of the radioactive plume, and calculate the overall effective dose equivalent values (TEDE). The meteorological parameters for one complete year 2020 (hourly) meteorological data were analysed in details such as wind direction, wind speed and temperature. The HotSpot code was used to model atmospheric dispersion and its application resulted in a radiation dose profile around the site using meteorological parameters specific to the area under study. This study used more than one scenario to investigate the role of various meteorological parameters. The radionuclide source term of Cs-137 was assumed to be 2.10E+15Bq. The results show that the maximum dose form all postulated scenarios is about 1.1E+4 Sv is observed at 10 m from the release source for weather stability class F, which is greater than the IAEA occupational exposure limit of 0.02 Sv per year. The results can also show that the time of accident is a major effect on the impact of accident and then on the consequences of emergency plan.

Gas mixing caused by interacting heat sources. Part ii: Modelling

The analyses of hypothetical accident scenario in various containment designs include the evaluation of the effectiveness of Passive Autocatalytic Recombiners (PARs), which are installed to mitigate the hydrogen risk, or are considered by utilities for retro-fitting the plants to implement revised regulatory requirements motivated by the Fukushima accident. The assessment requires validated computational tools. To build confidence in the codes, their capability must also be assessed against separate effect tests addressing specific phenomena under conditions that are representative of those expected in an accident scenario in a real plant. Under the conditions of a severe accident, the large amount of hydrogen release requires the installation of several units, and therefore the study of their interaction is necessary for the optimal layout. Within the OECD/NEA HYMERES project, four experiments have been performed in the large-scale PANDA facility, addressing the thermal effect of one or two PARs, which was simulated by means of heaters (Part I). The tests were of increasing complexity, to permit a step-by-step validation of the codes. These experiments have been analyzed with the GOTHIC code. In general, the GOTHIC code in conjunction with a coarse mesh could predict the mixing process reasonably well. The only notable discrepancy with the experiments was the overprediction of the mixing rate when the lower heater was activated. The analyses revealed that this was due to the incorrect representation of the interaction of the flow out of the lower heater with the obstruction, and better results could be obtained by refining the mesh around the heaters. It was concluded that a coarse mesh is adequate for representing the flow produced by multiple heat sources in an open geometry, but detailed models are generally required if the flows could interact with structures.

Uncertainty propagation in atmospheric dispersion models for radiological emergencies in the pre- and early release phase: summary of case studies

In the framework of the European project CONFIDENCE, Work Package 1 (WP1) focused on the uncertainties in the pre- and early phase of a radiological emergency, when environmental observations are not available and the assessment of the environmental and health impact of the accident largely relies on atmospheric dispersion modelling. The latter is subject to large uncertainties coming from, in particular, meteorological and release data. In WP1, several case studies were identified, including hypothetical accident scenarios in Europe and the Fukushima accident, for which participants propagated input uncertainties through their atmospheric dispersion and subsequent dose models. This resulted in several ensembles of results (consisting of tens to hundreds of simulations) that were compared to each other and to radiological observations (in the Fukushima case). These ensembles were analysed in order to answer questions such as: among meteorology, source term and model-related uncertainties, which are the predominant ones? Are uncertainty assessments very different between the participants and can this inter-ensemble variability be explained? What are the optimal ways of characterizing and presenting the uncertainties? Is the ensemble modelling sufficient to encompass the observations, or are there sources of uncertainty not (sufficiently) taken into account? This paper describes the case studies of WP1 and presents some illustrations of the results, with a summary of the main findings.

Accident modeling of toxic gas-containing flammable gas release and explosion on an offshore platform

Toxic gas-containing flammable gas leak can lead to poisoning accidents as well as explosion accidents once the ignition source appears. Many attempts have been made to evaluate and mitigate the adverse effects of these accidents. All these efforts are instructive and valuable for risk assessment and risk management towards the poisoning effect and explosion effect. However, these analyses assessed the poisoning effect and explosion effect separately, ignoring that these two kinds of hazard effects may happen simultaneously. Accordingly, an integrated methodology is proposed to evaluate the consequences of toxic gas-containing flammable gas leakage and explosion accident, in which a risk-based concept and the grid-based concept are adopted to combine the effects. The approach is applied to a hypothetical accident scenario concerning an H2S-containing natural gas leakage and explosion accident on an offshore platform. The dispersion behavior and accumulation characteristics of released gas as well as the subsequent vapor cloud explosion (VCE) are modeled by Computational Fluid Dynamics (CFD) code Flame Acceleration Simulator (FLACS). This approach is concise and efficient for practical engineering applications. And it helps to develop safety measures and improve the emergency response plan.

Application of Decision Support System during the Emergency Exercises for Nuclear Emergency Management

In India, two independent decision support system (DSS) systems are developed and implemented at NAPS (Narora Atomic Power Station) and MAPS (Madras Atomic Power Station) sites for emergency response. The simulation results of the DSS systems guides in identification of emergency zones and distances and implementation of the protective actions. This paper describes application / testing of the three decision support systems developed by IGCAR, NPCIL and SRI (AERB) for dose projections and decision on the implementation of the protective action in the public domain during the recently conducted emergency exercise at NPP site. Thus three DSS systems developed in India were applied to perform atmospheric dispersion and dose assessment in public domain around the nuclear power plants (NPPs) during the recent emergency exercise. The hypothetical accident scenario was considered for the exercise. The source term estimated based on plant parameters and pre calculated source term for large number of accident scenarios was used during the exercise. The results of the assessments by using these Decision support systems were presented to the decision makers for recommendation and implementation of protective actions such as evacuation, sheltering, KIO3 distribution, contamination control etc. The Numerical Weather Forecast model was used for the area around the NPP to produce the meteorological parameters that were further used by these DSS system. Predictive assessments of the radiological situation in the vicinity of the NPP site was performed during emergency exercise with various source terms. Taking into account the high uncertainties in the source term estimation and inputs to the simulation models, the simulated results from these three DSS show a reasonable agreement. The study demonstrated the utility of DSS systems for the assessment of the radiological consequences of hypothetical nuclear accidents during the emergency exercises at different NPP sites. The experience gained in using the DSS systems for operational application to the Indian NPPs will be further used by the exercise planners and developers to improve the system continuously and their adaptation to all NPP sites in India.

Coastal transport of river-discharged radionuclides: Impact of speciation and transformation processes in numerical model simulations

Following a potential nuclear accident, river run-off may potentially become a significant source of radionuclide contamination to the coastal marine environment. In the present work, code for radionuclide speciation and dynamic transfer of radionuclides between the different species was implemented in a Lagrangian marine dispersion model. A case study was performed where the model system utilized ocean circulation fields at relatively high spatial (160 m × 160 m in horizontal direction) and temporal resolution (1 hour), considering a hypothetical accident scenario including river discharges of 137Cs to the marine environment. Results from a number of simulations were compared to identify how factors associated with radionuclide speciation and transfer between the model compartments could affect the predicted radiocesium activity concentrations.The results showed that by including dynamic transfer of radionuclides between the model compartments, the total activity concentrations at far-field sites could vary with more than two orders of magnitude, demonstrating that this model configuration enables prediction of potential local hot-spots. However, the total activity concentration near the river outlets was less affected (< factor 10). The radionuclide speciation in the river discharges and the parameterization of 137Cs particle affinity greatly affected the specie distribution (> factor 103 increase in concentration of particle-associated 137Cs) as well as the settling of radionuclides towards the seabed (up to factor 102 increase in 137Cs sediment concentrations). These factors were therefore identified as important contributors to the overall uncertainty.

Simulation of radionuclide atmospheric dispersion and dose assessment for inhabitants of Tehran province after a hypothetical accident of the Tehran Research Reactor.

Radiological dose assessment is one of the main categories of safety assessment for nuclear reactors and facilities. The radiation risks to the public and to the environment that may arise from these facilities have to be assessed and, if necessary, controlled. The main objective of this paper is the assessment of radiation doses to residents of Tehran province after a hypothetical accident of the Tehran Research Reactor (TRR) including the determination of any protective actions that might be needed for the benefit of people's health. The concentration of radionuclides in air and deposited on the ground surface as a result of a hypothetical radionuclide release from the TRR, following a hypothetical accident scenario, have been calculated by the HYSPLIT computer code. Simulations were performed using selected source terms taken from the TRR Final Safety Analysis Report (FSAR). Meteorological data of the Air Resources Laboratory of the National Oceanic and Atmospheric Administration (NOAA) have been used in these calculations. The simulation results indicate that maximum annual total effective dose equivalent values for the residents of the Tehran province are less than the protective action dose limits. Thus, it is concluded that during this hypothetical accident in the TRR, required safety due to public radiation is achieved and the residents of Tehran province are safe under a TRR accident condition.

Atmospheric dispersion assessment of radioactive materials during severe accident conditions for Bushehr nuclear power plant using HYSPLIT code

In nuclear power plants and nuclear research reactors, radioactive material release into the environment and the dose received by individuals are the main concerns during a severe accident. Thus, calculation of expected dose at reactor perimeters and surrounding area in the event of an accident is a basic requirement for the safety of these facilities. This study uses the HYSPLIT code to simulate the consequences of the worst hypothetical accident scenario of Station Blackout (SBO) and Large Break Loss-of-Coolant Accident (LBLOCA) in Bushehr nuclear power plant unit-1 (BNPP-1). The concentration of released radioactive material and external effective doses received by populations within 30 km radius of facility are computed. Dispersion of radioactive materials is simulated using of Global Data Assimilation System (GDAS) meteorological data. Particle behavior in different stages of dispersion and annual dispersion along each of the 16 geographic directions are obtained from trajectory calculations. The concentration of different radionuclides is determined and the resulting annual external effective dose at different locations are computed as well. All calculations are performed for four different release time durations including 4, 12, 24 and 48 h. Given the distribution of population around the NPP facility, the highest doses are expected at 3 km north and 4 km northwest of the reactor location. The maximum dose for 4, 12, 24, and 48-h long emission at different points of Bushehr city is calculated and compared with the allowable dose limits.

What are the factors that contribute to road accidents? An assessment of law enforcement views, ordinary drivers’ opinions, and road accident records

What are the main contributing factors to road accidents? Factors such as inexperience, lack of skill, and risk-taking behaviors have been associated with the collisions of young drivers. In contrast, visual, cognitive, and mobility impairment have been associated with the collisions of older drivers. We investigated the main causes of road accidents by drawing on multiple sources: expert views of police officers, lay views of the driving public, and official road accident records. In Studies 1 and 2, police officers and the public were asked about the typical causes of road traffic collisions using hypothetical accident scenarios. In Study 3, we investigated whether the views of police officers and the public about accident causation influence their recall accuracy for factors reported to contribute to hypothetical road accidents. The results show that both expert views of police officers and lay views of the driving public closely approximated the typical factors associated with the collisions of young and older drivers, as determined from official accident records. The results also reveal potential underreporting of factors in existing accident records, identifying possible inadequacies in law enforcement practices for investigating driver distraction, drug and alcohol impairment, and uncorrected or defective eyesight. Our investigation also highlights a need for accident report forms to be continuously reviewed and updated to ensure that contributing factor lists reflect the full range of factors that contribute to road accidents. Finally, the views held by police officers and the public on accident causation influenced their memory recall of factors involved in hypothetical scenarios. These findings indicate that delay in completing accident report forms should be minimised, possibly by use of mobile reporting devices at the accident scene.

Modelling horizontal two-phase flows using generalized models

Gas–liquid two-phase flows in the hot leg of a pressurized water reactor (PWR) under hypothetical accident scenarios have received special attention in nuclear reactor safety research. The numerical simulation of such flows can be performed using phase-averaged multi-fluid models, such as the two-fluid approach, or non-phase-averaged alternatives. The method shown in this paper within the two-fluid framework is the Algebraic Interfacial Area Density (AIAD) model. It features the macroscopic blending between different interfacial area density and momentum transfer models depending on the local flow morphology. The work presented here strives to improve the turbulence modelling for free surface flows by introducing a model for sub-grid size waves induced by Kelvin-Helmholtz instability. A first CFD validation of the methodology is done by means of horizontal, adiabatic, stratified flow data from the WENKA facility. More verification and validation of the approach is required – more CFD grade experimental data is the key to further validation.

Multi-physics simulation of nuclear reactor core by coupled simulation using CUPID/MASTER

In order to visualize a realistic nuclear reactor behavior for abnormal control rod operation accident scenario, a multi-physics approach was performed. Multi-dimensional thermal hydraulics in the reactor core was calculated by coupling the reactor kinetics code. The reactor core for thermal hydraulic analysis is obtained by the CUPID which is aimed for the analysis of transient two-phase flows in nuclear reactor components by Korea Atomic Energy Research Institute (KAERI). In the present paper, a multi-physics simulation was performed by coupling CUPID with a three dimensional neutron kinetics code, MASTER which is a two-group, three-dimensional neutron diffusion code for microscopic depletion, xenon dynamics, on-line Departure of Nucleate Boiling (DNB) analysis and kinetics calculation. The MASTER code is merged into the CUPID code as a dynamic link library (DLL). The OPR1000 reactor core was taken into account by adopting porous media approach. Two hypothetical accident scenarios for malfunction of control element assembly (CEA) were numerically reproduced; (i) CEA drop and (ii) ejection accident. The coupled simulation shows a reasonable and consistent thermal hydraulic behavior. The multi-dimensional thermal hydraulics results show asymmetric temperature distribution which was not obtained by the best-estimated thermal hydraulics code before.

Impacts on the marine environment in the case of a hypothetical accident involving the recovery of the dumped Russian submarine K-27, based on dispersion of 137Cs

There is increasing concern regarding the issue of dumped nuclear waste in the Arctic Seas and in particular dumped objects with Spent Nuclear Fuel (SNF). Amongst dumped objects in the Arctic, the dumped Russian submarine K-27 has received great attention as it contains two reactors with highly enriched fuel and lies at a depth of about 30 m under water. To address these concerns a health and environmental impact assessment has been undertaken. Marine dispersion of potentially released radionuclides as a consequence of different hypothetical accident scenarios was modelled using the model NAOSIM. The outputs from the dispersion modelling have been used as inputs to food-chain transfer and environmental dosimetry models. The annual effective doses for subsistence fishing communities of the Barents-Kara seas region do not exceed 0.6 mSv for hypothetical accidents located at Stepovogo fjord or the Barents Sea. For high rate consumers of fish in Norway, following a potential accident at the Gremikha Bay, annual effects doses would be at around 0.15 mSv. Accumulated doses (over 90 days) for various organisms and for all release scenarios considered were never in excess of 150 μGy. The levels of 137Cs derived for marine organism in areas close to Norway were not values that would likely cause concern from a regulatory perspective although for subsistence fishing communities close to the considered accident locations, it is not inconceivable that some restrictions on fishing etc. would need to be introduced.