Severe Nuclear Accident Research Articles

Influence of SIC control rod material on the iodine release in case of nuclear severe accident – Chemical reactivity with fission products in thermal conditions of RCS

The effect of Silver-Indium-Cadmiun (SIC) control rod on the transport of volatile fission products (I, Cs, Mo) in conditions of a Reactor Coolant System under NPP severe accident was investigated within three semi-integral tests in a thermal gradient tube. The experiments addressed separately the effect of each SIC element. Nature of transported and deposited species was investigated by ICP MS, ESEM-EDX, Raman Microspectrometry, XRD and XPS techniques.For the first time, numerous metallic molybdates involving SIC components were evidenced. The formation of such metallic molybdates competes with the formation of Cs-molybdate and may in certain conditions strongly reduce the scavenging effect of Mo towards Cs. As a result the nature of transported iodine can be strongly modified, compared to chemical systems involving only Mo, Cs and I. The data obtained are useful to guide what species have to be added in the thermodynamic database used in the SA simulation software.

Deposition Behavior of Cesium Molybdate on Type 304 Austenite Stainless Steel in Severe Accident

The chemical behavior of cesium molybdate (Cs2MoO4) in light water reactors during severe nuclear accidents remains unexplored. This study demonstrated the deposition behavior of Cs2MoO4 on Type 304 stainless steel (SUS304) at 1530 to 530 K under dry (Ar) and humid (Ar + H2O) conditions. Cesium molybdate was partially decomposed on the SUS304 surface, thereby inducing the oxidation of iron (Fe) and chromium (Cr) under the dry condition. Molybdenum (Mo) metal and molybdenum dioxide (MoO2) were detected on the surface, while Cs coexisted with chromium in the oxide layer at 1500 K. Both Cs2MoO4 and Mo metal were identified on the SUS304 surface at 1230 K. Under the humid condition, the oxidation of the SUS304 was affected by Cs2MoO4 vapor. Molybdenum was detected in the form of spots in the iron oxide layer, while cesium was not detected above 1500 K. Molybdenum metal was detected on the surface of SUS304 oxide at 1230 K. Cesium molybdate was deposited on the SUS304 at 730 to 530 K under both the dry and humid conditions. The results are discussed in relation with the thermodynamic model of the Cs-Fe-Cr-Mo-O system. Thus, the chemical behavior of Cs2MoO4 at the interior of the reactor cooling system is elucidated.

Improving the estimation accuracy of multi-nuclide source term estimation method for severe nuclear accidents using temporal convolutional network optimized by Bayesian optimization and hyperband

During a nuclear accident, estimating the source terms using environmental measurements is vital for emergency decision-making. In this study, we propose a forecasting model based on a temporal convolutional network to estimate the release rates of seven radionuclides (Kr-88, Te-132, I-131, Xe-133, Cs-137, Ba-140, and Ce-144) based on off-site sequential gamma dose rates and meteorological monitoring data. To determine the best structure of the neural network, Bayesian optimization and hyperband (BOHB) was used on the hyperparameters of the model to reduce the testing loss. Additionally, a gradient boosting regression model was used to predict missing gamma dose rates to ensure the model offers a relatively reliable estimate under certain circumstances. The international radiological assessment system (InterRAS) was used to generate datasets for model training and testing. The results showed that the optimal hyperparameters selected by BOHB can reduce the valid loss of the model to 0.0153, and the mean absolute percentage error of prediction for the seven radionuclides was below 12%, three of which (Kr-88, Te-132, Cs-137) reached 8% at 10 h. When the first and second time-steps of the data were missing, the mean absolute percentage error of the prediction for all radionuclides was less than 30% after using a gradient boosting regression.

Revaporization Behavior of Cesium and Iodine Compounds from Their Deposits in the Steam-Boron Atmosphere.

This paper presents our investigation on cesium and iodine revaporization from cesium iodide (CsI) deposits on stainless steel type 304L, which were initiated by boron and/or steam flow. A dedicated basic experimental facility with a thermal gradient tube (TGT) having a temperature range of 1000–400 K was used for simulating the phenomena. In the absence of boron, it was found that the initially deposited CsI at 850 K could be revaporized as CsI vapor/aerosol or reacted with the carrier gas and stainless steel (Cr2O3 layer) to form Cs2CrO4. The latter mechanism consequently released gaseous iodine that was later accumulated downstream. After introducing boron to the steam flow, a severe revaporization occurred. This, in addition to the revaporized CsI vapor/aerosol, was caused by cesium borate (Cs2B4O7 and CsB5O8) formation, which then largely released gaseous iodine that was capable of reaching the TGT outlet (<400 K). In the case of a nuclear severe accident, our study has demonstrated that an increase of gaseous iodine in the colder region of a reactor could occur after late release of boron or a subsequent steam flow after refloods of the reactor, thus posing its inherent risk once leaked to the environment.

High-temperature ex-vessel corium spreading. Part 2: scaling principles for gravity-viscous spreading with slip at the melt–substrate interface

ABSTRACT Promoting the spreading of molten corium represents a key strategy in the dissipation of decay heat and in the maintainence of containment integrity in the aftermath of a severe nuclear accident. Precise repeatability is difficult to achieve during high-temperature melt spreading experiments, and so scaling factors are necessary to correct for differences in flow rate, viscosity, and density between comparative experiments. A gravity-viscous momentum balance derived for spreading in the two-dimensional spreading geometry employed during recent kg-scale VE-U9 experiments on ceramic and sacrificial concrete substrates establishes that no-slip spreading length scales with the square root of the mass flow rate. Scaling for the reduced mass flow rate and enhanced viscosity implied that the melt spread 37% further on the sacrificial concrete than on the inert ceramic substrate. The possibility of a lubricating film of molten concrete and ablation gases reducing friction at the lower boundary is investigated through the imposition of a slip velocity proportional to the shear at the melt–substrate interface. Simulations over a range of Navier slip lengths implied that spreading length scales with the ratio of slip length to the melt thickness and that spreading length can be augmented by as much as 68% in the case of a virtually frictionless interface.

ПРОГНОЗНАЯ ОЦЕНКА ПОСЛЕДСТВИЙ ТЯЖЕЛОЙ АВАРИИ НА БЕЛОРУССКОЙ АЭС ДЛЯ ЗДОРОВЬЯ НАСЕЛЕНИЯ

The article presents the results of assessment of the levels of emergency radionuclide discharge, gamma dose rates, radionuclide fallout, as well as the prognosis of public exposure doses to the population of Belarus as a result of a possible severe nuclear accident at the Belarusian NPP. It was found that, in the case of a severe nuclear accident at the Belarusian NPP with total activity of the accidental release of 4.5∙1015 Bq, the transfer of radioactive contamination to a distance up to 100 km from the site under various meteorological scenarios, will be less than 2 hours. The total contamination density of the territory with 131I and 137Сs isotopes within 100 km radius around the Belarusian NPP will be less than 29 kBq/m2 (0.78 Ci/km2) and less than 5.6 kBq/m2 (0.15 Ci/km2) respectivelly. Maximum value of the thyroid dose will be up to 93.8 mSv, and of the effective dose – 2.86 mSv. Based on the obtained results, the justification for public protective measures was made and recommendations were given for conducting emergency radiation monitoring, taking into account international recommendations and national radiation safety standards of the Republic of Belarus.

Applications of Particle Tracking Velocimetry to severe nuclear accident experimentation

Experimental research into severe nuclear accidents may entail the discharge of a very high-temperature lava-like molten fuel mixture, corium, either into a pool of less-dense, more-volatile coolant or onto a solid substrate where the corium will spread and cool. In both instances, remote, high-speed video imaging is usually required to interpret these transient interactions and PTV represents a powerful tool for the characterisation of the dynamic properties of discrete melt fragments or distinctive features in the surface of the melt during spreading. Nuclear fuel-coolant interactions present particular challenges for PTV analysis as a molten jet and its fragments can exhibit high rates of inter-frame deformation and undergo fragmentation with a relatively high frequency. A PTV algorithm, adapted to these challenges, is presented whereby a user-defined tolerance in the evolution of certain particle properties is used to refine the potential candidate particles prior to particle matching. This candidate refinement step is used to distinguish between acceptable levels of deformation between successive sightings of a given particle, and more substantial changes consistent with fragmentation or coalescence, requiring the tracking of a new particle. Implementation of the PTV algorithm is presented for (1) an X-ray video from the FCINA-30-1 experiment between a jet of molten stainless steel and liquid sodium, conducted at the JAEA’s MELT facility, and (2) video imaging of the VE-U9-ceramic experiment of a molten corium-thermite mixture spreading on a zirconium substrate, conducted at the CEA’s VULCANO facility. The latter case-study enabled the characterization of &gt; 70,000 local velocity vectors at locations corresponding to distinctive temperature heterogeneities in the surface of the spreading melt, providing extensive insight into the spreading dynamics for the validation of corium spreading models.

Tellurium transport in the RCS under conditions relevant for severe nuclear accidents

In the case of a severe nuclear power plant accident, tellurium is one of the more problematic and volatile fission products. If released it could become a health issue as it decays to iodine which accumulates in the thyroid gland. Research exists, that indicates tellurium likely interacts with caesium under severe accident conditions, thus it is important to further explore related phenomenon. In this work, tellurium was exposed to high temperature under oxidizing and inert conditions simulating severe accident conditions with and without airborne caesium iodide to determine the effect on the tellurium source term.The effect of caesium iodide was noticeable on tellurium transport behaviour in the gas phase under oxidizing and inert conditions. Under humid oxidizing conditions with caesium iodide, no significant impact on the total aerosol mass transport was noticed. However, less tellurium was transported through the model primary circuit and a potentially new compound was observed on the filter located after this model. Comparing inert dry to humid with caesium iodide showed an increase in the total aerosol mass transport whereas there was a decrease noticed of the tellurium reaching the filter after the model primary circuit. In the latter case, new unidentified compound(s) correlated to caesium, iodine and tellurium were observed on the filter located after the model.In this work, evidence was found that tellurium behaviour will be affected by caesium iodide under the investigated conditions. Moreover, it seems that under inert conditions the formed compounds may be stable at close to ambient temperatures. Unlike under oxidizing conditions, where dissociation likely occurred.

Structural Integrity Evaluation of a Reactor Cavity during a Steam Explosion for External Reactor Vessel Cooling

Nuclear power is a major source of electricity in the international community. However, a significant problem with nuclear power is that, if a severe nuclear accident occurs, radiation may leak and cause great damage. As such, research on nuclear safety has become increasingly popular worldwide. In this paper, the structural integrity of a reactor cavity during a steam explosion—one kind of the aforementioned severe nuclear accidents—was evaluated. Steam explosions are primarily caused by fuel–coolant interactions (FCI), and result from issues in the cooling system that discharges the melt from the reactor core to the outside. A steam explosion can damage the nuclear power plant, and radiation leakage, the greatest concern, may occur. In the Chernobyl or Fukushima Daiichi accidents, significant radiation leakages resulted in damages extending beyond the country of origin. In this paper, a steam explosion was simulated using values given by the transient analysis code for explosive reactions (TRACER-II)—the only steam explosion code in Korea. The walls of the reactor cavity were modeled after the APR-1400 currently operating in Korea. The integrity of the concrete, rebars, and liner plate in the reactor cavity during a steam explosion was evaluated in terms of stress and ductile failure strain limits.

A DFT study of iodine interaction with nuclear reactor cooling system surfaces under severe accident conditions

A severe nuclear accident can lead to the release of radiotoxic iodine compounds in either aerosol form (e.g. metal iodides or iodine oxides) or gaseous form (e.g. organic iodide as CH3I or inorganic as I2) species. 131I is particularly dangerous because of its possible absorption by the human body especially by the thyroid. Gaseous iodine is mainly formed in the nuclear containment building, is dispersed in the case of outside releases and may contribute in short term to long-distance contamination. Metallic iodide species are mainly formed at high temperature and partly condensed on the walls of the reactor coolant system (RCS), the rest being either deposited on the RCS or transported to the containment building. In this paper, we study theoretically, in severe accidental conditions, the adsorption of the metallic iodides on the surface of the primary circuit which is composed of Fe or Cr oxides. At high coverage, AgI and CdI2 form networks on the surfaces of the RCS whereas at low coverage the molecules are isolated. This study, setting out from the stable adsorbates, investigates the chemical mechanisms leading to the iodine re-vaporization. The formation of I2(g) from adsorbed AgI or CdI2 is thermodynamically and kinetically possible on over-oxidized chromium surfaces. On alternative surfaces, the co-adsorption of an oxidant, OH● issued from the steam radiolysis, is necessary to form I2(g). This study tends to show that delayed releases of gaseous iodine are likely to happen from the deposited iodide.

Investigation on the effect of low aspect ratio on heat transfer characteristics for light metallic layer

The nuclear safety and the integrity of the Reactor Pressure Vessel (RPV) could be ensured by the In-Vessel Retention (IVR) strategy during the severe nuclear accident. The HEat transfer behavior of Light Metallic layer experiment for the Low aspect Ratio (HELM-LR) has been established to investigate the heat transfer characteristics and the concentration factor of the light metallic layer in a three-dimensional molten pool. The melt simulant is water. The test section is heated from below and can be cooled from both top and side. The experimental results demonstrate that the melt temperature increases along the vertical direction and decreases along the radial direction. The possible natural convection in the light metallic layer is formed, due to the temperature difference between the melt near the center and the melt near the sidewall. However, the effect of the natural convection would be limited with the low aspect ratio (H/D). Increasing the top cooling could lead to reducing the concentration factor (qside/qinput). When the bottom heat flux is 1 MW/m2 and the concentration factor is below 204%, the heat flux transferred to the AP1000 sidewall would not exceed the Critical Heat Flux (CHF) and the effect of IVR strategy would be enhanced. Consequently, it could avoid the vessel failure during the nuclear severe accident.

Mental Health and Neuropsychiatric Aftermath 35 Years After the Chernobyl Catastrophe: Current State and Future Perspectives.

Objective:The Chernobyl nuclear power plant (ChNPP) disaster that happened in Ukraine on the 26th of April 1986 still represents the most severe nuclear accident in human history. Its consequences, especially those involving mental health are increasingly emerging as long-term detrimental effects. Therefore, the aim of the present paper was to review the results of some of authors’ studies and their personal reflections on this topic.Method:The authors selected and commented on the findings mainly derived from their contributions on the prevalence of long-term psychopathological symptoms and neuropsychiatric disorders in diferent groups of exposed and non exposed individuals, including the workers at the NPP the so-called liquidators (CUWs), the most exposed group, evacuees and people living in more or less contaminated areas.Results:The main findings derived from a series of studies carried out by the authors throughout the following decades after the disaster indicate the high prevalence of cerebrovascular diseases, organic mental and depressive disorders, cognitive impairment and even dementia that increase with the irradiation dose mainly amongst the liquidators. The organic disorders are probably related to a peculiar effect of radiation on left, dominant brain hemisphere. Interestingly, recent studies revealed abnormalities of the serotonin transporter and other genes disorders possibly at the basis of depression of exposed individuals.Conclusions:The high prevalence of neuropsychiatric disorders amongst irradiated subjects following the ChNPP disaster highlights the impact of radiation exposure on the lifelong onset of neuropsychiatric disorders, for too long neglected by international agencies. Such findings require to be deepened in the future possibly within the frame of the so-called “ecological psychiatry”.

Infrared matrix-isolation and theoretical studies of interactions between CH3I and water

Gaseous iodomethane are naturally emitted in the atmosphere over oceans through the algae and phytoplankton activities. The fate of naturally emitted iodomethane is of great interest because of the oxidizing properties of iodine in the atmosphere and its impact on the catalytic destruction of the ozone layer. Additionally, iodomethane is one of the gaseous species that can be emitted in the case of severe nuclear accident. The radiological impact of gaseous iodomethane is of concerns and requires knowledge about its behavior in the atmosphere. Water is one of the major species in the atmosphere which is responsible for atmospheric aerosol nucleation and thus, for cloud condensation nuclei (CCN). The fundamental knowledge concerning the interaction between methyl iodine and water at the molecular scale contributes to the better understanding of the fate of such species into the atmosphere and their role in CCN formation. Here the microhydration of iodomethane was investigated using cryogenic matrix experiments which were supported by theoretical DFT calculations. A large excess of water regarding CH3I was used in order to mimic atmospheric conditions. Dimers and trimers of CH3I are observed despite the high water amount in the initial mixture together with hetero aggregates between CH3I and water clusters. This may be explained by the low affinity of CH3I with water. Considering the concentration of iodomethane used in our experiments, the aggregates are rather formed in gas phase and not in the matrix cage. The interaction between CH3I and H2O molecules studied experimentally and supported by DFT calculation highlights that, in the atmosphere, gaseous iodomethane and water will likely form association between water and iodomethane aggregates instead of (CH3I)n-(H2O)m hetero complexes. Our results have important consequences for the understanding of the alkyl halide solvation in primary processes and contribute to the understanding of reactive halogen species in tropospheric chemistry. In the context of a nuclear severe accident, our work is contributing to better understand the fate of nuclear species in the atmosphere and thus, the radionuclide dispersion.

Effect of composition on thermal stability and microstructural behaviour of non-prototype material (CaO-Fe2O3)

The role of simulant materials becomes necessary for the predictive study of the nuclear severe accident phenomena due to its similarity with corium (a liquid form of UO2 and steel). Since simulant material is eco-friendly and has similar properties to corium, it has been widely used in the research field of severe accident management. In this study, material CaO-Fe2O3 a non-eutectic binary mixture is considered for characterization purpose to address the thermophysical properties at different compositional ratios. The CaO-Fe2O3 powder mixture was prepared in mortar for 40 minutes manually to form a homogeneous mixture and then cylindrical pellets prepared at five different ratios with the help of the phase diagram. Further, these pellets were heat-treated at 1200°C for three hours soaking time to address its thermal stability in a programmable electric furnace. Finally, pellets ground into powder form manually for further characterization. Initially, the weight loss analysis was reported by measurement of dimensions of pellets before and after heat treatment. The thermal properties, phase analysis, and morphological studies have been carried out through DSC, XRD and FE-SEM in laboratory and results were discussed in the context of the property of ideal simulant materials used for the study of nuclear severe accidents. The melting point of all the samples were found stable (1200°C-1230°C) and values of activation energy and specific heat were well synchronized between with and without heat-treated samples. Dislocation density of samples increases significantly with increasing the proportion value of calcium oxide after heat treatment.

Experimental investigation on vertical heat transfer characteristics of light metallic layer with low aspect ratio

In-Vessel Retention (IVR) strategy can ensure the nuclear safety and the integrity of the Reactor Pressure Vessel (RPV) during the severe nuclear accident. The HEat transfer behavior of Light Metallic layer experiment for the Low aspect Ratio (Height/Diameter less than 0.16) (HELM-LR) has been built to investigate the heat transfer characteristics of the light metallic layer. Water was used as the simulant and the test section was heated from below and cooled from both top and side. A new fitting correlation is recommended by HELM-LR experiment to simulate the vertical heat transfer of the light metallic layer when the aspect ratio is below 0.16. The experimental results indicated that the new fitting correlations could better predict the calculated results than Globe-Dropkin correlation under the low aspect ratio condition (1.72 × 107 < Ra < 4.05 × 109). The correlation Nu = 0.1995Ra0.284 with the temperature difference between the bottom and the top boundary is recommended.

Multi-nuclide source term estimation method for severe nuclear accidents from sequential gamma dose rate based on a recurrent neural network

When severe nuclear accidents at nuclear power plants release radioactive material into the atmosphere, the source term information is typically unknown. Estimating the emission rate of radionuclides is essential to assess the consequences of the accident before adequate decision-making can be performed. A recurrent neural network-based model, optimized with the Bayesian method, is proposed to estimate the emission rates of multi-nuclides using off-site sequential gamma dose rate monitoring data. Compared with the existing method that is based on least squares, this new model does not require a priori information and the complicated and time-consuming process of conducting atmospheric dispersion simulations following a nuclear accident, which is conducive to a faster response. Six typical radionuclides (Sr-91, La-140, Te-132, Xe-133, I-131, and Cs-137) were set as mixed source terms, combined with meteorological parameters, and input into the International Radiological Assessment System for simulation to generate data sets for model training. The results indicate that with the input of data describing the sequential gamma dose rate, the accuracy of the nuclide emission rates estimated by this new method is continuously improved, with a mean absolute percentage error for Te-132 of below 7% over 10 h.

A Batch Experiment of Cesium Uptake Using Illitic Clays with Different Degrees of Crystallinity

Radiocesium released by the severe nuclear accident and nuclear weapon test is a hazardous material. Illitic clays play a key role in the spatial distribution of radiocesium in groundwater environments due to selective uptake sites at the illite mineral, such as frayed edge sites. However, the cesium uptake capabilities of illitic clays are diverse, which could be associated with the illite crystallinity. This study was performed to determine the cesium uptake of illitic clays and evaluate the crystallinity effects on cesium uptake using statistical approaches. A total of 10 illitic clays showed various crystallinity, which was parameterized by the full width at half maximum (FWHM) at 10 Å XRD peak ranging from 0.15 to 0.64. The uptake behavior of illitic clays was well fitted with the Freundlich model (i.e., r2 &gt; 0.946). The uptake efficiency of illitic clays increased with the decrease in dissolved cesium concentrations. The cesium uptake was significantly correlated with the FWHM and cation exchange capacity, suggesting that the uptake becomes higher with decreasing crystallinity through expansion of the edge site and/or formation of ion-exchangeable sites.

Influence of temperature on thermo physical properties of non-prototype material (CaO-Fe2O3, 21:79 by wt. %) for nuclear

In severe accident management, the integrity of the containment structure is a key point to focus the study related to severe accident scenarios that need knowledge of thermo-physical properties of material composition. In the present nuclear accident scenario, nuclear severe accident management is one of the challenging tasks to mitigate the phenomena occurring inside the reactor pressure vessel (RPV). Numerous studies have been done in the past to predict the severe accident phenomena and tried to explore the incidence by using different prototype and non-prototype (simulant) materials. In this context, an initial effort has been made to study the actual phenomena during an accident scenario with non-prototype material CaO-Fe 2 O 3 . Initially, the present work involved the study of the material CaO-Fe 2 O 3 powder for thermo-physical property analysis and later, the study will be carried out via melt cool ability of this material in a hypothetical nuclear reactor for analysis and mitigation of nuclear severe accident phenomena. The non-eutectic binary powder mixture of CaO and Fe 2 O 3 has prepared manually by mixing in a mortar for 40 minutes in the ratio of 21:79 by wt. % (21C79F; C refers to CaO and F refers to Fe 2 O 3 ) which has been confirmed by the phase diagram. Further, pallets were prepared of the non-eutectic binary mixture and heat-treated at 1000°C, 1100°C, and 1200°C for 3 hours in a programmable furnace. The powder form of heat-treated pellets was characterized to analyze thermo-physical properties and to validate as a simulant material (non-prototype) used for the predictive study of a nuclear severe accident. The thermal properties, phase analysis, and morphological studies of the CaO-Fe 2 O 3 (21:79 by wt. %) have been reported in the current research work and analyzed accordingly.

Analysis of the Impact of Typical Severe Thermal Hydraulic Condition on the Evolution of Droplet Characteristics in a Spray: the Numerical Simulation

Numerical simulation has been widely applied in the research of spray systems during a severe nuclear plant accident. In order to improve the predictive accuracy of the calculation, the impact of the thermal hydraulic conditions on the development of the spray have been examined by the researchers. The current study presents the influence of the initial droplet distribution at the injection point of a full cone spray. By comparing the velocity of the droplets of different sizes with the experimental results, a reasonable initial droplet size distribution is suggested. Discussions are also performed to explain the divergence between the experiments and the numerical simulation.

Effects of water vapour and temperature on the retention of radiotoxic CH3I by silver faujasite zeolites

In this work, Ag/X and Ag/Y faujasite zeolites were evaluated as candidate sorbents for the retention of methyl iodide under conditions close to those expected in a severe nuclear accident. Different categories of tests were conducted from laboratory to semi-pilot scales. First, the effects of temperature and water vapour on the CH3I retention/decomposition mechanism were investigated under lab-scale conditions. More specifically, the CH3I adsorption capacities and the fate of its main decomposition products (oxygenated compounds and alkanes) were measured under dynamic conditions at different temperatures (35, 100 and 250 °C). Then, the decontamination factors of Ag/X and Ag/Y silver faujasites exchanged with different silver contents were monitored according to the time on stream using a very low CH3I concentration (1 ppmv). Finally, a parametric study was conducted at semi-pilot scale using realistic [CH3I]/[H2O] molar ratio ≈ 10−6 and temperatures in the range 20–90 °C. Those experiments were helpful in order to assess the effects of some important parameters relevant to the adsorbents or operating conditions.