Integrity Of Cladding Research Articles

Development of an advanced hydride reorientation model for Zircaloy cladding and its experimental validation

Hydride reorientation, which occurs under hoop stress during cooling, stands out as a primary mechanism for material degradation in spent fuel management. The radial hydride fraction (RHF) is strongly involved in the mechanical integrity of cladding, highlighting the necessity for a robust modeling framework for quantitative analysis. However, the predictability of previous thermodynamic models for hydride reorientation in reactor-grade Cold Worked Stress Relieved (CWSR) Zircaloy has been hindered due to the intricate nature of hydride reorientation and the difficulties in characterizing microstructures. Recent successful EBSD characterization of reactor-grade CWSR Zircaloy have revealed valuable insights into microstructural characteristics of hydrides, enabling advancements in the modeling framework of hydride reorientation. This study aims to develop a thermodynamic model specifically focused on predicting the RHF. The developed thermodynamic model, based on classical nucleation theory, integrates aforementioned microstructural findings, combined with the Hydride-Nucleation-Growth-Dissolution (HNGD) model to capture transient precipitation behavior during cooling. Extensive experimental validations demonstrate enhanced predictability of the model. Additionally, the study examines the sensitivities of hydride reorientation to hydrogen concentration, applied stress, and cooling rate. It also provides predictions on reorientation behavior for engineering implications such as extension of wet storage, matrix hardening, recrystallization, and thermal cycling, supported by plausible explanations rooted in the underlying physical mechanisms elucidated through the model.

Comprehensive integrity assessment of spent nuclear fuel cladding during normal and postulated conditions of transportation

As a part of Spent Nuclear Fuel (SNF) management, it is important to maintain the cladding integrity to prevent the potential release of hazardous radioactive materials. Accordingly, assessing the integrity of SNF under the generated shock and vibration loads during normal conditions is essential for safe transportation. In this study, failure criteria for two zirconium alloys were derived by utilizing data from tensile property tests, mechanical model calculations, and machine learning estimations. Integrity assessment under normal and postulated transportation conditions was also performed via finite element analyses. Loading conditions were generated by real time-acceleration data collected during transportation tests in the Republic of Korea. Shock and vibration loads were applied to a simplified light-water reactor assembly. Furthermore, postulated transportation conditions and pre-cracked cladding were assumed for more challenging conditions. Consequently, a comprehensive integrity assessment of SNF cladding under shock and vibration loads for both free-defect and defective (pre-cracked) cladding was conducted. In each case, the resultant maximum principal strains, stress intensity amplitudes, and stress intensity factors were significantly smaller than the strain-based failure criteria, S-N curve, and fracture toughness, respectively. Therefore, the integrity of SNF cladding under transportation conditions remained intact even when assuming further challenging postulated transportation load conditions.

High-fidelity numerical investigation on structural integrity of SFR fuel cladding during design basis events

A high-fidelity numerical analysis methodology was proposed for evaluating the fuel rod cladding integrity of a Prototype Gen IV Sodium Fast Reactor (PGSFR) during normal operation and Design basis events (DBEs). The MARS-LMR code, system transient safety analysis code, was applied to analyze the DBEs. The results of the MARS-LMR code were used as boundary condition for a 3D computational fluid dynamics (CFD) analysis. The peak temperatures considering HCFs satisfied the cladding temperature limit. The temperature and pressure distributions were calculated by ANSYS CFX code, and applied to structural analysis. Structural analysis was performed using ANSYS Mechanical code. The seismic reactivity insertion SSE accident among DBEs had the highest peak cladding temperature and the maximum stress, as the value of 87 MPa. The fuel cladding had over 40 % safety margin, and the strain was below the strain limit. Deformation behavior was elucidated for providing relative coordinate data on each active fuel rod center. Bending deformation resulted in a flower shape, and bowing bundle did not interact with the duct of fuel assemblies. Fuel rod maximum expansion was generated with highest stress. Therefore, it was concluded that the fuel rod cladding of the PGSFR has sufficient structural safety margin during DBEs.

Presentation of the xM3 Test Case of the P2M Simulation Exercise and Modeling with the Fuel Performance Code ALCYONE

This paper presents simulations of the xM3 power ramp with the fuel performance code ALCYONE performed during an international simulation exercise organized within the Organisation for Economic Co-operation and Development/Nuclear Energy Agency Power to Melt and Maneuverability project. The xM3 test involved a large-grain UO2 fuel from Mitsubishi Heavy Industries cladded with Zirlo and pre-irradiated in a Spanish pressurized water reactor up to an average burnup of 27 GWd/tU−1. It was then submitted to a staircase ramp protocol in the R2 reactor at Studsvik (Sweden) with 10 successive steps of 5 kW·m−1 up to a ramp terminal level of 70 kW·m−1. The fuel rodlet did not fail, and detailed post irradiation examinations performed during the Studsvik Cladding Integrity Project II evidenced recrystallization of the pellet center around a central hole, interpreted as signs of fuel melting. In this paper, simulations with ALCYONE of the xM3 power ramp, including an advanced model for fuel melting based on thermodynamic equilibrium calculations, are detailed. The model relies on the determination of the liquid fuel fraction evolution with temperature that is used to obtain a continuous description of the material properties during phase change. In consequence of the incorporation of rare earths and actinides in the bulk of the fuel, distinct solidus and liquidus temperatures are estimated. It is shown that the observed central hole and recrystallized central part of the pellet could be the consequence of totally melted fuel (liquidus is reached), partially melted fuel (solidus is reached), or pore migration only.

Experimental investigation of uranium release from spent nuclear fuels under conditions expected in a deep geological repository

Abstract. Direct disposal of spent nuclear fuel (SNF) in a deep geological repository is planned for the long-term management of the highly radioactive waste discharged from the light water reactors (LWRs) of nuclear power plants in Germany. The aim is to safely isolate the radionuclide inventory from the biosphere, ad infinitum, making use of a multi-barrier system consisting of technical, geotechnical and geological barriers interposed between the emplaced waste and the environment. Still, reversibility measures including retrievability of the waste products are considered in this concept. Internationally, in safety assessments for deep underground repositories (DGRs), the access of groundwater, followed by failure of canisters and finally loss of the SNF's cladding integrity is considered in the long term. Hence, the evaluation of the performance of SNF in the near field of a deep geological repository requires process understanding of SNF dissolution and rates, in addition to the quantification of the radionuclide release from SNF under reducing conditions of a breached container. With the aim to derive a radionuclide source term, the SNF dissolution and alteration processes can be assigned to two steps, namely the (i) instantaneous release of radionuclides upon cladding failure from gap and grain boundaries and (ii) a long-term release that results from the dissolution of the fuel grains themselves (Ewing, 2015). Since most radionuclides produced during irradiation of nuclear fuel in a reactor are trapped within the SNF matrix, understanding the processes leading to the dissolution of the matrix and its dissolution rates are of particular interest for post-closure safety assessments (Pastina and LaVerne, 2021). Research at the Institute for Nuclear Waste Disposal (INE) at the Karlsruhe Institute of Technology (KIT) has focused, for more than 25 years, on the behaviour of SNF (irradiated UO2 and mixed-oxide, MOX, fuels) under geochemical conditions (pH, redox milieu, gas atmosphere and ionic strength) representative of various repository concepts and radiolytic reactions on SNF dissolution mechanisms. This contribution will give an overview on the radionuclide release from dissolving SNFs under repository conditions. In particular, uranium released into solution from UO2 and MOX fuels during up to 1629 d of leaching in salt brines, granitic/bentonitic or cementitious groundwaters under Ar or Ar/H2 atmospheres. As observed in other published studies on SNF dissolution under reducing conditions, a large scatter of the uranium concentration is seen in the initial stages of our experiments. After about 1 year of leaching, the aqueous concentration of uranium slowly approaches the solubility limit of U(IV), independent of the type of studied SNF samples (various irradiated UO2 and MOX fuels). A similar behaviour is observed for other redox-sensitive actinides. The release rate of uranium decreases significantly in all experiments within first 400 d of leaching. Although the rate is very low, it is >0, and a continuous release is observed. A comparison of the results from SNF experiments under various redox conditions demonstrates that oxidative matrix dissolution is inhibited due to the presence of hydrogen. Our results are discussed in the context of the outcome of international research programmes on SNF corrosion. These findings are considered to be of relevance for the definition of source terms for various irradiated LWR fuels in safety investigations for a DGR in Germany.

Bushfire resistance of external light steel wall systems lined with fibre cement boards

Bushfires/wildfires have become a widespread and frequent occurrence in many parts of the world. With the adverse effects of climate change and the increasing population in the Bushland Urban Interface (BUI)/Wildland Urban Interface (WUI), ensuring adequate bushfire resistance of buildings in bushfire-prone areas has become very important. Improving the safety and integrity of the external building envelope is identified as a solution to minimise the bushfire related building damage and losses. This paper presents the details of bushfire resistance experiments conducted on external Light gauge Steel Framed (LSF) wall systems and the results. These LSF wall systems were externally lined with fibre cement boards and exposed to both bushfire radiant heat and flame zone fire conditions under indoor and outdoor test conditions. The results have shown that the rapid increment of the heat flux led to sudden explosive failure of the wall system whereas the gradual increment of the heat flux resulted in a progressive failure. This study has highlighted the importance of external cladding integrity and thermal shock resistance of building elements to enhance the bushfire resistance of buildings.

Inferential-Statistical Reevaluation of Spent Fuel Zircaloy Cladding Integrity

From the published results of experiments investigating the effects of delayed hydride cracking (DHC) on spent fuel Zircaloy cladding integrity, relevant data have been extracted and re-analyzed, taking advantage of inferential statistics and an information-theoretic model selection criterion. Statistical tolerance intervals, the method of maximum likelihood estimation, and the Akaike information criterion, corrected for small sample size, were applied to a small sample of measured values of the threshold stress-intensity factor . The purpose was to create a well-grounded probability density function for use in a mathematical model correlating random variates of with important conditions for the initiation of crack growth by DHC, specifically, cladding hoop stress and the depth and shape of surface flaws. A selection criterion purposely designed for small sample sizes and the robust nature of inferential statistics were ideally suited for the intended reevaluation. The fidelity of the mathematical model was protected by the exclusion of any simplifying approximations, e.g., substitution of constants or single-valued descriptive statistics for variables. The probabilistic effect of the random variable was thereby precisely mapped onto the linearly correlated variable, threshold cladding hoop stress, as a function of surface flaw depth and shape. Contour plots of the results constitute significant improvements over previous quantitative single-point estimates of the effects of DHC on spent fuel Zircaloy cladding integrity.

Analysis of Calculation Model for Primary Coolant Fission Products

The sources of radioactive contamination of the primary coolant by fission products when the unit is operating at the rated power are as follows: defect fuel elements with gas leakiness and substantial damages, surface contamination of the outer surfaces of fuel claddings, superficial contamination of structural materials of fuel assemblies. Initially in the reactor operation (if there are no manufacturing defects in fuel elements), the contamination of the coolant by fission products is determined by the release into the reactor coolant circuit of fission fragments of Uranium-235 (due to their kinetic energy) that is present on the outer surfaces of fuel elements as contamination in their manufacturing. During normal operation of the reactor, the integrity of cladding may fail due to various processes of corrosion fatigue type. These processes result in, first of all, micro-fissures and then in large defects in the claddings, which is accompanied by an increase in the release of fission products from fuel elements into the primary coolant.

Hydride reorientation and its impact on mechanical properties of high burn-up and unirradiated cold-worked stress-relieved Zircaloy-4 and ZirloTM fuel cladding

Precipitation of radial hydrides in zirconium-based cladding concomitant with the cooling of spent nuclear fuel can potentially compromise cladding integrity during its subsequent handling and transportation. Complementary experimental programs on unirradiated and irradiated cladding were performed to assess the conditions leading to the precipitation of radial hydrides and their detrimental effects on mechanical properties. This sequel paper to a previous publication [1] presents results for cold-worked, stress-relieved Zircaloy-4 and ZirloTM cladding used in pressurized water reactors.Cold-worked, stress-relieved Zircaloy-4 and ZirloTM behave very similarly, although irradiated ZirloTM was found to be slightly more prone to hydride reorientation (HRO) than irradiated Zircaloy-4. No distinct irradiation-induced effect on the HRO critical stress was detected. Depending on the initial high temperature that varied from 350 to 450 °C, HRO may cause a ductile-to-brittle transition when the cladding hoop stress, created by internal pressurization of the specimen, ranges from 70 to 120 MPa. Cracking of the oxide layer caused local preferential radial hydride precipitation. The impact of the decreasing temperature and internal pressure during cool-down was also investigated: it has been found that samples undergoing decreasing temperature and pressure cycles are less prone to hydride reorientation.

Sensitivity analysis for characterizing the impact of HNGD model on the prediction of hydrogen redistribution in Zircaloy cladding using BISON code

Hydrogen in zirconium cladding is able to precipitate into zirconium hydrides which impacts cladding integrity. The Hydride Nucleation-Growth-Dissolution (HNGD) model in the BISON code accounts for the precipitation and dissolution kinetics of hydride in Zircaloy material. This paper presents global sensitivity analyses of the HNGD model aiming to enhance our understanding of the hydride precipitation phenomena by quantifying the variance that key parameters have on the prediction of hydrogen behavior under various environmental conditions. Model predictions are compared to experimental data obtained under two different conditions: 1) with uniformly precharged specimens subjected to a linear thermal gradient, and 2) specimens precharged with a cathodically applied hydride rim at one end of the sample and subjected to an asymmetric thermal gradient. The Sobol sensitivity analysis identifies the key parameters in the HNGD model for both types of specimens. For linear temperature cases, the heat of transport dominates the accuracy of predictions when no precipitation occurs at the cold end, while Terminal Solid Solubility for Dissolution (TSSD) is the most important parameter when precipitation occurs. A large variation in the predicted hydrogen concentration profiles is found in the range of high TSSD due to the occurrence of precipitation. For asymmetric temperature cases, the solubility coefficient gives the largest impact on the predicted hydrogen distribution, as it determines the amount of solute hydrogen dissolved from the initially applied hydride rim. A large discrepancy in hydrogen distribution between simulations and experiments exists with the asymmetric specimens because BISON simulations fail to predict the precipitation of hydride at the cooler end. Comparative studies using former and updated models verifies the significant impact of the hydride growth mechanism on predicted hydrogen concentration profiles. In particular, when hydride initially exists, changes in TSSD generate a large variation in the predicted amount of precipitation by hydride growth, giving large uncertainty in predicting the hydrogen distribution over the sample length. The outputs characterize the significant impact of the hydride growth mechanism in the HNGD model on predicting hydrogen behavior, and improve the understanding of the precipitation of hydride in Zircaloy cladding within a range of expected environmental conditions. The analyses indicate work is still needed to improve the hydride solvus models in the BISON code to accurately predict experimentally observed hydride concentrations and distributions.

Experimental and mechanical research on CHF characteristics during combined/bottom quenching in narrow rectangular channel

Critical heat flux (CHF) phenomenon plays an important role during quenching phase when loss of coolant accident (LOCA) occurs in plate-type fuel reactor core. Heat transfer capability can be greatly enhanced with larger CHF value so that the cladding integrity can be maintained during quenching process. However, the related research on transient CHF during quenching process in narrow rectangular channel is extremely limited in literature, not to speak of combined quenching. Considered that quenching CHF mechanism may be totally different from that during heating process, experimental and mechanical researches were conducted comprehensively on CHF behavior during combined/bottom quenching in this article.Conclusion can be obtained that during combined quenching, bottom injection portion plays a dominant role because major portion of top injection may be expelled due to counter current flow limitation (CCFL) in narrow rectangular channel. Besides, boundary conditions such as injection methods, inlet subcooling, pressure and heating power have strong effect on CHF behavior. Based on previous visualization results, mechanism of quenching CHF can be attributed to dryout of liquid sublayer beneath coalesced bubble, and the proposed CHF model matches well with experimental data (RMS error 22.24%). The relevant findings are important for safety operation of plate-type fuel reactor and also can be applied in other industrial scenes where quenching phenomenon are encountered.

Numerical investigations on flow-induced vibration characteristics of wire-wrapped fuel rod

For the wire-wrapped fuel assembly in nuclear reactor, the high-speed scouring of coolant combined with the transverse flow caused by wrapped wire can lead to the vibration of fuel assembly, which in turn induce fretting wear and endanger the integrity of cladding. In this paper, computational fluid dynamics (CFD) method is used to simulate and analyze the flow induced vibration of a single wire-wrapped fuel rod at different flow velocities (2.5–6 m/s), the flow field parameters and solid vibration response characteristics are obtained. Through the analysis of spectrum and statistical method, the power spectral density (PSD) of amplitude is obtained and the vibration displacement, which mainly concentrated in the 3.0–3.4 μm, basically conforms to the normal distribution. The turbulence intensity increases significantly when the fluid passes through the wire, and the pressure pulsation within the boundary layer is enhanced, resulting in greater vibrational displacement of the rod.

Spectroscopic and chemical investigations on volatile fission and activation products within the fuel-cladding interface of irradiated pressurised water reactor fuel rod segments

Abstract. In Germany, the present waste management concept foresees the direct disposal of spent nuclear fuel (SNF) in deep geological repositories for high-level waste available by 2050, at best. Until then, SNF is encapsulated in dual-purpose casks and stored in dry interim storage facilities. Licenses for both casks and facilities will expire after 40 years following loading of the cask and emplacement of the first cask in the storage location. Yet, due to considerable delays in the site selection process and the estimated duration for construction and commissioning of a final repository of at least 2 decades, a prolonged dry interim storage of SNF is inevitable (ESK, 2015). Concerning these considerable timespans, integrity of the cladding is of utmost importance regarding the ultimately conditioning of the fuel assemblies for final disposal. Various processes strain the structural integrity of Zircaloy cladding during reactor operation and beyond such as delayed hydride cracking, fuel-cladding chemical interactions or irradiation damage induced by α-emitters present in the fuel pellet's rim zone (Ewing, 2015). Especially with higher burn-up, the gap between fuel and cladding closes and results in the formation of an interaction layer, in which precipitates of fission and activation products are present, displaying an interface for degradation processes. For chemical analysis and speciation of these agglomerates, Zircaloy-4 and SNF specimens were sampled from fuel rod segments irradiated in commercial pressurised water reactors during the 1980s. Zircaloy-4 specimens were taken from an UOX (50.4 GWdtHM-1) and mixed oxide fuel (MOX) (38.0 GWdtHM-1). In addition, SNF fragments were sampled from the closed gap of both fuel types to examine volatile activation and fission products, which had been segregated from the centre to the pellet periphery during irradiation and thus contribute to the possible chemically assisted cladding degradation effect of the precipitates within the fuel-cladding interface. Spectroscopic analysis of precipitates within the interface layer between fuel and cladding were performed by optical microscopy, X-ray absorption and X-ray photoelectron spectroscopy, as well as by energy-dispersive scanning electron microscopy. Moreover, the radionuclide inventory of the respective Zircaloy-4, fuel and interaction layers was determined using liquid scintillation counting, γ-spectroscopy, gas mass spectrometry, ion chromatography and inductive-coupled plasma mass spectrometry and compared to results received by MCNP/CINDER and webKORIGEN calculations. In this study, we provide results regarding the speciation and chemical composition of previously identified Cs-U-O-Zr-Cl-I bearing compounds found in the interaction layer of irradiated nuclear fuel and inventory analyses of radionuclides present therein, with particular emphasis on Cl-36 and I-129. Furthermore, the agglomerates within the fuel-cladding interface were characterised for the first time utilising synchrotron radiation-based Cl K-edge and I K-edge measurements, resulting in compounds with structural similarities to CsCl and CsI. The outcomes obtained from this study provide further insights into the complex chemistry within the fuel-cladding interface with respect to the aging management and integrity of SNF under the conditions of interim storage. In future studies we will examine whether the different compounds at the fuel-cladding interface have the potential to affect the mechanical properties of Zircaloy cladding.

Twenty years of research on spent nuclear fuel in the context of final disposal at KIT-INE in multinational collaborative projects

Abstract. Disposal of spent nuclear fuel (SNF) in deep geological repositories is considered a preferential option for the management of such wastes in many countries with nuclear power plants. With the aim to permanently and safely isolate the radionuclide inventory from the biosphere for a sufficient time, a multibarrier system consisting of technical, geotechnical and geological barriers is interposed between the emplaced waste and the environment. In safety assessments for deep underground repositories, access of water, followed by failure of canisters and finally loss of the cladding integrity is considered in the long-term. Hence, evaluating the performance of SNF in deep geological disposal systems requires process understanding of SNF dissolution and rates as well as quantification of radionuclides release from SNF under reducing conditions of a breached container. In order to derive a radionuclide source term, the SNF dissolution and alteration processes can be assigned to two steps: (i) instantaneous release of radionuclides upon cladding failure from gap and grain boundaries and (ii) a long-term release that results from dissolution of the fuel grains itself (Ewing, 2015). In this context, research at KIT-INE has focused for more than 20 years on the behavior of SNF (irradiated UO2 and MOX fuels) under geochemical conditions (pH, redox and ionic strength) representative of various repository concepts, including the interaction of SNF with backfill material, such as bentonite as well as the influence of iron corrosion products, e.g. magnetite and radiolytic reactions on SNF dissolution mechanisms. Since 2001, KIT-INE has contributed with experimental and theoretical studies on the behavior of SNF under repository relevant conditions to six Euratom projects viz SFS (2001–2004), NF-PRO (2004–2006), MICADO (2006–2009), RECOSY (2007–2011), FIRST-Nuclides (2012–2014) and DISCO (2016–2021). Moreover, since 2007, overall 4 consecutive projects for the Belgian waste management organization, ONDRAF-NIRAS, were performed on the behavior of SNF under conditions representative of the Belgian “Supercontainer” concept. In this contribution, we summarize major achievements of theses research projects to understand and quantify the radionuclide release from dissolving SNF under repository conditions. In particular, the dependence of radionuclide release on the chemical composition of the aqueous and gaseous phase in the proximity of repositories in different types of host rock is discussed.

Predicting the hydride rim by improving the solubility limits in the Hydride Nucleation-Growth-Dissolution (HNGD) model

During operation of a light water reactor, waterside corrosion of the Zircaloy nuclear fuel cladding causes hydrogen pickup. The absorbed hydrogen can redistribute in the cladding driven by existing concentration, stress, and temperature gradients. When the concentration reaches the solubility limit, hydrides precipitate. These hydrides can be more brittle than the Zircaloy matrix, so they can endanger the cladding integrity during a transient if their concentration is too high. In recent years, extensive efforts have been made to understand hydrogen behavior and to develop simulation tools able to predict hydrogen diffusion and hydride precipitation and dissolution. These efforts led to the development of the Hydride Nucleation-Growth-Dissolution (HNGD) model and its implementation into the nuclear fuel performance code Bison. While it offers a significant improvement and accurately predicts the amount of precipitates, this model fails to predict the thickness of the hydride rim under a temperature gradient. The current work presents the limitation of the HNGD model and proposes two hypotheses to improve the model’s accuracy. The first hypothesis introduces a time dependency to the supersolubility to reduce the nucleation barrier as hydrogen atoms find more favorable nucleation sites. The second one introduces a hydride content dependency to the solubility. These hypotheses were validated and implemented into Bison and are now available to the user community. The modified HNGD model accurately predicts the hydride rim thickness, and it was demonstrated that this updated model can be used in Bison to model Zircaloy cladding with a zirconium inner liner. Finally, potential experimental and numerical methods are discussed to further validate these hypotheses.

Flow boiling transient critical heat flux tests with stainless steel and FeCrAl: Transient correlation implementation, model calibration, and sensitivity analysis

Steady-state and transient internal-flow critical heat flux (CHF) experiments were carried out under two flow conditions, at atmospheric pressure, on 316 stainless steel (316-SS) and iron-chromium-aluminum (FeCrAl) tubes. Slow power transients at a low mass flow (300kg/m2−s) with very low subcooling (Xe=−0.0054) generated premature CHFs, which were prevented by faster power transients and high mass flow (1,000kg/m2−s). The measured transient CHFs increased linearly with increasing power transients, compared with the steady-state CHF. Yet, the wall superheat at the CHFs decreased with faster power transients. Transient CHF correlations highlighting heterogeneous spontaneous nucleation were calibrated to the measured CHFs and compared with other existing correlations. Transient CHF multipliers were acquired from pool and flow boiling empirical CHF correlations that were generated. The multipliers were applied to the RELAP5-3D nuclear system code to analyze the discrepancies between the measured data and the predicted CHF and post-CHF behavior, which improved peak cladding temperature predictions by 24.6%. A variance-based global sensitivity study perturbing the experimental uncertainties and cladding material thermal properties showed the diminishing influence of flow boiling heat transfer with increasing power transients, highlighting the significance of the volumetric heat capacity for the cladding integrity during transients. Transient CHF correlations were applied to the most limiting design basis accident: a hot-zero-power reactivity-initiated accident on a generic pressurized water reactor RELAP5-3D model.

Sensitivity analysis of BISON model for characterization of impact of experimental parameters on hydrogen migration and redistribution in zirconium-based alloys

The formation of hydrides, a result from excess hydrogen uptake by the cladding during nuclear reactor operation, can significantly impact cladding integrity. We present a sensitivity and uncertainty analysis of a hydrogen predictive model in the BISON fuel performance code to characterize the key input parameters involved in the model. This includes identifying the key parameters necessary to simulate hydrogen behavior in the fuel cladding, revealing the impact of environmental conditions on hydrogen distribution, and informing the envelope of conditions for ongoing experimental work conducted by the University of Michigan. The Sobol sensitivity analysis reveals the quantitative impacts of environmental conditions on the predicted total hydrogen concentration, as well as the respective impact on their sensitivity with respect to the physical parameters. Overall, the precipitation of hydrogen that occurred at the cold end of the sample is the most important phenomenon in the prediction of hydrogen concentration. The optimization study using the results from sensitivity analysis indicates that the BISON simulations produce accurate hydrogen predictions when the sets of parametric ranges are shifted to enable more precipitation to occur at the cold end. Lastly, the sensitivity and uncertainty (S/U) analysis for the ongoing benchmarking experiments supports the focus of experiments that lies on the measurement of Soret effect of hydrogen driven by linear temperature gradients. The outputs are expected to better characterize the various parameters involved in the hydrogen transport model in the BISON code, and improve the understanding of the hydrogen transport behaviors in zirconium-based fuel cladding in a range of expected environmental conditions.

Quantifying the effect of hydride microstructure on zirconium alloys embrittlement using image analysis

Because hydride microstructure can significantly influence Zr alloy nuclear fuel cladding’s ductility, a new metric has been developed to quantify hydride microstructure in 2D micrographs and relate it to crack propagation. As cladding failure usually results from a hoop stress, this new metric, called the Radial Hydride Continuous Path (RHCP), is based on quantifying the continuity of brittle hydride particles along the radial direction of the cladding tube. Compared to previous metrics, this approach more closely relates to the propensity of a crack to propagate radially through the cladding tube thickness. The RHCP takes into account hydride length, orientation, and connectivity to choose the optimal path for crack propagation through the cladding thickness. The RHCP can therefore be more closely linked to hydride embrittlement of the Zr alloy material, thus creating a relationship between material structure, properties, and performance. The new definition, along with previously proposed metrics such as the Radial Hydride Fraction (RHF), the Hydride Continuity Coefficient (HCC), and the Radial Hydride Continuity Factor (RHCF), have been implemented and automated in MATLAB. These codes are made available with this publication as supplementary materials. These metrics were verified by comparing their predictions of hydride morphology against expected values in simple cases, and the implementation of the new metric was validated by comparing its predictions with manual measurements of hydride microstructure performed on ImageJ. The RHCP was also validated against experimental measurements of fracture behavior and it was shown to correlate with cladding failure better than previous metrics. The information provided by these metrics will help accurately assess cladding integrity during operation, transportation, and storage.

Research of Integrity of Fire Insulation Cladding with Mineral Wool of Steel Beam under Fire Impact

In the article, the processes of integrity loss of fire-retardant cladding from mineral wool of a steel I-beam are considered. The relation between fire-retardant cladding integrity and its thermal insulating ability was studied. Computer model of a steel beam with fire-retardant cladding of mineral wool, which takes into account its own weight, current mechanical load and temperature mode of heating under the heat-insulating effect of the cladding, using the computer system LS-DINA, is developed in this article. It is shown that the fire-retardant cladding loses its integrity under the thermal influence of fire long before the onset of the limit state of loss of fire resistance in terms of bearing capacity. The conditions under which fire-retardant cladding loses its integrity and fire-retardant ability are investigated. Having considered the time of fire-retardant cladding integrity loss the fire resistance limit can be significantly clarified and allows to avoid early collapse of building structures cause by fire.

Experimental Verification of Phase Diagram Calculations of Zr-Based Alloys after High-Temperature Oxidation

Zirconium-based alloys are commonly used as a material for nuclear fuel claddings in the light water reactors. The cladding material must function to fix a huge number of pellets, while conducting heat into the coolant that flows turbulently around the fuel rods. Cladding tubes can contain gaseous fission products that escape the fuel. Thus, by functioning as a sealed unit, it prevents a contamination of the coolant water with high-radioactive fission products. The integrity of claddings is always a critical issue during reactor operation and wet or dry storage and transport of the spent fuel rods. Moreover, the role gains importance at Loss of Coolant Accidents (LOCA). After Fukushima accident, cladding materials are widely studied with the purpose to reduce the high-temperature oxidation rate and enhance accident tolerance. In our contribution, we introduce the studies on Zr-1Nb (E110) cladding tubes after high-temperature steam oxidation at 1350 °C. During the testing of claddings, microscopy analytical methods play an important role in experimental verification of pseudo-binary phase diagram Zr1Nb-O, i. e. particularly in oxygen content determination at phase transitions. Wave Dispersive Spectroscopy (WDS) with complementary nano-indentation method were used to characterize the Zr1Nb microstructure formed after LOCA. It includes the regions from an oxide and oxygen-stabilized α-Zr(O) to the acicular prior β-Zr phase. The decrease of hardness and Young's modulus corresponds with oxygen content measured in line-profiles by WDS. The oxygen level at transition points was partly determined from Fe, Nb β-stabilizers and significant change in mechanical properties in fine-grained prior β-Zr. The slight fluctuation of oxygen values in adjacent grains can be caused by preferential oxidation through the favorably oriented α-Zr(O) grains studied by WDS+EBSD. As well, the non-uniform oxygen-rich α-Zr(O) phase adjacent to the oxide was characterized by EBSD & WDS. Increasing hydrogen content in specimens, 10, 700 and 1000 ppm H, caused increasing solubility of oxygen in prior β-Zr phase upon high-temperature and the cladding material hardening.