•Bifurcating accident progression signatures contrast material interaction models.•Model biases are identified for material interaction models in MELCOR V2.2.•Eutectics model simulations exhibit ...accelerated fuel degradation.•Higher peak fuel rods and debris temperatures are exhibited by the eutectics model.•More extensive molten material formation is calculated by the eutectics model.
Single case comparisons between severe accident simulations can provide detailed insights into severe accident model behavior, however, they cannot offer insights into model uncertainty, sensitivity to uncertain parameters, or underlying model biases. In this analysis, the single case benchmark comparison of the MELCOR material interaction models for a station blackout (SBO) scenario of a boiling water reactor (BWR) using representative Fukushima Daiichi Unit 1 boundary conditions is expanded to include an uncertainty analysis. As part of this uncertainty analysis, 1200 simulations are performed for each material interaction model (2400 total), with random sampling of 14 uncertain MELCOR input parameters. Input parameters are selected for their impact on models representing core degradation processes. These include candling, fuel rod failure, debris quenching and dryout. The analysis performed here is not a traditional “best-estimate” uncertainty analysis that uses best-estimate parameters or identifies best-estimate figure of merit distributions. Instead, it is an exploratory uncertainty analysis that identifies and interrogates underlying model form biases of the two material interaction models (eutectics and interactive materials models). Uniform distributions are applied to all uncertain parameters to ensure coverage of the model parameter uncertainty space. Key findings from this study include underlying model form biases exhibited by material interaction models, and notable differences in accident progression outcomes between the material interaction models. This uncertainty study extends and confirms the conclusions from the first part of this study, which compared the impact of material interaction modeling on simulation of a short-term station blackout scenario with representative Fukushima Daiichi Unit I boundary conditions. In particular, this study confirms that the eutectics model generally exhibits accelerated degradation and failure of fuel components, the core plate, and the lower head. The eutectics model also has a tendency to exhibit a greater degree of core degradation, greater debris mass formation, and larger debris mass ejection. Finally, the eutectics model exhibits higher maximum temperatures for fuel, cladding, particulate debris, oxidic molten pool, and metallic molten pool components than the interactive materials model; interactive materials model simulations exhibit a soft “limitation” on maximum temperatures that is related to the temperature at which material relocation occurs.
A sample of plutonium tetrafluoride that was separated prior to 1966 at the Hanford Site in Washington State was analyzed at the Pacific Northwest National Laboratory (PNNL) in 2015 and 2016. The ...plutonium tetrafluoride, as received, was an unusual color and considering the age of the plutonium, there were questions about the condition of the material. These questions had to be answered in order to determine the suitability of the material for future use or long-term storage. Therefore, thermogravimetric/differential thermal analysis and X-ray diffraction evaluations were conducted to determine the plutonium's crystal structure, oxide content, and moisture content; these analyses reported that the plutonium was predominately amorphous and tetrafluoride, with an oxide content near ten percent. Freshly fluorinated plutonium tetrafluoride is known to be monoclinic. During the initial thermogravimetric/differential thermal analyses, it was discovered that an exothermic event occurred within the material near 414 °C. X-ray diffraction analyses were conducted on the annealed tetrafluoride. The X-ray diffraction analyses indicated that some degree of recrystallization occurred in conjunction with the 414 °C event. The following commentary describes the series of thermogravimetric/differential thermal and X-ray diffraction analyses that were conducted as part of this investigation at PNNL.
•Plutonium tetrafluoride appears to undergo self-induced radiolysis under long-term storage conditions.•Radiation damage in plutonium tetrafluoride appears to be reversible through thermal annealing.•Radiation damage in plutonium tetrafluoride induces a color change from a salmon pink to a greyish-brown.
Probabilistic fracture mechanics (PFM) simulates the behavior of cracked structures and propagates uncertainties from input parameters to a failure probability or its uncertain estimate. In nuclear ...technology, this approach supports the assessment of the rupture probability of highly reliable pipes, which is an important parameter for the safety analysis of a nuclear power plant. For the appropriate probabilistic modelling of a structure with consideration of uncertainties, but also for the analysis of PFM application cases, the question arises, which input parameter of a probabilistic model has a higher impact on the estimate of computed failure probability, and which has a minor impact. This question is associated with the sensitivity measures or importance factors of the input parameters and their ranking concerning their influence.
In this paper, six different approaches for the quantification of the sensitivity of parameters PFM evaluations are investigated: the amplification ratio, the direction cosine, the degree of separation, the analysis of the most probable failure point, the separation of uncertainty method, and the simple sample-based sensitivity study. Each method is described, visualized, applied to a common test case, and compared. The application case and the comparison are part of the Coordinated Research Project (CRP), “Methodology for Assessing Pipe Failure Rates in Advanced Water-Cooled Reactors (AWCRs)” by the International Atomic Energy Agency (IAEA), which is dedicated to the development of failure rates of piping in AWCRs. The participants used different PFM computer codes to analyze the test case and individual sensitivity methods to rank the input parameters, which motivated the comprehensive survey.
The predicted parameter ranking of the approaches is consistent between the methods and between different PFM codes, but the approaches differ in the scope and the required effort. A conclusion is drawn and recommendations for the six different approaches are given.
•Sensitivity measures in probabilistic fracture mechanics are compared.•Six parameter ranking methods with different starting points are evaluated.•The proposed sensitivity measures agree in their ranking.•The sensitivity ranking approaches differ in their scope and the required effort.
•Computation of failure frequency in the absence of operational experience.•Mechanistic models that account for the factor of improvement (FOI) concept.•Three different piping reliability modeling ...approaches for the computation of failure frequency.
Probabilistic failure metrics such as leak frequency and rupture frequency are commonly used to characterize piping reliability. The methodologies for calculating the failure metrics rely on a complex set of input parameters. Operating experience data and experimental data play an important role in informing the different input parameters. The paper describes results and conclusions of a coordinated research project to benchmark three different reliability models using a four-step procedure: reference case definition of relevance to advanced reactor designs, input parameter calibration, validation of results, and application of different methodologies upon completion of the calibration and validation steps. The reference case is a weld consisting of nickel-base alloy 152/52 and located within a primary pressure boundary of an advanced reactor. This alloy is a class of structural materials known to be highly resistant to stress corrosion cracking. Synergies between the different methods are noted and the importance of a multi-disciplinary approach to input parameter development is underscored. A key conclusion is that the three methods are equally suitable for estimating failure frequencies. In any specific application, a selection of the most practical or effective computational tool can be considered. The comparison of alternative models confirms and helps to gain confidence in the computed failure frequency estimates. The study was part of a coordinated research project organized by the International Atomic Energy Agency.
Plutonium tetrafluoride that was separated prior to 1966 at the Hanford Site in Washington State was analyzed at the Pacific Northwest National Laboratory (PNNL) in 2015 and 2016. The plutonium ...tetrafluoride, as received, was an off-normal color and considering the age of the plutonium, there were questions about the condition of the material. These questions had to be answered in order to determine the suitability of the material for future use or long-term storage. Therefore, Thermogravimetric/Differential Thermal Analysis and X-ray Diffraction evaluations were conducted to determine the plutonium’s crystal structure, oxide content, and moisture content; these analyses reported that the plutonium was predominately amorphous and tetrafluoride, with an oxide content near ten percent. Freshly fluorinated plutonium tetrafluoride is known to be monoclinic. During the initial Thermogravimetric/Differential Thermal analyses, it was discovered that an exothermic event occurred within the material near 414°C. X-ray Diffraction analyses were conducted on the annealed tetrafluoride. The X-ray Diffraction analyses indicated that some degree of recrystallization occurred in conjunction with the 414°C event. The following commentary describes the series of Thermogravimetric/Differential Thermal and X-ray Diffraction analyses that were conducted as part of this investigation at PNNL, in collaboration with the University of Utah Nuclear Engineering Program.
•3D neutron anisotropic scattering analysis with the export-controlled neutronics code system AGENT: assessment of angular and point-wise scalar flux.•Unit cell benchmarks and analysis of the effect ...of borated moderator in comparison to regular light water moderator.•Analysis of the University of Utah TRIGA with the P5 – AGENT model in comparison to MCNP6.
Neutron anisotropic scattering method of the AGENT (Arbitrary GEometry Neutron Transport) reactor neutronics modeling system is verified with a few representative benchmark examples in comparison to MCNP6. Benchmark examples include various unit cell types, and the University of Utah 100kWth TRIGA MARK-I reactor (UUTR). The effect of neutron anisotropic scattering is analyzed and visually assessed in showing that the AGENT anisotropic scattering of up to P5 order of Legendre expansion provide the expected agreements with MCNP6 and with the gained computational times.
In order to combine niobium (Nb) with lanthanum (La) and cerium (Ce), Nb ions
were deposited within a thin film of these two elements. According to the
Hume-Rothery rules, these elements cannot be ...combined into a traditional
crystalline metallic solid. The creation of an amorphous metallic glass
consisting of Nb, La, and Ce is then investigated. Amorphous metallic glasses
are traditionally made using fast cooling of a solution of molten metals. In
this paper, we show the results of an experiment carried out to form a
metallic glass by implanting 9 MeV Nb 3+ atoms into a thin film of La and Ce.
Prior to implantation, the ion volume distribution is calculated by Monte
Carlo simulation using the SRIM tool suite. Using multiple methods of
electron microscopy and material characterization, small quantities of
amorphous metallic glass are indeed identified.
nema
A novel polar angular quadrature set called the Kim-Jevremovic polar angular quadrature set is derived for the method of characteristics. It is based on neutron anisotropic scattering cross-sections ...in the Evaluated Nuclear Data File. This new set is implemented within the state-of-the-art neutron transport code AGENT and tested in comparison to MCNP6 as well as to other known quadrature sets for the UO2 unit cells, the well-known C5G7 benchmark, unreflected cylinders of uranyl-fluoride solutions in heavy water, and the University of Utah 100 kWth TRIGA MARK-I reactor core. These comparisons show that the newly proposed polar angular quadrature set provides better agreements than other quadrature sets for the lower order of anisotropic scattering expansions. This paper presents a complete derivation of the Kim-Jevremovic polar angular quadrature set and the analysis for the mentioned bench-mark examples.
Following a significant increase in the number of facilities in the world having and developing low- and high-linear energy transfer (LET) microbeams for experimental radiobiological studies, it is ...useful and demanding to establish reliable computational models to analyze such experiments. This paper summarizes initial MCNP5 calculations of the basic parameters needed to study X-ray microbeam penetration, dose deposition and dose spatial dissipation in tissue-like media of micro and macro scales. The presented models can be used to predict doses delivered to neighboring cells and analyze the cause of bystander cell deaths. In the case of low-LET radiation, dose distribution is more homogenized when compared to high-LET that deposits almost all of its energy in the cell hit by radiation. Results are presented for a microbeam of monoenergetic soft (2-10 keV) X-rays for two different micro-models: (a) single-cells of homogeneous and uniform chemical compositions, and (b) single-cells of heterogeneous structures (nucleus and cytoplasm) with different chemical compositions. In both numerical models, only one cell is irradiated and the electron and X-ray doses in all cells are recorded. It was found that surrounding cells receive approximately five orders of magnitude less dose than the target cell in the homogenized cell model. The more detailed, heterogeneous model showed that the nucleus of the target cell receives more than 95% of the dose delivered to the entire cell, while neighboring cell nuclei receive approximately 65% of their total cell dose. Results of the macroscopic behavior of a soft X-ray microbeam using a cylindrical phantom 5 cm tall and 1 cm in diameter are also presented. Three-dimensional dose profiles indicate the spatial dose dissipation. For example, a 10 keV X-ray microbeam dose scatters to a negligible level at 0.3 cm radially from the center while it reaches an axial depth of 2 cm.
•The eutectics model simulation exhibits a more accelerated accident progression.•Earlier fuel component degradation is observed for the eutectics model simulation.•The eutectics model simulation ...exhibits larger molten and refrozen debris masses.•The eutectics model simulation exhibits larger in and ex-vessel debris masses.•The interactive materials model simulation exhibits lower peak fuel temperatures.
In this analysis, the two material interaction models available in the MELCOR code are benchmarked for a severe accident at a BWR under representative Fukushima Daiichi boundary conditions. This part of the benchmark investigates the impact of each material interaction model on accident progression through a detailed single case analysis. It is found that the eutectics model simulation exhibits more rapid accident progression for the duration of the accident. The slower accident progression exhibited by the interactive materials model simulation, however, allows for a greater degree of core material oxidation and hydrogen generation to occur, as well as elevated core temperatures during the ex-vessel accident phase. The eutectics model simulation exhibits more significant degradation of core components during the late in-vessel accident phase – more debris forms and relocates to the lower plenum before lower head failure. The larger debris bed observed in the eutectics model simulation also reaches higher temperatures, presenting a more significant thermal challenge to the lower head until its failure. At the end of the simulated accident scenario, however, core damage is comparable between both simulations due to significant core degradation that occurs during the ex-vessel phase in the interactive materials model simulation. A key difference between the two models’ performance is the maximum temperatures that can be reached in the core and therefore the maximum ΔT between any two components. When implementing the interactive materials model, users have the option to modify the liquefaction temperature of the ZrO2-interactive and UO2-interactive materials as a way to mimic early fuel rod failure due to material interactions. Through modification of the liquefaction of high melting point materials with significant mass, users may inadvertently limit maximum core temperatures for fuel, cladding, and debris components.
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