Heat-up Phase Research Articles

Enhancing cladding mechanical modelling during DBA/LOCA accidents with FRAPTRAN: The TUmech one-dimensional model

The analysis of the cladding mechanical performance is essential for predicting rod failure during the heat-up phase of a loss-of-coolant accident (LOCA). Previous modelling and simulation exercises have shown that there is room for improvement in the accuracy of fuel performance simulations under such conditions. In this work, TUmech, a simplified standalone version of the mechanical model in TRANSURANUS, has been developed and coupled with FRAPTRAN-2.0. After highlighting the most significant differences between both approaches, particularly in their mechanical treatment and material property correlations, simulations have been performed for IFA-650.2 (fresh fuel) and IFA-650.10 (high burnup fuel) of the Halden LOCA test series. The results obtained from both FRAPTRAN versions have been compared with experimental data and benchmarked against TRANSURANUS estimates, focusing on rod inner gas pressure, post-test cladding outer diameter, and burst time. In both scenarios, FRAPTRAN-TUmech has proven to enhance the accuracy in predicting these three variables compared to the FRAPTRAN default version. However, further validation is required before drawing definitive conclusions based on the encouraging results.

Evaluation of temperatures and flow areas of the Phebus Test FPT0

The cladding temperatures and axial mass distribution computed by MAAP5 were compared with their measured values in the test bundle of the Phebus Test FPT0. The computed cladding temperatures were in good agreed with the measured values in the pre-transient phase. In the transient heat-up phase, the computed temperatures were overestimated by the Baker-Just correlation in MAAP5, but the computed temperatures could simulate the subsequently measured values. The computed mass distribution in the axial direction was in qualitative agreement with the measured one for post-test fuel damage observations. The calculated flow areas of inner and outer regions in the test bundle were compared with the photographic observations. MAAP5 computed them at the height of 0.2 m where the molten pool formed was in qualitative agreement with the photographic observations. It was found that the remaining steam flow paths might be caused by the gas-liquid two-phase flow counter-current flow limitation.

Xylan fast pyrolysis: An experimental and modelling study of particle changes and volatiles release

Biomass char particles produced by pyrolysis may have different morphologies, which has important implications on burning mode, conversion rate and boiler efficiency. These features are difficult to address due to the complexity of biomass structure and pyrolysis reaction models.The present work reports preliminary results on the morphological changes and volatile release that solid particles of Xylan experience upon fast heating in a Drop Tube Reactor (DTR) and in a Heated Strip Reactor (HSR) in a range of temperature between 1100 and 1573 K under inert atmosphere with heating rate in the order of 103 K/s. Two different Xylan samples were chosen as representative of Hemicellulose in angiosperm biomass: xylooligosaccharide extracted from Beechwood (hardwood biomass) and from Corn Cob (herbaceous biomass).During the heatup phase, Xylan particles do not retain the original shape and morphology, neither in DTR nor in HSR experiments. In the early pyrolysis stage, Xylan particles melt and form viscous droplets. As devolatilization proceeds these droplets in the DTR evolve into highly viscous sponge-like particles with spherical shape, which swell and eventually shrink, ultimately producing highly porous spherical char particles.The experimental results are compared with the predictions obtained by the Bio-CPD Xylan pyrolysis model. The model is fairly able to calculate the pyrolysis yields in the DTR, but predicts unexpectedly low extent of metaplast formation. To explain the remarkable melting phenomena and morphological changes observed in the experiments, tuning of some CPD parameters and inclusion of mass transfer and pressure build-up within the particles might be necessary in future work.

BISON validation to in situ cladding burst test and high-burnup LOCA experiments

The process for developing and qualifying nuclear fuels for commercial nuclear application requires fundamental material development, characterization, and design; out-of-pile testing on unirradiated materials; integral fuel rod irradiations, testing, and postirradiation examinations; and transient analyses. The historical approach depends on the generation of large empirical datasets and series of integral fuel rod irradiations, and this approach ultimately takes ∼20 years—or sometimes longer—to acquire data through extensive sequential testing. Thus, the qualification and eventual deployment of new fuel systems constitute a long process. However, recent technological advancements have provided researchers the opportunity to perform out-of-cell, in situ measurements to assess material performance for the duration of the experiment. One such example of this capability is the use of digital image coordination and thermal imaging to assess Zircaloy cladding performance under a simulated loss-of-coolant accident (LOCA) transient condition. In situ measurements generally provide high-fidelity strain, strain rates, and temperature surface maps. This is critical for the US nuclear industry, which is actively developing a technical basis to support extending the peak rod average burnup from 62 to ∼75 GWd/tU and the deployment of accident-tolerant fuel. However, the US Nuclear Regulatory Commission (NRC) outlined in its research information letter several technical issues that the industry must address before extending burnup. One topic of specific interest is understanding the cladding balloon and rupture geometry during the LOCA heat-up phase. By leveraging these advanced in situ capabilities, this work used in situ data generated from a simulated LOCA to better understand high-temperature creep and its effect on Zircaloy balloon and rupture performance. This work used the BISON fuel performance code to assess the high-temperature creep model predictions with in situ data.

Reaction kinetics of direct reduction of mineral iron carbonate with hydrogen: Determination of the kinetic triplet

Direct reduction of mineral iron carbonate with hydrogen is a CO2-lean technology for the production of elemental iron from iron carbonate ore. In this study, the reaction mechanism and reaction kinetics were investigated by thermogravimetric analysis and in a fixed-bed tubular reactor. The degree of metallization increases with increasing temperature from 773 to 1023 K. At 1023 K, the degree of metallization is 93 wt%, with the remaining iron species being wüstite. The reduction proceeds via two reaction pathways: calcination of iron carbonate to wüstite with consecutive reduction of wüstite to elemental iron and direct reduction of iron carbonate to elemental iron. The reaction steps occur simultaneously. During the first hour of reaction, which corresponds to the heat-up phase, calcination of iron carbonate to wüstite adopts the dominant reaction path. Then wüstite reduction and direct iron carbonate reduction with hydrogen to elemental iron, become dominant, facilitated by the increasing porosity of the ore due to the release of CO2. Towards the end of the reduction process the remaining wüstite is reduced to elemental iron. The kinetic triplet – solid phase reaction kinetic model, activation energy and frequency factor – was determined for each reaction step. The reaction kinetics can be described by a combination of an Avrami-Erofeyev model (A3 model for iron carbonate calcination to wüstite), and reaction-order models (F2 model for the reduction of iron carbonate to elemental iron and F3 model for the reduction of wüstite to elemental iron).

Uncertainty Quantification of the PHEBUS FPT-1 Test Modelling Results

One of the biggest experimental programs helping to improve nuclear installation safety is PHEBUS, an international cooperative research program that provides data for validating computer codes dedicated to the analysis of severe accidents and their calculation results. In the European Union, HORIZON 2020 EURATOM project MUSA (Management and Uncertainties of Severe Accidents) is in progress. Modelling of the PHEBUS FPT-1 test within the frame of this project is provided by project partners using different severe accident codes. Uncertainty quantification of the received results is provided using different methods and statistical tools. The Lithuanian Energy Institute (LEI) is taking part in the MUSA project and this uncertainty exercise. LEI is using the RELAP/SCDAPSIM severe accident code together with GRS methodology and SUSA statistical tool to evaluate the uncertainties of modelling results of PHEBUS FPT-1 test. In this article, uncertainty quantification of the PHEBUS FPT-1 test modelling results provided by LEI in the frame of MUSA project are presented. Provided uncertainty analysis for total hydrogen generation showed that upper and lower uncertainty limits are bounding the experimental data. At the end of the calculation, the upper and lower uncertainty limits of calculations are within the band of experimental uncertainties. Uncertainty analysis for Cs/I release fraction showed that uncertainty limits are bounding experimental data until the middle of the heat-up phase, but at the end of the experiment, the calculated upper uncertainty limit is lower than the experimental data. Results of sensitivity analysis showed that the thermal conductivity of the ZrO2 layer of the shroud has the dominant influence on hydrogen generation and Cs/I release fraction calculation results. Other important parameters are changing at the different experimental phases. In the future, it is planned to update the RELAP/SCDAPSIM model for the PHEBUS FPT-1 test using the results of the provided sensitivity analysis. This will allow a better agreement with experimental data.

Experimental analysis of the start-up of a natural circulation loop in single and two-phase flow

With reference to passive heat removal systems for advanced nuclear reactors, an experimental campaign has been carried out to study the start-up of a natural circulation loop in single and two-phase flow. The effect of the heat source power and initial mass inventory have been investigated. The facility consists of a loop 4.828 m high and 10.275 m long, that includes an electrically heated bayonet heat exchanger and a pool heat sink, and can operate at a maximum pressure of 20 bar. Both single- and two-phase flow (air and water) have been tested and typical oscillations have been observed. Characteristic transient phases have been defined by analyzing the time histories of pressures, temperatures and flow rate in the loop and in the water pool. During the transients, the bayonet heat-up phase is followed by a phase of natural circulation activation, then a natural circulation development phase and, in some tests, a nearly steady-state phase. The duration of the phases has been studied with different mass inventories and heat source power. It has been observed that the natural circulation is established more quickly at higher power; the final pressure increases with the power and decreases with the mass inventory. The natural circulation flow rate at the end of the transients depends on the heat source power, with a small influence of the mass inventory value. At high water filling of the loop, single-phase natural circulation is developed, a fast pressure transient occurs and the pressure in the loop has a diverging behavior. At lower water fillings, two-phase two-component flow (air, water and steam) occurs and the system pressure tends to reach a near-steady-state value.

A Two-Parameter Corresponding States Method for Calculating the Steady-State Evaporation Rate of C2\u2013C9 n-Alkane Droplets in Air for Elevated Pressures and Temperatures

Advanced gas turbine and internal combustion engine combustion chambers operate at highly elevated pressures and temperatures. Therefore, spray vaporization analysis cannot be limited to the atmospheric environment since evaporation strongly depends on ambient conditions. Presently, the effect of air pressure and temperature on droplet evaporation rate was investigated by using both a transient and a steady-state approach. A corresponding states model was derived for the steady-state evaporation rate for n-alkanes in the range of C2–C9 with an excellent fit quality and < 1% model uncertainty, considering the thermophysical data uncertainties. The model was tested for C1, C10, and C12 n-alkanes as well with low success. The ambient conditions were evaluated in terms of reduced pressures and temperatures, covering the range of 0.02–0.5 and 1.2–1.5, respectively. However, the applicability of the model was limited to reduced temperature of 1.3–1.5, as higher discrepancy was observed between the trends of the different n-alkanes at lower temperatures. Since the heat-up phase of practical sprays in combustion chambers is often short, the present model might significantly reduce the computational effort required for liquid evaporation calculations.

Hydrothermal Carbonization Kinetics of Lignocellulosic Agro-Wastes: Experimental Data and Modeling

Olive trimmings (OT) were used as feedstock for an in-depth experimental study on the reaction kinetics controlling hydrothermal carbonization (HTC). OT were hydrothermally carbonized for a residence time τ of up to 8 h at temperatures between 180 and 250 °C to systematically investigate the chemical and energy properties changes of hydrochars during HTC. Additional experiments at 120 and 150 °C at τ = 0 h were carried out to analyze the heat-up transient phase required to reach the HTC set-point temperature. Furthermore, an original HTC reaction kinetics model was developed. The HTC reaction pathway was described through a lumped model, in which biomass is converted into solid (distinguished between primary and secondary char), liquid, and gaseous products. The kinetics model, written in MATLABTM, was used in best fitting routines with HTC experimental data obtained using OT and two other agro-wastes previously tested: grape marc and Opuntia Ficus Indica. The HTC kinetics model effectively predicts carbon distribution among HTC products versus time with the thermal transient phase included; it represents an effective tool for R&amp;D in the HTC field. Importantly, both modeling and experimental data suggest that already during the heat-up phase, biomass greatly carbonizes, in particular at the highest temperature tested of 250 °C.

Monitoring of a dielectric barrier discharge-based process using the gas gap voltage

A method to monitor a temperature-sensitive process based on a sine-driven dielectric barrier discharge (DBD) has been developed and investigated. It uses the gas gap voltage in the active phases during the self-pulsing operation to obtain a value representing the gas temperature in the process chamber. The influence of pressure to the gas gap voltage is taken into account. By comparing the results to measurements by a thermistor and numerical simulations, it was found that the method in the considered application can deliver reasonable results that are robust against variations in the gas flow rates. Therefore, the generated data are potentially suitable as an input parameter to control processes such as the production of specific reactive species in flow-driven DBD-based devices. In particular, the method allows controlling a heat-up phase without gas flow prior to the actual usage of the device.

Development and Benchmarking of Mechanistic Channel Deformation Models in RELAP/SCDAPSIM/MOD3.6 for CANDU Severe Accident Analysis

In different stages of postulated severe accidents in CANDU reactors, the fuel channels may experience a series of thermomechanical deformations, some of which may have significant impacts on accident progression; however, they have not been mechanistically modeled by integrated severe accident codes such as MAAP-CANDU and SCDAP/RELAP5. This paper focuses on the development and benchmarking of mechanistic models for pressure tube (PT) ballooning and sagging phenomena during the fuel channel heatup phase as well as for the sagging of fuel channel assemblies during the core disassembly phase. These models, which are based on existing phenomena in literature, are coupled with RELAP5 and/or integrated into RELAP/SCDAPSIM/MOD3.6 as new SCDAP subroutines to provide more robust treatment of the deformation phases of severe accidents.The ballooning of a PT will lead to contact with its calandria tube (CT) and occurs during conditions where the coolant pressure is moderately high. At initial contact the high contact thermal conductance and the large temperature difference between the two tubes result in a large transient heat flux that challenges the channel integrity through potential film boiling on the outer calandria surface if moderator subcooling is low. A one-dimensional ballooning and contact model (BALLON) has been developed. BALLON calculates the ballooning-driven transverse strain of PT and CT and modifies the effective conductivity of the annulus before and after contact.Pressure tube sagging is the dominant deformation mechanism at low pressures and occurs at relatively high PT temperatures. A model based on simple beam theory (SAGPT) has been developed. SAGPT calculates the longitudinal strain and the deflection of PT, and it also determines PT-to-CT sagging contact. The sagging and disassembly of the entire fuel channel assembly occur when the fuel channels are uncovered and the moderator heat sink is lost; thus, the entire PT-CT assembly sags together, possibly contacting channels at lower elevations. A model named SAGCH is created to track fuel channel assembly sagging after moderator boil off and also determines the extent of channel-to-channel contact, channel disassembly, suspended debris bed characteristics, and eventual core collapse.This paper presents detailed descriptions of the models, the coupling schemes, and their benchmark against experiments, together with an extensive review of relevant studies in the literature.

Inactivation of Zika Virus By Solvent/Detergent Treatment or Pasteurization

▪ BACKGROUNDSince 2015 Zika virus (ZIKV) has spread within the Americas and even before in other regions. In 2016, the World Health Organisation (WHO) declared the ZIKV as a "Public Health Emergency of International Concern" (PHEIC). Therefore, it is very important to confirm the pathogen safety of blood plasma products. These products generally demand an effective donor selection as well as robust pathogen elimination/reduction steps within the manufacturing processes. Here, we provide experimental evidence that both solvent/detergent (S/D) treatment and pasteurization effectively inactivate ZIKV. STUDY DESIGN AND METHODSThe ZIKV inactivation capacity of the pasteurization step and the S/D treatment for different products and process conditions were investigated. Therefore, in-process material from commercial batches was collected and spiked with ZIKV in a ratio of approximately 1:10. Viral loads were determined prior to the safety steps and at pre-defined intervals during the individual processes to investigate the inactivation kinetics. Each process was investigated in duplicate runs. RESULTSSixty minutes after S/D treatment the viral titer was below detection limit during manufacturing of human plasma resulting in mean ZIKV reduction factor (LRF) of ≥ 6.78 log10. For immunoglobulin (IgG) after 240 min of S/D treatment the LRF was determined to be ≥ 7.00 log10. ZIKV was inactivated ≥ 6.18 log10 after 480 min by S/D treatment during the manufacture of factor VIII.During pasteurization of human albumin ZIKV infectivity was already below the detection limit after the heat-up phase. This finding was confirmed by subsequent test samples up to 2 hours. A LRF of ≥ 7.48 log10 was demonstrated. CONCLUSIONWe showed that different S/D treatment procedures inactivate the ZIKV to below the detection limit. This effect was seen independently of various product matrices or the choice of S/D reagent at various concentrations and temperatures, while taking product-specific treatment times into consideration. We also showed that pasteurization is a very efficient inactivation step for the ZIKV. These results demonstrate the effectiveness of S/D treatment against even newly emerging lipid-enveloped viruses like ZIKV in the manufacturing of human plasma and derivatives thereof, such as IgG or factor VIII. DisclosuresSchmidt:Octapharma: Employment. Kuehnel:Octapharma: Employment. Mueller:Octapharma: Employment. Pichotta:Octapharma: Employment. Radomski:Octapharma: Employment. Volk:Octapharma: Employment. Knaub:Octapharma: Employment.

Energy Efficiency Modelling of Residential Air Source Heat Pump Water Heater

The heat pump water heater is one of the most energy efficient technologies for heating water for household use. The present work proposes a simplified model of coefficient of performance and examines its predictive capability. The model is based on polynomial functions where the variables are temperatures and the coefficients are derived from the Australian standard test data, using regression technics. The model enables to estimate the coefficient of performance of the same heat pump water heater under other test standards (i.e. US, Japanese, European and Korean standards). The resulting estimations over a heat-up phase and a full test cycle including a draw off pattern are in close agreement with the measured data. Thus the model allows manufacturers to avoid the need to carry out physical tests for some standards and to reduce product cost. The limitations of the methodology proposed are also discussed.

The impact of CO and C3H6 pulses on PtOx reduction and NO oxidation in a diesel oxidation catalyst

A reversible deactivation of a diesel oxidation catalyst (DOC) due to the platinum oxide (PtOx) formation is observed under lean conditions. Among others, the gradual build-up of PtOx leads to a decreasing activity for NO oxidation. It is known that PtOx formation is caused by O2 and NO2 present in the gas mixture and that platinum oxide can be reduced by NO at lower temperatures or thermally decomposed at high temperatures. In this paper we present experimental and modeling results showing that PtOx can be reduced also by CO and C3H6 pulses.First, temperature ramps from 120 to 420°C and back to 120°C were performed with a constant inlet gas composition starting with a reduced catalyst. A higher NO oxidation activity was observed during the heat-up phase than during the subsequent cool-down phase, indicating formation of PtOx. The catalytic activity during the cool-down ramp was partially restored by pulses with an increased CO and C3H6 concentration (while still keeping lean conditions). The maximum effect of pulses was observed around the light-off temperature for CO and C3H6 oxidation.A global kinetic model was developed, considering PtOx existence and its reduction by CO and C3H6 pulses. The model was further validated by isothermal experiments, showing gradual decrease of NO oxidation activity and its recovery by CO and C3H6 at several different temperatures.

Characterization of thermomechanical damage on tungsten surfaces during long-duration plasma transients

A new experimental facility constructed at UCLA for the simulation of high heat flux effects on plasma-facing materials is described. The High Energy Flux Test Facility (HEFTY) is equipped with a Praxair model SG-100 plasma gun, which is nominally rated at 80kW of continuous operation, of which approximately 30kW reaches the target due to thermal losses. The gun is used to impart high intermittent heat flux to metal samples mounted within a cylindrical chamber. The system is capable of delivering an instantaneous heat flux in the range of 30–300MW/m2, depending on sample proximity to the gun. The duration of the plasma heat flux is in the range of 1–1000s, making it ideal for studies of mild plasma transients of relatively long duration. Tungsten and tungsten-copper alloy metal samples are tested in these transient heat flux conditions, and the surface is characterized for damage evaluation using optical, SEM, XRD, and micro-fabrication techniques. Results from a Finite Element (FE) thermo-elastoplasticity model indicate that during the heat-up phase of a plasma transient pulse, the majority of the sample surface is under compressive stresses leading to plastic deformation of the surface. Upon sample cooling, the recovered elastic strain of cooler parts of the sample exceeds that from parts that deformed plastically, resulting in a tensile surface self-stress (residual surface stress). The intensity of the residual tensile surface stress is experimentally correlated with the onset of complex surface fracture morphology on the tungsten surface, and extending below the surface region. Micro-compression mechanical tests of W micro-pillars show that the material has significant plasticity, failing by a “barreling” mode before plasma exposure, and by normal dislocation slip and localized shear after plasma exposure. Ongoing modeling of the complex thermo-fracture process, coupled with elasto-plasticity is based on a phase field approach for distributed fracture, and a discrete cracking approach, with cracks represented by Volterra dislocations.

Post-test calculation of the QUENCH-17 bundle experiment with debris formation and bottom water reflood using thermal hydraulic and severe fuel damage code SOCRAT/V3

The thermal hydraulic and SFD (Severe Fuel Damage) best estimate computer modeling code SOCRAT/V3 was used for the post-test analysis of the QUENCH-17 experiment performed at KIT on January 2013. The objective of this test was to examine the formation of a debris bed inside the completely oxidized region of the bundle without melt formation and to investigate the coolability behavior during the reflood. The test bundle for QUENCH-17 test was intentionally changed in comparison to basic QUENCH bundles (usually 21 heated rod simulators) with the emphasis to investigate debris behavior phenomena. Only 12 periphery fuel rod simulators were heated by centerline tungsten heaters. 9 unheated fuel rod simulators were located in the inner part of the test bundle. This is why the massive porous debris formation in the inner part of the bundle was not influenced by the presence of tungsten heaters.The QUENCH-17 test conditions simulated a hypothetical scenario of nuclear power plant severe accident sequence with debris bed formation in which the overheated up to 1800K core would be flooded from the bottom by ECCS (Emergency Core Cooling System). The QUENCH-17 test included the following phases: (1) heat-up phase (heat-up rate up to 0.25K/s); (2) oxidation phase (the cladding temperature about 1800K in hottest region, steam mass flow rate 2g/s); (3) bottom flood phase (characteristic cooling time about 600s, water mass flow rate 10g/s).SOCRAT/V3 computer modeling code was used for calculation of basic thermal hydraulic, oxidation and thermal mechanical behavior during all phases of the experiment. The calculated results are in a good agreement with experimental data which justifies the adequacy of modeling capabilities of SOCRAT code system.

Hydrothermal catalytic gasification of fermentation residues from a biogas plant

Biogas plants, increasing in number, produce a stream of fermentation residue with high organic content, providing an energy source which is by now mostly unused. We tested this biomass as a potential feedstock for catalytic gasification in supercritical water (T≥374°C, p≥22MPa) for methane production using a batch reactor system. The coke formation tendency during the heat-up phase was evaluated as well as the cleavage of biomass-bound sulfur with respect to its removal from the process as a salt. We found that sulfur is not sufficiently released from the biomass during heating up to a temperature of 410°C. Addition of alkali salts improved the liquefaction of fermentation residues with a low content of minerals, probably by buffering the pH. We found a deactivation of the carbon-supported ruthenium catalyst at low catalyst-to-biomass loadings, which we attribute to sulfur poisoning and fouling in accordance with the composition of the fermentation residue. A temperature of 400°C was found to maximize the methane yield. A residence time dependent biomass to catalyst ratio of 0.45gg−1h−1 was found to result in nearly full conversion with the Ru/C catalyst. A Ru/ZrO2 catalyst, tested under similar conditions, was less active.

Early phase fuel degradation in Phébus FP: Initiating phenomena of degradation in fuel bundle tests

The international Phébus Fission Product Programme investigated key phenomena occurring in light water reactor core meltdown accidents in a series of five in-pile experiments. Four of these tests focused on the degradation of fuel rod bundles, containing a central control rod, and on the resulting release of fission products, structural materials and actinides from the fuel rod bundle, their transport in the reactor coolant system (RCS) and their subsequent behaviour in the containment vessel. Various steam contents were used in the RCS, from highly oxidising conditions (in FPT0 and FPT1) to more reducing ones (in FPT2 and FPT3).The “early degradation phase” took place at the beginning of the Phébus driver core and fuel bundle heat-up phase, with a quasi-intact fuel bundle geometry. During this phase, the degradation of the control rod and the oxidation runaway due to the fast oxidation of the Zircaloy claddings of the fuel rods, were two major events which took place.The oxidation runaway locally increased the temperatures much above the temperatures resulting from the Phébus driver core heat transfer to the bundle and yielded a large hydrogen release, which amounted to 70–80% of the whole hydrogen production during the tests. The maximum hydrogen flow rates increased with increasing steam flow rates injected at the fuel bundle inlet.The failure mechanisms of silver–indium–cadmium (used in three tests) and boron carbide (used in one test) control rods involve eutectic interactions amongst the components of these control rods. Mechanical deformations of the control rod stainless steel cladding against the Zircaloy control rod guide tube are the main presumed mechanisms for the beginning of these eutectic formations. However, different post-failure scenarios can be postulated for the effect of control rod degradation on fuel bundle degradation for both types of control rods. The exothermic oxidation of the exposed boron carbide pellets led to the release of carbonaceous species (CO, CO2) as well as of additional hydrogen, but no significant methane release could be detected above the limits of detection.Overall, the results confirmed existing knowledge concerning early phase degradation phenomenology found in previous integral experiments such as CORA and QUENCH (Karlsruhe Institute of Technology) and Phébus SFD (IRSN Cadarache), and formed a sound basis for analysis of the late phase degradation subsequently observed. Quantitative analysis of boron carbide control rod degradation in FPT3 pointed to a need for improved modelling of chemical reactions involving this material, particularly its oxidation in steam; this has been studied in the BECARRE experiments conducted by IRSN in the International Source Term Programme, leading to better quantitative understanding and improved modelling in codes such as the European reference severe accident analysis code ASTEC.

The transient response of wicked heat pipes with non-condensable gas

Abstract Experiments were performed to evaluate the effect of non-condensable gases and axial conduction on the transient performance of copper-water wicked heat pipes. An existing transient network model for wicked heat pipes was extended to incorporate the effects of axial conduction and non-condensable gas. The different components were modeled by a larger number of smaller elements in both axial and radial directions. The model predictions of the steady and transient response of the vapour and wall temperature of the heat pipes were in good agreement with the experimental results. The non-condensable gases and axial conduction did not significantly affect the transient response during the heat-up phase; however, it significantly slows down the cool-down phase.

Some insights into the role of axial gas flow in fuel rod behaviour during the LOCA based on Halden tests and calculations with the FALCON-PSI code

A model for axial gas flow in a fuel rod during the LOCA is integrated into the FRELAX model that deals with the thermal behaviour and fuel relocation in the fuel rods of the Halden LOCA test series. The first verification was carried out using the experimental data for the inner pressure during the gas outflow after cladding rupture in tests 3, 4 and 5. Furthermore, the modified FRELAX model is implicitly coupled to the FALCON fuel behaviour code. The analysis with the new methodology shows that the dynamics of axial gas-flow along the rod and through the cladding rupture can have a strong influence on the fuel rod behaviour. Specifically, a delayed axial gas redistribution during the heat-up phase of the LOCA can result in a drop of local pressure in the ballooned area, which is eventually able to affect the cladding burst. The results of the new model seem to be useful when analysing some of the Halden LOCA tests (showing considerable fuel relocation) and selected cases of LOCA in full-length fuel rods. While the short rods used in the Halden tests only show a very small effect of the delayed gas redistribution during the clad ballooning, such an effect is predicted to be significant in the full-scale rods – with a power peak located sufficiently away from the plenum – resulting in a considerable delay of the predicted moment of cladding rupture.