Stefan Holmström

European Experiments on 2-D Molten Core Concrete Interaction: HECLA and VULCANO

This paper presents results from two ongoing European experimental programs on molten core concrete interactions: HECLA at VTT and VULCANO at the Commissariat à l’Énergie Atomique. In the HECLA experiments, metallic melt is poured into a cylindrical concrete crucible. The focus is on the initial, pouring phase of the interaction. Therefore, decay heat simulation is not required. The HECLA-2 experiment involved 50 kg of stainless steel at 1700°C and siliceous concrete. The final ablation depths were 25–30 mm in the basemat and ~15 mm in the side wall. The VULCANO VB experiments have been devoted to the study of the interaction of 28 to 45 kg of oxidic corium with silica-rich or limestone-rich concretes. These tests focus on long-term ablation and require the use of induction heating to simulate the decay heat fluxes. Anisotropic ablation between the horizontal and vertical direction has been observed with silica-rich concrete, confirming the CCI tests. A new series of experiments VULCANO VBS has been launched in which there are both oxide and metallic phases in the melt. In these tests, magnetic screening is used so that the induction power is provided almost only to the upper oxidic layer after stratification. Note: Some figures in this paper are in color only in the electronic version.

Creep-Fatigue Interaction Models for Grade 91 Steel

Different approaches for modelling creep–fatigue (CF) interaction are used on strain controlled creep fatigue data of 9Cr–1Mo-VNb (P91) steel and assessed with the target of finding suitable candidates for use in design rules. The assessed models include time, ductility, and strain energy-based creep-fatigue interaction methods and two simplified models. For the interaction diagram-based models, the challenge of acquiring representative creep damage fractions from the dynamic material response, i.e., cyclic softening with changing relaxation behaviour is addressed. In addition, the interaction diagram approaches are discussed in the light of known (fatigue) material scatter and defining representative cycles for CF data. The performance of the model are presented and also compared against the RCC–MR design code methodology. It is shown that the fitting accuracy of the complex interaction models vary significantly and that modified ductility based models seem to be less susceptible to changes in supporting creep and relaxation models. Successful and also superior prediction of the CF number of cycles to failure for Grade 91 steel was accomplished by simplified methods with much less fitting parameters. The practicality in using interaction diagram methods for design purposes, where simplicity is a key issue, is questioned.

Effect of short-term data on predicted creep rupture life – pivoting effect and optimized censoring

Abstract Fitting data to classical creep rupture models can result in unrealistically high extrapolated long-term strength. As a consequence, the standard strength values for new steel grades have frequently needed downward correction after obtaining more long-term test data. The reasons for non-conservative extrapolation include the influence of short-term data, which are easiest to produce but tend to pivot upwards the extrapolated values of creep rupture strength. Improvement in extrapolation could be expected by reducing this effect through model rigidity correction and censoring of very short-term data, but it may not be immediately clear how to justify the correction of particular models or censoring. Analogously to the instability parameter in the minimum commitment model for creep rupture, a rigidity parameter correction (RPC) is introduced to assess the pivoting effect of creep rupture models for the purpose of reducing potential to non-conservativeness in extrapolation. The RPC approach can be used with any creep rupture model for comparing the model rigidity and the potential benefit from censoring short-term data. The correction itself will never introduce non-conservatism, regardless of the model. The RPC approach is demonstrated by analyzing an ECCC data set for cross-welded 9%Cr steel (E911).

Modeling and Verification of Creep Strain and Exhaustion in a Welded Steam Mixer

Structures operating in the creep regime will consume their creep life at a greater rate in locations where the stress state is aggravated by triaxiality constraints. Many structures, such as the welded steam mixer studied here, also have multiple material zones differing in microstructure and material properties. The 3-dimensional structure as such in addition to interacting material zones is a great challenge for finite element analysis (FEA), even to accurately pinpoint the critical locations where damage will be found. The studied steam mixer, made of 10CrMo 9-10 steel (P22), has after 100 000 hours of service developed severe creep damage in the several saddle point positions adjacent to nozzle welds. FE-simulation of long term behaviour of this structure has been performed taking developing triaxiality constraints, material zones and primary to tertiary creep regimes into account. The creep strain rate formulation is based on the logistic creep strain prediction (LCSP) model implemented to ABAQUS, including primary, secondary and tertiary creep. The results are presented using a filtering technique utilising the formulation of rigid plastic deformation for describing and quantifying the developing “creep exhaustion”. INTRODUCTION The structural integrity of high temperature welded structures has been widely studied and published, often however as rather simple girth weld cases [1]-[10]. In these the development of stress (von Mises or maximum principal stress) and strains (axial or hoop) are followed and conclusions regarding critical locations are usually drawn from the locations of maximum stresses or strains. Furthermore simulations are often based on steady state creep strain rates and the impact of triaxiality constraints and multiaxial creep ductility are seldom taken into account. As a consequence these studies often fail to pinpoint the locations where service exposed components actually would develop creep damage. In the design stage this could become a problem emerging later in the life of the component, as it did with the mixer studied here at about half the desired (design) life. The influence of creep ductility exhaustion under multiaxial conditions has

Engineering models for softening and relaxation of Gr. 91 steel in creep-fatigue conditions

Purpose n n n n nThere are a number of different approaches for calculating creep-fatigue (CF) damage for design, such as the French nuclear code RCC-MRx, the American ASME III NH and the British R5 assessment code. To acquire estimates for the CF damage, that are not overly conservative, both the cyclic material softening/hardening and the potential changes in relaxation behavior have to be considered. The data presented here and models are an initial glimpse of the ongoing European FP7 project MATISSE effort to model the softening and relaxation behavior of Grade 91 steel under CF loading. The resulting models are used for calculating the relaxed stress at arbitrary location in the material cyclic softening curve. The initial test results show that softening of the material is not always detrimental. The initial model development and the pre-assessment of the MATISSE data show that the relaxed stress can be robustly predicted with hold time, strain range and the cyclic life fraction as the main input parameters. The paper aims to discuss these issues. n n n n nDesign/methodology/approach n n n n nEngineering models have been developed for predicting cyclic softening and relaxation for Gr. 91 steel at 550 and 600°C. n n n n nFindings n n n n nA simple engineering model can adequately predict the low cycle fatigue (LCF) and CF softening rates of Gr. 91 steel. Also a simple relaxation model was successfully defined for predicting relaxed stress of both virgin and cyclically softened material. n n n n nResearch limitations/implications n n n n nThe data are not yet complete and the models will be updated when the complete set of data in the MATISSE project is available. n n n n nPractical implications n n n n nThe models described can be used for predicting P91 material softening in an arbitrary location (n/Nf0) of the LCF and CF cyclic life. Also the relaxed stress in the softened material can be estimated. n n n n nOriginality/value n n n n nThe models are simple in nature but are able to estimate both material softening and relaxation in arbitrary location of the softening curve. This is the first time the Wilshire methodology has been applied on cyclic relaxation data.

A Comparison of Creep-Fatigue Assessment and Modelling Methods

Design codes, such as RCC-MRx and ASME III NH, for generation IV nuclear reactors use interaction diagram based method for creep-fatigue assessment. In the interaction diagram the fatigue damage is expressed as the ratio of design cycles over the allowable amount of cycles in service and the creep damage as the ratio of time in service over the design life. With this approach it is assumed that these quantities can be added linearly to represent the combined creep-fatigue damage accumulation. Failure is assumed to occur when the sum of the damage reaches a specified value, usually unity or less. The fatigue damage fraction should naturally be unity when no creep damage is present and creep damage should be unity when no fatigue damage is present. However, strict fatigue limits and safety factors used for creep rupture strengths as well as different approaches to relaxation calculation can cause a situation where creep-fatigue test data plotted according to the design rules are three orders of magnitude away from the interaction diagram unity line. Thus, utilizing the interaction diagram methods for predicting the number of creep-fatigue cycles may be inaccurate and from design point of view these methods may be overly conservative. In this paper the results of creep-fatigue tests carried out for austenitic stainless steel 316 and heat resistant ferritic-martensitic steel P91, which are included in the design codes, such as RCC-MRx, are assessed using the interaction diagram method with different levels of criteria for the creep and fatigue fractions. The test results are also compared against the predictions of a recently developed simplified creep-fatigue model which predicts the creep-fatigue damage as a function of strain range, temperature and hold period duration with little amount of fitting parameters. The Φ-model utilizes the creep rupture strength and ultimate tensile strength (UTS) of the material in question as base for the creep-fatigue prediction. Furthermore, challenge of acquiring representative creep damage fractions from the dynamic material response, i.e. cyclic softening with P91 steel, for the interaction diagram based assessment is discussed.Copyright

Observations on the Smith-Glasser index for self-heating of bituminous coal

Self-heating of coal depends partly on intrinsic, coal-related factors. This article aims to discuss the Smith–Glasser index as a potential indicator of the self-heating propensity of a large set of bituminous coals with similar heating value but different origins. For this purpose, recorded properties and experience were reviewed for 7.5 million tonnes of coal delivered to the same power plant operator. The results suggest that the propensity to self-heat can be conveniently indicated by Smith–Glasser index that only involves routinely measured moisture and volatile contents of coal. The results appear consistent with the observed incidences of self-heating in storage.

Experimental Study and Analytical Methods for Particle Bed Dryout With Heterogeneous Particles and Pressure Variation

The interest to study the dryout heat flux in particle beds is related to interest of quantify the debris coolability margins during a hypothetical severe reactor accident. When the molten core has relocated to the containment floor, one accident management concept is based on the cooling of the corium by the water injection on top. Earlier experimental and analytical work has concentrated on homogeneous particle beds at atmospheric pressures. For plant safety assessment in Finland, there is a need to consider heterogeneous particle mixtures, layered particle bed setups and varied pressures. A facility has been constructed at VTT to measure dryout heat flux in a heterogeneous particle bed. The bed dimensions are 0.3 m in diameter and 0.6 m in height, with a mixture of 0.1 to 10 mm particles. The facility has a pressure range from atmospheric to 6 bar (overpressure). The bed is heated by spirals of a resistance band. The preliminary experiments have been carried out, but a more systematic set of data is expected to be available in the spring 2002. To support the experiments analytical models have been developed for qualification of the experimental results. The first comparison is done against various critical heat flux correlations developed in 1980’ies and 1990’ies for homogeneous bed conditions. The second comparison is done against 1-D and 0-D models developed by Lipinski. The most detailed analysis of the transient process conditions and dryout predictions are done by using the two-dimensional, drift-flux based thermohydraulic solution for the particle bed immersed into the water. The code is called PILEXP. Already the first validation results against the preliminary tests indicate that the transient process conditions and the mechanisms related to the dryout can be best explained and understood by using a multidimensional, transient code, where all details of the process control can be modeled as well. The heterogeneous bed and stratified bed can not be well considered by single critical heat flux correlations.Copyright

Predicted life of P91 steel for cyclic high temperature service

Abstract The P91 steel is widely used in high temperature components of power plants, and it is a candidate material for Gen-IV reactors. The P91 steel has relatively attractive mechanical and physical properties combined with resistance to stress corrosion cracking in water–steam environments. This study aimed to explore the combined cyclic, creep and relaxation behaviour of P91 material. Uniaxial specimens were subjected to cyclic loadings with periodic forward creep or relaxation at peak stress. The results indicate that prior creep or intermediate relaxation periods up to 72 h will influence the subsequent cyclic softening of P91, but do not significantly reduce the cyclic life. In contrast, prior cycling has a detrimental effect on the subsequent creep life. A simplified creep-fatigue model is shown to predict life better than usual code-based approaches. Improved verification of all models would benefit from the availability of more extensive long-term data on P91 steel.

Early warning indicators for challenges in underground coal storage

Early warning or leading indicators are discussed for unexpected incidences in case of large-scale underground coal storage at a power plant. The experience is compared with above-ground stockpiles for which established procedures are available but where access for prevention and mitigation are much easier. It is suggested that while the explicit organization, procedures, and the general safety systems aim to provide the targeted levels of performance for the storage, representing new technology without much precedence elsewhere in the world, the extensive experience and tacit knowledge from above-ground open and closed storage systems can help to prepare for and to prevent unwanted incidents in the underground storage. This kind of experience has been also found useful for developing the leading or early warning indicators for underground storage. Examples are given on observed autoignition and freezing of coal in the storage silos, and on occupational hazards. Selection of the leading indicators needs to consider the specific features of the unique underground facility.