Heat-up Rate Research Articles

Degraded core reflood: Present understanding and impact on LWRs

The severe fuel damage (SFD) research at Forschungszentrum Karlsruhe started with the single rod experiments [Hagen, S., Malauschek, H., Peck, S.O., Wallenfels, K.P., 1983. Temperature Escalation in PWR Fuel Rod Simulator Bundles due to the Zircaloy/steam Reaction Test ESBU-1. Test Results Report, FZK-3508] already before the TMI-2 accident revealed that the knowledge on accident initiation and progression was scarce. The general focus on primary circuit research was the understanding of the relevant processes and their interactions, and of possibilities for countermeasures. However, even today, the most prominent countermeasure, the flooding of a degraded core, is not yet completely understood, namely the influence of adverse effects such as enhanced core degradation and hydrogen spikes under the most probable scenarios. Our work shows that the design basis accident procedures of core reflood can be extended to peak core temperatures of approx. 2200 K, if sufficient reflood mass flow rate can be supplied. Assuming an initial core heat-up rate of 0.5 K/s, this gives only approx. 12 min additional time with respect to design basis accidents (DBA) cases, but moreover a strategy for a successful core reflood is seen to be feasible before a large in-core pool is formed, that might be uncoolable.

Pressure-tube and calandria-tube deformation following a single-channel blockage event in ACR-700

One postulated accident scenario for the Advanced CANDU Reactor (ACR-700™) is the complete coolant flow blockage of a single pressure-tube (PT). The flow is not monitored within each individual PT, thus during the early stages of this accident the reactor remains at full power and full pressure, resulting in rapid coolant boil-off and fuel overheating. Melting of the Zircaloy (Zry) components of the fuel bundle can occur, with relocation of some molten material to the bottom of the PT, which may cause failure of the PT and/or the calandria-tube (CT). We analyzed several key phenomena occurring after the blockage, including coolant boil-off, cladding heat-up and melting, dripping of molten Zircaloy (Zry) from the fuel pin, thermal interaction between the molten Zry and the PT, localized bulging of the PT, and interaction of the bulged PT with the CT. The main findings of the study are as follows: (1) Most coolant boils off within 3 s of accident initiation. (2) The Zry cladding starts to melt between 7 and 10 s after accident initiation. (3) The very high heat-up rate typical of the flow blockage accident sequence ensures that the molten Zry would drip to the bottom of the PT. (4) After contacting the molten Zry, the PT and CT bulge out radially under the effect of the reactor pressure. (5) PT/CT failure occurs only if the postulated mass of molten Zry in contact with the PT is sufficiently large, i.e., >100 g. The characteristic time scales for this 100-g case are as follows: - PT bulging starts within 3 s of Zry/PT contact; - PT makes contact with the CT in another 3 s; - CT bulging starts in approximately 1 s; - CT failure occurs within another 6 s. Thus, the duration of the PT/CT deformation transient is 13 s, which gives a total duration of the accident (from PT blockage to PT/CT failure) of 20–23 s. The relatively simple models developed in this study and the estimates generated with these models provide a solid physical framework for the key phenomena in the single-channel flow blockage event in ACR-700. As such, they can also assist in the interpretation and verification of future analyses of this event conducted with more sophisticated codes and tools.

INVESTIGATION OF IMPINGEMENT HEAT TRANSFER COEFFICIENT AT HIGH TEMPERATURES

ABSTRACT Very little information exists for the impingement heat transfer coefficient at high temperatures. All available empirical correlations are mainly based on experiments conducted at relatively low impingement temperatures, and thus cannot describe the heat transfer characteristics of the impingement air at high temperatures with sufficient accuracy. A comprehensive study of the impingement heat transfer coefficient at high temperatures is carried out and presented in this paper. The aim of the study is to give a summary of the experimental results of the impingement heat transfer covering a large impingement air temperature range from 100 to 700°C. Heat transfer measurements were carried out on a laboratory-scale test rig. The main parts of the rig were a fan, a gas burner for air heating, a heavily insulated nozzle array with 300 × 500 mm impingement surface, a 40 mm thick and 300 × 500 mm sized aluminium plate for determination of heat transfer, and a data acquisition system. The heat transfer rate was determined from the heat-up rate of the aluminium plate due to the high temperature jet impingement.

Effect of weld thermal cycle and restraint stress on helium bubble formation in stainless steels

Helium bubble structure was examined in helium-implanted stainless steels after applying thermal cycles and tensile stresses using a weld thermal and stress cycle simulator. SUS304 specimens implanted with helium ions to 5 appm were heated at 1473 K for 2 s in an argon gas atmosphere. The heat-up rate and cooling rate were controlled to be 90 and 130 K/s, respectively. Tensile stresses ranging from 0 to 4 MPa were applied immediately after reaching a temperature of 1473 K. TEM observation revealed that bubble formation occurred even after short annealing times and that the size of the helium bubbles was strongly dependent on the tensile stress during heating.

Ground and space processing of single-crystalline organic thin films

Physical vapor transport (PVT) crystal growth experiments are being conducted with nonlinear optical organic materials in semi-closed ampoules which allow a vacuum to penetrate into the growth chamber, removing impurities and decomposition products from the growth interface. One material being grown is N,N-dimethyl-p-(2,2-dicyanovinyl) aniline (DCVA). Two samples of DCVA were flown in separate, seeded PVT chambers on Space Shuttle mission STS-59 in April 1994. The growth parameters were set to reproduce laboratory experiments that yielded small, bulk crystals. Surprisingly, however, the results from the STS experiment were, for both samples, the production of single-crystalline thin films. A second set of 8 PVT cells with DCVA was flown on STS-69 in September 1995. Several parameters were changed to study their influences on the film growth process and, in summary, the film growth process was found to be very robust. Films were obtained in all cases, albeit with varying quality. An evaluation of the flight- and ground-test data revealed a slower heat-up rate than was typical of the laboratory experiments. No background pressure measurements were made, but the temperature profile is an indication of a much higher background nitrogen pressure than the millitorr range used in the laboratory. A 4-year research grant has been obtained to study this film-growth phenomenon. Ground-based experiments are beginning to be carried out involving the varying of source and substrate temperatures, nitrogen background pressure, and the substrate material.

Electrically heated converters for automotive emission control: determination of the best size regime for the heated element

In view of the significant cold-start hydrocarbon emission reduction potential of the electrically heated converter (EHC) technology demonstrated in recent studies, there is considerable interest in better understanding the behavior and design aspects of an EHC during the cold-start portion of actual vehicle emission tests. Computer simulations based on the EHC model developed previously (Oh et al., 1993, Industrial and Engineering Chemistry Research 32, 1560–1567) show that although decreasing the volume of an electric heater generally improves the emission performance of the EHC system, there is a critical heater volume below which no significant emission benefit is obtained. This paper examines the EHC sizing issue in more detail by analyzing a simplified version of the EHC model to investigate why the emission benefit of decreasing electric heater volume eventually disappears and how the critical heater volume where this occurs is related to system parameters. Analysis of the simplified EHC model identifies this critical volume, V h , which is proportional to the exhaust flow rate through the EHC and inversely proportional to the product of the interphase (gas–solid) heat transfer coefficient within the heated element and its geometric surface area per unit volume. It is shown that the recommended heater size regime must be where the physical heater volume, V, and V h are comparable in magnitude. When V≫ V h , the electric heater is clearly too large because the emission performance suffers from the slow heat-up rate caused by the large thermal mass. Also, decreasing the heater volume to values much smaller than V h (i.e., V≪ V h ) is not desirable because it would simply lead to unnecessarily higher (potentially damaging) solid-phase temperatures without producing additional emission reductions. In this smallest size regime, the emission performance of the EHC system becomes insensitive to heater size variation because the higher solid-phase temperatures within the smaller heater tend to be compensated by its poor interphase (solid-to-gas) heat transfer arising from the insufficient heat transfer area available.

Nucleate pool boiling in the presence of an electric field: Effect of subcooling and heat-up rate

The purpose of the paper is to experimentally assess the effect exerted by subcooling degree and heat-up rate (i.e., the rate of increase of thermal flux) on the phenomenon of nucleate boiling in the presence of an electric field. To this aim, an experimental facility was set up to investigate pool boiling on a heated platinum wire of 0.2-mm diameter. The working fluid was R-113 (C 2Cl 3F 3). A cylindrical electric field (up to 21 MV/m at the heater surface) was imposed. The power was increased along a linear ramp, at a rate ranging from 2.5 mW/s to 6 W/s. Subcooling degrees up to 22 K (Ja = 146) were investigated. The results of zero-field measurements are in agreement with previous ones in the literature. The effect exerted by the electric field consists mainly of a strong increase in critical heat flux, whereas, for a given heat flux, the nucleate boiling performance can be either weakly degraded or slightly enhanced. The critical heat flux was found to remain nearly constant with increasing transient velocity up to a given heat-up rate and then to increase with it. The effect of subcooling on nucleate boiling is quite complex but, in any case, quite weak in the entire investigated range.

Interpretation of the results of the CORA-33 dry core boiling water reactor test

All boiling water reactor (BWR) degraded core experiments performed prior to CORA-33 were conducted under ‘wet’ core degradation conditions, in which water remains within the core and continuous steaming feeds metal-steam oxidation reactions on the in-core metallic surfaces. However, one dominant set of accident scenarios would occur with reduced metal oxidation under ‘dry’ core degradation conditions and, prior to CORA-33, this set had been neglected experimentally. The CORA-33 experiment was designed specifically to address this dominant set of BWR ‘dry’ core severe accident scenarios and to resolve partially phenomenological uncertainties concerning the behavior of relocating metallic melts that drain into the lower regions of a ‘dry’ BWR core (the ex-reactor experiments at Sandia National Laboratories will further address these uncertainties). CORA-33 was conducted on 1 October 1992, in the CORA test facility at Karlsruhe. A review of the CORA-33 data indicates that the objectives were achieved; i.e. core degradation occurred at a core heat-up rate (characterized by the absence of any temperature escalation caused by oxidation) and a test section axial temperature profile (at incipient structural melting) that are prototypic of full-core nuclear power plant simulations under ‘dry’ core conditions. Simulations of the CORA-33 test at Oak Ridge National Laboratory (ORNL) have required the modification of existing control blade-canister materials interaction models to include the eutectic melting of the stainless steel-zircaloy interaction products and the heat of mixing of stainless steel and zircaloy. The timing and location of canister failure and melt intrusion into the fuel assembly appear to be adequately simulated by the ORNL models. This paper will present the results of the post-test analyses carried out at ORNL based on the experimental data and the post-test examination of the test bundle at Karlsruhe. The implications of these results with respect to degraded core modelling and the associated safety issues are also discussed.

Electrical resistivity in Bi 2Sr 2CaCu 2O 8± δ compounds synthesized via melt-casting

We performed detailed measurements of electrical resistivity ϱ versus temperature T in superconducting Bi 2Sr 2CaCu 2O 8± δ compounds prepared by melt-casting followed by recrystallization at higher temperatures under v conditions. Differential thermal analysis (DTA) suggests that the recrystallization process starts near 460°C, and that the superconducting phase starts developing near 780°C. The porosity of the annealed materials depends strongly on the annealing temperature, time, and heat-up rate. Typically a sharp drop in ϱ at the onset of superconductivity is followed at lower T by either a shoulder-like feature, a tail-like feature, or both. The ϱ versus T data, SEM, and porosity analysis suggest that weak links with narrow or broad distribution of sustaining critical current densities J c are responsible for the latter behaviors, respectively.

Small scale isothermal age technique to determine exothermic onset temperatures

With the increased focus on safety in the chemical industry, there is a greater demand for reliable information as to exothermic onset temperatures before the scale-up of any process. The Small Scale Isothermal Age (SSIA) test is an isothermal DSC technique in which reusable metal crucibles are used to accurately determine exothermic onset temperatures. A typical solid sample, 50–75 mg, is held isothermally at a preset temperature for an extended period of time, usually 12–16h. The aged sample is then run at a standard heat-up ramp to determine any decrease in the size of the original exotherm relative to the unaged sample. The same DSC cell, metal crucible, heat-up rate and sample weight are used to determine the exotherm size in both the unaged and aged sample. The SSIA technique is applicable to both solids and liquids and has the advantages of: i) small sample size, (ii) handling of compounds in which exothermic activity is accompanied by large pressure increases without damaging experimental equipment, (iii) accurate determination of exothermic onset temperature, (iv) ease of experimental set-up, (v) ease of data interpretation, and (vi) rapid experimental turn-around time. The determination of the exothermic onset temperature using the small scale isothermal age technique will be presented and results compared to data obtained using standard and small dewar techniques.

Thermal simulation of quartz tube infra-red heaters used in the processing of thermoplastic composites

The use of infra-red heaters in the processing of thermoplastic composites is increasing due to the potential for quick process cycle times. Accurate modelling of the heating characteristics will help processors predict heat-up times for their processes and avoid the disadvantages associated with infra-red heating. This paper deals with the modelling of quartz tube infra-red heaters, taking into account both the characteristics of the emitted radiation and the absorption properties of the materials involved. Comparison of the model and experimental results shows that the model successfully predicts the heat-up rate and temperature distribution though different materials when heated using quartz tube infra-red heaters.

沸騰水型原子力発電プラント起動時の制御棒操作自動化方式の開発

This paper describes schemes for automated control rod operation during an approach to critical and heatup/pressurization regions in the BWR startup process. These schemes were developed to reduce the operators' work load. In the approach to critical region, a integral-plus-proportional control scheme, using estimated reactivity, is proposed. This scheme aims to control the reactor period to the specified level and determine the critical state. During the heatup/pressurization region, a non-linear integral-plus-proportional control scheme, where the logarithm of the neutron flux and the heatup rate deviation are fed back, has been developed to prevent heatup rate overshoot and to maintain the heatup rate at the specified value.The performance of these control schemes was evaluated using BWR simulators and compared with performances of a period feedback control and a neutron flux feedback cascade control scheme. In the approach to the critical region, the proposed method was able to hold the reactor period constant and achieved criticality faster than the period feedback control scheme. In the heatup/pressurization region, the proposed scheme also had a faster response than the cascade control scheme and showed a stable response for a disturbance caused by opening the main steam drain valves.

Kinetic studies of rapid oil shale pyrolysis: 2. Rapid pyrolysis of oil shales in a laminar-flow entrained reactor

Rapid pyrolysis of Kentucky New Albany shale was conducted in a laminar-flow entrained reactor (LFER) to obtain a fundamental understanding of thermal reactions, which occur during high-heating-rate retorting processes. The reactor configuration was designed to simplify operation and allow accurate modelling. Temperature characterization and flow visualization in the LFER were conducted to provide the data necessary to proceed with a kinetic study on the rapid pyrolysis of oil shale. The reactor with gas preheaters was constructed to achieve high particle heating rates and to feed oil shale fine particles generated from beneficiation. The rapid pyrolysis of raw and beneficiated oil shales was carried out in nitrogen at temperatures between 700 °C and 850 °C, with gas preheat temperatures up to 980 °C. For each temperature, the sampling probe was set at different positions along the length of the reactor tube to obtain different residence times. A pyrolysis kinetic model of LFER has been developed to calculate the particle heat-up rate and residence time under each set of conditions. Comparisons are presented to evaluate the effects of beneficiation, temperature and heating rate as well as residence time. The effects of particle size and gas environment on heat transfer rates and conversion yields were also studied, and the results were used to validate the heat transfer model and to evaluate the impact of devolatilization behaviour on shale combustion in a circulating fluidized-bed reactor.

Simultaneous determination of aramid fibre and polymeric matrices in advanced composites

The microelectronics industry has undergone significant technological advances in recent years. For example, the increase of circuit densities has required the introduction of the leadless chip carrier (LCC) and a redesign of conventional packaging. The extreme compactness of modern systems also requires weight reduction of the printed wire boards (PWBs). This weight reduction has been accomplished by the use of reinforced polymer/fibre composites which, because of their excellent mechanical properties, also improve the PWBs. The printed wire boards normally are made with epoxy and/ or polyimide polymers and are reinforced with E or S glass fabrics. Because some of the new speciality electronic systems must perform over a wide temperature range, they must be able to expand and contract during service. The major problem associated with mounting hermetic chip carriers (HCC) on PWBs is that the boards crack under thermal stresses because of unmatched coefficients of thermal expansion (CTEs). The HCC packages are usually made of alumina having a CTE value in the x, y directions of approximately 6 to 6.5 x 10-6/1m (/~m°C) ~ over the 5 0 to 130°C temperature range. On the other hand, the conventional polymer/fibre laminates have CTEs in the x, y directions of ,,~12 to 16 x 10-6/zm (/~m°C)1. The thermalexpansion characteristics of the PWB and the ceramic chip carrier can be more closely matched by replacing the glass fabric in the laminates with other materials, such as Kevlar 49, quartz, or ceramic fabrics, or with a combination of any two reinforcements which have lower coefficients of thermal expansion than glass. The ratio of polymer to reinforcement must be closely controlled to produce the required mechanical properties of the laminate and the CTE match. Conventional thermogravimetric analysis (TGA) is the preferred method to determine the amounts present in the polymer. However, when an organic reinforcement or a combination of organic and inorganic fibres are used, determining the laminate's composition is difficult, because under ordinary conditions all organic materials are lost during oxidation in air. More precisely, in the case of organic matrix composites containing aramid fibres, no accurate or accepted method of analysis exists. In this letter, we propose the use of differential thermogravimetric analysis (dTG) as an approach to obtain the desired measurements. However, complementary techniques such as Fourier-transform infrared spectroscopy (FTIR) and pyrolysis gas chromatography/mass spectrometry (GC/MS) must be used to describe fully the measured weight losses. The PWB samples used for this study were prepared by lamination techniques described elsewhere [1]. Several laminates were made, which contained quartz (J. P. Stevens and Co.) and/or Kevlar 49 (du Pont de Nemours and Co.) fabric as well as a fully imidized polyimide (Kerimid 601, Rhone Pomlene) and, in some instances, also an epoxy resin (proprietary formulation) to improve wetting of the Kevlar 49 fabric with the organic matrix. The TG measurements were made with a Mettler 3000 system operating under a stream of air at a heating rate of 20°C min -~. The dTG signals were obtained by data manipulation using a Mettler TC-10 microprocessor. Fig. 1 shows typical dTG curves, as a function of heat-up rate obtained for a laminate containing eight quartz layers, seven Kevlar-49 fabric layers, and cured Kerimid 601. At least three weight losses can be observed. The resolution of

Factors Affecting the Processing of Epoxy Film Adhesives III. Heat-Up Rate

Abstract Dielectric, thermomechanical (TMA), dynamic mechanical analysis (DMA) and mechanical testing were used to study the changes which occurred due to varying heat-up rate during fabrication of two commercially available epoxy film adhesives, 3M Co. AF-163 and Hysol EA-9649. The mechanical strength of bonded joints decreased by approximately twenty-five percent when processed above a critical heat-up rate. The glass transition temperature (Tg) of the cured adhesive bonds was found to change in opposing directions with increasing heat-up rate for the two adhesives. This different response has been interpreted in terms of the chemistry associated with the rubber additive and catalysts used in the adhesive formulations.

Conversion of uranium nuclear fuel into U 3O 8 at the head end of HTR reprocessing

Corresponding to the reference procedure for the head-end treatment of HTR fuel elements, separation of the moderator graphite from the materials uranium and plutonium is envisaged by combustion in the fluidized bed. Due to the defective silicon carbide layers of the uranium fuel particles a chemical conversion of the UO 2 kernel into U 3O 8 takes place in the oxidizing atmosphere of the combustion process. This reaction proceeds spontaneously and quantitatively, and causes a disintegration of the heavy metal kernel. It is observed that the degree of hardness of the kernel fragments is clearly dependent on the heat-up rate. In the commercial design of the head-end process step, attention must be paid to the cross-over of fuel from the stationary fluidized bed into the dust discharge.

Heating Curves During Roasting of Turkeys

Thirty-eight turkeys representing weight groups of 5.4, 6.8, 8.2, 9.5, and 10.9 kg (12, 15, 18, 21, and 24 Ib) were roasted at 163 C (325 F) to determine the effect of initial meat temperature, weight of bird, and presence or absence of stuffing on heating rate and time required for doneness.Approximately 40 to 60 min additional roasting time was required when the initial temperature of the turkey breast was −2 C compared to 3 C. The total time required for roasting completely thawed birds to doneness was 20 to 60 min longer than the times suggested in some of the existing roasting guidelines. The presence of warm stuffing caused a faster rate of heating at the beginning of the roasting process but later slowed heating and required additional time to attain doneness when compared to rates and times of unstuffed birds. Heat-up rate occurred more rapidly in the thigh than in the breast.Heating curves of temperatures measured in the breast are presented, which will allow temperature monitoring and oven adjustments by consumers during the turkey roasting process to assure doneness at the expected time.

The utilization of ZnSO 4 decomposition in thermochemical hydrogen cycles

We find that the ZnSO 4 decomposition reaction can be used as the high temperature step in a number of thermochemical cycles. It is especially applicable as a substitute for the H 2SO 4 decomposition step in H 2SO 4 based cycles, and significantly improves the efficiency of such cycles. In this study, we have taken an initial look at the effects of heat-up rate on the decomposition of ZnSO 4 as it is heated through an α-β transition at 1015 K. We find that a rapid heat-up of fine ZnSO 4 particulates through this transition leads to fracturing of the ZnSO 4 crystallites and significantly enhances its subsequent decomposition rate at ∼1043 K. We also find evidence for an autocatalytic decomposition process in fine ZnSO 4 particulates with either rapid or slow heat-up rates. We believe that a combination of (1) fracturing of the crystallites and (2) surface catalysis by ZnO to equilibrate the SO 3/SO 2/O 2 gaseous products contributes to the observed autocatalytic behaviour.

The flash hydropyrolysis of lignite and sub-bituminous coals to both liquid and gaseous hydrocarbon products

Lignite and sub-bituminous coals, ground to minus 100 mesh, have been hydrogenated with preheated hydrogen in a highly instrumented 1-in. I.D. × 8-ft, entrained downflow tubular reactor system designed to be operated at up to 900°C and 4000 p.s.i. The heat-up rate of the coal particles have been calculated to be in the order of 30,000 to 50,000°C/sec. Product yields of 10% BTX were observed when using lignite and 13% when using sub-bituminous coal, based on conversion of carbon contained in the coal, and when operating at 2000 to 2500 p.s.i. and 750° to 775°C. Methane yields in excess of 80% have been observed at 2500 p.s.i. and 875° to 900°C. Coal particle residence times are in the order of 4 to 12 sec and the effect of residence time is studied by the use of product sample taps located along the length of the reactor. Hydrogen to coal feed ratios as low as approximately 0.2 (lb/lb) were studied and methane concentrations as high as 34% in the process stream were obtained. Nitrogen and sulfur balances were made on the coal, char products and both the liquid and gaseous effluents. A reaction scheme and kinetics model have been developed to which the experimental data is applied to calculate rate constants and activation energies. Relative economic evaluations are made using the most recent experimental results assuming an integrated system using flash hydropyrolysis to produce: (a) motor fuel; (b) pipeline gas; and (c) motor fuel and pipeline gas.

The kinetics of helium release from irradiated samples of austenitic steels OKh16N15M3B and OKh16N15M3BR

Samples of a heat-resistant austenitic steel (both boron free and with 0.005% by weight boron added) were irradiated in the active zone of Reactor WWR-M with a neutron flux of 4 × 10 13 n/cm 2 · s (E ⩾ 3 MeV) and 6 × 10 13 n/cm 2 · s (E ⩽ 0.03 eV) at fixed temperatures in the range of 600°C to 850°C. The neutron fluence was 6 to 8 × 10 19 n/cm 2 . For the study of the helium release kinetics the samples were annealed with a constant heat-up rate. The experimental equipment and procedure were described previously. Most of the helium (70 to 90%) is released at temperatures of 1420 to 1490°C. This is connected with the melting of the samples. Helium release during the annealing in the range 20 to 1300°C is observed in the form of seven peaks on the curve of gas-release rate. Attempts to interpret the helium diffusion mechanism of each stage is given by determination of the activation energies and other data. Irradiation in a helium atmosphere leads to penetration of helium into the samples, which is proportional to the external gas density. A significant intensification of the generation of helium cavities occurs at irradiation temperatures of 750 to 850°C.