Heat and mass transfers in a concrete wall with composite liner under accidental conditions

The aim of this work was to study the behaviour of a concrete wall, covered with a composite liner and exposed to accidental conditions leading to high temperatures and pressures on a face of the material. In the laboratory, two practical levels of accidental situations (beyond design) have been considered. Firstly, the “SC1” scenario (accidental conditions) consisted of a rise from ambient conditions to a saturation point of 160 °C, and a pressure of 0.75 MPa in 12 h, using the maximum increase possible with the apparatus. This rise was then followed by cooling, leading to 0.22 MPa and 120 °C in 24 h. These conditions were maintained for several days. Secondly, a “SC2” scenario (severe accident conditions) consisted of a rise to a saturation point of 173 °C and a pressure of 1 MPa, these conditions were maintained for 24 h before cooling.A cylindrical specimen of 1.3 m of thickness was used. Thermocouples, pressure taps and moisture gauges were implemented before concreting. These devices provided local information, and were mostly distributed in the first 0.30 m of the concrete. The concrete composition (high performance concrete) was the same as that used for the construction of the CIVAUX 2 nuclear power station.Typical experimental results for the evolution of temperature, pressure and water content as functions of time are shown for the two test conditions. The concrete attached to the back of the composite dried, and a mass transfer was induced towards colder zones in the centre of the specimen. The liner acted as a heat insulator and the pressure acting on the back of the composite remained lower than that applied on the composite. The residual adhesion of the liner to the concrete was measured. Finally, the overall results allowed the comparison of situations where the wall was lined and unlined, during exposure to SC1 and SC2 conditions.     

Mathematical model for creep and thermal shrinkage of concrete at high temperature

Based on the existing limited test data, it is possible to set up an approximate constitutive model for creep and shrinkage at temperatures above 100°C, up to about 400°C. The model presented here describes the effect of various constant temperatures on the creep rate and the rate of aging, similar effects of the specific water content, the creep increase caused by simultaneous changes in moisture content, the thermal volume changes as well as the volume changes caused by changes in moisture content (drying shrinkage or thermal shrinkage), and the effect of pore pressure produced by heating. Generalizations to time-variable stresses and multiaxial stresses are also given. The model should allow more realistic analysis of nuclear reactor vessels and containments for accident situations, of concrete structures subjected to fire, of vessels for coal gassification or liquefaction, etc.     

Thermo-mechanical analysis of concrete in LMFBR programs

The R&D program described in this paper represents the structural mechanics effort underway and planned at ANL as an attempt to better understand the behavior of concrete structures in LMFBR plants where such structures are subjected to high temperature levels. This paper describes the analytical tools formulated and incorporated into the thermo-mechanical computer code called TEMP-STRESS.Emphasis in this paper is placed on describing the short-time and long-time constitutive formulation for concrete. The primary constitutive relation uses an elasto-plastic technique to simulate concrete nonlinearities, utilizes the von Mises ellipsoid as the loading surface, and assumes a four-parameter failure surface. Post-failure treatment is different when considering surface elements as opposed to internal elements. Creep of the concrete at temperatures up to about 400°C is approximated by a rate-type creep law using Maxwell chain technique. The creep model accounts for moisture and pore pressure disposition of the concrete.     

Fretting-wear of zirconium alloys

Fretting tests of Zircaloy fuel sheath bearing pads in contact with zirconium alloy (Zr–2.5Nb) pressure tube specimens were conducted at temperatures varying from 25 to 315 °C. The effects of motion type and amplitude, water chemistry, fuel sheath manufacturer and pressure tube surface finish were also investigated. The effect of temperature is the most significant. The pressure tube wear coefficient in the 225–286 °C for all four motions studied is considerably greater than that above 300 °C. The fretting rate for small amplitude motion representative of flow turbulence excitation is about equal at temperatures below 150 °C and above 300 °C, but is five to ten times greater in the 250–286 °C range.     

Finite element program for moisture and heat transfer in heated concrete

A new axisymmetric finite element program for the analysis of pore pressure, moisture content and temperature in heated concrete is described. The program is based on the diffusion equations for coupled heat and moisture transfer and uses a step-by-step time integration. The finite element scheme is based on Galerkin method. For time integration a step-by-step solution with iterations is used. The numerical analysis is complicated by the fact that the sorption isotherms exhibit a steep jump at saturation-nonsaturation transitions, and that the permeability dependence on temperature exhibits a jump of two orders of magnitude at 100°C. The mathematical model takes into account the release of chemically bound water due to dehydration and the associated changes in the pore space. The program may also be used at normal temperatures. Predictions of the program are compared with tests by HEDL as well as two other existing programs.     

Boiling characteristics of dilute polymer solutions and implications for the suppression of vapor explosions

Quenching experiments of hot solid spheres in dilute aqueous solutions of polyethylene oxide polymer and surfactant have been conducted for the purpose of investigating the physical mechanisms of the suppression of vapor explosions in this polymer solutions. Two spheres of 22.2 and 9.5 mm-diameter were tested in the polymer solutions of various concentrations and pool temperatures from 30°C to its boiling point. The minimum film boiling temperature in 30°C liquid rapidly decreased from over 700°C for pure water to about 150°C as the polymer concentration was increased up to 300 ppm for a 22.2 mm sphere, and it decreased to 350°C for a 9.5 mm sphere. This trend is observed consistently in the heated pool up to its boiling temperature, while the tests with surfactant solutions do not show an appreciable reduction in the minimum film boiling temperature. The ability of suppression of vapor explosions by dilute polyethylene oxide solutions against an external trigger pressure was tested by dropping molten tin into the polymer solutions at 25°C. It was observed that in 50 ppm solutions more mass fragmented than in pure water, but it produced weaker explosion pressures. The explosion was completely suppressed in 300 ppm solutions with the external trigger. The debris size distributions of fine fragments smaller than 0.7 mm were shown to be almost identical regardless of the polymer concentrations.     

Effect of temperature on graphite oxidation behavior

The temperature dependence of oxidation behavior for the graphite IG-11, used in the HTR-10, was investigated by thermogravimetric analysis in the temperature range of 400–1200 °C. The oxidant was dry air (water content <2 ppm) with a flow rate of 20 ml/min. The oxidation time was 4 h. The oxidation results exhibited three regimes: in the 400–600 °C range, the activation energy was 158.56 kJ/mol and oxidation was controlled by chemical reaction; in the 600–800 °C range, the activation energy was 72.01 kJ/mol and oxidation kinetics were controlled by in-pore diffusion; when the temperature was over 800 °C, the activation energy was very low and oxidation was controlled by the boundary layer. Due to CO production, the oxidation rate increased at high temperatures. The effect of burn-off on activation energy was also investigated. In the 600–800 °C range, the activation energy decreased with burn-off. Results of low temperature tests were very dispersible because the oxidation behavior at low temperatures is sensitive to inhomogeneous distribution of any impurity, and some impurities can catalyse graphite oxidation.     

Basic investigations on debris cooling

An experimental investigation of boiling phenomena in inductively heated particle beds has been performed. The major aim of these experiments is to provide data for validating numerical codes used in reactor safety. The experiments can be divided in three parts: boiling experiments, dryout experiments and quenching experiments. In boiling experiments, the pressure gradients have been measured along the bed height for different flow modes, different heat inputs and different system pressures. In dryout experiments, the minimum heat input has been determined for which the particle bed starts to superheat significantly above the saturation temperature. The final test series deals with the cool down behaviour of strongly superheated particles by flooding them with cold water. The initial temperatures ranged from 200 up to 900 °C in top-quenching experiments and from 230 up to 450 °C in bottom-quenching experiments. All experiments were performed with pre-oxidised stainless steel balls of 6 and 3 mm diameter in a cylindrical crucible. The bed height was 640 mm and the bed diameter was 125 mm for boiling and dryout experiments, respectively 150 mm for quenching experiments. The experimental results are compared with various available dryout models.     

Experimental investigations of pressure and temperature loads on a containment after a loss-of-coolant accident

For the design of an LWR containment one of the important conditions to be considered is the rapid rise of internal pressure and temperature caused by a loss-of-coolant accident (LOCA) of the primary cooling system. The phenomena occurring within a containment during a LOCA are currently investigated through experiments with a model containment. The experimental results are compared with the results of model calculations to improve the calculational methods.An experimental facility was built, consisting of a primary coolant circuit and a special model containment. The model containment, built in conventional reinforced concrete, has a diameter of 12 m, a height of 12.5 m, a capacity of 580 m³ and is designed for an internal pressure of 6 bar. The interior is divided by concrete walls and removable partitions into several compartments, which are interconnected through openings with adjustable cross sections. By exchanging the removable partitions it is possible to modify the interior of the containment and to simulate different containment shapes. For the first experiments a PWR configuration with nine compartments has been installed. The model scales of the compartment volumes and the overflow areas are about 1:64 compared to the 1200 MW PWR plant Biblis A.Up to now the test facility has been used for four trial runs and nine PWR LOCA experiments with single- and double-ended pipe ruptures of 100 mm dia. in a steam generator compartment and in the nozzle compartment. The initial conditions of the pressurized water in the coolant circuit before rupture were 140 bar and 290°C. About 0.1 sec after the rupture the flow rate at the site of rupture reaches its maximum of about 400 kg/sec (single-ended rupture) and 800 kg/sec (double-ended rupture). From the compartment where the rupture takes place a water-steam-air mixture streams through openings into the other compartments of the containment. Differential pressures between the compartments were measured with maximums of up to a few bar 0.15–0.5 sec after rupture, depending on the positions of rooms and transducers.Approximately 30–40 sec after rupture the blowdown has finished and the pressure in the containment has reached about 4–5 bar. The maximum pressure in a model containment is lower and the decrease of the pressure by condensation is faster than in a full-scale containment, due to the greater ratio of inner surface area to volume of a model containment. During blowdown the temperature of the containment atmosphere rises to about 150°C. Several minutes later the temperature of the concrete walls has increased non-uniformly causing considerable stress in the walls. Approximately 30 min after rupture measurements on the outside of the outer containment wall show a temperature-caused strain of about 30–60% of the maximum pressure-caused strain. A comparison between experiments and calculations shows discrepancies indicating the need for further development of calculational methods.     

Study of leaktightness integrity of containment wall without liner in high performance concrete under accidental conditions—I. Experimentation

When a partially saturated concrete wall is subjected to accidental conditions (high temperature and steam water pressure, as a LOCA or more severe conditions), water vapour penetrates the containment wall until saturation level of the containment atmosphere is achieved. The rate of penetration of water vapour through concrete is progressively reduced, leading to improvement of the leaktightness integrity of the concrete wall. In this paper, experimental studies involving the measurement of temperature, moisture propagation and pore pressures in a concrete containment wall are presented. The tests have been carried out on cylindrical specimens, made of high performance concrete (HPC) and having 1.3 m thickness (same thickness as a containment wall of a nuclear power plant). A finite element analysis is used to study the heat and mass transfer through the concrete wall. The results of this numerical modelling technique are presented in the second part of this study.     

III. PWR steam generator response to a scram at 50% load and an open grid transient (bugey-4 nuclear power plant)

Two transients, an open grid and a scram at 50% load, were conducted on unit 4 of the PWR power plant Bugey. The thermal hydraulic response of the steam generator was recorded. For the open grid test, the following observations are noted:No alarming phenomena are observed in the steam generator during the transient. Primary pressure oscillations were very mild, and did not exceed about 4.8 bar/min with a maximum amplitude of ±8 bar. This condition should not result in significant stress levels. Steam generator outer shell metal temperature gradients remained within very acceptable limits; a maximum amplitude of about +13°C and a rate not exceeding 0.8°C/min are obtained. This slow rate is explained by a fall in primary water temperature that allows for a temperature decrease inside the U-tube bundle. Similarly, the temperature rise on the tube sheet does not exceed an amplitude of 20°C with a rate of about 2°C/min. Again these conditions do not lead to any significant thermal shock on the tube sheet. The steam generator feed controls maintain the level within the normal operation range and the small addition of colder feedwater does not lead to great temperature changes because of the large mass of the recirculation water in the steam generator.For the scram at 50% load, the following observations are noted: no severe thermal or pressure transients are observed in this test. Fluid temperature fluctuations occur with rates not exceeding 1°C/s and a maximum amplitude of about 20°C in the downcomer and 10°C on the tube sheet. Steam generator outer shell temperature varies at a rate of about ±0.8°C/min with a maximum amplitude of about 16°C. These thermal transients should lead to thermally induced stresses of acceptable levels.     

Transient heating effect on high strength concrete

This study shows some differences in the properties and the behaviour at high temperature of two concretes (ordinary and high strength) made with the same calcareous aggregates. During heating tests at 1 °C min⁻¹, cylindrical samples of diameter 160 × 320 mm of high strength concrete, more dense, may explode in a critical temperature zone between 250 and 300 °C. Differences in behaviour between OC and HSC appeared at high temperatures: there were disparities especially in thermo-hydric transfer, porosity and thermal stability. The dense microstructure of high strength concrete was found to slow up the escape of vaporized water.     

The HTTR reactor pressure vessel and its integrity against a PTS event

Before manufacturing the real steel to be used in the reactor pressure vessel (RPV) of the high temperature engineering test reactor (HTTR) the vessel manufacturer and materials supplier made a sample steel by the same procedure as for the real steel (2.25Cr-1 Mo) and conducted many tests to obtain material strength data for its base and weld metals. The test results showed that the sample steel satisfied the HTTR design requirements. Vessel cooling panels are set on the inner surface of the biological shielding concrete around the RPV, and are circulated with cooling water at 0.5 MPa and 40°C to cool the shielding concrete during normal operation of the reactor. By supposing that the cooling panel breakes and the water discharges to the RPV outer surface heated at 400°C, the stress distribution generated in the vessel wall by a pressurized thermal shock (PTS) event can be calculated using a finite element method code. This paper describes some of the results obtained from the material testing of the sample steel and the estimated result using the scheme developed for a light water reactor pressure vessel, to clarify the integrity of the HTTR-RPV under a PTS event.     

Temperature effect on IG-11 graphite wear performance

IG-11 graphite, used in the 10 MW high temperature gas-cooled test reactor (HTR-10), was tested under different temperatures on an SRV standard wear performance tester. The experiment temperatures were room temperature, 100, 200, 300 and 400 °C. According to the reactor structure, the experiments were designed to test graphite–graphite and graphite–stainless steel wear. The wear debris was collected, and the worn surfaces and debris were observed under scanning electronic microscope (SEM). It was found that there were different wear mechanisms at different temperatures. The main wear mechanism at room temperature was abrasive wear; at 200 °C, it was fatigue wear; at 400 °C, adhesive wear was observed. This difference was mainly due to the change of stress distribution at the contact area. The distribution of wear debris was also analyzed by EDX particle analysis software.     

Design and experiment of hot gas duct for the HTR-10

A horizontal coaxial double-tube hot gas duct is a key component connecting the reactor pressure vessel and the steam generator pressure vessel for the 10 MW High Temperature Gas-cooled Reactor—Test Module. Hot helium gas from the core outlet flows into the steam generator through the liner tube, while helium gas after being cooled returns to the core through a passage formed between the inner tube and the duct pressure vessel. Thermal insulation material is packed into the space between the liner tube and the inner tube to resist heat transfer from the hot helium to the cold helium. The thermal compensation structure is designed in order to avoid large thermal stress because of different thermal expansions of the duct parts under various conditions. According to the design principal of the hot gas duct, the detailed structure design and strength evaluation for it has been done. A full-scale duct test section was then made according to the design parameters, and its thermal performance experiment was carried out in a helium test loop. With helium gas at pressure of about 3.0 MPa and a temperature over 900 °C, the continuous operation time for the duct test section lasted 98 h. At a helium gas temperature over 700 °C, the cumulative operation time for the duct test section reached 350 h. The duct test section also experienced 20 pressure cycles in the pressure range of 0.1–3.4 MPa, 18 temperature cycles in the temperature range of 100–950 °C. Thermal test results show an effective thermal conductivity of the hot gas duct thermal insulation is 0.47 W m⁻¹ °C⁻¹ under normal operation condition. In addition, a hot gas duct depressurization test was carried out; the test result showed that the pressure variation occurred on the liner tube was not more than 0.2 MPa for an assumed maximum gas release rate.     

A comparison between reactor experiments and SLEUTH-SEER code predictions of pellet-clad interactions in AGR fuel pins

Power cycling endurance experiments have been conducted in order to establish the relative endurance capabilities of commercial AGR type fuel pins and to determine the significance of the prime operating parameters. The information accumulated relates to 20/25/Nb and nitrided 20/25/Ti stainless steel clad fuel pins with hollow fuel pellets, operating at coolant pressures of 400 and 600 psi and clad temperatures in the range 680–840°C. The data has provided a basis for the validation of the SLEUTH-SEER model for this fuel type, and in the analysis the experimental observations are compared with model predictions in order to give an impression of the reliability of the code in commercial AGR applications. It is concluded that in the clad temperature range 700–800°C the SLEUTH-SEER code predicts the power cycling endurance of 20/25/Nb stainless steel clad fuel pins to within a factor 2.     

Moisture propagation and resulting stress in heated concrete walls

A mathematical model to simulate the coupled heat and mass transfer in heated porous media, as well as the resulting stress, is described. A finite element analysis, assuming homogeneous elastic material, is used to study the temperature and pore pressure distribution, and the rate of moisture propagation through a concrete containment wall under different time-dependent temperature boundary conditions. Results are also presented for the internal stresses caused by the presence of temperature gradients, pore pressure and the release of chemically bound water at high temperatures. Stress analysis calculations are superimposed over the calculations of the moisture propagation. The temperature, pore pressure and volume change resulting from the loss of bound water, as derived by the thermal mass transport calculation, are used as input for the stress analysis.     

Thermo-physical properties and transient heat transfer of concrete at elevated temperatures

The objective of this study is to produce our own experimental data of physical properties of domestic concrete used in Korean NPPs, and to study on the thermal behavior of concrete exposed to high temperature conditions. The compressive strength and chemical composition of the concrete used in the Yonggwang NPP units 3 and 4 were analyzed. The chemical composition of Korean concrete is similar to that of US basaltic concrete. The thermal properties of the concrete, such as density, conductivity, diffusivity, and specific heat were also measured with a wide temperature range of 20–1100 °C. Most thermo-physical properties of concrete decrease with an increase in temperature except for the specific heat, and particularly the conductivity and the diffusivity are a 50% lower at 900 °C as compared with the values at room temperature. The specific heat increases until 500 °C, decreases from 700 to 900 °C, and then increases again when temperature is above 900 °C. In this work, we also have performed CORCON analysis and MCCI experiments to simulate a transient thermal behavior of concrete exposed to high temperature conditions. The measured maximum downward heat flux to the concrete specimen was estimated to be about 2.1 MW m⁻² and the maximum erosion rate of the concrete to be 175 cm h⁻¹ with maximum erosion depth of about 2 cm. In the CORCON analysis, it is found that the concrete compositions have an important effect upon concrete erosion.     

Development of constitutive equations for computer analysis of stainless steel components

Engineering application makes conflicting demands of constitutive equations which are difficult to satisfy simultaneously, so forcing considerable approximation. The difficulty is compounded by the frequent need, in anything but room temperature application, to be able to describe the behaviour of the structural material over a range of temperatures. This is illustrated by considering the spectrum of behaviour of Type 316 stainless steel from room temperature to operation at 600°C. It is found that a simple plasticity model describes the behaviour well at 400°C but is less adequate at 20°C in the presence of “cold creep”. There is a discussion of the way plasticity and creep can both be described, with a systematic interaction but without the restrictions of a single “unified relation” for all inelastic deformation.     

Air vent in water chamber and surface coating on liner slides concerning auxiliary cooling system for the high temperature engineering test reactor

Experimental study was made to confirm the validity of new designs of the auxiliary cooling system for the high temperature engineering test reactor (HTTR). First, it is necessary to vent residence air in outlet side of water chamber of the auxiliary heat exchanger for the HTTR. Accordingly, we have proposed to mount a proper bend duct in the outlet side of the water chamber. Air vent is done by difference between pressures at both ends of the bend duct caused by the forced water circulation using the water pumps. From flow tests, it was confirmed that it is capable of venting the air through the bend duct by circulating the water in maximum capacity of the water pumps. Second, it is essential to prevent seizure and excessive wear of the liner slides of the auxiliary concentric hot gas duct for the HTTR at a service temperature of 950°C. Therefore, we have put forward to coat titanium nitride (TiN) on the surface of the liner slides made of nickel-based superalloy Hastelloy XR using the thermochemical vapor deposition method. As a result of seizure and wear tests, it was confirmed that the TiN coating film of 3 μm on the surface of Hastelloy XR is sufficient.