A review of spray-cooling and bottom-flooding work for lwr cores

Rewetting or the re-establishment of water films on the hot, fuel rod surfaces is of fundamental importance following a loss-of-coolant accident in a water cooled reactor. Two techniques are used: rewetting by a falling film of water (top sparay) and rewetting by an upflow of water (bottom flooding). Considerable theoretical and experimental work has been done to investigate the effects of different operating parameters on these techniques, and that work is summarized here.     

Experimental studies on rewetting of hot vertical annular channel

Studies on the rewetting behaviour of hot vertical annular channels are of interest in the context of emergency core cooling in nuclear reactors following LOCA. Experimental studies were carried out to study the rewetting behaviour of a hot vertical annular channel, with hot inner tube, for bottom flooding and top flow rewetting conditions. The length of the inner tube of the test section was 3030 mm for bottom flooding rewetting experiments and 2630 mm for top flow rewetting experiments. The tube was made of stainless steel. Experiments were conducted for water flow rates in the annulus upto 7 lpm (11.7×10⁻⁵ m³ s⁻¹). The initial surface temperature of the inner tube was varied from 200 to 500°C. The experimental studies show that for a given initial surface temperature of the tube, the rewetting velocity increases with an increase in flow rate of water and it decreases with an increase in the initial surface temperature for a given water flow rate. For a given water flow rate and initial surface temperature, the rewetting velocity is higher in the case of rewetting under bottom flooding conditions as compared to that in the case of rewetting under top flow conditions. These conclusions agree with the conclusions reported in the earlier literature. Using the experimental data of the present work, correlations for bottom flooding and top flow rewetting velocities are developed.     

The physics of rewetting in water reactor emergency core cooling

Surface rewetting is essential for the re-establishment of normal and safe temperature levels following dryout in rod clusters or boiler tubes, or following postulated loss-of-coolant accidents in water reactors. Rewetting experiments have been performed with tubes and rods with a wide range of materials and experimental conditions (surface temperatures 300–800°C, constant water flows 0.1–30 g s⁻¹). The physical processes involved in the rewetting of high temperature surfaces have been shown to be identical for both falling water films and bottom flooding. The variation of rewetting velocity with mass flow has been determined, and shown to be independent of hydraulic diameter over the range 0.2–6 mm of practical interest. Data have also been obtained on the mass ‘carryover’ fraction. Theoretical solutions for the rewetting velocities have been obtained by analysis of thermal conduction in the surface. At low mass flows, effectively one-dimensional (axial) conduction cools the surface ahead of the rewetting front, and gives agreement with experiment. At higher mass flows the rewetting velocity is substantially independent of surface thickness and conductivity. The present data and the available world data for rewetting are shown to be in agreement with the theory.     

Hydrodynamically-controlled rewetting

Transient cooling of a hot tube by a falling liquid film is analysed. Vapor production from the liquid film and the sputtered droplets can produce a countercurrent vapor velocity which exceeds the flooding limit, and rewetting becomes hydrodynamically-controlled rather than heat conduction-controlled. A criterion shows that conduction-controlled rewetting prevails at higher coolant flow rates and flooding conditions at lower flow rates. A solution is obtained for the liquid coolant vaporized during its fall from the sputtering film front. The required thermal radiation properties are also presented. Detailed calculation based on this analysis shows good agreement with experimental results.     

An analytical solution to a two-dimensional model of the rewetting of a hot dry rod

The model considers a hot dry rod of infinite length cooled by a film of liquid moving along its surface. The heat transfer coefficient is assumed to be constant on the wet side and zero on the dry side of the rewetting front, and the liquid film is assumed to move at constant speed. We derive an analytical formula relating the temperature difference in the rod, the temperature at the rewetting front, the wet side heat transfer coefficient, and the rewetting speed. The formula is thought to apply to the rewetting of a fuel rod during emergency cooling by flooding.     

On the process of rewetting a hot surface by a falling liquid film

A two-dimensional transient heat conduction model for rewetting a hot surface by a falling liquid film predicts that for stainless steel, Inconel or Zircaloy only a wall thickness of some 0.020 in. takes part in the rewetting process in steam at 100–1000 psia. The rewetting rate is nearly independent of heat flux and thermal conductivity, but increases with pressure and decreases with volumetric heat capacity.     

Response to G. Yadigaroglu''s letter

The rewetting or quench temperature is the temperature of a hot solid surface at which a liquid can reestablish contact with the dry surface. An estimation of this temperature is essential in predicting the rate at which the coolant quenches the core of a light-water reactor (LWR) after a loss-of-coolant accident. The present study reviews and evaluates previous work in this area and presents a model relating experiments to theory for the different possible types of reflood in LWRs. It is postulated that, with the exception of those cases of top reflood by a film in a single-rod geometry and bottom reflood with a very low mass flow rate, the quench temperature corresponds to either the minimum film boiling temperature or the Leidenfrost temperature. In cases where there are such exceptions, the quench temperature corresponds to the critical heat flux temperature. New correlations for the rewetting or quench temperature are presented.     

Experimental comparisons for bottom reflooding in tight LWHCR and standard LWR geometries

The NEPTUN test facility is currently being used for reflooding experiments in tight hexagonal geometry, representative of light water high conversion reactors (LWHCRs). Results of parametric studies, based on over sixty forced-feed bottom reflooding experiments carried out with the NEPTUN-III (p / d = 1.13) test bundle, show that flooding rate is the most important, single thermal-hydraulics parameter.Direct comparisons with earlier NEPTUN experiments in standard LWR geometry indicate — on the basis of pressure difference considerations — that much smaller flooding rates may be expected to occur in tight LWHCR cores. The corresponding NEPTUN-III experiments show long-lasting rod surface temperature excursions with relatively high maximum temperatures being reached, and some of the more detailed experimental data collected is used to explain this behaviour. In spite of the above, rewetting of the tight-LWHCR geometry bundle was found to occur in all experiments with reasonably LWHCR-representative values for the various thermal-hydraulics parameters.     

Analysis of Precursory Cooling in Quenching Phenomena

It has been noted that precursory cooling plays an important role in quenching phenomena. In this work, a new model is presented by assuming that the heat transfer in precursory cooling is mainly due to film boiling, which persists in a finite length. Then the quench velocity and temperature profile are obtained based on the three-region model for one- dimensional axial heat conduction in a heated tube. We applied our model to several existing experimental results and obtained a correlation to predict the effective length of precursory cooling region. It turns out that the correlation takes an identical form for both falling-film rewetting and bottom flooding.     

The top-down reflooding model in the CATHARE code

A top-down reflooding model was developed for the French best-estimate thermal hydraulic code cathare. The paper presents the current state of development of this model. Based on a literature survey and on compatibility considerations with respect to the existing cathare bottom reflooding package, a falling film top-down reflooding model was developed and implemented into cathare version 1.3U. Following a brief review of previous work, the paper describes the most important features of the model. The model was validated with the Winfrith single-tube top-down reflooding experiment and with the REWET-II simultaneous bottom and top-down reflooding experiment in a rod bundle geometry. The results demonstrate the ability of the new package to describe the falling film rewetting phenomena and the main parametric trends both in a simple analytical experimental set-up and in a much more complex rod bundle reflooding experiment.     

A correlation of rewetting data

Following a loss-of-coolant accident in a water reactor the fuel pins dry out and overheat and it becomes necessary to rewet them to restore normal temperatures. A thermal conduction analysis of rewetting is presented in which it is shown that the heat transfer coefficient associated with rewetting may be taken as an arbitrary function of surface temperature, rather than a constant, without changing the dependency of rewetting velocity on the other variables. An effective heat transfer coefficient then replaces the constant value used in previous expressions for the rewetting velocity. Experiments at atmospheric pressure show that the rewetting rate increases with inlet water subcooling. The available rewetting data at both atmospheric and elevated pressure have been analysed using an existing theoretical model. Taking the effective heat transfer coefficient as proportional to the product of mass flow rate and inlet subcooling a data fit has been achieved to within a factor of two. Expressions are given which predict rewetting rates for a wide range of pressures, wall temperatures, subcoolings, clad materials and geometries.     

Liquid film behavior and heat-transfer mechanism near the rewetting front in a single rod air–water system

ABSTRACT

The rewetting front propagation may occur when the fuel rod is cooled by the liquid film flow after it is dried out under accident conditions for boiling water reactor cores. Our previous study has revealed the importance of precursory cooling, defined as a rapid cooling just before the rewetting, which has a significant effect on the propagation velocity. To understand the mechanism of the precursory cooling, we conducted heat-transfer experiments using a single heater rod contained inside the transparent glass pipe to measure heat-transfer behavior with simultaneous observation using a high-speed camera. The results showed characteristic effects of the wall temperature on the liquid film flow and liquid droplets formation at the rewetting front, i.e. sputtering. Even when the liquid film flows in rivulets under adiabatic condition, horizontally uniformed rewetting front was observed with increasing wall temperature due to enhanced flow resistance by sputtering. This sputtering effect was also confirmed from observations of the liquid film thickness, which increased with approaching the rewetting front. Heat-transfer coefficients were predicted roughly well with a single-phase heat-transfer correlation with entrance effects, suggesting that the thinner thermal boundary layer downstream of the rewetting front may be one of the precursory cooling mechanisms.     

Development of a Transient Boiling Transition Analysis Method Based on a Film Flow Model

A new single-channel, transient boiling transition (BT) prediction method based on a film flow model has been developed for a core thermal-hydraulic code. This method could predict onset and location of dryout and rewetting under transient conditions mechanically based on the dryout criterion and with consideration of the spacer effect. The developed method was applied to analysis of steady-state and transient BT experiments using BWR fuel bundle mockups for verification. Comparisons between calculated results and experimental data showed that the developed method tended to predict occurrence of rewetting earlier, however, onset time of BT and maximum rod surface temperature were well predicted within 0.6 s and 20°C, respectively. Moreover, it was confirmed that consideration of the spacer effect on liquid film flow rate on the rod surface was required to predict dryout phenomena accurately under transient conditions.     

Simulation of the transient behaviour of LMEBR fuel pins under consideration of special burnup phenomena using the BREDA-II model

A new fuel pin model was developed to describe the influence of specific burnup phenomena on the behaviour of fuel pins under transient overpower conditions in a liquid metal fast breeder reactor (LMFBR). It has been used for transient fuel pin deformation analysis during hypothetical core disruptive accidents (HCDA) and for the purpose of interpreting fuel pin failure tests. The fuel pin model, designated as BREDA-II, is based on the equations of the quasi-static theory of thermal elasticity. The fuel is regarded as elastic and the cladding as elasto-plastic material. The equations for the stress-strain analysis are based on the plane strain approximation. A multiregion fuel pin model allows to simulate long-time and transient burnup phenomena. The long-time effects taken into account are the steady state swelling of fuel, the change in fuel porosity and the production and partial release of fission gases. During a power excursion transient fuel swelling and pressure increase due to transient fission gas behaviour are included in the deformation analysis. Potential fuel pin failure is indicated by the application of various criteria of failure. In subsequent model calculations the behaviour of an irradiated LMFBR fuel pin during an overpower transient corresponding to a reactivity ramp of $5/sec is simulated and interpreted from the point of view of reactor safety.     

The effect of thermal diffusion from fuel pellets on rewetting of overheated water reactor pins

This paper presents the results of a finite difference solution of a conduction equation for the rewetting of a hot tube containing a filler material. The results show that the effect of a filler is always to reduce the rewetting velocity compared with an empty tube and reasonable agreement is obtained with previous experimental work. The effects of a gas gap on the rewetting of a UO₂-filled Zircaloy tube are discussed. A simple physical model is also presented which shows that the dominant parameter in determining the effect of a filler is (kpc)^1/2. It is suggested that previous theories for rewetting rate derived for empty tubes can be modified to include the effects of a filler by the use of a conduction correction term.     

Investigation of the cause of peculiar irradiation behavior of 9Cr-ODS steel in BOR-60 irradiation tests

Four experimental fuel assemblies (EFAs) containing 9Cr-ODS steel cladding fuel pins were previously irradiated in the BOR-60 to demonstrate the in-reactor performance of 9Cr-ODS steel for use as fuel cladding tubes. One of the EFAs achieved the best data, a peak burn-up of 11.9at% and a neutron dose of 51 dpa, without any microstructure instability or any fuel pin rupture. On the other hand, in another EFA (peak burn-up, 10.5at%; peak neutron dose, 44 dpa), peculiar irradiation behaviors, such as microstructure instability and fuel pin rupture, occurred. Investigations of the cause of these peculiar irradiation behaviors were carried out. The detection sensitivity in an ultrasonic inspection test was shown to be low for the metallic Cr and metallic Fe inclusions. The peculiar microstructure change reappeared with high-temperature thermal-aging of the 9Cr-ODS steel containing metallic Cr inclusions. The strength and ductility of the defective part containing metallic Cr inclusions were appreciably lower than those of a standard part without the inclusions. The combined effects of matrix Cr heterogeneity (presence of metallic Cr inclusions) and high-temperature irradiation were concluded to be the main cause of the peculiar microstructure change in 9Cr-ODS steel cladding tubes in the BOR-60 irradiation tests. They contributed to the fuel pin rupture.     

Rewetting front propagation under anticipated operational occurrences for boiling water reactors – development of two-dimensional analytical model

An analytical model to predict a rewetting velocity applicable to high pressure and high flow rate condition during anticipated operational occurrences (AOOs) is developed by applying Wiener–Hopf technique coupled with appropriate kernel substitutions. The model considers the effects of enhanced cooling in the vicinity to liquid film front termed “precursory cooling” and heat input from fuel pellets on back side of wall as boundary conditions of a heat conduction equation. A simplified two-dimensional model neglecting an effect of axial heat conduction is also proposed. It is found through the comparison among the models and experimental data that the contribution of the heat conduction in the wall-depth direction is essential in the prediction of the rewetting velocity at the thermal-hydraulic condition simulating AOOs and the axial heat conduction has little influence when an enhanced heat transfer coefficient in the dried-out region is appropriately given as a function of distance from the liquid film front.     

Review and comparison among the different models for rewetting in LWR's

Rewetting of light water reactor fuel rods after a loss of coolant accident corresponds to the re-establishment of water contact with the hot surfaces. A considerable number of analytical and numerical models have been developed in order to solve the heat condition problem in the fuel pin and predict the rewetting velocity. A comparison among the existing models has been performed. Recommendations and suggestions are outlined.     

The effect of anisotropy on the ballooning of Zircaloy cladding

The effect of creep anisotropy on the ballooning of Zircaloy LWR fuel rod cladding tubes is investigated. A perturbation method for calculating the effect of temperature inhomogenities is developed further. The results are compared with a simple method that is not restricted to small deviations from axisymmetry. The perturbation method is shown to have only limited applicability to the Zircaloy ballooning problem. The other method which assumes that the cladding tubes retain a circular cross-section provides a more useful technique for fuel rod behaviour analysis. Studies of the bending of cladding tubes and the effect of restraint on deformation and failure are presented. Apart from cladding tube bending the effects of creep anisotropy on clad deformation and failure are not large.     

竖壁冷态降液膜流动统计特性实验研究

对冷态降液膜在大平板上的流动特性进行了实验研究,运用电容式精密测微仪测定垂直降液膜的厚度,考察了在无蒸发情况下雷诺数对降液膜厚度和流量对液膜覆盖率的影响规律,得到平均膜厚经验关系式,并认为液膜在达到临界膜厚后,随流量增加只会使覆盖率增大而厚度几乎不变,发现同流量下接触角较小的平板上液膜覆盖率较大。