Sheath Temperature Research Articles

CHARACTERIZING HIGH-TEMPERATURE DEFORMATION OF INTERNALLY HEATED NUCLEAR FUEL ELEMENT SIMULATORS

The sag behaviour of a simulated nuclear fuel element during high-temperature transients has been investigated in an experiment utilizing an internal indirect heating method. The major motivation of the experiment was to improve understanding of the dominant mechanisms underlying the element thermo-mechanical response under loss-of-coolant accident conditions and to obtain accurate experimental data to support development of 3-D computational fuel element models. The experiment was conducted using an electrically heated CANDU fuel element simulator. Three consecutive thermal cycles with peak temperatures up to ≈1000 °C were applied to the element. The element sag deflections and sheath temperatures were measured. On heating up to 600 °C, only minor lateral deflections of the element were observed. Further heating to above 700 °C resulted in an element multi-rate creep and significant permanent bow. Post-test visual and X-ray examinations revealed a pronounced necking of the sheath at the pellet-to-pellet interface locations. A wall thickness reduction was detected in the necked region that is interpreted as a sheath longitudinal strain localization effect. The sheath cross-sectioning showed signs of a “hard” pellet–cladding interaction due to the applied cycles. A 3-D model of the experiment was generated using the ANSYS finite element code. As a fully coupled thermal mechanical simulation is computationally expensive, it was deemed sufficient to use the measured sheath temperatures as a boundary condition, and thus an uncoupled mechanical simulation only was conducted. The ANSYS simulation results match the experiment sag observations well up to the point at which the fuel element started cooling down.

Plasma-wall interaction in an electrostatic sheath of plasma containing a monoenergetic electron beam

The plasma-wall interaction in the presence of a monoenergetic electron beam has been studied by taking into account the self-consistency among plasma transport in a collisionless electrostatic sheath, deposited energy flux at the wall and material thermal response for carbon and tungsten as wall materials. The variations of the potential drop across the sheath, ion velocity at the sheath edge, and surface temperature of material as a function of electron beam flux are explored in the presence of the electron emission. It is found that when electron beam does not dominate the sheath, potential drop across the sheath depends strongly on the material properties due to the impact of electron emission while the surface temperature of material shows monotonic variation. In the case of carbon wall, the electron beam may dominate the sheath at a certain electron beam concentration or energy. Under this circumstance, both the potential drop across the sheath and surface temperature of material demonstrate the sharp increasing transition. The development of local hot spot on the plasma facing material is caused by the enhanced ion energy flux instead of the electron beam energy flux. If the electron emission is not taken into account, as a smaller electron beam flux, both the potential drop across the sheath and surface temperature of material display the significant change and then it may be easier to develop for the local hot spot on the plasma facing material.

Assessment of ASSERT-PV for prediction of post-dryout heat transfer in CANDU bundles

Atomic Energy of Canada Limited (AECL) has developed the subchannel thermalhydraulics code ASSERT-PV for the Canadian nuclear industry. The recently released ASSERT-PV 3.2 provides enhanced models for improved predictions of subchannel flow distribution, critical heat flux (CHF), and post-dryout (PDO) heat transfer in horizontal CANDU fuel channels. This paper presents results of an assessment of the new code version against PDO tests performed during five full-size CANDU bundle experiments conducted between 1992 and 2009 by Stern Laboratories (SL), using 28-, 37- and 43-element bundles. A total of 10 PDO test series with varying pressure-tube creep and/or bearing-pad height were analyzed. The SL experiments encompassed the bundle geometries and range of flow conditions for the intended ASSERT-PV applications for existing CANDU reactors. Code predictions of maximum PDO fuel-sheath temperature were compared against measurements from the SL PDO tests to quantify the code's prediction accuracy. The prediction statistics using the recommended model set of ASSERT-PV 3.2 were compared to those from previous code versions. Furthermore, separate-effects sensitivity studies quantified the contribution of each PDO model change or enhancement to the improvement in PDO heat transfer prediction. Overall, the assessment demonstrated significant improvement in prediction of PDO sheath temperature in horizontal fuel channels containing CANDU bundles.

CFD SIMULATION OF VERTICAL SEVEN-ROD BUNDLE COOLED WITH SUPERCRITICAL FREON-12

In this paper, a seven-rod bare bundle was simulated using ANSYS Fluent 6.3.26 to accurately predict the fluid flow and heat transfer behaviour under supercritical flow conditions. Seven turbulence models were compared to identify the appropriate model to predict the experiments performed at the Institute of Physics and Power Engineering on a vertically oriented seven-rod bare bundle cooled with supercritical Freon-12. It was found that predictions of wall temperatures and heat transfer coefficients are sensitive to the choice of turbulence model as well as to the near-wall treatment. Overall, the CFD simulations were able to predict the measured sheath temperature profiles along the length of the bundle within reasonable accuracy.

ASSERT-PV 3.2: Advanced subchannel thermalhydraulics code for CANDU fuel bundles

Atomic Energy of Canada Limited (AECL) has developed the subchannel thermalhydraulics code ASSERT-PV for the Canadian nuclear industry. The most recent release version, ASSERT-PV 3.2 has enhanced phenomenon models for improved predictions of flow distribution, dryout power and CHF location, and post-dryout (PDO) sheath temperature in horizontal CANDU fuel bundles. The focus of the improvements is mainly on modeling considerations for the unique features of CANDU bundles such as horizontal flows, small pitch to diameter ratios, high mass fluxes, and mixed and irregular subchannel geometries, compared to PWR/BWR fuel assemblies. This paper provides a general introduction to ASSERT-PV 3.2, and describes the model changes or additions in the new version to improve predictions of flow distribution, dryout power and CHF location, and PDO sheath temperatures in CANDU fuel bundles.

Assessment of CFD for the Canadian SCWR bundle with wire wraps

Within the Generation-IV (Gen-IV) International Forum, Atomic Energy of Canada Limited (AECL) is leading the effort in developing a conceptual design for the Canadian supercritical water-cooled reactor (SCWR). AECL proposed a new fuel bundle design with two rings of fuel elements placed between the central flow tube and the pressure tube. In the current paper, both the bare-rod and wire-wrapped bundle geometries were tested for the proposed fuel bundle design at 25 MPa using STAR-CCM + CFD code to predict the sheath temperature variation along the axial direction. SST k–ω and Reynolds-stress based turbulence models were compared with each other to assess their capability in predicting wall-to-fluid heat transfer in supercritical flows including the heat transfer deterioration (HTD) phenomenon. Using the turbulence model identified from the sensitivity analysis, the mesh refinement for the bare-rod geometry was performed in accordance with the ASME CFD numerical accuracy guidelines. Following these assessments, the recommended CFD model was then applied to simulate the Canadian SCWR fuel bundle with wire wraps. Predictions for the velocity and temperature variation in bare-rod and wire-wrapped geometries were compared. As a result of this study, it was found that wire wraps reduces the maximum temperature variation significantly for the current SCWR bundle design.

Selected Papers from OECD-NEA PSBT Benchmark

Historically, the prediction of safety margins has been based on system level thermal-hydraulic calculations employing suitable empirical formulations for assembly specific geometries and fuel-element grid spacers. These works have assessed response, margins, and consequences for the system based on one-dimensional two-fluid or drift-flux type thermalhydraulics formulations with fuel-vendor specific hydraulic losses and heat transfer characteristics for various fuel assemblies, including the so-called hot channel. Analysis of the hot channel gives important information on flow rates, fuel element centerline temperature, fuel sheath temperature, and margin to the departure from nucleate boiling. Given the reliance of the above approaches on empirical formulations obtained from complex and often difficult experiments, there is significant interest in obtaining reliable and accurate results from computation tools which employ more fundamental empirical relationships which can be obtained from subsets of the domain or from other scaled experiments.

MANAGING A PROLONGED STATION BLACKOUT CONDITION IN AHWR BY PASSIVE MEANS

Removal of decay heat from an operating reactor during a prolonged station blackout condition is a big concern for reactor designers, especially after the recent Fukushima accident. In the case of a prolonged station blackout condition, heat removal is possible only by passive means since no pumps or active systems are available. Keeping this in mind, the AHWR has been designed with many passive safety features. One of them is a passive means of removing decay heat with the help of Isolation Condensers (ICs) which are submerged in a big water pool called the Gravity Driven Water Pool (GDWP). The ICs have many tubes in which the steam, generated by the reactor core due to the decay heat, flows and condenses by rejecting the heat into the water pool. After condensation, the condensate falls back into the steam drum of the reactor. The GDWP tank holds a large amount of water, about 8000 m3, which is located at a higher elevation than the steam drum of the reactor in order to promote natural circulation. Due to the recent Fukushima type accidents, it has been a concern to understand and evaluate the capability of the ICs to remove decay heat for a prolonged period without escalating fuel sheath temperature. In view of this, an analysis has been performed for decay heat removal characteristics over several days of an AHWR by ICs. The computer code RELAP5/MOD3.2 was used for this purpose. Results indicate that the ICs can remove the decay heat for more than 10 days without causing any bulk boiling in the GDWP. After that, decay heat can be removed for more than 40 days by boiling off the pool inventory. The pressure inside the containment does not exceed the design pressure even after 10 days by condensation of steam generated from the GDWP on the walls of containment and on the Passive Containment Cooling System (PCCS) tubes. If venting is carried out after this period, the decay heat can be removed for more than 50 days without exceeding the design limits.

Temperature Characteristics of Cathode Sheath in High-Pressure Volume Discharge Derived from Emanating Shock Wave

The temperature characteristics of cathode sheath in high-pressure volume discharge are investigated experimentally. The cathode sheath temperatures under various discharge conditions are derived from the speeds of the emanating shock waves which are measured by a Mach—Zehnder interferometry. It is found that the cathode sheath temperature and the ratio ΔT3/ΔT1 of temperature rise between cathode sheath and plasma bulk are determined by the specific energy deposition and the breakdown delay time respectively. These results are helpful for discharge stability improving and shock wave reducing.

Effective thermal conductivity of a heat generating rod bundle dissipating heat by natural convection and radiation

A numerical study of conjugate natural convection and surface radiation in a horizontal hexagonal sheath housing 19 solid heat generating rods with cladding and argon as the fill gas, is performed. The natural convection in the sheath is driven by the volumetric heat generation in the solid rods. The problem is solved using the FLUENT CFD code. A correlation is obtained to predict the maximum temperature in the rod bundle for different pitch-to-diameter ratios and heat generating rates. The effective thermal conductivity is related to the heat generation rate, maximum temperature and the sheath temperature. Results are presented for the dimensionless maximum temperature, Rayleigh number and the contribution of radiation with changing emissivity, total wattage and the pitch-to-diameter ratio. In the simulation of a larger system that contains a rod bundle, the effective thermal conductivity facilitates simplified modelling of the rod bundle by treating it as a solid of effective thermal conductivity. The parametric studies revealed that the contribution of radiation can be 38–65% of the total heat generation, for the parameter ranges chosen. Data for critical Rayleigh number above which natural convection comes into effect is also presented.

Relation between charge state distributions of peptide anions and pH changes in the electrospray plume. A mass spectrometry and optical spectroscopy investigation

A laser-induced fluorescence set-up profiling physico-chemical parameters of droplets and their changes in an electrospray plume is coupled to an Agilent single quadrupole mass spectrometer equipped with an Agilent Jet Stream electrospray source. This coupling allows investigating the evolution of the pH of droplets as they evaporate in the electrospray plume by measuring emission spectra of a pH-chromic dye. Simultaneous measurement of mass spectra and optical pH profiling permits to study the relation between the charge states of peptide anions in mass spectra and pH changes in the spray plume. In particular, due to changes in droplets evaporation rates, the source parameters (i.e., sheath gas flow and temperature) influence the pH of the droplets and relative intensities of charge states in peptide mass spectra.

Comparison between CAREB code calculations and LOCA test results in the FUMEX III project

Abstract The IAEA initiated a Coordinated Research Project (CRP) on improvement of computer codes used for fuel behaviour simulation under the name: FUMEX III. The Institute for Nuclear Research (INR) Pitesti participated at this CRP with ROFEM and CAREB computer codes. Recently, both codes have been improved with new models in order to extend their capabilities. The behaviour of fuel elements during high-temperature transients like LOCA is of importance to safety and licensing of power reactors. CAREB was developed for fuel transients analyses, such as LOCA and RIA. In this paper a comparison between CAREB code calculations and measured data from FIO-131 LOCA tests is presented. Several parameters were considered, including fuel sheath strains, internal element gas pressure, fuel centerline and sheath temperature, thicknesses of ZrO2 on the sheath. Fuel behavior during high-temperature transient was reasonably well modeled by CAREB code. New LOCA tests are planed to be performed in the C2-LOCA facility of the TRIGA research reactor at INR Pitesti in order to extend the experimental data base used for transient code validation.

A blind simulation of RD-14M small-break LOCA experiments using CATHENA code

The results of two Small Break Loss of Coolant Accident (SBLOCA) experiments in RD-14M test facility and their predictions by CATHENA code, which is used to analyze postulated events in CANDU reactors, are compared in this paper. Two specific SBLOCA experiments selected for the CATHENA code predictions are B9006 and B9802. Test B9006 is a 7-mm inlet header break experiment with pressurized accumulator emergency coolant injection and represents most complete SBLOCA test conducted in RD-14M. Test B9802 is a 3-mm inlet header break experiment with full channel power to study boiling in channels and condensation in steam generators in a slowly depressurizing loop rather than a blow-down. These blind simulations demonstrate that CATHENA code is capable of adequately predicting the primary pressure depressurization, channel flow rate, channel voiding for tests B9006 and B9802, and the high pressure core injection flow by CATHENA accumulator model and switching time from high pressure to low pressure injections for test B9006. The CATHENA predictions of fuel sheath temperatures for test B9006 are in much better agreement with the test measurements than those for test B9802, because CATHENA code could not capture the oscillatory behavior of channel flow and consequently sheath temperature when local fuel sheath surface was being intermittently dried and rewet under near stagnant flow and full power conditions of test B9802. Therefore, it is concluded that further works are required to appropriately predict the sheath temperature spike and its fluctuation during the transient of test B9802 where the critical heat flux and post-dryout heat transfer are important governing phenomena.

CZE and ESI-MS of Borate-Sugar Complexes

Capillary zone electrophoresis (CZE) with electrospray ionization (ESI) mass spectrometry (MS) was used to study borate (B−1) and sugar (L) complexes (LxB−1). Boric acid was adjusted to pH 10 with ammonium hydroxide to create an ESI-MS compatible CZE background electrolyte. We show for the first time that the electrophoretic peaks for each injected sugar contained both the substrates (i.e., sugar and/or multimers) and products (i.e., LxB−1). The effects of sheath liquid, temperature, and borate concentration were studied. The molecular mass information obtained from the ESI-MS provided new evidence on the mechanisms of borate-sugar complexation. Direct infusion ESI-MS and CZE-ESI-MS experiments strongly suggest that the formation of LxB−1 was from the direct reaction of a sugar or sugar multimer (Lx) and B−1. Larger LxB−1, where x > 2 were observed. Separation in the CZE dimension allows for the simultaneous analysis of a sugar mixture and simplified the ESI-MS analysis of sugars of the same molecular mass. The increase in sugar electrophoretic mobility caused by the increase in borate concentration was discussed in terms of the formation of LxB−1 complexes. In addition, the separation of five nucleosides by CZE using a borate electrolyte and detection using ESI-MS is demonstrated.

STUDY OF AN ARTIFICIAL VAGINA TO REDUCE THE MICROBIAL CONTAMINATION OF RABBIT SEMEN

Aim of the work was to evaluate and improve the efficiency of an artificial vagina with a short body which permits to collect semen almost directly into the test-tube (with the least contact with the elastic sheath). At the same time, also the test-tube is not in contact with the sheath. Ten adult hybrid bucks were collected during 10 weeks. The experiment consists of 2 successive phases: 1) analysis of microbial flora naturally present in the semen; 2) evaluation of effect of exogenous contamination. Each time, 2 collections were done, using alternatively the prototype and the standard vagina (as a control) at intervals of 20 minutes (maximum 5 times). Throughout the experiment data were collected, concerning operational activities (ejaculation time and reduced sheath temperature from 45 to 35 oC), biological quality of semen (volume, sperms concentration, live sperms) and microbial contamination rate (specific and total flora). The results showed that the time employed to collect the semen was not significantly affected by the different models of artificial vagina used. The microbial contamination of the semen collected with the prototype was lower than the other (phase a, b). The number of collections done using the same vagina did not affect biological characteristics of semen whereas increased progressively the microbiological contamination. A reduction of the contamination at the least 40% can be realized when the prototype is used (phase b).

Development and Applicatin of EMTP Based Power Cable Simulator for Underground Transmission Cables

This paper discusses the characteristics of sheath circulating current as well as the development and application of new software for underground power cable systems. Generally, in steady state, high sheath circulating current causes the increase of sheath temperature and thermal resistance which leads to the steeply reduction of the power capacity. Therefore, the exact calculation of sheath circulating current is required for analysis about the influence of high sheath current on permissible current. In this paper, Power Cable Simulator is developed for calculation of the sheath current. It can analyse the sheath current by real time. It is also easier to use than conventional software, such as EMTP and CabSim, because all the data for calculating the cable parameters are stored in a database(DB) within Power Cable Simulator. In addition, the accuracy of Power Cable Simulator is also proved through the comparison among the current calculated by Power Cable Simulator, EMTP and Cabsim with measured current.

Simulation of Plasma Focus Devices with Hemisphere Electrodes

A magneto-hydrodynamic simulation of a plasma focus device with hemisphere electrodes is constructed. The snowplow model is used with help of the momentum conservation equation to describe the motion of the plasma sheath between the two concentric hemispheres. The model simulates various plasma parameters like plasma temperature and plasma sheath velocity. The circuit equation is used to calculate the discharge current and electrodes voltage across the two hemisphere terminals. A comparison between the cylindrical and spherical devices is built. The results show that the current dip and the spike voltage is expected to be much pronounced in the spherical devices. It is found also that the plasma sheath velocity and temperature in the case of the cylindrical system are higher than that in the spherical one.

On the intrinsic thermal stability in HVDC cables

The paper deals with some important aspects of steady state thermal stability in the insulation of HVDC cables. The resistively graded electric field distribution and the temperature distribution, under steady state, in dc cables, are interrelated. The dc conductivity is known to be a function of stress as well as of temperature. Reasonably convincing theories of electric stress and temperature distribution are now available in the literature. Until recently, the thermal runaway was believed, primarily, to be a consequence of an inordinate run away in the sheath temperature due to dynamic interaction with the ambient. While this premise is partly true, there are other overriding factors controlling the stability. This paper presents a detailed investigation on this aspect and it is shown that the multi-factor dependent dc conductivity alone causes the thermal instability, boundary conditions at sheath having a marginal role.

A Robust Framework for Cable Rating and Temperature Monitoring

This paper introduces a robust framework for the temperature monitoring of power cables based on real-time current measurement only, or current measurement in conjunction with cable surface or sheath temperature measurement. The method is based on modeling the temperature response of a cable by summing an appropriate number of exponential functions. These functions are related to a thermal ladder circuit for installations that can be analyzed in such terms, or are mathematically fitted to numerically simulated results for those that cannot. The robustness of the method stems from the fact that exponential functions can be easily rendered into an inherently stable real-time formulation depending on the temperatures calculated at the previous increment and final conditions. The hypothetical final conditions are established at each time interval by simple steady-state calculations based on the present temperatures. This allows entry into territory that is forbidden for normal superposition methods, namely, a thermally unstable environment. Despite erroneous initial conditions, the algorithm will converge and it has the added merit of being extremely light, computationally speaking. The method provides an alternative method for transient cable rating, is inherently suited for real-time implementation, and provides a framework that can accommodate a changing thermal environment caused by, for example, such a phenomenon as moisture migration.

Nonuniformity modelling of particular aluminum sheath temperatures and losses

A nonlinear coupled electrothermal model of underground cables having aluminum sheaths is developed. The mathematical model deals with the numerical evaluation of losses, heating and ampacity. The filament method, the advancing front method as well as the finite-element method combined with the mapped infinite elements are used. In particular, an improved modelling of heat sources is carried out, taking into account the influence of the nonuniform losses distribution of a particular sheath on the cable heating. Moreover, the calculation of losses is performed with respect to the nonuniform temperature distribution of a particular sheath. Some illustrative computational results leading to clear-cut concluding remarks are given.