PWR Operating Conditions Research Articles

OpenFOAM CFD simulation of critical heat flux in vertical pipe under typically PWR conditions

This paper aims to see how well the open-source CFD code OpenFOAM can simulate the critical heat flux (CHF) in vertical pipe under extreme conditions, close to PWR operating conditions. The two-fluid approach combined with an extended wall boiling model was used to forecast CHF. The 2006 CHF look-up table was employed as benchmark data. The wall temperature jump was used as the criterion for predicting departure from nucleate boiling (DNB) type CHF. Good agreement was obtained between CHF from look-up table and calculated values. The effects of pressures, mass flow rates, and equilibrium qualities on CHF are investigated. CHF location and maximum volume fraction at corresponding CHF were also obtained. Most of the deviations are within 15% of error. The average absolute error of the simulated CHF was 7%. The findings indicate that OpenFOAM promises to forecast boiling flow and CHF in the PWR reactor fuel assembly geometry.

Fuel performance of chromium-coated zirconium alloy and silicon carbide accident tolerant fuel claddings

The U.S. Department of Energy’s Accident Tolerant Fuel program is focused on extending the time for fuel failure during postulated severe accidents compared to the standard UO2-Zr alloy fuel system. This paper investigates the feasibility of two different chromium-coated cladding concepts, one of which is zirconium-alloy based and the other is composite-SiC based. Both claddings had 50 µm coatings, deducted from the base layer thicknesses. The claddings were studied, using the multi-physics fuel performance tool BISON, under steady-state PWR operating conditions as well as under two transients: a power ramp and a loss-of-coolant accident (LOCA). The chromium-coated claddings showed comparable thermo-mechanical performance to the reference Zircaloy-4 cladding. As chromium is reported to provide an order of magnitude improvement in oxidation resistance, it is expected to be a better alternative in accident scenarios. Simulation results for both concepts show that further experimental investigation as well as modeling of beyond design-basis accidents is warranted.

Ion-Pair Formation Constants of Lithium Borate and Lithium Hydroxide under Pressurized Water Nuclear Reactor Coolant Conditions

A custom-made high-precision flow AC conductance instrument was used to measure the frequency-dependent molar electrical conductivities of aqueous solutions of lithium borate from T = 298 K to T = 548 K at a constant pressure, p = 20 MPa. Ion-pair formation constants of lithium borate, KA,m[LiB(OH)40], were derived from these measurements using the Turq–Blum–Bernard–Kunz (TBBK) conductivity model. Our results are consistent with previous low-temperature studies and are the first to be reported at temperatures above 318 K. Under ambient conditions, the degree of association of LiB(OH)40 is half an order of magnitude higher than that of NaB(OH)40 and KB(OH)40, but at T ≥ 448 K, the association constants of all three ion-pairs are equal within the combined experimental uncertainties. The results have been used to derive new equations to represent the temperature dependence of the limiting conductivity of lithium, and the ion-pair formation constant of lithium borate under PWR operating conditions. A new mode...

Uncertainty analysis of sub-channel code calculated ONB wall superheat in rod bundle experiments using the GRS methodology

Rod bundle experiments were performed for prototypical PWR operating conditions in the project “New Experimental Studies of Thermal-Hydraulics of Rod Bundles (NESTOR)”. The intent of the project was to improve the understanding of the Axial Offset Anomaly (AOA) through improved modeling of Onset of Nucleate Boiling (ONB) (EPRI, 2008) using sub-channel codes. Skewing of the axial power profile (AOA) is most likely driven by the deposition of boron in the crud layer on nuclear fuel rods, which is caused by boiling on the fuel rod surface (EPRI, 2008).VIPRE-I (Srikantiah, 1992), a sub-channel code, was chosen for the analysis of NESTOR tests and for which uncertainty analysis was performed. NESTOR experimental results were used to optimize grid-loss coefficients, friction-loss coefficients, and a single-phase heat transfer model in the code. By modeling NESTOR ONB tests, the VIPRE-I calculated wall superheat was determined at the experimental ONB locations. This calculated ONB wall superheat could be used as a criterion in VIPRE-I for the prediction of ONB; however, it is important to quantify the uncertainty of this calculated ONB wall superheat in order to know the accuracy of such a criterion. The VIPRE-I model optimization process, however, was a complicated one and involved interaction of both experimental and code modeling uncertainties. The propagation of these uncertainties was treated using the Gesellschaft für Anlagen und Reaktorsicherheit (GRS) methodology; a process which is detailed in this paper.

Steady-state fuel behavior modeling of nitride fuels in FRAPCON-EP

Fuel material properties and mechanistic fission gas models in FRAPCON-EP were updated to model the steady-state behavior of high-porosity nitride fuel operating at temperatures below half of the melting point. The fuel thermal conductivity and fuel thermal expansion models were updated with correlations for UN and (U,Pu)N fuels. Hot-pressing of the as-fabricated porosity was modeled as a function of the hydrostatic pressure and creep rate. The solid fission product swelling was assumed to increase linearly with burnup. Fission gas swelling constitutive models were updated to appropriately capture the intragranular gas bubble evolution in nitride fuel. Intergranular gas swelling was neglected due to the assumed high porosity of the fuel. The fission gas release behavior was modeled by fitting the fission gas diffusion coefficient in UN to FRAPCON’s default fission gas release model. This fitted gas diffusion coefficient reflects the effects of porosity, burnup, operating temperature, fission rate, and bubble sink strength. Fission gas release and fuel swelling benchmarks against irradiation data were performed. The updated code was applied to UN fuel in typical PWR geometry and operating conditions, with an extended cycle length of 24months. The results show that swelling of the nitride fuel up to 60MWd/kg burnup did not lead to excessive straining of the cladding. Furthermore, this study showed that a porous (>15% porosity) nitride fuel pellet could achieve a much higher margin to failure from the cladding collapse and grid-to-rod fretting.

Corrosion of M5 in PWRs: Quantification of Li, B, H and Nb in the Oxide Layers Formed Under Different Conditions

Abstract Until now, most of the detailed characterizations of the M5 corrosion behaviour were performed under standard PWR operating conditions, under moderate Li content and moderate temperature of the primary coolant. In this study, in addition to these standard conditions, two demanding operating conditions were explored: increased Li chemistry and elevated temperature. The objective is to establish whether these more demanding conditions have an impact on the structure of the oxide layers formed, on Nb, Li and B contents in these layers and on Hydrogen pickup of the cladding. The structure of oxide layers was studied by microscopy, the Nb content and distribution by Electron Probe Micro Analysis, the Li and B contents and distributions by Nuclear Reaction Analysis and the hydrogen pickup by gas extraction. It was observed that the stability of the corrosion behaviour of M5 is not affected by increased Li or elevated temperature conditions. The hydrogen pickup fraction of M5 is not modified by increased Li conditions or by irradiation temperature with measured contents (<100 ppm) below the solubility limit at irradiation temperature (586–631 K range). Moreover, no significant release of Nb from the oxide in the primary coolant was observed, whatever the operating conditions can be, with quantification accuracy below 0.05%. The respective Li and B contents reached in oxides are of the same order of magnitude under the all three conditions, with average Li content around 15 ppm and average B content varying from 70 to 140 ppm, corresponding to a range where enhanced corrosion is neither observed, nor expected.

Electrochemical behaviour of stainless steel in PWR primary coolant conditions: Effects of radiolysis

Few data are available in the literature on the role of the water radiolysis on the corrosion of stainless steel core components in PWR operating conditions (300 °C, 155 bar). The present approach uses a high energy proton beam to control the production of radiolytic species at the interface between a stainless steel sample and water in a high temperature and high pressure (HP–HT) electrochemical cell working in the range 25 °C/1 bar–300 °C/90 bar. The cell is designed to record the free corrosion potential of the AISI 316L/water interface mounted in line with a cyclotron delivering the proton beam. The evolution of the potential is compared before, during and after the proton irradiation. The first results are obtained with an aqueous solution containing boron, lithium and dissolved hydrogen, as in PWR primary coolant circuit. The stainless steel/water interfaces are irradiated between 25 °C and 300 °C with protons emerging at 22 MeV at the interface. The flux is varied by five orders of magnitude, from 6.6 × 10 11 to 6.6 × 10 15 H + m −2 s −1. The evolution of the free corrosion potential is highly dependent on the temperature and/or pressure. For a given temperature and pressure, it evolves with the flux and the ageing of the AISI 316L/water interfaces. An important role of the temperature of irradiation on the electrochemical response was observed. These results give a better understanding of the role of radiolysis on stainless steel corrosion in high temperature conditions.

CHF experiment and CFD analysis in a 2 × 3 rod bundle with mixing vane

In this study, the CHF enhancement using various mixing vanes is evaluated and the flow characteristics are investigated through the CHF experiments and CFD analysis. CHF tests were performed using 2 × 2 and 2 × 3 rod bundles and with R-134a as the working fluid. The test section geometry was identical to that of commercial PWR fuel assembly not including the heated length (1.125 m) and number of fuel rods. From the CHF tests, it was found that the CHF enhancement using mixing vanes under higher mass flux (1400 kg/m 2 s) and lower pressure (15 bar) conditions is larger than the CHF enhancements under other conditions. Among the mixing vanes used in this study, the swirl vane showed the best performance under relatively low pressure (15 bar) and mass flux (300–1000 kg/m 2 s) conditions and the hybrid vane performed best near the PWR operating conditions. The detailed flow characteristics were also investigated by CFD analysis using the same conditions as the CHF tests. To calculate the subcooled boiling flow, the wall partitioning model was applied to the wall boundary and various two-phase parameters were also considered. The reliability of the CFD analysis in the boiling analysis was confirmed by comparing the average void fractions of the analysis and the experiments: the results agreed well. From the CFD analysis, the void fraction flattening as a result of the lateral velocity induced by the mixing vane was observed. By the lateral motion of the liquid, the void fraction in the near wall was decreased and that of the core region was increased resulting in the void fraction flattening. The decrease of the void fraction in the near wall region promoted liquid supply to the wall and consequently the CHF increased. For the quantification of the void flatness, an index was developed and the applicability of the index in the CHF assessment was confirmed.

Elastic-plastic analysis of small cracks in tubes under internal pressure and bending

Elastic-plastic finite element analyses were conducted to generate new solutions of J-integral and crack-opening displacement (COD) for short through-wall cracks in pipes subjected to combined bending and tension loads. The results are presented in terms of the well-known GE/EPRI influence functions to allow comparisons with some limited results in the literature. Two different pipe pressures with values of 7.24 MPa (1050 psi) and 15.51 MPa (2250 psi) simulating BWR and PWR operating conditions, respectively, were used to evaluate the effects of pressure on J and COD. Pipes with various radius-to-thickness ratios, crack sizes, and material parameters were analyzed. Limited analyses were also performed to evaluate the effects of hoop stresses in pipes under pure pressure loads. The results suggest that the fracture response parameters can be significantly increased by pressure-induced axial tension for larger crack size, material hardening constant, and radius-to-thickness ratio of the pipe. The presence of pressure-induced hoop stresses also increases the fracture response, but in low-hardening materials their effects are insignificant due to small plastic-zone size that was expected for the intensity of pipe pressure and crack size considered in this study. However, for high-hardening materials when the plastic-zone size is not negligible, the hoop stresses can moderately increase J and COD.

A cycle-combining method for creep fatigue analysis: Debaene, J.P. and Permezel, P. Int. J. Pressure Vessels Piping 1989 36, (1), 75–90

Elastic-plastic finite element analyses were conducted to generate new solutions of J-integral and crack-opening displacement (COD) for short through-wall cracks in pipes subjected to combined bending and tension loads. The results are presented in terms of the well-known GE/EPRI influence functions to allow comparisons with some limited results in the literature. Two different pipe pressures with values of 7.24 MPa (1050 psi) and 15.51 MPa (2250 psi) simulating BWR and PWR operating conditions, respectively, were used to evaluate the effects of pressure on J and COD. Pipes with various radius-to-thickness ratios, crack sizes, and material parameters were analyzed. Limited analyses were also performed to evaluate the effects of hoop stresses in pipes under pure pressure loads. The results suggest that the fracture response parameters can be significantly increased by pressure-induced axial tension for larger crack size, material hardening constant, and radius-to-thickness ratio of the pipe. The presence of pressure-induced hoop stresses also increases the fracture response, but in low-hardening materials their effects are insignificant due to small plastic-zone size that was expected for the intensity of pipe pressure and crack size considered in this study. However, for high-hardening materials when the plastic-zone size is not negligible, the hoop stresses can moderately increase J and COD.

The effect of cyclic loading on the dislocation structure of fully pearlitic steel: Dollar, M., Bernstein, I.M., Daeubler, M. and Thompson, A.W. Metall. Trans. A Mar. 1989 20A, (3), 447–451

Elastic-plastic finite element analyses were conducted to generate new solutions of J-integral and crack-opening displacement (COD) for short through-wall cracks in pipes subjected to combined bending and tension loads. The results are presented in terms of the well-known GE/EPRI influence functions to allow comparisons with some limited results in the literature. Two different pipe pressures with values of 7.24 MPa (1050 psi) and 15.51 MPa (2250 psi) simulating BWR and PWR operating conditions, respectively, were used to evaluate the effects of pressure on J and COD. Pipes with various radius-to-thickness ratios, crack sizes, and material parameters were analyzed. Limited analyses were also performed to evaluate the effects of hoop stresses in pipes under pure pressure loads. The results suggest that the fracture response parameters can be significantly increased by pressure-induced axial tension for larger crack size, material hardening constant, and radius-to-thickness ratio of the pipe. The presence of pressure-induced hoop stresses also increases the fracture response, but in low-hardening materials their effects are insignificant due to small plastic-zone size that was expected for the intensity of pipe pressure and crack size considered in this study. However, for high-hardening materials when the plastic-zone size is not negligible, the hoop stresses can moderately increase J and COD.

Mass and heat exchange between adjacent channels in liquid-cooled rod bundles

Single-phase liquid energy-exchange experiments have been carried out in (1) a transparent test section containing two adjacent simplified reactor subchannels at low pressure and (2) a high pressure rig with a test section consisting of four heated cluster elements close to average PWR operating conditions. The amount of mixing was determined by measuring the enthalpy increase. The data were analysed by the eddy diffusivity approach. By means of the Prandtl mixing length theory an analytical explanation was obtained for the fact that the mixing coefficient is roughly inversely proportional to the gap width, as reported by many experimental investigators. The data from both test sections, when compared with existing mixing correlations, appear to be in much better agreement than when correlated with a Péclet number.