The core of a CANDU (CANada Deuterium Uranium) pressurized heavy water reactor includes several hundred horizontal fuel channels that pass through a calandria vessel containing the heavy water moderator. In each fuel channel, annulus spacers are used to maintain the gap between the cold calandria tube and the hot pressure tube, a pressurized vessel containing the nuclear fuel in contact with heavy water coolant. In order to carry the loads between the pressure tube and calandria tube, the annulus spacers are required to possess adequate structural strength throughout the operating life of the reactor. The Inconel X-750 spacers used in some reactor units are susceptible to irradiation induced degradation. As irradiation fluence increases with operating time, material embrittlement has been observed due to helium bubble formation in the X-750 spacer material. An engineering approach for assessing the structural strength of CANDU annulus spacers has been recently developed. When the ductility of the material is relatively low, the region susceptible to fracture under applied tensile stress may be adequately idealized as a strip-yield process zone surrounded by elastic material and associated with restraining stress. The engineering approach is based on applying the strip-yield process zone methodology to fracture at a nominally smooth surface. Finite element modeling was undertaken to simulate the strip-yield based fracture process zone. The finite element analyses and results are presented in this paper. The finite element results verify the engineering equations developed to assess the structural strength of annulus spacers.

Radiation-induced degradation of reactor pressure vessels and internals is a concern to the aging nuclear fleet and welding solutions will be required if repair of these irradiated components is deemed necessary. However, the weldability of highly irradiated austenitic materials is significantly diminished due to the presence of irradiation induced helium in the material matrix. Helium-induced weld cracking is a complex phenomenon that is related to the concentration of helium, the heat input from the welding process, and stresses generated during cooling of the weld. During conventional high heat input welding methods such as gas tungsten arc welding, helium bubbles can coalesce and grow on base metal grain boundaries within the heat-affected zone which subsequently causes intergranular cracking. The objective of this work was to obtain weldability data by characterizing welds made on highly activated, neutron irradiated Type 304 stainless steel containing both radiation-induced helium and microstructural damage such as void swelling. All irradiated materials welding was performed inside a Westinghouse hot cell utilizing a pulsed Nd:YAG laser with welds made on three rectangular samples of highly activated Type 304 stainless steel. The rectangular samples were cut and milled in-cell from sections previously obtained from two neutron reflector hex blocks. The hex blocks are U.S. Department of Energy owned material and were irradiated for approximately 13 years in the EBR-II sodium cooled fast reactor from 1982 until 1995. The three samples selected for welding have nominal damage doses of approximately 0.4, 11, and 28 dpa with corresponding estimated helium contents of 0.2, 3 and 8 appm helium, respectively. A number of different weld parameter sets were utilized and included variations of travel speed, wire feed speed and lens-to-work distances. The parameter sets allowed for a range of effective weld heat input levels to be compared. Single pass and multiple pass as well as wire fed and autogenous welds were made. This paper presents the results from post-weld evaluations performed on the three welded irradiated samples, focusing on the reduced tendency for cracks to form adjacent to the weld as a function of weld parameters, lens-to-work distance and helium content.

There is considerable interest in high fidelity simulation of both single phase incompressible flows and multiphase flows. Most commonly applied numerical methods include finite difference, finite volume, finite element and spectral methods. All of these methods attempt to capture the flow details by solving the Navier–Stokes equations. Challenges of solving the Navier–Stokes single phase incompressible flows include the non-locality of the pressure gradient, non-linearity of the advection term and handling the pressure-velocity coupling. Multiphase flow computations pose additional challenges, such as property and flow variable discontinuities at the interface, whose location and orientation is not known a priori. Further, capturing/tracking of the multiphase interface requires solution of an additional advection equation. Recently, the lattice Boltzmann method has been applied to compute fluid dynamics simulations both for single and multiphase configurations; it is considered a modern CFD approach with improved accuracy and performance. Specifically, we employ a multiple-relaxation time (MRT) technique for the collision term on a D3Q27 lattice. The multiphase interface is captured using the phase-field approach of Allen-Cahn. Test cases include lid driven cavity, vortex shedding for a double backward facing step, Rayleigh Taylor instability, Enright’s deformation test and rising bubble in an infinite domain. These test cases validate different aspects of the single and multiphase model, so that the results can be interpreted with confidence that the underlying computational framework is sufficiently accurate.

A theoretical model was explored for predicting cavitation generation from a solid-fluid interface with fluid-structure interaction. Predicting cavitation generation is crucial to evaluate the lifetime of fluid machines. Cavitation has been generated from a solid-fluid interface with tensile stress (pressure) wave propagation across the interface. It was revealed that cavitation generation was suppressed when the surface wettability of the solid in a solid-fluid interface was improved (hydrophilized). It means that a condition exists in which cavitation is not generated despite the existence of bubble nuclei in water. This phenomenon cannot be explained by the conventional theory of fluid mechanics. In this study, an analogy between the theory of crack propagation in fracture mechanics and cavitation generation and propagation from a solid-fluid interface with fluid-structure interactions is developed and applied. An impact experiment was conducted with a free-falling projectile that hit a cylindrical solid buffer placed on top of a water surface within an elastic tube standing on the ground. The projectile impact created a stress wave propagating through the buffer and across the interface of the buffer and water. During the experiments, cavitation bubbles were generated from the interface of the buffer and water due to tensile wave propagation across the interface. Cavitation intensity was controlled by adding a surfactant to water. A bubble was set on the solid-fluid interface beforehand, then its growth with stress (or pressure) wave propagation was observed. The formularization of cavitation occurrence was tested by using initial crack length and stress in fracture mechanics as an analogy for the diameter of pre-set bubble and pressure wave amplitude.

Hybrid friction diffusion bonding (HFDB) is a solid-state bonding process first introduced by Helmholtz-Centre Geesthacht, Germany, to join aluminium tube-to-tube sheet joints of coil-wound heat exchangers (CWHE) for liquefaction of natural gas (LNG). This study describes how HFDB was in a first step successfully transferred to create austenitic S32100 single hole tube-to-tube sheet joints. Process parameters are presented and results from subsequent non-destructive bubble leak testing and destructive tensile pull-out testing are discussed. After pull out testing the bonded areas were further investigated using optical microscopy as well as scanning electron microscopy. Leak tight joints were generated due to the formation of a metallic bond close to the planar friction area of the employed tools, and failure in pull-out tests occurred by ductile fracture. The results show that the HFDB approach developed for Al-alloys may well be transferred to steel, and in the future possibly to other high-temperature alloys. It thereby offers an alternative route for joining tube to tube-sheet connections in solid state, with the corresponding advantages, such as no open flames or arc, no spatter and no need for filler material.

Transient fluid velocity and pressure fields in a pressurized water reactor (PWR) steam generator (SG) secondary side during the blowdown period of a feedwater line break (FWLB) accident were numerically simulated employing the saturated liquid flashing model. This model is based on the assumption that compressed water in the SG is saturated at the beginning and decompresses into the two-phase region where saturated vapor forms, creating a mixture of steam bubbles in liquid by bulk boiling. The numerical calculations were performed for two cases where the outflow boundary condition is different from each other; one is specified as the direct blowdown discharge to atmospheric pressure and the other is specified as the blowdown discharge to an extended calculation domain with atmospheric pressure on its boundary. To effectively simulate the saturated water flashing from the SG following the FWLB accident, the physical SG model was simplified as a vertical once-through SG to which a feedwater pipe is attached. However, the physical geometry of the analysis model was modeled as realistically as possible in terms of the SG tube bundle height, the SG inner diameter and porosity, the inner diameter and length of broken feedwater pipe part, etc. It was considered that the SG shell-side and the attached feedwater pipe were initially filled with high pressure saturated water. The pressure in the steam space was 7.5 MPa. For the calculation of the two-phase flow during high pressure saturated water flashing from the SG through the broken feedwater pipe, the inhomogeneous two-fluid model was used. The present simulation results were discussed through a comparison with the predictions using a simple non-flashing model neglecting the effects of phase change. Based on the comparative discussions, the applicability of each of the non-flashing liquid discharge and saturated liquid flashing discharge models to the confirmatory safety evaluations of new SG designs was examined.

The Hanford Tank Waste Treatment and Immobilization Plant (WTP) will process waste slurries capable of generating a dispersed non-condensable gas phase. Under postulated conditions, the generated bubbles may coalesce to form flammable gas pockets. Ignition of such a gas pocket may result in a deflagration or detonation that can transmit a pressure pulse into the surrounding gas-liquid slurry. The transmitted pressure pulses will impose structural loads on the piping and associated supports. These loads, and their evolution as the pressure pulse propagates throughout the system, must be considered in the design of WTP piping. Recent work has demonstrated good agreement between experimental data and predictions of the pressure diminishment associated with the propagation of a pressure pulse through a bubbly medium using a simplified onedimensional pursuit model (i.e., model of rarefaction wave overtaking shock front). This paper describes the application of the pursuit model to flammable gas pocket scenarios postulated in the design of WTP piping. The model is adapted to consider the effect of the time-evolution of the ignited gas pocket and parametric simulations are used to demonstrate the effect of key variables on the propagation and diminishment of the pressure pulse in piping. The practical applicability of the pursuit model for WTP piping design / analysis is illustrated through the development of a correlation that appropriately represents the results of the pursuit model for a wide range of conditions and that could be readily incorporated into existing structural analysis tools. Arguments are presented supporting the applicability of the simplified model even for scenarios in which the ignited gas pocket does not satisfy the model’s onedimensional assumption.

The current work studies air-water flow through a ½-inch flow restricting orifice installed in a 1-inch pipe. Investigation of two phase flow downstream the orifice and its effects on vibration of the piping structure have been carried out. Several flow regimes from bubbly to stratified-wavy flow have been analyzed to evaluate the effects of flow pattern, phase redistribution, bubble frequency, and liquid flow rate on the vibration of the structure. The liquid velocity fields have been obtained using Particle Image Velocimetry (PIV) along with post processing algorithm for phase discrimination. Proximity sensors have been used to capture the pipe response in two orthogonal directions. Also, a capacitance sensor was used to measure the two-phase void fraction. The results show that the magnitude and nature of vibrations of the piping structure is largely affected by the frequency and size of the bubbles upstream, vortex creation by pressure fluctuation downstream, liquid flow rate, and the flow pattern upstream. Slug flow and stratified flow patterns induced significant vibrations in the examined structure. The location of the transition region of slug flow on flow pattern maps, play important role in the dynamic response of the structure to the flow.

Water Jet Peening (WJP) has been widely applied to nuclear power plants in Japan as one of mitigation techniques against Stress Corrosion Cracking (SCC) initiation [1]. WJP utilizes high pressure water flow including numerous cavitation bubbles and improves surface residual stress of susceptible materials used in reactor internals from tensile stress to compressive stress without significant plastic deformation, hardening, heating and furthermore retrieval of foreign materials. An inspection relief for the Primary Water SCC (PWSCC) concerned components, by means of peening technique application, has been discussed among PWR owners in the US for about last 10 years. The topical report on PWSCC mitigation by surface stress improvement (Material Reliability Program (MRP)-335, revision 3-A) was published through the above activities by Electric Power Research Institute (EPRI) MRP [2]. The target components, where PWSCC is concerned, are listed as Reactor Pressure Vessel Head Penetration Nozzles (RPVHPNs), such as Control Rod Drive Mechanism Nozzle (CRDMN), and dissimilar metal welds (DMWs) of Reactor Coolant System (RCS) nozzles, and performance criteria for peening are defined in the topical report. Moreover, the technical basis for PWSCC mitigation by surface stress improvement (MRP-267, revision 2) was published by EPRI MRP [3].The report details numerous data for each peening technique which show the effectiveness in mitigating the PWSCC initiation and its sustainability, i.e. state of stress. The report also includes the process control; covering nozzle diameter, water flow rate, application time, jet stand-off, impingement angle and stationary nozzle time for WJP [3]. RPVHPNs inner diameter (ID), such as CRDMN ID, is in narrower areas than the other target components of peening techniques. Hence the WJP nozzle should be set appropriate condition, e. g. sufficient stand-off distance or angle of the WJP nozzle, in line with the MRP-267 in order to ensure the stress improvement effect by WJP. Further, the reactor pressure vessel head, which has the RPVHPNs including the CRDMNs, is placed on the refueling floor and under atmosphere condition during outage, and therefore, the CRDMNs have to be filled with water by plugging etc. for WJP application on CRDMN ID. Thus the CRDMN ID becomes a closed narrow chamber. In such a closed narrow chamber, water flow might become complex and disturb the cavitation collapse on the target surface, resulting in decreased stress improvement. Additionally, WJP has been rarely applied in a narrow closed water chamber, and only a few residual stress measurement data are available for such a WJP treated specimen. For the above reason, we has conducted a WJP test utilizing the water chamber and measured the residual stress of the test coupon simulating the CRDMN ID before and after WJP application as our own research. As a result, an improvement in residual stress was ensured even in an application of WJP in a closed narrow water chamber, which assumes CRDMN ID configuration, and created a depth over the performance criteria (0.01” (0.25 mm) in depth) stated in MRP-335 [2]. As an another applicability study, we developed a WJP tool for Bottom Mounted Instrument (BMI) Nozzles and confirmed that the residual stress of BMI ID and Outer Diameter (OD) can be improved . The background of this study is that BMI nozzle is under discussion for inspection relief as one of the components which are concerned about PWSCC. Especially, BMI ID is narrow area for WJP application; on the other hand it does not need to become a closed chamber since the reactor pressure vessel, which has the BMI Nozzles on the bottom head, is filled with water during outage. As a result, it is ensured that the residual stress for BMI ID and OD is improved by WJP to a depth of at least 0.2mm which is deeper than the performance criteria for the depth of compressive residual stress of Austenitic Stainless Steel in Japan (3.9 × 10 −3 ” (0.1mm) in depth).

The understanding of hydrogen embrittlement (HE) is of significant importance and fundamental interest owing to its negative effects on industrial metallic materials. The effect of solute H on the void coalescence and growth needs to be clarified. Using molecular dynamics simulation, the evolution of preexisting nano voids is studied in the presence of H atoms. As the per unit area concentration of trapped H atom on void surface reaches 0.45 /Å 2 , the movement of void is observed. It proceeds along with the interdiffusion of H and Fe atoms around the voids. Strain-mediated diffusion of H atoms from void surface to the zone between nearest voids occurs at first. Then the Fe atoms are affected by migrated H and diffuse in the opposite direction following the principle of energy minimization. Such mechanism can help us understand the formation of high pressure bubble at nano scale. Based on this useful information, some methods could be obtained to prevent the growth of voids further, such as strengthening the stability of metal lattice around voids by dopant etc.

As the existing light water reactor (LWR) fleet ages, the weldability of structural materials used to construct the reactor pressure vessels (RPVs) and reactor internals is diminished. The decrease in the weldability in austenitic and ferritic materials is attributed to the formation of helium in the material microstructure. Helium (He) generation occurs during the service life of irradiated reactor internals from neutron transmutation reactions of boron and nickel in these materials. Welding on irradiated materials, if performed without appropriate consideration of fluence exposure and helium generation, can result in a heat affected zone cracking phenomenon termed helium induced cracking (HeIC). The heat input associated with welding is a major factor affecting the coalescence of the generated helium along grain boundaries. As the material cools, the tensile stresses generated from welding can cause cracking to occur along grain boundaries weakened by helium bubble coalescence. In some cases, the preferred or only method of repair or replacement of a reactor internal component is welding. For components located in regions of low thermal fluence, the welding process implementation may be relatively straightforward and only heat input control may be required. However, in high thermal fluence regions, weld repair of irradiated reactor internal components is complicated by the presence of high concentrations of helium and significant care must be taken in welding process selection and heat input control. This paper highlights envisioned applications for weld repair on irradiated reactor internals. It also summarizes recently completed guidance published by the EPRI Materials Reliability Program (MRP) and EPRI BWR Vessel and Internals Project (BWRVIP) which provides an improved basis for plants to assess the weldability of components at various locations within the reactor.

This study aims to clarify the effect of surface conditions of solid on wave propagation at solid-fluid interface with fluid-structure interaction. Although many studies have been done to develop the theoretical models of fluid-structure interaction caused by wave propagation, they do not take into account the surface conditions of the solid medium on the solid-fluid interface where interaction occurs. In this study, we experimentally investigated the wave propagation across the solid-fluid interface with several value of surface wettabilities and roughnesses of solid. We conducted an impact experiment with a free-falling projectile which hit the cylindrical solid buffer placed on top of the water surface within the elastic tube standing on the ground. During the experiments, cavitation bubbles were generated from the interface of the buffer and water. That generation was inhibited according to the decrease of the value of the contact angle (improve of the wettability) of the buffer surface. The amplitude of transmitted pressure wave from the buffer to water become smaller than the theoretical value according to the decrease of the value of the contact angle on the buffer surface (the smallest value was 55% of the theoretical value). Concerning the surface roughness, the amplitude of transmitted pressure wave becomes smaller than the theoretical value according to the shape of the buffer surface become more different from flat surface (the smallest value was 75% of the theoretical value). These experimental results indicate that by taking into account the surface condition of the solid on the interface, more accurate model of fluid-structure interaction or ways to reduce the damage of structures by fluid-structure interaction would be proposed.

Hydrogen desorption from 6061 aluminum alloy during deformation was investigated by a new type of ultrahigh vacuum apparatus. The new apparatus which combined tension and outgassing experiments can be used to measure hydrogen desorption from the specimen during deformation. Outgassing corresponded to the deformation process, which initially released surface-adsorbed hydrogen. After elastic deformation, the as-received specimen released hydrogen because of the rupturing of the surface oxide layer. The hydrogen-charged specimen showed lower hydrogen concentration and hydrogen release rate than the as-received specimen. The second hydrogen release peak was due to the hydrogen accumulation at the interface between the oxide layer and matrix. The irregular sharp hydrogen release peaks resulted from the hydrogen bubble exposed to the surface during the rupture of the surface oxide layer.

The Hanford Tank Waste Treatment and Immobilization Plant (WTP) will process waste slurries capable of both generating and retaining a dispersed non-condensable gas phase. Given sufficient time, the generated bubbles may coalesce to form pockets of flammable gas. An ignition of the gas pocket may result in a deflagration or detonation that may transmit a pressure pulse into the surrounding gas-liquid slurry. The transmitted pressure pulses will impose structural loads on the pipe and associated supports. This paper describes experiments that generated pressure pulses in a stainless steel pipe filled with a non-Newtonian simulant with various void fractions of non-condensable gas. The focus of this paper is on the measured pipe strains. The details of the propagation of the pressure pulses in the compressible media are described in a separate paper: PVP-2015-45970 [1]. The propagating pressure pulses produced both quasi-static and dynamic excitation of the pipe, and the differences in the nature of the response were determined to be related to the experimental methods used to generate and measure pressure pulses. Finite-element predictions qualitatively reproduced the pipe response characteristics from the experiments.

The Waste Treatment Plant (WTP), located at the U.S. Department of Energy’s Hanford Site, will process waste slurries capable of retaining a dispersed non-condensable gas phase while in a quiescent state. Under postulated conditions, a pressure pulse generated from a deflagration or detonation within a flammable gas pocket may be transmitted into piping systems that contain the gas-liquid slurry. It is necessary to understand how a pressure pulse diminishes as it propagates through the system in order to properly quantify the structural loading on the piping and its associated supports. Earlier work presented a set of conservation equations and discussed the dynamics of individual bubble oscillations but still required discretization within a numerical CFD solver (Reference PVP2015-45972). To simplify the computation requirements, a one-dimensional pursuit model that calculated the rarefaction wave catch-up was developed and compared to experimental data. This paper presents a one-dimensional model that utilizes the difference in speeds of the shock and acoustic rarefaction waves in the two-phase media to model the propagation and diminishment of a traveling pressure shock wave. The rarefaction waves overtake the shock front which is then rapidly diminished and decreases speed accordingly. Results of the pursuit model are compared to experimental data and shown to accurately capture the diminishment behavior and shock propagation.

A coupled CFD-PBM (population balance mode) model is adopted to investigate complex behavior in a rectangle bubble column. In this work The Euler–Euler (E–E) model was adopted for the liquid phase and gas phase, while accounting for bubble coalescence and breakup a PBM discrete model was employed. The total gas holdup for a range of superficial gas velocities were studied and compared with the literature and modest agreement was found. The simulation result shows that the superficial gas velocity has great effect on bubble size distribution, and a wider bubble size distribution is found at higher superficial gas velocity. This indicates an increasing of the superficial gas velocity increases the bubble coalescence and break-up rate.

In the United States of America all transportation of radioactive material is regulated by the Department of Transportation (DOT), along with input from the Nuclear Regulatory Commission. Beginning in 2008 a new type of sealed-source encapsulation package was developed at Oak Ridge National Laboratory (ORNL); these packages contain radioactive material and are regulated and transported in accordance with the requirements set for DOT Class 7 hazardous material. DOT provides regulations pertaining to specific package contents categorized as special form designs. The special form designation indicates that the encapsulated radioactive contents have a very low probability of dispersion even when subjected to significant structural conditions. All ORNL DOT designs have been certified by DOT as being special form materials. The special form designs have been shown to simplify the delivery, transport, acceptance, and receipt process. Simplification of the transportation process makes the sealed-source encapsulation designs very advantageous for shipment to various facilities throughout the lifetime of the special form material. To this end, DOT Certificates of Competent Authority (CoCAs) have been sought for the design suitable for containing high-alpha-activity actinide materials. This design consists of a core of porous zirconia matrix pre-encapsulated within triangular canister (ZipCan) tiles that are then enclosed by a spherical shell. This new ZipCan design and a similar rectangular ZipCube design were tested for compliance with the regulations found in Title 49, Code of Federal Regulations , Section 173.469, Tests for Special Form Class 7 (Radioactive) (49 CFR 173.469) materials . The spherical enclosure was subjected to 9 m impact, 1 m percussion, and 10-minute thermal tests. Before and after each test the designs were subjected to a helium leak check and a bubble test. The ZipCan tiles and core were subjected to the tests required for ISO 2919:1999(E), including a Class 4 impact test and heat test, and were subsequently subjected to helium leakage rate tests [49 CFR 173.469(a)(4)(i)]. The impact tile test unit contained a nonradioactive surrogate; however, the thermal test unit contained a radioactive source. All three designs are still undergoing regulatory special form testing, and all three sealed-source encapsulation designs are to be submitted to DOT for CoCAs.

The presence of entrained gas bubbles in a bubbly media leads to both dispersive and dissipative effects on a pressure wave traveling through the system. The complete set of equations used to model this process involves the combination of macroscopic pressure propagation and Rayleigh-Plesset oscillations of individual gas bubbles. This results in disparate temporal and spatial scales that are difficult to solve numerically inside of a CFD framework. This paper presents a simplification to the set of governing equations that specifically eliminates the need to model individual bubble oscillations by using a cycle-averaged approximation. Results generated with the simplified model are verified against equivalent results considering the full set of governing equations. The approximation is shown to capture the behavior of interest — e.g., the variation in gas phase volume that alters the bulk modulus of the bubbly media or the net transfer of mechanical energy to heat — without the additional effort required to model rapid dynamics that do not contribute substantially to the pressure wave decay.

This paper proposes a relation for the added mass coefficient of spherical bubbles depending on void fraction based on results obtained by a semi-analytical method. This information is essential to completely characterize finely dispersed bubbly flows, where small spherical gas bubbles are present in a continuous liquid phase. Most of the closure relations for Euler-Euler or Euler-Lagrange models are obtained from experiments involving single bubbles. Their applicability to systems with high void fraction is therefore questionable. This paper uses solid harmonics to solve 3D potential flow around bubbles. Several configurations were calculated for different numbers of particles and spatial arrangements. Our results are compared with previous studies. Depending on the model proposed by previous authors, added mass forces could increase or decrease with the void fraction. This paper solves these discrepancies. The main purpose of this work is to develop simple formulas fitting our semi-analytical results. These simple formulas are suitable for further use, particularly as added mass models for multiphase flow averaged equations.

This paper studies the effect of wall proximity and wall curvature on the added mass coefficient of a spherical bubble. Results are based on a semi-analytical method. This information is essential to completely characterize finely dispersed bubbly flows in two-phase cross flow. In such flows small spherical gas bubbles are present in a continuous liquid phase close to a cylinder. This paper uses solid harmonics to solve 3D potential flow around a bubble and a wall. A new technique is developed to calculate the flow potential around a sphere and a cylinder using solid harmonics. Several configurations were calculated: one bubble close to an infinite wall, one bubble close to a cylinder and one bubble close to a spherical wall. Our results are compared with previous studies. As expected added mass forces increase in the vicinity of the wall and for lower curvature. The main purpose of this work is to understand the effect of wall curvature and proximity on added mass. These results are suitable for further use, particularly as added mass models for multiphase flow averaged equations.