Flow-induced plastic collapse of stacked fuel plates

Flow-induced plastic collapse of stacked fuel plate assemblies was first noted in experimental nuclear reactors such as the Oak Ridge National Laboratory High Flux Reactor Assembly and the Engineering Test Reactor (ETR). The ETR assembly is a stack of 19 thin flat rectangular fuel plates separated by narrow channels through which a coolant flows to remove the heat generated by the nuclear fission of the fuel within the plates. The uranium alloyed plates have been noted to buckle laterally and plastically collapse at the system design coolant flow rate of 10.7 m/s, thus restricting the coolant flow through adjacent channels. In this paper a methodology and criterion are developed for predicting the plastic collapse of ETR fuel plates. The criterion is compared to some experimental results and the Miller critical velocity theory.     

Laminar flow instability in multiple channels

Temperature-viscosity-induced laminar flow instability (LFI) in two gaseous heated parallel channels with interchannel heat exchange is purely excursive, rather than oscillatory. Constant total flow always leads to a stable system, while constant pressure drop could have an instability, depending on the sign of (∂ΔP/∂W)_Q. The system was studied numerically with negative heat perturbations yielding bounded excursions, and positive heat perturbations giving unbounded excursions asymptotically approaching zero. A nine-channel system was probed, giving excursive behavior with the ultimate growth rate the same for single and multichannel systems.     

Laminar pipe flow at the entrance into transverse magnetic field

High-resolution numerical simulations are conducted to analyze transformation of a liquid metal flow in a pipe at the entrance into a transverse magnetic field. The case of laminar flow, perfectly insulating pipe walls, and Hartmann number up to 200 is considered. The simulations reveal detailed structure of velocity and electric current fields and distribution of forces with particular attention given to the flow with an M-shaped velocity profile. They also establish criteria for accurate computations of laminar magnetohydrodynamic flows in strong non-uniform magnetic fields.     

Irreversible energy losses of flow through porous and perforated plates

For a realistic numerical simulation of a hypothetical core disruptive accident (HCDA) it is often important to take into account perforated plates. Inclusion of the plates drastically changes the loading pattern of the containment and a significant amount of the fluid's energy is dissipated as it passes through the plates. At present the PISCES-2DELK program simulates the perforated plates explicitly by defining the geometric boundary and the fluid passages. This boundary is attached to a movable structure, thereby ensuring the correct fluid-structure interaction. Equally important as the correct fluid-structure interaction is the energy dissipation, associated with fluid flow through the narrow passages. This aspect will be treated in this paper. Simple PISCES-2DELK calculations will illustrate that the fluid undergoes an internal energy rise, well beyond the reversible component (de = −pdv). The excess energy can be marked as heating of the fluid and is therefore irreversible. Furthermore, as the flow through the narrow passages tends to a “stationary state”, an a posteriori computed energy loss factor C_D levels off to a value which corresponds to those common in the literature. The method of explicitly defining the geometric boundaries of perforated/porous structures is advantageous, for the following reasons: (a) it takes into account volume occupying effects of the perforated/porous plates; (b) it is equally applicable for the dynamic and for the more or less stationary state; and (c) it simulates the fluid-structure interaction and energy dissipation of the fluid, without any a priori assumptions about energy loss factors.     

Microstructural analysis of MTR fuel plates damaged by a coolant flow blockage

In 1975, as a result of a blockage of the coolant inlet flow, two plates of a fuel element of the BR2 reactor of the Belgian Nuclear Research Centre (SCK•CEN) were partially melted. The fuel element consisted of Al-clad plates with 90% ²³⁵U enriched UAl_x fuel dispersed in an Al matrix. The element had accumulated a burn up of 21% ²³⁵U before it was removed from the reactor. Recently, the damaged fuel plates were sent to the hot laboratory for detailed PIE.Microstructural changes and associated temperature markers were used to identify several stages in the progression to fuel melting. It was found that the temperature in the center of the fuel plate had increased above 900-950 °C before the reactor was scrammed. In view of the limited availability of such datasets, the results of this microstructural analysis provide valuable input in the analysis of accident scenarios for research reactors.     

Water flow experiments and analyses on the cross-flow type mercury target model with the flow guide plates

A mercury target is used in the spallation neutron source driven by a high-intensity proton accelerator. In this study, the effectiveness of the cross-flow type mercury target structure was evaluated experimentally and analytically. Prior to the experiment, the mercury flow field and the temperature distribution in the target container were analyzed assuming a proton beam energy and power of 1.5 GeV and 5 MW, respectively, and the feasibility of the cross-flow type target was evaluated. Then the average water flow velocity field in the target mock-up model, which was fabricated from Plexiglass for a water experiment, was measured at room temperature using the PIV technique. Water flow analyses were conducted and the analytical results were compared with the experimental results. The experimental results showed that the cross-flow could be realized in most of the proton beam path area and the analytical result of the water flow velocity field showed good correspondence to the experimental results in the case when the Reynolds number was more than 4.83×10⁵ at the model inlet. With these results, the effectiveness of the cross-flow type mercury target structure and the present analysis code system was demonstrated.     

The unsteady-state thermohydraulic calculation of a stacked core

NIKIÉT. Translated from Atomnaya Énergiya, Vol. 75, No. 4, pp. 265-269, October, 1993.     

Assessment of flow induced vibration in a sodium-sodium heat exchanger

The 500 MWe Prototype Fast Breeder Reactor (PFBR) is under construction at Kalpakkam. It is a liquid metal sodium cooled pool type fast reactor with all primary components located inside a sodium pool. The heat produced due to fission in the core is transported by primary sodium to the secondary sodium in a sodium to sodium Intermediate Heat Exchanger (IHX), which in turn is transferred to water in the steam generator. PFBR IHX is a shell and tube type heat exchanger with primary sodium on shell side and secondary sodium in the tube side. Since IHX is one of the critical components placed inside the radioactive primary sodium, trouble-free operation of the IHX is very much essential for power plant availability. To validate the design and the adequacy of the support system provided for the IHX, flow induced vibration (FIV) experiments were carried out in a water test loop on a 60° sector model. This paper discusses the flow induced vibration measurements carried out in 60° sector model of IHX, the modeling criteria, the results and conclusion.     

Wear in check valves due to flow induced motion

Experiments showing the frequency and amplitude of the flow induced motion of the gate for a 2- and a 4-in. swing check valve have been performed. The gate motion is due to turbulence in approach flow. We have found the dominant turbulent frequency of the approach flow is about half the natural frequency of the valves. The valves appear to be almost critically damped. Because of this, the valves respond almost as they would to a static force of the magnitude characteristic of the turbulent fluctuation in the flow. Both the dimensionless exciting force and the damping ratio have been found to be independent of valve size so the above statements are true for larger valves also. The recommended valve oscillation amplitudes and frequencies are used to calculate the wear at the shaft and at the stop. For an unpegged check valve, such as one of the 10-in. valves which was used at the San Onofre Nuclear Generation Station, it was found that shaft bearing wear would amount to 0.27 in.³/year and stop wear to 0.03 in.³/year.     

Analysis of flow induced valve operation and pressure wave propagation for single and two-phase flow conditions

The flow induced valve operation is calculated for single and two-phase flow conditions by the fluiddynamic computer code DYVRO and results are compared to experimental data. The analysis show that the operational behaviour of the valves is not only dependent on the condition of the induced flow, but also the pipe flow can cause a feedback as a result of the induced pressure waves. For the calculation of pressure wave propagation in pipes of which the operation of flow induced valves has a considerable influence it is therefore necessary to have a coupled analysis of the pressure wave propagation and the operational behaviour of the valves.The analyses of the fast transient transfer from steam to two-phase flow show a good agreement with experimental data. Hence even these very high loads on pipes resulting from such fluiddynamic transients can be calculated realistically.     

Influence of axisymmetric initial deflections on the thermal buckling of truncated conical shells

The thermal buckling of uniformly heated, clamped, truncated conical shells with axisymmetric initial deflections is theoretically studied. The axisymmetric initial deflections are represented by a cosine function. The Donnell-type basic equations governing the finite deformation of conical shells are solved by means of a finite difference procedure. It is found that the effect of axisymmetric initial deflections on the thermal buckling of truncated conical shells is quite significant for the axially constrained case, but negligible for the axially free one except very shallow cones. The thermal buckling of a clamped annular plate with the axisymmetric initial deflection is also examined.     

Direct contact condensation induced transition from stratified to slug flow

Selected condensation-induced water hammer experiments performed on PMK-2 device were numerically modelled with three-dimensional two-fluid models of computer codes NEPTUNE_CFD and CFX. Experimental setup consists of the horizontal pipe filled with the hot steam that is being slowly flooded with cold water. In most of the experimental cases, slow flooding of the pipe was abruptly interrupted by a strong slugging and water hammer, while in the selected experimental runs performed at higher initial pressures and temperatures that are analysed in the present work, the transition from the stratified into the slug flow was not accompanied by the water hammer pressure peak. That makes these cases more suitable tests for evaluation of the various condensation models in the horizontally stratified flows and puts them in the range of the available CFD (Computational Fluid Dynamics) codes. The key models for successful simulation appear to be the condensation model of the hot vapour on the cold liquid and the interfacial momentum transfer model. The surface renewal types of condensation correlations, developed for condensation in the stratified flows, were used in the simulations and were applied also in the regions of the slug flow. The “large interface” model for inter-phase momentum transfer model was compared to the bubble drag model. The CFD simulations quantitatively captured the main phenomena of the experiments, while the stochastic nature of the particular condensation-induced water hammer experiments did not allow detailed prediction of the time and position of the slug formation in the pipe. We have clearly shown that even the selected experiments without water hammer present a tough test for the applied CFD codes, while modelling of the water hammer pressure peaks in two-phase flow, being a strongly compressible flow phenomena, is beyond the capability of the current CFD codes.     

Analysis of interaction tracks in a stacked film track detector

In a previous paper we presented a method to analyze the data in a stacked film track detector and obtained a nuclear charge distribution for the incident heavy charged particles in cosmic rays.In the course of the analysis, several events of nuclear interaction were found. The present paper presents a method to analyze these interaction tracks by using the results of previous analysis for ordinary (non-interacting) tracks.     

pMOSFET多管级联结构辐照响应特性研究

ｐＭＯＳＦＥＴ剂量计多管级联结构电离辐射响应持性比单管能明显提高辐射响应灵敏度。多管共衬底级联与不共衬底级联灵敏度提高倍数较大。比较了多管共衬底级联结构在两种辐照偏置条件下的辐照响应差异,实验结果表明,辐照时,保持恒流注入条件,其响应灵敏度、线性度和稳定性均高于零偏置的结果。同时,研究了多管级联结构完全退火后的二次辐照响应,结果表明,响应灵敏度与线性度均高于第１次辐照的。     

骤冷网格对层流火焰影响的数值分析

利用计算流体力学(CFD)方法,研究二维简化情形下骤冷网格对层流火焰传播的影响。数值分析结果表明:随着火焰的传播,层流火焰前沿在接近骤冷网格的过程中,向金属网格的传热逐渐超过氢气燃烧产生的化学热,最终火焰的传播被骤冷网格终止。     

Bending of micropolar plates

This paper concerns itself with the finite element formulation of micropolar plates in bending and establishes the validity of the finite element model in the form of a flat rectangular element. Results are presented to show convergence of the solution and comparisons made with the classical theory of the bending of plates.     

Preliminary Studies of Practicability of Laminar Plasma Surface Treatment Process

The basic remelting and cladding tests with laminar plasma technology on metals have been conducted in order to demonstrate the possibility of the technology applied in material surface modification. The experimental results show that the properties of the modified layers of the cast iron surface can be improved notably by the remelting treatment and those of the stainless steel by the cladding treatment. The related results are also verified by microscopic studies such as scanning electron microscopic （SEM） observations, energy dispersive spectra （EDS） analysis and the Vickers hardness measurements of the surface modified layers.     

On the effect of boundary conditions on finite deflections of heated annuli exhibiting temperature-dependent properties

The present study concerns nonlinear deflection analyses of axisymmetrically heated annular plates for several boundary conditions at the outer radius. Temperature-dependent mechanical and thermal properties of plates are considered within the frame of orthotropic elasticity theory. It is pointed out by numerical calculations that the temperature-dependent effect on finite deflections differs markedly for distinct boundary conditions.     

Stability of the LIPAc beam dump to vibrations induced by the cooling flow

The beam dump of the LIPAc consists of a long and thin copper cone where the ions are stopped, cooled by water flowing along its outer surface. The high water velocity (between 4 and 8 m/s) and the turbulent regimes involved may compromise the mechanical stability of the slender beam dump structure and cause damages due to flow induced vibrations. Since the system is too complex to be studied theoretically, some tests have been carried out to evaluate its behavior in normal operating conditions. These tests, performed on a model built at 1:1 scale and replicating exactly the final version, have been focused on finding experimentally the main vibration modes and the responses to the different working flow rates. This modal analysis, together with the results of the measurements of the vibration characteristics obtained at several positions of the cone, is presented here. With amplitudes not greater than 500 mg rms in any case, the structure has proved its practical immunity to flow induced vibrations, thus validating its design and construction methodology.