Investigation of seal effects according to axial compression variation of metal seals for transport and storage casks

Abstract

Casks for the transport and storage of heat generating radioactive waste in Germany are normally provided with screwed lid systems, which are in most cases equipped with double jacket metal seals with an inner spring wire to provide long term resistance to the seal compression force. Preservation of the high sealing quality of those seals under operational and accidental stress conditions is essentially important to the safety of those casks. Relative displacements of the lid system surfaces caused by specific impact scenarios cannot be excluded and have to be evaluated with respect to a possible increase in the leakage rate.

To get representative data for such metal sealed lid systems, BAM has developed a special conceptualised flange system placed in an appropriate testing machine for relevant mechanical loading of the metal seals under static and cyclic conditions. Furthermore, the flange system enables continuous measurement of the standard helium leakage rate during each test.

The primary aim of the investigation is to identify the correlation between variation of installation conditions (axial displacements) caused by external loads and the standard helium leakage rate. An essential parameter in this case is the useable resilience r_u of a metal seal under relevant stress conditions. The useable resilience r_u is the vertical difference in the cross-section between the seal's assembling status and the point where the leakage rate, by means of external load relieving, exceeds the quality criterion of 10^–8 Pa m³ s^–1. Load relieving can instantly occur due to modification of the seal groove dimension caused by accident impacts and deformation of the lid system. Furthermore, component specific basis data for the development of finite element calculation models should be collected. In the tests, seals are subjected to static and cyclic loads. All tests are performed at ambient temperature.

This paper presents the test configuration, different test series and results of the current experiments. Typical load–displacement–leakage rate correlations are presented and discussed.     

The optimization of longitudinal convective fins with internal heat generation

The solution of the optimization problem for longitudinal convective fins of constant thickness, triangular or parabolic profile, and uniform internal heat generation, is presented. The cases considered are those of a given heat generation density, total heat generation, and heat generation per unit width of the fin, when either the heat dissipation or the width of the fin is prescribed. The results are set forth in a nondimensional form, which are presented graphically. The effect of the fin's thermal conductivity upon the optimum dimensions is discussed, and limiting values for the heat generation and the heat dissipation, which may be imposed on the fin for a feasible optimization, are also obtained.     

Verification by calculations of shock-like loads on transport and storage casks

In this paper, criteria for the verification of numerical results for type B tests are proposed. Furthermore, some well-known commercially available FEM codes are investigated by analysing benchmark problems especially designed for transport packaging and by analysing drop tests experimentally performed on an original transport and storage cask. The results are checked by the proposed verification criteria and are found to be reliable.     

Experimental investigation of heat transfer from a circular pipe to lead coolant with controlled oxygen content

The results of experimental investigations of heat transfer from a circular pipe to lead coolant with the oxygen content being controlled and monitored are presented. The heat-transfer investigations are conducted for Peclet numbers 800–3550, Prandtl numbers 0.0123–0.0211, and Reynolds numbers 40,000–190,000 with specific heat flux ~40 kW/m ² and thermodynamically active oxygen content in lead 10^-7 –10⁰ . The experimental dependences of the Nusselt numers on the Prandtl numbers with different oxygen content in the lead coolant are obtained.Translated from Atomnaya Énergiya, Vol. 97, No. 5, pp. 345–349, November, 2004.     

矩形通道干涸点传热特性试验研究

在中国核动力研究设计院流动传热基础试验平台上进行了矩形通道干涸点传热试验。通过对各种热工水力参数的试验研究,得出结论:(1)随着进口含汽率的增加,干涸点热流密度减小,含汽率增加,壁面温度降低,传热系数减小;(2)随着质量流速的增大,干涸点热流密度增大,含汽率减小,壁面温度升高,传热系数增大;(3)随着系统压力的升高,干涸点热流密度增大,含汽率增加,壁面温度升高,传热系数增大。由试验数据与现有经验关系式的比较,发现这些关系式适合中高压、中低质量流速工况,而对低压、高质量流速工况存在较大的偏差。在古塔杰拉奇关系式的基础上,引入矩形通道尺寸和进口焓等影响传热的因素,得出了适用于矩形通道的干涸关系式。关系式与试验数据吻合良好。     

窄环隙内强迫对流换热特性的实验研究

对窄环隙内强迫对流换热特性进行了实验研究。实验结果表明,在紊流区窄环隙可以强化换热;当Re<150时,发生传热恶化。对窄环隙内加热流体和冷却流体强迫对流换热特性进行分析,可以为窄环隙内对流换热的进一步研究提供基础。     

BWR spent fuel transport and storage with TN9/4 and TN24BH casks

Abstract

In 2001 the Swiss nuclear utilities started to store spent fuel in dry metallic dual purpose casks at ZWILAG, the Swiss interim storage facility. BKW FMB Energy Ltd, as the owner of the Mühleberg nuclear power plant, is involved in this process and has selected to store the spent fuel in a new high capacity dual purpose cask, the TN24BH. For the transport Cogema Logistics has developed a new medium size cask, the TN9/4, to replace the NTL9 cask, which has performed numerous shipments of BWR spent fuel in past decades. Licensed by the IAEA 1996, the TN9/4 is a 40 t transport cask, for seven BWR high burnup spent fuel assemblies. The spent fuel assemblies can be transferred to the ZWILAG hot cell in the TN24BH cask. These casks were first used in 2003. Ten TN9/4 shipments were made, and one TN24BH was loaded. After a brief presentation of the operational aspects, the paper will focus on the TN24BH high capacity dual purpose cask and the TN9/4 transport cask and describe in detail their characteristics and possibilities.     

Internal cask content collisions during drop test of transport casks for radioactive materials

Abstract

In transport casks for radioactive materials, significantly large axial and radial gaps between cask and internal content are often present because of certain specific geometrical dimensions of the content (e.g. spent fuel elements) or thermal reasons. The possibility of inner relative movement between content and cask will increase if the content is not fixed. During drop testing, these movements can lead to internal cask content collisions, causing significantly high loads on the cask components and the content itself. Especially in vertical drop test orientations onto a lid side of the cask, an internal collision induced by a delayed impact of the content onto the inner side of the lid can cause high stress peaks in the lid and the lid bolts with the risk of component failure as well as impairment of the leak tightness of the closure system. This paper reflects causes and effects of the phenomenon of internal impact on the basis of experimental results obtained from instrumented drop tests with transport casks and on the basis of analytical approaches. Furthermore, the paper concludes the importance of consideration of possible cask content collisions in the safety analysis of transport casks for radioactive materials under accident conditions of transport.     

Experimental study of heat transfer enhancement in narrow rectangular channel with longitudinal vortex generators

In order to enhance heat transfer in cooling channels of plate-type fuel elements in reactor cores, the experimental research is conducted on the heat transfer and pressure drop in horizontal narrow rectangular channels with mounted longitudinal vortex generators (LVGs) for water flow with Prandtl number Pr = 4–5. The parameters examined were: flow velocity from 0.5 to 3.4 m/s, Reynolds number from 3000 to 20,000, heat flux 43.6 kW/m², maximum system pressure 1.3 atm, and viscosity ratio from 1.05 to 1.2. It is found that the LVGs could greatly improve the heat transfer rate by 10–45%. Thermal performance is compared under three constraints, i.e., identical mass flow rate (IMF), identical pressure drop (IPD) and identical pumping power (IPP). It is found that the heat transfer performance of channel with LVGs on two sides are better than those on one side. Application of LVGs to plate-type fuel element is a potential technique for next generation advanced nuclear reactors concepts.     

Experimental investigations on single-phase heat transfer enhancement with longitudinal vortices in narrow rectangular channel

Four pairs of rectangular block as longitudinal vortex generators (LVG) were mounted periodically in a narrow rectangular channel to investigate fluid flow and convective heat transfer respectively in the narrow rectangular channel with LVG and without LVG. Both the channels have the same narrow gap (d) = 3 mm, the same hydraulic diameter (D_h) = 5.58 mm and the same length to diameter ratio (L/D_h) = 80.65. The experiments were performed with the channels oriented uprightly and uniform heat fluxes applied at the one side of the heating plate and single-phase water was used as test fluid. The parameters that were varied during the experiments included the mass flow rate, inlet liquid temperature, system pressure, and heat flux.In each of the experiments conducted, the temperature of both the liquid and the wall was measured at various locations along the flow direction. Based on the measured temperatures and the overall energy balance across the test section, the heat transfer coefficients for single-phase forced convection have been calculated. At the same time, in these experiments, the single-phase pressure drop across the channels was also measured. The correlations have been developed for mean Nusselt numbers and friction factors. Additionally, the visual experiments of infrared thermo-image recording the temperature on the outer wall of the heating plate have been conducted for validating the effects of LV.In these experimental investigations, both laminar regime and turbulent regime were under the thermo-hydraulic developing conditions, laminar-to-turbulent transition occurred in advance with the help of LV when Reynolds numbers vary between 310 and 4220. In laminar regime, LV causes heat transfer enhancement of about 100.9% and flow resistance increase of only 11.4%. And in turbulent regime, LV causes heat transfer enhancement of above 87.1% and flow resistance increase of 100.3%. As a result, LV can obviously enhance heat transfer of single-phase water, and increase flow resistance mildly.     

Experimental study on convective boiling heat transfer in narrow-gap annulus tubes

Since convective boiling or highly subcooled single-phase forced convection in micro-channels is an effective cooling mechanism with a wide range of applications, more experimental and theoretical studies are required to explain and verify the forced convection heat transfer phenomenon in narrow channels. In this experimental study, we model the convective boiling behavior of water with low latent heat substance Freon 113 (R-113), with the purpose of saving power consumption and visualizing experiments. Both heat transfer and pressure drop characteristics were measured in subcooled and saturated concentric narrow gap forced convection boiling. Data were obtained to qualitatively identify the effects of gap size, pressure, flow rate and wall superheat on boiling regimes and the transition between various regimes. Some significant differences from unconfined forced convection boiling were found, and also, the flow patterns in narrow vertical annulus tubes have been studied quantitatively.     

Experimental investigation on narrow gap heat transfer with porous media under downward-facing horizontal heated surface

An experimental study of horizontal narrow gap heat transfer of porous media under a round downward-facing heated plate has been carried out, using water as the working fluid. The boiling curves are obtained with different gap size, plate diameter and solid spherules. The results show that the heat transfer increases significantly with porous media in the gap especially under boiling condition, and the occurrence of pool boiling crisis would be brought forward when the gap size is very small. The results also indicate that the heat transfer in horizontal narrow gap can be enhanced by increasing the ratio of gap size to plate diameter and using porous media with high thermal conductivity. Based on the mechanism of heat transfer of porous media, the correlations for natural convection and nucleate boiling are proposed to predict the heat flux.     

Experimental study on flow pattern and heat transfer of inverted annular flow

Experimental results are presented on flow pattern and heat transfer in the regions from inverted annular flow to dispersed flow in a vertical tube using freon R-113 as a working fluid at atmospheric pressure to discuss the correspondence between them. Axial distributions of heat transfer coefficient are measured and flow patterns are observed. The heat transfer characteristics are divided into three regions and a heat transfer characteristics map is proposed. The flow pattern changes from inverted annular flow (IAF) to dispersed flow (DF) through inverted slug flow (ISF) for lower inlet velocities and through agitated inverted annular flow (AIAF) for higher inlet velocities. A flow pattern map is obtained which corresponds well with the heat transfer characteristic map.     

Thermal hydraulic analysis compared with tests of full-scale concrete casks

To develop a thermal analysis method for the concrete cask, numerical calculation based on thermal hydraulic phenomena was performed. In the present calculation model, calculation area was divided into two parts. One is inside of the canister and the other is outside of the canister. These two parts were combined at the surface of the canister. In the model of the outside, k– turbulence model was adopted for air flow region. Comparing calculation results with test results, it was found that the analysis method was valid for normal and accident conditions of the storage.     

Inverted-annular film boiling heat transfer from vertical surfaces

A theoretical model is developed which allows a consistent treatment of the wall heat flux in an inverted-annular film boiling regime. The model is based on an analytical solution of the energy conservation equations in the vapor and liquid regions downstream of the quench front. A constant vapor film thickness is assumed, which depends on the fluid equilibrium quality at the quench front. Good agreement is obtained in comparing the model with quasi-steady state experimental results.