Optimization of material and production to develop fluoroelastomer inflatable seals for sodium cooled fast breeder reactor

The feasibility of producing thin-walled fluoroelastomer profiles under continuous, atmospheric-pressure vulcanization conditions in air has been demonstrated by successful manufacture of ∼2 m diameter test inflatable seals for the 500 MWe, Prototype Fast Breeder Reactor (PFBR) using a 50/50 blend formulation of Viton^® GBL-200S/600S based on advanced polymer architecture (APA). A commercial cold feed screw extruder with 90 mm diameter screw was used along with continuous cure by microwave (2.45 GHz) and hot air heating (190 °C) at a line speed of 1 m/min to produce the seals. The blend formulation promises significant improvement in the performance and safety of the seals. This article depicts the relevant characteristics of the original inflatable seal compound that was used as reference to achieve the objectives through synchronized optimization of material and production technologies. The production trials are outlined and the blend formulation used with minor factory modifications to produce the test seals is reported. Progressive refinements of the original, Viton^® A-401C based compound to the blend formulation is presented along with an assessment of potential performance gains. Possible uses of the reported formulation and production technique for other large diameter, high temperature seals of PFBR are indicated along with the envisaged activities en-route the production of perfected reactor inflatable seals.     

Temperature tests on package lid seal materials over one year duration

Abstract

The design of lid seal system of any package transporting irradiated nuclear fuel is a major aspect of the containment safety case under normal and accident conditions. Consequently, when BNFL, now known as British Nuclear Group Sellafield, BN-GS, decided on a change in lid seal material, year long proving trials on the new material were conducted, these simulating actual service and accident temperature conditions. Several years ago, BN-GS with the Rubber and Plastic Research Association (RAPRA) developed an elastomer called EPDM 30H which could perform at temperatures below ?40°C and had excellent resistance to radiation, as later proven by long term testing. EPDM 30H material has been used in many site applications, with its first transport package application being on the Excellox 6 type, commissioned in 1991. Towards the end of the 1990s, BN-GS became aware that the fluorocarbon grade of lid seal material, used on several other packages, may be discontinued by the manufacturers and so they took the decision to replace the seals on the NTL 11 type packages with EPDM 30H. Consequently, BN-GS embarked on an extensive programme of seal testing which went beyond any it had previously carried out. Comprehensive data were available on radiation resistance and performance at low temperatures, but additional data were needed on its behaviour at elevated temperature over periods of about one year. A number of test sets were assembled comprising seals in representative lid seal grooves. Most of the test sets were fitted with EPDM 30H seals but another grade of EPDM seal was also included. Two test sets were continuously maintained at temperatures of 120 and 150°C, for one year. Other test sets were maintained at 60, 90, 120, 140, 160 and 180°C, but periodic inspection and compression set measurements were taken. At the end of one year the continuously heated seals were subjected to a thermal cycle corresponding to the thermal accident safety cases studies. These tests demonstrated that the EPDM 30H material had lower compression set characteristics than the other grade of EPDM and that EPDM 30H was a suitable seal material for all irradiated LWR fuel transport packages currently operated BN-GS.     

First tests results for determination of seal life of EPDM O-rings at high temperature (determined by unique method)

Abstract

A recurrent concern in the design of packaging for the transportation of radioactive material is to determine the life of elastomeric O-rings at a high temperature. Following a precedent paper presenting the TN International method for the determination of ethylene propylene diene (EPDM) O-rings seal life, a first series of tests results have been obtained. These first results are used with the previously mentioned method to see that the EPDM seal life did follow an Arrhenius curve. This Arrhenian behaviour is verified below a certain 'threshold temperature' that seems to be the upper limit for the use of such elastomeric O-ring grades. With the 'seal life versus temperature' curve obtained from these tests results, the time–temperature profile of O-rings in casks can then be benchmarked with the seal resistance and its 'maximum permissible damage rate'.     

Leakage behavior of inflatable seals subject to severe accident conditions

The paper describes tests to determine the leakage behavior of inflatable seals when subjected to containment pressures that exceed the design basis.² Inflatable seals are used to prevent leakage around personnel and escape lock doors in about 10% of the commercial nuclear power plant containment structures in the United States. All of the installations are in either Pressurized Water Reactor (PWR) or Boiling Water Reactor (BWR) Mark-Ill type containments. This work is a part of an overall effort at Sandia National Laboratories to develop proven techniques for evaluating the performance of Light Water Reactor (LWR) containment buildings for beyond design basis loadings.Inflatable seals were tested at both room temperature and at elevated temperatures representative of postulated severe accident conditions. Parameters that were monitored and recorded during each test were the internal seal pressure and temperature, chamber (containment) pressure, leakage past the seals, and temperature of the test chamber and fixture to which the seals were attached. An empirically based, analytical method is presented to predict the containment pressure at which significant leakage past inflatable seals can be expected.     

TN FLEX: a new generation of fluorocarbon O-rings developed for TN International casks with enhanced characteristics at low temperature (−40°C)

Abstract

Three main types of elastomers are used for the sealing of radioactive material transport casks with elastomeric gaskets: ethylene propylene diene monomer (EPDM), fluorocarbons type Viton^® (standard designation: FKM) and silicon rubbers. Each rubber has specific characteristics in terms of temperature range, permeability, coefficient of expansion, etc. For the casks where high temperatures can be reached (200°C in continuous using), FKM gaskets are generally used. The problem is that this type of gasket does not guarantee the leak tightness at ?40°C, which is a regulatory requirement. Two solutions are generally used to specify a minimum heat load or a minimum ambient temperature. The direct consequence is that it is impossible to get B(U) approvals on the new concepts when FKM gaskets are used but only B(M) approvals, which generate significant additional justification costs (multiple submittals of safety analysis reports, calculation of the minimum heat load or of the minimum ambient temperature, etc.). Therefore, it is important to develop gaskets with the same performance as FKM gaskets at high temperature but with enhanced performance at low temperature, which guarantee the leak tightness at ?40°C. TN International has qualified a new generation of fluorocarbon O-rings (TN^? FLEX gaskets) which can be used in continuous service within a temperature range between ?47 and +200°C. TN FLEX gaskets will be implemented on new casks designs.     

Development,computer simulation and performance testing in sodium of an eddy current flowmeter

Sodium is used as a coolant in Liquid Metal Fast Breeder Reactor (LMFBR). Sodium flow measurement is of prime importance both from the operational and safety aspects of a fast reactor. Various types of flowmeters namely permanent magnet, saddle type and eddy current flowmeters are used in FBRs. From the safety point of view flow through the core should be assured under all operating conditions. This requires a flow sensor which can withstand the high temperature sodium environment and can meet the dimensional constraints and be amenable to maintenance. Eddy current flowmeter (ECFM) is one such device which meets these requirements. It is meant for measuring flow in PFBR primary pump and also at the outlets of the fuel sub-assemblies to detect flow blockage. A simulation model of ECFM was made and output of ECFM was predicted for various flowrates and temperatures. The simulation model was validated by testing in a sodium loop. This paper deals with the design, simulation and tests conducted in sodium for the eddy current flowmeter for use in the Prototype Fast Breeder Reactor (PFBR).     

Testing of Elastomer Seals using Small-Size Rigs

Abstract

This paper looks at the use of small size seal leakage test rigs to demonstrate the compliance of full size container seals against the IAEA Transport Regulation's limits for activity release for normal transport and accident conditions. The detailed requirements of the regulations are discussed and it is concluded that an appropriate test programme to meet these requirements, using only small size test rigs, can normally be set up and carried out on a relatively short time scale. It is important that any small test rigs should be designed to represent the relevant features of the seal arrangement and the overall test programme should cover all of the conditions, specified by the regulations, for the type, classification and contents of the container under consideration. The parameters of elastomer O-rings, which affect their sealing ability, are considered and those which are amenable to small scale testing or have to be modelled at full size are identified. Generally, the seals used in leakage tests have to be modelled with a full size cross-section but can have a reduced peripheral length.     

Ageing of metallic gaskets for spent fuel casks: Century-long life forecast from 25,000-h-long experiments

An experimental programme is being carried out that aims at quantifying the relaxation of four types of metallic HELICOFLEX^® seals during their use in spent nuclear fuel storage casks. Two types of lining are taken into account: aluminium and silver. Tests longer than 10,000 h are implemented only for silver. For each type of lining, two different section diameters are investigated. The work aims at evaluating the minimum residual linear load that can be guaranteed for a seal after a particular time of relaxation. This relaxation depends on the evolution of the seal temperature with time. Therefore, holds of seals tightened between two flanges have been performed at several constant temperatures, including 100 and 200 °C. Residual load and ‘useful’ recovery have been measured after the holds. Results are interpreted according to two methods: a time extrapolation, and a time–temperature equivalence parameter. Both methods are based on linear relationships and are assessed through a statistical analysis (calculation of scatter) which is also used to determine a minimum guaranteed residual load. Finite element simulations of the relaxation of a seal have also been performed in order to justify qualitatively that the time extrapolation method is safe. For silver lining seals, the use of a time–temperature equivalence parameter equal to T (11 + log₁₀ (t)) appears justified and this enables us to assess the maximum temperature at which seals can be ‘safely’ used ‘up to a century’.Using the available ageing results (longest holds: 25,000 h), and the proposed prediction method, it can be proven that the two types of silver lining seals which are evaluated will retain a residual linear load of at least 100 N mm⁻¹ of seal perimeter after one century of use in a cask, if the initial temperature of the seal after closing the cask is less than or equal to 100 °C.     

国际核保障的封记系统及其薄弱性评价

封记,又叫篡改指示装置,已经广泛地应用于国际核保障体系和核安全监督管理,用来核实保护目标是否发生篡改或被未经授权的获取。封记实施计划、封记研制及其封记薄弱性评价、封记有效实施程序与方法 3方面内容一起构成封记的控制系统。本文针对封记信息化管理技术的发展需要,结合封记有效实施的任务分工、封记实物和数据信息管理等环节,明确提出了封记控制系统有效实施的技术要求。文中以热收缩膜封记的控制系统管理为例,对封记薄弱性的评价方法进行系统地归纳总结。封记系统的研究必然有效地促进封记在核安全保卫、核武器不扩散、放射性废物管理以及核材料衡算与控制等领域的应用。     

Comparison of FBTR hybrid core and Mark-II (carbide) core design and neutron irradiation in invessel shields

Fast Breeder Test Reactor (FBTR) in Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam, India is presently operating with a combination of Mark-I (PuC 70% and UC 30%) and Mark-II (PuC 55% and UC 45%) fuels. Recently it became necessary to review the type of fuel for the FBTR core due to the use of oxide fuel for the PFBR under construction at Kalpakkam. A full Mark-II core was earlier designed to take the reactor to higher power. An alternate strategy of having hybrid core with Mark-I fuel at the centre and MOX fuel at the periphery was also studied. MOX subassemblies have a fissile column length of 43 cm which is higher than Mark-I subassemblies (32 cm). The lower position of these subassemblies is closer to the grid plate and therefore there was a concern on the grid plate fluence below the MOX fuel. It is interesting to compare the dose rates to the grid plate in the two above design as the doses determine reactor life. Many 2-D R-Z transport calculations were carried out for FBTR including all regions of interest like borated graphite and carbon steel in the top shield and vault concrete to obtain 175 group neutron flux distribution in Vitamin J structure with IGC-S3 self shielded cross-section set. The radiation damage to grid plate below MOX part is found to be less than that below Mark-I part of hybrid core in spite of longer fissile column length. Similar behaviour is seen for helium production as well. Comparison showed that the difference in fluence in the grid plate region is less than 3% between the two core designs.     

Radioactive Material Package Seal Tests

Abstract

General design or test performance requirements for radioactive materials (RAM) packages are specified in Title 10 of the United States (US) Code of Federal Regulations Part 71. The requirements for Type B packages provide a broad range of environments under which the system must contain the RAM without posing a threat to health or property. Seals that provide the containment system interface between the packaging body and the closure must function in both high and low temperature environments under dynamic and static conditions. A seal technology programme, jointly funded by the US Department of Energy Office of Environmental Restoration and Waste Management and the Office of Civilian Radioactive Waste Management (OCRWM), was initiated at Sandia National Laboratories. Experiments were performed in this programme to characterise the behaviour of several static seal materials at low temperatures. Helium leak tests on face seals were used to compare the materials. Materials tested include butyl, neoprene, ethylene propylene, fluorosilicone, silicone, Eypel, Kalrez, Teflon, fluorocarbon, and Teflon/silicone composites. Because most elastomer O-ring applications are for hydraulic systems, manufacturer's low temperature ratings are based on methods that simulate this use. The seal materials tested in this programme with a fixture similar to a RAM cask closure, with the exception of silicone S613–6O, are not leak tight (1.0 × 10–7 std cm³.s^?1) at manufacturer's low temperature ratings.     

Packaging and security of radioactive material

Abstract

Elastomers are widely used as the main sealing materials for containers for low and intermediate level radioactive waste and as an additional component to metal seals in spent fuel and high active waste containers. The safe encapsulation of the radioactive container inventory has to be guaranteed according to regulation and appropriate guidelines for long term storage periods as well as for temperatures as low as ?40°C during transport. Therefore, the understanding of failure mechanisms that lead to leakage at low temperatures is of high importance. It is known that the material properties of elastomers are strongly temperature dependent. At low temperatures, this is caused by the rubber–glass transition (abbreviated: glass transition). During continuous cooling, the material changes from a rubber-like entropy elastic to a stiff energy elastic behaviour, which allows nearly no strain or retraction. Hence, rubbers are normally used above their glass transition, but the minimum working temperature limit is not defined precisely; this can cause problems during the above noted applications. Therefore, the lower operation temperature limit of elastomer seals must be determined in dependence of the material properties. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) are combined with standardised measurements to determine the compression set according to ISO 815. To reduce the test time of the standard tests, a faster technique than normally used was developed. Additionally, the breakdown temperature of the sealing function of complete O ring seals was measured in a component test set-up to compare it with the results of the other tests. The experimental set-up is capable of measuring the leakage rate at low temperatures by the pressure rise method. A fluorocarbon rubber (FKM) was selected for this investigation as it is often used for radioactive waste containers. The materials (seals and test sheets) were purchased from a commercial seal producer.     

An overview of polymer ageing studies in the nuclear power industry

Polymeric components are widely used in nuclear power plants (NPPs) in equipment which is important to the safety of the plant. The degradation of such components is therefore of considerable interest to the industry and its regulatory bodies, generating a large number of studies worldwide. Some of these components need to remain functional over the full operational life of the plant, which may span up to 60 years. Predictive modelling of their behaviour is therefore of key importance. This paper outlines the main areas of research, particularly relating to the use of elastomeric seals and polymeric cable insulation in NPP.     

流体动压型核主泵机械密封腔内流动与传热研究

为研究某型号流体动压型核主泵机械密封流场和温度场的分布规律,使用Pro/E软件建立了机械密封环及密封腔的三维实体模型。采用k-ε湍流模型,基于ANSYS Fluent软件求解了纳维-斯托克斯(N-S)方程和能量方程。研究了密封环生热与密封腔散热的规律。分析了流体流动与温度变化趋势。结果表明:该型核主泵机械密封的压力以密封端面为界,分为高压区和低压区。在密封端面液膜压力由外径到内径逐渐降低。最高温度出现在密封端面处,由密封端面向外温度逐渐降低。液膜粘性剪切热通过密封环的热传导及腔内流体的对流换热作用而带走。机械密封的泵送环强化了端面热量的散失。     

The 'Anti-Evidence' Approach to Tamper Detection

Abstract

Tamper-indicating seals are widely used for the transport, packaging and storage of nuclear material. Most existing seals operate under the same basic principle: once the seal is opened, information that the seal has indeed been unsealed is stored in or on the seal until such time as the seal can be inspected. This stored evidence of tampering, however, is often easy to hide or erase. A better approach, discussed theoretically in this paper, is to store information when the seal is first installed that tampering has not been detected; this anti-evidence is then erased once the seal is opened. Such anti-evidence seals may provide better security. They also have a number of potentially useful attributes, including an intrinsic check on whether the seal was actually inspected for evidence of tampering ('anti-gundecking').     

Plasma Modified Polypropylene Membranes as the Lithium-Ion Battery Separators

To reduce the thermal shrinkage of the polymeric separators and improve the safety of the Li-ion batteries,plasma treatment and plasma enhanced vapor chemical deposition（PECVD）of SiO_x-like are carried out on polypropylene（PP）separators,respectively.Critical parameters for separator properties,such as the thermal shrinkage rate,porosity,wettability,and mechanical strength,are evaluated on the plasma treated PP membranes.O₂ plasma treatment is found to remarkably improve the wettability,porosity and electrolyte uptake.PECVD SiO_x-like coatings are found to be able to effectively reduce the thermal shrinkage rate of the membranes and increase the ionic conductivity.The electrolyte-philicity of the Si Ox-like coating surface can be tuned by the varying O₂ content in the gas mixture during the deposition.Though still acceptable,the mechanical strength is reduced after PECVD,which is due to the plasma etching.     

Study of the Accident Environment During Sea Transport of Nuclear Material: Analysis of An Engine-Room Fire on a Purpose-Built Ship

Abstract

The programme goal was to show that the IAEA safe transport regulations adequately cover the thermal effects of an engine-room fire on plutonium transport packages stowed aboard a purpose-built ship. The packages are stored in transport containers located in a cargo hold of the ship. For this study, it was assumed that the packages in No 5 Hold, adjacent to an engine-room, could be subject to heating due to a fire in the engine-room. The No 5 hold and the engine-room are separated by a water-filled bulkhead. This study addressed the heat transfer from an engine-room fire that could heat and evaporate water out of the water-filled bulkhead and the resulting temperature conditions around the packages and inside the packages near their elastomeric seals.     

流体动压型核主泵机械密封腔内流动与传热研究

To study the distribution of the flow and temperature field in a type of hydrodynamic mechanical seal of a nuclear reactor coolant pump, a three-dimensional model of the mechanical seal and the seal chamber is established based on the software Pro/E. The N-S equations and energy equation coupling with the k-ε turbulence model are solved based on ANSYS Fluent. The heat generation between the mechanical seal rings and the heat transfer distribution in the sealing chamber is studied. The fluid flow and temperature field of the mechanical seals is analyzed. The results show that the pressure distribution of the mechanical seals is divided into the high-pressure zone and the low-pressure zone by the sealing end face. The liquid film pressure in the mechanical seal end faces gradually decreases from the outer radius to the inner radius. The highest temperature appears at the sealing face, and the temperature decreases gradually away from the sealing face. The viscous heat in the liquid film is transferred away through the seal rings and the heat convection of the fluid flow in the sealing chamber. The pumping ring of the mechanical seal strengths the heat convection of the end faces.     

Pressure drop and cavitation investigations on static helical-grooved square, triangular and curved cavity liquid labyrinth seals

Sodium cooled Fast Breeder Reactors (FBR) form the second stage of India's Nuclear power programme. Through a narrow annular space in the grid plate assembly of a prototype FBR, a very low leakage flow of liquid metal sodium should pass, experiencing a stipulated high pressure drop, and without much cavitation. To achieve this, a suitable labyrinth seal is required to be developed for use in the annulus. Water is employed as the model testing liquid which is estimated to experience a pressure drop ratio of 10.5 at the rated leakage flow. Previously studied circular or sinusoidal-grooved square, triangular or curved cavity labyrinth seals were unable to meet this value. In the present work, Computational Fluid Dynamics (CFD) analyses are carried out on Helical-grooved Square cavity Labyrinth Seals (HSLS), using commercial code Fluent. It is found that the geometrical configuration of the grooves plays a major role on the pressure drop. Experimental results reveal close agreement with CFD predictions. An optimal configuration of this square cavity seal is identified by applying genetic algorithm (GA) using commercial packages. It meets just about 24% of the targeted value. Later, using parametric CFD analyses, a Helical-grooved Triangular cavity Labyrinth Seal (HTLS) and different Helical-grooved Curved cavity Labyrinth Seals (HCLS) are analysed. The most favourable profile is tested and found to reach the required pressure drop. CFD cavitation analyses predict the intensity of cavitation in these seals to be below prohibitive levels.