Power plant design | Seals

PFBR backup seal development

20 June 2011

The design, material and procedures used in the development of a fluoroelastomer backup seal for the rotatable plugs of the 500 MWe Prototype Fast Breeder Reactor could be extended to all the elastomeric sealing applications of Indian nuclear reactors, resulting in considerable gains in development, operation and safety. By N. K. Sinha and Baldev Raj

The reactor assembly of the 500 MWe, sodium-cooled, Prototype Fast Breeder Reactor (PFBR) presently under construction at Kalpakkam requires elastomeric sealing applications to act as static and dynamic leak-resistant barriers during normal operation (NO), fuel handling (FH), accident and shutdown conditions. In addition, they also absorb the possible consequences of postulated accidents.

The seals, designed for a minimum life of ten years, are used in pairs to prevent the leakage of radioactive argon cover gas to the reactor containment building (RCB) atmosphere, and the ingress of RCB air into the cover gas space of main vessel. The internal space between each pair is provided with fresh argon gas at a pressure higher than the cover gas and RCB air pressure. The combination of fluoroelastomer inflatable and backup seals, used as primary and secondary leak-resistant barriers in the annular spaces of PFBR rotatable plugs (RPs), represents the most demanding polymeric application in the reactor assembly, and probably in the entire PFBR. The sealing system is classified as Safety Class I and Seismic Category I, safe shutdown earthquake (SSE) design.

The backup seal, held in a trapezoidal groove of carbon steel seal holder, is lowered by a suitable drive mechanism to engage with the carbon steel top faces (roughness: 3.2 µm) of the top and middle ring (TMR) and the bearing support ring (BSR), shown in Fig. 1. It seals an annular gap of 5 +/- 2 mm between the rings (considering a maximum level mismatch of 2 mm between the top faces) during NO along with the lower inflatable seal in each plug. The large rotatable plug (LRP), supported by the roof slab (RS), and the small rotatable plug (SRP), supported by the LRP, are located eccentric to each other and are mounted directly on bearings to enable intermittent oscillation during FH at a maximum speed of 75 mm/s. A pair of inflatable seals provides sealing of the annular space in each plug during FH as the backup seal is disengaged. Large diameter fluoroelastomer backup seals (~6.4 m for LRP; ~4.2 m for SRP), based on Viton A401C and manufactured by screw extrusion as well as autoclave cure, have been developed to withstand continuous exposure of temperature of 110°C, differential pressure of 25 kPa and ? dose rate of 23 mGy/h in the presence of RCB air, argon, Xe and Kr assuming a possible occurrence of a maximum differential pressure of 2 MPa and earthquake-induced structural deflections at any point during seal design life.

The seals have been installed in the PFBR recently based on validatory clearance of the seal design in a scaled-down test rig where seal compressive load and leakage were measured on 1m diameter test seals under simulated service conditions and were found to be within the stipulated limits of 5 kN/m length of seal circumference and 10-3 scc/s/m length of seal circumference respectively under a maximum seal squeeze of 6mm.

The development has provided an original solution to a specific problem of a critical nature through systematic exploitation of the advanced faculties of rubber engineering. Moreover, it has also contributed towards simplification and standardisation of design, development, production, operation and maintenance of static, elastomeric seals of Indian fast breeder, thermal and the Advanced Heavy Water Reactor (AHWR).

Conceptualisation and development

The cross-section of backup seal progressed from a solid rectangular geometry to a solid trapezoidal shape during conceptualisation to reduce the seal compressive load during engagement and avoid the possibilities of accidental fall during lifting and lowering. The thick, solid seal cross-section reduces the effects of synergistic degradation of thermal, radioactive and oxidative ageing during the design life. At the same time the design increases the seal compressive load and stress-field, but at the expense of simplicity of the plug and drive designs and the margin of safety available during incident and the postulated accidental conditions. The solid trapezoidal seal shape generates a maximum compressive load of 986kN for the LRP and 653 kN for the SRP seal during engagement under 20% squeeze based on a stiffness calculation approach using a Young’s modulus of 4 MPa and using a general fluorocarbon rubber (FKM) of 55° Shore A hardness as the seal material. The high engagement loads, coupled with the requirement of a number of parallel load application points on the seal holder for uniform sealing (40 for the LRP; 30 for the SRP) adds to the bulk and complexity of design, in addition to posing higher demands on synchronisation of load application as well as movement of the large-diameter seal holders.

The seal development programme, carried out as part of a planned initiative by Indira Gandhi Centre for Atomic Research (IGCAR) in collaboration with several other Indian agencies, employed orchestrated efforts in the fields of material development, seal sizing by finite element analysis (FEA), design optimisation, production trials in commercial extruders and validatory tests to arrive at a seal design shown in Figure 2 that minimises load and stress, maximises the factor of safety (FOS) and ensures failsafe operation under the most severe envisaged loadings at the end of seal design life [1].

The FKM formulation developed for the backup seal comprises of 100 parts per hundred parts of rubber (phr) of Viton A401C, 2 phr of medium thermal (MT) carbon black (N990), 3 phr of high activity magnesium oxide and 3 phr of calcium hydroxide. Viton A-401C is a dipolymer of hexa-fluoropropylene (HFP) and vinylidene fluoride (VDF) with nominal fluorine content of 66% by weight and specific gravity of 1.82. The compound has a hardness of 54° Shore A. Accelerated ageing studies of the formulation were carried out in air. The tensile stress Ts and elongation-at-break Eb values measured at 110°C on aged specimens show negligible variations on a minimum increase of about 10% while the hardness remains practically unchanged (considering experimental errors) during the ageing span of 32 weeks at all the three temperatures (Tables 1&2). This indicates the compound’s exceptional thermal stability [1].

The finalised seal design is a result of FEA and design optimisation of 14 geometric variations of the trapezoidal shape. Mooney-Rivlin material model, plane-strain and axisymmetric elements are employed in commercial FEA code and uniaxial tensile/compression stress-strain data, determined at 110°C with and without deformation cycling, to model and analyse the seal. A factor of safety of two is applied on the tensile strength value of the seal compound (determined at 110°C) to define the limiting principal/von Mises stresses in the seal. The FOS takes care of batch-to-batch and inter-laboratory variations of material data and accounts for the possible effects of ageing degradation coupled with the maximum accidental pressure of 2 MPa.

Figure 2 shows the deformation and Mises stress distributions in some of the geometries analysed. The trapezoidal seal design with hollow section and double O-ring contours on the sealing face is chosen because of higher wall thickness and narrower contact at seal-ring interface. This design is less sensitive to ageing effects and more robust under accidental loadings compared to the double-lip configuration in terms of contact pressure, deformation and stress, while meeting the stipulations of the compressive load during engagement [1].

Results of long-term accelerated ageing studies indicate some marginal gains in the FOS on Ts and Eb during seal operation under thermo-oxidative ageing. The cumulative ? dose of 2 kGy at seal location during design life is within the threshold that initiates damage in general elastomers and Viton rubbers (at room temperature in air) in terms of change in modulus, Ts, Eb and hardness as per the literature [2-4]. Research on combined effects of temperature, radiation and oxidation (in air) indicate that the bisphenol-cured, Viton A-401C-based fluorocarbon rubber formulation developed for the backup seal should not undergo any damage under the synergistic influence of the seal operating environment, and that the FOS should not be eroded by ageing effects during the design life [5]. Variations of compression set (CS) under accelerated ageing indicate that the seal should be failsafe during the design life, considering a failure criterion of attainment of 100% of CS.

A simple extrapolation of test results reported for bisphenol-cured VDF-HFP fluoroelastomers (similar to the Viton A-401-based compound) in product literature and other sources [6-7] indicate that the release and effects of fluorine-containing, volatile, low molecular weight products such as hydrogen fluoride (HF) during the backup seal operation is expected to be insignificant for all practical purposes.

Operating requirements of some of the important large diameter static elastomeric seals of PFBR shown in Table 3 indicate that the PFBR RP backup seal is representative of large-diameter static sealing applications of reactor assembly.

The O-ring seals are used in the chord diameter range of 20-30 mm and squeeze range of 20-30% to absorb the tolerance stack-ups involved in large diameters and to seal the machined carbon-steel/stainless-steel mating surfaces of 3.2 µm roughness by generating sufficient contact pressure and flow of seal material with the remaining squeeze. Even in the case of perfectly smooth surfaces, a minimum contact pressure higher than design pressure (DP) is required for complete sealing of by-pass leakage. The presence of roughness also demands that the micro-irregularities of the mating metallic surface be filled to the maximum extent by the flow of seal material so as to minimise the by-pass leakage through the unfilled roughness, which may require additional squeeze. The permeation leakage through seal material reduces with increasing squeeze but cannot be eliminated altogether. The low values of DP do not contribute to contact pressure or material flow.

Over time, ageing causes irreversible chemical changes in seal material (even at RT). As a result, the seal surface in contact with the mating surface assumes a permanent topography, which is a mirror image of the surface texture of the mating surface. For the same reason, the contact pressure relaxes (compression stress relaxation or CSR) and the initial squeeze imparted on the seal reduces (compression set) over time. The topography of the sealing surface ensures adequate static sealing even at 100% CS or CSR provided that the contacting surfaces are not disturbed by physical movements from thermal transients, vibrations and shock caused by impact or disengagement during service. The large-diameter elastomeric O-rings of the reactor assembly are not disturbed during design life in most cases and leakage during incidents and accidents is limited by controlling the flange movements under the postulated loadings. In the particular case of the PFBR RP backup seals, the effects of disengagement after every 240 days of NO could be compensated by additional axial movements of the seal holder with the drive mechanism. Large-diameter static elastomeric O-rings disturbed because of maintenance of other components could be replaced during that time. The tensile stress fields generated at the seal centre, in the vicinity of the free contours and at the extremities of contact should be kept within the limiting value (or Ts) to avoid cracking failures because of change in curvature of the seal cross-sectional contour induced by squeeze [8-10]. Eb at seal operating temperature should not fall far below 100% to retain the basic elastomeric functions required for sealing and also to avoid local failures as strain crosses the limit.

Extrapolating these results

Fluoroelastomers have significantly better permeation resistance than most other elastomer classes [11-12]. This resistance is necessary to minimise the permeation of fission product gases like Xe and Kr (which are possible because the PFBR is designed to operate with four failed fuel pins in the core). It also blocks possible ingress of oxygen impurities from fresh interseal argon gas into the annular and axial spaces at the cooler portion of the top shield.

Viton A-401C is supplied as a pre-compound containing accelerator and curative which is suitable for low CS applications such as O-ring seals [13-14]. The slower recovery rate of fluoroelastomers from strain, because of higher stiffness of fluorocarbon chains compared to that of the hydrocarbons, make them a natural choice for static sealing applications [15]. The effects of accelerated, thermo-oxidative ageing on Ts, Eb and CS (see also Table 2) indicate that the FKM formulation developed for the backup seal is applicable without any change for other large-diameter, static, elastomeric sealing applications of PFBR to be manufactured by extrusion.

The sodium aerosol-resistant, bisphenol-cured FKM formulation developed for the dynamic inflatable seals of PFBR RPs (suitable for screw extrusion and autoclave cure) comprise 100 phr of Viton A-401C, 20 phr of medium thermal (MT) carbon black (N990), 3 phr of high-activity magnesium oxide and 6 phr of calcium hydroxide. When measured at RT, it had a Ts of 12.28 MPa, Eb of 217% and hardness value of 68° Shore A [1]. The identical ingredients and curing system of inflatable and backup seal compounds suggest that the backup seal FKM formulation would also be resistant to sodium aerosol, which is also substantiated by the sodium vapour exposure data of Viton in a nitrogen atmosphere [16]. The maximum operating temperature and other condition of the seals (Table 3) also indicate that the release of fluorine-containing volatile products from the backup seal compound is expected to be insignificant under operating conditions of other seals. The Mooney-Rivlin constitutive model and the plane strain condition is applicable to the large-diameter, static elastomeric seals of PFBR in general considering identical FKM formulation, types of loading and boundary conditions and range of expected strain. Axisymmetric elements can be used considering possible slip between the seal and mating surfaces. Screw extrusion and steam curing in an autoclave are followed by post-curing and end-joining. End-joining involves holding the open ends of seal (cut at a 45° angle) under heat and pressure. This method is used for production of the backup seals, and is applicable for other large-diameter elastomeric seals of PFBR. This puts all the large diameter static elastomeric seals of PFBR on a single foundation of material formulation, FEA procedure and manufacturing technique developed and used for the RP backup seals. Multiple end joints are permitted in all the seals listed in Table 3 because of static operation and essentially compressive stress field at the end-joint locations.

The bisphenol-cured, Viton A-401C-based compound developed for the backup seal is also appropriate for the small-diameter, moulded, static elastomeric ring seals (mainly O-rings) of the reactor assembly, as their operating requirements do not exceed those listed in Table 3 and the compound is suitable for both extrusion and moulding. The compound provides higher safety margin and promises longer life for the moulded, static seals as the seal chord diameter and squeeze are lower (because of lower tolerance stack-ups) and the uniformity of cure as well as other properties are better compared to the large diameter seals. The suitability of Viton A-401C based inflatable seal compound for the dynamic O-ring/V-ring/Lip seal applications of PFBR reactor assembly has been reported [1]. Other grades of Viton based on advanced polymer architecture (APA) have also been established for inflatable seals based on their superior capabilities [1]. The overall elastomeric sealing applications of the PFBR reactor assembly could therefore be seen as a combination of three Viton-based formulations, one FEA approach and two manufacturing techniques involving extrusion and moulding. The relatively few components in this system simplify the design, development and operation of the seals considerably, in addition to adding safety and reliability to the sealing systems by standardising the whole process. A more detailed treatment of some of these aspects for Canadian-designed PHWRs has been given elsewhere [17].

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