Choice of rotatable plug seals for prototype fast breeder reactor: Review of historical perspectives

• Choice and arrangement of elastomeric inflatable and backup seals as primary and secondary barriers.
• With survey (mid-1930s onwards) of reactor, sealing, R&D and rubber technology.
• Load, reliability, safety, life and economy of seals and reactors are key factors.
• PFBR blends concepts and experience of MOX fuelled FBRs with original solutions.
• R&D indicates inflatable seal advanced fluoroelastomer pivotal in unifying nuclear sealing.

Choice and arrangement of elastomeric primary inflatable and secondary backup seals for the rotatable plugs (RPs) of 500 MW (e), sodium cooled, pool type, 2-loop, mixed oxide (MOX) fuelled Prototype Fast Breeder Reactor (PFBR) is depicted with review of various historical perspectives. Static and dynamic operation, largest diameters (PFBR: ∼6.4 m, ∼4.2 m), widest gaps and variations (5 ± 2 mm) and demanding operating requirements make RP openings on top shield (TS) the most difficult to seal which necessitated extensive development from 1950s to early 1990s. Liquid metal freeze seals with life equivalent to reactor prevailed as primary barrier (France, Japan, U.S.S.R.) during pre-1980s in spite of bulk, cost and complexity due to the abilities to meet zero leakage and resist core disruptive accident (CDA). Redefinition of CDA as beyond design basis accident, tolerable leakage and enhanced economisation drive during post-1980s established elastomeric inflatable seal as primary barrier excepting in U.S.S.R. (MOX fuel, freeze seal) and U.S.A. (metallic fuel). Choice of inflatable seal for PFBR RPs considers these perspectives, inherent advantages of elastomers and those of inflatable seals which maximise seal life. Choice of elastomeric backup seal as secondary barrier was governed by reliability and minimisation as well as distribution of load (temperature, radiation, mist) to maximise seal life. The compact sealing combination brings the hanging RPs at about the same elevation to reduce above-TS height and diameters substantially (vis-à-vis mid-1980s 4-loop, PFBR RP design with freeze-inflatable combination) which assures a significant step towards enhanced safety, economy and life (60 y at 85% capacity factor) of future FBRs, supported by stability maximisation, stress-minimisation and ease of critical component alignment. Closeness of inflatable seal operating requirements (25 kPa—120 °C—23 mGy/h) to TS barrier representative conditions (70 kPa—110 °C—23 mGy/h) are combined with key findings from R&D on elastomer, seal and coating of Indian FBRs to illustrate that critical elastomeric sealing of MOX fuelled FBRs, Pressurised Heavy Water Reactors and Advanced Heavy Water Reactor could be unified taking 50:50 blend formulation of Viton GBL 200S and 600S (developed for inflatable seal) as cornerstone and finite element based design as facilitator. It is further indicated that minimisation of synergistic ageing load on the fluoroelastomer inflatable-backup sealing combination by sodium dip seal could ensure 1 replacement during reactor life.