PSI - Issue 60

Rahul Mittal et al. / Procedia Structural Integrity 60 (2024) 535–540 Rahul Mittal/ StructuralIntegrity Procedia 00 (2019) 000 – 000

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1. Introduction Snubbers are safety related devices used to restrain piping and equipment against undesirable dynamic loads e.g. seismic event, pipe rupture, water hammer, relief valve opening etc. in Nuclear Power Plants (NPPs). Snubbers are dynamic restraints, which allow thermal movement of the attached piping/equipment without offering practically any resistance during normal operation and at the same time, snubbers provide requisite rigidity to take care of dynamic loads. In a nuclear power plant, nuclear piping systems need to have adequate flexibility to accommodate thermal expansion and at the same time be rigid enough to withstand loadings arising due to dynamic events. These contradicting requirements i.e. thermal flexibility during normal operation and rigidity during dynamic event (seismic etc.) have to be met in order to meet the codal qualification requirements of nuclear piping/equipment. In order to achieve this requirement, snubbers are provided on various nuclear piping and equipment. Location of snubbers and numbers of snubbers are finalized based on the detailed Finite Element Analysis (FEA), which is an iterative process & requires considerable efforts & time. Seismic loading is generally the governing dynamic loading and is site specific. NPPs in India are designed for two levels of earthquake, Operating Basis Earthquake (OBE) and Safe Shutdown Earthquake (SSE). Required capacity of the snubbers is decided based on the magnitude of dynamic loads evaluated using FEA of the piping system for both levels of earthquakes. Snubbers used in Indian PHWRs have a typical capacity ranging from 0.5Te to 250Te. Based on their working principle, snubbers are categorized into mechanical and hydraulic snubbers. In mechanical snubber, load is transmitted entirely through mechanical components. In hydraulic snubbers, load is transmitted through a hydraulic fluid. Because of more number of moving parts and being exposed to harsh working environment viz., high temperature, radiation etc., various degradations viz., high drag force, shift in dynamic characteristics, jamming have been observed in mechanical type snubbers in some of the older Indian PHWR units. These mechanical snubbers are being replaced by new hydraulic snubbers, which are considered to be more reliable due to less number of internal components and require less maintenance during operating life. No inadvertent jamming has been observed in hydraulic snubbers installed in Indian PHWR units. Snubbers are subjected to harsh environment during operation including high temperature, radiation, vibration and corrosive condition, which may adversely affect its performance. Periodic testing of Snubbers is required to ensure the proper functioning of snubbers during normal operating condition as well as during dynamic event. A dedicated In-Service Inspection (ISI) program exists in Indian PHWRs for snubbers to ensure their healthiness/functionality. ISI of snubbers include visual examination and functional testing of snubbers. For carrying out the functional testing, dedicated snubber test facilities are available at various PHWR sites. Additional design provision for online snubber health monitoring is also introduced to ensure the proper functioning of the snubbers. 2. Snubbers in Indian PHWRs In Indian PHWR units, Snubbers are provided on various critical nuclear equipment viz., Steam Generators (SGs), Primary Circulating Pumps (PCPs), Reactor Headers, Shut Down cooling system heat exchangers (SD HXs) /Pumps and nuclear piping of Main Primary Heat Transport (PHT) system and associated systems. The function of PHT system of PHWR is to extract the heat produced in the reactor core by circulating pressurized heavy water through the reactor coolant channels. This heat is used to generate steam for power production. The PHT system layout has been evolved based on design improvements with respect to higher safety margins for different service conditions, lower operating stresses, ISI requirements etc. Indian PHWR consists of 220 MWe, 540 MWe and 700 MWe units. Main PHT system of 220MWe Indian PHWRs has only single loop with four SGs, four PCPs, two Reactor Outlet Headers (ROH) and two Reactor Inlet Headers (RIH) along with feeder pipes connecting coolant channel assemblies to the reactor headers. The PHT system of 540/700 MWe Indian PHWR is divided in two independent and identical closed loops, each comprising of two SGs, two PCPs, two Reactor Outlet Headers (ROH) and two Reactor Inlet Headers (RIH) along