Abstract
With over 437 operational power plants, nuclear systems contribute 370705 MW(e) worldwide [1]. The Pressurized Water Reactors (PWR) constitute a two-third majority of the operational nuclear power plants while the nuclear reactors in planning and construction phases also show strong trend towards PWRs. These systems are mainly used as baseline load carriers while conventional fossil fueled systems are used for load adjustments and variations [2]. The PWRs have higher than average levels of radiation fields emanating from the corrosion and fission product activity [3] [4] [5]. This leads to prolongation of maintenance schedules entailing loss of revenues mounting to several million dollars per plant annually [6]. Consequently, the plant availability factors are also lowered. This situation is further aggravated due to strong shift of plant age profile toward over 25 years operational range. With plant aging, the fuel failures become more frequent which leads to enhancement of radiation levels in the primary circuits of PWRs. The levels of fission product activity (FPA) have been of concern both from the operational as well as from accidental perspectives. These levels are continuously monitored during the normal operation of PWRs. The fuel pins develop leakages with their burnup. When the failed fuel fraction exceeds a safe limit, replacement of defective assemblies by refueling becomes necessary. Therefore, low levels of leaked-out fission products (FPs) in primary coolant of PWRs are indicative of the core health [7]. In the accidental conditions, the total value of FPA serves as the available source term that potentially can escape into the surroundings [8] [9]. The fission products are released in the fuel matrix during burnup. They escape from the ceramic pellets into the gap between pellets and the clad regions. Hyun et al. [10] have developed an analytic method for the fuel rod gap inventory of unstable fission products during steady state operation of PWRs. The fission gas bubbles escape from grain corners and are interlinked in the open space. The release rate depends on the bubble interlinkage along with temperature and burnup. A generalized model for fission product transport in the fuel-to-sheath gap was given by Lewis [11]. Barrachin et al. presented a review of fission product behaviour in UO2 fuel [12].
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