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[en] The paper reports a computational study of the effects of distortions of the moderator cooling passages of an Advanced Gas-cooled Reactor (AGR) on the temperature of the graphite core using a finite element based Computational Fluid Dynamics (CFD) code FEAT. This study is an example of using CFD tackling complex practical engineering problems. Due to the complexity of the problem, significant simplifications were inevitable in order to obtain a manageable CFD model, which were done based on the understanding of the problem by the modellers plus engineering judgements. These were backed-up by sensitivity studies and benchmarking based on relevant simpler problems. The paper discusses the development of the CFD model and its validation, followed by presentation and discussion of the simulation results
[en] Fundamental mechanisms behind the molten core cooling strategies are revisited to provide an insight for a proper implementation of severe accident management guideline (SAMG) and a development of an engineered safety feature. From the results of a qualitative evaluation and a quantitative plant analysis, weak points of the current severe accident management guideline for an operating plant are identified and a revision of the molten core cooling strategies is proposed. In addition, technical issues for various kinds of core catcher concepts are discussed.
[en] Highlights: • The stability in a passive moderator cooling system of a unique system in the Canadian SCWR. • Identify and analyze unstable oscillations using flashing-driven natural circulation test results. • The flashing-driven oscillations categorized as a flashing-driven Type-I density wave instability including a geysering-like feature. • A stability map on the dimensionless plane with the Subcooling number and Phase Change number. - Abstract: This paper presents an examination of the instability mechanisms in a Passive Moderator Cooling System for the Canadian SCWR (Supercritical Water-cooled Reactor). The passive system is being developed at AECL using a flashing-driven natural circulation loop. Unstable intermittent and sinusoidal oscillations were identified from experimental data of the flashing-driven natural circulation passive moderator cooling system. The oscillation periods were correlated with the boiling delay time. A stability map for a flashing-driven two-phase natural circulation loop was established on the dimensionless plane with Subcooling number and Phase Change number. It was observed that there is thermal non-equilibrium in the single-phase and two-phase oscillation stages of the flashing-driven natural circulation
[en] A methodological and structured procedure to address the uncertainties related to passive safety functions is presented. The matter is treated with reference to a passive system designed for decay heat removal of advanced light water reactors, relying on natural circulation and provided with a heat exchanger immersed in a cooling pool, acting as heat sink, and connected to the pressure vessel via steam and condensate main lines. Two hazard identification used qualitative methods, as failure mode and effect analysis (FMEA) and hazard and operability study (HAZOP), are utilized and the relative results compared in order to assess the main sources of physical failure. The identification of the sources of uncertainties related to passive system performance, in terms of parameters which drive the failure mechanisms, follows. Finally the uncertainties are evaluated both for their assessment in probabilistic terms and for the determination of most contributors to the system thermal-hydraulic response
[en] The high temperature engineering test reactor (HTTR) is the first high-temperature gas-cooled reactor in Japan with reactor outlet gas temperature of 950 deg. C and thermal power of 30 MW. Sixteen pairs of control rods are employed for controlling the reactivity change of the HTTR. Each standpipe for a pair of the control rods, which is placed on the top head dome of the reactor pressure vessel, contains one control rod drive mechanism. The control rod drive mechanism may malfunction because of reduction of the electrical insulation of the electromagnetic clutch when the temperature exceeds 180 deg. C. Because 31 standpipes stand close together in the standpipe room, 16 standpipes for the control rods, which are located at the center, should be cooled effectively. Therefore, the control rod drives are cooled indirectly by forced air circulation through a pair of ring-ducts with proper air outlet nozzles and inlets. Based on analytical results, a pair of the ring-ducts was installed as one of structures in the standpipe room. Evaluation results through the rise-to-power test of the HTTR showed that temperatures of the electromagnetic clutch and the ambient helium gas inside the control rod standpipe should be below the limits of 180 and 75 deg. C, respectively, at full power operation and at the scram from the operation.
[en] Highlights: • Natural convection experiments for a finned plate in a chimney for RCCS application. • High Ra was achieved by using mass transfer experiments based on analogy concept. • Measured Nusselt numbers agreed with the other existing studies. • The chimney width is important due to bypass flow and internal circulation. • An empirical correlation was developed based upon the test results. - Abstract: We investigated natural convection heat transfer for a finned plate inside a chimney for application in reactor cavity cooling systems (RCCSs). To achieve a large Rayleigh number, the mass transfer rates were measured rather than the heat transfer rates, exploiting the analogy between heat and mass transfer systems. Experiments were carried out with systematically varied fin heights, fin spacings, chimney heights, and chimney widths, and for large Rayleigh numbers (of up to 2.91 × 10"7). The experimental results were consistent with the existing correlations, which were based on numerical models. Larger heat transfer rates were observed for larger fin heights and smaller fin spacings due to the increased heat transfer area. As the distance between walls and the fin tip increased, the heat transfer rate decreased, until the chimney effect was no longer observed, where the plume circulated within the duct. An empirical correlation was derived using the test results. The study concludes that the optimizations of chimney parameters as well as fin parameters are required for RCCS applications.
[en] The literature on hydrodynamics of forced convection of particle beds is reviewed and used to assess the characteristics of in-situ cooling of a degraded LWR reactor core under conditions representative of severe accidents. It is found that the pressure head required, for a given rate of liquid water flow, through a totally degraded core is one to two orders of magnitude higher than the case of intact core geometry. To remove decay heat of up to 1-2% of the reactor normal power, even with conservative assumptions, the pressure head is within the capability of the main reactor pumps. However, particles with very small diameter (less than 100 μm) will potentially be swept out by the flow. The sensitivity of the hydraulic characteristics to the allowed coolant temperature rise across the fragmented core is also investigated. (orig.)
[en] ANSALDO has conceived a reactor called ISIS (Inherently Safe Immersed System), an innovative light water reactor with easily understandable safety characteristics. The main targets are: Passively safe behaviour, no pressurization of the Reactor Containment under any accident condition, control of plant capital cost and construction schedule by virtue of the modular concept and the compact layout. The ISIS concept, described in general terms in the paper, builds up on the Density Lock concept originally proposed by ABB ATOM for the PIUS plant, featuring innovative ideas derived from ANSALDO experience and based on proven technology from both LWR and LMR. (orig.)
[en] To increase the capacity of decay heat removal of a LMR system that uses a natural air circulation cooling, feasibility of heat transfer enhancement has been studied for a planar air channel by introducing a new channel configuration using radiation-convection structures of compact heat transfer surface. For the new channel configuration, the heat transfer mechanism has been investigated and design guides for the radiation-convection structure have been developed based on the investigation results. Following the developed design guides, a new radiation-convection structure has been also devised. Analysis of the air channel cooling with the new radiation structures revealed substantial heat transfer enhancement and the feasibility of the heat transfer enhancement with the new channel configuration design has been confirmed. (orig.)