[en] Operating water-cooled reactors at supercritical conditions results in significant savings because of increased thermodynamic efficiency and plant simplifications. One of the supercritical water-cooled reactor concepts adopts the pressure tube design, and is a natural evolution of CANDU reactors. For the CANDU supercritical water reactor concept, an additional benefit can be realized due to the need to modify the pressure tube design to accommodate the higher coolant temperature and pressure. This modified pressure tube design allows the use of a passive moderator cooling loop to reject moderator heat under normal and accident conditions. This passive loop replaces the existing pumped loop design, and results in enhanced passive safety with the potential for additional cost reductions. (author)

[en] The separation between the low-pressure moderator and high-pressure coolant in CANDU reactors makes it possible to utilize the moderator as a heat sink for emergency heat removal. This paper presents a review of a passive moderator-cooling concept that can be used under both normal and upset conditions. The paper also presents recent work completed towards realization of this concept. (author)

[en] The CANDU^R-Supercritical Water Reactor (CANDU-SCWR) is one of the six reactor concepts being considered by the Generation-IV International Forum (GIF) for international collaborative R and D. With SCW coolant, the thermodynamic efficiency is increased to over 40%. The CANDU-SCWR is moderated using heavy water, and it has fuel bundles residing inside horizontal pressure tubes, similar to the current CANDU design. The coolant, however, is light water at 25 MPa, with an inlet temperature of 350 deg. C and an outlet temperature of 625 deg. C. Because of the high temperature and high pressure of the coolant, the standard CANDU pressure tube design cannot be used. This paper presents one of the insulated pressure tube designs being considered for the CANDU-SCWR fuel channels. Unlike current CANDU reactors, the proposed CANDU-SCWR fuel channel does not use calandria tubes to separate the pressure tubes from the moderator. Each pressure tube is in direct contact with the moderator, which operates at an average temperature of about 80 deg. C. The pressure tube is thermally insulated from the hot coolant by a porous ceramic insulator. A perforated metal liner protects the insulator from being damaged by the fuel bundles and erosion by the coolant. The coolant pressure is transmitted through the perforated metal liner and insulator and applied directly to the relatively cold pressure tube. The material selection for each fuel channel component depends on its function. The fuel sheaths and the perforated liner must have high corrosion resistance in SCW, although their resident times are significantly different. The insulator must have high thermal resistance and corrosion resistance in SCW, plus sufficient strength to bear the weight of the fuel bundles without significant thickness reduction during its design life. The pressure tube is the pressure boundary material, so it must have high strength to contain the coolant. One common requirement for all in-core fuel channel components is that they should be as neutron transparent as possible. The irradiation deformation of all these components must also be considered in their design. This paper presents the design of this fuel channel, reviews existing data for materials, indicates where more data are required, and summarizes our plans to obtain these data. (authors)

[en] The separation between the low-pressure moderator and high-pressure coolant in CANDU reactors makes it possible to utilize the moderator as a heat sink for emergency heat removal. This paper presents a review of a passive moderator-cooling concept that can be used under both normal and upset conditions. The paper also presents recent work completed realization of this concept. (author)

[en] This paper presents a method by which the passive safety features of the PIUS concept concerning decay heat removal could be improved. The proposed method involves direct passive cooling of the reactor pressure vessel by natural convection boiling of water surrounding the vessel. This makes it possible to substantially increase the passive decay heat removal time before outside intervention is needed. This method of passive decay heat removal makes it possible to design smaller pressure vessels than is currently possible. It was further found that indefinite cooling of the pressure vessel was possible by using atmospheric air as the ultimate heat sink after the water surrounding the vessel has evaporated. (author). 4 refs., 1 tab., 2 figs

[en] The results of an experimental study to investigate the feasibility of using a passive flashing-driven natural circulation loop for CANDU-reactor moderator heat rejection are presented. A scaled loop was constructed and tested at conditions approximating those of a CANDU calandria cooling system. The results showed that stable loop operation was possible at simulated powers approaching normal full power. At lower powers, flow oscillations occurred as the flow in the hot-leg periodically changed from two-phase to single-phase. The results from earlier numerical predictions using the CATHENA thermalhydraulics code showed good qualitative agreement with the experimental results. (author). 6 refs., 11 figs

[en] A flashing-driven passive moderator cooling system is being developed at AECL for CANDU reactors. Preliminary simulations and experiments showed that the concept was feasible at normal operating power. However, flow instabilities were observed at low powers under conditions of variable and constant calandria inlet temperatures. This finding contradicted code predictions that suggested the loop should be stable at all powers if the calandria inlet temperature was constant. This paper discusses a series of separate-effects tests that were used to identify the sources of low-power instabilities in the experiments, and it explores methods to avoid them. It concludes that low-power instabilities can be avoided, thereby eliminating the discrepancy between the experimental and code results. Two factors were found to be important for loop stability: (1) oscillations in the calandria outlet temperature, and (2) flashing superheat requirements, and the presence of nucleation sites. By addressing these factors, we could make the loop operate in a stable manner over the whole power range and we could obtain good agreement between the experimental and code results. (author)

[en] Detailed predictions of moderator flow patterns and the associated temperature distributions in the calandria of CANDU reactors are required as part of safety analysis of certain accident scenarios. The specialized computational fluid dynamics (CFD) code MODTURC_CLAS V2.9 1ST has been selected as the Canadian nuclear industry's tool for predicting moderator temperature distribution and flow patterns. The code requires extensive validation, which cannot be accomplished using only existing experimental data. To close gaps in the validation database, a program of three-dimensional moderator circulation experiments is being conducted for Ontario Power Generation Inc. (OPGI) at Chalk River Laboratories. The experimental program is divided into a separate-effects and component-testing stage, and a quarter-scale calandria model integral testing stage. One of the separate-effect tests consisted of a detailed characterization of a turbulent round jet in a confined tank. This paper presents the results of this test, and compares them with published results. The experimental results, in general, agree very well with published measurements and will be used in the validation of MODTURC_CLAS. (author)

[en] The Computational Fluid Dynamics code, MODTURC_-CLAS, has been validated against experimental data obtained in the Moderator Test Facility at the Chalk River Laboratories of Atomic Energy of Canada Limited. The MTF is an integral scaled facility built to investigate moderator behaviour, having the key characteristics of a full-scale CANDU nuclear reactor calandria vessel. Predicted three-dimensional fluid flow and temperature distributions within the MTF vessel are in good agreement with the experimental data, which represent a range of operating conditions of the moderator in the recent CANDU 9 design. (author)