Analysis of heat transfer characteristics of Canadian SCWR fuel assembly concept

Canada is participating in the Generation IV (Gen IV) International Forum with a main focus on the pressure-tube-type supercritical water-cooled reactor (SCWR) concept. The Canadian SCWR concept is a heavy water moderated and light water cooled reactor. One of the most important components in the development of the SCWR is the fuel assembly concept. The Canadian SCWR fuel assembly has a re-entrant or double flow pass configuration. Light water coolant flows from an inlet plenum into flow tubes located in the centre of each fuel channel. The outermost component of the fuel channel is a pressure tube. The fuel assembly consists of the fuel elements (bundle), central flow tube, encapsulated insulator, upper and lower fuel element supports, inlet/outlet flow exchanger and outlet flow tube. The coolant from the top of a fuel channel enters into and flows down through the central flow tube. When the coolant reaches the closed bottom, it is redirected into the fuel-containing annulus, or the fuel bundle region, where it flows up while being heated by the fuel. The subchannel code ASSERT-PV modified for SCWR applications is used to design this fuel assembly, specifically the fuel bundle. A multi-disciplinary and iterative approach is used to determine the optimum fuel bundle geometry given several geometric and physical constraints. The fuel cladding temperature is used as the main constraint. Several assumptions are required to model the fuel assembly, including the perfect insulation of (i) the central flow tube (i.e., no heat transfer through the central tube) and (ii) the pressure tube (i.e., no heat loss to the moderator). These two assumptions were considered as conservative, but they were not analysed or assessed for their validity or accuracy. To assess the assumptions, ASSERT-PV was upgraded to model the heat loss to the moderator, and an external CATHENA model was coupled to ASSERT-PV to model the heat transfer to the central flow tube. This paper describes these additional heat transfer components, and presents an assessment of these two assumptions for their impact on the prediction of maximum fuel cladding temperature. (author)