Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes On the characteristics of the flow and heat transfer in the core bypass region of a PWR I. Clifford ⁎ , M. Pecchia, R. Puragliesi, A. Vasiliev, H. Ferroukhi Laboratory for Reactor Physics and Systems Behaviour (LRS), Paul Scherrer Institut, 5232 Villigen PSI, Switzerland ARTICLE INFO Keywords: Computational fluid dynamics Core bypass Heat transfer Pressurized water reactor ABSTRACT The development of analysis models for the Swiss reactors is a key objective of the STARS project at the Paul Scherrer Institut (PSI). Within this context there is a need for the development of computational fluid dynamics (CFD) models of the Swiss reactors in support of future high fidelity investigations of steady-state and transient scenarios. This article presents initial results for the CFD analysis of a Siemens KWU PWR with a focus on the flow behaviour and heat transfer in the gap between the core shroud and core barrel. Temperatures and densities in this region of the reactor are important, for example, for accurate estimations of fast neutron fluence and activation in the steel structures of the core shroud, core barrel and reactor pressure vessel. The flow behaviour in this region may also be relevant for better understanding of ex-core detector responses. The flow conditions in the core bypass region were found to be in the transition-to-turbulence regime, with vortex shedding taking place downstream of the core formers as a result of flow instabilities. The non-stationary nature of the flow presented a challenge in terms of obtaining a solution within a reasonable time period. Two approaches were proposed to address this challenge: time-averaging of the flow-field information before solving the conjugate heat transfer problem; time-averaging of surface heat fluxes in order to derive detailed surface heat transfer coefficients. Both approaches yielded similar results with similar computational effort. Several characteristics and features of the core bypass flow are discussed. Updated Monte Carlo simulation results show that the in- fluence of the core bypass temperatures on the neutron fluence predictions is non-negligible. This highlights the importance of including accurate bypass temperatures in future Monte Carlo simulations focused on ex-core regions. 1. Introduction The topic of reactor life extension has gained interest over the past years as many reactors approach their 40 year design life span. There is renewed interest in understanding the behaviour of the structural ma- terials in the reactor core, in particular the reactor pressure vessel (RPV), core barrel and core shroud, which are under high neutron fluence and potentially high thermal stress due to their proximity to the reactor core. These operating conditions may lead to localized embrit- tlement of the steel and welds in these components. The flow behaviour in this region may also be relevant in the understanding of ex-core detector responses. Some general information on fluence and doses in PWR components is available in literature. Petrequin et al. (Petrequin et al., 1997), for example, give neutron fluence, dose and operating temperature values for selected steel components directly adjacent to the core for French, British and German PWRs. While useful, this information is not sufficiently detailed to build up a picture of the fluence distribution in these components. Altstadt et al. (Altstadt et al., 2004) obtained finite- element solutions for the temperatures and stresses in a German PWR core baffle, using heat sources derived from Monte Carlo simulations. Their analysis assumed a simple one-dimensional approach to model- ling the fluid flow. Rupp et al. (Rupp et al., 2009) performed detailed CFD and finite element stress analysis of the core baffle structure of a French PWR, with heat sources similarly derived from Monte Carlo si- mulations. Within the context of the STARS programme at PSI there is a need for the development of high fidelity simulation schemes, and integrate these into multi-physics and/or multi-scale computational schemes, in support of ageing and life extension studies. At the same time, there is a need to develop computational fluid dynamics (CFD) models of the Swiss reactors in support of future high fidelity investigations of steady- state and transient scenarios. Detailed studies of the fast neutron flu- ence in the RPV of a Siemens KWU reactor were completed by Dupré https://doi.org/10.1016/j.nucengdes.2018.01.039 Received 23 August 2017; Received in revised form 19 January 2018; Accepted 23 January 2018 ⁎ Corresponding author. E-mail addresses: ivor.clifford@psi.ch (I. Clifford), marco.pecchia@psi.ch (M. Pecchia), riccardo.puragliesi@psi.ch (R. Puragliesi), alexander.vasiliev@psi.ch (A. Vasiliev), hakim.ferroukhi@psi.ch (H. Ferroukhi). Nuclear Engineering and Design 330 (2018) 117–128 Available online 20 February 2018 0029-5493/ © 2018 Elsevier B.V. All rights reserved. T