1 Proceedings of the 7th International and 45th National Conference on Fluid Mechanics and Fluid Power (FMFP) December 10-12, 2018, IIT Bombay, Mumbai, India FMFP2018–PAPER NO. 071 Thermal-Hydraulic Analysis of Partial Loss of Forced Reactor Coolant Flow with Non-Uniform and Asymmetric Loop Flow Mixing in VVER-1000 Sanuj Chaudhary Directorate of Engg- LWR, Nuclear Power Corporation of India Ltd, Nabhikiya Urja Bhavan, Anushakti Nagar, Mumbai-400094 Email: sanujc@npcil.co.in P.Krishna Kumar Directorate of Engg-LWR, Nuclear Power Corporation of India Ltd, Nabhikiya Urja Bhavan, Anushakti Nagar, Mumbai-400094 Email: pkrishnakumar@npcil.co.in Y.K. Pandey Directorate of Engg-LWR, Nuclear Power Corporation of India Ltd, Nabhikiya Urja Bhavan, Anushakti Nagar, Mumbai-400094 Email: ypandey@npcil.co.in Gautam Biswas Directorate of Engg-LWR, Nuclear Power Corporation of India Ltd, Nabhikiya Urja Bhavan, Anushakti Nagar, Mumbai-400094 Email: gbiswas@npcil.co.in Abstract Partial loss of Reactor coolant flow can occur as a result of trip of one or more number of Reactor Coolant Pump (RCP) in Nuclear Power Plants, which affects heat removal from the core. Kudankulam Nuclear Power Plant (KKNPP) is VVER-1000 Pressurized Water Reactor having four main circulation loops. Each loop consists of a steam generator and a RCP set, which are connected to reactor core by cold and hot legs. VVER-1000 allows operation on partial running of RCP sets by decreasing power, in-order to match power to flow (P/F) ratio. In conservative simulation of accident scenario, the corresponding reactor power reduction is not considered on RCP trip. This causes a power to flow mismatch thereby worsening core cooling. The four loop are connected to the reactor asymmetrically and on trip of one or more RCP set in loops, the coolant flow re-distribution takes place in core which is highly non-symmetric. This has been analyzed using Thermal Hydraulic computer code RELAP-5/MOD 3.2 for Kudankulam Nuclear Power Plant (KKNPP). The model is developed in such a way that captures non-symmetric coolant re-distribution and mixing in core after such events. Keywords: Coolant Flow Loss; Asymmetric Loop Flow; Core mixing; Thermal-hydraulic analysis. I. INTRODUCTION The Kudankulam Nuclear Power Plant (KKNPP 1& 2) are 1000 MWe VVER-1000 Reactors. In order to investigate the variations in different parameters of the power plant under condition of partial loss of reactor coolant flow, trip of various number of RCP sets combinations were analyzed. It is an Anticipated Operational Occurrence (AOO) which can demonstrate the capability of safety systems to cope with such events. The present event result in primary pressure and temperature increase at reactor outlet. Minimum conservative DNBR in the core is checked during such events. The analysis has been done using system thermal hydraulics code RELAP5/MOD3.2. It has been extensively used in analysis of thermal hydraulic behavior of Water Cooled Reactors (LWR) during loss-of- coolant accidents and operational transients [1]. Additionally, the quality of the produced results and its flexibility in developing nodalization makes RELAP5 suitable to study such flow distribution. The objective of this study is to establish coolant flow asymmetric behavior and mixing in VVER core. Different number of RCP trip combinations has been taken as initial event and thermal hydraulic parameters are evaluated and checked against the applicable acceptance criterion, using the developed model in RELAP. II. RELAP-5 MODELLING OF KKNPP-1&2 REACTORS A. Description of KKNPP-1&2 Reactors Enriched uranium in oxide form is used as fuel with light water as moderator and coolant in VVER. It is a two circuit system namely primary and secondary circuit. Primary circuit contains radioactive coolant. It consist of a heterogeneous thermal neutron-reactor, four main circulation loops and a pressurizer connected to one of the