Experimental investigations on out-of-pile single rod test using fuel simulator and assessment of FRAPTRAN 2.0 ballooning model Ashwini Kumar Yadav a,⇑ , Chang Hwan Shin b , Chan Lee b , Sung Uk Lee b , Hyo Chan Kim b a Mechanical Engineering Department, Motilal Nehru National Institute of Technology Allahabad, Allahabad 211004, Uttar Pradesh, India b Nuclear Fuel Safety Research Division, Korea Atomic Energy Research Institute, 111, Daedeok-Daero 989beon-gil, Yuseong-gu, Daejeon 34057, Republic of Korea article info Article history: Received 7 December 2017 Received in revised form 21 September 2018 Accepted 1 October 2018 Keywords: Clad tube ballooning Loss of coolant accidents Nuclear fuel ECCS abstract The extent of clad tube ballooning is important when analyzing the upper limits of coolant-channel blockage and subsequent planning of emergency core cooling system (ECCS) design strategy. As per revised ECCS acceptance criteria, the safety-analysis code system should be able to predict precisely fuel rod behavior to simulate a realistic safety analysis under off-design conditions. Considering these aspects, the present investigation was carried out to access the capability of FRAPTRAN 2.0 code to predict the ballooning behavior of cladding at various heating rates and internal pressures. Three tests at (4.5, 5.5, and 6.5) MPa and heating rates of 1.7–4 K/s were performed on a facility named ‘FISRBIT’ (Facility to Investigate Single-Rod Behavior In Transient) under inert gas atmosphere. Transient temperature, pres- sure, and deformation were recorded during the experiment at three axial positions over the internally heated Zircaloy-4 clad tube. Ballooning started at the location with the highest temperature; then prop- agated in the axial directions. Under fast transient heating, the balloon was confined near the highest temperature site, but at slower rates, an axially elongated balloon was observed. The test time between initiation of ballooning and rupture varied from 25 to 100 s depending on heating rate and internal pres- sure. The maximum hoop-strain prediction based on the hoop-stress calculation by the Rosinger model was better than the FRACAS-I and BALON-2 models. Using the Rosinger model, the hoop stress increased gradually until burst; hence, it was judged to simulate the physics of ballooning adequately. The rupture timing prediction by the FRAPTRAN 2.0 code was sooner than in the experimental results. One reason for the early rupture prediction was the time-independent behavior of the plastic model adopted by the code for modeling the ballooning phenomenon. Ó 2018 Elsevier Ltd. All rights reserved. 1. Introduction Economic concerns are now compelling nuclear power utilities to consider the increase of average burn-up for fuel assemblies and to adopt new types of cladding materials to enhance thermal and safety margins. Sustaining such aggressive conditions for fuel and reactor cores requires new research on fuel rod behavior under ref- erence accidental conditions to meet current safety criteria and to provide new technical bases for modeling. As per revised emer- gency core-cooling-system (ECCS) acceptance-criteria, the safety analysis-code system should calculate the fuel behavior models. A loss of coolant accident (LOCA) analysis requires a thorough investigation of ballooning and burst behavior of fuel cladding to identify their potential interference with emergency cooling effectiveness. This is because ballooning and rupture locations at the same axial position could result in significant flow blockage. The deformation behavior for cladding has been widely investi- gated in the past to validate the models in the context of LOCA assessment. Chung and Kassner (1979) investigated the effect of the internal pressure, heating rate, steam, and temperature on the ballooning behavior of Zircalloy-4 cladding. Chapman et al. (1979) reported that clad deformation is extremely sensitive to small temperature variation at the surface. Karb et al. (1982) con- ducted experiments to investigate the influence of the nuclear environment on the cladding failure behavior. No significant effect was observed due to irradiation exposure over the cladding failure mechanism. Erbacher and Leistikow (1987) stated that a large azi- muthal temperature gradient around the periphery of a clad tube causes uneven wall thickness in the ballooned region resulting in a small burst strain. Kim et al. (2004) reported the role of phase transformation (from a to b phase) in the deformation behavior of Zircaloy-4 under transient conditions. Recent series of experi- ments conducted by Kekkonen (2005) at Halden Reactor provided https://doi.org/10.1016/j.anucene.2018.10.004 0306-4549/Ó 2018 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail addresses: ashwini@mnnit.ac.in (A.K. Yadav), shinch@kaeri.re.kr (C.H. Shin), Chanlee@kaeri.re.kr (C. Lee), leesunguk@kaeri.re.kr (S.U. Lee), hyochankim@kaeri.re.kr (H.C. Kim). Annals of Nuclear Energy 124 (2019) 234–244 Contents lists available at ScienceDirect Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene