Nuclear Engineering and Design 255 (2013) 68–76 Contents lists available at SciVerse ScienceDirect Nuclear Engineering and Design j ourna l ho me pag e: www.elsevier.com/locate/nucengdes Nuclear fuel rod fragmentation under accidental conditions Olivia Coindreau ∗ , Florian Fichot, Joëlle Fleurot Institut de Radioprotection et de Sûreté Nucléaire, IRSN/PSN-RES/SAG/LESAM, Cadarache Nuclear Center, BP 3, 13 115 St Paul Lez Durance cedex, France h i g h l i g h t s ◮ Fuel rods can be fragmented in case of reflooding during a core meltdown accident. ◮ The core coolability strongly depends on the extent of fuel rod fragmentation. ◮ Evaluation of the size and of the surface area of the fuel rod fragments. ◮ Fuel cracking in normal conditions leads to mean particle diameter higher than 2 mm. ◮ Lower diameter in case of additional fragmentation of highly irradiated fuel rods. a r t i c l e i n f o Article history: Received 15 May 2012 Received in revised form 13 September 2012 Accepted 16 September 2012 a b s t r a c t This paper deals with fuel rod fragmentation during a core meltdown accident in a Nuclear Power Plant. If water is injected on the degraded core to stop the degradation, embrittled fuel rods may crumble to form a reactor debris bed. The size and the morphology of the debris are two key parameters which determine in particular heat transfer and flow friction in the debris bed and as a consequence its coolability. To address this question, a bibliographic survey is performed with the aim of evaluating the size and the surface area of the fragments resulting from fuel rod fragmentation. On this basis, a model to estimate the mean particle diameter obtained in a reflooded degraded core is proposed. Modelling results show that the particle size distribution is very narrow if we only take into account fuel cracking resulting from normal operating conditions. It leads to minimum mean diameters of 2.5 mm (for fuel particles), 1.35 mm (for cladding particles) and 2 mm (for the mixing of cladding and fuel fragments). These results are obtained with fuel rods of 9.5 mm outer diameter and cladding thickness of 570 m. The particle size distribution is larger if fine fragmentation of the highly irradiated fuel rods during temperature rise is accounted for. This is illustrated with the computation by the severe accident code ASTEC, codeveloped by IRSN abd GRS, of the size of the debris expected to form in case of reflooding of a French 900 MW reactor core during a core meltdown accident. © 2012 Elsevier B.V. All rights reserved. 1. Introduction During a core meltdown accident in a pressurized water reac- tor (PWR), the original fuel rod bundle geometry is gradually lost. When temperature reaches about 800 ◦ C, the Zircaloy cladding of the fuel rods begin to balloon and to burst. Cladding ballooning leads to the first significant change in the initial fuel rod geometry with a potential blockage of a substantial part of the coolant flow in the reactor core. Nevertheless, it can be considered that the rod-like core configuration remains intact during this early phase of degra- dation. If temperature keeps increasing, molten materials from con- trol rods, guide tubes and spacer grids (at around 1400 ◦ C) flow and solidify at a lower, cooler zone in the core. At this temperature, the ∗ Corresponding author: Tel.: +33 4 42 19 92 63; fax: +33 4 42 19 91 65. E-mail address: olivia.coindreau@irsn.fr (O. Coindreau). exothermic oxidation of the cladding Zircaloy by steam can rapidly lead to the melting point of Zircaloy with subsequent dissolution of UO 2 pellets and ZrO 2 . The meltdown and relocation of control mate- rials and then of cladding and fuel materials causes an additional change in the geometry of the core with loss of the original rod-like geometry. A further evolution in the geometry is likely to occur if the reactor core is reflooded with water to stop the progression of the degradation. Indeed, the most important accident measure to terminate a core meltdown accident transient in a PWR is the injec- tion of water to cool the degraded core. But the quenching of highly oxidized fuel assemblies and/or molten cladding can lead to debris bed formation, as the analysis of the TMI-2 accident (Akers et al., 1986; McCardell et al., 1990) and results of various experiments like LOFT (Coryell et al., 1994; Hobbins and McPherson, 1990), PBF (Petti et al., 1989) and PHEBUS have shown. Indeed, the loss of integrity of unclad fuel rods (disappearance further to melting) or the embrittlement of oxidized cladding would leave the fuel pellets 0029-5493/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nucengdes.2012.09.021