In this paper, a reactor core thermomechanical analysis method is described to provide a tool to quantify the reactivity effect of the fuel subassembly distortion in the reactor core which is a relevant problem for certain reactor types such as Sodium-cooled Fast Reactors. The importance of such an analysis was made evident by the A.U.R.N. (negative reactivity event) [1] issue in the Phenix reactor as well as by the partial core meltdown of EBR-I [2]. The proposed method consists of two major phases: 1) The simulation of the core deformation through a CAD (Computer Aided Design) based finite element solver and 2) The measurement of the reactivity effect of the core distortion in Serpent 2 Monte Carlo code [3] through the obtained, deformed model. The technique makes it possible to obtain certain snapshots of the reactor core geometry in different points of time and to measure how the reactivity of the core is going to change as a response to a certain transient allowing an accurate prediction of the reactor behavior under the simulated conditions. In addition, the paper includes the verification process which was conducted to compare the accuracy of the finite element solver to the theoretical solutions regarding the deformation of a hexagonal subassembly. Moreover, the neutronics calculation accuracy has been demonstrated for the deformed and undeformed geometry subassemblies, equating the models obtained from the finite element solver with the model built directly in Serpent. As part of the investigation with the Monte Carlo code, the multiplication factors have been calculated and compared for different geometry subassemblies by carrying out a comprehensive analysis through the decomposition of the production and absorption terms at the various regions of the model. By means of this verification, it was demonstrated that, for the investigated conditions, the finite element solver accurately reproduces the theoretical deformation of the subassemblies and that the Serpent interface reads correctly the deformed CAD models. These conclusions open the path for the future validation work which is going to be performed on the Phenix Sodium-cooled Fast Reactor based on the data obtained from the core flowering End-of-Life test [4]. References: [1] – P. Dumaz et al., New investigations of the Phenix negative reactivity events, ICAPP 2012, Chicago, USA, June 24-28, 2012, Paper 12456 [2] – M. Ragheb, 2010, Experimental Breeder Reactor number I, EBR-I criticality accident. [3] – J. Leppänen, 2013. Serpent – a Continuous-energy Monte Carlo Reactor Physics Burnup Calculation Code. [4] – B. Fontaine, G. Prulhière, A. Vasile, P. Masoni, P. Barret, D. Rochwerger, J. Gros, R. Dupraz, N. Moussallam, M. Chassignet, “Description and preliminary results of PHENIX core flowering test”, Nuclear Engineering and Design 241 (2011) 4143– 4151.