A Methodology to Determinate the HTR (High Temperature Reactor) Core Seismic Behaviour: Experiments and Numerical Analyses

The Very High Temperature Reactor (VHTR) core is constituted of prismatic graphite blocks (fuel and reflectors), enclosed within a metallic core barrel. This design is common to several HTR reactor types such as AREVA ANTARES Project, GT-MHR [1] and HTTR [3]. The whole structure is constituted of nearly 2600 blocks and is cooled by pressurised helium gas. The necessity to withstand thermal dilatations and the probable occurrence of graphite shrinkage under irradiation implies the existence of significant gaps between adjacent block columns and clearances in the linking systems between piled up blocks. The seismic behavior of such assembly is largely non linear and includes frictional sliding, impacts and blocks rocking. Nevertheless, it requires a detailed understanding in order to: • Respect the safety criteria against seismic aggression, which include keeping the graphite blocks integrity and limiting the transversal displacements of the control rod channels. • Model and control the interactions between the core and the other internal components of the reactor vessel. A research program is performed by CEA, EdF and AREVA, with experimental and theoretical parts. Tests have been performed on a few graphite blocks. The interpretation of the experiments provides the opportunity to test and to choose numerical methods to describe the dynamic behavior and allows the determination of the corresponding parameters. Numerical simulations of the seismic behavior of the whole core lead to first estimations of the kind of movements the blocks can undergo. INTRODUCTION Many works have been done in the field of the seismic response of graphite nuclear core assemblies, since the 70’s, for different kinds of gas-cooled reactors. The seismic behavior is largely non-linear and is related with the non-linear problem of the dynamic of rocking-sliding blocks. The safety factors which have been considered are: the shear-bending strength of the dowel pins between the blocks, the functional operability in terms of maximal strain criterion and maximal displacements of the control rod channels, and the integrity of the lateral keying between the graphite blocks. Several authors have developed analytical models, where blocks are treated as rigid solids, allowing horizontal motion and rocking (face to face contacts, shear taken into account through the dowel-pins system) with lateral friction. These models used rigid solids lined by contact spring damper elements, with Coulomb’s friction. They frequently made the assumption that displacements and rotations are small, in order to simplify the models. Figure 1 shows an example of experiments and of numerical results of the movements of the blocks [2]. A common work program for the VHTR project is defined and carried out by the three partners of the present paper: AREVA, CEA, and EdF in France. As for previous studies on the seismic behavior of HTR and Gas reactors (GT-MHR [1], HTTR [3], AGR [4], [5].), the program is divided into an experimental and an analytical part: • The objectives of the experimental program are: the understanding of basic physical phenomena, such as blocks rocking, frictional sliding and collisions, the determination of input parameters to be used in the analytical studies, and eventually the experimental validation, at a reduced scale, of the whole stack behavior obtained by numerical simulations. • The objective of the analytical program is to provide an efficient tool for parametric numerical simulations, checked against experimental results, which could help the design of the VHTR core and surrounding structures. SMiRT 19, Toronto, August 2007 Transactions, Paper # CK01/2