TH2D : a computer code for the simulation of thermal-hydraulics during a Reactivity Initiated Accident

A. SEDOV, V. GAGIN (KURCHATOV INSTITUTE), V. BESSIRON, NURETH 10, 5-9 octobre 2003, Seoul, KOREA

The TH2D code has been developed by the Kurchatov Institute in the frame of a close collaboration with IRSN in order to simulate thermal-hydraulics under RIA conditions. The SCANAIR-TH2D coupling allows to have a fine description of a RIA from the fuel pellet to the coolant.
Specific features of clad-to-coolant heat transfer in fast transients include significant metastable overheating of the liquid coolant at the wall, absence of established nucleate boiling before the Critical Heat Flux (CHF) is reached and very fast transition to an inverse-annular film boiling regime. The simulation of RIA thermal-hydraulics has to take into account rapid changes in the fluid properties and the presence of steep temperature and density gradients in the vicinity of the cladding.
The TH2D code is based on the resolution of the mass, momentum and energy conservation equations for a homogeneous fluid that can be in liquid or vapor single phase or in two-phase conditions, depending on its enthalpy. Turbulence is taken into account on the basis of a mixing-length model.
The choice of particular conditions at the cladding-coolant boundary is controlled by local values of coolant enthalpy and heat flux. These particular conditions define five regimes of heat transfer at the cladding-coolant boundary: 1) single-phase liquid, 2) metastable-overheated liquid, 3) boiling, 4) single-phase vapor and 5) condensation.
The numerical scheme of TH2D is based on a 2D (r-z) approach with iterations between radial finite elements and axial finite-differences. The resolution of the energy conservation equation is first performed with given velocities in order to up-date the fluid properties. Then, iterations between the resolution of the variational form of the axial mass conservation and the finite-difference expression of the global mass conservation over the axial slices, allow to compute the pressure field along the channel length. The resolution of the local mass conservation equation finally gives the radial velocities at each node.
The coupling with the SCANAIR code is achieved at the cladding-coolant boundary. According to the global numerical scheme, iterations within one time-step are repeated until the computed TH2D and SCANAIR wall heat fluxes and temperatures converge.
The TH2D code has been validated on experimental data and well-known correlations under steady-state conditions (velocity profiles in laminar and turbulent regimes, heat transfer in single-phase and two-phase post dry-out regimes, Critical Heat Flux prediction in usual PWR conditions…).
Additionally, a global validation of the code on the PATRICIA experimental program simulating RIA conditions has been performed. It shows a good agreement of the code with the experimental results.