Comparison of 2D and 3D heat transfer models around the coolant channels in the HTR-PM side reflector

Abstract

In the HTR-PM pebble bed reactor heat is produced in a cylindrical core surrounded by a graphite reflector. The helium coolant flowing down through the core first flows up through 30 coolant channels in the reflector, cooling it. Heat is also transferred through the reflector in radial direction to the pressure vessel and other surroundings, which is the main heat loss mechanism during a loss of cooling accident. Usually heat transfer in the reflector region is modelled using a 2D axi-symmetric geometry, modelling the region containing the coolant channels as a homogenized mixture of coolant channel and graphite reflector using a porosity value, sometimes using very course meshes. In reality temperature gradients in azimuthal direction will exist around the coolant channels, possibly affecting both heat transfer to the coolant and heat transfer through the graphite around the coolant channels to the outer boundary. This paper investigates the accuracy of the 2D model by comparing calculations for a fine and course 2D mesh with a 3D mesh in which the coolant channel geometry is explicitly modelled. Two cases were investigated: one representing full power operation, and the other a loss of forced cooling incident with no helium flow through the coolant channels. The course 2D mesh resulted in large errors in the reflector temperature field for full power conditions, overestimating the temperature drop across the coolant channel region. The 2D fine mesh compared reasonably well with the 3D mesh, although it resulted in both an overestimation of the effective heat transfer rate to the coolant channels and an underestimation of the effective resistance to heat transfer in the reflector in the radial direction around the coolant channels. Especially the last can lead to an underestimation of reflector and core temperatures during a loss of coolant accident. To amend this problem, the conductivity of the graphite in the coolant channel region should be adjusted in the 2D porous model to compensate for the added effective resistance to heat transfer in radial direction due to the geometry.