Thermal-hydraulic evaluation of RBMK-1500 reactor graphite-pressure tube gas gap closure safety concerns

The RBMK-1500 reactor core consists of a graphite structure houses pressure tubes for fuel assemblies. There is a dimensional allowance between graphite bricks and pressure tubes referred to as the gas gap. The original gap between the outer pressure tube wall and the graphite bricks diminishes due to outwards expansion of the pressure tubes (from radiation and thermal creep of the zirconium-niobium tubes) and the radiation induced contraction of the graphite blocks surrounding the pressure tubes. However, it is necessary to point out that by the time the average gas gap was closed, a certain number of the pressure tubes in the core could be in the permanent gripped state. The transient analysis has been performed to evaluate operational conditions in the gripped pressure tubes and to estimate the corresponding pressure and temperature behavior in the pressure tubes with gas gap exhausted for most limiting heat-up and cool-down transients. Results of analysis have shown that elimination of gas gap leads to increasing of temperature of pressure tube wall of about 25-30degC and decreasing of temperature of outer graphite brick surface of about 50degC for normal operational conditions. The temperature differences between outer and inner surfaces of graphite bricks as well as between outer and inner surfaces of pressure tube in case without gas gap is about 2.6 higher as in the case with existing gas gap. Safety criteria are not exceeded for any analyzed limiting transients. (author)