Radiation heating of VVER-440 thermocouple
29th Symposium of AER on VVER Reactor Physics and Reactor Safety (2019, Energoland, Mochovce NPP, Slovakia)
advances in spectral and core calculation methods
Thermocouple located near the coolant outlet of the fuel assembly head may indicate a temperature biased by radiation heating and subsequently affect accuracy of incore measurements. The analysis splits into two parts; coupled neutron-photon Monte Carlo transport to determine radiation heating that serves as the input to heat transfer calculation resulting in temperature bias evaluation.
Various nuclear reactions contribute to radiation heating in the active volume of the thermocouple steel. Monte Carlo transport calculation of radiation heating for average fuel assembly in one of latest Dukovany NPP cycles was performed with MCNP code. Seven heating sources were compared; activated top nozzle steel photons, activated block of protective tubes photons, fission neutrons, secondary photons, fission photons, spent nucler fuel neutrons and SNF photons. It was found that dominant reactions are activated block of protective tubes and secondary photons and the analysis results can be easily extended to all fuel assemblies based on average fuel assembly model results. Heating power was compared to values published previously for Paks and Kola NPPs.
Heat transfer calculation were performed with TEPLO code developed in SKODA JS. The code calculates temperature field by finite element method in 2-D geometry. Radiation heating can be considered as negligible if the axial shift between thermocouple active part and thermocouple socket is below 15 mm.