Analysis of dust and fission products in a pebble bed NGNP

In the HTR pebble bed reactor graphite dust is generated during normal reactor operation due to pebble-to-pebble interactions. This dust will be deposited throughout the primary system. Furthermore, the dust will become radioactive due to sorption of fission products released during normal operation.This paper presents an analysis of dust transport and deposition and fission product transport and plate-out during normal operation of a NGNP (next generation nuclear plant) Westinghouse pebble bed design. The main objective is to determine the amount and location of deposited graphite dust in the system and the amount of radioactive isotopes adsorbed on the structures and the dust during normal operation. The results will be used in planning of maintenance activities. Moreover the present results may be used in a next step to perform a depressurized loss of forced cooling (D-LOFC) analysis and to determine the amount of radioactivity released to the atmosphere during the accident. The analysis was performed using the SPECTRA code.Based on the assumed dust source term, during the 60 years lifetime of the pebble bed reactor concept which was analyzed approximately 1630 kg dust enters the primary helium flow. It was determined:(1)That 86% of the dust settles on the graphite structures inside the reactor vessel. Of the remaining graphite dust 2/3 collects in the low temperature intermediate heat exchanger (IHX) and the remaining dust in the high temperature IHX and the connecting pipes of the primary system.(2)Agglomeration of dust from smaller into larger particles is observed at all locations. The resuspension of agglomerated dust particles is an important phenomenon that limits the build-up of a dust layer on the surface of the IHXs. A large deposited dust layer is observed on the control rod drive (CRD) walls in the graphite structures. The dust accumulates on the CRD walls because, due to very low gas velocities, resuspension of the agglomerated particles does not occur.(3)The dust binds a significant amount of fission product vapors. Although the vapor concentrations are typically smaller on the dust surface than on the metallic surfaces, the dust particles have a relatively large surface.