A case study: quantifying the radiation level at the core conditioning system of the PBMR reactor - 002

The Pebble Bed Modular Reactor (PBMR) being developed in South Africa, is a high-temperature helium-cooled graphite-moderated continuous-fuelled pebble bed reactor. Additionally, the power conversion unit in the PBMR is directly coupled to the reactor. The 400 MWth PBMR reactor design has reached a high level of maturity and then detailed calculations are needed to characterize the radiation levels at different positions in the plant. The Core Conditioning System (CCS) removes the core decay heat from the reactor during some particular operational modes, when the main turbine is not operating. It is required to know if the CCS room could be accessible during reactor operation, but it is also needed to establish the dose rate levels while performing some maintenance tasks with the reactor shutdown. To do so, the different radiation sources and the relative contribution of each one of them to the total dose rate need to be determined, both during reactor operation and shutdown. Three particularities of the PBMR must specifically be considered: the presence of several gaps and plenums in the graphite reflector, which could represent direct paths for radiation from the core to the CCS; the presence of an important opening in the concrete wall of the citadel enclosing the reactor pressure vessel; and the accumulation of radioactive dust and plate-out in some of the CCS components. As a consequence, the calculation of the radiation field at the CCS room must cover the neutron and gamma radiation transport through graphite, iron and gaps/ducts; the nuclear activation of structural materials; and nuclide transport calculations to determine the amount and characteristics of the dust and plate-out settled at the CCS. However, being the process of calculating the dust and plate-out sources of a very different nature as compared to that of radiation transport, we will only consider them here as given sources, without further explanation about how these sources have been calculated. Hence, the focus of this work will be on the description of the radiation transport from the core to the CCS, and results will be presented to show the relative contribution of different sources (here included the dust and plate-out ones) to the radiation level at the CCS room, both during reactor operation and shutdown. This complete case study is also a good example of how these analyses are performed at PBMR at this stage in the design of the plant. (authors)