Speaker
Description
In the frame work of shielding design for the OL3 nuclear power plant (NPP) a special design solution was required to shield the areas close to the circular wall inside the Reactor Building.
In the Reactor Building of the OL3 NPP, large equipment areas with high neutron and gamma radiation levels are separated from accessible areas by one circular shielding wall comprising the Reactor Coolant System. This shielding wall is penetrated by a large number of openings for cables and piping of different diameters and temperature requirements. The openings are generally realized during the construction phase by using sleeve pipes in the first concreting of the shielding wall. These openings may be used by a single pipe as well as by different pipes routed through the opening at different angles.
The installation of the piping has generally to include enough flexibility to allow moving of the piping due to thermal stress and other possible impacts. Furthermore, the installation of the piping shall include the option to remove and exchange piping during plant outage. These openings have to be sealed according to the radiological requirements of very low radiation levels given for the accessible areas. This was quite demanding taking into account the huge impact induced by the Reactor Coolant System in the large equipment areas.
After the sleeving – a very common standard technique - of each pipe (in some cases sleeving of pipes of small bore as a bundle) to allow enough flexibility after installation, the opening is filled with PE granules. This was done by a commonly used fan systems conveying the granules into the openings. These PE granules yield good neutron attenuation due to their high hydrogen content. Lead wool packages (used for OL3, comparable gamma attenuation materials can be used instead) were introduced in the openings at both ends to shield any gamma radiation impact.
The complete design yielded enough space to add additional sealing for room closure (HVAC) and fire protection sealing as final steps of the overall sealing procedure. The sealing procedure is quickly performed. The preparatory works as well as the impact on the areas around are quite low. The seals can easily be removed for exchange or addition of pipes etc.
It is shown on examples that the implementation method for the PE granules into the openings yielded a density that was high enough to provide sufficient neutron radiation attenuation. Calculations by Monte Carlo particle transport code, MCNP, showed that the total radiation dose rate attenuation of the complete seal is as good as of the unpenetrated wall. Radiological measurements during plant start-up measurements at OL3 at 60 % reactor power (commissioning of the plant and further increase of the reactor power is still ongoing) demonstrated in April that the neutron and gamma radiation dose rate levels at these openings are kept. According to the measurement results it can already be stated that the dose rates at these plugged openings will remain at the predicted low levels when reaching 100% reactor power. The patent applications for the plugging methodology are in progress. It is available under its Framatome product name “Flexible Radiation Plugging - Flex Rad Plug”.