Most of the low-level and very low-level waste packages held by ANDRA at its industrial repositories in the Aube department (CSA) are medium-sized; the largest are metal boxes with a volume of 10 cubic metres. However, there are a few cases for which such capacities are inadequate. Waste producers such as EDF expressed a wish to dispose of large waste components directly, for technical and economic reasons, and because not cutting them up decreases operators' exposure to radioactivity.
Disposal solutions were therefore needed for such outsized waste items. Accordingly, in 2000 the safety authorities authorised the disposal of baskets previously used to store spent reactor fuel, as well as 55 PWR reactor vessel heads.
Looking ahead, the planned dismantling of nuclear installations will be a major source of large waste items. The experience already acquired and the flexibility with which the CSA repository coped with the first such outsized objects augur well for the response to future dismantling waste requirements.
PWR reactor vessel heads
A vessel head is the upper section of the chamber that contains the core of a PWR reactor. It is designed to withstand the very high pressure (155 bars) of the water flowing between the fuel elements. The control rods and other control mechanisms as well as a number of safety devices are mounted on the vessel head.
Reactor vessel heads are replaced during a reactor shutdown, after a number of years in operation. This mechanical shell becomes slightly radioactive as a result of prolonged contact with reactor contact and the related neutron flux.
Due to their weight (91 and 120 tonnes, respectively, for 900 and 1300 MW reactors), it was not possible to dispose of vessel heads in normal disposal cells. As a result, special-purpose disposal cells had to be built, equipped with a 160-tonne crane and opening roof.
At the reactor site, the vessel head is placed inside a container, which is in turn placed inside another container for the transfer by road to CSA. Upon arrival, the vessel head and its container are unloaded from the transport container and placed in a bay in the disposal cell. A machine is then set up to inject cement slurry around and inside the vessel. The interior of the vessel is filled with cement through the control rod conduits on the vessel head.
The first in a series of 55 vessel heads was disposed of at the CSA repository in 2004. The procedure was subsequently repeated at a rate of 6 to 8 vessel heads per year.
ANDRA video on solutions for outsized waste packages: View video
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