DISMANTLING LARGE COMPONENTS | DECOMMISSIONING


of any initial weak points and to determine optimal process parameters before the equipment even gets to the reactor site. That way it is possible to minimise the risk of project delays and means it is better able handle tight deadlines for dismantling activities. Among the core requirements for the detailed design of the dismantling process is the weight information and radiological data provided by the operator, as well as an approved packaging concept. One area which can benefit from developing a mock-


up to model processes is in the decommissioning of large reactor components, such as the reactor pressure vessel. Such elements are typically unwieldy and cannot be removed as a single component. Furthermore, RPVs represent a radiological hazard. As a result, they warrant the development of a full-scale mock-up to allow work process to be explored in detail prior to on-site work commencing.


Dismantling an RPV The dismantling of an RPV and its peripheral components is normally performed sequentially. The dismantling process itself is split up in two major sub-procedures, which are called pre- and post-segmentation. This division enables parallel work and therefore allows for a more efficient overall execution of the task. The term pre-segmentation is used to describe the dismantling tasks, which are executed while the components are still in their original assembly situation. Conversely, post-segmentation includes all cutting processes creating segments of a size which is suitable for the containers they will be stored in. These tasks are carried out in a separate caisson, where the big segments cut in the pre-segmentation will be transported for further works. The pre- and post-segmentation of the components is always carried out in different rooms. This procedure reduces the dismantling time and the related personnel costs as well as the costs for maintaining the remaining operation. A thorough check of the equipment at the mock-up


facility guarantees that when the equipment is used for the actual on-site dismantling activities, it is already free from any initial flaws, and the optimal process parameters are defined. This makes it possible to adhere to tight dismantling schedules by minimising the risk of delays. The dismantling of the reactor pressure vessel (RPV) and the peripheral components is typically divided into three major phases:


● Fragmentation of the RPV lid, ● Fragmentation of the RPV skirt including separation of


the connected pipelines, control rod drive housing tubes (CRD housing tubes) and RPV calotte (spherical bottom of the reactor pressure vessel),


● Dismantling of the thermal insulation.


The dismantling process begins with pre-segmentation / cutting of RPV ring segments in their installation positions (in-situ). This is followed by the subsequent dismantling of the RPV ring segments and then placing them into packages suitable for radiological decay or final storage in the containers provided by the customer. x-head: Initial state vs final state For all nuclear power plants undergoing


decommissioning, the initial state is assumed to be as


follows: ● The RPV internals have been completely removed and taken out of the reactor building,


● The RPV has been decontaminated, ● All systems, including the RPV, are drained and free of residues,


● The RPV lid is placed on the RPV flange or already dismantled,


● The outgoing pipes from the RPV (feed water, main steam, reactor water purification system, etc.) are separated and sealed,


● The CRD housing tubes have been completely dismantled and transported away,


● The shielding bars have been removed and disassembled,


● Other covers such as the loading cover have been removed and disassembled,


● The storage areas on the reactor building operation floor are freely accessible,


● The reactor building crane is ready for operation, ● The refuelling machine is in the parking position


(if necessary, the refuelling machine is to be dismantled),


● The individual work areas, e.g. the reactor building operation floor and CRD housing tubes are recorded in their initial state (radioactive contamination, contamination by asbestos, etc.) and handed over to the customer. U


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Above right: Drilling device in the RPV


Above left: Lifting beam with the marking module for marking the start hole position


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