Mock up for dismantling reactor components

All nuclear facilities have to be decommissioned and dismantled at some point. Besides the maximum period of operation laid down by law, other reasons for decommissioning may be political decisions or economic drivers. When dismantling nuclear facilities, the technical and administrative conditions must be considered just as much as compliance with the safety goals set out in the relevant legislation. In Germany that means the Radiation Protection Ordinance and the Radiation Protection Act.

To work as cost-effectively as possible during decommissioning, attention must be paid to the type and quantity of the secondary waste generated, as well as to the reliability of the selected technology, the complexity of the tools used and the associated maintenance costs. Therefore, the machines and tools are ideally first tested in a test facility before they are used in the nuclear power plant. This kind of testing on full-size models of large components is known as a mock-up. Since repairs during the actual dismantling work in nuclear facilities can be risky or even impossible, all the needed work steps are first tested at the mock-up facility, similar to a dress rehearsal. Mock-up testing helps verify the functionality and can also support optimisation of the different processes. A test facility free of radiation-related risks enables the simplified implementation of optimisations. Meticulous testing of the equipment at the mock-up also makes it possible to get rid 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.

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.

After the RPV has been dismantled and packaged, the RPV and the reactor building are in the final state conditions as follows:

• The RPV components are removed from their installation positions, dismantled, and handed over to the customer in accordance with the approved disposal method,
• The contractor’s equipment as well as any machines and devices used by the contractor are removed from the dismantling area (clearance measurements and release are carried out by the customer; material that is not cleared is handed over to the customer as radioactive waste),
• Any enclosures or seals created by the contractor for ventilation purposes have been dismantled and disposed of by the contractor,
• The waste containers and the associated documentation have been handed over,
• Other components that do not require final disposal packaging have been packaged according to the existing requirements and handed over to the customer,
• At the customer’s request, the resulting open edges (especially the reactor cavity) are secured by the contractor by means of railings. Edges are also secured during the construction phase in accordance with the relevant safety regulations.
• Any other requirements and safeguards related to the work areas are prepared and carried out by the customer accordingly.

Pre-segmentation

Based on the pre-segmentation design and the associated cutting plan, the RPV is dismantled using the following work steps:

• Segmentation of the pool sealing

The segmentation is carried out by means of a plasma cutting system and/or an angle grinder, a saw or similar tooling.

• Removal of insulation per ring segment

The insulation is removed manually in the pre-segmentation phase in-situ using a shielded working cage and required pole tools (handling bars). The removed insulation bricks and metal structures are segmented again at a separate location and are assigned to their appropriate disposal route or packaged in such a way that they can be loaded into designated containers.

• Marking for flame cutting start holes

To perform thermal cutting, start holes need to be drilled first. The positions of the start holes are marked with the help of a marking module which is guided by the arms of the lifting beam.

• Drilling of start holes

Start holes are drilled by a specially developed drilling device. The drilling device moves from the inside to the outside. The process is carried out remotely and is controlled and monitored from the control room.

• Cutting of ring segments

The tool platform (also called the ‘working platform’) is positioned on the RPV ring by means of the lifting beam and the reactor building crane, and lifting holes are cut using the flame cutting method. These are used by the lifting beam to lift out the RPV ring segment after the final horizontal separation cut. To do this, the tool platform carries the guide system for the cutting system, which is used to perform the circumferential separation cut remotely. The reactor pressure vessel is pre-segmented one section after another from top to bottom. The tool platform is also used to extract the smoke produced during the separation cut.

• Lifting out ring segments and the calotte

After the final cut, the tool platform is removed and placed in the storage position. The ring segment is then lifted with the help of the lifting beam and placed at a cleaning location in the pre-segmentation area. The RPV ring segment is cleaned on the outside to remove any possible residues.

The dismantling activities on the RPV within the pre-segmentation area are completed with the removal of the calotte, as well as the subsequent removal of the support frame and the remaining control drive parts. The post-segmentation of the RPV rings into segments suitable for packaging takes place in the post-segmentation area.

Post-segmentation

The separated ring segments are placed on the reactor building operation floor or on the reactor pool bottom. The location of the post-segmentation area depends primarily on the possibilities and conditions on site and the resulting space conditions.

The reactor building crane is used to transport the cut ring segments of the RPV to the post-segmentation area. The pre-segmentation design ensures that the maximum weight of the RPV segments, in particular that of the RPV calotte, does not exceed the permitted operational load capacity of the reactor building crane.

At the post-segmentation area, the post-segmentation into smaller segments suitable for packaging begins. At the same time, the preparatory work is started for cutting off the next ring segment of the RPV.

The segmentation equipment includes a rotating cutting table and a cutting robot with an attached oxyacetylene torch. The packages are placed into packaging containers provided by the customer. As in the case of pre-segmentation, cutting at the post-segmentation area is carried out remotely by means of thermal cutting techniques.

The thermal cutting process is a proven cutting technology in the conventional dismantling of large components. In nuclear facilities, the spread of contamination by cutting fumes is prevented by local extraction systems and by enclosures for pre- and post-segmentation operations with directed airflow ventilation and filtering. Exhaust air filter systems are used to prevent the spread of hazardous substances.

As part of the dismantling process, regular contamination prevention and cleaning measures are applied, both within the post-segmentation area and near the reactor cavity. These measures include both the extraction and filtering of fumes and tiny particles by means of smoke extractors and the capture of slag particles close to their formation by means of collection funnels, insofar as this is technologically feasible. In addition, cleaning of the affected areas is carried out after each segmentation campaign. After segmentation, all of the affected areas are cleaned once again to remove contaminants.

Although the dismantling equipment was extensively tested in a mock-up facility beforehand, improvements were made on site.

Lessons Learned

NUKEM Technologies has successfully dismantled and packaged two complete RPVs in the last 24 months. The first rings of a third RPV have been cut out and temporarily stored for further dismantling. The equipment is currently being moved to another site, where the dismantling of two more pressure vessels is planned in the coming months.

Even though these reactors are not the same, there is regular exchange between the different project teams to share findings. Reduction in dismantling times and the associated cost reductions can be achieved through the experience of previous projects. The goal is to save costs and time and to increase the overall quality of work. Within the projects, experience is also exchanged between the construction site and the engineering departments in lessons-learned sessions so that these findings can be incorporated into follow-up projects as early as in the planning and design stage.

Furthermore, as the dismantling equipment is used at the site, further modifications and improvements are made. There are always some problems that remain undetected even during the extensive mock-up testing phase. One reason is that it is very hard to achieve identical reproduction of the conditions on site. The equipment also has to be adapted to the structural conditions on site, which often deviate from the design documents that are available. As a result, practical solutions are found which ensure a significantly higher level of safety for the processes and shortened operational times during the dismantling of the following reactor disassembly projects. For example, the dismantling time of the second RPV could be halved to six months, plus assembly and disassembly of equipment. The reduction in the cutting times results in significant cost savings due to the lower personnel costs on the construction site. In addition, the customer benefits on a case-by-case basis from the shorter dismantling time as the reactor room can be released earlier to be used for further activities.

Authors: Burkhard Ko¨nning, Deputy Head of Projects & Phillip Thalmann, Project Engineer, NUKEM Technologies