Update 3 April

HFR repairs progressing according to plan

Concrete removal started

Radiation protection during the repairs

HFR repairs progressing according to plan

The repairs to the High Flux Reactor are proceeding well and according to plan. There have been, no safety incidents during the period and the radiation dose levels for internal and external employees has remained below the planned limits.
The Densimet radiation shielding has been fitted in and around the reactor vessel. Two submersible pumps have been fitted to manage the water that leaks from the adjacent storage basins into the reactor basin. Also two membrane pumps have been installed as a back up system. The concrete upper shield has been fitted above the reactor basin and the basin has been drained. After draining the reactor basin, the radiation levels were measured at the work zones and the HFR site boundary. No measurable increase was measured at the site boundary. At the work zones, the measurements were mostly in accordance with the values that had been calculated in advance. In some places, extra shielding provisions have been added in conformity with the ALARA principle (As Low As Reasonably Achievable). The Dutch Competent Authorities were present during the draining of the reactor basin and have given the approval that the project can proceed to the next phase.
The preparations for installing the support frameworks to remove the concrete in the sub-pile room have started and are running to plan. The maintenance platforms have been removed and the dismantling of other pipework is proceeding as expected. A frame has been built in the space under the reactor vessel to support it during the concrete drilling and repair activities.
Employees of subcontractor Strukton have started removing the concrete. After this, the aluminium cooling water pipework will be repaired. NRG’s top priority is the safety and health of all the employees working on the repair and in the surrounding area during this work. The restart of the reactor is planned for the second half of August 2010. During the repair process, NRG will issue a progress update every month. A special page has been added to the NRG website that contains background information, images and a video of the repair project: www.nrg.eu/hfr-repair. Updates 1 and 2 are also available on the website.

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Concrete removal started

The reducers of the High Flux Reactor that are to be repaired are located within a very thick concrete floor. This concrete must be removed to allow the repairs to be carried out. This civil engineering phase of the repair was started on Friday 9 April. As Civil Sub Project Manager, Edwin Bach is closely involved with the preparation and the execution of the ‘concrete part’ of the repair. The removal and replacement of the heavy concrete around the Bottom Plug Liner has been extensively tried and tested in recent months. “We made a replica from the same concrete as that of the original HFR floor at the premises of the main contractor Strukton,” explains Bach. “We practised drilling into this and also assessed the quality of the new concrete. We carried out a casting test on the site, using the new low-compaction heavy concrete in a mock-up.” The demolition work is particularly complicated because the concrete must be removed from underneath, so the demolishion team have to work above their heads. Investigation revealed that core-drilling is the most suitable method of concrete removal. In the space of four weeks, more than two hundred cores are drilled out from under each of the the reducers, then the remaining concrete is removed with a special hammer. During the work, the drills are cooled with water, which must be removed after careful monitoring. The very last pieces will be chipped away with an electric hand drill .
Using the mock-ups, the team were able to test and improve the drilling process experimentally and the practice runs also gave a good indication of the time needed. “At first, we only drilled a couple of holes a day,” explains Bach. “But gradually, with more experience we found that we can drill many holes in a day. We have marked out the definitive pattern for the holes using a template on the ceiling of the sub-pile room. Colour codes indicate exactly how deep we have to drill so that the execution of the task can proceed smoothly.” The core drilling is carried out during twelve hour daytime shifts of four people. During the night, employees of the NRG Decontamination and Waste Treatment department collect the concrete that has been removed and monitor the concrete and cooling water for any signs of contamination before transporting them away safely. Bach: “The plan is to be ready by the 6th May for the next phase of the process: the repairs to the reducers. Once that is successfully completed, we will start the next ‘concrete part’ of the programme: the application of new heavy concrete.”

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Radiation protection during the repairs

In the past year, part of the project organisation has been the preparation of the radiation protection precautions necessary for the HFR repair. Many of these documents and directives are part of the existing NRG Health and Safely structure, but a number have been specially prepared for this repair. Local Radiation Safety Officer Joop Lutgerink: “We started with a base document in which we included the task risk analysis for the actual repairs as well as the preparations, the test repairs and the commissioning and restart of the Reactor. In addition, documents were also drafted, for example, for the Technical Specifications for the radiation shielding and the legally required Health and Safety Plan for all staff”. During the whole repair period, around forty external workers will come on to the site. To ensure the safety of these people, a number of special radiation safety directives are in force. All new external workers receive comprehensive training on their first day. During their work they are watched over continuously by an NRG supervisor. “To make all this possible, we also trained our own personnel so that they know what their duties and responsibilities are,” states Lutgerink. “As well as the Health and Safety Plan mentioned earlier, we prepared additional safety regulations for third parties. These cover subjects such as work permits and emergency situations and also indicate what our sanctions are if the safety rules are breached.” Both the individual radiation dose levels of workers and the collective dose for each task are being accurately recorded in a dose register. Lutgerink: “We take responsibility for the safety of our internal and external employees and the environment. In practice, the plan will be continuously monitored and any improvements implemented if required; as we have made good preparations we have the scope to do this.”

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