The High Flux Reactor (HFR) is an important player in the field of radioisotope production for the medical sector and plays a prominent role in international energy research. In August 2008 a routine in-service-inspection detected a jet of gas bubbles in one of the reducers, a part of the HFR primary cooling water system. Although there was no external water leakage; NRG decided not to restart the reactor before conducting a more detailed evaluation of the bubbles and the extent, cause and possible consequences of the deformations. On February 12th 2009 NRG received permission to restart and temporarily put into service the reactor, under a regime of extra safety measures for the reducers and having assessed the prospects for carrying out a full repair. As part of the temporary HFR return-to-services programme; NRG generated a plan to restart the HFR and in parallel, prepared for a full final repair of the cooling water system.

The repair preparations took 12 months. During that period, the repair techniques and the assembling of the radiation protection that would be required were extensively checked and tested. The staff, the materials and the repair methods were qualified by external organisations in consultation with the competent authorities. At the request of NRG and the competent authorities, various independent Dutch and foreign experts independently evaluated every aspect of the repair work, including providing supervision during the execution of work.

On February 19^th 2010 the final repair started. The parts of the primary cooling system that needed repair were located under the reactor core and were set in a thick concrete floor. The difficult location and the strict safety demands made the task exceptionally demanding. During the repair, the safety and health of all employees involved were of the highest consideration at all times.

To protect the workers against residual radiation emanating from the reactor core area, a specially tailored radiation shield was fitted in and around the reactor vessel with the aid of remote-controlled manipulators. Afterwards, the reactor basin was drained to work in a 'dry' environment. Subsequently the reducers were revealed from underneath by concrete core drilling. The inspection and measurement results of the parts of the primary cooling system that were revealed confirmed the condition of the corroded reducers.

In consultation with the competent authorities NRG decided to execute a local repair. The corroded parts were cut out and new aluminum pipe sections were welded back in place. The welding phase was successfully completed on June 30^th 2010. Afterwards, the concrete floor was repaired using heavy concrete. The removal of the radiation protection proceeded according to plan as well. After extensive safety tests including a five-day containment test to verify the leak tightness of the HFR dome, NRG successfully returned the High Flux Reactor back into service on September 9^th 2010.