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Cooling tower, where the inner components have been removed for inspection. © FRM II / TUM
Ten years after its first criticality regulations require the FRM II to do extensive tests on its main systems. Goal of these tests is to prove the safe operation of the FRM II for the years to come.
Types of test are mainly pressure tests and visual inspection of the principal components close to the reactor core. Since such checks cannot be done under operational condition the reactor has been shut down on Feb. 9th, 2014.
The components to be tested are mainly the central channel, the moderator vessel, the tips of the neutron beam tubes on the reactor side and their flanges on the experiment side, the cold and hot sources and parts of the heavy water system including the fast closing valves in the moderator cooling circuit. Also, calculations are under way to prove sufficient safety margins are still present for the aluminum components that suffer embrittlement under the influence of neutron irradiation. Most of these checks are to be carried out with participation of the expert organization; their results have to be approved by the licensing authority of the FRM II.
The shutdown period is also used for work on other components that require a longer time without reactor operation. The biggest of which is doubtlessly the cooling tower, where all the inner parts have been removed to inspect the steel frame and to improve the rust-protective coating. Arguably the most important components are the shutdown rods, where extensive maintenance is carried out. However, numerous other parts and components indispensible for the successful operation of the FRM II underwent thorough inspection, maintenance or even replacement, whatever appeared appropriate or was required by law.
The maintenance break started with the usual tests that routinely need to be performed after every shutdown. Then, to get ready for the inspection and to minimize contamination and irradiation of both personnel and equipment, the heavy water (D2O) was drained off the moderator vessel. It then was dried and filled with light water (H2O). Pressure tests were carried out on the moderator vessel, the central channel and the beam tubes, all accompanied by visual inspection. Also for the secondary sources the mandatory checks were successfully performed and confirmed their excellent condition.
Limited accessibility due to space and irradiation limits made some of these tests a challenging task. Most of these have never been carried out at the FRM II before and are highly unusual due to the very special nature of the FRM II.
Meanwhile, all major tests have been concluded and demonstrated the impeccable condition of the FRM II.