Damage to the compensator of the central channel

During maintenance work, damage to the compensator of the central channel was detected. This is now being removed and replaced.

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FRM II staff is carrying out work on the central channel. Here, the removal of individual components such as the flow shroud and control rod damping tube. © FRM II

During the current maintenance break, a small amount of water was detected by the automatic level measurement of the leakage extraction and collection system of the moderator tank (JFT) and automatically reported as designed. The subsequent visual inspection revealed a maximum of 1 drop of water per three minutes. The water was sampled and identified as water from the reactor pool.

To locate the leak, FRM II employees pressurized the JFT system with helium at a slight overpressure. Using camera inspection, they determined that bubbles were leaking in the central channel in the area of the double compensators.

The analyses showed that only the barrier between water from the reactor pool (H2O) and the JFT system was affected. In contrast, the barrier to the moderator tank, which is filled with heavy water (D2O) during operation, was not.

The central channel is located vertically in the moderator tank and carries the fuel element. It separates the primary cooling circuit (water from the reactor pool) from the heavy water in the moderator tank which is located around the outside. The double compensator is used to compensate for normal operating vibrations and thermal length changes, i.e. movements between the central channel and the moderator tank.

The space between the two compensators is continuously monitored automatically in order to verify the integrity of the compensators at all times and to be able to react accordingly if necessary, as has now been done.

The finding had no impact on the safety of the neutron source, operating personnel or the surrounding area. No radioactivity was released.

The central channel is now to be removed and replaced with a spare. This was routinely scheduled for 2024 and is now being brought forward. The installation of the new central channel will be followed by commissioning tests. Only then will regular reactor operation be resumed.

The event was notified to the relevant supervisory authority, the Bavarian State Ministry for the Environment and Consumer Protection, in according with the reporting criterion N (normal notification) and classified in the lowest level 0 of the international evaluation scale INES (no or very low safety significance).