The following maintenance tasks were performed during mini-shutdown of 2022 with the goal of resolving the Fault 14966 (target water flow rate low).

• Replace the o-ring, paddle wheel, and shaft on the following flow rate sensors:
  • Q1 (FGTGT)
  • Q2
  • Q4
• Replace the demineralizing flow metering valve with a new needle valve (the old one was flaky and appeared to occasionally restrict flow)
• Re-tune the demin. flow to 1.15 gpm
• Top up the expansion tank due to water loss during the maintenance job (from 27.1 cm level to 34.8 cm)

To test the theory that the demin. flow metering valve was faulty and had an effect on the target water flow rate during operations, the valve was closed to determine if it stopped flow to the target. The result is that it did not, and only flow in the demin. circuit was affected. Therefore, it is likely that the root cause of Fault 14966 was not due to the demin. metering valve, but rather an issue with the pump, heat exchanger, pressure in the lines, or another form of obstruction. It should be noted that when the pump is turned off, all flow rate and pressure sensors drop to zero (except the pressure sensor located in the BL1A Service Tunnel) - so if the fault occurs again and symptoms show this case, it is likely that there is an issue with the pump intermittently turning off or similar.

As an aside: it is recommended in the future when changing circuit boards on the flow rate sensors to use metal screws instead of the plastic screws supplied with the parts.

Dose rates observed in the work area: 300-500 uSv/h at the target cooling package area, 1300 uSv/h at the target monolith area.