Common cause mechanical deficiency in control rods

During an outage, on 26 September 2013, NRG reports the discovery of a deficiency in one of the six control rods of the High Flux Reactor (HFR) during an outage. Based on this discovery NRG decided to delay the restart of the reactor until the root cause and necessary measures were established and carried out. During the investigation two more control rods were found to show comparable deficiencies. The deficiencies could be traced back to an unreliable joining method of two main parts of the control rods. This has caused a limited amount of flexible kinking of these control rods inside the core.

In the event of the tripping of the main coolant pumps, for example due to a failure of the external power grid, a minimum of three of the six control rods is required to shut-down the reactor and reduce the produced heat in the reactor core. The presence of a common cause failure mechanism in all control rods that may hinder the free movement of the rods seriously reduces the reliability of this safety system.

NRG has performed an in-depth root-cause analysis of the discovered deficiencies. Based on the findings of this analysis it has improved the design of the control rods and taken measures to prevent reoccurrence of the event. Furthermore, NRG has initiated a large scale improvement program on technology, safety systems and the organization. On 14 February 2014 the reactor has resumed operation.

Location: High Flux Reactor / Petten
Event date: Thu, 26-09-2013
Nuclear event report
Legenda & explanation