An Update of the Assessment Methodology for Civil Ageing Management for LTO/CSO Based on International Standards and Engineering Judgement
Authors: de Haan - de Wilde F. H. E. , Janssen M. J. 
Published: October 2021

For many nuclear power plants worldwide the operation period will be extended to 60 or 80 years in the coming years. As the operation period increases, the importance of knowledge of ageing mechanisms increases. In the framework of LTO there is limited knowledge about ageing and structural integrity of concrete structures. Recent developments have shown that ageing of civil structures receive more attention internationally. In order to increase the knowledge in the field of civil structures, this paper focusses on investigation of ageing of civil structures and determining an ageing management strategy. Knowledge of the ageing mechanisms of civil structures and especially concrete, will lead to improvement of ageing management and assessment methods of concrete.

As a first step international information was gathered on civil structures ageing issues and management thereof (see PVP2019-93029). In addition a highlevel assessment methodology was proposed. In the next step the initially proposed assessment methodology has been tested by application to a nuclear reactor. The resulting list of relevant AMPs has been verified with the outcome for another PWR with a steel containment. With this experience the assessment methodology is tested, compared and improved (see PVP2020-21838). The results indicated that the method can be used to obtain a list of plant specific AMPs. What was added to the assessment method is the link to the TLAAs for civil structures.

In this follow up step the transition is made from a high level of IGALL AMPs to a practical AMPs that will deal with the right mechanism at the right location. The detailing to a level of practical work instructions for the maintenance of the plant has to be made in order to make real life implementation possible. In this step studying of relevant degradation mechanisms, relevant AMPs (like IAEA AMP305 and AMP306 ) and applicable literature in combination with the practical knowledge from operation of a reactor, has taken place. The international developments on ageing management of concrete will be included.

The goal of the project is to obtain more knowledge on ageing management of civil structures and especially concrete. It will lead to an assessment method for civil ageing management and ageing management programs dealing with the relevant mechanisms at the various locations in a practical manner.

The results of this ongoing work are presented in this report. For the research reactor all SSCs in scope of the Continued Safe Operation could be linked to the relevant AMP(s) and a resulting set of plant specific AMPs for civil ageing management was obtained. Including the international developments, literature and guidelines, a more general applicable list was created (Table 5 through Table 13).

The conclusion is that Figure 2 represents a practical method for obtaining a set of plant specific civil AMPs ready for implementation. For representation in this paper the final outcome is given in as a generic list of actions for a generic reactor (Table 5 through Table 13). In these tables the relevant SSCs, ageing mechanisms and actions are listed. The tables represent an generic list of actions for civil ageing management that might others help develop their ageing management program.

Future steps are shifting the focus from the general but practical assessment methodology to finite element modelling techniques for concrete. The assessment criteria for concrete (e.g. in ASME III, ASME XIII or Eurocode) will be investigated and investigation on the modelling of the concrete for ageing are planned.