LTO support projects of Borssele NPP and Ringhals NPP

There is a need for a generally accepted approach for LTO assessment of nuclear power plants (NPPs), as an increasing part of the nuclear fleet approaches the original design lifetime. The IAEA developed Safety Standards and other guidance to provide for such approach. Two examples will be given of LTO assessment projects which used this IAEA guidelines and some additional analyses. Borssele NPP in the Netherlands used IAEA Safety Guide NS-G-2.12 and Safety Report No. 57 (later incorporated into and superseded by IAEA Safety Guide SSG-48) as a basis for their LTO assessment project. The experience gained in the Borssele LTO assessment project was used for the set-up of the LTO assessment project for Ringhals NPPs in Sweden.

The Borssele nuclear power plant (in Dutch ‘Kerncentrale Borssele’ or KCB) planned for operation until 2034 (60 years). On 16 June 2006, a covenant between the owners and the government was signed in which operation until 31 December 2033 was agreed upon. 40 years of operation was assumed in the original safety report (SAR). A formal license change application process was submitted based on a comprehensive LTO assessment project to revalidate the SAR for 60 years of operation.

In the Netherlands, the nuclear regulatory requirements are contained in the Nuclear Energy Act. Within the Nuclear Energy Act the so called Nuclear Safety Rules (NVRs = Nucleaire Veiligheids Regels) provide the basis for a system of more detailed safety regulations for NPPs. This set of rules is based on the IAEA Safety Standard Standards. Application of the NVRs is monitored by the ‘Kernfysische Dienst’ (KFD), the Dutch nuclear inspectorate. The IAEA guidelines available at that time were applied for this project:

The IAEA Safety Report No.57 was largely based on US NRC rules, which assume that any plant implementing LTO also applies the Maintenance Rule (10 CFR 50.65). The Maintenance Rule ensures proper ageing management of active components, however, the Maintenance Rule is not mandatory in NPPs that do not fall under the regulations of the US NRC. Therefore, evaluation of active components was included in the LTO project in line with the methodology of the Maintenance Rule. The active components are classified into three groups: mechanical, structural/civil and electrical.

Note: Currently valid IAEA Safety Guide SSG-48 already clearly recommends to manage ageing of both passive and active SSCs.

To be able to revalidate the fatigue analyses for LTO including the incorporation of possible environmental influence, best estimate calculations of the fatigue life are needed including realistic assumptions of the (thermal) loads. The FAMOS system was implemented for this reason during the yearly outage in 2010. It abled to precisely monitor thermal loads including stratification. A systematic review of fatigue assessment was performed for all component locations in the scope. A cumulative usage factor for expected number of transients in 2034 (CUF2034) was calculated for every in-scope component location.

Detailed assessment was performed for the locations where CUF2034<1 could not be demonstrated for LTO period to prove that adequate safety margins against crack initiation by fatigue are in place. Calculations and additional measures, a sound basis was given for the prevention of crack initiation by fatigue for the period of LTO.

LBB is part of the break preclusion concept at Borssele NPP. The scope for break preclusion for LBB is:

The goal of the review was to demonstrate that the Break Preclusion concept (Bruchausschluß), as implemented in 1997, remains valid for LTO until 2034. The assessment concluded that the time dependent assumptions in LBB TLAAs are not restricting operation for 60 years.

NRG used the experience of the Borssele NPP LTO assessment project to set up the LTO assessment project of Ringhals NPP.
After development and agreement upon the LTO approach, Ringhals NPP and NRG had set-up a more detailed LTO methodology for Ringhals units. During several workshops, NRG consultants trained Ringhals NPP staff to understand related IAEA guidance and suitable way how to implement it. Based on these workshops and with support of NRG consultants, Ringhals staff defined their LTO methodology. The IAEA IGALL TLAAs were used as a basis for identification and revalidation of Ringhals NPP TLAAs. The Ringhals NPP LTO methodology was positively evaluated during several IAEA SALTO peer missions.