Long Term Operation for Borssele and Ringhals

LTO EPZ & Ringhals

27 mei 2020

A complete LTO approach was set up

Performed on both Borssele and Ringhals NPPs

Both LTO projects have led to regulator approval

LTO at NRG

The LTO team of NRG led by Frederic Blom is involved in many LTO projects. These are spread far and wide: from Sweden to Argentina. The LTO team offers:

  • Reliable and independent LTO services
  • Tailored to each specific nuclear power plant
  • Consultants with global experience
  • Our in-house developed TULIP approach

Long term operation: searching the best approach

There is need for a generally accepted approach for Long Term Operation (LTO) assessment of Nuclear Power Plants, as an increasing part of the nuclear fleet approaches the original design lifetime. Specific IAEA guidelines are written to provide such an approach, but these do not include all beneficial analyses. Two examples will be given of LTO assessment projects which have used these IAEA guidelines, plus some additional analyses. Borssele NPP in The Netherlands used IAEA safety guide 57 as a basis for their LTO assessment project. The experience gained in the Borssele LTO assessment project has been used for the set-up of the LTO assessment project for Ringhals NPPs in Sweden.

EPZ (2)
Jan Willem Spoelstra Borssele

Borssele LTO assessment project

The Borssele Nuclear Power Plant (“KernCentrale Borssele”, KCB) planned for operation until 2034 (60 years). On june 16th 2006 a covenant between the owners and the government was signed in which operation until December 31st 2033 was agreed upon. In the original safety report (SAR) 40 years was assumed. To revalidate the SAR for 60 years of operation a formal license change application process was performed based on a comprehensive LTO assessment project.

Performing to governmental standards

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 VeiligheidsRegels) provide the basis for a system of more detailed safety regulations for nuclear power plants. This set of rules is based on the IAEA Safety Standard Series (SSS). Application of the NVRs is monitored by the “Kernfysische Dienst” (KFD). KFD is the Dutch nuclear inspectorate. During this project, we had to comply to IAEA guidelines:

  • IAEA safety report No.57
  • IAEA safety guide No.NS-G-2.12
Performing To Governmental Standards 480X480

Scoping, screening and Ageing management Review

As a starting process for the assessment in LTO “Bewijsvoering” the scoping was performed. Within this step, all SSCs in Scope of LTO were identified on a system level based on the criteria from IAEA Safety Report  No.57. The Ageing Management Review (AMR) involved detailed technical evaluation of in-scope passive long-lived components (e.g. Main Coolant piping) as well as passive subcomponents of active longlived SCs (e.g. Main Coolant Pump casing) to demonstrate that the effects of ageing are adequately managed such that the intended function(s) will remain consistent with the KCB licensing basis during Long-Term Operation. All reports were reviewed by the Dutch nuclear regulatory authority.

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Active Components 480X480

Active components

Safety Report 57 is largely based on USNRC 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, this aspect is not addressed in SR57. The Maintenance Rule is not mandatory in NPPs that do not fall under the regulations of the US-NRC. Therefore, evaluation of active components is included in the project scope, in line with the methodology of the Maintenance Rule. The active components are classified into three groups: Mechanical, Structural/Civil and Electrical.   

Revalidation of reactor pressure vessel

The TLAAs identified at KCB are: Reactor Pressure Vessel, Fatigue, Leak Before Break and Qualification of Design Base Accident resistant electrical Equipment (EQDBA).

The time limited ageing mechanism for the KCB Reactor Pressure Vessel is irradiation embrittlement. A safety assessment of the Reactor Pressure Vessel (RPV), including the assessment of irradiation induced ageing of the KCB RPV, has been carried out. The structural integrity of the RPV with respect to operation, irradiation surveillance and Pressurized Thermal Shock (PTS) analysis is assessed using fluence calculations validated by shadow calculations and scraping samples from the RPV. Finally, the RPV safety of KCB is evaluated in terms of the up-to-dateness of the assessment methods used (including Master Curve) and by a general benchmark of the KCB results with RPV safety assessment data worldwide. It could be proven that the safe operation of the KCB RPV is guaranteed by comprehensive state-of-the-art methods for all load cases with large safety margins.

Fatigue Revalidation 480X480

Revalidation of fatigue

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 on the (thermal) loads. For this reason during the yearly outage in 2010 the FAMOS system was implemented which is able to precisely monitor thermal loads including stratification. For all component locations in the scope, a systematic review is performed on the available fatigue assessments. Based on a comparison of the number of transients in the analysis with the expected number of transients in 2034 an expected CUF2034 is calculated for every in-scope component location.

Further assessment is performed for the locations where CUF2034<1 could not be demonstrated before going into LTO to prove that adequate safety margins against crack initiation by fatigue are in place also during LTO. With the assessment and the follow-up of the recommendations a sound basis is given for the prevention of crack initiation by fatigue for the period of LTO.

Revalidation of leak before break

Leak Before Break (LBB) is part of the break preclusion concept at KCB. The scope for Break Preclusion for LBB is:

  • Primary piping
  • Main steam and feedwater lines within the secondary containment

The goal of the review is to demonstrate that the Break Preclusion concept (Bruchausschluß) as entered in 1997 remains valid in case of life time extension to 2034. From the assessment it is concluded that the time dependent assumptions in LBB TLAAs are not restricting operation for 60 years

Revalidation Of Leak Before Break 480X480

Qualification of Design Base Accident resistant electrical Equipment (EQDBA)

Due to the Harrisburg accident it was realized in the mid-1980s that the electrical components didn’t have a qualification for harsh environment conditions. Subsequently, a list of electrical equipment needed to manage the various accidents was developed based on design base accident scenarios and required safety functions. The selected hardware was qualified in conformity with the German KTA standards. The EQDBA TLAA is assessed by the implementation of a method to establish the qualified life of each component with a harsh environment qualification for LTO. The EQDBA project has led to the qualification of design base accident resistant electrical equipment, where for components with insufficient data requalification and replacement programmes are carried out.

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Applying the lessons learned elsewhere: Ringhals LTO

NRG has used the experience of the Borssele LTO assessment project to set up a plan of approach for the LTO assessment project of Ringhals NPPs.

After the plan of approach Ringhals and NRG have set-up a more detailed LTO methodology for all four Ringhals NPPs. In several workshops NRG explained the LTO activities mentioned in Safety           report   57. Based on these                workshops Ringhals, in consultation with NRG, defined their LTO methodology. Ringhals decided to make use of the IAEA IGALL project to define the TLAAs for the LTO project. The LTO methodology of the Ringhals NPPS has also been assessed during several IAEA SALTO peer reviews.

  • Primary piping
  • Main steam and feedwater lines within the secondary containment

The goal of the review is to demonstrate that the Break Preclusion concept (Bruchausschluß) as entered in 1997 remains valid in case of life time extension to 2034. From the assessment it is concluded that the time dependent assumptions in LBB TLAAs are not restricting operation for 60 years

Isi Ringhals

Conclusion

The IAEA guidelines provide a generally and globally accepted methodology for LTO assessment. Two examples of LTO projects which have used these guidelines were presented. Borssele NPP in The Netherlands used IAEA safety guide 57 “Safe Long Term Operation of Nuclear Power Plants” as a basis for their LTO assessment project. The outcome of the LTO assessment project was submitted to the Dutch regulator for a license change application to revise the Safety Report for long term operation until 2034. In 2013 the license change was approved by the Dutch regulator. The experience gained in the Borssele LTO assessment project is used for the set-up of the LTO assessment project for Ringhals NPPs in Sweden. Ringhals also adopted IAEA Safety report 57 for the general structure of their LTO project which have led to approval of Ringhals LTO.

More information?

Contact the LTO team of NRG led by Frederic Blom.

Lto At Frederic Blom Nrg 640X640

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