THE BB2 PROJECT — THE THIRD and final phase of Krško’s (NEK) essential safety upgrade — was assigned to Ansaldo Nuclear in August 2017 following an international tender. It was initiated after a review of Slovenian Nuclear Safety Administration (SNSA) requirements and to enable a plant life extension. It began in September 2017 and is due for completion in April 2021.

The scope of the phase-three project is to supply two additional safety systems, to guarantee an alternative and independent line of defence against core damage. The upgrade includes construction of a new building to house the two safety systems, which have an independent power source, providing an autonomous line of defence against core damage should the existing safety systems fail.

All the structures and components are designed to withstand the maximum likely earthquake or aircraft crash.

During the BB2 Project, Ansaldo Nuclear will perform two outages at NEK, at scheduled 18-month intervals, so work can take place inside the reactor building.

The first outage took place in October 2019. During the outage, Ansaldo Nuclear engineers installed piping and supports to connect the reactor with the new BB2 building. There were 70 supervisors, engineers and subcontractors on site for 24 hours per day from 7 October to 24 October. All components were fixed as planned with zero accidents and all 23 field design change requests (FDCRs) were completed in-situ.

Giampiero Lapenna, director of the NEK Safety Upgrade Project, describes his team’s preparations for the outage: “Thorough planning and forecasting is paramount when it comes to executing a successful outage, so it was important to factor every possible detail into our preparations.

The steps of the overall project are as follows:

  • Prepare required documentation as per Slovenian construction code.
  • Construction of the pit, which consists of protection diaphragm wall and excavation works.
  • Construction of the BB2 building.
  • Detailed seismic analysis.
  • Design as per ESP-2.602 (plant design modification) for each of the sub-projects.
  • Project implementation, which consists of BB2 pit excavation, BB2 building construction and equipment installation, project coordination & quality assurance and licensing support.

“There are real costs associated with outage management, not just in terms of delays and lost revenues, but also safety. All prospective refurbishment activities were planned with the ultimate intention of minimising the duration of the outage and ensuring that we would all return home safely at the end of the day.

“We achieved everything we planned during the outage. By employing quality assurance measures throughout the planning and execution phases, we ensured that the piping and supports we installed would have longevity, both in terms of quality and safety.”

The outage at NEK presented a number of unique challenges which had to be addressed in real time. The team was permitted to work inside the reactor seven days after the plant had been disconnected from the grid (although some preliminary activities were allowed in the first couple of days). This meant they had between 16 and 18 days to complete all contracted activities to deadline. This was addressed by assembling a specialist team of supervisors, engineers and quality controllers who were able to quickly respond to any FDCRs, ultimately installing all the required components without cutting or modifying any existing element of the reactor in any way.

Despite using this large team in a tight environment, Ansaldo Nuclear maintained high safety standards during the outage by working with experts in nuclear plant procedure, locally subcontracted. While space inside the reactor was extremely limited, the expertise and professionalism of the workforce, with regular safety briefings, allowed the installation to be completed even in the most secure environment.

To maximise safety, the full team followed a radiological educational session with a specialised course on activities in confined spaces and height. All personnel were provided with specific DPI and security belts for activity at height, along with a personal dosimeter. Welding and drilling activities were performed within tents with ventilation systems and air recirculation.

The BB2 Project commenced on-site in October 2018. The full scope is to erect a bunkerised 48/38x32m BB2 building to contain an alternative auxiliary feedwater (AAF) system to provide demineralised water to the steam generators and an alternative safety injection system to provide borated water to the reactor vessel — both tanks can also be refilled by the water from an underground well located in the vicinity of the BB2 building or by mobile means. Completion is due in March 2020.

The project scope also includes the installation of an AAF pump with the same capacity as the existing auxiliary feed pump and alternative safety injection pump with head and flow capacity similar to the existing safety injection pump. All equipment will be operated and controlled from the main control room and the emergency control room.

The installation of all the systems inside the BB2 building, and final connection of the AAF and ASI systems to the reactor building, is scheduled to be completed following the second planned outage at Krško. This is schdeulde for April 2021.

Above Image: Aerial view of the Krško nuclear plant