Above: Olkiluoto features two BWR nuclear power units and a new PWR located near Rauma in Finland

In late April 2022 Finland's nuclear power plant operator Teollisuuden Voima Oy (TVO) began its regular annual outage at unit 1 of the Olkiluoto nuclear plant. Named after Olkiluoto island in western Finland, the plant has two operating 890MWe BWRs that were commissioned in 1978 and 1980 with commercial operations following on a year or two later. Collectively the units supply around a fifth of the country’s electricity demand. Designed and built by ASEA- Atom, now part of Westinghouse, the plant was returned to service in early June.

‘Our normal strategy is that we have two kinds of outages,” explains Sauli Suoniemi, Maintenance Manager of OL1 And OL2. “A fuel outage takes place every year for one unit and service outage for another. For the fuel outage we change fuel and perform mandatory tests and some pre- maintenance. We also deal with those forward notifications which we must handle in an outage. In a service outage we make practically all pre-maintenance, all inspections and other modifications. Standard lengths for these outages are seven days for fuel outage and 14 days for service outage. Then every five or six years we do a longer service outage when we do bigger modifications like we did for example this year. For unit 1, this took 32 days and was extra-long for us,” he adds.

The strategy sees TVO ‘save up’ various modifications over the previous two or three outages which can be undertaken during the longer outage. During the latest outage a whole range of preventive maintenance activities, inspections and modifications were carried out. The most significant work packages included, for example, the replacement of pumps and valves in the shutdown cooling system, renewal of electrical bushing assemblies in containment, inspection of the reactor pressure vessel bottom, maintenance activities in the service water channel and concreting one of the service water channels as well as replacement of impellers in feedwater pumps, installation of a new recirculation line and a pressure test of the reactor primary circuit.

“We don’t typically need extra-long outages, we have a spread out pre-maintenance programme so that we can do all our free maintenance during a normal 14 day service outage. These extra-long outages are just for modification projects when we have to, for example, change our big primary circulation pumps and large valves. When the modification takes too long for the normal service outage then we make this extra-long service outage. Actually, practically the only maintenance-related work is our primary circuit pressure test as that is too long for a normal two-week outage. However, that’s just once in every eight years so these longer outages are just for big modification projects.

Avoiding obsolescence

Nuclear plants are typically very long-lived assets and given this longevity – OL1 and 2 have already been operating for well over 40 years – one issue that can demand modification and upgrading is the potential for key components to become obsolete. The original equipment manufacturers may even go bust or be taken over during the decades of operational lifespan. At a certain point it’s better to simply replace a unit for a more modern equivalent. This was behind a key upgrade during the latest outage in which saw a number of water circulation pumps replaced.

As Suoniemi says: “These pumps which we did this year are from late 1970s and those were the original pumps. They worked fine and there were no problems from that perspective but it’s hard to get spare parts. Those are technically old pumps and that’s normally the primary reason to make these modifications. Of course, when we are changing to new components we check to see if we can get more efficient equipment and we normally do get more efficiency. We make these modifications also for the future, when we see that we are going to change some systems, let’s say in five years for example, then we have to make these additional modifications so that they are still valid when we do something else.”

Although modernisation and replacement are one weapon in the armoury for TVO, it may also be the case that a stronger maintenance regime could be more economically advantageous. “Obsolescence is one point of concern when we make these decisions but it’s also vice versa.

As with any normal asset management programme it’s a balancing act, you have to check to see if the maintenance costs are too big sometimes or if it’s cheaper to do more maintenance than to change a piece of equipment for a new one,” says Suoniemi, adding: “When we are planning or making decisions if we have to change some equipment then we also check if it’s more efficient to carry out more maintenance for example, a big overhaul project or something. We have done that also, for example, for large valves which can be intact but have a worn pivot bearing. Then we can carry out a major overhaul project to make the old existing equipment like new.”

When looking to replace or upgrade a component though, it is becoming increasingly difficult to source appropriate components. “It is clear that nowadays in the nuclear business it’s not a buyer’s market. Sometimes it’s very hard to find suppliers which are willing to deal with our demands because we are kind of a small market and we have extra needs, so it’s not easy,” says Suoniemi, adding: “It’s not black and white but there is this phenomenon because even in Europe the situation has been affected by Fukushima, which added to the supply chain strain when some countries left the nuclear sector. That may change now we’re seeing a nuclear being reappraised in light of the climate issue.”

Any part replacement programme must also tie in with the existing regulatory requirements to ensure that any modifications are qualified. “When we are dealing with the safety-related systems then all the equipment and any modifications must be qualified by our own officials. For reliability, if we are dealing with the safety-related systems, for example, in unit one and two we have four subsystems. Once you’ve found a piece of equipment that meets your specs then you trial it for quite a long time.

We install one component in the equipment for one subsystem then we get operational experience and then when we are sure that it’s working well and we won’t have any surprises then we can roll it out on other three subsystems once it’s proven. It’s a potential nightmare if we install all four subsystems at the same time and then find there’s some surprises so we ensure reliability in this way,” notes Suoniemi.

Using maintenance to improve efficiency While replacing components can prevent potential problems with obsolescence, the comprehensive maintenance programme put in place by TVO also allows the overall system to be improved over time. Upgrades support efficiency improvements by reducing parasitic loads from pump motors and the like. During the five-year modernisation outage TVO therefore assesses potential gains in plant output. Suoniemi explains: “Motors can be more efficient and the equipment can be more efficient so one point is that we check to see if we can make our own processes more effective in the modification process. If we can get more efficiency from our plant by putting in new equipment then it’s very important because we have to aim that we have a more efficient plant and that’s also wise in terms of economics if we can get more efficiency from our plant than we can get more with the same assets.”

He cites a upgrade undertaken during the previous extended outage: “Five or six years ago we made a modification project and changed our main sea water condensers. The old ones were of a technical age where we thought we were at the end of their lifespan and we got more efficient main condensers and more net power out of our plant. Those new condensers will pay for themselves and also we changed our high pressure heat exchangers that also gave us a bit more net power.”

Replacing the impellers for the feed water pumps is another modification that has improved efficiency but has also improved safety. “The old ones were wearing and so it was time to change them and we changed them more effective next-generation design ones from our OEM partner. Now we can get more from our feed water pumps and we don’t need so many pumps for normal running.

We have four main feed water pumps and with these new impellers in all normal circumstances we can use just three of these pumps. We had a situation beforehand that we could drive the system with the three pumps but not in all circumstances. Now we can use tree pumps in every case,” says Suoniemi. This leaves a significant reserve capacity potentially but also represents a 25% reduction in main feed water pump power demand which in turn saves quite a substantial volume of power used in plant systems. “Beforehand if the one pump failed then it wasn’t possible to get the full power output from the plant. It not only improves the safety margin but also improves performance. From that perspective it’s a good move too,” he says.

“This project is quite a good example where we had options to change the whole pumps or change just the impellers and there were other choices also. We made a decision that we would keep these old pumps and we overhauled some elements. Those pumps are driven using hydraulic clutches and we also made some optimizations for the clutches too as well as changed the impellers. We’re also going to make some erosion repairs to the pumps. This kind of project has many steps but afterwards we get almost like new pumps without changing the whole unit.”

There has been an on-going process of improvement for the plant and follows on from projects such as the steam turbine replacement that took place some 17 years ago under the Turbine Island Modernisation project (TIMO). Timed to coincide with the annual outages in 2005-2006, this programme saw the replacement of the reheaters, low pressure turbines, steam dryers and the 6.6 kV switchgear as well as the modernization of turbine process automation in both plants. “When these projects were combined we got quite a lot more net power out from the plant,” says Suoniemi, who noted that while these major modernisations take place every five or 10 years the biggest modification process made during the latest outage were changes to the pumps.

Tech development

Alongside the modernisation of plant systems, TVO also works with its partners to develop novel tools to support its O&M and inspection programme. For this outage a special camera system that was developed to inspect the reactor pressure vessel of unit 1 was deployed, for example. “The vessel tree project is a materials-related project.

When we took all the fuel out of the core during our primary circuit pressure test we had camera manipulators that we used to inspect the bottom of the vessel to see if there was any contamination or foreign bodies. This was the first time since the plant started up that we were able to inspect the whole bottom of the reactor vessel,” says Suoniemi.

He adds: “We worked with a partner and supplier to develop the special equipment for that purpose based on monitor camera manipulators. We took part in that and it went very well. We developed that for unit two and after that we and they made some changes from the lessons learned and now it is an even better system. We have developed some special systems for maintenance but it’s in inspections that it is more useful when they use a lot of manipulators for inspection and not so much in maintenance.”

Suoniemi continues: “Although there had been some damaged fuel removed from the unit 1 reactor the camera inspection wasn’t developed to find bits of fin or anything like that that had dropped through the fuel channels. Instead, it was for finding small components that had down to the bottom of the vessel, for example pins or little nuts and similar kinds of things. Practically we didn’t find anything, it was all clean.”

Similar inspections are planned for the future, but TVO hasn’t decided when they are to be carried out. “Perhaps, for example, when we take all the fuel out from the core for the pressure test. That could be the window to do this but we’ll see we haven’t made any decisions,” says Suoniemi. The operators don’t normally change all of the fuel during an outage but change just around a fourth of it.

Another aspect of modernization at Okiluoto is the use of modern data systems. More digitalization with embedded sensors forms part of the ongoing systems upgrading and modernization as is a part of this maintenance process:

“I think that is one thing which is coming more and more because, of course, in an older plant like this we don’t have almost anything of that kind. All we have we built ourselves afterwards, for example the vibration monitoring systems and that kind of thing. But in the future the equipment will rely on these things much more. I’m sure that it will come also for the old plants that we have to get more this kind of data which we can use, for example, for maintenance planning. If we can get more data and info then we can make changes also with that knowledge. It’s coming and I’m positive about that,” says Suoniemi. He continues: “In our current maintenance and uprating programme we actually have a project for that. There is a lot of digitalisation and these kinds of sensors and sources of data and I think that we will learn a lot from unit 3. In the future we can also use this knowledge for units 1 and 2 to learn which things are important and where to put these kinds of assets to develop that knowledge base but we don’t right now routinely include that capability in new equipment that we install such as valves.”

Workflow management

Although 14 days is a narrow window in which to execute a whole raft of different maintenance activities and special projects, TVO has a novel approach to its workflow management. “We have quite a unique system in how we handle these maintenance outages in which one maintenance engineer is in charge of a whole project, for example all the turbine island valves,” explains Suoniemi. “For an outage this single engineer will handle the whole pre-maintenance programme and plan all the outage works for their equipment. Then they will get the resources from vendors and partners to execute the works and during the outage is in charge of all these works. They are the field and project manager for maintenance works for, in this example, turbine island valves. We will have another maintenance engineer for the reactor island valves and another one for pumps, another one for instrumentation and so on. So, we have divided all of the plant equipment into small portions or sectors and each has its own maintenance engineer who is responsible. They plan all his own or her own works during the winter time and then, before the outage, does all the planning work preparation to make sure they get all the spare parts and all the resources and that they are all in the field during an outage,” says Suoniemi.

He adds: “This is quite rare because we do all the maintenance planning and maintenance engineering in maintenance. In most plants the technical department does for example the pre-maintenance planning and this kind of things but in TVO we do it in maintenance, which is not just for execution it’s also for maintenance planning.”

“It’s very effective when you have to make all the planning beforehand yourself, secure any necessary spare parts and keep up the whole pre-maintenance system and then are in charge in the field too. You learn a lot when you manage all your own equipment and then during the following outage next winter you may remember, for example, that a particular valve saw extra wear in a part. Now they have to take that information as it concerns the next outage and make a full report annually at the end of the year. It’s a very good system in terms of both maintenance needs and the outage efficiency needs,” he says.

The various lead engineers also coordinate between each other to ensure a smooth workflow during the outage process. “Definitely they coordinate their works within operations and they know each other very well and it’s easy to make contact. We have a single organisation for both units 1 and 2 as they are more or less identical so the same individuals manage unit one and two with the same equipment and spares,” he says.

This approach builds an in-depth knowledge base that is really important but builds efficiency between the two units where particular problems, such as accelerated ware of a particular valve that is likely to be seen in both units.

“It’s quite motivational too when you are doing the planning as you keep up the pre-maintenance programme and the spare parts. Then you make all the planning and preparation during the winter months,” Suoniemi concludes.

The outage period is also kept to a minimum by effecting repairs to non-operating components while spare inventory is put into service. “We do have a quite a lot of spare equipment for the units because, for example, if you have a major repair on a big valve it can take a very long time. Instead we change the whole insert of the valve then we have this insert which we have taken out and we can perform the necessary maintenance for that, for example, over the next winter in our hot works. Then we put it back to stores and use that as a replacement during the next outage or in a couple of years. This is one way we can get shorter outages when we have a changeable equipment or inserts and we can do the overhauls not in outage time but in online time,” Suoniemi notes.

Knowledge continuity

A perennial issue for the nuclear business to recruit new engineers to replace the depth of experience and knowledge that long-serving staff have. TVO has a personnel of about 1000 but there is inevitably a constant turnover of staff.

“When I started back 20 years ago there were still those guys who actually built these plants back in the late 1970s and early 1980s. They were still on-line but there were a few years when they all then retired and we got a new generation. Of course, they didn’t have that kind of know- how of those people who actually built the plants. It went well but there were certain risks. In that situation it’s very important that we have good data systems that covers a lot of this operational experience. New engineers can go to the data system and check previous experience, for example noting there was the same kind of phenomena back in 1999 and at that time it was handled in this way,” Suoniemi explains, adding: “In these kinds of plants which are going to be in use for maybe 60 years or more it’s very important that we have this historical data and also the cultural heritage. That’s the foundation of an approach where we can start to make the decisions to make things more effective. When you don’t have this kind of foundation it’s very difficult to make any kind of change for the better.”

TVO is also looking at uprating and digitizing these historical records so they can also be accessed and used more effectively.

“We have started that process. Now we are pulling out key bits of the paper record and assessing its helpfulness for future maintenance and getting that digitised if required. We have a very good operational data, for example, for maintenance use from the early 1990s and then we have a project to develop more data systems but it’s very long-term because all the plant documents have been on paper originally, for example all the blueprints. We have quite a lot of data already for units one and two but there’s a long way to go before we’re going to have everything.”

Throughout its existence, TVO has worked to acquire nuclear competence by uprating Olkiluoto 1 and Olkiluoto L2 and modernising the plants. Over that last four decades these modernisation projects have improved the safety, production capacity and economy of the power plant units and pave the way not only for an expended lifespan but also realiability of operations. Since the early 1990s both units have had capacity factors of well above 90% for example. The latest annual outage for OL1 and OL2 consisted of refuelling and various annual preventive maintenance activities, inspections, repairs and tests and the plant was returned to service in June.