Plant life management
The Oskarshamn extra15 April 2009
The increased availability and power output from plant life extension work at Oskarshamn equates to building another reactor similar in output to unit 1. By Anders Helmersson
Sweden’s 12 operating nuclear reactors were all commissioned between 1972 and 1985. Three of these are located at Oskarshamn on the southeast coast. Unit 1, output 487MWe, was officially opened on 8 May 1972. Unit 2, output 620MWe, was first connected to the grid on 2 October 1974. Unit 3, output 1195MWe, opened on 3 March 1985. The Oskarshamn site also hosts the central interim storage facility for spent fuel from all Swedish nuclear power stations, known as CLAB.
All three units are BWRs built and designed by Asea Atom, which later joined with Brown Boveri to form ABB. Initially the power stations at Oskarshamn were built for a 40-year lifespan. But with time the ambition of owner OKG has changed. Today a 60-year life span is anticipated.
Extending the life span of a nuclear power station by 20 years is complex. In our experience two major issues decide what measures need to be taken. One issue is the ageing of components; the other is keeping up with safety standards and regulations.
Until 2002, upgrading unit 1 had been a key focus area for OKG. Once that project was completed, our attention turned to the two remaining units. OKG now had a wealth of experience that could be used to extend the life of units 2 and 3.
In November 2004, OKG’s plant life extension (PLEX) group was formed to coordinate a number of upgrade projects planned for unit 2. The same year, the Swedish Radiation Safety Authority (then named SSI) had established a new safety standard that resulted in the need for adaptations. At the same time, many components in the 30-year-old plant were ageing. In addition to this, pilot studies were started to investigate the feasibility of thermal power uprating.
Very soon we understood that each of the adaptations or replacements we were planning were too complex to be handled one by one. There were certainly effects in terms of synergy to be gained by coordinating them and, after some time, we had settled for a plan to complete the project in three steps during the outages of 2007, 2009 and 2011.
With a total investment close to 5 billion SEK (460 million EUR), PLEX is currently the largest project for Eon Nordic, the main shareholder in OKG.
In winter 2006/07, OKG signed the two major life extension contracts. The reactor contract was signed by a consortium consisting of Areva, Siemens and Heitkamp, and the turbine contract was signed by Siemens.
The overall scope of the project can be divided into three major areas – reactor safety, availability and thermal power uprate.
Oskarshamn 2 was designated to be fully adapted to the new SSI safety regulations by 31 December 2012. Transforming a nuclear power station designed and built in the early 70s to comply with modern standards requires several measures to be taken, including introducing new technology and reconstructing the actual building.
In 2007, during the first leg of the PLEX, a software-based control system was implemented for the turbine. In 2011 we are planning to take the system one step further by building a software-based reactor protection system (RPS) that can replace the legacy relay technology currently being used.
Implementing software technology requires an extensive rebuild of the control room. Many of the old control panels have been replaced by large computer screen interfaces. Similar interface changes were carried out for the turbine operation. Judging by control room operator opinions, it has worked out well.
When the software technology is extended to the RPS, safety information will be gathered on a safety panel to give a better overview. Operator experience and human factor engineering provide important input to the design of the panels and process pictures for the screens. Introducing software technology for the RPS also has the advantage of providing more process information for maintenance technicians, often facilitating troubleshooting in the event of operational problems at the plant.
Ten years ago, the RPS for Oskarshamn 1 was the single most difficult and time-consuming part of the whole modernisation programme – but the experience we gained was invaluable. Today the reactor consortium uses personnel from OKG as a source of expertise in the development process .
The control room modifications need to be designed far in advance to enable training of control room operators in a simulator before the changes are implemented in the plant control room. In 2006 the simulator for unit 2 was moved from a facility at Studsvik, 80km south of Stockholm, to a new building on site. The strategy to move the simulator close to the plant has helped consecutive alterations before each leg of the upgrade and assisted intercommunication between design engineers, operators and training staff.
During the upgrade of unit 1 we also learned that redesigning the simulator process in advance reveals potential construction errors before they are implemented in the actual plant, saving us both time and trouble during the test and startup period.
Safety-related equipment must have redundant back up in order to prevent a failure leading to a serious accident. In the early 1970s when Oskarshamn 2 was built, the safety regulations only demanded two parallel systems. Now, four redundant systems are required. These four sources and all their respective wiring are physically separated from each other to prevent a fire from simultaneously destroying all back-ups. Because containing all the new equipment in four divisions naturally demands more space, two new eight-storey buildings are necessary. Simultaneously, the original buildings will be reinforced to meet new requirements for earthquakes and flooding.
We are also reinforcing our auxiliary power system by replacing two diesel generators with new ones, and adding two more generators. This, in combination with two gas turbines, will provide us with a perfectly satisfactory system of auxiliary power in case of a grid failure.
Reactor safety will also be improved by a number of upgrades in process systems. Some have already been implemented but most still remain to be done. In 2007, the feedwater system was upgraded to modern pipe rupture requirements and during the 2009 outage an entirely new system will be implemented for venting the reactor vessel in the event of an accident.
In 2011 many other process and reactor safety systems will be redesigned or upgraded to meet new safety requirements or new conditions as a result of the thermal power uprate. For example, the pressure relief system for the reactor vessel will be upgraded with new valves; the hydraulic scram system is going to be upgraded as well. We are planning to build a new system for residual heat removal. Also, the steam dryer and the main steam valve lines will be exchanged. We will perform functional and structural verifications on internal reactor parts and reactor systems, not to mention the vast number of safety analyses that need to be carried out.
Currently we are moving from basic design to detail design on the 2011 scope and consequently we are in a decisive part of the project with a lot of work to be done both in design and analysis.
With many of its best annual production results actually reached after the turn of the century, Oskarshamn 2’s availability may not seem to be a major problem. But its prospects of an extended lifespan make the preparation necessary.
The turbine components and power transmission systems are most prone to suffer from ageing and, before the life extension was initiated, a plan was established for the exchange of the main transformer and the generator. The transformer was exchanged during the outage in 2005 and the generator was exchanged one year later, in 2006. Both were dimensioned for a potential thermal power uprate. In hindsight, that proved to be a wise choice. During the 2009 outage the first part of the turbine exchange programme will be performed, installing three new low-pressure turbines.
Also during the outage, a new reactor vessel accident venting system will be installed. This system is a consequence of new accident management regulations. A feedwater pipe crack could potentially lower the water level inside the reactor vessel below the core, causing a core meltdown. One way to cool down the core during an accident is to flood the entire containment (including the reactor) with water. In an accident, gases created by the core meltdown would rise to the top of the vessel, blocking the path of cooling water. The new system would direct the gases to the top of the containment, relieving the pressure inside the vessel, and allowing water to enter.
Many of the major components due to be installed in summer 2009 are already manufactured. New components are not only more resistant to ageing; they are also designed to be more efficient than the original ones. As a result of this, the electrical output from Oskarshamn 2 is estimated to increase in future by approximately 50MWe while the thermal power remains constant. In 2007, the feedwater system was strengthened to cope with a higher load, and some pipes and valves were replaced.
The goal is to reach a plant availability of 93% after completion of the PLEX work. This high figure more or less requires uninterrupted operation between outages. Less need for maintenance is also expected to lead to shorter outages in the future. But the availability goals are set very high even during the installation years. We are aiming at an availability of 87% during the period between 2005 and 2011. To make this plausible it is vital not to have any delays during outages.
In the early days of life extension, a feasibility study was carried out on the prospects of a thermal power uprate at Oskarshamn 2. The idea of an uprate was scrapped for a while but the shortage of electricity on the Nordic market caused OKG owners Eon and Fortum to rethink their position. A new feasibility study was started and in May 2007 the board of OKG decided to apply for a thermal power uprate.
In Sweden, utilities apply to both the Environmental Court and the government through the Swedish Radiation Safety Authority for thermal power uprates. The applications are currently being handled. An environmental court order is expected during the summer of 2009, with a government decision later in the year. So far thermal uprates in existing plants have been treated well by the government; we are optimistic about our chances.
The thermal power uprate we are planning is of 500MWt, that is, from 1800MWt to 2300MWt, which is an increase of 28%. It will raise the electrical output by 180MWe. Combined with efficiency advances that have already been mentioned, we are looking at a total power increase from 620MWe today to 840MWe.
Oskarshamn 2 was initially built with substantial margins for power uprates but the large scale of this uprate calls for adaptations of the process systems. These adaptations will be assimilated in the last leg of the modernisation programme in 2011. Not only process systems, but also the power transmission systems and surrounding grid will be subject to adaptation.
At Oskarshamn 3, several major components were in need of replacement once the technical life span was increased from 40 years to 60 years. Given the high cost of an upgrade, an investigation was launched to see whether a thermal uprate might be possible to pay back the investment. The feasibility study proved that there were margins for a substantial power uprate, provided that the process systems were adapted to the new load. The Oskarshamn 3 upgrade began in an outage starting 2 March 2009. The 600MWt (250MWe) upgrade scope of work includes improved capacity on cooling systems, new main circulation pumps, new isolation valves, new high- and low-pressure turbine, new generator and transformer and a new secondary cooling system. The outage is expected to last until mid-June.
Today, Oskarshamn 1 and 2 take cooling water from a surface inlet in the Baltic Sea south of the plant. The cooling water is released in a bay north of the plant. The water released back into the Baltic is approximately 10°C warmer than it was at the inlet.
The planned thermal power uprates would raise the temperature of the released water even further.
To prevent a hazard to marine life, the environmental court published a directive in 2006 demanding that Oskarshamn build a new cooling water inlet at a depth of 18m, where the average temperature is lower.
Oskarshamn 3 already uses a similar inlet, and there are indisputable advantages. In summer the surface water is often at rather high temperatures, which leads to poor cooling efficiency. An inlet placed at a depth of 18m is not as vulnerable to seasonal temperature changes and will render better efficiency with a higher output.
The construction of a new cooling water inlet has been assimilated into the PLEX work and will begin in May 2009. A pilot study has picked a favourable location for the inlet, 600m offshore. The cooling water will be led through a tunnel to a reservoir close to the present inlet before entering the cooling systems.
To build the inlet, the Oskarshamn team will need to drill an underground tunnel 600m long below the sea bed, remove the excavated rock, and connect the intake pipe to the tunnel. These tasks are made more difficult by prevailing poor weather conditions.
If everything runs according to plan, the new cooling water inlet will be ready for use early in 2011.
Anders Helmersson was appointed OKG project manager for the last leg of the modernisation of Oskarshamn 1. After two years as head of the OKG Technology Department he was chosen to become project manager for plant life extension at unit 2.
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