Reinforcing safety

Safety work takes precedence over almost everything else in a nuclear power station. Every component in a nuclear power plant must be absolutely top quality. At Oskarshamn, where OKG AB runs one of Sweden’s four nuclear power stations, BUMAX has been selected as the fastener for the most exposed areas.

“When it comes to safety in a nuclear power plant, there’s no room for compromise,” explain Magnus Antonsson (left) and Dieter Schmatz at the nuclear power station in Oskarshamn. The pipes in the screening station are suspended on brackets fitted with BUMAX products from Bufab Stainless.

A whole range of different fasteners are used at the plant in Oskarshamn, including carbon steel and stainless grades of steel, all with different strength classes. The type of fastener to be used in a specific environment and for a specific function is stipulated by OKG’s design engineers, technicians and safety department. “There are technical provisions for all mechanical devices here and they are based on interpretations of the rules and requirements set by SKI,” says Magnus Antonsson, group manager of Mechanical Engineering, a unit of OKG, which works with materials, calculations and design etc. SKI, the Swedish Nuclear Power Inspectorate, is the supervisory body whose job it is to monitor that all nuclear activity in the country is conducted safely. The size of the load on the screw joint and the environment in which it has to work determine the quality requirement of the fastener. In other words, strength and corrosion resistance are two of the main demanded for the screws.

Stainless was just the ticket
Stainless steel screws are the norm at the Oskarshamn nuclear power station and the majority of products are required to be of SS 2343 steel grade, i.e. the Bumax grade. Examples of applications for which Bumax screws are used are flange joints in stainless pipe systems, different types of valves, purification filters, pipe mounting brackets etc. There are probably around 3,000 pipe mountings at O2 (Oskarshamn plant 2) alone, each containing 4 – 8 screws. And then there’s O1 and O3… In the surrounding tanks (for example in the water plant), the screws are in constant contact with fresh water. “It’s obvious we need the 2343 grade here,” says Dieter Schmatz, technical manager responsible for welding and material matters at the central maintenance unit at OKG.

“A general rule of thumb here is that everything around the reactor section has to be corrosion-resistant and everything around the turbine section must be Carbon and low alloyed steels. And we always use stainless fasteners on stainless material,” he continues.

FSpecial material is used for components located near the nuclear fission process.

Inspected supplier
OKG has used Bufab Stainless products since the early seventies. The company is, of course, included in the register of approved suppliers, jointly produced by all four of Sweden’s nuclear power stations. Supplier approval, which lasts for three years, is conducted during a site visit to the supplier in accordance with the ISO 9000 standard.

“All companies which supply classified equipment to us must be assessed and approved. The four nuclear power stations make use of each other’s supplier approvals. Bufab Stainless was approved in 1997 by Barsebäck,” reports Mikael Ahlström, quality engineer at OKG.

Approved suppliers are part of the safety work at a nuclear power plant. Maintenance is another. Each of the three nuclear reactors at the Oskarshamn plant has its own maintenance unit. There is also general maintenance (GU) available as a central and collective resource.

“Our task is to back up the maintenance units in each of the relevant blocks,” says Dieter Schmatz.
 
Safety is part of the system
In addition to the double, sometimes multiple, automatic safety systems and various protective barriers, the safety work comprises inspections and continuous maintenance. The inspections are many, thorough and frequent at the Oskarshamn plant. For example, every day, staff do their ‘rounds’ according to a special plan.

Also, once a year, a very carefully planned, more extensive review is done of each reactor. This requires the relevant reactor to be shut down so that it can be inspected and parts replaced. The review, which lasts 3-5 weeks, involves not just the plant’s staff – a good 500 external staff are also brought in for these occasions.

Products from Bufab Stainless AB
Number of screw types: approx. 150. Hexagon head screws, socket head cap screws and nuts ranging from M6 to M30, class Bumax 88.
Special products: Stud-bolts and nuts made from high alloyed steel grades.
Steel grade: Swedish steel SS 14.2343, EN 1.4436 with low carbon content
Also: All fasteners supplied are class Bumax 88.

Facts about OKG AB
Site: Simpevarp peninsula, thirty km north of Oskarshamn
Business: production of electricity using nuclear power and operation of the central intermediate store for used nuclear fuel owned by SKB, Svensk Kärnbränslehantering (Swedish Nuclear Fuel and Waste Management Company).
Number of reactors: 3 – O1 (Sweden’s first in commercial operation, 1972), O2 and O3 (the country’s most recent and biggest).
Production capacity: O1 – 460 MW gross output, O2 – 630 MW and O3 – 1200 MW. The latter reactor’s capacity equals the annual requirement of 315,000 electrically heated homes.
Process: the fuel (uranium-based) is used to heat water in the reactor tank; the steam from the boiling water drives turbines; the rotation of the turbines is converted into electricity in generators.
Net turnover: SEK 3,287 million (1998)
No. of employees: 1,165 (1998)