You are here

SWEDEN: CRACKED CONTROL RODS; GAPING HOLES IN THE ‘CULTURE OF SAFETY’

Nuclear Monitor Issue: 
#679
10/11/2008
Article

(November 10, 2008) A routine annual control of Unit 3, a boiling water reactor, at Sweden’s Oskarshamn nuclear power plant in early October turned up a control rod that had broken off. On closer inspection other rods were found to be cracked. The Swedish Radiation Safety Authority (SSM) immediately ordered an inspection of all Swedish BWRs of the same construction, and especially a ‘twin’ reactor (Unit 3) at Forsmark.

 

(679.) WISE Sweden - Once the investigation ordered by the safety authority got under way (26 October), almost immediately cracks were found there, as well. The operator, a subsidiary of Vattenfall, was quick to assure the press that “even broken rods will work”.  Forsmark expected the reactor to be back on line after about 10 days. But 3 November, spokespersons for SSM confirmed the severity of the problem. All the control rods at Oskarshamn 3 and more than 60 rods at Forsmark will be inspected  in coming weeks. All cracked rods will have to be replaced. Just how many is at present unknown. Both reactors are now expected to stay off line through November.

A reactor typically has 160-170 control rods. Used to regulate temperature in the reactor core, they are the first line of defence should it be necessary to interrupt criticity. (The second line of defence is to pump in boron.) In an interview with the Swedish technical weekly Ny Teknik Anders Bredfell, press officer for SSM, stressed the seriousness of the situation. The rods are an essential safety barrier; any risk of malfunction puts the safety of the reactor at risk, he said.

 

Many question-marks         

The problems discovered at Forsmark and Oskarshamn are rarities. Lars Skånberg, also of SSM, said they were unique in his experience. Nor is it clear why both units should begin failing at the same time. The design lifetime of the control rods is ten years, but cracks were found in rods ranging from 3 to 10 years of service and delivered in different batches over a span of years. All, however, were manufactured and supplied by Westinghouse, who are reported to have investigators on site at Oskarshamn.

The cracks and the total break are concentrated around the nexus of shaft and rod. Investigators are trying to determine whether the cause is a manufacturing or materials fault or so-called thermic fatigue, i.e. stress due to alternating extremes of hot and cold. Further study will determine if standard operating procedures will have to be revised.

“The safety of Swedish reactors is based on multiple lines of defence. Each line must be fully functional whenever a reactor is in operation,” Mr. Skånberg stated. Seemingly commonplace, the comment has a cutting edge. Having reviewed past safety reports SSM has discovered that neither plant has checked the status of its reactors’ control rods since last year. That is not acceptable: “It goes without saying that all systems and components that are of importance to a reactor’s safety have to be checked on a regular basis,” Mr. Skånberg said on Swedish Television 28 October.

SSM has asked both operators to explain why the cracks have occurred. They have also been asked to explain why the problems went undetected. The reactors will not be allowed to come on line again until SSM is satisfied that the situation has been rectified.

 

Not the first time, hardly the last ...

Both the Oskarshamn and Forsmark plants have repeatedly been faulted for poor safety routines. In Summer 2006 Forsmark was close to a serious calamity (potentially a meltdown) due to a fundamental design error in the auxiliary power system that had gone undetected. In 2007 plastic foil was found to have clogged vital pipes at Oskarshamn.

This past August an annual inspection at Forsmark (Unit 2) revealed that a broken valve in the reactor’s emergency cooling system had been known to be out of commission but left unattended-to since the previous year’s inspection. No routine checks of vital components are carried out between annual “revisions” – in violation of the plant’s own safety rules. SSM officials have demanded that Forsmark explain how the valve could have been ignored such a long period of time. And, once again they demand a report on the measures Forsmark’s management is taking to revive an obviously ailing ‘culture of safety’.

Most recently, Oskarshamn has come under fire for using untrained hired night watchmen to stand in for malfunctioning automatic alarm systems. In its evaluation SSM points out that the practice is “a serious breach of the company’s safety policy. And it is especially serious inasmuch as it has been going on for quite a while”. The decision to do this was not documented in any company protocols. Worse, the relevant protocol was found to be “outright misleading”. “All in all, [these are] indications that the ‘culture of safety’  needs to be improved with respect to physical security,“ SSM’s press release concludes.

 

Sources: The Swedish Radiation Safety Authority: www.ssm.se (pressrum) / Sveriges Radio, News department (Ekot): www.sr.se / Sveriges Television, regional news: www.svt.se / Forsmark Kraftgrupp AB: www.vattenfall.se/forsmark / Gefle Dagblad (Gävle; 3 Nov): www.gd.se / Östran (Kalmar; 3 nov): www.ostran.se /  Ny Teknik, 7 November 2008

Contact: WISE Stockholm/Charly Hultén