Unterweser (Germany)

Map of Unterweser

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PWR 1300 MW constructed by KWU, operation started in 1978

Facilities in Unterweser

plantreactor typconstruction startoperation startshut down

Error in the adjustment of the end switch of the regulating valve of a residual heat exchanger.


In 2004 six events rated INES 0 occured according to the BfS annual report.


In 2003 seven events rated INES 0 occured according to the BfS annual report.


Operators restarted Unterweser with the main steam isolation and safety valves on one SG inoperable, a fact not discovered until the plant scrammed two days later for other reasons.

However, the German public, usually highly sensitive to matters of nuclear risk, has so far completely ignored the event.

A key, which is used to manually disconnect secondary-side SG safety valves from normal control lines during maintenance, had not been turned to reconnect one of the four trains. The MSIV and safety relief valves for one of four SG were, accordingly, still locked out of operation when the reactor restarted.

The key to reactivate the valves in the SG-3 train had not been returned to the control room and logged in before the restart, a serious violation of procedures.
According to the INES report submitted to the IAEA by Gesellschaft für Anlagen- und Reaktorsicherheit mbH. (GRS), the event itself would have been rated as level 1 "because of the amount of available redundancy and diversity of the safety function affected." However the rating was upgraded by one level due to deficiencies in administrative procedures and control of safety equipment availability."
Lother Hahn, a reactor safety export at the Institute for Applied Ecology (Öko-Institut) in Darmstadt said the discovery of the missing key raised three major issues which must be cleared up before the plant is allowed to restart. They are: how the plant technically was able to restart with the safety valves locked out, possibly overriding alarms and/or safety interlocks; how procedures allowed restart to go forward; and how operator training treats such events.

Preussenelektra AG´s PWR will be shut until at least August while event investigations are completed and staffers are retrained, in the wake of an event in which operators restarted the plant without realizing that a key safety system, disconnected for maintenance, was still inoperable.

There were too many human errors involved to allow the reactor to restart without a careful review of staff reliability and retraining , said a spokeswoman for the Ministry of Environment in the state of Lower Saxony.

Officials at France's Institute de Protection et de Surete Nucleaire (IPSN) said the non-availability of a major safety system would be judged severely by French regulations.

Experts suggested this week that the root of the problem at Unterweser may be in the design of the Siemens AG convoy PWR.

At Unterweser, a transient would have resulted in the design pressure being exceeded, and the worst case would be a SG tube rupture followed by a main steam line rupture.


Leak in 1 of 4 PRZ safety valves ( 15 kg /h - leaked steam was fed into the PRZ discharge container). Same valves are installed in Stade, Obrigheim, Neckar (cause of the leak probably generic)