The 1,402 megawatt plant, which is jointly operated by the Swedish/German utility Vattanfall utility group and by Germany's E.ON has been closed since June 28, 2007 when a fire at a tranformer substation caused a short circuit.

The Social Affairs Ministry in the northern state of Schleswig Holstein, which supervises nuclear safety, said the incident took place around 0700 GMT and was resolved shortly after 0800 GMT.

The internal fire brigade had dealt with the fire so there had been no need to involve public fire fighters, it said. There were no injuries.

Vattenfall confirmed the details in a statement, saying there had been a smouldering fire at a ventilation system.

Germany is in the process of phasing out nuclear power by 2020 under plans agreed by the previous coalition of Social Democrats (SPD) and Greens.

The plans are being contested by the conservative parties CDU/CSU, which are currently in a coalition government with the SPD. Each safety-related incident is helping to weaken their arguments and reignite fears over the safety of the technology.

A statement from the ministry said the cause of the fire had not yet been determined but its experts were investigating at the site.

The 24-year old Kruemmel reactor is about 20 km (12.43 miles) southeast of Hamburg on the River Elbe.

Adjacent Brunsbuettel, another nuclear plant operated jointly by Vattenfall and E.ON, with 806 megawatts of capacity, also remains shut since the incidents last summer.

Environmental organisation Greenpeace called for the permanent closure of the two plants.

"The latest fire at Kruemmel shows that reactors cannot be operated safely," it said in a statement, adding that power markets were managing without the supply.

Utilities say they need to prolong nuclear operations to win time to meet increasingly ambitious goals for curbing emissions of climate-warming carbon dioxide.

Nuclear-derived power generation emits no carbon dioxide, although the process of mining and enriching uranium does. Alternative renewable energies are not yet fully commercially viable.

Before Monday's incident, Kruemmel had been expected to reopen possibly in mid-May, and Brunsbuettel at the end of March at the earliest.