The German branch of environmental group, Friends of the Earth, BUND, demanded "full transparency in the investigation of the causes of the fire and possible dangers" from the plant's operator, European energy group, Vattenfall.
The blaze broke out last Thursday at the Krümmel power plant in Geesthacht, 30 kilometers southeast of northern city of Hamburg.
Separately, another nuclear power plant in Schleswig-Holstein, Brunsbüttel, was temporarily shut down last Thursday about two hours before the Krümmel fire because its capacity was overloaded.
The incident triggered a fresh national debate about the safety of nuclear energy just days before an annual energy summit in Berlin focused on exploring ways to reduce greenhouse gas emissions and curb global warming.
Environmental organization BUND demanded the immediate closure of both plants.
Greenpeace also accused Vattenfall and the local government in Kiel of withholding important information on the consequences of the incident. According to the environmental organization, this was an obvious attempt to avoid conflict at Germany's third annual energy summit in Berlin.
Contrary to initial reports, the fire had in fact led to problems at the plant's nuclear reactor, the Schleswig-Holstein state social affairs ministry, which is responsible for the region's power plants, said in a statement released Thursday.
Last week, local police had stated that the fire, which started when coolant in a large electric power transformer substation ignited due to a short circuit, had been isolated from the atomic reactor.
However experts investigating the incident found "several unusual things when the reactor was shut down" including evidence of damage related to the fire. But they said there was no radiation leak.
Politicians from Germany's opposition Green Party as well as some from the ruling Christian Democratic Party have said they want energy company Vattenfall to explain why it remained silent for almost a week on the consequences of the incident. Some say Vattenfall's stance is undermining the credibility of the nuclear power industry in Germany.
Vattenfall defends itself
European energy group Vattenfall however defended itself, saying it had acted quickly.
A company spokesman dismissed the criticism, saying it had provided quick and comprehensive information on the accident to the authorities.
"We informed the public immediately," said CEO Bruno Thomauske in Hamburg Wednesday. "At no point was the public at risk."
"This was nothing to do with the summit," said Oliver Breuer from Schleswig-Holstein's state social affairs ministry in an interview with Spiegel Online. "We have no intention of covering anything up."
Nuclear phase-out still an incendiary issue
Germany has begun a long-term phase-out of its 17 nuclear energy reactors and expects to mothball its last plant around 2020.
The plan was approved by the previous Social Democrat and Green government, but Chancellor Angela Merkel's conservatives argue that abandoning nuclear energy would seriously undermine the country's chances of slashing greenhouse gas emissions, as well as increase reliance on Russian fossil fuel and make it harder to keep in check energy prices.
The European Union has set a goal of a 20-percent cut in greenhouse gas emissions by 2020 compared with 1990 levels, but Germany is aiming for a cut of up to 40 percent.
Merkel has come under increasing pressure from Brussels to renegotiate the phase-out deal.