The shutdown on Wednesday led to the EU raising a Europe-wide radiation alert for the first time since the system was put in place in the aftermath of the Chernobyl disaster 22 years ago.
After detecting a loss in the reactor's cooling system mid-afternoon Wednesday, Slovenian authorities decided to manually shut the plant down, and correctly alerted the European Commission -- but erroneously told neighbouring countries the incident was an exercise.
The Slovenian government apologised Thursday but insisted there was no safety threat from the water coolant problem, and that the reactor at Krsko, 120 miles (75 kilometres) east of the capital, Ljubljana, would be fixed within days.
Slovenian Environment Minister Janez Podobnik told other EU environment ministers he was sorry for the mistaken alert at a meeting in Luxembourg.
Neighbouring Italy said the incident was now "closed" but Austria, which also has a border with Slovenia, was furious at the mix-up, which Slovenian authorities blamed on using the wrong paperwork.
Austrian Environment Minister Josef Proell, whose country is deeply opposed to nuclear power, said: "It's not okay to set off an alarm in Europe and inform Austria, Italy and Hungary that it's only an exercise," he said.
"There is no absolute security when it comes to nuclear power," he added.
Podobnik said Slovenia's nuclear agency had "used the wrong form. It used a form that had 'exercise' on it. It was a mistake that was a genuine human error."
He said the error was spotted "in a few minutes" and corrected.
German Environment Minister Sigmar Gabriel said he would demand answers of Slovenia, but added: "I prefer to have an unnecessary alert, than to have too few alerts."
On Thursday the European Commission rejected accusations it had been "sowing the seeds of panic" by issuing its unprecedented radiation alert.
"I don't think we gave you any information that would cause people to panic. We just gave you the facts," Ferran Tarradellas, spokesman for EU Energy Commissioner Andris Piebalgs, told journalists in Brussels.
Slovenian Interior Minister Dragutin Mate said there was no environmental fall-out from the incident.
"The environment is not polluted, everything is OK. It's a stable situation," he said.
Italy and Austria both said radiation levels tested at their borders were normal.
The European Union issued a special radiation alert -- the European Community Urgent Radiological Information Exchange (ECURIE) system -- late Wednesday for the first time since its alarm system was set up after the 1986 Chernobyl disaster.
The Krsko plant was still cooling down Thursday and plant director Stane Ruzman said repairs would start Friday. The reactor was expected to be operating again by Tuesday or Wednesday.
Krsko, east of Ljubljana, was reopened last November after being shut down for a month for maintenance work.
In the town itself, local residents were phlegmatic about the potential risks from the nuclear power plant.
"We are too close to the plant to worry about it. The whole of Slovenia is too close and we could not help ourselves even if something would happen," said Andreja, the owner of a local tourist agency, who did not want to give her surname.
"I was not scared by yesterday's incident, but I do not like to live near a plant," added 17-year Uros, again without giving his full name.
Krsko's plant is jointly owned by Slovenia and Croatia. It produces 20 percent of all electricity used in Slovenia and 15 percent of Croatia's power needs.