Water leaking from the cooling system at the Ringhals plant, near Gothenburg, increased to twelve litres an hour, compared to the normal six, but the problem is described as not being too serious.
The reactor was taken off-line so staff investigating the leak are not exposed to too much radiation.
The Swedish Nuclear Power Inspectorate is criticising bosses there for waiting almost two weeks before turning the reactor off, despite not knowing until now exactly what the problem was.
The stoppage follows months of problems at Sweden’s Forsmark plant which have led to its Chief Executive resigning and a police investigation into an emergency shut down last July.
This week bosses were banned from re-starting one of its three reactors which was taken off-line earlier this month because of another safety scare. A second reactor there can not be re-started until a report on safety controls is presented to the Swedish Nuclear Power Inspectorate.