* French 2011 autumn: 2nd hottest since 1900
* Nuclear plants use water to cool their reactors
* Chooz plant already forced to stop in July due to weather
PARIS, Nov 29 (Reuters) - Dry weather conditions are starting to hit output at France's nuclear reactors with EDF forced to stop one reactor in northern France to protect river flows, EDF said on Monday.
France, the European Union's biggest power exporter, this year experienced its driest March-May spring period in 50 years and its hottest since 1900. While rain fell over the summer, France experienced another dry bout this autumn.
Autumn 2011 was the second hottest since the start of the 20th century and rainfall in October was 45 percent lower than average, according to French weather forecaster Meteo France.
Nuclear plants use water to cool their reactors. French power producer EDF, which operates the country's 58 reactors, is not allowed to keep reactors operating if water temperatures rise beyond a set level or if flows fall below authorised limits.
A spokesman at EDF's Chooz nuclear plant, located close to the Belgian border, said the utility had not restarted the 1,450-megawatt reactor 1 as planned on November 28 to safeguard minimum river flows.
"There is an agreement between France and Belgium whereby France owes Belgium a minimum of 20 cubic metre per second on a 12-day average," the spokesman said.
Each of the plant's two reactor uses 1 cubic metre per second of water for cooling purposes, he said, adding the Chooz plant had already been forced to halt operations in July.
In case of a very tight supply situation, the plant would be able to operate for a while thanks to a damn system built around the plant, he said without giving further details.
Reporting By Muriel Boselli and Abdoul-Karim Cisse; editing by Keiron Henderson