France does not have enough nuclear capacity for high winter electricity demand and it has too much nuclear capacity relative to domestic demand in temperate weather. It ends up reliant on imports from Germany in winter time and exports surplus electricity at times of low demand. As I write this, France is importing 6.7 GW from Germany: http://www.gridwatch.templar.co.uk/france/
If France were forced to disconnect from its neighbors, its electricity would be more expensive and/or more emissions-intensive. It would need more fossil combustion to deal with seasonal demand swings and/or it would get less utilization out of its nuclear reactor fleet. The same goes for Germany, of course: electrically isolating itself would also increase costs/emissions. Germany and France both rely on international electrical connections to improve utilization of their non-combustion electricity sources. Both use imports to meet part of their electricity demands. Both are net electricity exporters over the course of a full year.