The fine is probably not a big deal, but the damage they might be sued for.
Some people do not grasp why France is throttling down nukes in summer. They claim: "to safe wildlife". Already idiotic to think wildlife has no value.
Point is: it is France. 90% of Frances population has 6 weeks holidays during mid summer. They make vacation. For example at the rivers above. If the rivers are full with stinking dead fish, then it has several drawbacks, e.g.: no fish to eat in the restaurants. In case you like fresh water fish like Trouts. So, you go there this year and can not bath in the river, because everywhere are dead fish, then you go next year to a lake in the mountains. And the restaurants, the fishers, and the hoteliers will sue the owner(s) of the plant(s).
It is not a "because of the fish" decision that France is throttling down plants: it is a damn decision of _common_sense_ to avoid billions of damage to the tourist industry. After all: those pay all taxes, and the plants run on tax money anyway. Why not buy cheap power from Germany or Portugal or Spain, and use the relaxed time to overhaul the plants. Oh, that is exactly what they did recent decades: every summer.
But just wait for atomicalgebra, sonlas and that MadMan to call this a conspiracy ...