France is the the largest nuclear energy exporter, but it only exports about 12% of its nuclear energy, which might be enough to move the domestic price.
If that 12% is the annual average, also keep in mind that it's presumably ~25% sometimes and ~0% at other times.
Now suppose the domestic production is 100 GW, the domestic consumption is 90 GW and you're not exporting anything. Compare this to when the domestic production and consumption are still the same but you're exporting 20 GW. You go from having 10 GW to spare to being 10 GW short and having to bid for it against the foreign market.
And the prices aren't linear. In oversupply you could be paying barely anything. At 10% undersupply you could be paying twenty times as much if that's how much it takes to reduce demand by 10%.
Now suppose the domestic production is 100 GW, the domestic consumption is 90 GW and you're not exporting anything. Compare this to when the domestic production and consumption are still the same but you're exporting 20 GW. You go from having 10 GW to spare to being 10 GW short and having to bid for it against the foreign market.
And the prices aren't linear. In oversupply you could be paying barely anything. At 10% undersupply you could be paying twenty times as much if that's how much it takes to reduce demand by 10%.