It's illustrative of the scale of U extraction that would be needed. A single 1 GW(e) burner reactor would require a field of U absorbers covering 170 km^2 of continental shelf (and to supply 18 TW of primary thermal energy would need about 6000 such reactors). The power/area would be considerably worse than solar (with solar capacity factor taken into account).

Anyway, I don't believe France (or anyone else) has any major program to bring seawater uranium extraction to market either.