For 1.2GW in panels and 20GWh of batteries you'd currently pay about a billion for the panels and 2-3 billion for the batteries (price for a turn-key solution, not just cells). Assumptions:

- 25% capacity factor for panels

- 3 days of storage (at 300MW)

- $120/kWh for storage

- $1/W (peak) for panels

I'd argue that such a setup is a big upgrade over the reactor since you have much higher peak power; you basically get more dispatchability at a comparable capacity factor (90%-ish). I suspect maintenance to be cheaper as well.

But an actual solar setup would probably install more panels and less storage (giving you cheaper power at a lower capacity factor).

Edit: The problem in practice for the nuclear reactor is that it has to be competitive with almost the panels alone, because otherwise your consumers are just gonna go "I wont buy nuclear energy 24/7, I'll just use much cheaper solar electricity whenever the sun shines, pay maybe for a few hours of batteries and fall back to gas when I really need to (very cheap per peak MW, but not per MWh)".

If you want to be fully paid for constant power output, you have to be fully competitive at all times, not just when the sun is down.