There's typically a range of operation, so you can adjust a hundred MW but you can't drop to 0 or spin up from standstill without a time consuming process.

Edit: also, the economics are such that you rarely want to drop load from a nuclear plant unless it's offline or for system reasons. The fuel cost is negligible so you'd rather turn off your gas plant or lower the coal plant and save on those fuels.

That assume we still allow coal, oil or gas power plant to exist in the power grid. We should probably not assume that to be the case, especially after the temperatures rises to a break point and some of the major climate change crisis occurs.

None of the commercially available (Western) reactor designs today are fast load followers, so you are dependent on having gas or hydro when you project for new nuclear power plants. That's one part of the reason why this stuff is politically charged.

From what I understand it's not a theoretical constraint, but mostly a lack of enough commercial interest for any other design. But it is what it is.

> Cant control rods can be lifted or inserted to meet demand?

Thermally it is difficult to dial a reactor up and down. Generally the way nuclear power is modified is by not-sending the steam to generators through a by-pass and quenching their heat in some fashion.

So thermal generation stays at 100% (or whatever), but electrical generation output can be dropped.

Usually you'd vary the concentration of boric acid being injected in the reactor's core instead, since that doesn't involve wear and tear on safety-critical elements of the reactor.

Nuclear reactors absolutely can vary their output to match demand, this is what France has been doing for 50+ years (and what Germany was doing before switching back to coal). It's not as reactive as coal/gas, but you can still vary within 30-100% of output power at a speed of 5% change per minute. Way more than enough to react to 1-day-ahead forecasted supply/demand, and way more than enough to react minute-by-minute if you've got a tiny bit of storage to stabilize the grid's frequency (e.g. pumped hydro).

It is a hugely complicated system where reactors which are earlier in their fuel cycle ramp more leaving the later ones to run at 100% around the clock.

Ramping once is easy. Ramping continuously through the entire fuel cycle requires a meticulously planned fleet.

France is balancing their reactors with a massive amount of water power, so they are not doing it alone on the reactor side. But I think France is indeed one of the countries with the highest relative amount of nuclear in the grid. Germany went never over 30% nuclear in the mix, so demand matching was way less of a problem. But also, both countries tried to make demand mostly constant like with pushing inefficient heating systems which would consume electricity at night.

This is very far off from working well together with a mostly renewable grid, where renewables can cover 100% of the load on most days, but there are larger gaps to be quickly filled.

They can, though it depends a little on the plant design.

It just doesn't make any sense to use reliable nuclear as the "backup" to unreliable renewables.

Because this "backup" is already CO2 free. It is also reliable. And cheap to run. So just run it all the time (nuclear tends to have >90% capacity factor).

You then simply don't need the "primary".

A nuke operated at 50% of capacity costs the same as one operating at 100%. Thus, power from it at 50% costs twice as much per kWh. But nukes are already not competitive even at 100%, and get less so with each passing day.