By reactants I assume you mean fission daughters which do decay and give off heat in the process.
The decay of fission daughter account for 5% or less of the reactors power out put.
Fuel can be conserved by reducing power. But as I said this is not the preferred way to operate a nuclear power plant. Besides complicating the operation of the plant raising and lowering reactor output places thermal stresses on the reactor vessel that can cause cracking in metal components and the reactor vessel itself.
Another problem with lowering reactor power to follow load is that it messes with the planned fuel burn up. Reactors are either on a 2 year fuel cycle or a 18 month fuel cycle. The burn plan has the plant running out of fuel in either the spring or fall when the electric grid is at its lowest demand. Mess with the burn out rate and the plant will shut down for refueling with unburned fuel. That means wasted money and higher cost for electricity for the consumer.
The best way to operate a nuclear power plant is to take the plant to 100% power and leave it there as much as possible.
Like I said, you OBVIOUSLY know this stuff better than me. So, thanks. My thoughts were in the right place, but I needed someone who knows the business to help me out.
Bottom line - it is silly for France, of all countries, to be doing this stunt.