It's already solved insofar as a fission or fusion heated steam engines can be solved. Pumped hydro and caes is scalable at lower prices than fission could ach

It's already solved insofar as a fission or fusion heated steam engines can be solved. Pumped hydro and caes is scalable at lower prices than fission could achieve. DT fusion will be much lower power density with much more exotic materials and much higher maintenance burden. It's just that renewables have to live in the real world where customers aren't strongarmed into paying $200/MWh for 50 years to pay for them to do whatever they want. As such work needs to be done to make solar/wind+storage economically dominate gas because governments are not powerful enough to make fossil fuels pay for their externalities.