European power prices rose to fresh records on Thursday as a heat wave limits energy supplies and wildfires rage across France. The gains reflect a tight market for natural gas—used to fuel power plants—as Russia cuts supplies just as Europe works to replenish stockpiles for winter. A drop in nuclear-reactor output, as well as low wind and hydro generation, has exacerbated the squeeze, raising the specter of intervention to reduce demand. “If Russia cuts off the gas and there might not be enough gas for the whole demand, then there would be rationing,” Annegret Groebel, president of the Council of European Energy Regulators, said in a Bloomberg Television interview.