A nuclear reactor has just achieved criticality at 10-8% reactor power during a reactor startup from xenon-free conditions. The operator establishes a 0.5 decade per minute startup rate to increase power. Over a period of ten minutes, startup rate decreases to zero and then becomes increasingly negative. Which one of the following is a possible cause for these indications? Fuel depletion Burnable poison burnout Reactor power reaching the point of adding heat Inadvertent boration of the reactor coolant system