We talk of the fuel irradiation as so many Megawatt.days/tonne, and that is a limiting factor on how much energy you will get from a tonne of fuel. However pile irradiation must represent the total Megawatt.days achieved from a particular reactor. In the early days of reactor experiments reactors were often referred to as piles, as they were literally piles of graphite bricks arranged with holes for fuel rods and control rods.