Of course, the milk will eventually spoil, even when stored inside the refrigerator. The refrigerator is therefore no guarantee against spoilage, but it does delay the inevitable. You hope to use the milk before it goes bad. This spoiled milk saga is only one of countless examples of aging. Living creatures have lifetimes defined by birth and death.
Inanimate objects have lifetimes defined by their usefulness.
For many objects, usefulness ends with failure as when milk goes bad or a light bulb burns out. The simple act of refrigerating a carton of milk postpones, but does not prevent, entry into Region C. Conversely, leaving the carton on the kitchen counter hastens the arrival of Region C and spoilage. In today's nuclear plants and in those proposed to be fueled by MOX, power is created by the splitting, or fissioning, of uranium and plutonium atoms. The splitting of plutonium atoms releases more neutrons at higher energy levels than the splitting of uranium atoms. More neutrons traveling at greater speeds may increase the number that strike the metal reactor vessel containing the reactor core.