The energy contained in 1 g of fissile material being about equivalent to the energy contained in 1 ton of oil, refueling a Nuclear Power Plant occurs about every year. But it takes about one month, so such refueling phases should be carefully planned. This is why we shall dedicate this chapter to fissile material consumptionConsumption and fuel evolution that leads to reactivity decay of fuel assemblies. Ideally, refueling should occur when the reactivity of the core is not sufficient for the core to diverge. (The elapse time between two refuelings is called a cycle).