During certain periods the load demands are very high. Example the morning when all factories operate. During these time certain power plants like thermal plants gives this extra power required. These are the Peak load stations, which operates at certain periods.
But during the rest of period that is when there is no peak power demand there are some power stations, which cater to the base load always runs giving power to the grid. These stations are producing power at constant rate. These stations can not be easily stopped or restated. Nuclear power station comes under Base load power station category.
