A conventional nuclear fuel assembly for boiling water reactor comprises a bundle of fuel rods and a water channel arranged in a fuel channel, the water channel being surrounded by the fuel rods. In operation, water flows through the water channel and through the fuel channel, from lower end to upper end of the fuel assembly.
Water serves as a moderator for the nuclear reaction and as a coolant. Water is progressively heated, so that water is in vapour-liquid phase nearby the upper end of the fuel assembly.
The water channel enables to increase the moderator (water) to fuel ratio and the coolant amount near the central region of the fuel assembly.
The moderator (water) to fuel ratio tends to decrease toward the upper end of the fuel assembly as the proportion of vapour increases in the water flow, namely in the water channel. The decrease of said ratio leads to a less efficient burning of the fuel in the upper region.
Providing fuel rods of shorter length enables to reduce the amount of nuclear fuel in the upper region of the fuel assembly and to improve burning of the nuclear fuel in said upper section.
Providing a water channel having an upper section of larger cross-section area enables to increase the amount of water in the upper region, to compensate the increase of vapour in the water flow, and to thus improve burning of the fuel rods.
U.S. Pat. No. 5,202,085 describes a nuclear fuel assembly having a water channel surrounded by fuel rods. The water channel has a lower section having a cruciform cross-section occupying a region equivalent to five fuel rod cells and an upper section of square cross section occupying a region equivalent to nine fuel rod cells. The fuel rods comprise shorter fuel rods disposed adjacent the water channel beneath the upper region.
However, such an arrangement makes it difficult to catch the shorter fuel rods located beneath the upper section, e.g. in view of replacing one of these fuel rods.