The present invention relates to a nuclear fuel assembly for a boiling water reactor comprising a base, a head and a bundle of full length fuel rods and partial length fuel rods, said bundle extending longitudinally between the base and the head, at least one partial length fuel rod having a lower end received in a housing provided in the base and delimited by a closed bottom larger than the lower end.

The full length fuel rods (FLFRs in the following specification) are received between the base and the head of the fuel assembly with a small longitudinal gap allowing the FLFRs to expand in their longitudinal direction during operation of the nuclear reactor.

Usually, the upper ends of the FLFRs are attached to the nuclear fuel assembly head.

The partial length fuel rods (PLFRs in the following specification) are used in boiling water reactors (BWR in the following specification) for reasons of thermo- hydraulic stability and neutron moderation.

PLFRs extend upwardly from the fuel assembly base and stop at a distance from the fuel assembly head. Usually, the PLFRs length is comprised between 25% and 75% of the FLFRs length.

The fuel rods are positioned and held longitudinally and transversally by a plurality of spacer grids spaced along the fuel rods. These spacer grids allow for local and limited sliding movement of the fuel rods in the spacer grids to accommodate the fuel rod expansion under irradiation.

Nevertheless, under specific operating conditions, the longitudinal holding force applied by the spacer grids to the bundle of fuel rods may be not sufficient to avoid any longitudinal global displacements of some of the rods.

Accordingly, in order to prevent the PLFRs from lifting-off during the nuclear reactor operation and reaching undesired positions, PLFRs are usually attached to the fuel assembly base.

DE-201 05 913 discloses a fuel assembly where such an attachment is made, for each PLFR, through an elastic bushing welded in a through hole provided in the base of the fuel assembly. However, such a design requires the welding of additional parts to the nuclear fuel assembly base and is therefore complicated and expensive to manufacture.

US-2008/101528 teaches a simpler and cheaper arrangement. In this arrangement, which corresponds to the preamble of claim 1 , the lower ends of the PLFRs are attached to the nuclear fuel assembly base through clips integral with the base and forming said housings. However, such an arrangement still requires specific machining to achieve adequate tolerances for the clips. Therefore, while such an arrangement is satisfactory, it is still desirable to simplify its design and lower its price.

An object of the invention is therefore to provide a fuel assembly which reduces the risk of lift-off of the PLFRs during operation of the nuclear reactor and which has a simpler design and induce lower costs.

To this end, the invention relates to a fuel assembly according to claim 1 .

According to specific embodiments, the fuel assembly may comprise one or several of the features of the dependent claims 2-1 1 .

The invention and its advantages will be better understood upon reading the following description, given solely by way of example and with reference to the appended drawings, in which:

- fig. 1 is a schematic perspective view of a BWR nuclear fuel assembly according to the invention,

- fig. 2 is a schematic perspective view illustrating the lower debris filter of the nuclear fuel assembly of fig. 1 and its connection with two PLFRs, and

- fig. 3 is an enlarged view of portion III of fig. 2.

Fig. 1 shows a nuclear fuel assembly 1 for a boiling water reactor (BWR). This fuel assembly 1 extends along a vertical longitudinal direction L. Such a fuel assembly 1 is intended to be placed in a core of a nuclear reactor where coolant flows upwardly during operation of the nuclear reactor.

The fuel assembly 1 conventionally comprises:

- a base 2 intended to rest on a lower plate of the core,

- a head 3,

- a bundle 4 of fuel rods, said bundle 4 extending longitudinally between the base 2 and the head 3, - a water channel 5 placed inside the bundle 4 and connecting the base 2 to the head 3,

- a plurality of spacer grids 6 spaced apart along the longitudinal direction L and maintaining longitudinally and transversally the bundle 4,

- a fuel channel 7 surrounding the bundle 4 and fixed to the base 2 and the head 3.

Only a portion of the fuel channel 7 is shown on fig. 1 .

The bundle 4 comprises full length fuel rods (FLFRs) 9 and partial length fuel rods (PLFRs) 1 1 not shown on fig.1 .

Only two of these PLFRs 1 1 are partially shown on fig. 2 and 3. In the disclosed embodiment, the fuel assembly 1 comprises 20 PLFRs 1 1 and 92 FLFRs 9. However, these numbers may change from one embodiment to the other.

Each FLFRs 9 and PLFRs 1 1 comprises a cladding containing nuclear fuel pellets and closed by upper and lower plugs. A lower plug 12 can be seen on fig. 2 and 3.

The FLFRs 9 are attached through their upper ends to the head 3 in a conventional way, e.g. by a grid with holes receiving pins provided on the upper plugs of the FLFRs 9.

The FLFRs 9 extend downwardly from the head 3 up to the base 2 while maintaining a longitudinal gap between the lower ends 23 of the FLFRs 9 and the base 2 to allow longitudinal expansion of the FLFRs 9 during operation of the nuclear reactor.

The PLFRs 1 1 are shorter than the FLFRs 9. They extend upwardly from the base 2 and stop at a distance from the head 3. In some embodiments, the PLFRs 1 1 within the fuel assembly 1 may have different lengths.

The base 2 includes a debris filter 13 shown on fig. 2 and 3. This debris filter 13 has, for example, a first set of parallel transversal bars 15 and a second set of parallel transversal bars 17. The bars 17 are placed above the bars 15 and extend e.g. perpendicular with respect to the bars 15.

The bars 17 have enlargements 19 where housings 21 are provided in the form of upper recesses opening upwardly. Each housing 21 receives the lower end 23 of a respective PLFR 1 1 . The housings 21 have generally similar shapes. Only one housing 21 and its relationship with the respective PLFR 1 1 is disclosed hereafter with reference to fig. 3.

The housing 21 is delimited by a closed bottom 25 having substantially a disk shape and a peripheral wall 27 of substantially circular base. The peripheral wall 27 extends upwardly from the bottom 25. The bottom 25 and the peripheral wall 27 are integral with the enlargements. In other embodiments, the housings 21 can be delimited by parts affixed to the debris filter 1 3.

The diameter of the bottom 25 and of the peripheral wall 27 is slightly greater than the diameter of the lower end 23 of the corresponding PLFR 1 1 . The lower end 23 of the PLFR 1 1 is received in the housing 21 and rests freely on the bottom 25. This means that the base 2 does not longitudinally retain the PLFR 1 1 from moving upwardly with respect to the debris filter 1 3.

On fig. 3, the right PLFR 1 1 is shown spaced from the base 2 only to ease the description, but all PLFRs 1 1 have their lower ends 23 received in their respective housing 21 .

The debris filter 13 has a central portion 29 connecting some of the bars 17 and e.g. integral with these bars 1 7. A through hole 31 is provided in the central portion 29 for receiving the lower end of the water channel 5. The lower end of the water channel 5 is optionally screwed in the through hole 31 .

Each housing 21 being delimited by a closed bottom 25 larger than the corresponding PLFR lower end 23 and by a peripheral wall 27 surrounding this lower end 23, the lower end 23 is shielded from the vertical and upward flow of the coolant illustrated by arrows 33 on fig. 3.

Each lower end 23 is thus protected from the direct flow forces applied by the coolant and that would lead to impact forces resulting from power transients occurring during operation of the nuclear reactor.

Therefore, the longitudinal holding force applied by the spacer grids 6 remains sufficient and the risk of lift-off of the PLFRs 1 1 is reduced while avoiding specific arrangements for positively retaining the PLFRs 1 1 against lift-off.

The fuel assembly 1 has therefore a simple design which is cheap to manufacture.

The engagement of the lower ends 23 of the PLFRs 1 1 in the housings 21 prevents the debris filter 1 3 from rotating around a central longitudinal axis L with respect to the water channel 5. Therefore, a screw connection can optionally be provided between the water channel 5 and the debris filter 13, as disclosed previously, without the need of an additional anti-rotation feature. This helps further reducing the manufacturing costs of the fuel assembly 1 .

Moreover, the lower ends 23 of the PLFRs 1 1 do not require a specific design and they can therefore be of a similar shape or even have identical lower plugs 12 than the lower ends 23 of the FLFRs 9, thereby reducing further the costs of the fuel assembly 1 .

The above mentioned relationship between the base 2 and the PLFRs 1 1 can be implemented through other parts of the base 2 than a debris filter 13.

Also, only some or even only one of the PLFRs 1 1 may exhibit such a relationship.

In the disclosed embodiment, the lower ends 23 have a substantially circular base, but other shapes can be contemplated.