Patent Description:
A nuclear fuel assembly (or "fuel assembly"), such as ones designed for VVER reactors (acronym for Russian "Vodo-Vodianoï Energuetitcheski Reaktor"), comprises a skeleton supporting nuclear fuel rods (or "fuel rods") arranged in an organized hexagonal array.

Each fuel rod comprises a tubular cladding containing nuclear fuel (e.g. nuclear fuel pellets, in particular UO<NUM> pellets), the two ends of the tubular cladding being closed by respective end plugs.

The skeleton comprises a bottom nozzle and a top nozzle spaced along the assembly axis, a plurality of guide tubes along the assembly axis connecting the bottom nozzle and the top nozzle together, spacer grids spaced longitudinally and secured to the guide tubes, and one instrumentation tube.

The fuel rods extend between the bottom nozzle and the top nozzle by passing through the spacer grids. The function of the spacer grids is to support the fuel rods along the assembly axis in an organized hexagonal array.

In operation, the fuel assembly is inserted into a reactor core received into a reactor vessel, with the longitudinal axis of the fuel assembly extending substantially vertically, and a coolant flows vertically through the fuel assembly in order to moderate the nuclear reaction retrieve heat produced by the fuel rods.

Small debris such a metallic chips may be created during maintenance operations, and such debris may be transported in the coolant flow and consequently damage the fuel rods of the fuel assembly, e.g. by generating fretting wear.

The provision of a debris filter in the bottom nozzle increases the complexity of the nuclear fuel assembly and its manufacturing cost, and may also increase pressure drop in the coolant flow.

<CIT>, <CIT> and <CIT> disclose bottom nozzles for nuclear fuel assemblies.

One of the aims of the invention is to provide a bottom nozzle that ensures a good resistance to fretting due to debris with providing easy of maintenance.

To this end, the invention proposes a bottom nozzle as defined in claim <NUM>.

Optional features of the bottom nozzle are defined in claims <NUM> - <NUM>.

The invention also pertains to a nuclear fuel assembly as defined in claim <NUM>.

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

The nuclear fuel assembly <NUM> of <FIG> is positioned along an assembly axis L. In operation, the nuclear fuel assembly <NUM> is inserted in a reactor core with the assembly axis L being vertical.

In the following, the terms "upper", "lower", "bottom", "top", "longitudinal" and "transversal" refer to the position of the nuclear fuel assembly in operation, with the assembly axis L being longitudinal and vertical.

The nuclear fuel assembly <NUM> comprises a bundle of nuclear fuel rods <NUM> supported by a skeleton <NUM>. The fuel rods <NUM> are parallel to each other and to the assembly axis L.

The skeleton <NUM> comprises a bottom nozzle <NUM> and a top nozzle <NUM> spaced along the assembly axis L.

The skeleton <NUM> comprises guide tubes <NUM> connecting the bottom nozzle <NUM> to the top nozzle <NUM> with maintaining a predetermined spacing along assembly axis L between the bottom nozzle <NUM> and the top nozzle <NUM>. The guide tubes <NUM> are parallel to the assembly axis L. The fuel rods <NUM> are positioned between the bottom nozzle <NUM> and the top nozzle <NUM>.

The skeleton <NUM> comprises a plurality of spacer grids <NUM> distributed along the guide tubes <NUM> and fixedly attached to the guide tubes <NUM>. The fuel rods <NUM> extend through the spaced grids <NUM>. Each spacer grid <NUM> is configured for supporting the fuel rods <NUM> along the assembly axis L and transversely to the assembly axis L.

Each spacer grid <NUM> is configured for supporting the fuel rods <NUM> in a transversely spaced relationship.

In one exemplary embodiment, the fuel rods <NUM> are maintained transversely at the nodes of an imaginary hexagonal network. In such case, in a plane perpendicular to the assembly axis L, the bundle formed by the fuel rods <NUM> has for example a hexagonal contour. The guide tubes <NUM> are positioned at some of the nodes of the imaginary hexagonal network.

The nuclear fuel assembly <NUM> is for example designed for a VVER (acronym for Russian "Vodo-Vodianoï Energuetitcheski Reaktor"). The nuclear fuel assemblies <NUM> for VVER generally have fuel rods <NUM> arranged in a hexagonal array.

In operation, the nuclear fuel assembly <NUM> is inserted in a reactor core housed inside a reactor vessel, between a lower core plate and an upper core plate, with the assembly axis L being vertical.

The bottom nozzle <NUM> is configured to rest onto the lower core plate containing a flow opening that allows a coolant entering the bottom nozzle <NUM> and flowing upwardly through the nuclear fuel assembly <NUM> in order to moderate the nuclear reaction and retrieve heat from the fuel rods <NUM> (see arrow F on <FIG>).

As illustrated on <FIG>, the bottom nozzle <NUM> comprises a tubular nozzle body <NUM> extending along a nozzle axis N, having a lower opening <NUM> and an upper opening <NUM>. In use the nozzle axis N is substantially collinear with the assembly axis L.

The lower opening <NUM> is configured for being positioned in register with a flow opening in the lower core plate, consequently receiving the coolant flow (see arrow F on <FIG>).

The nozzle body <NUM> defines an internal duct <NUM> extending along the nozzle axis N from the lower opening <NUM> to the upper opening <NUM> for allowing coolant to flow through the nozzle body <NUM>, from the lower opening <NUM> to the upper opening <NUM>.

The bottom nozzle <NUM> comprises a lower tie plate <NUM> extending across the upper opening <NUM>. The lower tie plate <NUM> is fixedly attached to the nozzle body <NUM> and configured for fixing the guide tubes <NUM> to the lower tie plate <NUM>.

The lower tie plate <NUM> has a lower face 24A and a top face 24B.

The lower tie plate <NUM> is configured to allow coolant to flow through the lower tie plate <NUM>. The lower tie plate <NUM> has for example a plurality of flow apertures <NUM> extending through the lower tie plate <NUM> allowing the coolant to flow through the lower tie plate <NUM>.

The bottom nozzle <NUM> comprises a debris filter <NUM> received inside the nozzle body <NUM>, below the lower tie plate <NUM>. The debris filter <NUM> is located axially between the lower tie plate <NUM> and the lower opening <NUM> of the bottom nozzle <NUM>. The debris filter <NUM> is upstream the lower tie plate <NUM> when considering the coolant flow.

The debris filter <NUM> has a lower face 28A and an upper face 28B.

The debris filter <NUM> is configured for allowing coolant to flow through the debris filter <NUM> whilst retaining debris that may be present in the coolant.

The debris filter <NUM> comprises flow passages <NUM> for allowing coolant to flow through the debris filter <NUM>. Each flow passage <NUM> extends from the lower face 28A to the upper face 28B of the debris filter <NUM>.

The debris filter <NUM> comprises for example flow passages <NUM> that exhibit small transverse dimensions retaining debris having transverse dimensions larger and/or higher than that of the flow passages <NUM>.

The debris filter <NUM> comprises flow passages <NUM> extending axially. Alternatively or optionally, the debris filter <NUM> comprises tortuous flow passages <NUM>. Such flow passages <NUM> are adapted for retaining elongated debris.

The bottom nozzle <NUM> comprises connection members <NUM>, each connection member <NUM> being configured for securing a lower end of a respective guide tube <NUM> of the nuclear fuel assembly <NUM> to the lower tie plate <NUM>. Each connection member <NUM> is preferably configured for securing the guide tube <NUM> axially and transversely to the lower tie plate <NUM>.

Each connection member <NUM> is configured to be operable from below the debris filter <NUM> for attaching the guide tube <NUM> to the lower tie plate <NUM> and/or detaching the guide tube <NUM> from the lower tie plate <NUM>.

As may be seen on <FIG>, each connection member <NUM> comprises for example a connection rod <NUM> extending through the lower tie plate <NUM> and the debris filter <NUM>, the connection rod <NUM> having an upper end <NUM> configured to be connected to a lower end <NUM> of a guide tube <NUM> and a lower end <NUM>.

More specifically, the connection rod <NUM> passes through a connection orifice <NUM> of the lower tie plate <NUM> and through a corresponding connection orifice <NUM> of the debris filter <NUM>.

The upper end <NUM> of the connection rod <NUM> is for example configured for screwing the connection rod <NUM> to the lower end <NUM> of the guide tube <NUM>.

The upper end <NUM> of the connection rod <NUM> is for example threaded for screwing the lower end <NUM> to the guide tube <NUM> that is provided with a thread that is complementary to that of the upper end <NUM> of the connection rod <NUM>. The upper end <NUM> of the connection rod <NUM> is for example externally threaded, the lower end <NUM> of the guide tube <NUM> being internally threaded.

The lower end <NUM> of the connection rod <NUM> is provided with a connection head <NUM> that is configured for holding the connection rod <NUM> under the lower tie plate <NUM>.

In one example, the connection head <NUM> is screwed to the connection rod <NUM>. Alternatively, the connection head <NUM> is integrated to the connection rod <NUM>. In such case, the connection head <NUM> is not detachable from the connection rod <NUM>. In a particular example, the connection head <NUM> and the connection rod <NUM> may be welded together or made in one single piece of material.

Advantageously, the connection head <NUM> is configured for operating the connection rod <NUM> and therefore attaching the guide tube <NUM> to the lower tie plate <NUM> and/or detaching the guide tube <NUM> from the lower tie plate <NUM>.

The connection head <NUM> is for example configured for turning the connection rod <NUM>, e.g. for screwing the connection rod <NUM> to the lower end <NUM> of the guide tube <NUM> and/or unscrewing the connection rod <NUM> from the lower end <NUM> of the guide tube <NUM>.

Screwing / unscrewing the connection rod <NUM> is operated from the underside of the lower tie plate <NUM>, through the lower opening 18of the nozzle body <NUM>.

The connection head <NUM> is for example provided with facets <NUM> for cooperating with a tool such as a wrench. In a particular example, the connection head <NUM> may have a hexagonal outer shape.

Advantageously, each connection member <NUM> is configured for assembling the debris filter <NUM> to the underside of the lower tie plate <NUM>.

In one example, the connection head <NUM> is configured for retaining the debris filter <NUM> by interfering with the periphery of the connection orifice <NUM> on the lower face 28A of the debris filter <NUM>.

Advantageously, the bottom nozzle <NUM> is configured for maintaining an axial spacing S between the debris filter <NUM> and the lower tie plate <NUM>.

Due to the axial spacing S, an intermediate space <NUM> is maintained between the debris filter <NUM> and the lower tie plate <NUM>, more specifically between the upper face 28B of the debris filter <NUM> and the lower face 24A of the lower tie plate <NUM>.

The spacing S provided between the lower tie plate <NUM> and the debris filter <NUM> allows a flexible design of the debris filter <NUM> without penalizing the pressure loss coefficient of the bottom nozzle <NUM>. Indeed, the geometry of the debris filter <NUM> does not have to be congruent with the lower tie plate <NUM>. The debris filter <NUM> may e.g. have an offset with regard to the lower tie plate <NUM>.

The bottom nozzle <NUM> is thus modular with the possibility to associate different debris filters <NUM> to a same lower tie plate <NUM>, in particular with different debris filter <NUM> imparting different pressure loss coefficient to the bottom nozzle <NUM>.

In the bottom nozzle <NUM>, a debris filter <NUM> may be easily replaced by another one that is identical or different during maintenance operations.

In one example, the bottom nozzle <NUM> comprises columns <NUM> extending between the the lower face 24A of the lower tie plate <NUM> and the debris filter <NUM>, each column <NUM> being configured such as to maintain the spacing S between the lower tie plate <NUM> and the debris filter <NUM>.

Preferably, each connection orifice <NUM> of the lower tie plate <NUM> extends through a respective column <NUM> that is thus tubular. Each connection rod <NUM> extends through a respective tubular column <NUM>.

In one example, the outer surface of each column <NUM> is provided with a stop surface <NUM> located at a distance from the lower face 24B of the lower plate <NUM> and configured to stop an upward movement of debris filter <NUM> along the column <NUM>.

The spacing S between the lower corresponds to the length of each column <NUM> between the lower face 24A of the lower tie plate <NUM> and the stop surface <NUM>.

Preferably, the stop surface <NUM> of each column <NUM> is located at a distance above the lower extremity of the column <NUM>. Each column <NUM> has a lower portion extending through a connection orifice <NUM> of the debris filter <NUM> with the stop surface <NUM> abutting the upper face 28B of debris filter <NUM> when the debris filter <NUM> moves upwardly.

The debris filter <NUM> is retained axially between the connection head <NUM> of each connection member <NUM> and the stop surface <NUM> of the corresponding column <NUM>.

Preferably, the bottom nozzle <NUM> is configured for securing each connection member <NUM> to a guide tube <NUM> thought the lower tie plate <NUM> without clamping the debris filter <NUM> against the lower tie plate <NUM>.

To that end, the connection head <NUM> of each connection member <NUM> abuts for example the lower extremity of the corresponding column <NUM> without clamping the debris filter <NUM>.

Optionally, the connection head <NUM> of each connection member <NUM> is hydrodynamically profiled to limit pressure losses of the coolant. In one example, the connection head <NUM> tapers downwardly. Preferably, the connection head <NUM> is ogive shaped.

Each column protrudes from the lower face 24B of the lower tie plate <NUM>.

Preferably, each column <NUM> is integral with the lower tie plate <NUM>. In particular, each column <NUM> is preferably made in one piece of material with the lower tie plate <NUM>. Alternatively, each column <NUM> is fixedly secured to the lower tie plate <NUM>, e.g. by welding.

As visible on <FIG>, in one example, the lower tie plate <NUM> is positioned across the upper opening <NUM> of the nozzle body <NUM> with abutting a peripheral edge <NUM> of the upper opening <NUM>.

For example, the lower tie plate <NUM> has a peripheral shoulder <NUM> abutting the peripheral edge <NUM>.

The peripheral shoulder <NUM> is located at a junction between a lower portion <NUM> and an upper portion <NUM> of the lower tie plate <NUM>, the lower portion <NUM> being nested into the upper opening <NUM>.

The upper opening <NUM> and the debris filter <NUM> are configured for insertion of the debris filter <NUM> into the nozzle body <NUM> from the top of the nozzle body <NUM>, through the upper opening <NUM>.

In one example, the nozzle body <NUM> is provided inside the nozzle body <NUM> with a stop feature <NUM> configured for stopping the downward movement of the debris filter <NUM> inserted inside the nozzle body <NUM> through the upper opening <NUM>.

In one exemplary embodiment, the stop feature <NUM> is an internal peripheral shoulder provided on an inner surface of the nozzle body <NUM>.

Preferably, each connection member <NUM> is positioned such as to be operable through the lower opening <NUM> of the nozzle body <NUM>.

In one exemplary embodiment, each connection member <NUM> is positioned in such a way that the connection member <NUM>, and in particular the connection head <NUM>, is axially in register with the lower opening <NUM> of the nozzle body <NUM> along the nozzle axis N.

Preferably, each connection member <NUM>, and in particular the connection head <NUM> of each connection member <NUM>, is located within an imaginary cylinder C formed by projecting the contour of the lower opening <NUM> along the nozzle axis N.

The contour of the lower opening <NUM> is preferably circular in a plane perpendicular to the nozzle axis N.

Preferably, the lower tie plate <NUM> is to be secured to the nozzle body <NUM>.

To this end, the bottom nozzle <NUM> comprises for example connection elements <NUM> (diagrammatically represented by mixed lines), which are e.g. screws.

In view of assembling the bottom nozzle <NUM>, the debris filter <NUM> is inserted inside the nozzle body <NUM> through the upper opening <NUM>.

Then the lower tie plate <NUM> is positioned across the upper opening <NUM>, with optionally inserting the lower portion <NUM> of the lower tie plate <NUM> into the upper opening <NUM> and the lower tie plate <NUM> is secured to the nozzle body <NUM>, e.g. via the connection elements <NUM>.

Then, the connection members <NUM> are inserted into the nozzle body <NUM> through the lower opening <NUM> and operated from below the nozzle body <NUM> to be connected to the lower ends <NUM> of the guide tubes <NUM> through the debris filter <NUM> and the lower tie plate <NUM>.

Once the connection members <NUM> are secured to the guide tubes <NUM>, the guide tubes <NUM> are fixedly secured to the lower tie plate <NUM>, and preferably, the debris filter <NUM> is also secured to the lower tie plate <NUM>, advantageously with a spacing S between the debris filter <NUM> and the lower tie plate <NUM>.

In view of disassembling the bottom nozzle <NUM>, the connection members <NUM> are operated through the lower opening <NUM> of the bottom nozzle <NUM> such as to unsecure the guide tubes <NUM> from the lower tie plate <NUM>.

Before securing the connection members <NUM> to the tube guides <NUM>, the connection members <NUM> may be temporarily secured to the lower tie plate <NUM> using nuts screwed onto the upper ends <NUM> of the connection rods <NUM> of the connection members <NUM> inserted through the lower tie plate <NUM>.

Such a temporary mounting may be useful for transporting the bottom nozzle <NUM> in an assembled stated before securing to the guide tubes <NUM>.

Owing to the invention, it is possible to obtain a bottom nozzle <NUM> for a nuclear fuel assembly <NUM> that can be secured easily to the lower ends of the guide tubes <NUM> of the nuclear fuel assembly <NUM>.

The assembly/disassembly is performed easily through the lower opening <NUM> of the nozzle body <NUM>.

In addition, the bottom nozzle <NUM> is provided with a debris filter <NUM> inside the nozzle body <NUM>, below the lower tie plate <NUM>. This reduces the risk of debris reaching the fuel rods <NUM> and thus limits the risk of fretting wear of the fuel rods <NUM> due to debris.

The optional spacing S between the debris filter <NUM> and the lower tie plate <NUM> allows providing a debris filter <NUM> with a specific design that is more freely adaptable, in terms of filtering performances and/or pressure loss coefficient for the coolant flowing through the bottom nozzle <NUM>.

The optional spacing S is obtained in a simple manner, with combining the connection of the lower tie plate <NUM> to the guide tubes <NUM> using connection members <NUM> extending through the debris filter <NUM> and the lower tie plate <NUM> and the spacing S of the lower tie plate <NUM> and the debris filter <NUM> thanks to the columns <NUM>.

The invention is not limited to the exemplary embodiments that have been described above. Other embodiments may be contemplated.

In one exemplary embodiment above, the debris filter <NUM> is secured to the lower tie plate <NUM> by the connection members <NUM>, in particular by the connection head <NUM> interfering with the debris filter <NUM>.

Alternatively, debris filter <NUM> is maintained between the lower tie plate <NUM> and the stop feature <NUM>, in particular between the stop surfaces <NUM> of the columns <NUM> and the strop feature <NUM>, when a spacing S is provided between the lower tie plate <NUM> and the debris filter <NUM>.

In such a case, the stop feature <NUM> is preferably positioned axially relative to the lower tie plate <NUM> in such a way that the debris filter <NUM> is held between the stop feature <NUM> and the columns <NUM>, advantageously with a small or no axial play.

Besides, the invention is applicable to a nuclear fuel assembly <NUM> in which the nuclear fuel rods <NUM> are arranged at the nods of an imaginary hexagonal network, in particular by forming a bundle of fuel rods <NUM> with a hexagonal contour.

The invention also applies to a nuclear fuel assembly <NUM> in which the nuclear fuel rods <NUM> are arranged at the nods of an imaginary square network, in particular by forming a bundle of fuel rods <NUM> with a square contour.

Claim 1:
Bottom nozzle for a nuclear fuel assembly, the bottom nozzle comprising a tubular nozzle body (<NUM>) having a lower opening (<NUM>) and an upper opening (<NUM>), a lower tie plate (<NUM>) extending across the upper opening (<NUM>) and connection members (<NUM>), each connection member (<NUM>) being configured for securing a lower end of a respective guide tube (<NUM>) of the nuclear fuel assembly to the lower tie plate (<NUM>), wherein the bottom nozzle comprises a debris filter (<NUM>),
each connection member (<NUM>) being operable from below the debris filter (<NUM>) for attaching the guide tube (<NUM>) to the lower tie plate (<NUM>) and detaching the guide tube (<NUM>) from the lower tie plate (<NUM>),
characterised in that
the debris filter (<NUM>) is received inside the nozzle body (<NUM>) below the lower tie plate (<NUM>).