Patent Description:
A reactor core of a BWR is formed of a plurality of fuel assemblies. Each fuel assembly is elongated along a fuel assembly axis. The fuel assemblies of the reactor core are placed side-by-side with their fuel assembly axis being substantially vertical. Each fuel assembly of the reactor core is loaded into the reactor core or unloaded from the reactor core by lifting the fuel assembly.

A fuel assembly for a BWR generally comprises a base including a lower tie plate, a head including an upper tie plate and a lifting handle, the base and the head being spaced one from the other along the fuel assembly axis, a bundle of fuel rods, the fuel rods extending axially between the lower tie plate and the upper tie plate and each fuel rod containing fissile material, at least one tubular water channel (or water rod) arranged inside the bundle of fuel rods with connecting the lower tie plate to the upper tie plate and a tubular fuel channel encasing the bundle of fuel rods and each water channel located within the bundle of fuel rods. The fuel rods are e.g. supported axially and transversely by spacer grids which are distributed along the water channel(s) and connected to the water channel(s).

The lifting handle is configured for lifting the fuel assembly, e.g. for loading the fuel assembly into the reactor core or unloading the fuel assembly from the reactor core.

The lifting handle is connected to the upper tie plate, the lower tie plate being connected to the upper tie plate via the water channel(s), such that fuel assembly can be lifted as a unit, the load of the fuel channel being transferred via the water channel(s) forming a main load chain.

However, a water channel may break such that the lower tie plate is no longer connected to the upper tie plate and the lifting handle and may fall and damage, e.g. to other fuel assemblies, or generate debris that may potentially damage other fuel assemblies or the BWR.

<CIT> discloses a nuclear fuel assembly in which a load transfer between a lower part and an upper tie plate is performed via a fuel channel sleeve.

<CIT> discloses nuclear fuel assemblies in which a load transfer is carried out by load bearing water rods or by lifting rods.

<CIT> and <CIT> each disclose a nuclear fuel assembly comprising a lower tie plate and an upper tie plate connected by tie rods.

<CIT> discloses a nuclear fuel assembly comprising an upper fitting and a lower fitting connected via guide tubes.

One of the aims of the invention is to provide a fuel assembly that is more reliable, in particular when lifting the fuel assembly.

To this end, the invention proposes a fuel assembly for a boiling water reactoras defined in claim <NUM>.

The provision one or several tie rods allows to form an auxiliary load chain (or redundant load chain) that is operative to connect the lower tie plate to the upper tie plate in case of a breakage of the main load chain formed by one or several water channel(s).

Preferably, the stopping member is active only in case of breakage of the water channel. In particular, the (inactive) stopping member is spaced from the corresponding abutting surface when the water channel is operative to connect the lower tie plate to the upper tie plate, and the (active) stopping member abuts against the corresponding abutting surface for stopping a downward movement of tie rod relative to the abutting surface when lifting the fuel assembly when the water channel is inoperative to axially connect the lower tie plate to the upper tie plate, the water channel being e.g. broken. The auxiliary load chain is thus active only when the main load chain is inoperative.

In specific embodiments, the fuel assembly comprises optional features as defined in claims <NUM> - <NUM>.

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> shows a nuclear fuel assembly <NUM> for a boiling water reactor (BWR). The fuel assembly <NUM> is elongated along a fuel assembly axis L.

The fuel assembly <NUM> is intended to be placed with the fuel assembly axis L oriented vertically in a reactor core of a nuclear reactor where coolant flows upwardly during operation. In the following, the terms "lower" and "upper" refer to the position of the fuel assembly <NUM> in the reactor.

The fuel assembly <NUM> comprises a base <NUM> comprising a lower tie plate <NUM>, a head <NUM> spaced axially from the base <NUM> and comprising an upper tie plate <NUM> and a lifting handle <NUM>, a bundle of nuclear fuel rods <NUM> extending axially between the lower tie plate <NUM> and the upper tie plate <NUM>, a tubular water channel <NUM> extending axially between the lower tie plate <NUM> and the upper tie plate <NUM> with being inserted within the bundle of fuel rods <NUM> and connecting the lower tie plate <NUM> to the upper tie plate <NUM>, and a tubular fuel channel <NUM> extending axially between the lower tie plate <NUM> and the upper tie plate <NUM> with encasing the bundle of fuel rods <NUM>.

The fuel rods <NUM> are maintained transversely in a spaced relationship with being arranged for example at the nodes of an imaginary lattice. In such case, the water channel <NUM> occupies for example one or several nodes of the imaginary lattice.

The fuel assembly <NUM> comprise for example a plurality of spacer grids <NUM> distributed along the longitudinal axis L, each spacer grid <NUM> maintaining the fuel rods <NUM> longitudinally and transversely.

Each spacer grid <NUM> is for example rigidly connected to the water rod <NUM>. Each spacer grid <NUM> comprises for example fuel rod cells, each fuel rod <NUM> supported by the spacer grid <NUM> extending through a fuel rod cell.

Each fuel rod <NUM> is elongated axially. Each fuel rod <NUM> contains fissile material. Each fuel rod <NUM> comprises for example a tubular cladding filled with nuclear fuel pellets stacked inside the cladding, the ends of the cladding being closed by end plugs.

The base <NUM> optionally comprises a tubular transition piece <NUM> that is connected to the lower tie plate <NUM>. The transition piece <NUM> is configured for channeling water from an inlet of the transition piece <NUM> towards the lower tie plate <NUM> such that the water can flow axially upwardly within the water channel <NUM> and within the fuel channel <NUM> along the fuel rods <NUM>.

The lifting handle <NUM> is rigidly connected to the upper tie plate <NUM>. The lifting handle <NUM> is configured for grabbing the fuel assembly <NUM> for lifting the fuel assembly <NUM>.

Upon lifting the fuel assembly <NUM> via the handle <NUM>, the load of the fuel assembly <NUM> is transmitted between the lower tie plate <NUM> and the upper tie plate <NUM> via the water channel <NUM> that axially connects the lower tie plate <NUM> to the upper tie plate <NUM>.

The water channel <NUM> defines a main load chain between the lower tie plate <NUM> and the upper tie plate <NUM>.

The fuel assembly <NUM> further comprises at least one tie rod <NUM> extending axially between the base <NUM> and the head <NUM>, each tie rod <NUM> having a lower end <NUM> axially fixed to the base <NUM> and an upper end <NUM> axially connected to the head <NUM> via a connection assembly <NUM>, such as to limit an axial downward movement of the base <NUM> relative to head <NUM>.

Each tie rod <NUM> forms part of an auxiliary load chain that is configured for connecting the base <NUM> to the head <NUM> in case of failure of the water channel <NUM>, in particular in case of breakage of the water channel <NUM>.

Each tie rod <NUM> is distinct from each fuel rod <NUM>. Each tie rod <NUM> is deprived of fissile material. Each tie rod <NUM> is for example located at a node of the imaginary lattice at the nods of which the fuel rods <NUM> and the water channel <NUM> are placed.

Each tie rod <NUM> is also distinct from each water channel <NUM>.

The lower end <NUM> of each tie rod <NUM> is axially fixedly connected to the base <NUM>. The lower end <NUM> of each tie rod <NUM> is for example fixedly connected to the lower tie plate <NUM>.

As illustrated on <FIG>, a lower end <NUM> of each tie rod <NUM> is axially fixedly connected to the lower tie plate <NUM> via at least one fixing nut <NUM>. The fixing nut <NUM> is for example screwed onto the lower end <NUM> that is e.g. externally threaded.

The connection assembly <NUM> associated to each tie rod <NUM> is preferably configured for allowing an axial displacement of the upper end <NUM> of tie rod <NUM> relative to the head <NUM> on a limited stroke and for preventing axial displacement of the upper end <NUM> of the tie rod <NUM> relative to the head <NUM> further than the limited stroke.

The possible limited stroke of the upper end <NUM> of each tie rod <NUM> allows accommodating a differential thermal elongation between the tie rods <NUM> and the water channel <NUM>.

The connection assembly <NUM> associated to each tie rod <NUM> connects for example the upper end <NUM> of said tie rod <NUM> to the upper tie plate <NUM> or to the lifting handle <NUM>. In the following, the connection assembly <NUM> is described with referring in a general manner to the head <NUM> but may apply to the upper tie plate <NUM> or the lifting handle <NUM>.

<FIG> schematically illustrates an example of a connection assembly <NUM> connecting an upper end <NUM> of a tie rod <NUM> to the head <NUM>.

The tie rod <NUM> is slidably connected to the head <NUM>. The tie rod <NUM> extends for example through a hole <NUM> of the head <NUM> with being slidably received in this hole <NUM>. The hole <NUM> may be provided on the upper tie plate <NUM> or in the lifting handle <NUM>.

The connection assembly <NUM> comprises a stopping member <NUM> that is axially fixedly attached to an upper end <NUM> of the tie rod <NUM>, the stopping member <NUM> being configured to abut against an abutting surface <NUM> of the head <NUM> for limiting a downward movement of the upper end <NUM> of the tie rod <NUM> relative to the head <NUM>.

The abutting surface <NUM> may be provided on the upper tie plate <NUM> or on the lifting handle <NUM>.

The stopping member <NUM> is for example screwed to the upper end <NUM> of the tie rod <NUM>.

In such case, the upper end <NUM> of the tie rod <NUM> is for example threaded. In one embodiment, the stopping member <NUM> is a nut and the upper end <NUM> is externally threaded.

The stopping member <NUM> is positioned axially along the tie rod <NUM> such as to allow the limited downward stroke of the stopping member <NUM> downwardly relative to the head <NUM> before the stopping member <NUM> abuts the abutting surface <NUM> such as to prevent a further downward movement of the tie rod <NUM> (and the base <NUM>) relative to the head <NUM>.

As indicated above, the possible limited stroke of the stopping member <NUM> allows accommodating a differential thermal elongation between the tie rods <NUM> and the water channel <NUM>.

Optionally, the connection assembly <NUM> comprises a return member <NUM> configured for urging the stopping member <NUM> away from the abutting surface <NUM>, preferably in a permanent manner. In particular, the return member <NUM> is configured for pushing the stopping member <NUM> away from the abutting surface <NUM>, preferably in a permanent manner.

The return member <NUM> is for example interposed axially between the stopping member <NUM> and the head <NUM> such as to oppose to a downward movement of the stopping member <NUM> relative to head <NUM> and push the stopping member <NUM> upwardly relative to the head <NUM>.

The return member <NUM> is for example elastically deformable spring. In a particular embodiment, the return member <NUM> is a helical spring fitted onto the upper end <NUM> of the tie rod <NUM> with being interposed between the stopping member <NUM> and the head <NUM>.

In operation, the base <NUM> is axially rigidly connected to the head <NUM> via the water channel <NUM>. Each tie rods <NUM> is rigidly connected to the base <NUM> and is connected to the head <NUM> with an axial play via the associated connection assembly <NUM>.

Upon lifting the fuel assembly <NUM>, here via the lifting handle <NUM>, the load of the base <NUM> is transferred to the head <NUM> via the water channel <NUM> which defines a main load chain.

In case of a breakage of the water channel <NUM>, the load of the base <NUM> is no longer transferred to the head <NUM> via the water channel <NUM>. Upon lifting the fuel assembly <NUM>, here with using the lifting handle <NUM>, the base <NUM> tends to move downward relative to the head <NUM>.

In such case, the tie rods <NUM> limit a downward movement of the base <NUM> relative to the head <NUM>. Each tie rod <NUM> slides downwardly relative to the head <NUM> until the stopping member <NUM> abuts the abutting surface <NUM>. The load of the base <NUM> is then transferred to the head <NUM> via the tie rod <NUM>.

Hence, although the water channel <NUM> is broken, the fuel assembly <NUM> can still be lifted with remaining unitary and with limiting the risk of parts falling down and/or the risk of generating debris in the nuclear reactor.

Each tie rod <NUM> forms an auxiliary load chain (or redundant load chain) that transfers load between the base <NUM> and the head <NUM> in case of failure or breakage of the main load chain defined by the water channel <NUM>.

<FIG> schematically illustrates another example of a connection assembly <NUM> that differs from that of <FIG> in that it is configured for the stopping member <NUM> to wedge between the upper end <NUM> of the tie rod <NUM> and the abutting surface <NUM>, in particular when the tie rod <NUM> is pulled downwardly relative to the head <NUM>.

The stopping member <NUM> comprises for example an axially tapering outer surface 30A. The outer surface 30A tapers downwardly. The outer surface 30A is for example frustoconical.

The abutting surface <NUM> is for example tapering axially. In particular, the abutting surface <NUM> tapers downwardly. The abutting surface <NUM> is for example frustoconical.

The stopping member <NUM> is for example ring shaped and fitted onto the upper end <NUM> of the tie rod <NUM>.

Advantageously, the stopping member <NUM> is slidably fitted onto the upper end <NUM> of the tie rod <NUM>, the stopping member <NUM> being configured to slide along the upper end <NUM> of the tie rod <NUM> when the stopping member <NUM> is not wedged and to axially stop the upper end <NUM> of the tie rod <NUM> when the stopping member <NUM> is wedged between the upper end <NUM> of the tie rod <NUM> and the abutting surface <NUM>.

This allows an axial movement of the upper end <NUM> of the tie rod <NUM> relative to the head <NUM>, e.g. to accommodate for differential thermal elongation as explained above, when the stopping member <NUM> is not wedged.

In one example, the stopping member <NUM> is fitted onto the upper end <NUM> of the tie rod <NUM> with a friction fit. Hence, slow movement of the upper end <NUM> of the tie rod <NUM>, such as a slow movement due to thermal expansion, will not wedge the stopping member <NUM> between the upper end <NUM> and the abutting surface <NUM>, whereas a faster movement, such as provoke by pulling down the tie rod <NUM>, will wedge the stopping member <NUM> between the upper end <NUM> and the abutting surface <NUM>.

Preferably, as illustrated on <FIG>, the ring shaped stopping member <NUM> is slotted such as to be radially compressible. This eases wedging of the stopping member <NUM> between the upper end <NUM> of the tie rod <NUM> and the abutting surface <NUM>.

In one example, as illustrated on <FIG>, the tie rod <NUM> extends through a hole <NUM> of the head <NUM>, the abutting surface <NUM> being defined inside the hole <NUM>.

The stopping member <NUM> is for example mounted captive on the head <NUM>. The stopping member <NUM> is for example retained captive inside the hole <NUM>. In particular, axial movement of the stopping member <NUM> is axially restricted relative to the head <NUM>.

The connection assembly <NUM> comprises for example a retaining member <NUM> attached to the upper tie plate <NUM>, the stopping member <NUM> being retained axially between the retaining member <NUM> and the abutting surface <NUM> such that the stopping member <NUM> is mounted captive on the head <NUM>.

The retaining member <NUM> is for example ring shaped, with being externally threaded such as to be screwed onto the head <NUM>, e.g. into an internally threaded section of the hole <NUM> and with having an orifice <NUM> allowing the tie rod <NUM> to extend slidably through the retaining member <NUM>. The retaining member <NUM> does not interfere with the tie rod <NUM> that is free to slide axially relative to the retaining member <NUM>.

Advantageously, the connection assembly <NUM> optionally comprises a return member <NUM> that is configured to bias the stopping member <NUM> away from the abutting surface <NUM>.

The return member <NUM> is for example an elastically deformable spring, in particular an helical spring fitted onto the tie rod <NUM>, axially between the stopping member <NUM> and an internal shoulder <NUM> of the hole <NUM>, such as to push the stopping member <NUM> away from the abutting surface <NUM>.

The return member <NUM> helps maintaining the stopping member <NUM> in a position in which the stopping member <NUM> is not wedged between the tie rod <NUM> and the head <NUM> in normal operation (when the main load chain is operative).

During normal operation, a slight and/or slow movement of the tie rod <NUM> relative to the upper tie plate <NUM>, e.g. due to thermal elongation of the tie rod <NUM>, is not prevented by the stopping member <NUM>.

In case of a failure (e.g. breakage) of the water channel <NUM>, the tie rod <NUM> tends to be pulled downwards relative to the head <NUM>. The tie rod <NUM> thus moves the stopping member <NUM> downwardly, e.g. by friction, and the stopping member <NUM> wedges between the tie rod <NUM> and the abutting surface <NUM>, thus stopping the tie rod <NUM> relative to the head <NUM>.

Once the stopping member <NUM> is wedged, the tie rod <NUM> is stopped axially relative to the head <NUM> and the tie rod <NUM> can transmit load from the base <NUM> to the head <NUM>. Any increase in the load applied to the tie rod <NUM> wedged the stopping member <NUM> further.

During normal operation, when the return member <NUM> is provided, the return member <NUM> helps keeping the stopping member <NUM> not wedged, even though the tie rod <NUM> may slide relative to the head <NUM>, e.g. due to differential thermal elongation.

In case of a failure (e.g. breakage) of the water channel <NUM>, the stopping member <NUM> is wedged between the tie rod <NUM> and the abutting surface <NUM> against action of the return member <NUM>.

Once the stopping member <NUM> is wedged between the tie rod <NUM> and the head <NUM> and the fuel assembly <NUM> has been removed from the reactor core, the wedged stopping member <NUM> may prevent easy dismounting of the fuel assembly <NUM>. The return member <NUM> also helps dismounting the fuel assembly <NUM> when the stopping member <NUM> is wedged.

As illustrated on <FIG>, optionally, the retaining member <NUM> comprises at least one protrusion <NUM> configured for interfering with the stopping member <NUM> upon rotating the retaining member <NUM> about the axis of the tie rod <NUM>, in particular upon unscrewing the retaining member <NUM> from the head <NUM>, such as to drive the stopping member <NUM> in rotation jointly with the retaining member <NUM>. Thus can help releasing the wedged stopping member <NUM> upon dismounting the fuel assembly <NUM>.

Each protrusion <NUM> is provided on a lower face <NUM> of the retaining member <NUM> facing the stopping member <NUM>. In the illustrated example, the stopping member <NUM> is pushed axially by the return member <NUM> towards the lower face <NUM>, thus promoting engagement of the stopping member <NUM> with the protrusion(s) <NUM>.

Each protrusion <NUM> is configured for example has an abutting surface <NUM> configured to interfere with the stopping member <NUM> and to drive the stopping member <NUM> in rotation jointly with the retaining member <NUM> upon unscrewing the retaining member <NUM> from the head <NUM>.

Preferably, each protrusion <NUM> has a sliding surface <NUM> configured for sliding onto the stopping member <NUM> without driving the stopping member <NUM> in rotation upon screwing the retaining member <NUM> onto the head <NUM>.

Each protrusion <NUM> is thus configured for driving the stopping member <NUM> in rotation upon unscrewing the retaining member <NUM> and not driving the stopping member <NUM> in rotation upon screwing the retaining member <NUM>.

If the stopping member <NUM> is slotted, each protrusion <NUM> may be configured for example to interfere with one of the two edges of the slot.

Alternatively or optionally, the stopping member <NUM> is also provided with at least one protrusion <NUM> onto a surface of the stopping member <NUM> facing the retaining member <NUM>, each protrusion <NUM> of the stopping member being configured for cooperating with each protrusion <NUM> of the retaining member <NUM> for the retaining member <NUM> to drive the stopping member <NUM> in rotation upon unscrewing retaining member <NUM> from the head <NUM> and not driving the stopping member <NUM> in rotation upon screwing the retaining member <NUM> onto the head <NUM>.

Each protrusion <NUM> of the stopping member <NUM> comprises for example an abutting surface <NUM> for abutting onto an abutting surface <NUM> of a protrusion <NUM> of the retaining member <NUM> when screwing the retaining member <NUM> and a sliding surface <NUM> for sliding onto a sliding surface <NUM> of a protrusion <NUM> of the retaining member <NUM> when unscrewing the retaining member <NUM>.

Owing to the invention, the fuel assembly <NUM> is provided with a main load chain defined by a water channel <NUM> and an auxiliary load chain formed of tie rods <NUM>, the auxiliary load chain being not active in normal operation and becoming active upon failure of the main load chain, to prevent a downward movement of the base <NUM> relative to the head <NUM> upon lifting the fuel assembly <NUM> via the head <NUM>.

The auxiliary load chain is formed of tie rods <NUM> extending between the base <NUM> and the head <NUM> with being fixedly attached to the base <NUM>, in particular to the lower tie plate <NUM>, and connected to the head <NUM> via a connection assembly <NUM> allowing with an axial play. The axial play allows accommodating differential thermal elongation between the tie rods <NUM> and the water channel <NUM> during normal operation, and an activation of the auxiliary load chain upon failure of the main load chain.

The connection assembly <NUM> comprising a stopping member <NUM> for abutting an abutting surface <NUM> is reliable. The axial play is easily adjustable. A wedging stopping member <NUM> allows a reliable string stopping of the tie rod <NUM> relative to the head <NUM>.

The connection assembly <NUM> is arranged between the upper end <NUM> of the tie rod <NUM> and the head <NUM>. More specifically it may be arranged between the upper end <NUM> of the tie rod <NUM> and the handle <NUM> of the head <NUM>, in which case the hole <NUM> and/or abutting surface <NUM> is provided on the handle <NUM>, or between the upper end <NUM> of the tie rod <NUM> and the upper tie plate <NUM> of the head <NUM>, in which case the hole <NUM> and/or abutting surface <NUM> is provided on the upper tie plate <NUM>.

The fuel assembly <NUM> has been described above with reference to one water channel <NUM>, but the fuel assembly may of course comprises several water channels <NUM> forming together a main load chain for transferring the load of the base <NUM> to the head <NUM>.

Claim 1:
A fuel assembly for a boiling water reactor, the fuel assembly extending along a fuel assembly axis (L) and comprising a base (<NUM>) including a lower tie plate (<NUM>), a head (<NUM>) including an upper tie plate (<NUM>) and a lift handle (<NUM>), a bundle of fuel rods (<NUM>) extending axially between the lower tie plate (<NUM>) and the upper tie plate (<NUM>), and a water channel (<NUM>) extending within the bundle of fuel rods (<NUM>) with axially connecting the base (<NUM>) to the head (<NUM>) such that the load of the base (<NUM>) is transferred to the head (<NUM>) via the water channel (<NUM>) forming a main load chain, wherein the fuel assembly further comprises a tie rod (<NUM>) extending between the base (<NUM>) and the head (<NUM>) and forming an auxiliary load chain that transfers load between the base (<NUM>) and the head (<NUM>) in case of breakage of the main load chain defined by the water channel (<NUM>), the tie rod (<NUM>) being axially fixed to the base (<NUM>) and connected to the head (<NUM>) via a connection assembly (<NUM>) comprising a stopping member (<NUM>) configured to abut an abutting surface (<NUM>) of the head (<NUM>) for limiting a downward movement of the base (<NUM>) relative to the head (<NUM>) during lifting of the fuel assembly (<NUM>), in case of a breakage of the water channel (<NUM>).