Patent Description:
A nuclear fuel assembly (or "fuel assembly") comprises nuclear fuel rods (or "fuel rods") arranged in a bundle and a skeleton supporting the fuel rods.

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 for example a bottom nozzle and a top nozzle spaced along an assembly axis, guide thimbles extending along the assembly axis between the bottom nozzle and the top nozzle with connecting the bottom nozzle and the top nozzle together, and spacer grids attached to the guide thimbles with being distributed along the guide thimbles, between the bottom nozzle and the top nozzle.

The fuel rods extend between the bottom nozzle and the top nozzle with passing through the spacer grids. The function of the spacer grids is to support the fuel rods along the assembly axis and transversely to the assembly axis with maintaining the fuel rods in a spaced relationship.

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 for moderating the nuclear reaction and for retrieving heat produced by the fuel rods.

The coolant flowing upwardly through the fuel assembly generates a force that tends to lift-off the fuel assembly. This force is generally called the "hydraulic lift force". The hydraulic lift force can overcome the weight of the fuel assembly.

In view of counteracting the hydraulic lift force, the fuel assembly is generally provided with a hold-down device that is configured to bias the fuel assembly downwardly.

The hold-down device is for example provided on the top nozzle and is configured to bear onto a reactor upper plate for pushing the fuel assembly downwardly. The hold-down device comprises spring members that generate a force that is substantially linearly proportional to the deflection of the spring members.

The hold-down force required to prevent fuel assembly lift-off varies with different operating conditions since the hydraulic lift force depends on the coolant temperature (since the coolant temperature influence the coolant density and thus the hydraulic lift force) and the volume flow rate of the coolant through the reactor.

During cold reactor startup, the coolant density is high which results in a high hydraulic lift force and the required hold-down force is high. In normal power operation, the coolant density is low and the required hold-down force is low.

Besides, due to differential thermal expansion between the fuel assembly and the reactor vessel, the hold-down force generated by the hold-down device varies with the temperature. For example, an increase in temperature of the nuclear reactor results in a reduction of the hold-down force generated by the hold-down device.

Despite this effect, the hold-down force generated by the hold-down device during hot operating conditions is generally higher than necessary since the lower coolant density also results in a lower hydraulic lift force.

Besides, irradiation of the fuel assembly generally causes a decrease of the hold-down force due to spring relaxation during the lifetime of the fuel assembly. This is compensated by providing a hold-down device that generates a higher hold-down force than necessary before irradiation of the fuel assembly.

As a consequence, the hold-down device exerts unnecessarily high levels of mechanical stress onto the fuel assembly structure, which is one of the main causes for fuel assembly bow.

The consequences of fuel assembly bow include difficulties during handling operations such as loading / unloading the fuel assembly into or out from the reactor core and safety concerns such as incomplete insertion of control rods inserted into the guide thimbles for lowering the nuclear reactivity of the nuclear fuel assembly.

<CIT> discloses a nuclear fuel assembly having a top nozzle comprising an upper plate, a lower plate attached to guide tubes and springs arranged between the upper plate and the lower plate for moving the upper plate away from the lower plate.

One of the aims of the invention is to propose a hold-down device that allows better tuning of the hold-down force.

To this end, the invention proposes a top nozzle for a nuclear fuel assembly, the top nozzle extending along a nozzle axis and comprising:.

The top nozzle incorporating such a hold-down device provides better tuning of the hold-down force for various operating conditions and during the lifetime of the fuel assembly, thus allowing a reduction of the risk for fuel assembly bow.

The hold-down device provides for sufficient and need-tailored hold-down force at cold operating conditions (e.g. starting phase of an operating cycle) during which the main spring member(s) and at least one of the auxiliary spring member(s) is(are) active to generate the hold-down force, while allowing for a decrease of the hold-down force at hot operating conditions (e.g. normal operating cycle), during which only the main spring member(s) is(are) active, the auxiliary spring member(s) no longer acting onto the intermediate plate.

In specific embodiments, the top nozzle comprises one or several of the following optional features, taken individually or according to any technically feasible combination:.

The invention also relates to a nuclear fuel assembly extending along a longitudinal axis and a bottom nozzle, a top nozzle as defined above, guide thimbles extending between the bottom nozzle and the top nozzle with connecting the bottom nozzle and the top nozzle, nuclear fuel rods positioned between the bottom nozzle and the top nozzle, and spacer grids fixedly attached to the guide thimbles, the nuclear fuel rods extending between the bottom nozzle and the top nozzle with passing through the spacer grids.

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> comprises a bundle of nuclear fuel rods <NUM> and a skeleton <NUM> for supporting the fuel rods <NUM>. The fuel rods <NUM> extend parallel to each other and to an assembly axis L.

The skeleton <NUM> comprises a bottom nozzle <NUM>, a top nozzle <NUM>, a plurality of guide thimbles <NUM> and a plurality of spacer grids <NUM>.

The bottom nozzle <NUM> and the top nozzle <NUM> are spaced along the assembly axis L. The guide thimbles <NUM> extend parallel to the assembly axis L and connect the bottom nozzle <NUM> to the top nozzle <NUM> with maintaining a predetermined spacing along the assembly axis L between the bottom nozzle <NUM> and the top nozzle <NUM>. The fuel rods <NUM> are received between the bottom nozzle <NUM> and the top nozzle <NUM>.

The spacer grids <NUM> are distributed along the bundle of fuel rods <NUM>. Each spacer grid <NUM> is fixedly attached to the guide thimbles <NUM>, with the guide thimbles extending through the spacer grid <NUM>.

The fuel rods <NUM> extend through the spaced grids <NUM>. Each spacer grid <NUM> is configured for supporting the fuel rods <NUM> in a transversely spaced relationship. Each spacer grid <NUM> is configured for supporting the fuel rods <NUM> along the assembly axis L and transversely to the assembly axis L.

In operation, the nuclear fuel assembly <NUM> is inserted in a reactor core inside a reactor vessel, between a lower core plate and an upper core plate. The bottom nozzle <NUM> is configured to rest onto the lower core plate. The top nozzle <NUM> is configured to bear against the upper core plate with urging the nuclear fuel assembly <NUM> downwardly against the hydraulic lift-up force.

As visible on <FIG>, the top nozzle <NUM> extends along a nozzle axis N. The nozzle axis N coincides with the assembly axis L.

The top nozzle <NUM> comprises a lower part <NUM> that is fixedly attached to the guide thimbles <NUM> and an upper part <NUM> that is movable along the nozzle axis N relative to the lower part <NUM>. The upper part <NUM> is configured for resting onto the upper core plate.

As illustrated on <FIG>, the top nozzle <NUM> comprises a hold-down device <NUM> configured for pushing the upper part <NUM> upwardly relative to the lower part <NUM> along the nozzle axis N.

Hence, when the upper part <NUM> abuts the upper core plate, the lower part <NUM> is pushed downwardly and the fuel assembly <NUM> is thus pushed downwardly to hold-down the fuel assembly <NUM> against hydraulic lift force.

The hold-down device <NUM> comprises a lower plate <NUM>, an upper plate <NUM> axially spaced from the lower plate <NUM>, and an intermediate plate <NUM> provided between the lower plate <NUM> and the upper plate <NUM>, the intermediate plate <NUM> being axially movable along the nozzle axis N relative to lower plate <NUM> and the upper plate <NUM>, between and upper position (<FIG>) and a lower position (<FIG>).

Preferably, the lower plate <NUM> and the upper plate <NUM> are maintained at fixed positions along the nozzle axis N.

In one example, the lower plate <NUM> and the upper plate <NUM> are connected one to the other via connection elements, the intermediate plate <NUM> moving axially between the lower plate <NUM> and the upper plate <NUM> with sliding along the connection elements.

Preferably, the lower plate <NUM> and the upper plate <NUM> are connected via the connection elements such that the lower plate <NUM> and the upper plate <NUM> are maintained at respective fixed positions along the connection elements.

In a particular embodiment, the upper plate <NUM> is fixed axially relative to the connection elements and/or the lower plate <NUM> is permanently pushed downwardly by at least one elastic member in abutment against at least one lower stop member limiting a downward movement of the lower plate <NUM> along the connection elements.

As illustrated on <FIG>, the connection elements comprise for example guide thimbles <NUM> of the nuclear fuel assembly <NUM>.

The guide thimbles <NUM> extend through the lower plate <NUM>, the upper plate <NUM> and the intermediate plate <NUM>.

The lower plate <NUM> is retained axially along the guide thimbles <NUM> such as to limit a downward movement of the lower plate <NUM> relative to the guide thimbles <NUM>.

In this view, at least one guide thimble <NUM> or each guide thimble <NUM> is for example provided with a lower stop member limiting a downward movement of the lower plate <NUM> along the guide thimble <NUM>. The lower stop member is for example a shoulder 12A of the guide thimble <NUM>.

As it will be explained later, the lower plate <NUM> is permanently pushed downwardly by one or several spring member(s).

The upper plate <NUM> is for example maintained at a fixed position axially along the guide thimbles <NUM>.

In this view, the upper plate <NUM> is for example mounted on at least one guide thimble <NUM> or each the guide thimble <NUM> with being axially fixed between an upper shoulder 12B of the guide thimble <NUM> and a nut <NUM> screwed onto an external thread of the guide thimble <NUM>.

The intermediate plate <NUM> is slidable along the guide thimbles <NUM>, between the lower plate <NUM> and the upper plate <NUM>.

The hold-down device <NUM> comprises at least one main spring member <NUM>, each main spring member <NUM> being arranged between the lower plate <NUM> and the intermediate plate <NUM> such as to permanently push the intermediate plate <NUM> towards the upper plate <NUM>.

Each main spring member <NUM> is permanently active along the entire stroke of the intermediate plate <NUM> between the upper position (<FIG>) and the lower position (<FIG>).

The lower plate <NUM> is permanently pushed downwardly by each main spring member <NUM>, preferably in abutment against each lower stop member 12A. The lower plate <NUM> is thus maintained in a fixed position along the guide thimbles <NUM>.

The hold-down device <NUM> comprises one or several auxiliary spring member(s) <NUM>, each auxiliary spring member <NUM> being arranged such as to push to intermediate plate <NUM> towards the upper plate <NUM> when the intermediate plate <NUM> is located between the lower position and an intermediate position associated to this auxiliary spring member <NUM> (<FIG>), the intermediate position being located between the lower position and the upper position.

Each auxiliary spring member <NUM> is arranged such as to be active to push the intermediate plate <NUM> towards the upper plate <NUM> when the intermediate plate <NUM> is located between the lower position and the associated intermediate position and to be inactive when the intermediate plate <NUM> is located between the associated intermediate position and the upper position.

When the intermediate plate <NUM> is located between the lower position and the intermediate position associated to one or several auxiliary spring member(s) <NUM>, the intermediate plate <NUM> is pushed towards the upper plate <NUM> by each main spring member <NUM> and by each auxiliary spring member <NUM> associated to this intermediate position.

When the intermediate plate <NUM> is located between an intermediate position associated to one or several auxiliary spring member(s) <NUM> and the upper position, the intermediate plate <NUM> is pushed towards the upper plate <NUM> by each main spring member <NUM> without being pushed towards the upper plate <NUM> by each auxiliary spring member <NUM> associated to this intermediate position.

The hold-down device <NUM> comprises a preload element <NUM> associated to each auxiliary spring member <NUM> and arranged such that the auxiliary spring member <NUM> abuts against the upper plate <NUM> via the preload element <NUM> when the intermediate plate <NUM> is located between the intermediate position associated to this auxiliary spring member <NUM> and the upper position.

The preload element <NUM> associated to each auxiliary spring member <NUM> is configured such that the auxiliary spring member <NUM> abutting the upper plate <NUM> via the preload element <NUM> is preloaded.

The preload element <NUM> extends for example through the intermediate plate <NUM>, the auxiliary spring member <NUM> being located between the lower plate <NUM> and the intermediate plate <NUM> and abutting the upper plate <NUM> via the preload element <NUM> extending through the intermediate plate <NUM> when the intermediate plate <NUM> is between the intermediate position associated to the auxiliary spring member <NUM> and the upper position.

In such an example, advantageously, when the intermediate plate <NUM> is between the intermediate position associated to an auxiliary spring member <NUM> and the lower position, this auxiliary spring member <NUM> abuts the intermediate plate <NUM> via the preload element <NUM>.

Each guide thimble <NUM> has an upper axial portion <NUM> extending axially between the lower plate <NUM> and the upper plate <NUM>.

The upper axial portion <NUM> of each guide thimble <NUM> extends through the intermediate plate <NUM>. The intermediate plate <NUM> is provided with a respective opening 26A for the upper axial portion <NUM> of each guide thimble <NUM>.

Each main spring member <NUM> is for example a helical spring, having a lower end abutting the lower plate <NUM> and an upper end abutting the intermediate plate <NUM>.

Each main spring member <NUM> is for example fitted onto an upper axial portion <NUM> of a guide thimble <NUM> such that the main spring member <NUM> is guided by the upper axial portion <NUM> of the guide thimble <NUM>. Each main spring member <NUM> is for example provided as a helical spring extending around the upper axial portion <NUM> onto which the main spring member <NUM> is fitted.

Each auxiliary spring member <NUM> is for example a helical spring, having a lower end abutting the lower plate <NUM> and an upper end abutting the preload element <NUM>.

Each auxiliary spring member <NUM> is for example fitted onto an upper axial portion <NUM> of a guide thimble <NUM> such as to be guided by the upper axial portion <NUM> of the guide thimble <NUM>.

Each auxiliary spring member <NUM> is for example provided as a helical spring extending around the upper axial portion <NUM> onto which the auxiliary spring member <NUM> is fitted.

Each preload element <NUM> is for example tubular and slidably fitted onto a guide thimble <NUM>, axially between an auxiliary spring member <NUM> and the upper plate <NUM>.

Each preload element <NUM> has for example a lower bearing surface 32A onto which the auxiliary spring member <NUM> pushes to urge the preload element <NUM> upwardly, an upper bearing surface 32B abutting the upper plate <NUM> when the intermediate plate is between the intermediate position and the upper position and an activation bearing surface 32C abutting the intermediate plate <NUM> when the intermediate plate <NUM> is located between the intermediate position and the lower position.

The intermediate plate <NUM> is slidable along the preload element <NUM> between the intermediate position and the upper position.

When the intermediate plate <NUM> is between the intermediate position associated to an auxiliary spring member <NUM> and the lower position, the auxiliary spring member <NUM> pushes onto the intermediate plate <NUM> via the preload element <NUM>. The auxiliary spring member <NUM> is bearing onto the lower bearing surface 32A and the intermediate plate <NUM> is bearing onto the activation bearing surface 32C.

When the intermediate plate <NUM> is between the intermediate position associated to an auxiliary spring member <NUM> and the lower position, the preload element <NUM> is moved downwardly towards the lower plate <NUM> jointly with the intermediate plate <NUM> such that the preload element <NUM> no longer abuts the upper plate <NUM> (<FIG>).

The lower plate <NUM> is permanently urged downwardly by each auxiliary spring member <NUM>, preferably in abutment against each lower stop member <NUM>.

In a preferred example, in the different positions, the main spring member(s) <NUM> and the auxiliary spring member(s) <NUM> permanently urge the lower plate <NUM> downwardly with maintaining lower plate <NUM> in abutment against each lower stop member 12A, in a fixed position along the guide thimbles <NUM>.

Optionally, the top nozzle <NUM> comprises a retaining member <NUM> configured to limit the axial movement of the intermediate plate <NUM> towards the upper plate <NUM> relative to the lower plate <NUM>. Such a retaining member <NUM> is useful e.g. during mounting/dismounting operations, in particular in the present case upon removing nuts <NUM>.

The retaining member <NUM> is for example elongated axially and comprised a fixed end <NUM> fixedly attached to one of the lower plate <NUM> and the intermediate plate <NUM>, the retaining member <NUM> sliding axially through a hole provide in the other one of the lower plate <NUM> and the intermediate plate <NUM>.

Optionally, the other end <NUM> of the retaining member <NUM> is provided with a stop member <NUM> for limiting the spacing between the lower plate <NUM> and the intermediate plate <NUM>.

In one exemplary embodiment, the fixed end <NUM> is fixedly attached to the intermediate plate <NUM>, the retaining member <NUM> sliding axially through a hole provided in the lower plate <NUM>, the other end <NUM> of the retaining member <NUM> being provided with the stop member <NUM>.

The lower part <NUM> of the top nozzle <NUM> is rigidly connected to the lower plate <NUM> and the upper part <NUM> of the top nozzle <NUM> is rigidly connected to the intermediate plate <NUM>, such that the intermediate plate <NUM> moves axially with the upper part <NUM>.

The lower part <NUM> and the upper part <NUM> are preferably tubular and telescopically mounted one onto the other, the lower plate <NUM> being received inside the lower part <NUM> and the upper plate <NUM> and the intermediate plate <NUM> being received inside the upper part <NUM>.

The operation of the hold-down device <NUM> will now be described with reference to <FIG>.

The upper part <NUM> of the top nozzle <NUM> is for example initially in an uppermost position, the intermediate plate <NUM> being thus its upper position adjacent the upper plate <NUM>.

When the upper part <NUM> of the top nozzle <NUM> abuts the upper core plate, the upper part <NUM> tends to move downwardly, thus moving the intermediate plate <NUM> downwardly towards the lower plate <NUM>.

As long as the intermediate plate <NUM> is located between the upper position and the intermediate position associated to an auxiliary spring member <NUM>, the main spring member(s) <NUM> push the intermediate plate <NUM> and the lower plate <NUM> away from each other but the auxiliary spring member <NUM> is inactive and does not act on the intermediate plate <NUM>. The hold-down force generated by the hold-down device <NUM> is generated only by the main spring member(s) <NUM>.

As long as the intermediate plate <NUM> is located between the upper position and the intermediate position associated to an auxiliary spring member <NUM>, the intermediate plate <NUM> does not abut against the preload element <NUM> associated to this auxiliary spring member <NUM> which abuts against the upper plate <NUM> via the preload element <NUM>.

When the intermediate plate <NUM> reaches the intermediate position associated to an auxiliary spring member <NUM> and is located between the intermediate position and the lower position, the main spring member(s) <NUM> and the auxiliary spring member <NUM> are simultaneously active and cooperate to push the intermediate plate <NUM> upwardly.

Below a given intermediate position of the intermediate plate <NUM>, the hold-down force generated by the hold-down device <NUM> is generated jointly by the main spring member(s) <NUM> and by each auxiliary spring member <NUM> associated to this intermediate position or to a lower intermediate position.

Hence, the hold-down force generated by the hold-down device <NUM> becomes higher when the intermediate plate <NUM> moves below each intermediate position, with a step in the hold-down force generated when the intermediate plate <NUM> passes the intermediate position due to the preload of the auxiliary spring member(s) <NUM>.

Due to differential thermal expansion of the reactor vessel and the nuclear fuel assembly, the intermediate plate <NUM> tends to move up relative to the lower plate <NUM> in normal operating conditions and to move down in cold startup condition.

In cold startup condition, the main spring member(s) <NUM> and auxiliary spring member(s) <NUM> are simultaneously acting to provide the maximum hold-down force.

With temperature increases, the intermediate plate <NUM> tends to move up relative to the lower plate <NUM>. The force generated by each auxiliary spring member <NUM> decreases until the intermediate plate <NUM> reaches the associated intermediate position and the auxiliary spring member <NUM> abuts against the upper plate <NUM> via the associated preload element <NUM>.

At each intermediate position of the intermediate plate <NUM>, the hold-down force is decreased by a force step corresponding to the preload of the auxiliary spring member(s) <NUM> associated to this intermediate position.

Using the hold-down device <NUM> with a stepped force relative to the stroke of the upper part of the top nozzle <NUM>, the hold-down force follows the load required to avoid a lift off of the nuclear fuel assembly in the different operating conditions.

The stepped hold-down system can be dimensioned in a way that in the lifetime of the fuel assembly, due to irradiation-induced lengthening of the fuel assembly, the auxiliary spring member(s) <NUM> become active during the operating conditions such as to compensate a loss of hold-down force due to relaxation of spring members <NUM> and <NUM> can be compensated. The overall hold-down force can stay relatively constant over the irradiation time.

While avoiding fuel assembly lift-off in both cold startup conditions and hot operating conditions, the stepped hold-down system reduces the excess hold-down forces in normal (hot) operating conditions. The mechanical stresses exerted over the lifetime of the fuel assembly are significantly reduced, which results in lower fuel assembly bow.

The invention is not limited to the examples and variant discussed above. Other examples and variants may be contemplated within the scope of the invention as defined by the claims.

In the illustrated example, the lower plate <NUM> and the upper plate <NUM> are connected via guide thimbles <NUM>.

Alternatively or optionally, the lower plate <NUM> and the upper plate <NUM> are connected one to the other with connection elements distinct from the guide thimbles <NUM>. In such case, the main spring member(s) <NUM> and the auxiliary spring member(s) <NUM> are optionally fitted onto the connection elements such as to the guided by said connection elements.

Hence, in a general manner, the lower plate <NUM> and the upper plate <NUM> are connected one to the other via connection elements, the main spring member(s) <NUM> and the auxiliary spring member(s) <NUM> being optionally fitted onto the connection elements such as to the guided by said connection elements.

Besides, the hold-down device <NUM> may be configured such that the hold-down force generated by the hold-down device <NUM> has one single step or several successive steps along the axial movement of the intermediate plate <NUM>.

In this view, the hold-down device <NUM> comprises for example one single intermediate position or several successive intermediate positions with the hold-down force generated by the hold-down device <NUM> exhibiting a step at the or each intermediate position, with one or several auxiliary spring member(s) <NUM> being associated to each intermediate position.

When successive intermediate positions are provided, the intermediate positions are distinct from each other.

Claim 1:
Top nozzle for a nuclear fuel assembly, the top nozzle extending along a nozzle axis and comprising :
- a lower plate (<NUM>);
- an upper plate (<NUM>) axially spaced from the lower plate (<NUM>);
- an intermediate plate (<NUM>) axially positioned between the upper plate (<NUM>) and the lower plate (<NUM>) with being axially movable between an upper position and a lower position;
- one or several main spring member(s) (<NUM>), each main spring member (<NUM>) being arranged such as to permanently push the intermediate plate (<NUM>) towards the upper plate (<NUM>);
- one or several auxilary spring member(s) (<NUM>), each auxiliary spring member (<NUM>) being arranged to push to intermediate plate (<NUM>) towards the upper plate (<NUM>) when the intermediate plate (<NUM>) is located between the lower position and an intermediate position associated to this auxiliary spring member (<NUM>); and
- a preload element (<NUM>) associated to each auxiliary spring member (<NUM>) with being arranged such that the auxiliary spring member (<NUM>) abuts against the upper plate (<NUM>) via the preload element (<NUM>) when the intermediate plate (<NUM>) is located between the intermediate position associated to said auxiliary spring member (<NUM>) and the upper position.