Patent Description:
A nuclear reactor is comprised of a reactor core housed in a nuclear reactor vessel, which includes a vessel shell and a vessel top head closing the top end of the vessel shell. The reactor core contains fissile material. In use, a coolant fluid flows through the reactor core. The coolant fluid has the function of moderating the nuclear reaction and retrieving heat from the reactor core.

The nuclear reactor also comprises control rods that are movable inside and outside the reactor core for adjusting the reactivity of the reactor core. The control rods contain material that absorbs neutrons generated by the fissile material.

Inserting the controls rods into the reactor core diminishes the reactivity of the reactor core and removing the control rods from the reactor core increases the reactivity of the reactor core.

The nuclear reactor comprises control rod drive mechanisms configured for moving the control rods. The control rods are gathered into control rod assemblies. Each control rod assembly includes several control rods rigidly attached together. Each control drive mechanism drives one respective control rod assembly.

Each control rod drive mechanism is located outside the reactor vessel and drives a control rod assembly via a shaft that extends though the vessel top head via a penetration.

Each penetration extends through the vessel top head along a penetration axis and comprises a penetration tube fixed to the vessel top head, along with a thermal sleeve located inside the penetration tube with limited clearance along the penetration axis. This limited clearance allows movement of the thermal sleeve relative to the penetration tube along the penetration axis.

The thermal sleeve suspends inside the penetration tube. The thermal sleeve has an enlarged upper end portion that sits on a bearing surface of the penetration tube, which is for retaining the thermal sleeve axially.

The lower end of the thermal sleeve protrudes inside the penetration tube that is inside the nuclear reactor vessel. The lower end of the thermal sleeve is in register with the upper end of a control rod guide tube. The drive shaft extends inside the thermal sleeve and into the control rod guide tube.

As the thermal sleeve suspends inside the penetration tube with an axial clearance, premature wear may occur in the contact area between the thermal sleeve and the penetration tube.

<CIT> discloses a thermal sleeve having a frustoconical engagement piece equipped with legs protruding downwardly below a lower edge of the engagement piece for contacting a plate.

<CIT>, which is prior art only under article <NUM>(<NUM>) EPC, discloses a thermal sleeve having a frustoconical engagement piece provided with slots or notches extending from a lower edge thereof.

One of the aims of the invention is to better control this premature wear phenomenon.

To this end, the invention proposes a nuclear reactor head according to claim <NUM>.

The spacer provided at the lower end of the thermal sleeve limits the downward movement of the thermal sleeve relative to the penetration tube, thus avoiding wear at the contact area between the thermal sleeve and the penetration tube.

It is possible to retrofit the spacer at the lower end of the thermal sleeve in parallel to maintenance operations that require opening the nuclear reactor vessel by removing the reactor vessel head.

The addition of a spacer at the lower end of a thermal sleeve is not critical and has no impact on safety of the nuclear reactor. It is easy to manufacture the spacer with a low cost.

Optional features are defined in claims <NUM> - <NUM>.

The invention also relates to a nuclear reactor according to claim <NUM>. The invention also relates to a method of maintenance of a nuclear reactor according to claim <NUM>.

The best understanding of the invention and its advantages comes by reading the following description given solely as a non-limiting example, and made with reference to the appended drawings, in which:.

The nuclear reactor <NUM> of <FIG> comprises a nuclear reactor vessel <NUM> containing a reactor core <NUM>.

The nuclear reactor vessel <NUM> has a substantial vertical central axis A. In the following, the terms "vertical", "horizontal", "upper", "lower", "bottom" and "top" are in reference to the vertical central axis of the reactor vessel.

The nuclear reactor vessel <NUM> comprises a substantially cylindrical vessel shell <NUM> extending along the central axis A and a vessel bottom head <NUM> closing the bottom end of the vessel shell <NUM>. The vessel shell <NUM> has an opening at the top end thereof.

The nuclear reactor vessel <NUM> comprises a nuclear reactor head <NUM> that includes a vessel top head <NUM> closing the top end of the vessel shell <NUM>. The vessel top head <NUM> is removable and attached to the vessel shell <NUM> via screws <NUM>.

The reactor core <NUM> includes fissile material. The reactor core <NUM> is made of a plurality of nuclear fuel assemblies <NUM> that are arranged side-by-side inside the nuclear reactor vessel <NUM>. Each nuclear fuel assembly <NUM> typically comprises a bundle of fuel rods supported by an armature in a spaced relationship with each fuel rod containing fissile material.

The nuclear reactor vessel <NUM> has coolant fluid inlets <NUM>, coolant flow outlets <NUM>, and an internal sleeve <NUM> surrounding the reactor core <NUM>. In use, coolant fluid entering the coolant fluid inlets <NUM> flows downwardly, outside the internal sleeve <NUM>, enters the internal sleeve <NUM> at a bottom end, and then flows vertically upward inside the internal sleeve <NUM> through the reactor core <NUM>, before exiting the nuclear reactor vessel via the coolant flow outlets <NUM>.

The nuclear reactor <NUM> comprises control rods <NUM> that are movable inside and outside the reactor core to adjust the reactivity of the reactor core <NUM>. The control rods <NUM> contain neutron-absorbing material.

Inserting the controls rods <NUM> into the reactor core <NUM> diminishes the reactivity of the reactor core <NUM>, and removing the control rods <NUM> from the reactor core <NUM> increases the reactivity of the reactor core <NUM>.

The control rods combine into control rod assemblies <NUM>. Each control rod assembly <NUM> comprises a bundle of control rods <NUM>.

The nuclear reactor <NUM> comprises control rod guiding tubes <NUM>. Each control rod guiding tube <NUM> is located inside the nuclear reactor vessel <NUM> above the reactor core <NUM>, and receives a respective control rod assembly <NUM> that slides vertically inside the control rod guiding tube <NUM>.

Each control rod guiding tube <NUM> provides guidance for the corresponding control rod assembly <NUM> to ensure that each control rod <NUM> remains aligned with the corresponding space provided in the reactor core <NUM>.

The nuclear reactor comprises control rod drive mechanisms <NUM> that are configured for moving the control rods <NUM>, specifically the control rod assemblies <NUM>. Each control drive mechanism <NUM> is associated to one respective control rod assembly <NUM>.

Each control rod drive mechanism <NUM> is located outside the reactor vessel <NUM> and drives a respective control rod assembly <NUM> via a drive shaft <NUM> (<FIG>) that extends though the vessel top head <NUM>. Each drive shaft <NUM> connects a respective control rod drive mechanism <NUM> to the corresponding control rod assembly <NUM> through the vessel top head <NUM>.

The nuclear reactor head <NUM> comprises penetrations <NUM> extending through the vessel top head <NUM>. Each penetration <NUM> has the configuration to receive a respective drive shaft <NUM>. Each drive shaft <NUM> is slidably received inside the corresponding penetration <NUM>.

Each penetration <NUM> extends through the vessel top head <NUM> along a penetration axis B, and is comprised of a penetration tube <NUM> fixedly attached to the vessel top head <NUM> and a thermal sleeve <NUM> that is received inside the penetration tube <NUM>.

Each thermal sleeve <NUM> receives the corresponding penetration tube <NUM> with a limited clearance along the penetration axis B. This limited clearance allows a movement of the thermal sleeve relative to the penetration tube along the penetration axis.

Each thermal sleeve <NUM> suspends inside the corresponding penetration tube <NUM>, via an upper end portion <NUM> of the thermal sleeve <NUM>. The thermal sleeve <NUM> has an enlarged upper end portion <NUM> that seats onto the seating surface <NUM> of the penetration tube <NUM> for retaining the thermal sleeve <NUM> axially.

The penetration tube <NUM> is comprised of an enlarged upper end portion <NUM>. The upper end portion <NUM> of the thermal sleeve <NUM> is received inside the enlarged upper end portion <NUM> of the penetration tube <NUM>. The seating surface <NUM> of the penetration tube <NUM> is located inside the enlarged upper end portion <NUM>.

The lower end of the thermal sleeve <NUM> protrudes from the penetration tube <NUM> inside the nuclear reactor vessel <NUM>. The lower end of the thermal sleeve <NUM> for each penetration <NUM> is in register with the upper end of the corresponding control rod guide tube <NUM>. The drive shaft <NUM> extends inside the thermal sleeve <NUM> as well as into the control rod guide tube <NUM>.

As illustrated on <FIG>, the upper end <NUM> of the control rod guide tube <NUM> is closed by a cover plate <NUM> that is provided with an opening <NUM> for the passage of the drive shaft <NUM>. The opening <NUM> is aligned with the penetration axis B.

The thermal sleeve <NUM> has successively along the penetration axis B the upper end portion <NUM>, an intermediate portion <NUM>, and a lower end portion <NUM>. The intermediate portion <NUM> extends between the upper end portion <NUM> and the lower end portion <NUM>.

The lower end portion <NUM> of the thermal sleeve <NUM> diverges downwardly. The lower end portion <NUM> of the thermal sleeve <NUM> is frustoconical. The lower end portion <NUM> of the thermal sleeve <NUM> is a circular cross-section with a diameter that gradually increases towards the lower end of the thermal sleeve <NUM>.

The intermediate portion <NUM> is cylindrical with a constant cross-section along the penetration axis B. The intermediate portion <NUM> has a circular cross-section.

In one example, the intermediate portion <NUM> is made of a thermal sleeve tube <NUM>, the lower end portion <NUM> being made of a frustum <NUM> attached to the thermal sleeve tube by welding or by means of a threaded connection.

The thermal sleeve <NUM> is provided with a spacer <NUM> attached at the lower end of the thermal sleeve <NUM>. The spacer <NUM> is configured to maintain an axial spacing between the lower end of the thermal sleeve <NUM> and the corresponding control rod guide tube <NUM>, at least after a predetermined wear of the thermal sleeve <NUM> located at the interface of the seating surface <NUM> for the penetration tube <NUM>.

The spacer <NUM> is configured to contact the control rod guide tube <NUM> after a predetermined amount of wear between the upper end section of the thermal sleeve <NUM> and the seating surface <NUM> of the penetration tube <NUM>.

The spacer <NUM> is configured for allowing coolant fluid to flow radially, relative to the penetration axis B between the lower end of the thermal sleeve <NUM> and the corresponding control rod guide tube <NUM>.

The spacer <NUM> comprises openings <NUM> distributed circumferentially around the penetration axis B for allowing coolant fluid to flow radially through the spacer <NUM> relative to the penetration axis B.

The spacer <NUM> comprises spacing legs <NUM> attached to the lower end of the thermal sleeve <NUM>, and extending downwardly from the lower end of the thermal sleeve <NUM> to abut on the control rod guide tube after wearing in.

In one exemplary embodiment, the spacer <NUM> comprises a seating ring <NUM> spaced from the lower end of the thermal sleeve <NUM> along the penetration axis B, the seating ring <NUM> being configured to the corresponding control rod guide tube <NUM> after wearing in.

As illustrated in <FIG> and <FIG>, the spacer comprises a seating ring <NUM> that attaches to the lower end of the thermal sleeve <NUM> via a plurality of spacing legs <NUM> extending downwardly from the lower end of the thermal sleeve <NUM>.

The spacing legs <NUM> are distributed circumferentially. Flow openings <NUM> are delimited axially between the lower end of the thermal sleeve <NUM> and the seating ring <NUM> and circumferentially between the spacing legs <NUM>.

Several flow openings <NUM> are distributed circumferentially. Each flow opening <NUM> is delimited circumferentially between two adjacent legs <NUM> and axially between the lower end of the thermal sleeve <NUM> and the seating ring <NUM>.

The flow openings <NUM> allow coolant fluid to enter or exit the control rod guide tube <NUM> during normal operation. When dropping the control rods <NUM> in an emergency stop of the nuclear reactor <NUM>, the control rods <NUM> fall and the thermal sleeve raises up, allowing coolant fluid flow upwards in to the control rod drive mechanism.

The spacer <NUM> initially separates from the thermal sleeve <NUM> and attaches to the thermal sleeve <NUM>.

The spacer <NUM> attaches to the thermal sleeve <NUM> with fasteners, such as rivets and/or screws, in particular threaded fasteners, such as screws. Alternatively or optionally, the spacer <NUM> attaches to the thermal sleeve <NUM> by welding.

A method of maintaining a nuclear reactor <NUM> is applied to the nuclear reactor <NUM> in which at least one thermal sleeve <NUM>, and in particular each thermal sleeve <NUM>, is not equipped with a spacer <NUM> attached to the lower end of the thermal sleeve <NUM>.

The maintenance method is implemented during a stop of the nuclear reactor <NUM>. The maintenance method comprises removing the vessel top head <NUM> from the vessel shell <NUM> and attaching a spacer <NUM> to a lower end of a thermal sleeve <NUM>.

Claim 1:
A nuclear reactor head (<NUM>) comprising a vessel top head (<NUM>), a penetration (<NUM>) extending through the vessel top head (<NUM>) along a penetration axis (B) for allowing passage of a control shaft (<NUM>) of a control rod drive mechanism (<NUM>) through the vessel top head (<NUM>) and to a corresponding control guide tube (<NUM>) of the nuclear reactor (<NUM>), the penetration (<NUM>) comprising a penetration tube (<NUM>) extending through the vessel top head (<NUM>) and a thermal sleeve (<NUM>) extending inside the penetration tube (<NUM>) and coaxially with the penetration tube (<NUM>) with an axial play between the thermal sleeve (<NUM>) and the penetration tube (<NUM>), wherein the nuclear reactor head further comprises a spacer (<NUM>) attached to a lower end of the thermal sleeve (<NUM>) for maintaining minimal spacing with the upper end of the corresponding control rod guide tube (<NUM>), wherein the thermal sleeve (<NUM>) comprises a lower end portion (<NUM>) diverging downwardly and the spacer is attached to a lower edge of the lower end portion (<NUM>), characterized in that the spacer (<NUM>) has an openwork structure.