GB2135108A

GB2135108A - Nuclear fuel replacement device

Info

Publication number: GB2135108A
Application number: GB08402534A
Authority: GB
Inventors: William Clarence Ritz; Robert Mcdowell Robey; John Frederick Wett
Original assignee: Westinghouse Electric Corp
Current assignee: Westinghouse Electric Corp
Priority date: 1983-02-04
Filing date: 1984-01-31
Publication date: 1984-08-22
Also published as: GB8402534D0; JPS59147299A; FR2540662A1; DE3403110A1; GB2135108B; FR2540662B1

Abstract

A fuel handling arrangement for a liquid metal cooled nuclear reactor having a single rotating plug (2) eccentric to the fuel core (4), and a fuel handling machine (13) radially movable along a slot (9) in the plug (2) with a transfer station (6) disposed outside the fuel core (4) but covered by the eccentric plug (2) and within range of movement of said fuel handling machine (13) to permit transfer of fuel assemblies (8) between the core (4) and the transfer station (6). <IMAGE>

Description

SPECIFICATION Nuclear fuel replacement device This invention relates to nuclear reactors, especially liquid metal cooled fast breeder reactors.

A nuclear reactor produces heat by fissioning of nuclear materials which are fabricated into fuel elements and assembled within a nuclear core situated in a pressure vessel. In commercial nuclear reactors, the heat produced thereby is used to generate electricity. Such nuclear reactor typically comprises one or more primary flow and heat transfer loops, and a corresponding number of secondary flow and heat transfer loops to which conventional steam turbines and electrical generators are coupled. A typical energy conversion process for commercial nuclear reactors, therefore, involves transfer of heat from a nuclear core to the primary coolant flow system, to a secondary coolant flow system and finally into steam from which electricity is generated.

In a liquid cooled nuclear reactor, such as a liquid metal cooled breeder reactor, a reactor coolant, such as liquid sodium, is circulated through the primary coolant flow system. A typical primary coolant flow system comprises a nuclear core within a reactor vessel, a heat exchanger, and a circulating pump. In nuclear reactors having more than one primary flow loop within the primary system, the nuclear core and the reactor pressure vessel are common to each of the primary loops.

The heat generated by the nuclear core is removed by the reactor coolant which flows into the reactor vessel and through the reactor core. The heated reactor coolant then exits from the reactor vessel and flows through the heat exchangers which transfer the heat to secondary flow systems associated therewith. The cooled reactor coolant exits from the heat exchangers and flows to a circulating pump which again circulates the coolant to the pressure vessel, repeating the described flow cycle.

The nuclear reactor pressure vessel is generally sealed at its top by a cover known generally as a closure head. In fast-neutron energy reactors, such as a liquid metal cooled breeder reactor, it is customary for the closure head to include one or more rotatable members known as plugs. By suitable rotation of these plugs, it is possible for the instrumentation, control, and handling equipment located on these plugs to be positioned above all desired locations in the nuclear core. In this manner, it is possible to provide under-the-plug refueiing; that is, refueling of the nuclear core occurs while the closure head is maintained in its location atop the pressure vessel and core.

In liquid metal cooled breeder reactors, it is customary to provide a positive top core holddown to help maintain the fuel elements in their position during reactor operations, or in the unlikely event of a core disruptive accident. This core holddown structure, and the upper internals of which it is an integral part, also functions to guide, protect and maintain alignment for the various control mechanisms and instrumentation.

The core holddown structure or "upper internals structure" (UIS), must be positioned on top of the nuclear core during normal reactor operations. During refueling operations, the UlS must permit accessibility of the nuclear core by the fuel handling equipment.

The prior art attempts to solve this problem by attaching the UIS to a small rotating plug. During normal reactor operations, the UIS is seated on top of, and completely covers, the nuclear core. For refueiing, this small plug with attached UIS is mechanically raised, and moved out of its position atop of the nuclear core by the rotation of the large plug. Refueling then occurs by means of a grapple, or an expandable arm, which is located underneath the large rotating plug. The problem associated with this method is that, in order to free the top of the nuclear core, the UIS must be rotated out from atop the nuclear core. This means that a large free space is necessary around the core for the displacement of the UIS during refueling.This free space has to occur within the reactor pressure vessel, and necessitates the building of a nuclear reactor pressure vessel having a diameter much larger than that of the nuclear core.

Another method utilized in the prior art, as exemplified by U.S. Patent No. 3,773,626, is to attach the UIS to the small rotatable plug eccentric to the axis of the small plug. In this position, the UIS could be rotated away from its position atop the nuclear core by the rotation of the small plug. Although this method reduces somewhat the size requirements of the pressure vessel, it nevertheless requires additional space within the pressure vessel for the movement of the UIS.

Other refueling arrangements are described by: U.S. Patent No. 4,142,935 (two rotatable plugs and a split UIS), and U.S. Patent No. 3,948,724 (one rotatable plug, a slotted UIS and a movable hoist).

The above-described systems and other prior art arrangements are operable and feasible when applied to reactors containing fuel cores of conventional sizes typically 18 feet in diameter. Present research efforts are directed at the development of fuel cores which will have extended useful lives (5, 10, perhaps 30 years) without fuel replacement. These long life cores will be physically much larger than present cores for the same power level, and may be over 30 feet in diameter. Extrapolation of the plug size from prior art systems to accommodate such large cores results in plug diameters of over 60 feet which is considered unfeasible.

Consequently, it is the principal object of the present invention to provide an improved nuclear fuel replacement system.

With this object in view, the present invention resides in an arangementfor handling fuel assemblies of a nuclear reactor comprising a reactor vessel, a cylindrical core barrel containing said fuel assemblies in the reactor vessel, a circular rotary plug on the reactor vessel, means for rotating the rotary plug, an upper internals structure above said fuel assemblies, said plug and upper internals structure having slots formed therethrough, and a hoisting fuel handling machine disposed on said plug so as to be movable along said slot, characterized in that said plug has a diameter somewhat larger than said core and is eccentric with respect to said core but fully covers said core whereby rotation of said plug combined with movement of said fuel handling machine along said slot permits access by said fuel handling machine to all fuel assemblies in said reactor, said plug overlapping said core barrel at one side when a fuel transfer station is provided adjacent the core barrel with said slot extending radially outwardly sufficiently to overlap said transfer station to permit passage of fuel assemblies between said transfer station and said fuel handling machine when the plug is in the proper angular position.

A refueling system for a fast breeder reactor having a single rotating plug eccentric to the fuel core to which plug the UIS is rigidly attached, and a fixed offset arm fuel handling machine radially movable along a radial slot in the plug and in the UIS. A combination of plug rotational motion and fuel handling machine position shifting provides access to all core positions.

Figure lisa partial schematic top view of a nuclear reactor; and Figure 2 is a schematic view of a complete reactor.

Detailed description The detailed description of a reactor of the type with which this invention is concerned is found in U.S. Patent No. 3,948,724 to Rothfuss, incorporated herein by reference, in which the rotary cover 3 corresponds herein to plug 3, and hoisting unit 13 corresponds herein to fuel handling machine 13.

In Figure 1, circle 1 is the outside diameter of the vessel, are 5 is part of the outside diameter of the UIS, and circle 2 is the outside diameter of rotating plug 3. The core of the reactor is the assembly of fuel assemblies 8 and control rods 7. Fuel handling machine 13 is movable along slot 9 in the plug 3 and UIS.

Circles 2 and 4 do not have identical centers, meaning that plug 3 rotates eccentrically with respect to the core barrel and UIS diameter 4 and consequently to the reactor fuel core.

Rotating plug 3 is larger than the core and fully covers the core and a fuel access station 6 outside the core so that rotation of plug 3 and movement of fuel machine 13 along slot 9 allow access by fuel machine 13 to all fuel assemblies 8 and control rods 7. Arc 10 shows a path for movement of a fuel assembly 8 to the fuel transfer station 6 outside the core barrel circumference 4.

The bottom plate of the UIS may perform a holddown fuction for fuel assemblies 8. Slot 9 must be greater in width than fuel assembly 8 to permit passage therethrough, but may be less in width than the distance between the centers of two adjacent fuel assemblies and the angular rotational position of plug 3 during normal reactor operation can be chosen so that slot 9 is disposed substantially between two rows of fuel asemblies 8 resulting in 100% core assembly holddown.

Claims

1. An arrangement for handling fuel assemblies (8) of a nuclear reactor comprising a reactor vessel (1), a cylindrical core barrel (4) containing said fuel assemblies (8) in the reactor vessel (1), a circular rotary plug (2) on the reactor vessel (1), means for rotating the rotary plug (2), an upper internals structure above said fuel assemblies (8), said plug (2) and upper internals structure having slots (9) formed therethrough, and a hoisting fuel handling machine (13) disposed on said plug (2) so as to be movable along said slot (9), characterized in that said plug (2) has a diameter somewhat larger than said core (4) and is eccentric with respect to said core (4) but fully covers said core (4) whereby rotation of said plug (2) combined with movement of said fuel handling machine (13) along said slot (9) permits access by said fuel handling machine (13) to al all fuel assemblies (8) in said reactor, said plug (2) overlapping said core barrel (4) at one side when a fuel transfer station (6) is provided adjacent the core barrel (4) with said slot (9) extending radially outwardly sufficiently to over lap said transfer station (6) to permit passage of fuel assemblies (8) between said transfer station (6) and said fuel handling machine (13) when the plug (2) is in the proper angular position.

2. An arrangement according to claim 1, char acterized in that said upper internals structure is supported by said plug (2) so as to rotate therewith.

3. An arrangement according to claim 1 or 2, characterized in that said slot (9) is disposed radially in the plug (2).

4. An arrangement according to claim 1, 2 or 3, characterized in that said slot (9) is of a width less than the distance between the centers of two adjacent fuel assemblies (8) but greater than the width ofonefuel assembly (8).