Induced nuclear reactions: processes – systems – and elements – Fuel component structure – Plural fuel segments or elements
Reexamination Certificate
1999-10-06
2001-08-07
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fuel component structure
Plural fuel segments or elements
C376S347000, C376S438000, C376S442000, C376S443000, C376S462000
Reexamination Certificate
active
06272197
ABSTRACT:
The present invention relates to fuel rod assemblies for nuclear reactors.
Fuel rod assemblies, or “elements”, for nuclear reactors comprise a plurality of parallel fuel rods or pins which are maintained a set distance apart from each other and mutually parallel by grids at the top, bottom and usually at one or more intermediate positions therebetween in the fuel assembly. A further function of the grids is to maintain the fuel pins apart and thereby to prevent fretting of the fuel pins leading to mechanical damage.
Grids employed to the present time are generally constructed by welding together for example many individual components of sheet material to form an array of individual apertures each receiving a fuel pin and having resilient locating means such as “spring fingers” for example formed from the sheet material within the apertures so as to locate the fuel pin as centrally as possible within the aperture. Such grids generally also have vanes integrally formed from the sheet material to induce turbulence in the gas or liquid coolant which flows through the fuel assemblies in a direction generally parallel to the axis of the fuel assembly. The purpose of inducing turbulence in the coolant is to improve the heat extraction from the fuel pins by improved coolant mixing and thus to prevent overheating thereof.
Grids such as those described above are generally fabricated from sheet material wherein the plane of the sheet material is generally parallel to the axis of the fuel assembly and comprise substantial quantities of metal in their construction which is detrimental in that the metal is parasitic in absorbing neutrons from the fuel and reducing power output from the reactor. A typical example of such a mixing grid is described in U.S. Pat. No. 5,183 629.
In a pressurised water reactor (PWR) for example, the individual fuel pins may be about 4 m in length. The fuel pin comprises an outer tubular sheath known as the “cladding” made from a metal alloy such as “Zircaloy”™ for example, within which cladding is the fuel per se. Owing to the high temperature reached by the fuel pin in operation, the outer surface of the cladding is subject to corrosion and oxidation where it is in contact with the coolant. The maximum depth of corrosion or thickness of oxide corrosion product of the cladding occurs at about 80% up the length of the pin from the point of entry of the coolant into the fuel pin assembly. The maximum allowable thickness of the oxide layer is a potential life-limiting factor of the fuel pin and consequently of the fuel assembly. Therefore, it is desirable to improve the cooling of the cladding in at least this region so as to reduce the rate of corrosion of the cladding.
An improvement of the cooling of the cladding to reduce the rate of corrosion also may have the effect of delaying or providing a greater safety margin prior to the onset of departure from nucleate boiling (DNB) and allowing the fuel assembly to be operated at a higher power level than would otherwise be possible. Nucleate boiling is the most efficient form of heat extraction. DNB occurs where a film of steam occurs at the surface of the fuel pin and heat transfer from the pin to the coolant decreases dramatically resulting in failure of the pin within a very short time.
According to a first aspect of the present invention, there is provided a fuel assembly for a nuclear reactor, the fuel assembly including: a plurality of fuel pins extending substantially parallel to the axis of the assembly and to each other; at least two structural grids spaced apart from each other, the grids being in contact with said fuel pins and maintaining said fuel pins substantially mutually parallel and preventing contact therebetween, wherein the fuel assembly further comprises at least one mixing grid situated intermediate said at least two structural grids, the fuel assembly being characterised in that said mixing grid is positioned and fixedly located out of substantial contact with said fuel pins, the mixing grid also having turbulence inducing means to promote turbulence in a coolant flowing through said fuel assembly in use and in that the mixing grid is formed from sheet metal wherein the plane of the metal sheet from which the mixing grid is formed lies in a plane which is transverse to the axis of the fuel pin assembly.
The mixing grid of the fuel assembly according to the present invention may alternatively be formed from wire for example and being joined by welding for example at positions where wires cross and also having turbulence inducing means such as vanes for example attached to the wires.
However, in a preferred embodiment of the fuel assembly of the present invention, the mixing grid may be formed from sheet metal by for example pressing or stamping wherein the plane of the metal sheet from which the mixing grid is initially formed lies in a plane which is transverse to the axis of the fuel pin assembly.
The thickness of the sheet material may be in the range from 0.5 mm to about 1 mm for example. However, the thickness is not considered to be critical and may be chosen so as to be resistant to forces imposed by the coolant flow whilst allowing easy mechanical forming thereof.
The mixing grid may be in the form of a framework having an array of apertures of predetermined size and shape, such as square, triangular or hexagonal for example, and through which the fuel pins extend, preferably without making any significant contact with the surrounding framework of each aperture.
Where formed from sheet metal as described above by pressing or stamping, turbulence inducing means such as vanes may be integrally formed during such a forming operation and deformed away from a position lying in the sheet plane to a desired angle so as to provide the optimum turbulence inducing effect. Such turbulence inducing means may be formed on an inner edge or edges of each or any apertures as desired consistent with producing the optimum desired turbulence. In view of the many different designs of fuel pin assembly in existence, the optimum configuration and distribution of turbulence inducing means may be determined by experimentation.
Some or all of the framework members surrounding each aperture may be twisted about the plane of the sheet so as to form turbulence inducing features per se. Such twisting may also reduce the pressure increase necessary to pump coolant through the fuel pin assembly.
The overall outer boundary shape of the mixing grid will correspond to the particular fuel pin assembly into which it is being assembled and may for example be square or hexagonal.
The mixing grid of the fuel pin assembly according to the present invention may not extend to and encompass the outer peripheral ring of full pins. The reason for this is that the fuel pins in the outer peripheral ring tend to run cooler than inner fuel pins and thus, the degree of corrosion is less under normal operating conditions. A further advantage of the mixing grid not extending to the outer ring of fuel pins is that the risk of snagging of the fuel assembly during insertion into and removal from the reactor core is lessened.
The mixing grid of the fuel pin assembly of the present invention may be held in position within the fuel pin assembly by, for example, so-called thimble tubes in which reactor control rods run; the appropriate grid apertures being sized so as to be located by welding or swaging thereto for example. Alternatively, in a preferred embodiment of the present invention, short stub tubes may be fixed to the mixing grid by welding, for example, at positions which correspond to some or all of the thimble tubes such that the short tubes fit over the thimble tubes and are in turn fixed, by crimping or welding for example, to the thimble tubes.
Determination of the optimum position or positions of mixing grids within the fuel pin assembly may be determined by experimentation and will vary according to the fuel pin assembly design, the type of coolant and/or the type of reactor in question. Sufficient mixing grids may be employed s
Abram Timothy James
Gillespie John Woodside
British Nuclear Fuels PLC
Carone Michael J.
Myers Bigel & Sibley & Sajovec
Richardson John
LandOfFree
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