METHOD AND FUEL DESIGN TO STABILIZE BOILING WATER REACTORS

Abstract

A method of stabilizing density wave oscillations in boiling water reactor cores is disclosed. The invention introduced a thin metallic fuel element made of fissile isotope baring metal encased and bonded with metallic cladding. The thin construction and the metallic material guarantee very short thermal time constant compared with the oscillation period. Although the feedback of the processes involved in density waves are negative, i.e. oppose any initial perturbation, sufficiently strong feedback may result in unstable behavior because of the time delay inherent in the propagation of the density wave and the heat conduction delay in the traditional fuel rods made of ceramic pellets encased in zircaloy tubes. The new fuel element of this invention introduces fast, not delayed, thermal energy to the coolant in response of any neutron flux perturbation, and thus introduces a stabilizing feedback. Boiling water reactor fuel bundles may benefit from this invention by including the new fuel element as part of its array of fuel rods, preferably filling in the space vacated by part-length fuel rods.

Claims

1. An (invention) comprising: method for introducing a damping effect to stabilize neutron-reactivity-coupled density wave flow and power oscillations in a boiling water reactor core comprising the introduction into at least one fuel bundle of at least one thin fuel element, bearing a fissile isotope, by which a flow perturbation which in turn creates a proportional neutron flux perturbation produces fast power response in the coolant which opposes the original perturbation thus stabilizing the reactor.

2. An (invention) . . . depending from claim 1 and further comprising: the fissionable material, in the at least one thin fuel element, is natural uranium or uranium enriched in the isotope U-235 alloyed with zirconium.

3. An (invention) . . . depending from claim 1 and further comprising: the fissionable material, in at least one thin fuel element, contains the fissile isotope plutonium-239.

4. An (invention) comprising: a nuclear fuel element in the shape of a rod with rectangular cross section of small thickness in the order of 1 to 3 mm such that its thermal time constant is in the order of less than 500 ms, further comprised of a zirconium alloy cladding bonded to core metallic alloy bearing fissile isotope and completely encasing it.

5. An (invention) . . . depending from claim 4 and further comprising: the cross section of the fuel element is cruciform.

6. An (invention) . . . depending from claim 4 and further comprising: the cross section of the fuel element is multi-flanged shape such as a star or a union jack.

7. An (invention) comprising: a nuclear fuel element made of a plurality of small rods or wires where each of the said small rods is made of metallic alloy bearing fissile material and encased in metallic cladding.

8. An (invention) . . . depending from claim 7 and further comprising: the nuclear fuel element is made of 7 or 8 or 9 small rods as shown in FIG. 4.

9. An (invention) comprising: a nuclear fuel bundle where fuel rods made of tubes filled with uranium oxide pellets are placed in a regular lattice, further having some of the said fuel rods replaced by metallic fuel elements.

10. An (invention) . . . depending from claim 8 and further comprising: the metallic fuel elements are of a reduced length compared to other fuel rods.

11. An (invention) . . . depending from claim 8 and further comprising: the metallic fuel elements are attached to part length fuel rods.

12. An (invention) comprising: a nuclear fuel bundle where fuel rods are placed in a regular lattice, further having some rod positions shifted by half lattice period thus creating widened spaces between rods; said wide spaces occupied by thin metallic fuel elements as shown in FIG. 6

Description

BRIEF DESCRIPTION OF THE FIGURES

[0017] The foregoing and other features and advantages of the present invention will become more readily appreciated as the same become better understood by reference to the following detailed description of the preferred embodiments of the invention when taken in conjunction with the accompanying drawings, wherein:

[0018] FIG. 1 illustrates the preferred embodiment fuel element (100) showing the cross section which is a star or a union jack shape. The inner material (101) is made of fissile isotope containing metal. The outer part (102) is metallic cladding.

[0019] FIG. 2 is 3-D representation of the fuel element (100) for which FIG. 1 is a cross section.

[0020] FIG. 3 is a variant of the metallic fuel element shown in FIG. 2, where the cross section of the fuel element (200) is x-shaped.

[0021] FIG. 4 is a sketch representation of part of a fuel bundle showing fuel rods (300) arranged in a regular lattice. The fuel element (100) which is an embodiment of this invention is shown in a lattice position. Other embodiments of thin metallic elements are shown where each fuel elements is placed in a lattice position: fuel element (110) is made of a group of 9 small metallic rods, fuel element (120) is made of 8 small metallic rods, and fuel element (130) is made of 7 small metallic rods.

[0022] FIG. 5 shows the cross section of fuel bundle (400) where fuel rods (300) are arranged in a regular lattice, thin metal fuel element (140) in the shape of a thin metal strip is placed to substitute 2 fuel rods.

[0023] FIG. 6 shows the cross section of fuel bundle (500) where fuel rods (300) are arranged in a regular lattice, rods (310) are shifted by half lattice period to create gap (320) where thin metal fuel element (140) in the shape of a thin metal strip is placed.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The patents and publications referred to herein are provided herewith in an Information Disclosure Statement in accordance with 37 CFR 1.97.

[0025] The Invention is a method for stabilizing the reactivity coupled mode of density wave oscillations in BWR. The structure of the embodiment of this method is a fuel element characterized with fast thermal response. The new fuel elements constitute a part of the fuel bundle of traditional design and produce a small percentage of power relative to the total bundle power that is comparable to the percentage of fission energy deposited directly in the active coolant via gamma ray absorption and neutron slowing down. The new fuel element mimics the stabilizing effect of the direct energy deposition as it releases power to the coolant in direct proportion to the neutron flux and the energy release occurs nearly instantaneously. The heat conduction time constant in the new fuel element should be significantly less than the oscillation period to achieve the desired stabilizing effect.

[0026] The simplest embodiment of the new fuel element is made of a thin strip of fissile plutonium-zirconium or uranium-zirconium alloy with a cladding layer of Zircaloy. The total thickness of the strip is in the order of 1-3 mm (approximately), for which the conduction time constant is of the order of a tenth of a second, which is sufficient to satisfy the fast thermal response requirement. A union-jack or cruciform cross section of the fuel element is preferred for its mechanical strength and for providing more surface area compared with a metal strip. Other embodiments include making a fuel element of small rods or wires where each rod is made of fissile-containing metal alloy covered with an outer layer of metal cladding. The fuel element made of small metal rods may contain 7 or 8 or 9 or as may be suitable for designing the needed fissile mass and fuel element surface area and the heat conduction time constant. Several of the new fuel elements of the preferred or other embodiments are placed in a fuel bundle of any existing design to substitute some of the fuel rods. The new fuel elements preferred positioning is in fuel bundle lattice positions vacated by part-length rods. The new fuel element can be optionally attached to the top of the part-length rods. For a fuel bundle design with, for example, 12 part-length rods there is room for 12 new fuel elements. The maximum length of the new fuel element equals the length of the full-length fuel rod minus that of a part-length rod. The actual length of the new fuel element and the number of array positions to be used can be varied as design parameters subject to overall optimization of the fuel bundle. The alloy concentration and enrichment of the fissile material in the new fuel element can be also varied by a fuel designer. The preferred base metal used in alloying to make the fissile core of the new fuel elements is zirconium, but other materials can be used in general as long as they satisfy requirements of mechanical strength and chemical compatibility and low neutron absorption.

[0027] Embodiments and obvious variants of methods of stabilizing and of fuel rod configurations include the following:

[0028] 1. A method for introducing a damping effect to stabilize neutron-reactivity-coupled density wave flow and power oscillations in a boiling water reactor core comprising the introduction into at least one fuel bundle of at least one thin fuel element, bearing a fissile isotope, by which a flow perturbation which in turn creates a proportional neutron flux perturbation produces fast power response in the coolant which opposes the original perturbation thus stabilizing the reactor. The fissionable material is in at least one thin fuel element and is natural uranium or uranium enriched in the isotope U-235 alloyed with zirconium. Alternatively, the fissionable material, in at least one thin fuel element, contains the fissile isotope plutonium-239.

[0029] 2. A nuclear fuel element in the shape of a rod with rectangular cross section of small thickness in the order of 1 to 3 mm such that its thermal time constant is in the order of less than 500 ms, further comprised of a zirconium alloy cladding bonded to core metallic alloy bearing fissile isotope and completely encasing it. An embodiment has a cross section of the fuel element which is cruciform. An alternative and obvious variant is a cross section of the fuel element which is multi-flanged shape such as a star or a union jack.

[0030] 3. A variant fuel element nuclear is comprised of a plurality of small rods or wires where each of the said small rods is made of metallic alloy bearing fissile material and encased in metallic cladding. The nuclear fuel element is may be made of 7 or 8 or 9 small rods as shown in FIG. 4.

[0031] 4. A variant nuclear fuel bundle may comprise or be composed of fuel rods made of tubes filled with uranium oxide pellets which are placed in a regular lattice, further having some of the said fuel rods replaced by metallic fuel elements. Alternatively, the metallic fuel elements will be of a reduced length compared to other fuel rods or the metallic fuel elements can be attached to part length fuel rods.

[0032] 5. Another embodiment will be a nuclear fuel bundle where fuel rods are placed in a regular lattice, further having some rod positions shifted by half lattice period thus creating widened spaces between rods; said wide spaces occupied by thin metallic fuel elements as shown in FIG. 6.