Electric heating – Metal heating – For deposition welding
Reexamination Certificate
2000-07-20
2002-07-30
Dunn, Tom (Department: 3641)
Electric heating
Metal heating
For deposition welding
Reexamination Certificate
active
06426476
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a laminated article of manufacture and a method of making wherein at least one layer of the laminate comprises at least one rare earth element. More particularly, this invention relates to electrospark depositing a rare earth coating on a metal substrate wherein the coating may be subsequently bonded to another metal substrate. Still more particularly, this invention relates to electrospark depositing the rare earth element erbium on a zirconium alloy substrate that can be subsequently formed into a fuel assembly component for neutronic control in a light water reactor. “Laminated” herein is defined as composed of layers of metallurgically-bonded material with at least one substrate layer and at least one coating layer. “Rare earth” element is defined in the conventional manner, that is, an element of the lanthanide series.
BACKGROUND OF THE INVENTION
The operation of a nuclear power plant requires that the reactor core maintain criticality throughout the duration of its operating cycle. In order to operate for an extended period of time, the reactor core must initially have excess reactivity (i.e., an excess amount of fissile material). This excess reactivity changes over time such that by the end of its operating cycle, the excess reactivity approaches zero, and the reactor core can no longer remain critical. At this point, the reactor is shut down and the core is refueled.
The amount of excess reactivity in a reactor core is limited to maintain a safe, controlled nuclear chain reaction. The primary method of reactivity control is to fuel the reactor core with a number of fuel “batches,” each batch generally having been operated a cycle more than the succeeding batch. Ideally, the reactivity of each batch would be designed such that the average of the whole core allows the core to be just critical. When a particular fuel batch does not have sufficient reactivity to meaningfully contribute to the excess reactivity of another fuel cycle, the batch is discharged to the spent fuel pool and a fresh fuel batch takes its place.
Because fresh fuel must typically last for ~1200-2000 effective full power days in the reactor (depending on the particular cycle design), fresh fuel must be loaded with far more reactivity than would be required if the fresh fuel only needed to last for 1 cycle. These high levels of excess reactivity require design measures to maintain the reactor core within acceptable safety margins. One of these design measures is the incorporation of a burnable neutron absorption material (sometimes called “burnable poison,” referred to hereinafter as simply “absorber”) within the fuel assemblies that provide “negative” reactivity to the batch in an amount that is able to help control the excess reactivity as the reactor cycle proceeds (U.S. Pat. No. 5,241,571, No. 5,267,290, and No. 5,872,826, referred to hereinafter as patents '571, '290, and '826, respectively).
Absorbers typically comprise one or more of the following high neutron absorption cross-section elements: boron, cadmium, silver, indium, hafnium, and the rare earth elements of gadolinium, erbium, and samarium. Some of these absorbers have been incorporated in “discrete” absorber pins that occupy fuel pin lattice positions in a fuel assembly, as a coating on fuel pellets, as a constituent of the fuel, and as an alloying element of a component of the fuel assembly (e.g., fuel cladding or structural members). All of these methods, however, have shortcomings. For example, discrete absorber pins and absorber-containing fuel displace power-producing fuel, operate at lower linear heat generation rates than standard fuel pins, and require more stringent controls in material handling and fabrication during fuel assembly manufacture. Furthermore, the alloying approach restricts the range of options available to the designer for choosing the optimum amount and spatial distribution of the absorber within the fuel assembly component to meet reactivity needs.
A more attractive and versatile approach is provided by patent '826 which discloses a fuel assembly design comprising absorbers as sheets that are embedded in the structural channel box of a fuel assembly using a variety of encapsulation, rolling, and pressing techniques. Such an approach provides flexibility in the amount and location of the absorber within the fuel assembly and keeps the absorber from directly contacting the reactor coolant. In addition, this approach offers a method to increase fuel burnup. By replacing the absorber-containing structural member (e.g., guide tube, channel, duct) of a fuel assembly during a reactor shutdown with another member containing a lower amount of absorber (without replacing the fuel pins) and reinserting the assembly in the reactor, fuel lifetime can be increased. The absorber sheets disclosed in patent '826 were made of cadmium, samarium, boron, gadolinium, silver, indium and hafnium.
An optimum reactivity profile for each fuel assembly would be one that has a flat reactivity curve throughout its life and then drops off to zero just prior to the assembly being discharged. Practically, this would require that the negative reactivity of the absorber in the assembly burn out at exactly the same rate as the fissile fuel, and that all of the absorber is depleted at the end of the cycle. Any absorber that remains at the end of the life of the fuel assembly contributes to a residual negative reactivity that can shorten assembly (and therefore core) life. In practice, it is very difficult to achieve a flat reactivity curve with no absorber left at the end of assembly life. Each absorber has its own nuclear characteristics, and every reload batch is a compromise between competing alternatives.
In this regard, the designer has two considerations to achieve the compromise in designing a core load. First, any residual negative reactivity from absorber that remains at the end of assembly life results in a loss of economic value of the assembly. There is no way to mitigate the presence of residual negative reactivity except to add more fissile material to the initial fuel load. Clearly, the best designs would minimize residual negative reactivity. Second, the amount of fuel assembly excess reactivity controlled by the absorber during the life of the assembly may be highly variable. This is because there are a variety of methods that can be used to control overall core reactivity, including control rods, water flow, etc. In addition, there is generally sufficient thermal margin in fuel designs to allow reasonably wide assembly power/reactivity swings (~25%) through the life of the assembly. However, there are limits to what can be accommodated from a safety standpoint in a core design. It is clear, however, that it would be more economically desirable to design an assembly that has larger swings in reactivity than a large amount of residual negative reactivity.
FIG. 1
provides a graphical comparison of some reactivity calculations for several sample fuel assembly designs containing a variety of absorbers. The “Gadolinium Pins” curve is the baseline curve representing a fuel assembly designed with gadolinium mixed with fuel in several fuel pins in the assembly. This design represents the current state of the art of absorber application in boiling water reactors (BWRs). Fuel pins with gadolinium have been used for a number of years, and has provided a reasonable balance of reactivity and residual negative reactivity. The other curves shown in
FIG. 1
use an absorber incorporated in the structural member of the fuel assembly. As discussed previously, incorporation of the absorber in a structural member (as an non-alloying element) has advantages over other approaches.
The common basis for each curve in
FIG. 1
is that each absorber analyzed is placed in an assembly with the same initial amount of U-235. The amount of absorber is adjusted to try to obtain a reactivity curve that is as constant as possible with a peak reactivity of <1.2 and a minimum r
Johnson Roger N.
Larson Sandra
Love, Jr. Edward F.
Prichard Andrew W.
Reid Bruce D.
Battelle (Memorial Institute)
Dunn Tom
May Stephen R.
Stoner Kiley
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