BACKGROUND

As shown inFIG. 1, a conventional fuel assembly10of a nuclear reactor, such as a Boiling Water Reactor (BWR), may include an outer channel12surrounding an upper tie plate14and a lower tie plate16. A plurality of full length fuel rods18and/or part length fuel rods19may be arranged in a matrix within the fuel assembly10and pass through a plurality of spacers (also known as spacer grids)15axially spaced one from the other and maintaining the rods18,19in the given matrix thereof. The fuel rods18and19are generally continuous from their base to terminal, which, in the case of the full length fuel rod18, is from the lower tie plate16to the upper tie plate14. Outer channel12encloses the fuel rods18/19within the assembly10and maintains water or other coolant flow within assembly10about fuel rods18/19and in contact with the fuel rods18/19to facilitate heat transfer from the fuel to the coolant. Outer channel12is traditionally uniform in mechanical design and material for each other assembly10provided to a particular core, to aid in assembly design standardization and manufacturing simplicity. Outer channel12may be fabricated conventionally of a material compatible with the operating nuclear reactor environment, such as a Zircaloy-2.

As shown inFIG. 2, a conventional reactor core, such as a BWR core, may include a plurality of cells40in the reactor core. Each cell may include four fuel assemblies10having adjacent fuel channels12. Other fuel assemblies10may be placed in the reactor core outside of cells40and not adjacent to control blades. The fuel assemblies10inFIG. 2are shown in section to illustrate control blades45, which are conventionally cruciform-shaped and movably-positioned between the adjacent surfaces of the fuel channels12in a cell40for purposes of controlling the reaction rate of the reactor core. Conventionally, there is one control blade45per cell40. As a result, each fuel channel12has two sides adjacent to the control blade45and two sides with no adjacent control blade.

The control blade45is formed of materials that are capable of absorbing neutrons without undergoing fission itself, for example, boron, hafnium, silver, indium, cadmium, or other elements having a sufficiently high capture cross section for neutrons. Thus, when the control blade45is moved between the adjacent surfaces of the fuel channels12, the control blade45absorbs neutrons which would otherwise contribute to the fission reaction in the core. On the other hand, when the control blade45is moved out of the way, more neutrons will be allowed to contribute to the fission reaction in the core. Conventionally, only a fraction of all control blades45within a core will be exercised to control the fission reaction within the core during an operating cycle. As such, only a corresponding fraction of fuel assemblies will be directly adjacent to an extended control blade, or “subject to control,” during an operating cycle.

After a period of time, a fuel channel12may become distorted as a result of differential irradiation growth, differential hydrogen absorption, and/or irradiation creep. Differential irradiation growth is caused by fluence gradients and results in fluence-gradient bow. Differential hydrogen absorption is a function of differential corrosion resulting from shadow corrosion on the channel sides adjacent to the control blades45and the percent of hydrogen liberated from the corrosion process that is absorbed into the fuel channel12; this results in shadow corrosion-induced bow. Irradiation creep is caused by a pressure drop across the channel faces, which results in permanent distortion called creep bulge. As a result, the distortion (bow and bulge) of the fuel channel12may interfere with the movement of the control blade45. Channel/control blade interference may cause uncertainty in control blade location, increased loads on reactor structural components, and decreased scram velocities. Conventionally, if channel/control blade interference has become severe, the control blade is declared inoperable and remains fully inserted.

SUMMARY

Example embodiments are directed to fuel assemblies useable in nuclear reactors and methods of optimizing and fabricating the same. Example embodiment fuel assemblies include an outer channel having a physical configuration determined based on a position of the fuel assembly within a core of the nuclear reactor, such as the position of the fuel assembly with respect to a control blade in the nuclear reactor that will be used to control core reactivity. When example embodiment fuel assemblies are to be directly adjacent to an inserted control blade, the outer channel may be thickened, reinforced, and/or fabricated of a material more resistant to deformation than Zircaloy-2, such as Zircaloy-4, NSF, and VB, so as to reduce or prevent distortion of the channel against the control blade and interfering with operation of the same. When example embodiment fuel assemblies are not in a controlled location, the outer channel may be thinned so as to increase water volume and reactivity in the assembly. As such, a reactor core including example embodiment fuel assemblies will include fuel assemblies having unique outer channels, in thickness, material, etc., unlike conventional power reactor cores.

Example methods of configuring fuel assemblies include determining operational characteristics of the fuel assembly, such as the likelihood that the fuel assembly is controlled via control blade insertion in the nuclear reactor in a current or future fuel cycle, and physically selecting or modifying the outer channel of the fuel assembly based thereon. For example, if the fuel assembly is in a controlled location during the fuel cycle, the outer channel may be fabricated of a material more resistant to deformation than Zircaloy-2, such as Zircaloy-4, NSF, or VB, and/or thickened. Or, for example, if the fuel assembly is not in a controlled location, the outer channel may be approximately 20 mils (thousandths of an inch) or more thinner than outer channels of conventional fuel assemblies. Example methods are useable with or may further include configuring outer channel characteristics in order to meet desired neutronic properties of the fuel assembly.

