The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In a nuclear reactor, a fissile fuel atom, such as U-235, absorbs a neutron in its nucleus that results in a nuclear disintegration which produces on the average two fission fragments of lower atomic weight with kinetic energy and several neutrons at high energy. In a typical nuclear reactor, fuel is in the form of fuel rods, each of which contains stacked sintered pellets of a nuclear fuel arranged within an elongated cladding tube. Each fuel rod can be of the same length or a different length. Typically, each fuel rod has a fuel enrichment distribution in the vertical/axial direction of the rod and is often designed for a uniform enrichment across the axial length of the rod.
Groups of fuel rods are coupled together and often enclosed within a casing to form fuel bundles (also referred to as fuel assemblies). The fuel assemblies are placed within the reactor core and are supported between upper and lower core plates within the core. A plurality of fuel assemblies are arranged in a matrix to form the nuclear reactor core that is capable of a self-sustained fission reaction.
The kinetic energy of the fission products is dissipated as heat in the fuel rods. Energy is also deposited in the fuel assemblies and moderated by the neutrons, gamma rays, and other radiation resulting from the fission process. During operation of the reactor, water (that serves both as a coolant and as neutron moderator) enters the bottom of the fuel assembly and flows upwards through the fuel assembly past the fuel rods. Heat is given off by the fuel rods and is taken up by the water which boils and is transformed into steam. The coolant (liquid and steam) rise upward through the upper portion of the fuel assembly and the steam exits the top of the fuel assembly where it is collected for delivery to a turbine for generating electrical energy.
As the water and steam rise, the coolant reduces in liquid content and increases in steam content. At the upper portion of the fuel rods, the coolant is primarily steam content. This results in the fuel at the top of the fuel assembly not being utilized as efficiently in the generation of steam from the liquid as the fuel at the bottom of the fuel assembly. Additionally, the higher steam content at the top of the fuel assembly results in less cooling of the fuel rods by the coolant than at the lower portions where the ratio of liquid to steam is higher. If the heat from the fuel rod becomes excessive as compared to the available coolant at the top of the fuels rods, there is a risk of dryout. Additionally, the higher percentage pressure drop from bottom to top of the fuel assemblies and core increases the instability of the core. When there is a higher percentage of steam, the neutron moderation of the coolant is reduced because steam is an inferior moderator compared to water.
During operation, the percentage of steam voids increases towards the top of the reactor, leading to decreased moderation in the top regions of the core and about the fuel rods and assemblies. As such, the power distribution within the reactor core is generally skewed toward the lower regions of the core. It is a known practice to compensate for this by distributing a burnable absorber in an axially inhomogeneous manner and to enrich the uranium in the middle and/or top axial portions of the core. A burnable absorber is a neutron absorber which is converted by neutron absorption into a material of lesser neutron absorbing capability. A number of the fuel rods are often provided with a burnable absorber with a distribution in the fuel rod skewed toward the axial region of hot operating maximum reactivity. A well-known burnable absorber is gadolinium, normally in the form of gadolinia. The burnable absorbers available for use in design have an undesirable end-of-refueling cycle neutron absorption reactivity residual due to residual isotopic neutron absorption by small neutron cross section absorbers. For example, if gadolinium is used as a burnable absorber, the high cross section isotopes (Gd-155 and Gd-157) deplete rapidly but residual absorption remains due to continued neutron capture in the even isotopes (Gd-154, Gd-156, and Gd-158). As such, the use of a burnable absorber is not the most effective or desirable method of reactor core design and results in fuel cycle inefficiencies.
Additionally, when the reactor is in the cold shutdown condition, the top of an irradiated boiling water reactor core is more reactive than the bottom due to greater plutonium production at the top and less U-235 destruction in the top during operation (greater conversion ratio and smaller burnup occurs in the top of the core). In the cold shutdown condition, the steam voids in the upper part of the core are eliminated, thus making the top of the core more reactive than the bottom.
As noted, axial power shaping within the core and fuel assemblies is traditionally provided by including greater amounts of burnable absorber in the lower portions of the reactor core. However, the optimum burnable absorber shaping for full power optimization to maintain a desired shutdown margin is not adequately maintained during cold shutdown. In order to meet cold shutdown margin objectives, it is typically necessary to design fuel assemblies with excess burnable absorber residual that penalizes the initial enrichment and uranium ore requirements, reduces fuel cycle efficiency and therefore increases the fuel cycle cost of the reactor.
A further problem is that available burnable absorbers such as gadolinia reduce the thermal conductivity of the fuel rods and increases fission gas release. Consequently, the gadolinia-containing rods are frequently the most limiting rods in the fuel assembly, and have to be down-rated in power with a correspondingly adverse effect on local power distributions. The amount of power down-rating that is required depends on the gadolinia concentration, but becomes a serious problem in extended burn up fuel bundle designs and/or high energy cycle designs where increased gadolinia concentrations are required in order to provide adequate cold shutdown margins.
Thus, the current methods of axial shaping of fuel assemblies by having higher enrichments in the middle or upper portions and using burnable absorbers in the lower portion have significant negative effects on obtaining optimal core configurations, on fuel cycle efficiencies and on operating costs of a nuclear reactor.