This invention relates to fuel elements for nuclear reactors, and more particularly to braces, spacers, or support lattices which are usually placed at predetermined distance(s) along the length of the fuel elements in order to provide lateral bracing and spacing, and to maintain the fuel elements in fixed positions.
In a nuclear reactor, the reactor core contains nuclear fuel which is typically in the form of fuel rods grouped together in fuel assemblies. The fuel assemblies are usually mechanically identical and can therefore, be interchangeable. Groups of fuel assemblies are arranged into a matrix to form a core capable of controlled fission reaction.
Each fuel rod is typically a long member approximately 0.4 inches in diameter and 8 to 15 feet long containing fuel usually in the form of a stack of fuel pellets which are surrounded by tubular cladding. The fuel rods which make up an assembly are grouped together to form a plurality of longitudinally extending members which are supported vertically by two parallel end plates, an upper and a lower tie plate. These plates are usually connected to one another by tie rods, or other structural elements.
Each fuel assembly or bundle may also include non-fuel bearing members. Examples include guide tubes to form passageways for control rods which assist in controlling the rate of fission, instrumentation tubes for in-core instrumentation, spacer capture rods, and water rods to modify the neutron moderation in the assembly. The spaces between adjacent fuel rods create flow channels through which coolant and/or moderator can circulate. In light water reactors, the coolant and moderator is water. Lateral bracing and spacing of the fuel rods in the fuel assembly are provided by spacers or spacer grids.
The fuel assembly or bundle, whether in a pressurized water reactor, boiling water reactor, high temperature gas cooled reactor, or any other type of reactor, functions in part to maintain the fuel rods in a fixed position, ideally free of vibration and restrained from bowing or other movement during normal and other operating conditions. In addition, by maintaining the fuel rods in fixed positions, proper cooling and neutron moderation can be achieved. Devices that assist in maintaining the fuel rods in fixed positions in the fuel assembly or bundle and which thereby facilitate proper fuel cooling are spacers.
Spacers or spacer grids which provide lateral bracing are typically designed to allow differential axial expansion of the fuel rods. Springs incorporated in the spacer grids are most frequently used to permit some sliding of the fuel rods with respect to the spacer grids. In some of the designs, the spacer grid is free to move axially a small amount to accommodate minor changes in the axial length of the fuel rods during irradiation.
If spacers wee to be rigidly connected to the fuel rods as well as to structural members of the fuel assembly, then relative axial movement due to rod growth and thermal expansion of adjacent rods can cause local fuel rod skewing and bowing.
By being positioned at regular intervals, spacers maintain rod-to-rod spacing along the length of the fuel assembly. Spacers are typically made of zirconium based alloy sheet material or sometimes from Inconel or stainless steel, and are built up from a relatively large number of different intricately shaped strips that are fitted together by hand and subsequently welded. Sometimes, short sections of tubing are used that are welded to one another along parts of their edges. The spacers have an egg crate shape and each spacer cell includes dimples and/or springs to maintain the desired rod-to-rod spacing. Thus, the springs and dimples keep the fuel rods in their proper lateral positions. But, under the influence of irradiation, the springs are prone to relax and this can lead to undesirable changes in fuel rod pitch (i.e. rod-to-rod spacing) or it may cause gaps or spaces to develop between fuel rods and the springs and dimples, and increases the likelihood that the rods and/or spacer grids will vibrate. Such gaps, changes in fuel rod pitch, and vibration may lead to fuel rod fretting and failure.
Springs necessarily contact the fuel cladding and can cause fretting even if they are not subject to relaxation. Furthermore, as the fuel is irradiated, the fuel rods undergo a shrinkage or diameter reduction known as "creepdown" which can result in gaps between the fuel rod cladding and the springs or dimples which in turn can cause or contribute to fuel rod fretting.
Since even the slightest repeated relative movement between contacted fuel rods and spacers can result in fretting of the fuel cladding, minimization of such relative movement is desired. Thus, loads due to thermal expansion of fuel rods, particularly fuel cladding, and associated tensile loads from such expansion must be considered in spacer design.
Spacers should be thin members and have minimal cross-sectional area. Ideally, they are invisible to moderator and coolant flow. Spacer designs which reduce the flow area, also increase flow resistance and restrict coolant flow causing undesirable pressure drops. Thus, the particular physical configuration of a spacer can create or contribute to local or even non-local undesirable flow redistribution, restriction, or distortion.
Typically, the fabrication of spacers requires extensive labor in shaping the separate parts and in assembling and welding these parts to form a spacer grid. Many of these operations can be automated. However, even with automated spacer fabrication, assembly and welding, extensive labor is required in inserting the fuel rods through the spacers to form a fuel bundle. Even with simplified spacer designs, the assembly of spacer and fuel rods is intricate, time consuming and costly. Whenever spacer designs include springs and/or dimples, even non-intricate assembly techniques become particularly labor intensive. Labor costs can be substantial, particularly in light of the rigorous quality control standards that are applied to nuclear reactor components. When spacer design and fuel rod-spacer assemblies include spring or dimple elements, assembly of the fuel rods into a fuel bundle often result in scratching or damaging the fuel rod cladding surface which may lead to rejection of fuel assemblies during pre-operational quality control tests and inspections and/or fuel rod cladding failures during reactor operation.
It would thus be an advantage if spacers did not include springs and dimples or similar contact devices which relax from irradiation.
It would be an additional advantage to limit the repeated relative movement between spacers and the contacted fuel rod cladding.
It would be a further advantage if the actual physical connection between each spacer and fuel rod was not mechanical but was welded, fused, or bonded thereby avoiding the possibility of fretting of the fuel cladding by its spacer.
It would be a further advantage if the effective coolant flow area surrounding each fuel rod is maximized by minimizing the spacer cross sectional surface area which impedes coolant flow.
It would be yet a further advantage if the flow characteristics of a completed fuel assembly with fuel rod spacers could be improved by providing effectively unhindered coolant (e.g. water and steam) flow along the length of the fuel rod between the fuel cladding and its spacer and unhindered coolant flow within the area in the center of adjacent spacers by keeping the center free from any restrictions.
It would be a further advantage to minimize radial changes in the spacer structure in order to minimize changes in the pitch of the fuel rods which may affect the neutronic and thermal performance of the assembly.
It would be yet another advantage if spacers could be made of materials having low neutron cross section as well as having small mass.
It would be an advantage if each fuel rod was connected to its own spacer which remained integral with the fuel rod.
It would be an advantage if each fuel rod/spacer was mechanically identical.
It would be an advantage if the spacers were not rigidly connected to one another but were left free to move in the axial direction and thereby accommodate rod growth and thermal expansion differences between adjacent fuel rods.
It would also be an advantage if the spacer is not connected to the fuel assembly tie or spacer capture rod or to the guide tubes or other structure members so that axial movement of the rods is unrestricted and axial forces are not exerted on the rods thereby minimizing fuel rod bow.
It would also be an advantage if a portion of the spacer could deform in order to accommodate the reduction in diameter of the fuel rods due to creep without changing the distance between the spacer and fuel rod cladding as well as the distance between adjacent spacers.
It would be yet a further advantage if the fuel rod surface would not be exposed to any spacer parts that may cause fretting and if any fretting is to take place, that it would occur on the spacers and not the fuel rods.
It would be an additional further advantage if a spacer having each of the above advantages could be fabricated, tested and inspected at a lower cost than conventional spacers and at the same time improving overall quality and reliability.
It would be another advantage to have spacers which could be attached to and form a sleeve around fuel rods and avoid the prior art method of inserting the fuel rods through and interlacing with the spacer grids.
It would be yet a further advantage if each fuel rod and its integral spacer was such that their assembly into a fuel bundle merely involved stacking of individual fuel rods one on top of another without scratching any of the fuel rods.