Nuclear power reactors are well known and are discussed, for example, by M. M. El-Wakil in "Nuclear Power Engineering", McGraw-Hill Book Company, Inc., 1962.
In a known type of nuclear power reactor, for example, as used in the Dresden Nuclear Power Station near Chicago, Ill., the reactor core is of the heterogenous type. In such reactors the nuclear fuel comprises elongated rods formed of sealed cladding tubes of suitable material, such as a zirconium alloy, containing uranium oxide and/or plutonium oxide as the nuclear fuel, for example, as shown in U.S. Pat. No. 3,365,371. A number of such fuel rods are grouped together and contained in an open-ended tubular flow channel to form a separately removable fuel assembly or bundle as shown, for example, in U.S. Pat. No. 3,431,170. A sufficient number of fuel assemblies are arranged in a matrix, approximating a right circular cylinder, to form the nuclear reactor core capable of self-sustained fission reaction. The core is submerged in a fluid, such as light water, which serves both as a coolant and as a neutron moderator.
A typical fuel assembly is formed by an array of spaced fuel rods supported between upper and lower tie plates, the rods being several feet in length, on the order of one-half inch in diameter and spaced from one another by a fraction of an inch. To provide proper coolant flow past the fuel rods it is important to maintain the rods in spaced position and restrain them from bowing and vibrating during reactor operation. A plurality of fuel rod spacers spaced along the length of the fuel assembly are provided for this purpose.
Design considerations of such fuel rod spacers include the following: retention of rod-to-rod spacing; retention of fuel assembly shape; allowance for fuel rod thermal expansion; restriction of fuel rod vibration; ease of fuel bundle assembly; minimization of contact areas between the spacer and fuel rods; maintenance of structural integrity of the spacer under normal and abnormal (such as seismic) loads; minimization of reactor coolant flow distortion and restriction; maximization of thermal limits; minimization of parasitic neutron absorption; minimization of manufacturing costs including adaptation to automated production. Thus the need to provide such fuel rod spacers creates several significant problems.
Any material, in addition to the nuclear fuel, that must be used in the construction of the reactor core unproductively absorbs neutrons and thus reduces reactivity with the result that an additional compensating amount of fuel must be provided. The amount of such parasitic neutron absorption is a function of the amount of the non-fuel material, of its neutron absorption characteristics, that is, its neutron absorption cross section, and of the neutron flux density to which it is exposed.
To remove the heat from the nuclear fuel, pressurized coolant is forced through the fuel assemblies of the reactor core. The fuel rod spacers in the assemblies act as coolant flow restrictors and cause an undesirable though inevitable coolant flow pressure drop. To maintain proper cooling of the fuel rods along their length and to minimize the required coolant pumping power it is desirable that spacer coolant flow resistance be minimized. The flow resistance of a spacer is a strong function of its projected or "shadow" area. Therefore, the flow resistance of a spacer can be minimized by minimizing the projected area of the structure of the spacer. Tests have shown that spacers employing minimized projected area also have the highest thermal limits.
The coolant flow resistance of a spacer is also a strong function of the surface or "wetted" area of the spacer because of the fluid flow friction between the spacer surfaces and the coolant flowing therethrough. Therefore, the flow resistance of a spacer can be minimized by reducing the height of the spacer.
As a practical matter the desire to minimize both parasitic neutron absorption and coolant flow restriction presents a conflict in fuel rod spacer design.
To minimize coolant flow restrictions, spacer members should be thin and of minimal cross section area. However, very thin members must be formed of high strength material to provide suitable spacer strength. Also, high strength material with suitable resilience characteristics must be used for any spring member portions of the spacer. It is found that such suitable materials have relatively high neutron absorption characteristics.
On the other hand, materials of desirably low neutron absorption characteristics are found to be of relatively low strength, difficult to form and lacking the resiliency desired for the spring member portions of the spacer.
An approach toward the resolution of the foregoing design conflict is a "composite" spacer wherein the structural members are formed of a material having a low neutron absorption cross section and the spring members thereof are separately formed of suitably resilient material whereby the amount of high neutron absorption cross section material is minimized.
A variety of such fuel rod spacers have been proposed and used. An example is shown in U.S. Pat. No. 3,654,077. The spacer shown therein (especially the embodiment of FIGS. 5 and 6 thereof) has enjoyed long commercial success. In the spacer thereof the peripheral support member and the divider members are formed of low neutron cross section material such as a zirconium alloy. The divider members are skeletonized, i.e., formed with cutouts, to further reduce neutron loss. To minimize the amount of high neutron cross section spring material in the spacer, a single spring member projects into each of the fuel rod passages, the springs being in the form of four-sided assemblies.
Another example of a spacer design is shown in U.S. Pat. No. 3,886,038.
A further example of a spacer design is a spacer of the ferrule type (a spacer formed of an array of cojoined tubular ferrules) as shown by Matzner et al in U.S. patent application Ser. No. 410,124, filed Aug. 20, 1982, now U.S. Pat. No. 4,508,679 issued April 2, 1985, assigned to the same assignee herein, which application is hereby incorporated by reference herein.
In the past, fuel assemblies were designed for a residence time in the core of in the order of four years. Recent trends toward longer fuel burnup require fuel assembly residence times in the order of six years or more.
This increased residence time in the core gives rise to a further spacer design problem, namely, an increase in the amount of hydrogen picked up by the spacer from the environment of the core to the extent that the hydride concentration in the material of the spacer can cause embrittlement thereof and consequent decrease in its strength. If the hydride concentration becomes too high, there is a possibility of spacer failure.
The amount of hydrogen picked up by the spacer is proportional to the spacer surface area which is exposed to the coolant, i.e., the "wetted" surface area. The rate of hydrogen diffusion into the relatively thin spacer members is sufficient to give a substantially uniform hydride concentration throughout the volume of the spacer material. Therefore, the hydride concentration is proportional to the ratio of the wetted surface area to the volume of the spacer material.
Thus the hydride concentration can be reduced by increasing the cross section area of the spacer members by an increase in their width or thickness.
As discussed hereinbefore, coolant flow resistance through a spacer is a function of both cross section area and height of the spacer members, therefore, to maintain a desirably low coolant flow resistance, any increase in thickness of the spacer members must be compensated for by a decrease in the height of the spacer members.
Thus, for minimizing hydride concentration, a spacer of minimum height is indicated.
In many spacer designs the minimum height is limited by the design of the spacer springs. In spacers where the springs are vertically (axially) oriented, such as shown in the above-mentioned patent application Ser. No. 410,124, minimum height of the spacer is dictated by the spring length needed for the required spring flexibility and force.
An object of the invention is a nuclear fuel element spacer with spacer members having sufficient cross section area to maintain hydride concentrations therein at suitable levels over extended residence time of the spacer in a nuclear reactor core.
Another object is to minimize the height of a spacer to minimize the resistance to coolant flow therethrough.
Another object is a laterally or horizontally oriented spring member for a spacer so that the required spring length does not dictate the minimum height of the spacer.
Another object is a lateral spacer spring which spans two adjacent fuel element passages for lateral support of the elements extending through the passages.
Another object is a lateral spacer spring which is retained by horizontal slots in the spacer members whereby a substantial portion of the spring is within the "shadow" of the spacer members so that the contribution of the spring to coolant flow resistance is minimized.
Another object is a lateral spacer spring wherein the stress distribution is such as to make efficient use of the spring material whereby the amount of spring material in the spacer is minimized.
Another object is a spacer formed of an array of cojoined ferrules wherein the ferrules have outer surfaces of octagonal shape and inner surfaces of circular shape.