Patent Number: 062263404
Section: description

BEST MODE FOR CARRYING OUT THE INVENTION Referring now to the drawings, and particularly to FIGS. 1 and 2, there is illustrated a control rod assembly 10 of conventional construction. The control rod assembly includes a generally cruciform control body 12 with each wing 14 of the cruciform shaped body 12 including a plurality of elongated absorber tubes 16 extending substantially parallel to the longitudinal axis of the control rod. The absorber tubes 16 carry neutron absorbing material, which may be round hafnium rods and or round boron carbide powder in cylindrical, stainless steel capsules. FIG. 2 illustrates a conventional BWR control rod absorber tube 16. The control rod assembly 10 also includes at its upper end handle 18, a coupling release handle 20 and a velocity limiter 22 along with a coupling socket 24. It will be appreciated that the control rod is adapted for insertion between fuel channels containing fuel bundles arranged in quadrants, such that the perpendicularly related wings of the control rod lie adjacent the corresponding sides of a fuel channel. The neutron absorber tubes 16 contain neutron absorbing material in the form of round hafnium rods and/or round boron carbide filled capsules (generally indicated at 26). Typically, capsules of the same or different length may contain the boron carbide material in powder form while the upper end of the tube may be closed by a short rod formed of another neutron absorbing material, i.e., hafnium. The individual capsule segments may be of the same or different lengths and preferably have uniform loadings of neutron absorbing material, although non-uniform loadings may also be employed. In accordance with the present invention, referring to FIG. 3, the absorber tube 16' is loaded with a hermaphroditic absorber, i.e., a poison mass that incorporates two types of absorbers, the first being a strong thermal absorber 26' near the surface of the mass, and the second being a strong resonance absorber 26" in the interior of the poison mass such that the thermal absorber serves to screen the resonance absorber. The outer regions of the poison mass are comprised of a strong "1/v" thermal absorber. A strong "1/v" absorber refers to a poison that has a high absorbtivity for thermal neutrons. It has also been referred to as a black thermal absorber. The phrase "strong `1/v` absorber" is an art-recognized term as evidenced by its use in, for example, U.S. Pat. No. 3,255,092, U.S. Pat. No. 3,103,479 and U.K. Patent No. 1,067,523, the disclosures of which are hereby incorporated by reference. In the present invention, the thermal absorber is preferably boron carbide. The inner region of the poison mass is comprised of a resonance absorber, which is preferably hafnium. Alternatively, a mixture of thermal and resonance absorber, such as a mixture of hafnium, dysprosium and europium or a single material that has both large thermal and resonance neutron absorption cross sections may be used as the resonance absorber in the interior of the poison mass. The relative amounts of each component in the mixture of hafnium, dysprosium and europium as the resonance absorber can be determined in at least two different ways. The neutron worth effect of various mixtures of the elements in the proper geometry may be calculated with a benchmarked computer code that solves the neutron transport problem with a Monte Carlo methodology. Such a code is MCNP.TM. from Los Alamos National Laboratory as described in "MCNP.TM.--A General Monte Carlo N-Particle Transport Code," LA-12625-M, November, 1993, Briesmeister, J. F., Ed. Alternatively, experiments may be performed in a critical assembly facility that determine the neutron worth of various mixtures in the desired geometrical arrangement. It will also be appreciated that the adjustment of neutron worth of such combinations of materials would be readily determined by those of ordinary skill in the art. As an example, Japanese Publication 56-79993 provides specific ranges of rare earth oxide mixtures used to obtain a viable control rod design. Similarly, U.S. Pat. No. 2,859,163 gives a prescription for combining cadmium-rare earth oxide glass with a zirconium matrix to obtain a worth equivalent to a reference absorber. An example of a mixture obtained by experimentation is disclosed in U.S. Pat. No. 3,923,502. Each of the disclosures in the above-mentioned publications is hereby incorporated by reference. The creation of the hermaphroditic poison mass permits an increase in the control material worth while maintaining the external dimensions of the structure containing the control material, such as the control rod. The resonance absorber more appropriately exploits the hardened characteristics of the neutron spectrum (FIG. 4) within the absorber mass by selectively absorbing the epi-thermal neutrons. That is, worth increases are obtained by placing neutron absorbers according to their spectral characteristics. It should be appreciated that while the cited example is applicable to a conventional BWR control rod, the invention is applicable to any nuclear system that uses lumped absorbers for parasitic absorption of neutrons, including not only thermal reactors, but also fast or epi-thermal reactors, and the invention is not meant to be limited to the specific embodiments. In accordance with the invention, higher worth control rods can be obtained, which are important to obtain adequate control for thermal reactors that incorporate mixed oxide fuels. Moreover, the higher worth control materials generate an economic benefit in uranium fueled reactors. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.