Patent Number: 042749220
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to shield materials and structures for nuclear reactors which generate substantial amounts of fast neutron radiation during operation and more particularly to shield materials and structures for use in nuclear reactors of a type employed for example in power generation. In the design of nuclear reactors involving either fission or fusion reactions, provision must be made for the attenuation of escaping nuclear radiation by means of suitable shield structure. The shielding must be capable of moderating and absorbing the various types of radiation generated within the reactor. Normally the most significant types of radiation for which such shielding is required are primary neutrons and gamma rays originating within the core and secondary gamma rays produced by neutron interaction with materials external to the core such as reflector components, coolant or even materials within the shield itself. The reactor shielding must of course provide irradiation protection for personnel in the vicinity of the reactor. However, shielding is also necessary for various other functions. For example, radiation from the reactor may interfere with the satisfactory functioning of instruments employed in various operations and control aspects of the reactor and associated components such as vapor generators and the like. Furthermore, radiation encountering the shield materials may produce internal heating and tend to cause radiation damage in various components of the shield. Accordingly, the selection of materials and the structural design of a reactor shield is primarily dependent upon the purpose or application for the reactor itself. For example, a reactor employed to generate sufficient energy for the operation of a power station may be considered as a stationary system with the shield being a substantial or massive structure. Accordingly, the materials and structural design of the shield must be selected to moderate and absorb radiation from the reactor as well as to provide structural support for itself and related components of the reactor while also being designed to permit adequate heat transfer in order to maintain temperature levels of the shield itself within satisfactory limits. The selection of material and structure of the shield is also largely dependent upon the specific type of radiation generated by the reactor during operation. As noted above, the present invention is contemplated for use in connection with a gas cooled, fast neutronic reactor where fast neutrons are produced in a fission process. However, it will be apparent from the following description that the invention is equally applicable for use in a variety of reactors involving substantial amounts of fast neutron generation, either from fission or fusion processes. In a neutronic reactor of the type referred to above, neutrons generated within the reactor core may experience scattering collisions, mainly elastic, as a result of which their energy is decreased. Thereafter, they may be absorbed by various materials within the reactor core or they may escape. Depending upon the design of the reactor, neutron captures leading to fission reactions within the reactor tend to occur within specific energy ranges. If most of the fission results from the capture of thermal neutrons or neutrons of an intermediate energy range, the system may be referred to as a thermal reactor or intermediate reactor. However, if the fission process results primarily from the capture of fast neutrons, the system is generally referred to as a fast reactor. Details concerning these various types of reactors are generally well known in the art. For purposes of the present invention, it is sufficient as noted above to indicate that the invention is particularly directed toward nuclear reactors involving substantial amounts of fast neutron generation which must be contained by suitable shielding. It will be obvious that such a reactor may also generate other types of irradiation such as thermal neutrons and gamma rays for example. For purposes of the present invention, fast neutrons are defined as having a flux or energy of about 70 keV or greater. In selecting the materials and structural design for suitable shielding in such reactors, it should also be kept in mind that the shielding involves a substantial volume of the overall reactor. For this reason and because of the need to assure adequate containment while avoiding excessive temperatures or radiation damage over prolonged periods of operation, efficiency and economics of a reactor may be due in large part to design and material components of the shield. Accordingly, there has been found to remain a need for nuclear reactor shields including suitable materials and structure which permit efficient containment of reactor cores generating substantial quantities of fast neutrons. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an improved nuclear reactor shield capable of efficient containment for fast neutron irradiation sources or irradiation sources involving substantial amounts of fast neutron generation. More specifically, the invention is directed toward the selection of materials and structural design for shields in large reactors of a type employed for example in electrical power generation. Within such reactors, the shield is necessarily a massive structure with materials and overall design of the shield being suitable to provide structural support for itself and related reactor components. In addition, the shield must include suitable components for moderating and absorbing various types of irradiation from a fission or fusion source in addition to the predominant fast neutrons. Finally, the materials and structure of the shield must be selected to permit adequate heat transfer from the shield to avoid excessive temperatures, the materials and overall shield structure being selected also to resist damage due to the effects of irradiation therein. According to the present invention, it has been found that a particularly efficient and economical reactor shield structure may be formed for use in applications involving substantial fast neutron radiation. For such applications, the present invention contemplates the use of magnesium oxide as a moderator material. The magnesium oxide may be employed alone or in combination with other moderator materials such as graphite and/or iron for example. Magnesium or magnesium oxide has not been considered a particularly effective moderator material in the prior art. However, the total macroscopic cross section for magnesium between approximately 70 keV and 1 meV is significantly greater than that of carbon or graphite as will be discussed in greater detail below. Below that energy range, the total cross section for magnesium is only somewhat less than that of carbon or graphite while above the noted energy range, the cross sections for these materials are about equal. Thus, the present invention contemplates that magnesium preferably in the form of magnesium oxide may be employed by itself as a moderator material in applications involving neutron radiation at energy levels of approximately 70 keV or greater. On the other hand, in applications involving a broad energy range of neutron radiation, an efficient and economical shield may be constructed employing at least a portion of magnesium oxide together with other moderator materials such as graphite and/or iron. Within such a combination, the magnesium provides efficient moderation for fast neutron flux above an energy level of about 70 keV. The other moderator material with which the magnesium is combined may be selected to produce particularly effective moderation within the same energy ranges as magnesium and/or in energy ranges below 70 keV. It is also known that graphite and iron have been employed as moderator materials in reactor shields in the past due to physical properties such as mechanical strength and resistance to deterioration from high temperatures or interaction with radiation. Generally, magnesium materials such as magnesium oxide do not have mechanical strength as great as other moderator materials including graphite. However, it may be employed where great mechanical strength is not essential. Also, the magnesium oxide may be combined with other moderator materials in order to develop substantial mechanical strength throughout the shield. Additional objects and advantages of the invention are made apparent in the following description having reference to the accompanying drawings.