Patent Number: 042082471
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a typical thermal nuclear reactor including a sealed reactor vessel 10 housing a nuclear core 12 comprised of a plurality of fuel assemblies 14. A reactor coolant, such as one including water, enters the vessel through inlet nozzles 16, passes downward in an annular region between the vessel and a core support structure, turns and flows upward through a perforated plate 20 and through the core 12, and is discharged through outlet nozzles 22. The heat energy imparted to the coolant while passing through the core is then transferred in apparatus, not shown, ultimately for the purpose of electric power generation. The reactor coolant serves as a moderator to thermalize neutrons for the fission process. A fuel assembly 14 is shown in FIG. 3, and includes a plurality of fuel pins 24, containing nuclear fuel pellets 26, arranged in a bundle. The assembly also includes a plurality of guide thimbles 28 which provide skeletal support for the assembly and which are sized to removably receive control rods 29 of elements 30, positionable above and within the core area by means such as electromagnets 32 which act upon shafts 34 (FIG. 1) removably connected to the elements 30. A control element 30 can include one or more control rods 29. Although a "spider-type" control element is shown, including webs 35, it will be understood that many other control element and control rod configurations, including rods, bars, X-shaped and so forth, are compatible with the teachings of this invention. The control elements 30 are typically comprised of a material black to thermal neutrons, such as alloys of tantalum, silver, indium and cadmium, or boron carbide (B.sub.4 C) elements sealed within a cladding. The control elements 30 are reciprocatingly insertable into the core, between or within fuel assemblies, to control the neutron flux and hence the reactor power output. Typically the control elements 30 include those referred to as "full-length", having neutron absorbing material along their entire length, or "part length" having neutron absorbing material only along their bottom portions. The neutron flux within the core is continuously monitored by detection apparatus such as the neutron detectors 36 (FIG. 1) which are located at an elevation aligned with the elevation of the core 12. The detectors, located external to the vessel, may be fixed or laterally movable by positioning bars 38. The detectors must be operable under substantially all reactor conditions, including shutdowns for maintenance and refueling. Neutron flux detection is also accomplished on an intermittent basis by use of movable flux detectors 40 which are inserted at a predetermined rate into thimble tubes 42 which are removably positionable within the core 12 and within selected fuel assemblies 14. The thimble tubes 42, in accordance with an embodiment of this invention, are extendable above the core 12. The guide thimbles 28 of the fuel assemblies 14, in addition to receiving control rods 29, are sized to receive neutron sources 44. A typical source is shown in FIG. 4. In the prior art, the sources are, for example, shaped similar to the control rod spider element 30, but have a shorter length. The arrangement of a neutron source 44 in accordance with one embodiment of this invention is shown in FIG. 5. The source includes a fast neutron emitting material 46, sealingly encapsulated in a cladding 48 essentially black to thermal neutrons. The preferred source material 46 is a combination of plutonium-238 and beryllium, and the preferred cladding 48 includes cadmium, such as a combination of silver, indium and cadmium. The plutonium-238, which has a half-life of approximately 89 years, emits alpha particles. The alpha particles strike the beryllium, and result in (.alpha., n) reactions, emitting fast neutrons. The fast neutrons can readily pass through the cladding and may be thermalized in the surrounding medium or can continue through the reactor vessel to activate the neutron detectors 36. Cadmium is preferred as the source cladding 48 as it significantly reduces neutron-induced heat generation within the source 44. The heat generation results primarily from fission of plutonium-238 nuclei by thermal and resonance energy neutrons that penetrate the cladding 48 during power operation. Preliminary calculations indicate that the heat generation rate due to fissioning of plutonium-238 by low energy neutrons in, for example, a 100 curie source will be reduced to an acceptable maximum of 20 watts by encapsulation in cladding 48 having a wall thickness as shown in Table I. Pure cadmium is listed for comparison purposes only, as the melting point of cadmium, 321.degree. C. (610.degree. F.), limits its structural integrity within a reactor environment. The relative percentages of the compounds are given by weight, and although adding up to 100%, do not preclude some impurities. TABLE I ______________________________________ Required Cladding Material Thickness, Inches ______________________________________ Cadmium 0.008 65% Silver, 35% Cadmium 0.023 80% Silver, 15% Indium, 5% Cadmium 0.075 ______________________________________ The source 44 also preferably includes a plenum 50 within the sealed cladding 48, in flow communication with the source material 46. The plenum 50, or void space, serves to collect gas within the source, most particularly helium resulting from the emitted alpha particles. The cladding 48 can be sealed in a variety of manners, including the use of welded end plugs 52. The described neutron source will have an extended life relative to prior art sources as a result of the long half-life of plutonium-238 and the protection afforded the plutonium by the cadmium cladding. However, the life of the described source, or prior art sources, can be further extended if the source does not reside in the core region 12 continuously throughout a fuel cycle. Another embodiment is an arrangement which provides an extended life, and is shown in FIG. 5. The neutron source 44, or any source, is here affixed to a control rod 29 or control element 30, which is slidingly movable within the core region. The rod 29 can specifically be received within one of the tubular guide thimbles 28 of a fuel assembly 14. Preferably the source 44 is sealed within a control rod cladding 54, which also seals a neutron absorbing control material 56 as well known in the art. As the control rods or elements are reciprocatingly inserted and withdrawn from the reactor core 12 in order to control reactor power and shutdown reactivity, a neutron source affixed to an element insertable at shutdown conditions and withdrawn at power operation will extend the source operating life. Additionally, as indicated by FIG. 6, a singular rod 29 can be separately fabricated and field assembled to a complete control element 30. The rod 29 can, for example, be field welded to the web 35 or a web receptacle 60. It will be apparent that the extended life benefit provided by the incorporation of a source with a control means is applicable to other source types including those of the prior art, in addition to the preferred source disclosed. Dependent upon the strength of the source and the configuration of the core and the surrounding components, the source 44 is radially positioned so as to activate the detectors 36 at low power. Typically a source can be included in one or more of the outermost rods 58 of a radially outermost control element 30, as shown in FIG. 2. The control elements utilized are preferably within the boundary of the outer three rows of fuel assemblies within the exemplary core. It will also be apparent that the control elements 30 including neutron sources, for example, two in a reactor core, can advantageously be interconnected to the balance of the control elements and reactor control systems to achieve desired operational movements. For example, when the reactor is subcritical, the source-bearing control elements can be electrically disconnected from the normal control banks or groups with which they are associated to avoid potential inadvertent movement of the sources and disruption of neutron flux level monitoring. During an approach to criticality, the source-bearing elements would remain disengaged, and within the core, while shutdown and first control groups are withdrawn from the core to establish criticality. The flux level accordingly increases to well above the minimum source level range during this operation. At this point, the source bearing elements should then be withdrawn to form a normal operating configuration, and electrically connected to the normal control means. Alternatively, a separate source shutdown control element group, including the source-bearing elements, can be defined within the normal control element programming control system. Additional uses of the inventive teachings can easily be visualized. For example, the source 44 can be used for calibration of the movable flux detectors 40. This can be accomplished by orienting a thimble tube 42 proximate a control element 30 including the neutron source. The thimble tube 42 would be inserted so that its extremity rises above the top of the core, and the movable detector inserted to also rise above the core. The neutron source can then be laterally aligned with the movable detector 40, and used as a source of calibration for the detector 40. It will be apparent that utilization of the inventive teachings provides distinct advantages over prior art neutron sources. The invention permits reduction of secondary source use with an associated reduction in operating costs and core complexity. It can eliminate the need for special orificing which has been required when fixed neutron sources are placed in fuel assemblies. A combination neutron source and control element also markedly reduces the neutron and gamma radiation exposure of the source and improves its long term mechanical integrity. Refueling procedures are also simplified by reducing the number of components which must be handled. And, in addition to extending the life of neutron emitting sources, the teachings can also be utilized for calibration of movable in-core flux detectors. It will be further apparent that additional modifications and benefits are possible in view of the above teachings. It therefore is to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.