Patent Number: 045086770
Section: summary

This invention relates to nuclear reactors for large-scale energy generation. In particular, a self-contained nuclear heat supply module is disclosed which is fabricated at a factory and which is assembled at a field location to form a nuclear reactor. The reactor module is shown in its factory construction, in its on-site assembly, and in the operation of the assembled power plant. PRIOR ART Typical nuclear reactor construction includes a heat supply containing a nuclear fuel, a primary coolant system, and a secondary coolant system. The primary coolant receives heat from the nuclear heat supply and delivers the received heat to the secondary coolant through a heat exchanger. The heat of the secondary coolant is delivered to a means for generating electricity, such as a steam turbine. For commercial energy generation nuclear generating facilities are typically large installations including one or more independent nuclear reactors, each of which can have an installed capacity as large as 1200 megawatts. Such reactor systems are typically housed within large containment buildings, which enclose the reactor and auxiliary and backup cooling and control systems as well as such facilities for refueling and servicing the reactor as will be needed over the lifetime of the plant. The containment building must, of course, be designed for entry by humans for servicing and refueling operations. Aside from the large-scale nuclear reactors intended to power commercial energy-generating plants, small-scale nuclear reactors have also been designed to operate in an entirely different environment--namely, aboard ship. A nuclear generating plant for shipboard use must necessarily be more compact than a land-base plant, but the power requirements are much less. Various designs for compact nuclear generating units suitable for shipboard use are disclosed in U.S. Pat. Nos. 3,170,846; 3,245,879; 3,255,088; 3,401,082; 3,941,187; 4,124,064; and 4,288,196. In the reactor units disclosed in these patents, the reactor core is contained within a close-coupled pressure vessel and containment vessel. For compactness a boiler or steam-generating unit is frequently included within the containment vessel. In typical shipboard construction the containment vessel and associated shielding are permanently assembled within the steam-generator compartment of the ship. The reactor core and associated cooling and heat-exchange systems are then lowered into the containment vessel. A nuclear generating unit for shipboard use must, accordingly, be constructed with the reactor core, steam-generating unit, heat exchangers and the like mounted so that they can all be removed through the top of the containment vessel for servicing and refueling. In contrast to the compact shipboard operating units, land-based nuclear power plants were initially constructed on a large scale because it was believed they would achieve an economy of scale. Basic geometrical considerations teach that as the reactor volume increases, the ratio of the reactor surface area to the reactor volume decreases, so that greater energy generation capability was expected per unit of shielding, cooling system, and containment vessel which had to be constructed. Simply stated, as the plant became larger, it was expected that the marginal amount of time, money, and effort devoted to the containment vessel, confinement building, site operating personnel and other necessary services would generally decrease. Unfortunately, these expectations have not generally been borne out in practice. Recent events have demonstrated that an unscheduled shutdown of one such unit can have very substantial economic consequences. Where a large reactor suffers radioactive contamination, plant capacity suffers substantially and cleanup costs conceivably outweigh the benefit otherwise derived from use of a nuclear fuel. Furthermore, government-imposed safety regulations require that nuclear power plants be constructed to the highest possible standard. With current construction methods this high standard must be "transported to" the remote construction sites in which nuclear plants are commonly located and imposed on construction operations in the field. Recruiting construction personnel and training them in the field to the high and exacting standards of modern nuclear licensing procedures has proved expensive, time-consuming, difficult--and sometimes impossible. In short, with current construction methods, quality assurance has been a problem. As it is ultimately disposed within a large nuclear energy-generation facility, the reactor relies upon active safety measures in the event of substantial malfunction. These measures include active heat rejection systems oftentimes coupled to auxiliary or backup systems through extensive piping networks. The integrity of such piping networks and their associated pumping stations during unpredictable seismic events has oftentimes been questioned. Additionally, such plant designs have essentially been unalterable once the plant is placed on-line. Plant capacity is fixed within a prescribed range for economical operation, making nuclear plants suitable for only large-scale base load power generation. A plant shutdown means complete interruption of base load enery supply. Finally, all such plants must be over-designed in an attempt to withstand possible malfunctions and must be repeatedly inspected for flawed components by X-ray techniques and the like to assure continued safety against such malfunctions. Some of the above-cited U.S. patents suggest that compact nuclear generating units may also play a role in land-based energy generation. U.S. Pat. No. 4,289,196, in particular, suggests that a number of auxiliary systems associated with typical land-based reactors can be eliminated if multiple small-scale modular nuclear steam generating units are connected to a single turbine for generating electricity. SUMMARY OF THE INVENTION The present invention efficiently merges into a large-scale land-based energy generating facility the concept of a compact nuclear generating unit. The invention provides a method by which all the critical elements of the compact nuclear generating unit are constructed and assembled at a central factory location, transported to a field location, outfitted with a biological shield, and incorporated into the generating plant. In particular, the invention provides a prefabricated nuclear heat supply module which includes all critical nuclear components of the compact generating unit and which can be completely nuclear-certified at the central factory and incorporated into the generating plant with only minimal non-critical assembly in the field. Briefly, the nuclear heat supply module as assembled at the factory includes the primary vessel which is surrounded by an outer vessel close-coupled to the primary vessel to define an interstitial region between the two for containing an inert gas. The outer vessel has dimensions which are sized to enable the factory-assembled module to be shipped on a railway car. An unloaded reactor core unit including a plurality of control rods is mounted within the primary vessel. For compactness without sacrificing power output, the reactor core unit is of the fast-breeder type. Also mounted within the primary vessel is a heat exchanger having an inlet and outlet for secondary coolant. Inlet means and outlet means are provided for communicating to the outer and primary vessels with the heat exchanger inlet and outlet. The inlet means and outlet means are adapted to be connected to the balance of a conventional secondary cooling system, which is constructed at the field location. A pump is mounted within the primary vessel for pumping a primary coolant through the reactor core unit and heat exchanger. The pump and the reactor core unit communicate directly with an inlet plenum so as to define a primary coolant flow path directly from the pump to the reactor core unit. Means are also provided within the primary vessel for defining a plenum-like primary coolant flow path from the reactor core unit to the heat exchanger and from the heat exchanger to the pump, the flow-path-defining means being contained entirely within the primary vessel and including no piping subject to leakage or rupture. A control rod drive unit is mounted within the outer vessel overlying the reactor core unit and is operatively connected through the primary vessel to the control rods within the reactor core. It is an object of the invention to provide a prefabricated nuclear heat supply module which contains within all critical nuclear components which are subject to compulsory nuclear-certification procedures, so that a fully certified module can be factory-produced and shipped to the generating plant field location. It is a feature of the invention that the nuclear heat supply module, as it leaves the factory, is sized especially to be transported on a conventional railway car. After the module is unloaded from the railway car at the field site, a segmented surrounding cylindrical shell is assembled about the module. Each segment of the shell defines an interstitial region for the pouring and curing of a concrete biological shield. Each segment also includes a plurality of passive, free-convection cooling loops, providing a shutdown heat-removal system. Shell assembly and pouring and curing of a cementatious biological shield is performed on a movable pad, for example, an airlift pad, and a concrete shield seal plug is poured at this time. In the referred plant design the assembled module with concrete shield is moved to a service building wherein the outer vessel head and primary vessel head are removed enabling the reactor core to be charged in a conventional open-head manner. The heads are then bolted and hermetically seal-welded in place, and the concrete biological seal plug is installed. The assembled and charged module is then moved into position for connection to a conventional steam generation circuit. OTHER OBJECTS, FEATURES AND ADVANTAGES An object of this invention is to provide a nuclear heat supply module which can be assembled at a factory location and thereafter shipped by railroad to a plant site in the field. Simply stated, maximum construction of the reactor occurs at the factory with minimum assembly occurring at the plant site. This disclosure hereafter sets forth: the reactor construction in the plant; the reactor module structure which is shipped from the factory; the process of shielded reactor assembly at the plant site; the resultant reactor at the plant which comprises a discrete steam generation unit; and an overall plant layout adapted to accommodate on a permanent site of infinite life the reactor disclosed herein. It is an object of the invention to disclose the construction of a nuclear heat supply module in which as many assembly steps as possible can be carried out in a factory under controlled condition by trained workers. An advantage of the disclosed apparatus is that where assembly occurs at a factory, quality assurance can be maximized. Critical nuclear components can be intimately inspected and verified before licensing. Proceeding in this manner, greater inspection can be carried out in a central location, thereby providing for more reliable certification with reduced administrative costs. Yet another advantage of the disclosed assembly of components is that the efficiencies of factory automation can be enjoyed. In effect, an almost completely hermetically sealed primary loop is shipped from the factory, with the primary-coolant pumps already installed in position. Only a minimum number of seal welds must be made on-site to complete the hermetic seal of the primary vessel after loading of the fuel assembies and charging of the interstitial regions with inert gas. Another object of the invention is to provide for easy assembly of a biological shield about the nuclear heat supply module at the field site. According to this aspect of the invention, a four-segment upstanding cylindrical annular shell is prefabricated at the factory and shipped to the site. The shell has discrete, vertically extending, serpentine free-convection loops mounted preferably eight to a shell segment. The inner, heat-receiving branches of each loop are in intimate contact with the outer vessel, and the heat-dissipating branches of each loop are exterior to the shell. According to one embodiment of the invention, the inner coolant branch of the shutdown heat removal circuits adjacent the outer vessel are supplemented with a water jacket vented to atmospheric pressure, which provides for greater passive heat removal capability in the event of shutdown. Venting the water jacket to the atmosphere assures that in the event of convective loop failure, the biological shield will not be exposed to temperatures above the boiling point of water so as to destroy the concrete. The jacket is conveniently refillable to provide for continual heat-removal capability in the event of total system failure. A further object of this invention is to provide a process of charging and refueling the reactor core unit. According to this aspect of the invention, the nuclear heat supply module with biological shield in place is transported to a service building. A concrete plug capping the biological shield is removed, followed by removal of the outer vessel head and primary vessel head. A conventional open-head charging or refueling operation is then carried out within the on-site service building after which the heads are re-attached and re-sealed to their respective vessel bodies. The biological shield cap is then re-positioned and the charged reactor module is transported to its operating location whereupon the secondary coolant inlet and outlet means are connected to the balance of the secondary cooling system. The disclosed process results in a steam generation unit of high integrity. All critical components of the reactor module are assembled without piping. Only the necesary flow of the intermediate heat exchanger occurs through a piping network, and that is exterior to the nuclear module. The disclosed module design takes advantage of the improved scram performance provided by gravity-actuated primary and secondary control rods to render the unit subcritical. No feedwater cooling or other active cooling system is relied upon in a scram condition. Instead the scrammed reactor is entirely passive providing for the dissipation of heat to the atmosphere through the free-convection coolant loops integrally included within the biological shield. An additional advantage of the disclosed construction is that the reactor module itself with its massive biological shield provides an ambient heat sink significantly contributing to the module's passive shutdown capability. An additional advantage of the invention is that during operation the shutdown heat-removal system accounts for less than three-tenths of a percent of the loss of the total reactor heat. The minimal heat loss through the shutdown system is sufficient to provide a continual flow in the sealed free-convection loops so as to maintain them in an operative condition, yet it does not reduce significantly the useful heat carried by the secondary coolant. The dimensions of the prefabricated nuclear heat supply module specifically set forth herein are directly related to the utility of the invention. The smaller reactor modules of the present invention taken in combination with one another render a significantly safer, more economical nuclear power plant which can be more thoroughly subjected to nuclear-certification procedures. The smaller units are amenable to the free-convection shutdown heat-removal loop disclosed herein for energy dissipation upon a scram condition. Moreover, a smaller unit can be constructed to take advantage of the thermal expansion of core material so as to aid in bringing the reactor to a subcritical disposition in the event of shutdown. Not only can a smaller unit be substantially assembled at the factory and charged at the field site, but additionally when subjected to casualty at the field site, it forms a manageable unit which can simply be moved away from the installation and left to cool down while the remainder of the plant continues to operate. The nuclear heat supply module achieving the above results will preferably be 14 feet in diameter and 70 feet in height. A nuclear heat supply module of these dimensions can be conveniently shipped to the field site on a conventional railway car. The unit assembled in the field with biological shield is 30 feet in diameter and 80 feet in height. The unit can be transported on a conventional air-lift pad and detachably connected to a steam generation system. An advantage of the disclosed reactor is that in the event of casualty, it can easily be moved off-site to an isolated area. The site is thus furnished with effectively infinite lifetime. The fuel elements can then be removed from the isolated reactor, or alternatively the reactor can be left intact for a radioactive decay period or more and then the fuel elements removed.