Generation of electrical power by means of nuclear energy has long been proven feasible; but even so, opposition against such activity is consistent because of varying safety considerations. One issue that opponents of nuclear energy raise is the possible release to atmosphere of radioactivity, such as might occur in an overheating malfunction situation incidental to a coolant blockage, a power excursion, an electrical power failure, or the like. To counteract this opposition, reactor development has established redundant cooling systems to dissipate any build-up of heat in the reactor core. One reactor design would use a primary reactor coolant, such as high pressure water or molten sodium that flows through the core; and a secondary or isolated coolant of water-steam that cools the primary coolant remote from the core. Since only the secondary coolant is expanded through the electrical power generating apparatus, it is relatively free of radioactive contaminants.
Redundant cooling systems mean that the reactor operation can continue or be stopped safely even in the event of a complete failure of one of the cooling systems. However, one limiting factor to a redundant design is its dependence on electrical power, including standby or emergency power; whereupon even a redundant cooling system could fail or become severely degraded if it were the "active" type and required electrical input power.
Another problem associated with reactor design is excessive thermal expansion incurred upon the reactor components being subjected to wide variations of temperatures. In this regard, the reactor might include an open-top tank perhaps 50-80 feet in diameter and within which the reactor core and primary reactor coolant (sodium) would be confined; and a heavy deck to close and seal the open top of the reactor tank. The deck is structural in nature and suspends from it reactor components such as heat exchangers, primary coolant flow pumps, control and safety instruments, and fuel rod control and loading and unloading mechanism. These components are specifically positioned and cooperate with one another within the reactor tank, so that excessive differential thermal expansion of the deck structure can be amplified significantly to cause misalignment of these components or separation of the seals and/or conduits isolating cooling flow between these components.
The deck commonly has been fabricated of vertically-separated upper and lower horizontal deck plates and interconnecting vertical walls between the deck plates. The lower deck plate overlies the primary reactor coolant liquid confined in the vessel at temperatures as high as 600.degree.-1000.degree. F., and is thereby subjected to a significant heat input. The upper deck plate is exposed to ambient air of a reactor containment building; and consequently has a capacity to dissipate heat. Although radiation shielding and thermal insulating materials are supported proximate the underside of the lower deck plate, nonetheless a large temperature difference would exist between the upper and lower deck plates if adequate cooling were not provided.
Conventional deck design attempts to establish and maintain a generally small temperature differential between the upper and lower deck plates. One system provides for circulating coolant through appropriate coolant passages formed in the deck structure. The coolant has been either a gas such as air or nitrogen, or a liquid such as water. This approach, however, requires an active source, typically electric pump means, to force the coolant through the passages. Consequently, under an electric power failure design comparison, the cooling capacity drops off dramatically to produce excessive temperature differences between the upper and lower plates. Inasmuch as the upper and lower deck plates are normally separated from one another by, for example 10 or 12 feet, any temperature differentials beyond a designed amount can cause significant thermal movement between the deck plates and misalignment of the components supported by the deck.
A passive dual concept design variation provides for convective flow of coolant through the deck structure. This is not totally satisfactory since this design generally has required draft chimneys to assure adequate cooling and moreover, the hollow deck design must be open to the atmosphere. This is contrary to a preferred design concept that confines the deck coolant within a sealed hollow deck structure.