Patent Number: 047160136
Section: summary

BACKGROUND OF THE INVENTION Kimbrell discloses a nuclear reactor including a calandria which is mounted above the upper internals. The control-rod drives are passed through the hollow members of the calandria. The coolant is conducted generally axially (vertically) through the upper internals and it flows transversely past the outer surfaces of the hollow members of the calandria. These hollow members are constructed to minimize the stresses produced by the transverse flow of the coolant notwithstanding that the velocity of the coolant may be high (40 ft./sec.). Failure of the guides in the upper internals and of any control rods or control-rod drives of the reactor is thus precluded. The calandria is at a substantially higher level of the reactor than the outlet nozzles. While the Kimbrell nuclear reactor is on the whole satisfactory, its operation requires that after passing through the calandria, the coolant must flow through a relatively narrow channel between the calandria and the outlet nozzles. There is a substantial drop in the pressure of the coolant between the region where it leaves the calandria and the region where it flows out of the nozzles. In addition, there is substantial resistance to the inflow of cooling water through the outlet nozzles during an emergency. On the occurrence of a steam bubble, the steam would be collected in the reactor head and calandria and block the natural circulation of coolant through the core, calandria and outlet nozzles. It is an object of this invention to overcome the above-described disadvantages of the Kimbrell reactor. SUMMARY OF THE INVENTION In accordance with this invention a nuclear reactor is provided in which there is no substantial pressure drop in the coolant between the calandria and the outlet nozzles. Specifically, the outlet nozzles are substantially at the level of the upper calandria. The calandria includes a plurality of vertical hollow members supported between upper and lower horizontal supports. The drive rods pass through, and are protected by, these hollow members. The coolant flows in through holes in the lower support. The portion of the hollow members between the supports has a length such that the outlet area through the outlet nozzles spans only hollow members and is not blocked by parts of the supports. These hollow members also have a length such that excessive non-uniform pressure distribution of the coolant is precluded. Such non-uniform pressure distribution produces undesirable non-uniformity of the coolant pressure in the core. In nuclear reactors according to this invention, substantial losses in the outlet pressure of the coolant and outlet nozzle vortices are avoided. Nuclear reactors in which the inlet nozzles as well as the outlet nozzles are at the level of the calandria have additional advantages. The elevation of the coolant piping and specifically of the crossover leg; i.e, the leg between the steam generator and the primary coolant pumps, with respect to the core prevents core uncover for a high range of conditions of small breaks in the piping. The formation of a steam bubble does not preclude effective natural circulation when the primary coolant pump may be disabled. The steam bubble forms at the top and, in the Kimbrell reactor, would block the flow of coolant through the outlet nozzle. In the case of large breaks in the cold legs of the reactor, which cause reverse flow of coolant in the core, the large volume of coolant in the rod-guide region affords an additional measure of protection. The additional coolant permits prolonged core flow and effectively reduces the peak cladding temperature. During core reflood after blowdown a large static head exists on the coolant in the downcomer, i.e., the annulus through which the incoming accumulator flow passes, and this materially aids the core reflood rate.