Patent Number: 054935911
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to an internal pump for a nuclear reactor and, more particularly, to a wet motor type reactor internal pump which is disposed in the bottom of a reactor pressure vessel for circulating the reactor coolant. The present invention is especially applied to a rotary portion floating device for the reactor internal pump which has its rotor weight increased by using a flywheel at its rotary portion. In a known reactor internal pump of the prior art, as disclosed in Japanese Published Patent Application No. 211595/1983, there is fixed on the shaft in a motor chamber a thrust disc acting as an auxiliary impeller. A fixed orifice is formed between the outer circumference of the thrust disc and an external ring facing the same whereas a variable orifice is formed between the inner lower face of the thrust disc and an internal ring facing the same, to define a pressure regulating chamber between the two orifices. In the prior art technology described above, the upward thrust is balanced by finely adjusting the pressure in the pressure regulating chamber through the variable orifice. Since the reactor internal pump of the prior art has no flywheel, it is not concerned with the substantial weight increase due to the flywheels. Reactor internal pumps may be required to have a high inertia by the system used in a plant. For this high inertia, a flywheel is added to the motor unit so that the weight of the rotary portion is accordingly increased. This increase of weight leads to an increase in the load upon the lower thrust bearing, thus raising a problem that the bearing lifetime is adversely affected. SUMMARY OF TEE INVENTION An object of the present invention is to provide a reactor internal pump which is equipped with rotary portion floating means for lightening the load upon the lower thrust bearing during operational running so as to prolong the lifetime of the thrust bearing without adding any complicated and costly peripheral devices. In order to achieve the above-specified object, the present invention contemplates a reactor internal pump assembly comprising: a pump unit disposed in a pressure vessel; and a motor unit disposed coaxially with the pump unit in a motor casing joined to the pressure vessel, which motor unit includes a motor, a flywheel fixed on the shaft of the motor at one side, and a thrust disc fixed on the motor shaft at the other side and acting as an auxiliary impeller; wherein the thrust disc has a plurality of coaxial cylinder portions of different external diameters for forming respective fixed orifices between the outer circumference of the respective cylinder portions and a larger ring and a smaller ring disposed therearound to thereby define a pressurized chamber between the fixed orifices at the large and smaller diameter sides. In especially preferred embodiments, the pressurized chamber is formed below the bulging cylinder large diameter portion of the thrust disc. In addition, the thrust disc further has two sets of channels for circulating motor cooling water and for boosting the pressure in the pressurized chamber, and the pressurized chamber pressure boosting channels have their suction ports positioned at the discharge side of the motor cooling water circulating channels. According to preferred embodiments of the present invention, the pressurized chamber is formed by both the thrust disc acting as an auxiliary impeller fixed on the lower portion of the motor rotary portion of the reactor internal pump and the orifice formed in the stationary portion facing the outer circumference of the former. The auxiliary impeller has its outlet leading to the pressurized chamber and its inlet leading to the fluid support portion below the auxiliary impeller. The thrust disc is connected to the rotary portion of the pump. Moreover, the thrust disc has the function of a piston so that it is lifted upward as the pressure in the pressurized chamber rises, to thereby float the rotary portion. When the reactor internal pump is operated, the auxiliary impeller is rotated by the motor. The fluid supplied from the suction chamber below the auxiliary impeller is pressurized by the auxiliary impeller to reach the pressurized chamber. Then, the liquid flows upward and downward of the thrust disc through the clearance between the thrust disc and the orifice of the stationary portion. At this time, the thrust disc is lifted upward by the pressure difference between the inside and outside of the pressurized chamber and the difference between the pressurizing areas of the orifice portions. Then, the thrust disc floats the pump rotary portion because these two are connected to each other. Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.