Patent Number: 043893686
Section: summary

DESCRIPTION Background of the Invention This invention relates to high pressure fluid cooled nuclear reactors and means for minimizing the effects of a loss-of-coolant through a cold leg break or loss-of-power to the reactor coolant pumps. In the event of an accident in which there is a break in the cold leg of the reactor coolant system, it has been postulated that the entire coolant medium which absorbs and removes the heat generated in the nuclear core will be lost or at least considerably decreased. If sufficient coolant water does not reach the reactor core within the first minute following the accident, the entire reactor core, fuel and supporting structure begins to melt down and slump to the bottom of the reactor vessel. Emergency cooling water injected at this stage may well amplify the disaster as the now molten metals can react violently with water, generating large quantities of heat, releasing steam and hydrogen in amounts and at pressures that can themselves breach the containment. If the containment vessels themselves do not burst, it has been postulated that the molten mass of fuel would continue to melt downward, fed by the heat generated by fission product radioactivity. If a break occurs in the cold leg between the reactor vessel and the coolant pump causing the loss of coolant accident, then the first path for pressure relief is down through the middle of the reactor core and up through the outer annulus between the core support barrel and the pressure vessel walls and out through the cold leg to the break. This flow is opposite to the normal flow of reactor coolant as well as the flow of emergency core cooling water being injected into that leg, thereby impeding the flooding of the reactor core from the bottom. The second path for pressure relief is from the plenum above the reactor core out through the hot leg to the steam generator, through the steam generator and through the coolant pump to the cold leg break where the steam is discharged to the atmosphere. The steam that flows through this path is being driven by the pressure differential between the high pressure in the reactor plenum above the core and the containment buliding pressure which is initially atmospheric. Consequently, the steam flow rate is very high and the steam tends to drive the reactor pump like a turbine. This circumstance produces the substantial danger of over speeding the reactor coolant pump to the point where the massive flywheel connected to the reactor coolant pump shaft disintegrates and eventually would cause severe damage to the surrounding equipment. Flywheels store the energy required to maintain sufficient cooling flow after a loss-of-power to the driving motors. Absence of the flywheels would allow the motor and its coupled pump to slow, or stop, more rapidly, such that the slowing, or stationary pump impeller would seriously restrict the coolant flow. This desirable flow is traversing in the desired direction due to the previously established momentum of the fluid and the thermal driving head established by the fluid being heated in the reactor and being cooled by the steam generators. Restriction of this flow is undesirable. It is to the solution of these problems that the present invention is directed. One prior art solution to the problem is represented by U.S. Pat. Nos. 4,017,217 issued Apr. 12, 1977 to Robert P. Lamers and 4,036,561 issued July 19, 1977 to Elmar Harand et al. According to the teachings of these patents, brake means are provided to prevent overspeeding of the coolant pump and motor. However, in the event of a loss of coolant accident, it is desirable to maintain the flow established by the fluid momentum and the thermal driving head to reduce the temperature of the reactor. The braking of the coolant pumps according to the teaching of the prior art will at least limit the flow of coolant and will tend to actually impede the flow of coolant under these emergency conditions and will be no benefit during the loss-of-power casualty. Other prior art solutions are represented by U.S. Pat. No. 4,064,001, issued Dec. 20, 1977 to Richard J. Duncan and the article in the April 1970 issue of "Power Magazine" on pages 90-91. According to these solutions, valve means are provided to help maintain normal flow direction through the reactor vessel. While these would reduce the amount of stored energy which would cause pump overspeed, it is expected that these reductions would be insufficient to solve the overspeed problem. These solutions are expected to have no effect on the loss-of-power casualty. SUMMARY OF THE INVENTION According to this invention, an improvement is provided in a pressurized fluid nuclear steam supply system which not only provides overspeed protection to prevent the failure of a flywheel caused by an overspeed condition of the pump, but also enhances the flow of fluid under other emergency conditions. An analysis of a pump fitted with this means might indicate that the pump motors could be fitted with smaller flywheels and might further indicate that the flywheel could be eliminated. The improvement comprises a unidirectional drive means interposed between the coolant pump and the motor driving the pump. The unidirectional drive means is adapted to enable the pump to operate in the pumping direction only at a greater speed than the motor which normally drives it. That is, it allows the impeller to free-wheel in the pumping direction.