JPH0452431B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an improvement in an in-core pump for circulating primary cooling water in a nuclear reactor without leading it out of the reactor.

[Technical background of the invention and its problems]

Generally, in boiling water nuclear power plants, the primary coolant (pure water) of the reactor is forced to circulate in the reactor in order to increase the thermal efficiency of the reactor. This method involves first guiding a portion of the primary coolant out of the furnace, increasing the pressure with a pump, and then guiding it back into the furnace to drive the jet pump installed inside the furnace. One method is to circulate the primary coolant, and the other is to install an in-core pump inside the reactor and drive it directly with a motor from outside the reactor pressure vessel, thereby circulating the primary coolant without leading it outside the reactor.

In the latter method, the primary cooling water is circulated by an in-furnace pump, but in this in-furnace pump, purge water is passed in order to prevent the cooling water from flowing into the gap between the pump casing and the pump shaft. .

However, since there is a temperature difference between the cooling water and the purge water, this temperature difference may generate thermal stress in the constituent members of the in-furnace pump, reducing the reliability of the pump.

[Purpose of the invention]

In view of these points, it is an object of the present invention to provide an in-furnace pump that improves reliability by suppressing thermal stress caused by the temperature difference between cooling water and purge water.

[Summary of the invention]

In order to achieve the above-mentioned object, the in-furnace pump according to the present invention has a pump impeller for primary cooling water circulation protruding from the upper end of a pump shaft inserted into a pump casing, and this impeller is inserted from an outer cylinder and an inner cylinder. The pump is housed in a diff user and allows purge water to flow between the pump casing and the pump shaft, and the temperature of the purge water is set to a predetermined temperature, for example, 60° C. or higher.

[Embodiments of the invention]

The present invention will be explained below with reference to embodiments shown in the drawings.

FIG. 1 shows an in-reactor pump according to the present invention, in which a pump nozzle 2 extending vertically is formed in a reactor pressure vessel 1, and the upper end of the pump nozzle 2 has a small diameter on the upper side. A tapered portion 3 is formed. The upper end of a motor casing 4 is attached to the inner circumferential surface of the tapered part 3 of this pump nozzle 2. This motor casing 4 extends downward inside the pump nozzle 2, and the large diameter part 5 at the lower end is It faces the outside of the reactor pressure vessel 1. A stator 6 is fixedly disposed within the large diameter portion 5 of the motor casing 4, and a rotor 7 is disposed opposite to the center of the stator 6, forming a motor section of the in-furnace pump. A pump shaft 8 is protruded from the upper end of the rotor 7 and extends through the motor casing 4 to reach the inside of the reactor pressure vessel 1 . At the upper end of this pump shaft 8 is a pump impeller 9.
is provided protrudingly, and an outer cylinder 11 of a diff user 10 faces the outer periphery of this impeller 9, forming a pump section of an in-furnace pump. An inner cylinder 12 of a diff user 10 connected to the outer cylinder 11 is seated on the tapered portion 3 of the pump nozzle 2, and the diff user 10 is fixed by a stretch tube 13. That is, the stretch tube 13 interposed between the pump nozzle 2 and the pump shaft 8 has flanges 1 at its upper and lower ends, respectively.
4, 15 are provided around each flange 14, 1.
5 is engaged with stepped portions 16 and 17 formed on the inner cylinder 12 and the pump casing 5, and is held hermetically.

The pump impeller 9 and the differential user 10
from between the inner cylinder 12 of the pump shaft 8 and the inner cylinder 1
A purge water introduction pipe 18 is connected to the pump casing 5 to prevent reactor water, which is primary cooling water, from entering between the pump casing 5 and the stretch tube 13, so that the purge water is introduced between the pump casing 5 and the stretch tube 13. It's getting old. Note that the differential user 1
A reactor internal structure 19 faces near the outer cylinder 11 of No. 0.

According to the above-described configuration, the pump impeller 9 rotates as the pump shaft 8 rotates, and the reactor water flows through the arrow A.
As shown, the fuel is introduced from above the pump impeller 9, discharged downward, detoured below the reactor internals 19, and circulated to the reactor core. At this time, part of the reactor water tries to enter the pump shaft 8 and inner cylinder 12 from between the pump impeller 9 and the inner cylinder 12, but purge water (arrow B) with a water pressure slightly higher than that of the reactor water A fixed amount of purge water is supplied into the pump casing 5 from the purge water introduction pipe 18 and rises between the pump casing 5 and the stretch tube 13,
Reactor water is pushed out from inside the inner cylinder 12 and merges with this reactor water. Since the purge water, which is lower temperature than the reactor water, flows along the upper end of the pump shaft 8, the pump impeller 9, and the diffuser 10 surfaces, which are in the reactor water temperature atmosphere, these surfaces are instantly cooled by the purge water and the temperature is lowered. Fluctuations occur. Furthermore, it is fully expected that temperature fluctuations on the surfaces of the pump shaft 8, pump impeller 9, and diff user 10 will occur periodically due to fluctuations in reactor water pressure, vibrations of the in-reactor pump, and the like.

By the way, when a low-temperature liquid comes into contact with a material that has been kept at a high temperature, that area tends to contract.
On the other hand, since the deformation of that part is restrained by other surrounding parts, tensile thermal stress is generated at that part. If this phenomenon occurs repeatedly, the location will be subjected to repeated thermal stress, the material at that location will suffer fatigue damage, and cracks may occur. Here, the amplitude Δσ of the thermal stress generated on the surface of the material is the amplitude of temperature fluctuation on the material surface ΔT (℃), the coefficient of linear expansion α (11.2×10 ^-6 m/m℃),
If the longitudinal elastic modulus is E (2.0×10 ⁴ Kg/mm ² ) and Poisson's ratio is ν (0.3), it can be expressed as Δσ=1EαΔT/2(1−ν).

The magnitude of the generated thermal stress increases in proportion to the temperature difference between reactor water and purge water. The temperature difference between the reactor water and purge water is affected by the rate of heat transfer between the reactor water and purge water and the material surface because heat transfer occurs between them and the material surface. It also depends on the repetition frequency at which reactor water and purge water alternately contact the material surface. That is, the temperature fluctuation on the material surface becomes smaller in inverse proportion to the repetition frequency. Considering the above two effects, the temperature fluctuation ΔT on the material surface can be considered to be about 50% of the temperature difference between reactor water and purge water.
Therefore, the amplitude Δσ of thermal stress generated on the material surface is 0.08ΔT.

On the other hand, since the rotational speed of the in-reactor pump is constant, the flow rate of reactor water is also constant. On the other hand, since the flow rate of the purge water is also constant, the temperature of the purge water is set at the three points mentioned above so that the thermal stress is as low as possible and the cooling effect of the pump shaft 8 by the purge water is advantageous. This will improve the reliability of the pump.

It is desirable that the in-furnace pump not be replaced or repaired for as long as possible, for example 40 years. To determine the change in the thermal stress amplitude Δσ that occurs when the purge water temperature is changed and the number of times until cracking occurs due to repetition of the thermal stress amplitude Δσ of the materials used in the pump shaft 8, impeller 9, and diff user 10. This determines the lower limit of the purge water temperature for the intended service life.

The range of the set temperature of the purge water when the reactor water temperature is, for example, 288°C is shown below.

Figure 2 shows the magnitude of thermal stress that occurs due to temperature fluctuations on the material surfaces of the upper end of the pump shaft 8, the pump impeller 9, and the diffuser 10 as the purge water temperature changes. It is something. In other words, the higher the temperature of the purge water, the smaller the magnitude of thermal stress.

On the other hand, FIG. 3 shows the upper end of the pump shaft 8,
The lower limit of the experimental value obtained in an atmospheric environment is shown for the relationship between the repeatedly applied stress σ and the number of repetitions until failure N _f for stainless steel used in the pump impeller 9 and the differential user 10. According to this figure, the repeated stress Δσ is 19
Below Kg/ ^mm2 , the fatigue limit does not depend on the number of repetitions. Furthermore, the cycle of temperature fluctuations on the upper end of the pump shaft 8, the surfaces of the impeller 9, and the differential user 10 is considered to be on the order of about 0.1 Hz. Therefore, it is thought that thermal stress will be repeatedly applied to the above three locations approximately 1.26×10 ⁸ times in 40 years. From this point of view, as shown in FIG. 3, in order to prevent the occurrence of cracks, it is necessary to suppress the thermal stress to 19 kg/mm ² or less. Thermal stress can be reduced by reducing the difference between the purge water temperature and the reactor water temperature, but from the relationship between the thermal stress and the purge water temperature shown in Figure 2, it is possible to reduce the purge water temperature to a predetermined temperature, for example 60°C. If above, the design life of the plant is 40
There has been no cracking in the relevant part of the in-furnace pump for over 20 years, and there is no need for repair or replacement work during the loan period, ensuring high reliability.

〔Effect of the invention〕

As explained above, since the in-furnace pump according to the present invention sets the temperature of the purge water to a predetermined temperature, for example, 60°C or higher, the temperature difference between the primary cooling water and the purge water becomes small, and therefore the purge water Thermal stress on components that are subject to temperature fluctuations due to water is reduced, and high reliability can be maintained.

[Brief explanation of the drawing]

Fig. 1 is a longitudinal cross-sectional view showing an embodiment of the in-furnace pump according to the present invention, Fig. 2 is a graph showing the relationship between purge water temperature and thermal stress, and Fig. 3 is a graph showing the thermal stress up to crack initiation and its It is a graph showing the relationship with the number of repetitions. 1...Reactor pressure vessel, 2...Pump nozzle,
4...Pump casing, 8...Pump shaft, 9...Pump impeller, 10...Diff user, 13...Stretch tube.

Claims (1)

[Claims] 1. A pump impeller for primary cooling water circulation is protruded from the upper end of the pump shaft inserted into the pump casing, and this impeller is housed in a differential user consisting of an outer cylinder and an inner cylinder, and purge water is passed between the pump casing and the pump shaft. In the in-furnace pump, which is designed to flow purge water, the temperature of the purge water is
An in-furnace pump characterized by being set at a temperature of 60℃ or higher.