confirm that the design criteria in the applicable regulations and codes are satisfied.

3. Performance objectives

The reactor coolant which is heated to about 321°C in the reactor core enters the SG at a flow rate of about 15xl0³ ton/h. The reactor coolant leaves the SG at about 284° C after transferring its heat to the secondary coolant, and flows back to the reactor. In the secondary side, feed water from the turbine system enters the SG at a temperature of about 220° C with a flow rate of 1.7xl0³ ton/h, and it is converted to saturated steam at about 5.4 MPa (gage) pressure and circulated back to the turbine system. The steam generating capacity of a steam generator is equivalent to about 250 to 295 MWe.

Although these operating parameters slightly differ from one plant to another, sufficient margins are considered for the heat transfer capabilities of the SGs.

3. Operating experience

Since 1970, when the first PWR power plant went into operation in Japan, the following types of heat transfer tube failures have been experienced, (f) Failures due to the accumulation of phosphate

used for water treatment in the secondary system (tube thinning), and failures caused by alkaline accumulation due to volatile alkali chemical additives (inter-granular corrosion of heat transfer tubes at tube support plate locations, known as a “denting” phenomenon).

(D Stress corrosion cracking due to high residual stresses developed during the fabrication and assembling of the heat transfer tubes (for example, failures found in the small radius U-bend sections of heat transfer tubes in some SGs of the US).

(3) Tube failures due to abrasion of the tube wall in contact with anti-vibration bars, caused by flow-induced vibration.

4. Improvements of SGs

In order to keep the expected performance of the SGs over their service life, it is important to improve the reliability of the heat transfer tubes. Therefore, the SG operating experiences from plant operators all over the world have been shared and the following improvements have been made to increase the tube reliability.

(D The secondary water treatment technology was improved.

If some impurities enter the secondary system, their effect should be kept to a minimum. The configuration of mechanical parts in the areas where the potentials for impurity precipitation and accumulation exist were improved to change the flow patterns in such areas to prevent the water from stagnating.

2. The materials of heat transfer tubes and tube support plates were changed to high corrosion­resistant materials.

3. The fabrication methods were improved to minimize residual stresses in tubes.

4. The structural design of anti-vibration bars was improved to firmly support heat transfer tubes and materials having better resistance to abrasion were selected

5. The heat transfer areas were enlarged to have adequate heat transfer margins to compensate for potential reductions in the heat conductivities of some tube materials.

These improvements have been implemented to actual plants after their positive effects were confirmed by various demonstration tests and careful analyses performed over a long time. Table 3.4.3 summarizes the history of the SG improvements.

The design specifications for a typical SG are summarized in Table 3.4.4.

5. Tests and inspections of SGs

In manufacturing SGs, the same as for the RV, their expected performance and quality were assured by various tests and inspections made on their raw materials, as well as at different stages of the manufacturing processes. Manholes provided on both the primary and secondary sides, allow the inspection of the internal surfaces of SGs during in-service inspections. The integrity of heat transfer tubes is confirmed by the eddy current flow detection tests over their service life, and failed tubes are repaired appropriately.

3. Pressurizer

The pressurizer, as shown in Figure 3.4.6, is a vertical cylindrical shell with hemispherical upper and bottom heads, equipped with electric heaters and piping nozzles. The electric heaters are located

NSRA, Japan

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Chapter 3 Systems of PWR Nuclear Power Plants Table 3.4.3 History of steam generator improvement * History of steam generator improvement in japan (Scale up) (Improvement Type5IF Type 52F Type 54F of thermal Material & hydraulics) improvement Manufacturing improvement > ''Crevice Concentration ^improvement^ Tube material improvement Channel head improvement Antivibration bars improvement z- Increased heat-transfer area Improved tube support plate hole design □Pursue higher reliability □Improve on the basis of proven type 51 ■ Special features of each type	Type 51		Type 51M		Type 51F	Type 52F		Type 54F
	Initial	Later	Initial	Later
Hydraulics in the 2ry side	Conventional		Improved
Shape of tube support plate	Conventional		Partially Improved		Improved
Design of tube support plate hole	Improved type (Quatrefoil)								Improved type (Improved quatrefoil)
Antivibration bars (AVS)	Inconel 660 + Cr plating 2 bars combined						Ferritic SS 3 bars combined
Tube material	Inconel 600 (no heat treatment)			T.T Inconel 600 (with heat treatment)			T.T Inconel 690 (with heat treatment)
Stress relief on small U bend	Without stress relief			With stress relief
Support plate material	Carbon steel			Ferritic stainless steel
Tube expansion method	Partial	Full depth expansion
Moisture separator	Conventional		Partially Improved	Improved
Channel head	Cast steel hemisphere						Low alloy steel hemisphere with cylinder