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Research Papers: Fluid-Structure Interaction

# Root Cause Analysis of SI Nozzle Thermal Sleeve Breakaway Failures Occurring at PWR Plants

[+] Author and Article Information
Jong Chull Jo1

Korea Institute of Nuclear Safety, 19 Kusung-dong, Yusung-gu, Taejon 305-338, Koreajcjo@kins.re.kr

Myung Jo Jhung, Seon Oh Yu, Hho Jung Kim, Young Gill Yune

Korea Institute of Nuclear Safety, 19 Kusung-dong, Yusung-gu, Taejon 305-338, Korea

1

Corresponding author.

J. Pressure Vessel Technol 131(1), 011304 (Nov 24, 2008) (14 pages) doi:10.1115/1.2980017 History: Received May 11, 2006; Revised June 05, 2007; Published November 24, 2008

## Abstract

At conventional pressurized water reactors (PWRs), cold water stored in the refueling water tank of emergency core cooling system is injected into the primary coolant system through a safety injection (SI) line, which is connected to each cold leg pipe between the main coolant pump and the reactor vessel during the SI operation, which begins on the receipt of a loss of coolant accident signal. In normal reactor power operation mode, the wall of SI line nozzle maintains at high temperature because it is the junction part connected to the cold leg pipe through which the hot main coolant flows. To prevent and relieve excessive transient thermal stress in the nozzle wall, which may be caused by the direct contact of cold water in the SI operation mode, a thermal sleeve in the shape of thin wall cylinder is set in the nozzle part of each SI line. Recently, mechanical failures that the sleeves are separated from the SI branch pipe and fall into the connected cold leg main pipe occurred in sequence at some typical PWR plants in Korea. To find out the root cause of thermal sleeve breakaway failures, the flow situation in the junction of primary coolant main pipe-SI branch pipe and the vibration modal characteristics of the thermal sleeve are investigated in detail by using both computational fluid dynamics code and structure analysis finite element code. As a result, the transient response in fluid pressure exerting on the local part of thermal sleeve wall surface to the primary coolant flow through the pipe junction area during the normal reactor operation mode shows oscillatory characteristics with the frequencies ranging from $15Hzto18Hz$. These frequencies coincide with the lower mode natural frequencies of thermal sleeve, which has a pinned support condition on the outer surface with the circumferential prominence set into the circumferential groove on the inner surface of SI nozzle at the midheight of thermal sleeve. In addition, the variation of pressure on the thermal sleeve surface yields alternating forces and torques in the directions of two rectangular axes perpendicular to the longitudinal axis of cylindrical thermal sleeve, which causes both rolling and pitching motions of the thermal sleeve. Consequently, it is seen that this flow situation surrounding the thermal sleeve during the normal reactor operation can induce resonant vibrations accompanying the shaking motion of the thermal sleeve at the pinned support condition, which finally leads to the failures of thermal sleeve breakaway from the SI nozzle.

## Figures

Figure 1

Path of separated thermal sleeve movement

Figure 2

Comparison between unfailed and failed thermal sleeves

Figure 3

Schematic geometry of thermal sleeve and nozzle

Figure 4

(a) Local cross-sectional view of intact thermal sleeve meshes for CFD analysis; (b) local cross-sectional view of loose-fitting thermal sleeve meshes for CFD analysis

Figure 5

Transient responses in velocity at the local monitoring locations

Figure 6

Local coordinate system for the computation of hydraulic loads acting on the thermal sleeve

Figure 7

Velocity vectors

Figure 8

Local velocity vectors

Figure 9

Stream lines (normal flow)

Figure 10

Stream lines (SI flow)

Figure 11

Annulus velocity vectors (normal flow)

Figure 12

Annulus velocity vectors (SI flow)

Figure 13

Contour of pressure on the thermal sleeve (normal flow)

Figure 14

Contour of force acting on the thermal sleeve (normal flow)

Figure 15

Locations of the velocity and pressure monitoring points in the flow field

Figure 16

Pressure variations at the monitoring point PC

Figure 17

Pressure variations at the monitoring point P1

Figure 18

Pressure variations at the monitoring point P6

Figure 19

Pressure variations at the monitoring point P8

Figure 20

Variation in pressure on the outer wall surface of thermal sleeve during 1.0s

Figure 21

Transient responses in force components acting on the thermal sleeve

Figure 22

Time history of the fluctuating resultant force exerted on the thermal sleeve wall

Figure 23

Time history of the direction of fluctuating resultant force exerted on the thermal sleeve wall

Figure 24

Trajectory of the fluctuating X and Y directional force components exerted on the thermal sleeve wall by recirculating turbulent swirl flow during 1s

Figure 25

Transient response in torque applied to the thermal sleeve

Figure 26

Possible Y-directional maximum negative and positive amplitude motions of loosely fitted thermal sleeve with a pinned support line on the outer wall surface at about slightly above the midway between the bottom and top of thermal sleeve

Figure 27

Finite element models of thermal sleeve and nozzle

Figure 28

Mode shapes of shell in the axial direction

Figure 29

Mode shapes of shell in the circumferential direction

Figure 30

Figure 31

Natural frequencies of shell in air for fixed condition at z=L1

Figure 32

Natural frequencies of shell in water for simply supported condition at z=0

Figure 33

Variation of frequencies of shell mode (1, 2), (2, 3), and (1, 1) with water for simply supported condition at z=L1

Figure 34

Typical mode shapes of the shell for simply supported condition at the bottom

Figure 35

Amplitude response of the harmonically excited single-DOF oscillator

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