atw - International Journal for Nuclear Power | 04.2019

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atw Vol. 64 (2019) | Issue 4 ı April

OPERATION AND NEW BUILD 214

significant cracking can be tolerated

without loss of essential jet pump

safety functions. Therefore, it is

important to evaluate the remaining

structural integrity when a riser is

cracked.

2 Statement

of the problem

The development of helical cracks at

the weld between the riser and the

riser brace compromise the structural

integrity of the jet pumps. Therefore,

it is very important to evaluate the

critical size of a crack that could

be tolerated, before a complicated

reparation has to be introduced. It can

be considered that the failure can be

in the range of brittle and ductile

conditions. So, a methodology which

considers both conditions of failure is

required.

3 Materials and methods

In order to obtain the critical size of

the crack, the loads applied on the jet

pump arrangement were evaluated.

The hydrodynamic loads are included.

It has to keep in mind that aging could

take place as hours of operation are

accumulated. For this purpose, brittle

and ductile failures were evaluated

with fracture mechanics and net

section collapse analysis approaches,

respectively. Then, these results were

compared against those obtained with

Failure Assessment Diagrams.

The hydraulic loads considered,

were the following: by cross flow, the

impulse loading of the pump of the

Reactor Recirculation Core (RRC)

system and the vibration induced by

fluid flow (fatigue). In the last case,

the dynamic loads are generated by

the bend located at the lower end of

the riser, the ram head at the top of

the riser and the mixer of the jet

pump. The thermohydraulic analysis

was carried out with the RELAP/

SCDAPSIM code [7, 8].

Another source of vibration are the

dynamic loads from strong earthquakes.

However, strong earth quakes

are not a source of fatigue because of

these events do not happen everyday

at the same place. Summarizing, it is

important to evaluate the impact of

the dynamic loads which will take

place on the structural integrity of the

jet pumps.

3.1 Cross flow

The simplified method, described in

the Part N1324.1 “Avoiding Lock-In

Synchronization” of Section III of the

ASME Code [9], was followed. Initially,

the Vortex Shedding frequency is

calculated with the following relationship.

(1)

Where: S is the Strouhal number

and it is a function of the Reynolds

number, U is the velocity of the cross

flow and D is the lower diameter of the

assembly of the jet pumps. The calculations

show that the Vortex Shedding

frequency was 10.7 Hz. In accordance

with the criterion of the ASME code

mentioned above, 1.3f s must be lower

than the first natural frequency

(26.3 Hz), in order to avoid “Lock-In

Synchronization” with the first mode.

So, as a conclusion, cross flow vibration

resonance did not take place.

3.2 Impulse loading of the

pump of the external

Reactor Recirculation Core

(RRC) system

In accordance with the open literature

[10, 11, 12], the centrifugal pump

of each circuit of RRC operates at

1,800 RPM. As a result, its frequency

is 30 Hz. The impeller of the centrifugal

pump has five blades. Therefore,

the impulse frequency is 5 (30 Hz) =

150 Hz. If this parameter is compared

with the range of the first 5 natural

frequencies (26.3 Hz – 67 Hz), it can

be concluded that resonance in operation

is not induced.

3.3 Flow-Induced Vibration

(fatigue)

The sources of fatigue on the jet pump

arrangement are the dynamic forces

and moments generated by the internal

flow of water.

Forces at the lower elbow of the riser:

These forces are generated by the inlet

flow of water at the elbow of the riser.

They were calculated by the following

relationships (Figure 2):

(2)

(3)

ρ is the water density. p 1 and p 2 are the

pressures at the inlet and outlet of the

bend, respectively. A 1 and A 2 are the

cross sections at the inlet and outlet of

the bend and θ is the angle of the

bend. For a 90° elbow, the forces

are resulting. F x = 15,500 lb and

F y = 15,500 lb horizontal and vertical

respectively.

Forces over the mixer nozzles of the

jet pumps (Figure 3): This force is

| | Fig. 2.

Forces on the bend.

| | Fig. 3.

Forces on the bend.

| | Fig. 4.

Forces generated by the ram head over the riser.

developed by the flow discharge,

which comes from the Reactor Recirculation

Core System, and is mixed

with the suctioned flow of the condensed

steam. The vertical force is:

(4)

ΔP is the differential pressure and A i is

the cross section of the nozzle. For the

jet pump assembly under study are

186.7 pound/inch 2 and 26.1 inch 2 ,

respectively. So, the resultant force is

4881 pounds upwards.

Forces generated by the ram head

over the riser (Figure 4): The vertical

loads over the riser, which are generated

by both elbows of the ram head,

were calculated with the following

equation:

(5)

F y is the vertical force, ρ is the water

density, p 1 and p 2 are inlet and outlet

Operation and New Build

Failure Analysis of the Jet Pumps Riser in a Boiling Water Reactor-5 ı

Pablo Ruiz-López, Luis Héctor Hernández-Gómez, Juan Cruz-Castro, Gilberto Soto-Mendoza, Juan Alfonso Beltrán-Fernánde and Guillermo Manuel Urriolagoitia-Calderón

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