atw - International Journal for Nuclear Power | 04.2019

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atw Vol. 64 (2019) | Issue 4 ı April

Failure Analysis of the Jet Pumps Riser

in a Boiling Water Reactor-5

Pablo Ruiz-López, Luis Héctor Hernández-Gómez, Juan Cruz-Castro, Gilberto Soto-Mendoza,

Juan Alfonso Beltrán-Fernánde and Guillermo Manuel Urriolagoitia-Calderón

The arrangement of a riser coupled with two jet pumps is an important element in the Reactor Recirculation Core

system of a Boiling Water Reactor. Its operational objective is to force the flow of water through the core for load variation

and for safety is to keep the core flooded. In order to avoid thermal stresses, they are supported in a flexible way. In

this case, the material expansion does not introduce stresses. The riser is only fixed at the upper zone welded at the

riser brace. An important concern happens when the assembly vibrates under a torsional mode around its longitudinal

axis. Under these conditions, helical cracks can be initiated at the riser brace weld. In this paper, a methodology for the

evaluation of the structural integrity of the cracked riser is presented. It is considered that failure can take place in a

brittle or in a ductile manner. For its evaluation, such cracks are projected along the axial and circumferential axis and

evaluated in both conditions. In the first case, Fracture Mechanics are used. The ductile failure is evaluated applying the

Collapse Limit Load analysis. The allowable crack length was determined when the flow through the core varied between

95% and 107%. These analyses were carried out when one or two recirculation circuits were in operation. In

order to demonstrate the application of these results, three cases were analyzed. The results were evaluated with the

Failure Assessment Diagram R6.

213

OPERATION AND NEW BUILD

1 Introduction

The Reactor Recirculation Core System

in a Boiling Water Reactor (BWR) plays

an important role. It induces a forced

flow of water through the core to

increase the power density and the

safety function is to provide coolability

for the core to maintain the water level

at two thirds of the height of the core.

There are twenty jet pumps in a

BWR-5. They are arranged in ten pairs

in the annular region between the

inner wall and the core shroud.

Each pair is joined with a riser pipe

(Figure 1).

In order to avoid thermal expansion

stresses, this arrangement is

supported in a flexible way. In other

words, the upper part of the riser is

stiff welded to a flexible brace, which

is clamped on the inner surface of the

reactor vessel. The bottom of the riser

is joined with an elbow, which connects

this arrangement to a circular

manifold. Besides, the bottom of each

jet pump has a slip joint. Thus, axial

displacement can take place without

any restriction. It has to be kept in

mind, that the two pumps and the

riser are joined by a joke. The stiffness

is increased while the three elements

are maintained together by such joke.

However, this stiffness is reduced

when wear of the wedge of the joke

has taken place. The worst condition

is when such wedge is completely

loosen.

The jet pumps are subjected to an

internal flow and an external cross

flow of water. Therefore, structural

vibrations are exacerbated when the

jet pumps are not completely tight. If

this situation arises, the weld at the

| | Fig. 1.

BWR-5 3-D view and detailed view of the jet pump section.

riser brace has to support the fatigue

loads which are developed. In the

open literature [1], it has been reported

that a 6.6 inch crack was developed

at the riser, close to the weld of the

riser of a brace of the unit 1 of the

Kousheng Nuclear Power Plant. It was

during the 16 nd outage in March of

2003.

In a 2014 work [2], the first five

modes of vibration were calculated. It

was observed that the fourth mode is

torsional around the axial axis of the

riser. Its resonance frequency is

43.4 Hz, which can induce helicoidal

cracks at the zone of the welds mentioned

above. These calculations were

done with SAP 2000 code [3]. In this

analysis, the mass of the riser and

the two jet pumps was considered.

Besides, the mass of water inside and

outside of this arrangement was also

taken into account. The considerations

for this purpose were based on

the works of Blevins [4]. The flexibility

of the bends, which took place

during its ovalization, was introduced.

The stiffness matrix was modified.

The boundary conditions at the riser

brace, riser bracket and slip joint were

introduced in the numerical model.

For the purpose of this work, a helical

crack was analyzed.

This analysis was also carried out

with ANSYS 14.5 code. The same

mode was obtained at 39.5 Hz,

following the same considerations.

These results are in agreement with

those reported by Stevens and

coworkers [5].

The analysis of potential cracks in

jet pumps has attracted attention [6].

Information on potential failure

locations in BWR/3-6 jet pumps is

provided in this document. Fatigue

and Intergranular Stress Corrosion

Cracking plays an important role.

Such document also mentions that

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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