atw - International Journal for Nuclear Power | 04.2019
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<strong>atw</strong> Vol. 64 (2019) | Issue 4 ı April<br />
Failure Analysis of the Jet Pumps Riser<br />
in a Boiling Water Reactor-5<br />
Pablo Ruiz-López, Luis Héctor Hernández-Gómez, Juan Cruz-Castro, Gilberto Soto-Mendoza,<br />
Juan Alfonso Beltrán-Fernánde and Guillermo Manuel Urriolagoitia-Calderón<br />
The arrangement of a riser coupled with two jet pumps is an important element in the Reactor Recirculation Core<br />
system of a Boiling Water Reactor. Its operational objective is to <strong>for</strong>ce the flow of water through the core <strong>for</strong> load variation<br />
and <strong>for</strong> safety is to keep the core flooded. In order to avoid thermal stresses, they are supported in a flexible way. In<br />
this case, the material expansion does not introduce stresses. The riser is only fixed at the upper zone welded at the<br />
riser brace. An important concern happens when the assembly vibrates under a torsional mode around its longitudinal<br />
axis. Under these conditions, helical cracks can be initiated at the riser brace weld. In this paper, a methodology <strong>for</strong> the<br />
evaluation of the structural integrity of the cracked riser is presented. It is considered that failure can take place in a<br />
brittle or in a ductile manner. For its evaluation, such cracks are projected along the axial and circumferential axis and<br />
evaluated in both conditions. In the first case, Fracture Mechanics are used. The ductile failure is evaluated applying the<br />
Collapse Limit Load analysis. The allowable crack length was determined when the flow through the core varied between<br />
95% and 107%. These analyses were carried out when one or two recirculation circuits were in operation. In<br />
order to demonstrate the application of these results, three cases were analyzed. The results were evaluated with the<br />
Failure Assessment Diagram R6.<br />
213<br />
OPERATION AND NEW BUILD<br />
1 Introduction<br />
The Reactor Recirculation Core System<br />
in a Boiling Water Reactor (BWR) plays<br />
an important role. It induces a <strong>for</strong>ced<br />
flow of water through the core to<br />
increase the power density and the<br />
safety function is to provide coolability<br />
<strong>for</strong> the core to maintain the water level<br />
at two thirds of the height of the core.<br />
There are twenty jet pumps in a<br />
BWR-5. They are arranged in ten pairs<br />
in the annular region between the<br />
inner wall and the core shroud.<br />
Each pair is joined with a riser pipe<br />
(Figure 1).<br />
In order to avoid thermal expansion<br />
stresses, this arrangement is<br />
supported in a flexible way. In other<br />
words, the upper part of the riser is<br />
stiff welded to a flexible brace, which<br />
is clamped on the inner surface of the<br />
reactor vessel. The bottom of the riser<br />
is joined with an elbow, which connects<br />
this arrangement to a circular<br />
manifold. Besides, the bottom of each<br />
jet pump has a slip joint. Thus, axial<br />
displacement can take place without<br />
any restriction. It has to be kept in<br />
mind, that the two pumps and the<br />
riser are joined by a joke. The stiffness<br />
is increased while the three elements<br />
are maintained together by such joke.<br />
However, this stiffness is reduced<br />
when wear of the wedge of the joke<br />
has taken place. The worst condition<br />
is when such wedge is completely<br />
loosen.<br />
The jet pumps are subjected to an<br />
internal flow and an external cross<br />
flow of water. There<strong>for</strong>e, structural<br />
vibrations are exacerbated when the<br />
jet pumps are not completely tight. If<br />
this situation arises, the weld at the<br />
| | Fig. 1.<br />
BWR-5 3-D view and detailed view of the jet pump section.<br />
riser brace has to support the fatigue<br />
loads which are developed. In the<br />
open literature [1], it has been reported<br />
that a 6.6 inch crack was developed<br />
at the riser, close to the weld of the<br />
riser of a brace of the unit 1 of the<br />
Kousheng <strong>Nuclear</strong> <strong>Power</strong> Plant. It was<br />
during the 16 nd outage in March of<br />
2003.<br />
In a 2014 work [2], the first five<br />
modes of vibration were calculated. It<br />
was observed that the fourth mode is<br />
torsional around the axial axis of the<br />
riser. Its resonance frequency is<br />
43.4 Hz, which can induce helicoidal<br />
cracks at the zone of the welds mentioned<br />
above. These calculations were<br />
done with SAP 2000 code [3]. In this<br />
analysis, the mass of the riser and<br />
the two jet pumps was considered.<br />
Besides, the mass of water inside and<br />
outside of this arrangement was also<br />
taken into account. The considerations<br />
<strong>for</strong> this purpose were based on<br />
the works of Blevins [4]. The flexibility<br />
of the bends, which took place<br />
during its ovalization, was introduced.<br />
The stiffness matrix was modified.<br />
The boundary conditions at the riser<br />
brace, riser bracket and slip joint were<br />
introduced in the numerical model.<br />
For the purpose of this work, a helical<br />
crack was analyzed.<br />
This analysis was also carried out<br />
with ANSYS 14.5 code. The same<br />
mode was obtained at 39.5 Hz,<br />
following the same considerations.<br />
These results are in agreement with<br />
those reported by Stevens and<br />
coworkers [5].<br />
The analysis of potential cracks in<br />
jet pumps has attracted attention [6].<br />
In<strong>for</strong>mation on potential failure<br />
locations in BWR/3-6 jet pumps is<br />
provided in this document. Fatigue<br />
and Intergranular Stress Corrosion<br />
Cracking plays an important role.<br />
Such document also mentions that<br />
Operation and New Build<br />
Failure Analysis of the Jet Pumps Riser in a Boiling Water Reactor-5<br />
ı Pablo Ruiz-López, Luis Héctor Hernández-Gómez, Juan Cruz-Castro, Gilberto Soto-Mendoza, Juan Alfonso Beltrán-Fernánde and Guillermo Manuel Urriolagoitia-Calderón