Induced nuclear reactions: processes – systems – and elements – Reactor protection or damage prevention – Expansion means
Reexamination Certificate
2002-08-21
2003-11-11
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Reactor protection or damage prevention
Expansion means
C376S372000, C376S285000, C254S104000
Reexamination Certificate
active
06647083
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to nuclear reactors and, more particularly, to a method and apparatus for stiffening a riser brace that lends lateral support to a jet pump assembly of a boiling water reactor.
2. Related Art
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically-shaped shroud.
FIG. 1
is a schematic, partial cross sectional view, with parts cut away, of a reactor pressure vessel (RPV)
20
for a boiling water reactor. RPV
20
has a generally cylindrical-shape and is closed at one end by a bottom head and at its other end by removable top head (not shown). A top guide (not shown) is situated above a core plate
22
within RPV
20
. A shroud
24
surrounds core plate
22
and is supported by a shroud support structure
26
. A downcomer annulus
28
is formed between shroud
24
and sidewall
30
of RPV
20
.
An annulet nozzle
32
extends through sidewall
30
of RPV
20
and is coupled to a jet pump assembly
34
. Jet pump assembly
34
may include a thermal sleeve
36
which extends through nozzle
32
, a lower elbow (only partially visible in FIG.
1
), and a riser pipe
38
. Thermal sleeve
36
is secured at a first end (not shown) to a second end of the lower elbow. The first end of thermal sleeve
36
is welded to the second end of the lower elbow. A first end of the lower elbow similarly secured, or welded, to one end of riser pipe
38
. Riser pipe
38
extends between and substantially parallel to shroud
24
and sidewall
30
. A riser brace assembly
40
stabilizes riser pipe
38
within RPV
20
. The riser brace assembly
40
may be fabricated of type
304
stainless steel which, after periods of use, is susceptible to cracking at welded joints. The riser brace assembly
40
is connected between riser pipe
38
and sidewall
30
.
FIG. 2
illustrates the riser brace assembly
40
of
FIG. 1
in further detail. Riser brace assembly
40
primarily provides lateral support to the jet pump assembly
34
via riser pipe
38
, and includes a riser brace block
43
and two riser brace leaves, an upper riser brace leaf
41
and a lower riser brace leaf
42
. Leaves
41
and
42
are attached to riser brace block
43
by welds, and riser brace block
43
is welded to a support pad
130
which in turn is affixed to RPV sidewall
30
. At the other end, leaves
41
and
42
of the riser brace assembly
40
are connected to a yoke such as brace plate
49
. Brace plate
49
in turn is welded to the riser pipe
38
.
The riser brace assembly
40
is designed to accommodate the differential thermal expansion that results from reactor start-up and heat-up, and to accommodate the flow-induced vibration that is incumbent in the reactor water recirculation system (not shown) due to reactor recirculation pumps. Accordingly, a concern with the riser brace assembly
40
is that the natural frequency of the riser brace assembly
40
be greater than the vane passing frequency of the recirculation pumps at any pump speed. If the vane passing frequency of the recirculation pumps equals or exceeds the natural frequency of the riser brace assembly
40
, the riser brace assembly
40
goes into resonance. The resonating riser brace assembly
40
could potentially cause the riser pipe
38
to become unstabilized, adversely affecting the jet pump assembly
34
.
SUMMARY OF THE INVENTION
The present invention provides a method and clamp apparatus for stiffening a riser brace assembly so as increase the natural vibration frequency (e.g., first harmonic natural frequency) of the riser brace assembly above the vane passing frequency of a recirculation pump frequency at any pump speed, Installation of the vibration mitigation clamp apparatus should not require removal of any installed reactor hardware, such as jet pump assembly hardware or shroud repair hardware.
In an embodiment, the vibration mitigation clamp apparatus may include a first plate, a second plate and a wedge assembly. The vibration mitigation clamp apparatus is attached to upper and lower riser brace leaves of the riser brace assembly, at a location near a weld that attaches the leaves to a riser brace block of the riser brace assembly that is affixed to an RPV sidewall. The wedge assembly is expandable to apply forces on inside surfaces of the riser brace leaves, countering clamping forces applied to the first and second plate to fixedly secure the vibration mitigation clamp apparatus on the riser brace assembly.
REFERENCES:
patent: 401368 (1889-04-01), Piper
patent: 1006974 (1911-10-01), Moore et al.
patent: 1435811 (1922-11-01), Cuntala
patent: 2547789 (1951-04-01), Skeel
patent: 3021112 (1962-02-01), Forman
patent: 3390862 (1968-07-01), Schreffer
patent: 4142646 (1979-03-01), Wienpahl
patent: 4184665 (1980-01-01), Queen, Jr.
patent: 4747996 (1988-05-01), Hale et al.
patent: H906 (1991-04-01), Baggett et al.
patent: 5427349 (1995-06-01), Obrecht
patent: 6099199 (2000-08-01), Mullenberg
patent: 6463114 (2002-10-01), Wivagg
patent: 1551408 (1979-08-01), None
patent: 2126190 (1984-03-01), None
patent: 6-46885 (1993-04-01), None
Carone Michael J.
General Electric Company
Harness & Dickey & Pierce P.L.C.
Palabrica R
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