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Description  |
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FIELD OF THE INVENTION
This invention relates to maintenance and repair of nuclear reactors. In
particular, the invention relates to the repair of the fuel core shroud of
a boiling water reactor.
BACKGROUND OF THE INVENTION
A conventional boiling water reactor is shown in FIG. 1. Feedwater is
admitted into a reactor pressure vessel (RPV) 10 via a feedwater inlet 12
and a feedwater sparger 14, which is a ring-shaped pipe having suitable
apertures for circumferentially distributing the feedwater inside the RPV.
A core spray inlet 11 supplies water to a core spray sparger 15 via core
spray line 13. The feedwater from feedwater sparger 14 flows downwardly
through the downcomer annulus 16, which is an annular region between RPV
10 and core shroud 18. Core shroud 18 is a stainless steel cylinder which
surrounds the core 20, which is made up of a plurality of fuel bundle
assemblies 22 (only two 2.times.2 arrays of which are shown in FIG. 1).
Each array of fuel bundle assemblies is supported at the top by a top
guide 19 and at the bottom by a core plate 21. The core top guide provides
lateral support for the top of the fuel assemblies and maintains the
correct fuel channel spacing to permit control rod insertion.
The water flows through downcomer annulus 16 to the core lower plenum 24.
The water subsequently enters the fuel assemblies 22, wherein a boiling
boundary layer is established. A mixture of water and steam enters core
upper plenum 26 under shroud head 28. Core upper plenum 26 provides
standoff between the steam-water mixture exiting core 20 and entering
vertical standpipes 30, which are disposed atop shroud head 28 and in
fluid communication with core upper plenum 26.
The steam-water mixture flows through standpipes 30 and enters steam
separators 32, which are of the axial-flow centrifugal type. The separated
liquid water then mixes with feedwater in the mixing plenum 33, which
mixture then returns to the core via the downcomer annulus. The steam
passes through steam dryers 34 and enters steam dome 36. The steam is
withdrawn from the RPV via steam outlet 38.
The BWR also includes a coolant recirculation system which provides the
forced convection flow through the core necessary to attain the required
power density. A portion of the water is sucked from the lower end of the
downcomer annulus 16 via recirculation water outlet 43 and forced by a
centrifugal recirculation pump (not shown) into jet pump assemblies 42
(only one of which is shown) via recirculation water inlets 45. The BWR
has two recirculation pumps, each of which provides the driving flow for a
plurality of jet pump assemblies. The pressurized driving water is
supplied to each jet pump nozzle 44 via an inlet riser 47, an elbow 48 and
an inlet mixer 46 in flow sequence. A typical BWR has 16 to 24 inlet
mixers. The jet pump assemblies are circumferentially distributed around
the core shroud 18.
The core shroud 18 (see FIG. 2) comprises a shroud head flange 18a for
supporting the shroud head 28; a circular cylindrical upper shroud wall
18b having a top end welded to shroud head flange 18a; an annular top
guide support ring 18c welded to the bottom end of upper shroud wall 18b;
a circular cylindrical lower shroud wall 18d having a top end welded to
top guide support ring 18c; and an annular core plate support ring 18e
welded to the bottom end of lower shroud wall 18d. The diameter of upper
shroud wall 18b is greater than the diameter of lower shroud wall 18d. The
entire structure is supported by RPV shroud support 51, which is welded to
shroud 18, and by annular shroud support plate 52, which is welded at its
inner diameter to RPV shroud support 51 and at its outer diameter to RPV
10.
Stress corrosion cracking (SCC) is a known phenomenon occurring in reactor
components, such as structural members, piping, fasteners, and welds,
exposed to high-temperature water. The reactor components are subject to a
variety of stresses associated with, e.g., differences in thermal
expansion, the operating pressure needed for the containment of the
reactor cooling water, and other sources such as residual stress from
welding, cold working and other asymmetric metal treatments. In addition,
water chemistry, welding, heat treatment, and radiation can increase the
susceptibility of metal in a component to SCC.
Stress corrosion cracking has been found in the top guide support ring of a
BWR. The cracks in top guide support ring 18c extend radially inwardly
from the outer circumferential surface thereof and radially outwardly from
the inner circumferential surface thereof in the vicinity of the welds
which join top guide support ring 18c to shroud walls 18b and 18d.
Stress corrosion cracking in top guide support ring 18c diminishes the
structural integrity of shroud 18, which vertically and horizontally
supports core top guide 19 and shroud head 28. There is a need for a
method and an apparatus for repairing a shroud which has cracks in the top
guide support ring.
SUMMARY OF THE INVENTION
The present invention is a method and an apparatus for repairing a shroud
in which the top guide support ring has experienced SCC. The method
involves the placement of a plurality of clamps around the outer
circumference of the shroud at a plurality of azimuthal positions between
jet pump assemblies. The clamps are intended to structurally replace the
welds that join the top guide support ring to the upper and lower shroud
walls.
The shroud repair clamps in accordance with the invention are designed to
support the top guide, the fuel bundle assemblies and the shroud head. The
clamps are further designed to withstand the thermal and radiological
conditions which the shroud is subjected to. The shroud repair clamps are
bolted to the shroud above and below the top guide support ring in a
manner which will prevent relative movement across the top guide support
ring welds during all normal and upset conditions. Further, the shroud
repair clamps of the present invention are designed and installed such
that removal of jet pump inlet mixers and RPV beltline inspection can be
performed without removal of the repair clamps.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic showing a partially cutaway perspective view of a
conventional BWR.
FIG. 2 is a sectional view of a portion of the core shroud of the BWR shown
in FIG. 1.
FIG. 3A is an elevational view of a portion of the core shroud showing the
hole pattern by which the shroud repair clamp of the invention is
installed.
FIG. 3B is a sectional view of an installed shroud repair clamp assembly in
accordance with a preferred embodiment of the invention.
FIGS. 4A, 4B and 4C are bottom, side and plan views, respectively, of a
bracket incorporated in the shroud repair clamp assembly shown in FIG. 3B.
FIGS. 5A and 5B are top and side views, respectively, of an upper bolt
incorporated in the shroud repair clamp assembly shown in FIG. 3B.
FIG. 6 is a plan view of a shim washer incorporated in the shroud repair
clamp assembly shown in FIG. 3B.
FIGS. 7A and 7B are plan and side views, respectively, of a tang washer
incorporated in the shroud repair clamp assembly shown in FIG. 3B.
FIG. 8 is a plan view of a lower bolt incorporated in the shroud repair
clamp assembly shown in FIG. 3B.
FIG. 9 is a top view showing the placement of clamps in accordance with the
shroud repair method of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is a method and an apparatus for repairing a core
shroud in which the top guide support ring has been subjected to SCC. The
purpose of the shroud repair clamps is to structurally replace welds W1
and W2 which are potentially undermined by cracks C in the top guide
support ring (see FIG. 3B). Each shroud repair clamp is bolted to the
shroud above and below the top guide support ring in a manner which will
prevent relative movement across the top guide support ring welds during
all normal and upset conditions. The bolts are installed in holes machined
into the shroud walls, as shown in FIG. 3A.
An installed shroud repair clamp in accordance with the preferred
embodiment is shown in FIG. 3B. The clamp comprises a rigid bracket 54
which is attached to the upper shroud wall 18b by means of a pair of upper
bolts 56 and to the upper shroud wall 18d by means of a pair of lower
bolts 66 (only one of each pair of bolts is visible in FIG. 3B). Each
upper bolt 56 is tensioned by a corresponding nut 62, which nut is locked
in the tensioned position by a corresponding nut keeper 64 tack welded to
the shank of upper bolt 56. Similarly, each lower bolt 66 is tensioned by
a corresponding nut 68, which nut is locked in the tensioned position by a
corresponding nut keeper 64 tack welded to the shaft of lower bolt 66.
As shown in FIGS. 4A-4C, bracket 54 comprises a lower portion 80, an upper
portion 82 and a stiffening rib 84, all of which are integrally formed
from a metal alloy (e.g., F304, F304L, F316 or F316L material per ASTM
A182) having a carbon content not to exceed 0.02 wt. %. To improve
corrosion resistance, the material should be annealed at 1900.degree. to
2100.degree. F. followed by quenching in circulating water to a
temperature below 400.degree. F. A pair of circular cylindrical holes 76
are formed in upper portion 82 of bracket 54 for receiving upper bolts 56
respectively. The axes of holes 76 are mutually non-parallel and disposed
perpendicular to the cylindrical upper shroud wall 18b when the clamp is
installed. A pair of circular cylindrical holes 78 are formed in bracket
lower portion 80 for receiving lower bolts 66 respectively. The axes of
holes 78 are mutually non-parallel and disposed perpendicular to the
cylindrical upper shroud wall 18b when the clamp is installed. Each
bracket portion 80 and 82 has a pair of contact surfaces disposed
generally parallel to the tangent of the opposing portion of the upper or
lower shroud wall.
To install the shroud repair clamps, holes must be formed in the upper and
lower shroud walls 18b and 18d. The shapes and positions of these holes
can be seen in FIG. 3A. The lower holes 4 and 4' are simple circular
cylindrical bores for receiving the circular cylindrical shafts of bolts
66 (shown in FIG. 8). The upper holes 2 and 2' are non-circular and shaped
to allow passage therethrough of the head of upper bolt 56, the latter
being in the shape of a rectangle with rounded corners. The use of bolts
with non-circular heads is necessary because the top guide 19 installed
inside the upper shroud wall 18b blocks installation of upper bolts 56
from the interior of the shroud. Instead upper bolts 56 must be installed
headfirst from the exterior of the shroud.
The holes in shroud 18 are formed by electrode discharge machining (EDM).
The holes are machined parallel to the azimuth line on which the hole
pattern is centered. The hole patterns are to be located at the twelve
reference bracket location azimuths (see FIG. 9) shown by attaching an EDM
alignment fixture yoke onto the shroud lugs. When attached, the EDM
alignment fixture is centered in the lugs within 0.25 inch. The dimension
is confirmed if the EDM alignment fixture is touching the top guide
support ring at both ends of the alignment fixture. Hole locations are
controlled by as-built measurements of the alignment fixture and field
runout and position measurements of the EDM electrodes. Therefore field
verification of the hole locations and sizes is not required.
As shown in FIGS. 5A and 5B, each upper bolt 56 has a bolt head 57 of
rectangular section with rounded corners and a shaft with a threaded
section 55 which cooperates with internal threads on nut 62 (see FIG. 3B).
Before bracket 54 is mounted on lower bolts 66, the head of each upper
bolt 56 is oriented into alignment with respective holes 2, 2' and then
passed therethrough until the bolt head 57 clears the upper shroud wall
18b. Once clear of the upper shroud wall, the upper bolts 56 can be
rotated 90.degree. to an orientation of nonalignment with holes 2, 2' in
which orientation the bolt head 57 cannot pass back through holes 2, 2'.
To lock bolt 56 so that its head remains oriented in non-alignment with
upper holes 2, 2' a specially designed locking washer 60 (see FIGS. 7A and
7B) is installed in each hole 2, 2'. Each locking washer 60 comprises a
thin annular disk 61 which supports a pair of diametrally opposed, axially
extending projections 63 and 63'. When locking washer 60 is installed in
each upper hole 2, 2', projections 63 and 63' occupy the interstices
between the circular shaft of bolt 56 and the non-circular boundary of the
upper holes. Each projection in turn has a respective tang 86, 86'
extending axially from its end surface. The projections and tangs have
lengths such that when locking washers are fully inserted in upper holes
2, 2' (with annular disk 61 flush against the external surface of upper
shroud wall 18b), tangs 86 and 86' will project into respective spaces on
opposing sides of rectangular bolt head 57. In these positions, the tangs
will interfere with and thereby block rotation of upper bolt 56 into a
position where head 57 will be aligned with the corresponding upper hole.
Each of lower bolts 66 has a bolt head 67 of circular section with an outer
diameter greater than the diameter of lower holes 4, 4'. The shaft of each
bolt 66 is inserted through the respective lower hole from the inside,
i.e., core side, of the shroud.
To install the shroud repair clamp, bolts 56 are inserted in upper holes
76, 76' of bracket 54. Then the bracket is mounted on the shroud by
installing bolts 56 headfirst holes 2, 2' in upper shroud wall 18b. Bolts
66 are inserted in holes 4, 4' of lower shroud wall 18d and lower holes
78, 78' of bracket 54 from the core side. Thereafter, an internally
threaded nut 68 is screwed onto the threaded shaft 65 of each lower bolt
66 and then tensioned.
With the bracket secured in position, the space between bracket upper
portion 82 and annular disk 61 of locking washer 60 is measured. A
U-shaped shim washer 58 (see FIG. 6) is then trimmed by machining to a
thickness equal to the measured dimension. Then shim washers 58 are
installed in the space between annular disk 61 and bracket upper portion
82. The shaft of each upper bolts 56 is received in the opening between
the legs of the U-shaped shim washer. Each leg has a shoulder 59, the
shoulders being in mutual opposition to form a narrowing of the opening
having a dimension slightly less than the outer diameter of the bolt
shaft. This enables the shim washers to be clipped onto the bolt shafts so
that the washers will not fall off.
After shim washers 58 are properly installed, upper bolts 56 are tensioned
as required by tightening of nuts 62. Nuts 62 and 68 are then locked in
position by installing and tack welding nut keepers 64 to the shafts of
bolts 56 and 66. After tensioning and prior to tack welding, the upper
bolt nut keepers are machined to a length such that the end of the keeper
is 0.20.+-.0.05 inch from the end of upper bolt 56.
In accordance with the preferred embodiment of the invention, the bolts and
nuts are made of XM-19 material per ASTM A479 with a carbon content not to
exceed 0.04 wt. %. The XM-19 material should be annealed at
2000.degree..+-.25.degree. F. followed by cooling at a rate of at least
200.degree. F. per minute. The washers and nut keepers may be made of Type
304, 304L, 316 or 316L stainless steel alloy per ASTM A240 or A276.
The placement of a plurality of clamps 50 around the periphery of shroud 18
in accordance with the invention is shown in FIG. 9. In a typical BWR,
twelve clamps are installed at azimuths of 15.degree., 45.degree.,
105.degree., 135.degree., 165.degree., 195.degree., 225.degree.,
255.degree., 285.degree., 315.degree. and 345.degree.. The majority of
clamps 50 are installed in the spaces between adjacent jet pump assemblies
42.
The preferred embodiment of the shroud repair clamp has been disclosed for
the purpose of illustration. Variations and modifications of the disclosed
structure which do not depart from the concept of this invention will be
readily apparent to mechanical engineers skilled in the art of tooling.
For example, the heads of the upper bolts can have non-circular shapes
different than rectangles with rounded corners, with corresponding shapes
for the upper holes machined in the upper shroud wall. All such variations
and modifications are intended to be encompassed by the claims set forth
hereinafter.
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