Seal for a joint or juncture – Process of static sealing – Pipe – conduit – or cable
Reexamination Certificate
2000-01-28
2001-07-24
Mah, Chuck Y. (Department: 3626)
Seal for a joint or juncture
Process of static sealing
Pipe, conduit, or cable
C277S603000, C277S607000, C277S632000, C029S402020, C285S015000, C285S420000, C376S260000, C376S372000
Reexamination Certificate
active
06264203
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to boiling water reactors and, more particularly, to methods and apparatus for repairing a cracked jet pump riser in a boiling water reactor.
2. Discussion of the Related Art
A typical boiling water nuclear reactor
10
, as illustrated in
FIG. 1
, includes a reactor vessel
12
, a core
14
made up of a plurality of fuel assemblies
16
, and a core shroud
18
disposed concentrically within the reactor vessel around the core. Core shroud
18
includes upper and lower cylindrical sections
20
and
22
connected by a horizontal plate or ring
24
extending radially inward from a bottom edge of the upper cylindrical section to a top edge of the lower cylindrical section. A shroud head flange is welded to the top edge of the upper cylindrical shroud section and extends radially inward to support a shroud head or lid
26
of generally hemispherical or dome-shaped configuration, the lid being secured to the top of the shroud with bolts threadedly or otherwise engaged by lugs mounted in angularly spaced relation about the shroud periphery adjacent the top edge of the shroud.
Fuel assemblies
16
are supported at the bottom by a core plate
28
mounted on a core plate support ring or ledge
29
, best shown in
FIG. 4
, extending radially inward from the bottom edge of the lower cylindrical shroud section and at the top by a top guide
30
mounted on horizontal plate
24
. Control rod guide tubes
32
are provided within vessel
12
at locations above a control rod driving mechanism extending through nozzles located at the bottom of the reactor vessel beneath the shroud. Lower ends of corresponding control rods are detachably connected to the driving mechanism and are arranged to move up and down within the guide tubes.
Feedwater enters the reactor vessel via a feedwater inlet
34
. The feedwater is distributed circumferentially within the reactor vessel by a ring-shaped pipe
36
, known as a feedwater sparger, disposed above the shroud and having suitable apertures for circumferentially distributing the feedwater inside the reactor vessel. The feedwater mixes with other water coming from steam separators of the reactor and flows downwardly from feedwater sparger
36
through the downcomer annulus
38
, that is, the annular region or space between the reactor vessel and the core shroud, and enters the core lower plenum
40
. Boiling is produced in the core creating a mixture of water and steam which enters the core upper plenum, that is, the space under the shroud lid, and is directed into steam plenum heads or stand pipes
46
mounted vertically on the shroud lid in fluid communication with the core upper plenum. The mixture of water and steam flows through stand pipes
46
and enters a respective plurality of steam separators
48
, which are shown as being of the axial-flow centrifugal type. The separated liquid water then mixes with incoming feedwater and flows downwardly to the core via the downcomer annulus. The steam, on the other hand, passes through a steam drying assembly or dryer
50
disposed above the steam separators and is withdrawn from the reactor vessel via a steam outlet
52
.
Boiling water reactors also include a coolant recirculation system providing the necessary forced convection flow through the core. A portion of the water flowing through the downcomer annulus is withdrawn from the reactor vessel via a recirculation water outlet
42
and is fed under pressure into a plurality of jet pump assemblies
44
distributed circumferentially about the exterior of the core shroud to produce a forced convection flow through the core, thusly providing the required reactor core water flow. Boiling water reactors typically include between six and twelve jet pump assemblies; however, most reactors include ten jet pump assemblies. Referring to
FIG. 2
, it can be seen that jet pump assemblies
44
are distributed circumferentially about core shroud
18
in annular space
38
between the core shroud and reactor vessel
12
. Each jet pump assembly
44
is positioned adjacent a recirculation inlet nozzle
54
formed on the exterior of the reactor vessel and, as best seen in
FIGS. 3 and 4
, the jet pump assemblies each include a riser assembly
56
extending upwardly from the recirculation inlet nozzle to a transition piece
58
, and two inlet mixers
60
extending downwardly from the transition piece to a pair of diffusers
62
mounted over holes (shown in
FIG. 1
at
64
) in a pump deck
66
connecting a bottom portion of the shroud with the reactor vessel. Riser assembly
56
typically includes a riser pipe
68
oriented vertically in parallel relation to shroud
18
and a riser elbow
70
extending downwardly from the bottom of the riser pipe and bending outwardly to a circumferential weld
72
connecting the elbow with a thermal sleeve
74
in the recirculation inlet nozzle. The riser assembly is supported near the top by a riser brace
76
, which is welded to riser pipe
68
and to pads (not shown) on reactor vessel
12
and/or shroud
18
. Transition piece
58
extends in opposite directions from the top of riser pipe
68
to connect with inlet mixers
60
on opposite sides of the riser pipe. Inlet mixers
60
extend downwardly from transition piece
58
in parallel relation to riser pipe
68
, with lateral support for the inlet mixers being provided by jet pump restrainer brackets
78
attached between respective lower ends of the jet pump inlet mixers and the riser pipe. The entrance or upper end of each inlet mixer
60
is clamped to the riser transition piece
58
by a beam-bolt assembly. The exit or lower end of each inlet mixer
60
forms a slip joint with the entrance or upper end of one of the diffusers
62
, the interface between the inlet mixers and the diffusers providing additional lateral support for the riser assembly. The top of the slip joint is located near the bottom of the fuel assemblies; the exact elevation of the slip joint being dependent upon the particular boiling water reactor in question. Diffusers
62
extend downwardly from inlet mixers
60
to pump deck
66
and are of increasing diameter in the downward direction.
The riser elbow is typically a 10-inch diameter, 90 degree radius elbow fabricated of Type 304 stainless steel which, after periods of use, is susceptible to cracking along welded joints. Cracking is particularly common in the heat-affected zone of the weld joining the riser elbow with the thermal sleeve since this weld is typically made in the field and not in a shop where conditions are less likely to result in cracking. Separation of the jet pump riser piping in this area could adversely impact safety in some boiling water reactors under certain accident conditions. When more than one jet pump assembly is affected, the jet pump piping must either be replaced or repaired. Repair is certainly the preferred alternative in view of the fact that replacement involves significant expense, relatively long shut-down time, and the potential for radiation exposure to personnel.
Since weld repairs in the downcomer annulus are typically not practical due to inaccessibility, helium cracking, and the potential for excessive radiation exposure to personnel, a need exists for a method of repairing cracked jet pump riser assemblies involving little or no in-vessel welding.
SUMMARY OF THE INVENTION
Accordingly, it is a primary object of the present invention to provide methods and apparatus for repairing cracked jet pump risers in a boiling water reactor to ensure an adequate flow of water through the reactor core.
It is a more specific object of the present invention to assure continued core flood and low pressure coolant injection in a boiling water reactor having cracked jet pump riser piping.
Another object of the present invention is to limit leakage from cracked jet pump riser piping over a predetermined range of crack widths.
An additional object of the present invention is to limit horizontal displacement of a shroud and jet pump riser assembly
Sylvester William E.
Weems Sterling J.
Mah Chuck Y.
MPR Associates, Inc.
Patel Vishal
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