Induced nuclear reactions: processes – systems – and elements – Handling of fission reactor component structure within...
Reexamination Certificate
1998-05-07
2001-04-10
Poon, Peter M. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Handling of fission reactor component structure within...
C376S245000, C376S249000, C073S623000, C073S633000, C073S640000, C324S220000
Reexamination Certificate
active
06215836
ABSTRACT:
BRIEF DESCRIPTION OF THE INVENTION
This invention relates generally to the maintenance of nuclear steam supply systems. More particularly, this invention relates to a technique of ultrasonically examining remotely located welds in cast stainless steel nuclear steam supply systems.
BACKGROUND OF THE INVENTION
FIG. 1
illustrates a nuclear steam supply system
20
in accordance with the prior art. As known in the art, the system
20
includes a reactor vessel
22
with a core
24
. The system
20
also includes a set of coolant loops
26
A-
26
D. Each coolant loop
26
includes a steam generator
28
and an associated reactor coolant pump
30
. Each steam generator
28
includes a feedwater inlet
32
and a steam outlet
34
.
The operation of a nuclear steam supply system
20
is well known in the art. Coolant leaving the reactor vessel
22
enters a steam generator
28
where it imparts its heat to a working fluid which exits as steam through a steam outlet
34
. The steam is then used to drive a turbine (not shown) to produce electricity. The coolant leaves the steam generator
28
via the reactor coolant pump
30
and is pumped back to the reactor vessel
22
.
The present invention is directed toward the maintenance of reactor coolant pumps
30
associated with nuclear steam supply systems.
FIG. 2
is a cut-away view of a prior art reactor coolant pump
30
. The reactor coolant pump
30
includes a pump shaft
40
connected to an impeller
42
. A seal housing
44
surrounds the pump shaft
40
. The seal housing includes a set of seal injection lines or pipes
46
A,
46
B.
The reactor coolant pump
30
also includes a casing
48
. A set of cooling lines or pipes
50
A,
50
B pass through the casing
48
and into a thermal barrier
52
. The casing
48
and thermal barrier
52
are formed of cast stainless steel.
FIG. 3
is a top view of a cooling line
50
passing into the casing
48
.
FIG. 4
illustrates the seal injection line
46
terminating in a thermal barrier
52
. A weld
60
is used to connect the seal injection line
46
to the thermal barrier
52
.
The present invention is directed toward identifying a flaw
62
associated with a weld
60
. The flaw may be in many forms, for example, a crack or an incomplete weld penetration. The flaw may be in a cooling line
50
, a seal injection line
46
, or any other remote location. The invention is most useful in relation to cast stainless steel components associated with nuclear steam supply systems.
There are no known prior art techniques for identifying flaws in attachments to reactor coolant pumps of nuclear steam supply system coolant loops. More particularly, there are no known techniques for identifying flaws in seal injection lines and cooling lines of reactor coolant pumps. The present practice is to wait for a failure and then shutdown the plant. Repairs and welds are then made during shutdown. The expense associated with an unplanned shutdown of this type is typically about $500,000 per day. Thus, it would be highly desirable to provide a technique for detecting flaws in remotely located nuclear steam supply system components. Such a technique would save critical operation time and prevent unscheduled plant outages.
It is difficult to identify defects of the type described above for a number of reasons. First, the defects are remotely located. Therefore, a special apparatus must be contrived to reach the remote location. Another problem is that the seal injection lines
46
and cooling lines
50
are connected to cast stainless steel components.
Conventional ultrasonic examination of such components is not possible for the following reasons. First, the properties of the cast stainless steel material from which the thermal barrier is fabricated are not conducive to the transmission of ultrasonic energy due to large gain structure, ultrasonic beam redirection, and ultrasonic scattering. Second, access to the thermal barrier for ultrasonic examination is limited by component geometry. Third, the welds are typically not examined due to inherent reflectors formed during fabrication of the weld joint. Fourth, the orientation of the crack is not conducive to a conventional examination procedure, even if the material properties would permit ultrasonic transmission and subsequent reflection from the crack face.
Cast stainless steel material typically consists of large, randomly orientated grain structure that tends to scatter and otherwise disperse ultrasonic energy. The nuclear power industry has spent a great deal of money trying to identify ultrasonic examination techniques for cast stainless steel material, with very limited success. Present techniques for ultrasonic examination of cast stainless steel material is unreliable, at best. The most promising results, although very limited, have been obtained by using low frequency, large diameter, dual-element ultrasonic transducers. The available access to the thermal barrier is inadequate for this size of probe. In addition, even if the ultrasonic energy were to reach the crack location, the ultrasonic wave length would be too large (with low resolution) for efficient reflection from the small crack face. The beam size of conventional probes is also larger than this particular weld and would result in an ineffective examination.
In view of the foregoing, it would be highly desirable to provide an ultrasonic examination technique to detect remotely located flaws in nuclear steam supply systems prior to leakage. Such a technique would allow repair of the flaws during scheduled refueling outage, instead of during unplanned outages.
SUMMARY OF THE INVENTION
An apparatus for ultrasonically examining a weld in a nuclear steam supply system includes an elongated guide rod for positioning within a pipe of the nuclear steam supply system. The pipe may be, for example, a seal injection line or cooling line of a reactor coolant pump. An ultrasonic transducer is positioned at an end of the elongated guide rod. A collapsible shoe encloses the ultrasonic transducer. The collapsible shoe includes a biasing mechanism to allow the collapsible shoe to pass through the pipe while the pipe is at a first circumference and while the pipe is at a second circumference. The collapsible shoe continuously contacts the pipe to establish ultrasonic coupling for the ultrasonic transducer.
The invention can detect weld flaws without the need of passing the ultrasonic beam through the complex grain structure of the cast stainless steel. The invention can be used to identify weld flaws during scheduled outages so that unexpected damaging weld leaks and concomitant plant outages can be avoided.
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Caban Adam Roy
Kenefick Steven Allen
Lowery Robert Jeffery
Walker Stanley Maurice
Electric Power Research Institute Inc.
Flehr Hohbach Test Albritton & Herbert LLP
Keith Jack
Poon Peter M.
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