Information Notice No. 85-13: Consequences of Using Soluble Dams
SSINS No. 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D. C. 20555
February 21, 1985
INFORMATION NOTICE NO. 85-13: CONSEQUENCES OF USING SOLUBLE DAMS
All boiling water reactor (BWR) and pressurized water reactor (PWR)
facilities holding an operating license (OL) or construction permit (CP).
This notice is provided to alert licensees and applicants of the potential
for introducing insoluble fibers into the reactor coolant system as a result
of the use of soluble dams during pipe replacement. Recipients are expected
to review the information for applicability to their facilities and consider
actions, if appropriate, to preclude similar problems occurring at their
facilities. However, suggestions contained in this notice do not constitute
NRC requirements; therefore, no specific action or written response is
Description of Circumstances:
Following an extended outage to replace major portions of the recirculation
l system piping, the Monticello plant experienced excessive scram times
during surveillance testing of the control rod system. The excessive scram
times were caused by clogged inner filters in the control rod drive
mechanisms (CRDMs). The filters were clogged by fibers from soluble dams
used to contain an inert gas for welding during the pipe replacement.
During hydrostatic testing following the pipe replacement, the licensee
found scram times approaching 10 seconds on several control rods. Technical
Specifications require the rods to be 90% inserted within 3.8 seconds.
Investigation by the licensee, including friction testing, verified that
there was no mechanical binding of the rods. Additional investigation
involved the examination of the filters in the hydraulic flow path on the
CRDMs. These examinations revealed that the inner filters in the flow path
were plugged, thus increasing the scram insertion time.
There are three aspects of this event worth noting. The first is the impact
of foreign material in the reactor coolant system on BWR scram times. The
second is the more general implications to both PWRs and BWRs regarding
insoluble material from soluble dams. The third is the importance of
ensuring the cleanliness of reactor coolant system water following major
maintenance on system piping components.
February 21, 1985
Page 2 of 3
BWR CRDM INNER FILTER
In a BWR, a scram is slowed by a clogged movable inner filter because water
must pass through the filter during a scram and fill the volume beneath the
spud and over the top of the stop piston. If water does not pass through the
movable inner filter and the reactor is at full pressure, a large differ-
ential pressure develops across the filter during a scram that retards rod
The purpose of the inner filter is to remove particulates from the reactor
coolant system entering the CRDMs, thus reducing wear to the seals of the
stop piston. A clogged movable inner filter will affect scram times only
while the reactor is at pressure. If the reactor is not at pressure, the
differential pressure across a clogged filter is not sufficient to signif-
icantly retard rod motion. A clogged inner filter will not prevent a scram;
it can only slow the scram because there is a small amount of bypass flow
around a movable inner filter. Neither will a clogged inner filter affect
normal rod movement.
Before the recirculation pipe was replaced, all but two of the CRDMs at
Monticello had movable inner filters mounted beneath the spud. Some plants
of a more recent design have a fixed inner filter mounted to the top of the
stop piston. The fixed filter will not inhibit scram insertion if it becomes
clogged. All but a few licensees of older plants have modified their CRDMs
with a fixed filter using kits supplied by General Electric.
As a corrective measure at Monticello, the licensee is replacing all of the
CRDM inner filters. Of the 121 CRDMs, 64 have now been modified to accept
the fixed inner filter. Movable inner filters were used for the remaining 57
CRDMs. Changing the filters in the CRDMs (6 to 8 rods per day) resulted in
significant radiation exposure, approximately 3.0 man-rem per day.
After replacing some of the movable filters, the licensee discovered that
the replacement filters had an incorrect mesh size (2-mil), even though the
filters came from General Electric in boxes marked as the correct
replacement movable inner filter which should have a 10-mil mesh. As a
consequence, the CRDMs with these filters were removed a second time and
replaced with filters of the proper mesh size. It should be noted, however,
that a fixed inner filter should have a 2-mil mesh.
The clogging of the inner filters at Monticello was caused by soluble dams
that did not fully dissolve. The soluble dams, called DISSOLVO paper, have a
starch binder that is soluble and fibers of cotton and rayon that are not
soluble but deteriorate to soluble products at elevated system temperatures.
One gram of this paper produces about 1 million fibers 1/4 inch in length
and 5 microns in diameter. The fibers become trapped on the inner filter and
effectively caused the 10-mil filter to act as a much finer filter, which
trapped oxides and particulates that accumulated during the long outage. The
manufacturer's current recommendation is to dissolve the dams and remove the
water used for flushing.
Tests show that the soluble dams require temperatures greater than about
400F for a period of 10 hours for the fibers to deteriorate and change
February 21, 1985
Page 3 of 3
dioxide. This explains why the fibers did not deteriorate because the
reactor coolant system was never hotter than about 185F during the
hydrostatic testing prior to the scram time surveillance testing.
Although not a problem at Monticello, another potential problem with the use
of soluble dams is associated with the glue used to attach them. If the dams
are installed too close to the weld, the heat of welding can cause the norm-
ally soluble glue to become insoluble. Attempts to remove the product have
been only partially successful.
The NRC has previously issued Information Notice No. 81-07: "Potential
Problem With Water-Soluble Purge Dam Materials Used During Inert Gas
Welding" on March 12, 1981. That information notice described other problems
with a different type of soluble dam material.
CLEAN UP OF REACTOR WATER
At Monticello, during the outage to replace the recirculation piping, the
licensee took measures to keep foreign material out of the control rod drive
mechanisms. These measures included keeping a flow of at least 10 gpm
through the drives, except during the time the vessel was drained.
After the pipe replacement, but before refueling and performing scram time
surveillance, the licensee took measures to remove foreign material from the
reactor coolant system. For example, the licensee vacuumed portions of the
reactor coolant system including the control rod guide tubes. Despite these
measures, the licensee found it necessary to operate the reactor water
clean-up system (RWCUS) for about 2 days before refueling in order to
restore water clarity.
Subsequent to the filter clogging problem and to prevent recurrence of the
clogged filters, the licensee has operated the recirculation pumps to sweep
foreign material in the piping into the reactor coolant system and operated
the condensate system with its filter demineralizers.
No specific action or written response is required by this information
notice. If you have any questions about this matter, please contact the
Regional Administrator of the appropriate regional office or this office.
Edward L. Jordan, Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: Eric Weiss, IE
List of Recently Issued IE Information Notices
Page Last Reviewed/Updated Friday, May 22, 2015