United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 85-13: Consequences of Using Soluble Dams

                                                          SSINS No. 6835 
                                                            IN 85-13 
  
                               UNITED STATES 
                       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 
 
Addressees: 

All boiling water reactor (BWR) and pressurized water reactor (PWR) 
facilities holding an operating license (OL) or construction permit (CP). 
  
Purpose: 

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 
required. 

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. 

Discussion: 

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. 


8502190137 
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                                                          IN 85-13  
                                                          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 
motion. 

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. 

SOLUBLE DAMS 

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 
to carbon  

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                                                          IN 85-13  
                                                          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 
                    (301) 492-9005 

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