United States Nuclear Regulatory Commission - Protecting People and the Environment

Modifications to Boiling Water Reactor Control Rod Drive Systems (Generic Letter 80-17)

                                                                   GL 80-17





Enclosed you will find a copy of our Jan. 28, 1980 letter to General Electric
which discusses the NRC staff's conclusion regarding proposed control rod
drive (CRD) system modifications related to the elimination of cracking in the
CRD return line nozzle.  You will also find a copy of our Feb. 11, 1980 letter
to GE regarding additional analyses of boiloff rates and CRD system makeup
capability.  This letter also responds to a GE-proposed draft procedure for
optimizing CRD pump flow to the reactor vessel. 

You should especially not our request that modifications not be performed on
operating reactors until complete guidance has been issued in NUREG-0619.  We
anticipate issuing this document in its "For Comment" form in April 1980.
However, if an operating reactor is scheduled for a refueling outage in the
near future, and if applicable CRD system modifications or adjustments are
scheduled prior to the final issuance of NUREG-0619, please obtain NRC
guidance by contacting your Project Manager.  The staff will provide
assistance as necessary.


                                    OR/BWR Branch Chief

As stated



                                UNITED STATES 
                        NUCLEAR REGULATORY COMMISSION 
                           WASHINGTON, D.C.  20555 

                                       January 28, 1980

Generic Technical Activity A-10

Mr. Richard Gridley, Manager
Fuel and Services Licensing
General Electric Co
175 Curtner Ave
San Jose, CA 95215

Dear Mr. Gridley:

Since the initial discovery of cracking in boiling water reactor (BWR) control
rod drive return line (CRCRL) nozzles in early 1977, General Electric (GE) has
proposed a number of solutions to the problem in the course of which several
documents were submitted for NRC staff review.  These documents were as

1.    Letter of 03/14, 1979, G.G. Sherwood (GE) to V. Stello and R. Mattson
(NRC) regarding calculation of CRD system return flow capacity;

2.    Letter of 04/09/79, G.G. Sherwood (GE) to V. Stello and R. Mattson (NRC)
forwarding results of CRD system solenoid valve endurance testing; and

3.    Letter of 05/01/79, G.G. Sherwood (GE) to V. Stello and R. Mattson (NRC)
forwarding results of CRD system solenoid valve performance testing; and

4.    Letter of 11/02/79, G.G. Sherwood (GE) to R. P. Snaider (NRC) forwarding 
additional information as requested regarding CRD hydrulic system performance,
especially with regard to corrosion products emanating from carbon steel

All concerned the GE rationale for the latest proposed system modification to
prevent nozzle cracking; namely, total removal of the CRDRL and cutting and
capping of the CRDRL nozzle.  Previous submittals had presented the bases for
the other modification proposals discussed herein.



Mr. Richard Gridley                  - 2 -                January 28, 1980

Specifically, your March 14, 1979 letter discussed the GE analysis performed
after the NRC's selection of a base case for use in comparing capability to
inject high pressure water into the reactor vessel when other water sources
were isolated.  This base case was the 1975 incident at Browns Ferry Unit No 1
during which the CRD system sometimes was one of the only capable sources of
high pressure water injection to keep the reactors core covered.  The staff
recognizes that the pressure of this capability had not been directly assumed
in any previous safety analysis.  However, the critical need for the system
was again revealed during the early 1979 incident at the Oyster Creek Nuclear
Station.  During this incident the reactor vessel also was isolated from other
sources of high pressure water and the CRD system makeup capability helped
prevent uncovering of the active fuel.

Your analysis of March 14, 1979, included several assumptions which the NRC
staff has found acceptable.  Principal among these was that concurrent
operation of the two CRD pumps was possible at any plant.  This of course
implies that there will be no electrical supply limitations and no pump net
positive suction head (NPSH) limits that will be reached.  Licensees and
applicants will be required to demonstrate this to be valid, by testing, prior
to our approving CRD return line removal.

The letters of April 9 and May 01, 1979, discussed the solenoid valve testing
program initiated in response to earlier NRC concerns.  The original analysis
of CRDRL removal without rerouting determined that return flow to the reactor
vessel from drive operation would enter CRD cooling water lines and return to
the vessel through the CRD mechanisms themselves.  During testing, however,
you discovered that the actual path would be a reverse flow path through the
insert exhaust directional control valves of the non-actuated Hydraulic
Control units.  The long-term cycling of the control valves in the reverse
direction was a cause of NRC concern with regard to possible deleterious
effects upon the operation of the CRD hydraulic system.

In response to this concern, GE tested ten valves which had been removed from
an operating reactor on which the return line had been isolated for six
months.  These valves were then compared against tests performed on five new
valves.  The results showed that the reverse flow characteristics of all valve
were similar and that degradation of the valves to the point of causing system
malfunction would not be expected during long-term normal operation of the
system.  The NRC staff is satisfied with these results.

Simulated life cycle testing also was performed on five valves, resulting in
the determination that no adverse effects were caused by the backflow.  The
NRC staff has found this acceptable.


Mr. Richard Gridley                  - 3 -                January 28, 1980

Your final letter of Nov. 2, 1979, discussed in detail your response to staff
concerns regarding possible degradation of the CRD system and individual CRD
mechanisms because of corrosion problems from carbon steel piping.  Certain
modifications were suggested to solve these problems.  You also discussed your
recommendations regarding the installation of pressure equalizing valves in
the CRD system to prevent, under a hypothetical transient, a large pressure
differential across the CRD system which could result in excessively fast
movement of a selected control rod.  The valves also prevent flow from the
carbon steel piping of the normal exhaust water header to the drive cooling
water header.

We have reviewed your submittals and have concluded the following:

1.    Only licensees of the following classes of plants will be allowed at
this time to implement the recommendation to cut and cap with no re-routing of
the CRDRL and without further analysis.  Each applicable plant must
demonstrate, by testing, concurrent two CRD pump operation (with one
exception), satisfactory CRD system operation, required flow capability, and
each will be required to install the system modifications listed in 4. below.

      a.    218" BWR/6

      b.    251" BWR/6

      c.    183" BWR/4 (only one pump needed to satisfy base case requirement)

      d.    251" BWR/4

      No modifications should be performed on operating reactors prior to
issuance of the "For Comment" issue of NUREG-0619, scheduled for release in
January 1980.

2.    We do not accept the hypothesis that the calculations for the above
plants were bounding.  Therefore, prior to our approval of modification of
other plant classes, we shall require analysis similar to that performed on
the plant classes of 1. above.  The same testing and system modifications will
also be required.

3.    We found the 251* BWR/5 (the fifth case analyzed in the March 14, 1979
letter) presently to be unacceptable for modification in that its calculated
flow fell below the acceptable base case value.  Further analysis or
plant-specific testing could prove flow capacity to be acceptable.


Mr. Richard Gridley                  - 4 -                January 28, 1980

4.    We will require that the following modifications be implemented on all
plants requesting the removal of the CRDRL without rerouting and those which
reroute but choose to operate with CRD return line flow valved out;

      a.    Installation of equalizing valves between the cooling water header
            and the exhaust water header.

      b.    Flush ports installed at high and low points of exhaust water
            header piping run if carbon steel piping is retained; and

      c.    Replacement of carbon steel pipe in the flow stabilizer loop with
            stainless steel and rerouting directly to the cooling water

5.    Each licensee must establish readily-available operating procedures for
achieving maximum CRD flow to an otherwise isolated reactor vessel.

6.    Licensees who choose to reroute the CRDRL, either with or without
continuous return line flow to the system being tapped into, must add the
GE-recommended pressure control station to the cooling water header.  This
station acts to buffer hydraulic perturbations from any connected system in
order to prevent pressure fluctuations in the CRD system.

Modification 4.c is based upon our decision not to accept the "do-nothing"
alternative addressed in your November 2, 1979 letter.  We consider the "more
absolute solution" (your characterization) to be the correct one and agree
with your recommendation, mad in accordance with this "more absolute
solution", that the carbon steel piping should be eliminated.  We do not
accept the option of filter installation as a means of trapping corrosion
particles that have a deleterious effect on the CRD mechanisms.  Our concern
is that improperly maintained filters on the cooling  water header could
result in heatup of drive mechanisms and the possibility of multiple drive
failures of a type not previously analyzed.

Note that we have discussed only the acceptability of the latest GE
recommendation discussed in the four letters.  We continue to a accept CRDRL
re-routing to a line outside containment that in turn provides the return flow
to the reactor vessel (valving out after re-routing results in other
requirement - see 4. and 6. above).  We also find acceptable, as a strictly
interim measure, the valving out of the CRDRL.  However, this will require
inspection, during each refueling outage, of that portion of the line
containing stagnant water.  No matter which option is chosen, we will require
complete inspection, by dye penetrant techniques, of the CRDRL nozzle, the
apron area beneath the nozzle, and the subsequent removal of any cracks found
during the inspection.


Mr. Richard Gridley                  - 5 -                January 28, 1980

For the BWRs undergoing licensing review and designed and constructed without
the CRDRL and its nozzle or modified with the CRDRL cut and capped without
rerouting, we will require testing (similar to that for operating plants) to
prove satisfactory system operation, return flow capability equal to or in
excess of the base case requirement discussed above, and two pump operation. 
Applicable modifications of 4. above also must be implemented.  We shall
require the establishment of operating procedures for achieving maximum CRD
flow to an otherwise isolated vessel. Calculations with regard to base case
return flow requirements should be submitted, but in lieu of such
calculations, the staff may accept reference to a bounding analysis if
necessary justification is provided.

Additional guidance on this subject will be contained in NUREG-0619.  This
document is tentatively scheduled for publication in February 1980.


                                        Darrel G. Eisenhut, Acting Director
                                        Division of Operating Reactors
                                        Office of Nuclear Reactor Regulation

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