United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 81-16: Control Rod Drive System Malfunctions

                                                            SSINS No.: 6835 
                                                            Accession No.: 
                                                            8103300372     
                                                            IN 81-16       

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF INSPECTION AND ENFORCEMENT
                           WASHINGTON, D.C. 20555
                                     
                               April 23, 1981

Information Notice No. 81-16: CONTROL ROD DRIVE SYSTEM MALFUNCTIONS 

Description of Circumstances: 

Continued NRC evaluation of BWR control rod drive (CRD) systems operating 
experience has identified several operating events which highlight the need 
for timely operator action if certain CRD system malfunctions occur during 
specific modes of reactor operation. In each event, operator action was 
taken when needed and scram capability was maintained at all times. However, 
if timely operator action were not taken (or if other circumstances existed)
scram capability might have been degraded. This notice is provided to inform
reactor operators of these events and re-emphasize the reliance on timely 
operator action (IEB 80-17 Supplement 4, Confirmatory Order dated October 2,
1980, and Safety Evaluation Report dated December 1, 1980.) 

On February 24, 1981, at Brunswick Unit 2, the reactor was manually scrammed
from 1.5% power after the group 4 control rods had received three scram 
signals. (Group 4 contains 33 control rods and is one of four control rod 
groups.) The first scram signal for group 4 occurred when surveillance 
testing caused a trip of RPS "B" channel. Reactor power decreased from 7% to
1.5% and the RPS "B" trip was reset by the operator. Another group 4 scram 
signal, received when an intermediate range monitor (IRM) drifted upscale, 
was reset by the operator. A third scram signal, received when the IRM 
drifted upscale again, caused the operator to initiate a manual scram. 
Subsequent investigation revealed that a relay contact (K14c) in group 4 RPS
"A" had failed open. Thus, group 4 rods received a scram signal each time 
RPS "B" was tripped. 

We note that rod group scrams of this type have been previously addressed by
the NRC (December 1, 1980 Safety Evaluation Report pages 22-24). For plants 
like Brunswick with good communication between the SDV and instrument volume
(IV), operator action is not needed to maintain scram capability. However, 
for those BWRs with poor communication between the SDV and IV, CRD seal 
leakage from the scrammed control rods (with open scram outlet valves) could
potentially result in filling the SDV before level switches in the IV 
initiate an automatic scram. In this case, timely operator action is needed 
to prevent a temporary loss of scram capability. Indications are available 
to alert the operator to scrammed CRDs and accumulation of water in the SDV.
These indications include control rod position indication, rod drift 
indication (with annunciator), high level in SDV (with annunciator), high 
level in IV (with rod block and annunciator), and change in reactor power 
with attendant indications. With current equipment and requirements (IEB 
80-17 and Confirmatory Orders) we expect the operator would initiate a 
manual scram while sufficient capacity remains in the SDV. 
.

                                                             IN 81-16      
                                                             April 23, 1981 
                                                             Page 2 of 2   

Two other operating events involved operator action following CRD system 
malfunctions not related to SDV performance. At Brunswick 1 on August 27, 
1980, both CRD pumps became inoperable due to low suction pressure caused by
high pressure drop across the suction filter coincident with low level in 
the condensate storage tank. In accordance with procedures the operator 
manually scrammed the reactor which was in startup, subcritical with some 
control rods not fully inserted, and at approximately 5 psig pressure. At 
Oyster Creek on November 30, 1980, operability of both CRD pumps was 
challenged by seal water piping leaks on each pump. This condition was 
detected and corrected by operators during routine power operation. There 
was no direct threat to loss of scram capability in this event since the 
reactor was pressurized, however, this event is of interest since similar 
failures affected both pumps. Scram capability was maintained at all times 
during both events. 

Evaluation of these two events and possible CRD system failure modes show 
the need for operator action to maintain scram capability. Under conditions 
of reactor low pressure, such as those encountered during startup, control 
rod scram capability could be lost in an event in which complete failure of 
CRD hydraulic flow occurred simultaneously with gross leakage from the scram
accumulators. The CRD pumps maintain the pressure on the accumulators and, 
provide motive force for single rod drive operations. Failure of CRD 
hydraulic flow can be caused by (1) inoperability of both CRD pumps caused 
by power failure; (2) plugging of CRD pump suction strainers; (3) lack of an
adequate condensate storage tank supply; or (4) other failures in the CRD 
hydraulic system. Scram capability under these conditions is designed to be 
provided by the scram accumulators. Extensive deterioration of the 
accumulator charging line check valves could cause a sufficient number of 
accumulators to discharge and result in a loss of scram capability if the 
operator does not take appropriate action. In the event of such multiple 
failures, reactor shutdown would have to be accomplished by use of the 
liquid control system. 

This information is provided as a notification of a possibly significant 
matter that is still under review by the NRC staff. In case the continuing 
NRC review finds that specific licensee actions would be appropriate, a 
bulletin or circular may be issued. In the interim, we expect that licensees
will review this information for applicability to their facilities paying 
particular attention to their operating procedures. The operating procedures
should include specific actions (i.e., initiation of full scram) to be taken
by the operator in response to a scram of a portion of the control rods. 
Procedures should also include the required response (i.e., again to 
initiate a scram) on recognition of loss of operability of both CRD pumps 
especially during the initial stages of plant startup when reactor pressure 
is insufficient to accomplish a scram. It is noted that current licensing 
requirements, as reflected in BWR Standard Technical Specifications, include
surveillance testing at least once every eighteen months to check the leak 
tightness of the scram accumulators to hold pressure for at least 20 
minutes. 

No written response to this information is required. If you need additional 
information regarding this matter, please contact the Director of the 
appropriate NRC Regional Office. 

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