Headquarters Daily report

                         APRIL 08, 1994

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                    REPORT             NEGATIVE            NO INPUT
                    ATTACHED           INPUT RECEIVED      RECEIVED
HEADQUARTERS        X                                      
REGION I                               X                   
REGION II                              X                   
REGION III                             X                   
REGION IV                              X                   
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PRIORITY ATTENTION REQUIRED  MORNING REPORT - REGION      APRIL  8, 1994

MR Number: H-94-0034

                            NRR DAILY REPORT ITEM
                            SIGNIFICANT EVENTS
                         EVENTS ASSESSMENT BRANCH
                    DIVISION OF OPERATING REACTOR SUPPORT

Subject: INCORRECT CONTAINMENT EMERGENCY SUMP SCREEN CONFIGURATION

The NRR/AEOD Events Assessment Panel on March 22, 1994, classified the   
containment emergency sump screen integrity issues at Arkansas Nuclear   
One as a Significant Event for the NRC Performance Indicator Program.    
                                                                         
On October 1, 1993, personnel at Arkansas Nuclear One (ANO), Unit 1,     
found several breaches in the integrity of the reactor building sump.    
These breaches consisted of 22 semicircular holes (6" x 3" scuppers) at  
the base of the sump curb; four conduit penetrations in the sump screen; 
a small conduit penetration in the sump curb; two defects in the screen  
mesh covering the sump, one L-shaped cut and one straight cut; and       
several reactor building drain headers that lacked protective screen     
material. On October 22, 1993, seven holes approximately 1" x 3" were    
found in the grout in the Unit 2 containment sump curb.                  
                                                                         
Although these containment sumps had been inspected on several occasions,
the breaches of the sump screens and curbs and lack of reactor building  
drain header screens had gone unnoticed. The licensee had focused its    
inspections on debris and did not question the existence of penetrations 
through the screen and curb. In Unit 1, the scuppers were part of the    
original design and, while shown on plant drawings, the drawings were    
unclear with respect to screen installation. In Unit 2, the holes in the 
grout at the base of the containment sump curb were not identified in the
design nor on drawings. These holes in Unit 2 were only visible after    
close examination because of the construction of the sump curb. The      
penetrations in the curb and cuts in the screen were not recognized as a 
problem until an electrician questioned the existence of the penetrations
while working in the Unit 1 containment.                                 
                                                                         
On December 3, 1993, the NRC issued Information Notice 89-77, Supplement 
1, "Debris in Containment Emergency Sumps and Incorrect Screen           
Configurations," to inform licensees of the problems at ANO. On December 
14, 1993, the NRC issued a Severity Level III Notice of Violation for    
failure to assure that the reactor building sump screens were designed,  
installed, and maintained in accordance with design specifications       
described in the Final Safety Analysis Reports. No civil penalty was     
proposed due to mitigative factors, i.e., self-identification of the     
problem, prompt and comprehensive corrective actions, and past evidence  
of good performance in identifying and correcting problems.              
                                                                         
Based on preliminary information from the licensee and screening values  
from the Accident Sequence Precursor (ASP) models for human error        
probabilities, the increase in core damage frequency (CDF) is estimated  
to be approximately 7 x 10-5/year for Unit 1, and 2 x 10-5/year for Unit 
2. The Unit 1 estimation assumes that the presence of holes in the sump  
screens and curb would lead to plugging the decay heat removal (DHR) heat
exchanger outlet valves, which, the licensee reports, have a restrictive 
labyrinth flow path. This assumes that other available pathways would not
become plugged, due to more favorable flow path internal geometry, and   
that the DHR pumps would not be damaged by passage of insulation, or by  
deadheading when their discharge paths became plugged. Although the      
inadequate sump screening condition has existed for an extended period,  
fibrous insulation has been present in the reactor building only since   
May 1992. Therefore the total probability of core damage for the         
applicable period would be about 1 x 10-4. The Unit 2 estimation assumes 
that the presence of the holes in the containment sump curb would result 
in a failure rate of 0.05 for each high pressure injection pump due to   
ingestion of debris during the recirculation mode for all loss of coolant
accidents. The inadequate sump screening condition has apparently existed
since the reactor entered commercial service about 15 years ago.         
Therefore, the total incremental risk of core damage over that period is 
about 3 x 10-4.                                                          
                                                                         
CONTACT:    Eric J. Benner, NRR/DORS/OEAB                                
            (301) 504-1171