Information Notice No. 82-51: Overexposures in PWR Cavities

                                                       SSINS No.: 6835  
                                                       IN 82-51 

                               UNITED STATES 
                           WASHINGTON, D.C. 20555 

                             December 21, 1982 

Information Notice No. 82-51:   OVEREXPOSURES IN PWR CAVITIES 


All nuclear power plant facilities holding an operating license (OL) or 
construction permit (CP). 


This information notice is provided as notification of a significant, 
whole-body exposure in excess of regulatory limits to a shift engineer while
inspecting for water leaks into the cavity beneath the reactor vessel 
(hereafter called reactor cavity). This is the sixth overexposure since 1972
that has occurred under similar circumstances; consequently, each licensed 
senior reactor operator (SRO) should be provided a copy of this information 
notice. SROs are specified because they frequently make the decision whether
a cavity entry is needed and they supervise facility operations. 

Although the radiation doses received by personnel in this and previous 
incidents have not been greater than about 10 rems, extremely high radiation
fields are created in reactor cavities by withdrawn, irradiated incore 
instrumentation thimbles. Radiation levels of thousands of roentgens per 
hour (R/hr) are possible and, in at least one of the incidents, an 
individual entered a field of at least 2000 R/hr. Entry into radiation 
fields of this magnitude seriously jeopardizes the health and safety of 
personnel. All SROs should be cognizant of this information. 

It is expected that licensees will review the information for applicability 
to their facilities. No specific action or response is required at this 

Description of Circumstances: 

Commonwealth Edison's Zion Unit 1 was in cold shutdown for refueling and 
maintenance. Incore instrumentation thimble retraction started during the 
evening shift on March 23, 1982, and was completed about six hours later at 
approximately 0400 hours on March 24. The governing maintenance procedure 
for retracting and inserting incore instrumentation thimbles required that 
all access doors to the reactor cavity be locked and all incore detectors be 
in the storage position before the thimbles were retracted. Control of keys 
to the locks was administratively assigned to the shift engineer on duty. 

After thimble retraction was completed on March 24, the licensee began to 
flood the refueling cavity in preparation for refueling. At about 1030 
hours, it was determined that the water level in the refueling cavity was 
decreasing. At about noon, a shift foreman entered the reactor cavity in an 
effort to 


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locate the leakage source. The shift foreman saw that the leakage was 
massive. The licensee decided to lower the water in the refueling cavity, 
reinstall the reactor vessel head, and investigate the leakage source. At 
about 2300 hours, the licensee found an excore nuclear instrumentation cover
gasket had slipped and was apparently the cause of the leak. 

After the gasket was replaced, the licensee raised the vessel head and 
partially flooded the refueling cavity. At about 1800 hours on March 25, the
shift engineer entered the reactor cavity to determine if there was further 
leakage. During this entry which only took about 70 seconds, the shift 
engineer received a whole-body radiation dose of approximately five rem. 

"Power Reactor Events", Vol. 4, No. 4, published in November, 1982 describes
the event at Zion in more specific detail." 

The Zion overexposure resulted from failure to follow good radiation 
protection practices and programmatic weaknesses in the radiation protection
program. The following specific weaknesses contributed to the overexposure: 

1.   Failure of Shift Operations Personnel in Leadership Positions to 
     Exhibit Good Radiation Protection Practices 

     Shift operations personnel in leadership positions failed to exhibit 
     good radiation practices. On March 24, a shift foreman entered the 
     unsurveyed cavity area without observing survey instrument readings 
     until he had, descended the cavity ladder; at the bottom of the ladder 
     (a 50 R/hr radiation field) the shift foreman noted his survey 
     instrument was offscale, high. On March 25, a shift engineer entered an
     unsurveyed area (moved closer to the bottom of the reactor vessel) 
     fully aware that exposure rates would increase significantly as he 
     approached the reactor vessel. Of all the personnel directly involved 
     in the two cavity entries, these two managers were the most 
     knowledgeable of the specific cavity radiological hazards. 

2.   Lack of Preplanning and Communication Among Individuals and Work Groups

     There was a lack of preplanning and briefing of all participants prior 
     to the start of the job. No Radiological Work Permit (RWP) was 
     completed which could have: defined the intended actions (the shift 
     engineer went further into the cavity than expected); communicated the 
     "stay time" allowed (the shift engineer was not told his "stay time"); 
     assured that precautions were identified (the plant health physicist 
     and radiation/chemistry foreman each assumed the other had discussed 
     precautions with the shift engineer); and provided for proper equipment 
     (the shift engineer only had a film badge and a [0-200 mr] self-reading 
     pocket dosimeter). Under Zion's procedures an RWP was not required 
     since a radiation/chemistry technician (RCT) was to provide continuous 
     job coverage. 

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3.   Lack of Understanding by Radiation Protection Personnel of Reactor 
     Cavity Radiological Hazards 

     The RCT and foremen involved had a general lack of understanding of the
     reactor cavity's specific radiation hazards. The RCT and RCT trainees 
     providing job coverage for the cavity entries were not familiar with 
     the nature and strength of the radiation sources present with the 
     incore thimbles withdrawn. RCT training prior to the overexposure 
     described reactor cavity hazards only in general terms, with no 
     specific description of the radiation sources or the expected exposure 
     rates. The RCT thought the radiation source strength was uniformly 
     distributed along the length of the incore tubes (which run the entire 
     length of the reactor cavity); thus, the RCT, did not warn the engineer 
     to stop advancing into higher radiation fields. 

As a result of this event, the licensee has initiated certain corrective 
actions which include the following: 

1.   The incore thimbles will be required to be re-inserted prior to 
     personnel entry into the reactor cavity area beyond the base of the two
     ladders extending down to the reactor cavity area. A special lock will 
     be placed on the door to the reactor cavity when the thimbles are 
     removed and the procedure for insertion and withdrawal of the incore 
     thimbles will be revised to include a sign off for rad-chem department 
     notification. An information sign showing the locations of the thimbles
     and incore detectors during outages will be posted in the rad-chem 

2.   All jobs where greater than 50 mrem could be received will require an 

3.   Operator and radiation department training and retraining programs will
     include special emphasis on the incore instrumentation system and its 
     radiations hazards. In addition, the licensee will evaluate the need to
     upgrade the radiation department training program on plant systems and 
     their operations. 

4.   The licensee will evaluate other major problem areas where a potential 
     for high radiation doses exist. 


Since 1972 there have been five other overexposures and one near 
overexposure (see Table 1) associated with individuals entering the reactor 
cavity where extremely high radiation dose rates have been present. The 
purpose of most of these entries was to check for water leakage while 
filling the refueling pool. The major causes of these overexposures were 
nearly identical in every case and included: (1) failure of operations 
personnel in leadership positions to follow good radiation protection 
practices, (2) inadequate preplanning for the entries including a breakdown 
in communication among Health Physics (HP) and Operations Groups, and (3) 
inadequate training of the HP technicians in the radiological aspects of the 
incore detection system operation and anticipated dose rates in the cavity. 

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In September of 1976, the NRC issued IE Circular No. 76-03 "Radiation 
Exposures in Reactor Cavities" which described the first three events listed
in Table 1 and specified controls to be implemented to prevent their 
recurrence. Licensees' responses to that circular, including improved 
training of the staff and establishment of administrative controls, have not
been totally effective as indicated by the four subsequent similar 

A particular concern of the NRC is that the person charged with the 
responsibility for implementing these controls, the Shift Supervisor, has 
frequently been the individual directly involved. Three of the four exposure
incidents that have occurred since the issuance of Circular 76-03 have 
resulted from Shift Supervisors entering the cavity to check for leaks. 

It appears that Shift Supervisors and other licensed senior reactor 
operators should exert greater control over reactor cavity entries if 
serious overexposures are to be avoided. Therefore, each licensee and CP 
holder is requested to provide a copy of this information notice to each 
licensed senior reactor operator. 

A civil penalty of $100,000 was proposed and was paid by Commonwealth Edison
following the March 1982 event. NRC considers continued overexposures to be 
unacceptable and intends to apply its full enforcement authority for future 

If you need additional information about this matter, please contact the 
Regional Administrator of the appropriate NRC Regional Office or this 

                              Richard C. DeYoung, Director 
                              Office of Inspection and Enforcement 

Technical Contacts: R. Pedersen, NRR 
                    (301) 492-7541 

                    J. Wigginton, IE 
                    (301) 492-4967 

1. Table 1 
2. List of Recently Issued IE Information Notices 
                                                           Attachment 1 
                                                           IN 82-51  
                                                           December 21, 1982
                                 TABLE 1 

DATE                          PLANT                              DOSE 

October 1972                  Point Beach                        5 rems 

March 1976                    Zion                               8 rems 

April 1976                    Indian Point                       10 rems 

May 1978                      Kewaunee                           2.8 rems* 

April 1979                    Surry 2                            10 rems 

April 1980                    Davis Besse                        5 rems 

March 1982                    Zion                               5 rems 

*Near overexposure 


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