United States Nuclear Regulatory Commission - Protecting People and the Environment

Steam Generator Overfill (Generic Letter 81-28)



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

                                July 31, 1981

ALL LICENSEES OF OPERATING PLANTS AND HOLDERS OF CONSTRUCTION PERMITS 

Gentlemen: 

SUBJECT:  STEAM GENERATOR OVERFILL (GENERIC LETTER 81-28) 
          (Formerly Issued July 1, 1981, as Generic Letter 81-16) 

In a letter dated March 28, 1980 from H.R. Denton, we informed you of the 
revised criteria to be used by the staff in evaluating reactor operator 
training and licensing that could be implemented under the current 
regulations. We also advised you that Commission review in the area of 
operator training and qualification was continuing and it could be expected 
to result in additional criteria. 

The NRC Office of Analysis and Evaluation of Operational Data has produced a 
report entitled, "AEOD Observations and Recommendations Concerning the 
Problem of Steam Generator Overfill and Combined Primary and Secondary Side 
Blowdown," dated December 17, 1980, a copy of the report is enclosed. This 
report documents results of studies completed to data by the Office of 
Analysis and Evaluation of Operational Data with regard to the team 
generator overfill problem. 

This report expresses concerns in the following area: (1) increased dead 
weight and potential seismic loads placed on the main steamline and its 
support should this line become flooded; (2) the load placed on the main 
steamlines due to the potential for rapid collapse of steam voids resulting 
in water hammer; (3) the potential for secondary safety valves sticking open
following discharge of water or two-phase flow; (4) the potential for 
rupture for weakened tubes in the once-through-steam-generator (OTSG) on B&W 
NSSS plants due to tensile loads caused by the rapid thermal shrinkage of 
the tubes relative to the generator shell. 

From the examining experiences of the Operator Licensing Branch, operators 
at nuclear power plants are aware of the need to avoid overfilling steam 
generators and not operating steam systems with water accumulation. However,
there may be a general lack of appreciation of the potential seriousness of 
situations that can arise from these events. 
.

                                    - 2 -                     July 1, 1981 

While this issue is being studied further, we request that you determine 
which scenarios are credible for your plant and that you include in your 
overall training program, plant-specific information stressing the 
importance of feedwater flow as well as the possible consequences of steam 
generator overfill. This information should be factored into your initial 
operator training programs and the operator requalification programs. 

                              Sincerely, 


                              Darrell G. Eisenhut, Director 
                              Division of Licensing 
                              Office of Nuclear Reactor Regulation 

Enclosure:
As stated
Page Last Reviewed/Updated Monday, June 17, 2013