IE Circular No. 78-06, Potential Common Mode Flooding of ECCS Equipment Rooms at BWR Facilities

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

IE Circular No. 78-06
Date: May 31, 1978
Page 1 of 2


Georgia Power Company (GPC), by letter dated March 22, 1978, reported that 
during a review of the design of the ECCS corner room equipment drain
systems  at the Hatch facilities, the equipment drain lines for Hatch 1
were found  piped into the top of an open 6-inch pipe (standpipe) in each
corner room.  These corner room standpipes are interconnected by a 6-inch
pipe to a common  sump in one corner room. This sump has redundant pumps
which are powered from  different emergency buses. The equipment drain
system does not have any  isolation valves within any of the piping
between corner rooms. Therefore, in  the event of a pipe break of
sufficient size to flood sump pumps, the other  ECCS corner rooms would
be subject to flooding via the equipment drain piping.

Since one of the reasons for installing the ECCS equipment in separate, 
watertight rooms was to eliminate the potential for common flooding, GPC
is  "hard piping" all of the equipment drain lines into the side of the
corner  room standpipe and capping the standpipe. This limits the size of
the  potential flooding backflow to the small size (approximately 1/2")
piping of  the equipment leakoff drain line.

The above described design for Hatch 1 and 2 was part of the A-E balance-
of-plant design provided by Bechtel.

All holders of reactor operating licenses should assure that similar
pathways,  which could lead to common flooding of redundant equipment, do
not exist in  their facilities. It is recommended that the following items
be considered in  your review of this matter:

1. The specific design and installation for floor and equipment drains
should  be reviewed to verify that a flood in any one room or location
would not  result in flooding redundant equipment in other ECCS equipment
rooms or in  areas at a lower elevation.


IE Circular No. 78-06 
Date: May 31, 1978 
Page 2 of 2

2. If your installation depends on isolation valves and limit switches to 
prevent common flooding, your surveillance program should be reviewed to 
verify adequate maintenance of protection. Your Technical Specifications 
should be reviewed to assure that necessary surveillance of isolation
valves,  level switches, or sump pumps are included as appropriate.

3. Your administrative controls should also be reviewed to assure that 
separation criteria is maintained and watertight doors and/or hatches are 
closed as appropriate.

This Circular is also issued to holders of construction permits for 
information or use as appropriate to the status of plant construction.

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


Page Last Reviewed/Updated Thursday, March 25, 2021