Installation of a Hardened Wetwell Vent (Generic Letter 89-16)

.                                September 1, 1989



As a part of a comprehensive plan for closing severe accident issues, the 
staff undertook a program to determine if any actions should be taken, on a 
generic basis, to reduce the vulnerability of BWR Mark I containments to 
severe accident challenges.  At the conclusion of the Mark I Containment 
Performance Improvement Program, the staff identified a number of plant 
modifications that substantially enhance the plants' capability to both 
prevent and mitigate the consequences of severe accidents.  The improvements 
that were recommended include (1) improved hardened wetwell vent capability, 
(2) improved reactor pressure vessel depressurization system reliability, (3) 
an alternative water supply to the reactor vessel and drywell sprays, and (4) 
updated emergency procedures and training.  The staff as part of that effort 
also evaluated various mechanisms for implementing of these plant improvements 
so that the licensee and the staff efforts would result in a coordinated 
coherent approach to resolution of severe accident issues in accordance with 
the Commission's severe accident policy.  

After considering the proposed Mark I Containment Performance Program 
(described in SECY 89-017, January 1989), the Commission directed the staff to 
pursue Mark I enhancements on a plant-specific basis in order to account for 
possible unique design differences that may bear on the necessity and nature 
of specific safety improvements.  Accordingly, the Commission concluded that 
the recommended safety improvements, with one exception, that is, hardened 
wetwell vent capability, should be evaluated by licensees as part of the 
Individual Plant Examination (IPE) Program.  With regard to the recommended 
plant improvement dealing with hardened vent capability, the Commission, in 
recognition of the circumstances and benefits associated with this 
modification, has directed a different approach.  Specifically, the Commission 
has directed the staff to approve installation of a hardened vent under the 
provisions of 10 CFR 50.59 for licensees, who on their own initiative, elect 
to incorporate this plant improvement.  The staff previously inspected the 
design of such a system that was installed by Boston Edison Company at the 
Pilgrim Nuclear Power Station.  The staff found the installed system and the 
associated Boston Edison Company's analysis acceptable. 

A copy of Boston Edison Company's description of the vent modification is 
enclosed for your information.  For the remaining plants, the staff has been 
directed to initiate plant-specific backfit analyses for each of the Mark I 
plants to evaluate the efficacy of requiring the installation of hardened 
wetwell vents.  Where the backfit analysis supports imposition of that 
requirement, the staff is directed to issue orders for modifications to 
install a reliable hardened vent.

.Generic Letter 89-16                    -2-                 September 1, 1989

The staff believes that the available information provides strong incentive 
for installation of a hardened vent.  First, it is recognized that all 
affected plants have in place emergency procedures directing the operator to 
vent under certain circumstances (primarily to avoid exceeding the primary 
containment pressure limit) from the wetwell airspace.  Thus, incorporation of 
a designated capability consistent with the objectives of the emergency 
procedure guidelines is seen as a logical and prudent plant improvement.  
Continued reliance on pre-existing capability (non-pressure-bearing vent path) 
which may jeopardize access to vital plant areas or other equipment is an 
unnecessary complication that threatens accident management strategies.  
Second, implementation of reliable venting capability and procedures can 
reduce the likelihood of core melt from accident sequences involving loss of 
long-term decay heat removal by about a factor of 10.  Reliable venting 
capability is also beneficial, depending on plant design and capabilities, in 
reducing the likelihood of core melt from other accident initiators, for 
example, station blackout and anticipated transients without scram.  As a 
mitigation measure, a reliable wetwell vent provides assurance of pressure 
relief through a path with significant scrubbing of fission products and can 
result in lower releases even for containment failure modes not associated 
with pressurization (i.e., liner meltthrough).  Finally, a reliable hardened 
wetwell vent allows for consideration of coordinated accident management 
strategies by providing design capability consistent with safety objectives.  
For the aforementioned reasons, the staff concludes that a plant modification 
is highly desirable and a prudent engineering solution of issues surrounding 
complex and uncertain phenomena.  Therefore, the staff strongly encourages 
licensees to implement requisite design changes, utilizing portions of 
existing systems to the greatest extent practical, under the provisions of 
10 CFR 50.59.

As noted previously, for facilities not electing to voluntarily incorporate 
design changes, the Commission has directed the staff to perform 
plant-specific backfit analyses.  In an effort to most accurately reflect 
plant specificity, the staff herein requests that each licensee provide cost 
estimates for implementation of a hardened vent by pipe replacement, as 
described in SECY 89-017.  In addition, licensees are requested to indicate 
the incremental cost of installing an ac independent design in comparison to a 
design relying on availability of ac power.  In the absence of such 
information, the staff will use an estimate of $750,000.  This estimate is 
based on modification of prevalent existing designs to bypass the standby gas 
treatment system ducting and includes piping, electrical design changes, and 
modifications to procedures and training.

The NRC staff requests that each licensee with a Mark I plant provide 
notification of its plans for addressing resolution of this issue.  If the 
licensee elects to voluntarily proceed with plant modifications, it should be 
so noted, along with an estimated schedule, and no further information is 
necessary.  Otherwise, the NRC staff requests that the above cost information 
be provided.  In either event, it requests that each licensee respond within 
45 days of receipt of this letter.

.Generic Letter 89-16                    -3-                 September 1, 1989

This request is covered by Office of Management and Budget Clearance Number 
3150-0011, which expires December 31, 1989.  The estimated average burden 
hours are 100 person hours per licensee response, including searching data 
sources, gathering and analyzing the data, and preparing the required letters.  
These estimated average burden hours pertain only to the identified 
response-related matters and do not include the time for actual implementation 
of the requested actions.  Send comments regarding this burden estimate or any 
other aspect of this collection of information, including suggestions for 
reducing this burden, to the Record and Reports Management Branch, Division of 
Information Support Services, Office of Information Resources Management, U.S. 
Nuclear Regulatory Commission, Washington, D.C.  20555; and to the Paperwork 
Reduction Project (3150-0011), Office of Management and Budget, Washington, 
D.C. 20503.

If you have any questions regarding this matter, please contact the NRC Lead 
Project Manager, Mohan Thadani, at (301) 492-1427.


                                   James G. Partlow 
                                   Associate Director for Projects 
                                   Office of Nuclear Reactor Regulation

1.  Description of Vent Modification at the Pilgrim Nuclear Power Station
2.  List of Most Recently Issued Generic Letters


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