Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
. September 1, 1989
TO: ALL HOLDERS OF OPERATING LICENSES FOR NUCLEAR POWER REACTORS
WITH MARK I CONTAINMENTS
SUBJECT: INSTALLATION OF A HARDENED WETWELL VENT (GENERIC LETTER 89-16)
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.
8909010375
.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.
Sincerely,
James G. Partlow
Associate Director for Projects
Office of Nuclear Reactor Regulation
Enclosures:
1. Description of Vent
Modification at the Pilgrim
Nuclear Power Station
2. List of Most Recently
Issued Generic Letters
