[Federal Register Volume 82, Number 137 (Wednesday, July 19, 2017)]
[Notices]
[Pages 33161-33165]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 2017-15136]
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NUCLEAR REGULATORY COMMISSION
[Docket Nos. 50-498 and 50-499; NRC-2016-0092]
STP Nuclear Operating Company; South Texas Project, Units 1 and 2
AGENCY: Nuclear Regulatory Commission.
ACTION: Exemption; issuance.
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SUMMARY: The U.S. Nuclear Regulatory Commission (NRC) is granting
exemptions from certain portions of the acceptance criteria for
emergency core cooling, and the general design criteria for emergency
core cooling, containment heat removal, and atmosphere cleanup for the
use of a risk-informed analysis to evaluate the effects of debris in
containment following a loss-of-coolant accident (LOCA) for the South
Texas Project (STP), Units 1 and 2, located in Matagorda County, Texas,
Docket Nos. 50-498 and 50-499, respectively. The exemptions are in
response to a request dated June 19,
[[Page 33162]]
2013, from the STP Nuclear Operating Company (STPNOC, the licensee)
related to STPNOC's proposed approach to resolve a generic safety
concern for pressurized water reactors (PWRs) associated with potential
clogging of emergency core cooling and containment spray system
strainers during certain design basis events.
DATES: The exemption was issued on July 11, 2017.
ADDRESSES: Please refer to Docket ID NRC-2016-0092 when contacting the
NRC about the availability of information regarding this document. You
may obtain publicly-available information related to this document
using any of the following methods:
Federal Rulemaking Web site: Go to http://www.regulations.gov and search for Docket ID NRC-2016-0092. Address
questions about NRC dockets to Carol Gallagher; telephone: 301-415-
3463; email: [email protected]. For technical questions, contact
the individual listed in the FOR FURTHER INFORMATION CONTACT section of
this document.
NRC's Agencywide Documents Access and Management System
(ADAMS): You may obtain publicly-available documents online in the
ADAMS Public Documents collection at http://www.nrc.gov/reading-rm/adams.html. To begin the search, select ``ADAMS Public Documents'' and
then select ``Begin Web-based ADAMS Search.'' For problems with ADAMS,
please contact the NRC's Public Document Room (PDR) reference staff at
1-800-397-4209, 301-415-4737, or by email to [email protected]. For
the convenience of the reader, the ADAMS accession numbers are provided
in a table in the ``Availability of Documents'' section of this
document.
NRC's PDR: You may examine and purchase copies of public
documents at the NRC's PDR, Room O1-F21, One White Flint North, 11555
Rockville Pike, Rockville, Maryland 20852.
FOR FURTHER INFORMATION CONTACT: Lisa Regner, Office of Nuclear Reactor
Regulation, U.S. Nuclear Regulatory Commission, Washington DC 20555-
0001; telephone: 301-415-1906, email: [email protected].
SUPPLEMENTARY INFORMATION:
I. Background
The licensee is the holder of Facility Operating License Nos. NPF-
76 and NPF-80, which authorize operation of the STP Units 1 and 2,
respectively. The licenses provide, among other things, that the
facility is subject to all rules, regulations, and orders of the NRC
now or hereafter in effect. The facility consists of two PWRs located
in Matagorda County, Texas.
In 1996, the NRC identified Generic Safety Issue (GSI)-191
associated with the effects of debris accumulation on PWR sump
performance during design-basis accidents. As part of the actions to
resolve GSI-191, the NRC issued Generic Letter (GL) 2004-02,
``Potential Impact of Debris Blockage on Emergency Recirculation during
Design Basis Accidents at Pressurized-Water Reactors,'' dated September
13, 2004, to holders of operating licenses for PWRs. In GL 2004-02, the
NRC staff requested that licensees perform an evaluation of their
emergency core cooling systems (ECCS) and containment spray system
(CSS) recirculation functions considering the potential for debris-
laden coolant to be circulated by the ECCS and the CSS after a LOCA or
high energy line break inside containment and, if appropriate, take
additional actions to ensure system function. The GL required that
licensees provide a written response to the NRC, pursuant to section
50.54(f) of title 10 of the Code of Federal Regulations (10 CFR),
describing the results of their evaluation and any modifications made,
or planned, to ensure the ECCS and CSS remain functional.
II. Request/Action
By letter dated June 19, 2013, as supplemented by letters dated
August 20, 2015, and April 13, 2016, STPNOC submitted requests for
exemptions pursuant to 10 CFR 50.12, ``Specific exemptions,'' from the
requirements of 10 CFR 50.46, ``Acceptance criteria for emergency core
cooling systems for light-water nuclear power reactors,'' and 10 CFR
part 50, appendix A, General Design Criterion (GDC) 35, ``Emergency
core cooling,'' GDC 38, ``Containment heat removal,'' and GDC 41,
``Containment atmosphere cleanup,'' to use a risk-informed methodology
instead of the traditional deterministic methodology, to resolve the
concerns associated with GSI-191 and respond to GL 2004-02.
Specifically, the licensee requested an exemption from 10 CFR
50.46(a)(1)(i), which, in part, requires ECCS cooling performance to be
calculated in accordance with an acceptable evaluation model, as
described in 10 CFR 50.46(a)(1), for postulated LOCAs of different
sizes, locations and other properties sufficient to provide assurance
that the most severe LOCAs are evaluated in order to demonstrate that
acceptance criteria in 10 CFR 50.46(b) are met. The NRC staff
interprets 10 CFR 50.46(a)(1) requirement to calculate ECCS performance
for ``other properties'' as requiring licensees to consider the impacts
of debris generation and transport in containment. The most significant
form of debris in nuclear power reactor containments is piping and
component insulation that becomes debris during LOCAs, is transported
and accumulates in the sumps, and clogs the sumps strainers, thus
creating resistance to coolant flow. Fibrous debris from this
insulation can also enter the reactor core and directly impede heat
transfer from the fuel to the coolant. The licensee also requested
exemptions from GDC 35, which contain ECCS performance requirements,
and GDCs 38 and 41, which respectively set performance requirements for
reactor containment heat removal following a LOCA and for containment
atmosphere cleanup following postulated accidents.
The approval of a risk-informed methodology would require
exemptions from 10 CFR 50.46(a)(1)(i) and GDCs 35, 38, and 41 because
the NRC has interpreted these regulations as requiring a deterministic
approach and bounding calculation to show compliance with ECCS and CSS
performance criteria in 10 CFR 50.46(b) and GDCs 35, 38 and 41.
Issuance of exemptions is an appropriate means to grant relief from the
use of a deterministic approach to show compliance with these
requirements.
The licensee's 10 CFR 50.46 deterministic analysis considered the
debris in containment and demonstrated that the debris loading could
prevent acceptable ECCS and CSS operation and core cooling for certain
pipe ruptures. Based on its analysis, the licensee concluded that the
amount of debris in the STP containment would need to be reduced to
demonstrate compliance with 10 CFR 50.46 criteria using a deterministic
analysis for certain large-break LOCA sizes because, for those breaks,
the plant-specific testing threshold for generation and transport of
debris was exceeded.
Additionally, the licensee's deterministic thermal-hydraulic (TH)
analysis could not show that hot-leg LOCAs greater than 16 inches could
maintain adequate cooling. While not all large-break hot-leg LOCAs
resulted in a loss of in-core cooling due to strainer blockage, the
licensee categorized all hot-leg breaks greater than 16 inches as
assumed to fail in order to simplify the TH analysis.
The licensee requested exemptions from the requirement to use a
deterministic analysis for specific scenarios of LOCA breaks producing
and transporting debris in excess of the plant-specific tested debris
limits and
[[Page 33163]]
for large hot-leg breaks. Since it determined that the probability of
consequences from debris effects is very low, the licensee requested an
exemption to use a risk-informed analysis to show adequate assurance of
ECCS and CSS functionality, in accordance with the criteria in
Regulatory Guide (RG) 1.174, ``An Approach for Using Probabilistic Risk
Assessment in Risk-Informed Decisions on Plant-Specific Changes to the
Licensing Basis.'' The RG 1.174 was developed in consideration of the
Commission's Policy Statements on safety goals and the use of
probabilistic risk assessment methods in nuclear regulatory activities
(``Safety Goals for the Operations of Nuclear Power Plants; Policy
Statement,'' August 4, 1986, 51 FR 30028; and ``Use of Probabilistic
Risk Assessment Methods in Nuclear Activities; Final Policy
Statement,'' August 16, 1995, 60 FR 42622, respectively). Therefore, RG
1.174 provides an acceptable method for licensees and NRC staff to use
in assessing the impact of licensing basis changes when the licensee
chooses to use risk information.
The GDC 35, in part, requires that the ECCS safety system functions
adequately to transfer heat from the reactor core following a LOCA and
in the presence of a worst single failure, at a rate such that (a) fuel
and clad damage that could interfere with continued effective core
cooling is prevented and (b) clad metal-water reactor is limited to
negligible amounts. The licensee stated in its submittal that the
function of the ECCS emergency sump is assumed to fail for debris that
exceeds the amount determined in acceptable plant-specific testing.
Failure of the sump and strainers result in loss of cooling to the
core. The licensee requested an exemption from the deterministic
requirements of GDC 35 to use a risk-informed approach to show ECCS
function for those LOCA breaks that exceed the plant-specific testing
debris threshold, and for large hot-leg breaks. The use of a risk-
informed analysis, in accordance with the criteria in RG 1.174, would
allow the licensee to show that the risk from debris effects is very
low.
The GDC 38 requires containment heat removal, rapid reduction of
containment pressure and temperature, and maintenance of pressure and
temperature at an acceptably low level following a LOCA, and in the
presence of a single failure, to preserve containment function. The
STPNOC proposed that an exemption be granted from the deterministic
requirements in GDC 38, for those LOCA breaks that exceed the plant-
specific testing debris threshold. Current STP design basis
calculations are based on the reactor containment fan coolers
functioning in conjunction with the CSS and ECCS, both of which can be
affected by debris. Using deterministic assumptions, STPNOC's analysis
and testing does not assure that the emergency sump strainers will be
available to support the CSS and ECCS function considering the effects
of debris produced by those breaks that can generate and transport
debris amounts greater than the plant-specific testing threshold. The
licensee requested an exemption from the deterministic requirements of
GDC 38 to use a risk-informed analysis, in accordance with the criteria
in RG 1.174, to show that the risk from debris effects is very low.
The GDC 41, in part, requires containment atmosphere cleanup to
control substances that may be released into the reactor containment,
to reduce the concentration and quality of fission products released to
the environment following postulated accidents, and to control the
concentration of hydrogen or oxygen and other substances in the
containment atmosphere following postulated accidents, assuming a
single failure. The licensee stated that using deterministic
assumptions, STPNOC's analysis and testing cannot demonstrate that the
emergency sump strainers will be available to support the CSS function
considering the effects of debris produced and transported by breaks
not bounded by acceptable plant-specific testing. The licensee
requested an exemption from the deterministic requirements of GDC 41 to
use a risk-informed analysis, in accordance with the criteria in RG
1.174, to show that the risk from debris effects is very low.
III. Discussion
Pursuant to 10 CFR 50.12, the Commission may, upon application by
any interested person or upon its own initiative, grant exemptions from
the requirements of 10 CFR part 50, when (1) the exemptions are
authorized by law, will not present an undue risk to public health or
safety, and are consistent with the common defense and security; and
(2) when special circumstances are present. Under 10 CFR
50.12(a)(2)(ii), special circumstances are present ``when application
of the regulation in the particular circumstances would not serve the
underlying purpose of the rule or is not necessary to achieve the
underlying purpose of the rule.''
The licensee proposed to use a risk-informed methodology instead of
a deterministic approach to account for the effects of debris in
containment for portions of the LOCA analysis applicable to breaks that
exceed the STP plant-specific debris testing threshold and large hot-
leg piping breaks. The STPNOC methodology, termed Risk over
Deterministic, or RoverD, divides the loss of core cooling design-basis
analysis into two portions: the ``deterministic analysis'' and the
``risk-informed analysis.'' The risk-informed analysis is used by the
licensee for breaks that generate and transport debris exceeding the
plant-specific testing threshold. These breaks result in low density
fiber glass fiber fines estimated to arrive in the ECCS sump post-LOCA
in amounts that are equal to or greater than the amount of fines used
in acceptable strainer testing. The acceptable limit was determined
using testing methods intended to determine the maximum ECCS strainer
head loss for the tested condition.
Also, the licensee evaluated the in-core TH aspects of fibrous
debris to prevent adequate fuel cooling, finding that hot-leg breaks
greater than 16 inches have the potential to prevent adequate in-core
cooling. In order to simplify its TH evaluation, the licensee assumed
that all large breaks greater than 16 inches in the hot-leg will result
in the loss of the cooling function. For ECCS and CSS analyses other
than the postulated large-break LOCAs in the hot-leg piping in
containment and those breaks that exceed the STP plant-specific testing
limit, STPNOC applied a deterministic methodology. If the exemptions
were granted for these postulated breaks, the requirement to use a
deterministic methodology for all other postulated LOCA breaks would
continue to apply.
A. Special Circumstances
Under the regulations in 10 CFR 50.12, the Commission may grant
exemptions from the requirements of 10 CFR part 50 provided certain
findings are made; namely, that special circumstances are present, the
exemptions present no undue risk to public health and safety, the
exemptions are consistent with the common defense and security, and the
exemptions are authorized by law. The exemptions would allow the
licensee to use a risk-informed methodology to show compliance with 10
CFR 50.46(b), and GDCs 35, 38, and 41, specifically for the analyses of
debris in containment impacting emergency cooling function during
postulated large-break hot-leg LOCAs and those breaks that exceed the
plant-specific testing threshold.
The licensee requested exemptions citing the special circumstances
criteria
[[Page 33164]]
of 10 CFR 50.12(a)(2)(ii), because compliance in the particular
circumstances would not serve the underlying purpose of the rule or is
not necessary to achieve the underlying purpose of the rule. The
licensee stated that these special circumstances are common to all of
the requested exemptions.
The licensee stated that an objective of each of the regulations
for which an exemption is proposed is to maintain low risk to the
public health and safety through the adequate functioning of the ECCS
and CSS safety systems. These systems must be supported by adequate
functioning of the containment sumps. The regulations in 10 CFR
50.46(a)(1)(i) and GDCs 35, 38, and 41 are met when the licensee is
able to demonstrate, using a bounding calculation or other
deterministic method that the ECCS and CSS are capable of functioning
during design basis events. The STPNOC stated that its risk-informed
analysis to show adequate functioning of ECCS and CSS considering the
impacts of debris during certain LOCA events demonstrates that the risk
of failure of these systems is very small. The licensee stated that
special circumstances exist because the underlying intent of the
regulations, to ensure adequate protection of public health and safety
is met when applying a risk-informed approach to address GSI-191 and
respond to GL 2004-02. Further, it states that the risk-informed
approach is consistent with RG 1.174, and supports operation of those
functions with a high degree of reliability. Thus, the licensee
concludes that the underlying intent of each regulation is met and the
special circumstances described in 10 CFR 50.12(a)(2)(ii) apply to each
of the exemptions proposed by STPNOC.
The NRC staff evaluated the STPNOC submittal and supplements, and
discussed the details of its evaluation of the risk-informed approach
in an NRC safety evaluation available in ADAMS under Accession No.
ML17019A001. Although 10 CFR 50.46(a)(1) requires a deterministic
approach, the GDCs do not specify that a risk-informed methodology may
not be used to show compliance; however, because the NRC has
interpreted each of these regulations as requiring a deterministic
approach, an exemption is an appropriate means to grant the licensee
relief to use an alternative approach. The underlying purpose of each
regulation is to protect public health and safety in the event of a
LOCA by establishing criteria for emergency core cooling, containment
cooling and containment atmosphere cleanup system performance. In its
safety evaluation, the NRC staff concluded, in part, that the licensee
adequately demonstrated that the change in risk attributable to debris
in postulated hot-leg LOCAs greater than 16 inches, and those breaks
that exceed the plant specific threshold, is very small. The NRC staff
also concluded that the licensee's proposal for demonstrating
compliance with the ECCS and CSS performance requirements meet the risk
acceptance guidelines in RG 1.174 because the approach is related to a
permissible exemption request, is consistent with defense-in-depth
philosophy, maintains sufficient safety margins, results in a small
increase in risk, and the impact of this approach is monitored by the
licensee using performance measurement strategies. Therefore, the
licensee's use of the risk-informed analysis to consider the impacts of
debris meets the underlying requirements of 10 CFR 50.46 and GDCs 35,
38, and 41, to ensure that a licensee demonstrates that the ECCS and
CSS will provide adequate cooling for the reactor core and containment,
as well as containment atmosphere cleanup following postulated design-
basis accidents.
Based on the above, the NRC staff concludes that special
circumstances under 10 CFR 50.12(a)(2)(ii) exist because compliance
with the deterministic requirements of 10 CFR 50.46(a)(1)(i), and GDCs
35, 38, and 41 is not necessary to achieve the underlying purpose of
each rule.
B. The Exemption Presents No Undue Risk to Public Health and Safety
The provisions of 10 CFR 50.46 and GDCs 35, 38, and 41 establish
criteria for the emergency core cooling, containment cooling, and
containment atmosphere cleanup system performance. As part of the
amendment requests, the STPNOC submitted exemption requests to change
its design-basis analysis specified in the Updated Final Safety
Analysis Report (UFSAR) to use new risk-informed and deterministic
methodologies to specifically account for the impacts of debris in
containment. The licensee justified its use of the risk-informed
approach by stating that the proposed risk-informed approach meets the
key principles in RG 1.174 in that it is consistent with defense-in-
depth philosophy, maintains sufficient safety margins, results in a
small increase in risk, and is monitored by the licensee using
performance measurement strategies.
Additionally, the licensee stated that the proposed exemptions to
use the risk-informed method are consistent with Key Principle 1 in RG
1.174 that requires a proposed change to the licensing basis (or
amendment) to meet current regulations unless the change is explicitly
related to a requested exemption. The licensee's probabilistic risk
analysis results provided by the licensee and evaluated by the NRC
staff in its safety evaluation, showed that the increase in risk
associated with debris generation and transport on ECCS and CSS
function following postulated LOCAs is very low, in accordance with the
criteria in RG 1.174.
The NRC staff concluded that the risk is consistent with the
guidance in RG 1.174 and with the Commission policy statements on
safety goals and the use of probabilistic risk assessment methods in
nuclear regulatory activities; therefore, the requested exemption
presents no undue risk to public health and safety.
C. The Exemption Is Consistent With the Common Defense and Security
The requested exemptions to use a risk-informed methodology allow
STPNOC to resolve a generic safety concern for PWRs associated with
potential clogging of the ECCS and CSS strainers during certain design-
basis events. The change is adequately controlled by safety acceptance
criteria and technical specification requirements and is not related to
security issues. Because the common defense and security is not
impacted by the exemption, the exemption is consistent with the common
defense and security.
D. The Exemptions Are Authorized by Law
The exemptions to use a risk-informed methodology allow STPNOC to
show compliance with 10 CFR 50.46(a)(1)(i), and GDCs 35, 38, and 41,
when considering debris in containment generated and transported during
postulated hot-leg LOCA breaks greater than 16 inches, and those breaks
that exceed the plant-specific testing threshold. These regulations
were promulgated under, and are consistent with the Commission's
authority under Section 161 of the Atomic Energy Act. Because the
application of a risk-informed methodology to show compliance with 10
CFR 50.46, and GDC 35, 38, and 41 would not violate the Atomic Energy
Act of 1954, as amended, or the Commission's regulations, the
exemptions are authorized by law provided all requisite findings are
made.
[[Page 33165]]
E. Environmental Considerations
Pursuant to 10 CFR 51.21, ``Criteria for and identification of
licensing and regulatory actions requiring environmental assessments,''
the NRC has prepared an Environmental Assessment (EA) summarizing the
findings of its review of the environmental impacts of the proposed
action under the National Environmental Policy Act. The NRC staff
determined that special circumstances under 10 CFR 51.21 exist to
warrant preparation of an EA because STP is the pilot plant to propose
a risk-informed approach to resolve GSI-191 as recognized in Staff
Requirement Memorandum SECY-12-0093, ``Closure Options for Generic
Safety Issue-191, Assessment of Debris Accumulation on Pressurized-
Water Reactor Sump Performance,'' dated December 14, 2012. Because this
is the first approval of a risk-informed approach, the NRC staff
considered preparations of an EA to be a prudent course of action that
would further the purposes of the National Environmental Policy Act.
Based on its review, the NRC concluded that an environmental impact
statement is not required and that the proposed action will have no
significant impact on the environment.
The NRC published a final EA on the proposed action in the Federal
Register on May 9, 2017 (82 FR 21568).
IV. Conclusions
Accordingly, the Commission has determined that, pursuant to 10 CFR
50.12, exemptions are authorized by law, will not present an undue risk
to the public health and safety, are consistent with the common defense
and security, and special circumstances are present pursuant to 10 CFR
50.12(a)(2)(ii). Therefore, the NRC hereby grants STPNOC a one-time
exemption from 10 CFR 50.46(a)(1), and 10 CFR part 50, appendix A, GDCs
35, 38, and 41 to use a risk-informed methodology in lieu of a
deterministic methodology to show conformance with the ECCS and CSS
performance criteria accounting for debris in containment for large-
break hot-leg LOCAs and those breaks that exceed the plant-specific STP
testing threshold.
V. Availability of Documents
The documents identified in the following table are available for
public inspection through the NRC's Agencywide Documents Access and
Management System (ADAMS).
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ADAMS
Title Date accession No.
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NRC Generic Letter 2004-02, Potential 9/13/2004 ML042360586
Impact of Debris Blockage on Emergency
Recirculation During Design Basis
Accidents at Pressurized-Water Reactors
STPNOC letter to NRC, Revised STP Pilot 6/19/2013 ML131750250
Submittal and Requests for Exemptions (Package)
and License Amendment for a Risk-
Informed Approach to Resolving Generic
Safety Issue (GSI)-191.................
STPNOC letter to NRC, Supplement 2 to 8/20/15 ML15246A125
STP Pilot Submittal and Requests for (Package)
Exemptions and License Amendment for a
Risk-Informed Approach to Address
Generic Safety Issue (GSI)-191 and
Respond to Generic Letter (GL) 2004-02.
STPNOC letter to NRC, South Texas 4/13/2016 ML16111B204
Project, Units 1 and 2--Revision to
Proposed Exemption to 10 CFR 50.46
Described in Pilot Submittal and
Requests for Exemptions and License
Amendment for a Risk-Informed Approach
to Address Generic Safety Issue (GSI)-
191 and Respond to Generic Letter (GL)
2004-02................................
Regulatory Guide 1.174, Revision 2, ``An 5/2011 ML100910006
Approach for Using Probabilistic Risk
Assessment in Risk-Informed Decisions
on Plant-Specific Changes to the
Licensing Basis''......................
NRC letter to STPNOC, South Texas 7/11/2017 ML17019A001
Project, Units 1 and 2--Issuance of (Package)
Amendment Nos. 212 and 198--Risk-
Informed Approach to Resolve Generic
Safety Issue-191 (includes Safety
Evaluation)............................
Commission SRM-SECY-12-0093, Staff 12/14/2012 ML12349A378
Requirements--SECY-12-0093--Closure
Options for Generic Safety Issue--191,
Assessment of Debris Accumulation on
Pressurized-Water Reactor Sump
Performance............................
NRC Letter to STPNOC, South Texas 5/02/2017 ML16278A598
Project, Units 1 and 2--Letter,
Environmental Assessment and Finding of
No Significant Impact, Revise Licensing
Basis as Documented in the UFSAR and
Request for Exemptions, Risk-Informed
approach to Address GSI[dash]191.......
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Dated at Rockville, Maryland, this 11th day of July 2017.
For the Nuclear Regulatory Commission.
Eric J. Benner,
Deputy Director, Division of Operating Reactor Licensing, Office of
Nuclear Reactor Regulation.
[FR Doc. 2017-15136 Filed 7-18-17; 8:45 am]
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