NRC Generic Letter 04-02
Potential Impact of Debris Blockage on Emergency Recirculation During Design Basis Accidents at Pressurized-Water Reactor
https://www.nrc.gov/docs/ML0423/ML042360586.pdf
text
OMB Control No.: 3150-0011
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555
September 13, 2004
NRC GENERIC LETTER 2004-02: POTENTIAL IMPACT OF DEBRIS BLOCKAGE ON
EMERGENCY RECIRCULATION DURING DESIGN
BASIS ACCIDENTS AT PRESSURIZED-WATER
REACTORS
Addressees
All holders of operating licenses for pressurized-water nuclear power reactors, except those
who have ceased operations and have certified that fuel has been permanently removed from
the reactor vessel.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this generic letter to:
(1) Request that addressees perform an evaluation of the emergency core cooling system
(ECCS) and containment spray system (CSS) recirculation functions in light of the
information provided in this letter and, if appropriate, take additional actions to ensure
system function. Additionally, addressees are requested to submit the information
specified in this letter to the NRC. This request is based on the identified potential
susceptibility of pressurized-water reactor (PWR) recirculation sump screens to debris
blockage during design basis accidents requiring recirculation operation of ECCS or
CSS and on the potential for additional adverse effects due to debris blockage of
flowpaths necessary for ECCS and CSS recirculation and containment drainage.
(2) Require addressees to provide the NRC a written response in accordance with
Background
In 1979, as a result of evolving staff concerns related to the adequacy of PWR recirculation
sump designs, the NRC opened Unresolved Safety Issue (USI) A-43, “Containment Emergency
Sump Performance.” To support the resolution of USI A-43, the NRC undertook an extensive
research program, the technical findings of which are summarized in NUREG-0897,
“Containment Emergency Sump Performance,” dated October 1985. The resolution of
USI A-43 was subsequently documented in Generic Letter (GL) 85-22, “Potential for Loss of
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Post-LOCA Recirculation Capability Due to Insulation Debris Blockage,” dated December 3,
1985. Although the staff’s regulatory analysis concerning USI A-43 did not support imposing
new sump performance requirements upon licensees of operating PWRs or boiling-water
reactors (BWRs), the staff found in GL 85-22 that the 50-percent blockage assumption (under
which most nuclear power plants had been licensed) identified in Regulatory Guide (RG) 1.82,
Sumps for Emergency Core Cooling and Containment Spray Systems, Revision 0 should be
replaced with a more comprehensive requirement to assess debris effects on a plant-specific
basis. The 50-percent screen blockage assumption does not require a plant-specific evaluation
of the debris-blockage potential and may result in a nonconservative analysis for screen
blockage effects. The staff also updated the NRC’s regulatory guidance, including Section
6.2.2 of the Standard Review Plan (NUREG-0800) and RG 1.82 to reflect the USI A-43
technical findings documented in NUREG-0897.
Following the resolution of USI A-43 in 1985, several events challenged the conclusion that no
new requirements were necessary to prevent the clogging of ECCS strainers at operating
BWRs:
! On July 28, 1992, at Barsebäck Unit 2, a Swedish BWR, the spurious opening of a
pilot-operated relief valve led to the plugging of two containment vessel spray system
suction strainers with mineral wool and required operators to shut down the spray
pumps and backflush the strainers.
! In 1993, at Perry Unit 1, two events occurred during which ECCS strainers became
plugged with debris. On January 16, ECCS strainers were plugged with suppression
pool particulate matter, and on April 14, an ECCS strainer was plugged with glass fiber
from ventilation filters that had fallen into the suppression pool. On both occasions, the
affected ECCS strainers were deformed by excessive differential pressure created by
the debris plugging.
! On September 11, 1995, at Limerick Unit 1, following a manual scram due to a
stuck-open safety/relief valve, operators observed fluctuating flow and pump motor
current on the A loop of suppression pool cooling. The licensee later attributed these
indications to a thin mat of fiber and sludge which had accumulated on the suction
strainer.
In response to these ECCS suction strainer plugging events, the NRC issued several generic
communications, including Bulletin 93-02, Supplement 1, “Debris Plugging of Emergency Core
Cooling Suction Strainers,” dated February 18, 1994;Bulletin 95-02, “Unexpected Clogging of a
Residual Heat Removal (RHR) Pump Strainer While Operating in Suppression Pool Cooling
Mode,” dated October 17, 1995; and,Bulletin 96-03, “Potential Plugging of Emergency Core
Cooling Suction Strainers by Debris in Boiling-Water Reactors,” dated May 6, 1996. These
bulletins requested that BWR licensees implement appropriate procedural measures,
maintenance practices, and plant modifications to minimize the potential for the clogging of
ECCS suction strainers by debris accumulation following a loss-of-coolant accident (LOCA).
The NRC staff has concluded that all BWR licensees have sufficiently addressed these
bulletins.
However, findings from research to resolve the BWR strainer clogging issue raised questions
concerning the adequacy of PWR sump designs. In comparison to the technical findings of the
earlier USI A-43 research program on PWRs, the BWR research findings demonstrated that
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the amount of debris generated by a high-energy line break (HELB) could be greater, that the
debris could be finer (and thus more easily transportable), and that certain combinations of
debris (e.g., fibrous material plus particulate material) could result in a substantially greater
head loss than an equivalent amount of either type of debris alone. These research findings
prompted the NRC to open Generic Safety Issue (GSI) 191, “Assessment of Debris
Accumulation on PWR Sump Performance.” The objective of GSI-191 is to ensure that
post-accident debris blockage will not impede or prevent the operation of the ECCS and CSS in
recirculation mode at PWRs during LOCAs or other HELB accidents for which sump
recirculation is required.
On June 9, 2003, having completed its technical assessment of GSI-191 (summarized below in
the Discussion section of this generic letter), the NRC issued Bulletin 2003-01, “Potential
Impact of Debris Blockage on Emergency Recirculation During Design-Basis Accidents at
Pressurized-Water Reactors.” As a result of the emergent issues discussed therein, the bulletin
requested an expedited response from PWR licensees on the status of their compliance on a
mechanistic basis with regulatory requirements concerning the ECCS and CSS recirculation
functions. Addressees who chose not to confirm regulatory compliance were asked to describe
any interim compensatory measures that have been implemented or will be implemented to
reduce risk until the analysis could be completed. All licensees have since responded to
Bulletin 2003-01. In developing Bulletin 2003-01, the NRC staff recognized that it may be
necessary for addressees to undertake complex evaluations to determine whether regulatory
compliance exists in light of the concerns identified in the bulletin and that the methodology
needed to perform these evaluations was not currently available. As a result, that information
was not requested in the bulletin, but addressees were informed that the staff was preparing a
generic letter that would request this information. This generic letter is the follow-on to the
bulletin.
In response to Bulletin 2003-01, PWR licensees that chose not to confirm regulatory
compliance implemented or planned to implement compensatory measures to reduce risk or
otherwise enhance the capability of the ECCS and CSS recirculation functions. Addressees’
understanding of their facilities’ ECCS and CSS recirculation capabilities may change when
they resolve the potential concerns identified in this generic letter, and revise their analyses of
sump performance. Therefore, addressees may find it necessary to reevaluate the adequacy
of their compensatory measures in light of the new information and take further action as
appropriate and necessary. Upon resolution of the potential concerns identified in this generic
letter and the completion of any corrective actions resulting from that resolution, addressees
may consider continuing, revising, or retiring their compensatory measures as appropriate.
The NRC has developed a Web page to keep the public informed of generic activities on PWR
sump performance at (http://www.nrc.gov/reactors/operating/ops-experience/pwr-sump-performance.html).
This page provides links to information on PWR sump performance issues, along with
documentation of NRC interactions with industry (industry submittals, meeting notices,
presentation materials, and meeting summaries). The NRC will continue to update this Web
page as new information becomes available.
Discussion
In the event of an HELB inside the containment of a PWR, energetic pressure waves and fluid
jets would impinge upon materials in the vicinity of the break, such as thermal insulation,
coatings, and concrete, damaging and dislodging them. Debris could also be generated
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through secondary mechanisms, such as severe post-accident temperature and humidity
conditions, flooding of the lower containment, and the impact of containment spray droplets. In
addition to debris generated by jet forces from the pipe rupture, debris could be created by the
chemical reaction between the materials in containment and the chemically reactive spray
solutions used following a LOCA. These reactions might generate additional debris such as
disbonded coatings and chemical precipitants. Through transport methods such as
entrainment in the steam/water flows issuing from the break and containment spray washdown,
a fraction of the generated debris and foreign material in the containment would be transported
to the pool of water formed on the containment floor. Subsequently, if the ECCS or CSS
pumps took suction from the recirculation sump, the debris suspended in the containment pool
would begin to accumulate on the sump screen or be transported through the associated
system. The accumulation of this suspended debris on the sump screen could create a roughly
uniform covering on the screen, referred to as a debris bed, which would tend to increase the
head loss across the screen through a filtering action. If a sufficient amount of debris
accumulated, the debris bed would reach a critical thickness at which the head loss across the
debris bed would exceed the net positive suction head (NPSH) margin required to ensure the
successful operation of the ECCS and CSS pumps in recirculation mode. A loss of NPSH
margin for the ECCS or CSS pumps as a result of the accumulation of debris on the
recirculation sump screen, referred to as sump clogging, could result in degraded pump
performance and eventual pump failure. Debris could also plug or wear close-tolerance
components within the ECCS or CSS systems. This plugging or wear might cause a
component to degrade to the point where it could not perform its designated function (i.e., pump
fluid, maintain system pressure, or pass and control system flow.)
The primary object of the NRC’s technical assessment of GSI-191 was to assess the likelihood
that the ECCS and CSS pumps at domestic PWRs would experience a debris-induced loss of
NPSH margin during sump recirculation. The NRC’s technical assessment culminated in a
parametric study documented in Volume 1 of NUREG/CR-6762, “GSI-191 Technical
Assessment: Parametric Evaluations for Pressurized Water Reactor Recirculation Sump
Performance,” dated August 2002. This study was a mechanistic treatment of phenomena
associated with debris blockage using analytical models of domestic PWRs generated with a
combination of generic and plant-specific data. The GSI-191 parametric study concluded that
recirculation sump clogging was a credible concern for domestic PWRs. As a result of the
limitations of plant-specific data and other modeling uncertainties, however, the parametric
study did not definitively show whether particular PWR plants were vulnerable to sump clogging
when phenomena associated with debris blockage were modeled mechanistically.
The methodology employed by the GSI-191 parametric study is based upon the substantial
body of test data and analyses that are documented in technical reports generated during the
NRC’s GSI-191 research program and earlier technical reports by the NRC and the industry
during the resolution of the BWR strainer clogging issue and USI A-43. Four of these NRC
technical reports on debris generation, transport, accumulation, and head loss, are incorporated
by reference into the GSI-191 parametric study:
! NUREG/CR-6770, “GSI-191: Thermal-Hydraulic Response of PWR Reactor Coolant
System and Containments to Selected Accident Sequences,” August 2002
! NUREG/CR-6762, Vol. 3, “GSI-191 Technical Assessment: Development of Debris
Generation Quantities in Support of the Parametric Evaluation,” August 2002
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! NUREG/CR-6762, Vol. 4, “GSI-191 Technical Assessment: Development of Debris
Transport Fractions in Support of the Parametric Evaluation,” August 2002
! NUREG/CR-6224, “Parametric Study of the Potential for BWR ECCS Strainer Blockage
Due to LOCA-Generated Debris,” October 1995
In addition to demonstrating the potential for debris to clog containment recirculation sumps,
operational experience and the NRC’s technical assessment of GSI-191 have also identified
three integrally related modes by which post-accident debris blockage could adversely affect
the sump screen’s design function of intercepting debris that could impede or prevent the
operation of the ECCS and CSS in recirculation mode.
First, as a result of the 50-percent blockage assumption, most PWR sump screens were
designed assuming that relatively small structural loadings would result from the differential
pressure associated with debris blockage. Consequently, PWR sump screens may not be
capable of accommodating the increased structural loadings that would occur due to
mechanistically determined debris beds that cover essentially the entire screen surface.
Inadequate structural reinforcement of a sump screen may result in its deformation, damage, or
failure, which could allow large quantities of debris to be ingested into the ECCS and CSS
piping, pumps, and other components, potentially leading to their clogging or failure. The
credibility of this concern for screens and strainers that have not been designed with adequate
reinforcement was shown by the ECCS strainer plugging and deformation events that occurred
at Perry Unit 1 (further described in Information Notice (IN) 93-34, “Potential for Loss of
Emergency Cooling Function Due to a Combination of Operational and Post-LOCA Debris in
Containment,” dated April 26, 1993, and License Event Report (LER) 50-440/93-011,
“Excessive Strainer Differential Pressure Across the RHR Suction Strainer Could Have
Compromised Long-Term Cooling During Post-LOCA Operation,” submitted May 19, 1993).
Second, in some PWR containments, the flowpaths by which containment spray or break flows
return to the recirculation sump may include chokepoints, where the flowpath becomes so
constricted that it could become blocked with debris following an HELB. Examples of potential
chokepoints are pool drains, cavities, isolated containment compartments, and constricted
drainage paths between physically separated containment elevations. Debris blockage at
certain chokepoints could hold up substantial amounts of water required for adequate
recirculation or cause the water to be diverted into containment volumes that do not drain to the
recirculation sump. The holdup or diversion of water assumed to be available to support sump
recirculation could result in an available NPSH for ECCS and CSS pumps that is lower than the
analyzed value, thereby reducing assurance that recirculation would successfully function. A
reduced available NPSH directly concerns sump screen design because the NPSH margin of
the ECCS and CSS pumps must be conservatively calculated to determine correctly the
required surface area of passive sump screens when mechanistically determined debris
loadings are considered. Although the parametric study (NUREG/CR-6762, Vol. 1) did not
analyze in detail the potential for the holdup or diversion of recirculation sump inventory, the
NRC’s GSI-191 research identified this phenomenon as an important and potentially credible
concern. A number of LERs associated with this concern further confirm its credibility and
potential significance:
! LER 50-369/90-012, “Loose Material Was Located in Upper Containment During Unit
Operation Because of an Inappropriate Action,” McGuire Unit 1, August 30, 1990
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! LER 50-266/97-006, “Potential Refueling Cavity Drain Failure Could Affect Accident
Mitigation,” Point Beach Unit 1, February 19, 1997
! LER 50-455/97-001, “Unit 2 Containment Drain System Clogged Due to Debris,”
Byron Unit 2, April 17, 1997
! LER 50-269/97-010, “Inadequate Analysis of ECCS Sump Inventory Due to Inadequate
Design Analysis,” Oconee Unit 1, January 8, 1998
! LER 50-315/98-017, “Debris Recovered from Ice Condenser Represents Unanalyzed
Condition,” D.C. Cook Unit 1, July 1, 1998
Third, debris blockage at flow restrictions within the ECCS recirculation flowpath downstream of
the sump screen is a potential concern for PWRs. Debris that is capable of passing through
the recirculation sump screen may have the potential to become lodged at a downstream flow
restriction, such as a high-pressure safety injection (HPSI) throttle valve or fuel assembly inlet
debris screen. Debris blockage at such flow restrictions in the ECCS flowpath could impede or
prevent the recirculation of coolant to the reactor core, thereby leading to inadequate core
cooling. Similarly, debris blockage at flow restrictions in the CSS flowpath, such as a
containment spray nozzle, could impede or prevent CSS recirculation, thereby leading to
inadequate containment heat removal. Debris may also accumulate in close-tolerance
subcomponents of pumps and valves. The effect may be either to plug the subcomponent,
thereby rendering the component unable to perform its function, or to wear critical closetolerance subcomponents to the point at which component or system operation is degraded and
unable to fully perform its function. Considering the recirculation sump screen’s design function
of intercepting potentially harmful debris, it is essential that the screen openings be adequately
sized and that the sump screen’s current configuration be free of gaps or breaches which could
compromise the ECCS and CSS recirculation functions. It is also essential that system
components be designed and evaluated to be able to operate as necessary with debris laden
fluid post-LOCA.
Section 50.46(c)(2) of Title 10 of the Code of Federal Regulations (10 CFR 50.46(c)(2)) defines
an evaluation model as the calculational framework for evaluating the behavior of the reactor
system during a postulated LOCA. An evaluation model includes one or more computer
programs and all other information necessary for applying the calculational framework to a
specific LOCA (the mathematical models used, the assumptions included in the programs, the
procedure for treating the program input and output information, the parts of the analysis not
included in the computer programs, values of parameters, and all other information necessary
to specify the calculational procedure). Although not traditionally considered as a component of
the 10 CFR 50.46 ECCS evaluation model, the calculation of sump performance is necessary
to determine if the sump and the ECCS are predicted to provide enough flow to ensure longterm cooling.
Based on the new information identified during the efforts to resolve GSI-191, the staff has
determined that the previous guidance used to develop current licensing basis analyses does
not adequately and completely model sump screen debris blockage and related effects. As a
result, due to the deficiencies in the previous guidance, an analytical error could be introduced
which results in ECCS and CSS performance that does not conform with the existing applicable
regulatory requirements outlined in this generic letter. Therefore, the staff is revising the
guidance for determining the susceptibility of PWR recirculation sump screens to the adverse
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effects of debris blockage during design basis accidents requiring recirculation operation of the
ECCS or CSS. In light of this revised staff guidance, it is appropriate to request that
addressees perform new, more realistic analyses and submit information to confirm the
functionality of the ECCS and CSS during design basis accidents requiring recirculation
operations.
To assist in determining, on a plant-specific basis, the impact on sump screen performance and
other related effects of extended post-accident operation with debris-laden fluids, addressees
may use the guidance in Regulatory Guide (RG) 1.82, Revision 3, “Water Sources for LongTerm Recirculation Cooling Following a Loss-of-Coolant Accident,” dated November 2003.
Revision 3 enhanced the debris blockage evaluation guidance for PWRs provided in Revision 1
of the regulatory guide to better model sump screen debris blockage and related effects.
Revision 1 replaced the 50-percent blockage assumption in Revision 0 with a comprehensive,
mechanistic assessment of plant-specific debris blockage potential for future modifications
related to sump performance, such as thermal insulation changeouts. This was in response to
the findings of USI A-43. The staff issued Revision 2 of the RG after evaluating blockage
events such as the Barsebäck Unit 2 event mentioned above but for BWRs only. The NRC
staff determined after the issuance of Revision 2 that research for PWRs indicated that the
guidance in that revision was not comprehensive enough to ensure adequate evaluation of a
PWR plant’s susceptibility to the detrimental effects of debris accumulation on debris
interceptors (e.g., trash racks and sump screens). This led to the issuance of Revision 3 to
address the PWRs. In addition, the NRC staff is reviewing generic industry guidance and will
issue a safety evaluation endorsing acceptable portions or all of the generic industry guidance.
Once approved, this guidance may also be used to assist in determining the status of
regulatory compliance. For areas not addressed in the industry guidance, the NRC will provide
guidance in the safety evaluation. Individual addressees may also develop an alternative to the
approaches mentioned in this paragraph for responding to this generic letter; however,
additional staff review may be required to assess the adequacy of such approaches.
The timeframes for addressee responses in this generic letter were selected to (1) allow
adequate time for addressees to perform an analysis, (2) allow addressees to properly design
and install any identified modifications, (3) allow addressees adequate time to obtain NRC
approval, as necessary, for any licensing basis changes, (4) allow addressees adequate time to
obtain NRC approval, as necessary, for any exemption requests, and (5) allow for the closure of
the generic issue in accordance with the published schedule. These timeframes are
appropriate since all addressees have responded to Bulletin 2003-01 and will, if necessary,
implement compensatory measures until the issues identified in this generic letter are resolved.
The staff has assessed whether existing PWRs should continue operation while responding to
this generic letter in light of the GSI-191 resolution schedule, proposed through December 31,
2007, and determined that continued operation is justified. The staff released a justification for
continued operation in the “Summary of July 26-27, 2001, Meeting with Nuclear Energy Institute
and Industry on ECCS Strainer Blockage in PWRs,” dated August 14, 2001. As discussed in
this justification, continued plant operation is still justified for several reasons. First, the
probability of the most severe initiating event (i.e., large and intermediate break LOCAs) is
extremely low. More probable (although still low probability) small LOCAs would require less
ECCS flow, take more time to use up the water inventory in the refueling water storage tank
(RWST), and in some cases may not even require the use of recirculation from the ECCS sump
because the flow through the break would be small enough that the operator will have sufficient
time to initiate RHR operation and depressurize the reactor coolant system to terminate the loss
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of reactor coolant system inventory for higher elevation breaks. In addition, there are PWR
design features which would tend to prevent blockage of the ECCS sumps during a LOCA.
These features would tend to be effective for insulation and coating debris. For instance, the
containments in PWRs tend to be very compartmentalized making the transport of debris to the
sump screens more difficult. In addition, PWRs typically do not need to switch over to
recirculation from the sump during a LOCA until greater than 20-30 minutes after the large
break LOCA initiation and the elapsed time for all LOCAs will allow time for some of the debris
to settle in other places within the containment. Coating debris, which is a major contributor to
the latent debris in containment, would have a significant amount of time to settle. In addition,
all PWRs have received approval by the staff for leak-before-break (LBB) credit on their largest
RCS primary coolant piping. While LBB is not acceptable for demonstrating compliance with
10 CFR 50.46, it does demonstrate that LBB-qualified piping is sufficiently tough that it will most
likely leak (even under safe shutdown earthquake conditions) rather than rupture. This would
allow operators adequate opportunity to shut the plant down safely. Additionally, the staff notes
that there are sources of margin in PWR designs which are not always credited in the licensing
basis for each plant. For instance, NPSH analyses for most PWRs do not credit containment
overpressure (which may be present during a LOCA). Any containment pressure greater than
assumed in the NPSH analysis provides additional margin for ECCS operability during an
accident. Another source of margin is that it has been shown that low pressure ECCS pumps
would be able to continue operating in many cases for some time under cavitation conditions.
Some licensees have vendor data demonstrating this. Such design margins such as these
examples may prevent complete loss of ECCS recirculation flow or increase the time available
for operator action (e.g., refilling the RWST) prior to loss of flow. Moreover, in response to
Bulletin 2003-01, addressees have implemented or will implement interim compensatory
measures to reduce the risk.
The staff has also determined that addressees are not required to be in compliance with the
newly issued analysis using a NRC-approved methodology, until after all plant modifications (if
required) are completed in accordance with the resolution schedule (i.e. December 31, 2007),
which is located below in paragraph 2(b) of the Requested Information section, and addressees
have changed their licensing basis, as appropriate. However, if a non-compliance with the
existing licensing design basis that affects the operability of an ECCS or CSS design feature is
identified while taking actions in response to the generic letter, addressees should comply with
established regulatory requirements.
Applicable Regulatory Requirements
NRC regulations in Title 10, of the Code of Federal Regulations Section 50.46,10 CFR 50.46,
require that the ECCS have the capability to provide long-term cooling of the reactor core
following a LOCA. That is, the ECCS must be able to remove decay heat, so that the core
temperature is maintained at an acceptably low value for the extended period of time required
by the long-lived radioactivity remaining in the core.
Similarly, for PWRs licensed to the General Design Criteria (GDCs) in Appendix A to 10 CFR Part 50, GDC 38 provides requirements for containment heat removal systems, and GDC 41
provides requirements for containment atmosphere cleanup. Many PWR licensees credit a
CSS, at least in part, with performing the safety functions to satisfy these requirements, and
PWRs that are not licensed to the GDCs may similarly credit a CSS to satisfy licensing basis
requirements. In addition, PWR licensees may credit a CSS with reducing the accident source
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1 The NRC staff is currently reviewing evaluation guidance developed by the industry. The NRC
staff intends to document its review in a safety evaluation which licensees can reference as
regulatory guidance.
term to meet the limits of 10 CFR Part 100 or 10 CFR 50.67. GDC 35 is listed in 10 CFR 50.46(d) and specifies additional ECCS requirements. PWRs that are not licensed to the GDCs
typically have similar requirements in their licensing basis.
Applicable Regulatory Guidance1
Regulatory Guide 1.82, Revision 3, “Water Sources for Long-Term Recirculation Cooling
Following a Loss-of-Coolant Accident,” November 2003.
Requested Action
All addressees are requested to take the following actions:
Using an NRC-approved methodology, perform a mechanistic evaluation of the potential for the
adverse effects of post-accident debris blockage and operation with debris-laden fluids to
impede or prevent the recirculation functions of the ECCS and CSS following all postulated
accidents for which the recirculation of these systems is required. Individual addressees may
also use alternative methodologies to those already approved by the NRC; however, additional
staff review may be required to assess the adequacy of such approaches.
Implement any plant modifications that the above evaluation identifies as being necessary to
ensure system functionality.
Requested Information
All addressees are requested to provide the following information:
1. Within 90 days of the date of the safety evaluation report providing the guidance for
performing the requested evaluation, addressees are requested to provide information
regarding their planned actions and schedule to complete the requested evaluation.
The information should include the following:
(a) A description of the methodology that is used or will be used to analyze the
susceptibility of the ECCS and CSS recirculation functions for your reactor to the
adverse effects identified in this generic letter of post-accident debris blockage and
operation with debris-laden fluids identified in this generic letter. Provide the
completion date of the analysis that will be performed.
(b) A statement of whether you plan to perform a containment walkdown surveillance
in support of the analysis of the susceptibility of the ECCS and CSS recirculation
functions to the adverse effects of debris blockage identified in this generic letter.
Provide justification if no containment walkdown surveillance will be performed. If
a containment walkdown surveillance will be performed, state the planned
methodology to be used and the planned completion date.
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2. Addressees are requested to provide the following information no later than
September 1, 2005:
(a) Confirmation that the ECCS and CSS recirculation functions under debris loading
conditions are or will be in compliance with the regulatory requirements listed in
the Applicable Regulatory Requirements section of this generic letter. This
submittal should address the configuration of the plant that will exist once all
modifications required for regulatory compliance have been made and this
licensing basis has been updated to reflect the results of the analysis described
above.
(b) A general description of and implementation schedule for all corrective actions,
including any plant modifications, that you identified while responding to this
generic letter. Efforts to implement the identified actions should be initiated no
later than the first refueling outage starting after April 1, 2006. All actions should
be completed by December 31, 2007. Provide justification for not implementing
the identified actions during the first refueling outage starting after April 1, 2006. If
all corrective actions will not be completed by December 31, 2007, describe how
the regulatory requirements discussed in the Applicable Regulatory Requirements
section will be met until the corrective actions are completed.
(c) A description of the methodology that was used to perform the analysis of the
susceptibility of the ECCS and CSS recirculation functions to the adverse effects
of post-accident debris blockage and operation with debris-laden fluids. The
submittal may reference a guidance document (e.g., Regulatory Guide 1.82,
Rev. 3, industry guidance) or other methodology previously submitted to the NRC.
(The submittal may also reference the response to Item 1 of the Requested
Information described above. The documents to be submitted or referenced
should include the results of any supporting containment walkdown surveillance
performed to identify potential debris sources and other pertinent containment
characteristics.)
(d) The submittal should include, at a minimum, the following information:
(i) The minimum available NPSH margin for the ECCS and CSS pumps with
an unblocked sump screen.
(ii) The submerged area of the sump screen at this time and the percent of
submergence of the sump screen (i.e., partial or full) at the time of the
switchover to sump recirculation.
(iii) The maximum head loss postulated from debris accumulation on the
submerged sump screen, and a description of the primary constituents of
the debris bed that result in this head loss. In addition to debris
generated by jet forces from the pipe rupture, debris created by the
resulting containment environment (thermal and chemical) and CSS
washdown should be considered in the analyses. Examples of this type
of debris are disbonded coatings in the form of chips and particulates and
chemical precipitants caused by chemical reactions in the pool.
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(iv) The basis for concluding that the water inventory required to ensure
adequate ECCS or CSS recirculation would not be held up or diverted by
debris blockage at choke-points in containment recirculation sump return
flowpaths.
(v) The basis for concluding that inadequate core or containment cooling
would not result due to debris blockage at flow restrictions in the ECCS
and CSS flowpaths downstream of the sump screen, (e.g., a HPSI
throttle valve, pump bearings and seals, fuel assembly inlet debris
screen, or containment spray nozzles). The discussion should consider
the adequacy of the sump screen’s mesh spacing and state the basis for
concluding that adverse gaps or breaches are not present on the screen
surface.
(vi) Verification that close-tolerance subcomponents in pumps, valves and
other ECCS and CSS components are not susceptible to plugging or
excessive wear due to extended post-accident operation with
debris-laden fluids.
(vii) Verification that the strength of the trash racks is adequate to protect the
debris screens from missiles and other large debris. The submittal
should also provide verification that the trash racks and sump screens
are capable of withstanding the loads imposed by expanding jets,
missiles, the accumulation of debris, and pressure differentials caused by
post-LOCA blockage under predicted flow conditions.
(viii) If an active approach (e.g., backflushing, powered screens) is selected in
lieu of or in addition to a passive approach to mitigate the effects of the
debris blockage, describe the approach and associated analyses.
(e) A general description of and planned schedule for any changes to the plant
licensing bases resulting from any analysis or plant modifications made to ensure
compliance with the regulatory requirements listed in the Applicable Regulatory
Requirements section of this generic letter. Any licensing actions or exemption
requests needed to support changes to the plant licensing basis should be
included.
(f) A description of the existing or planned programmatic controls that will ensure that
potential sources of debris introduced into containment (e.g., insulations, signs,
coatings, and foreign materials) will be assessed for potential adverse effects on
the ECCS and CSS recirculation functions. Addressees may reference their
responses to GL 98-04, "Potential for Degradation of the Emergency Core Cooling
System and the Containment Spray System after a Loss-of-Coolant Accident
Because of Construction and Protective Coating Deficiencies and Foreign Material
in Containment," to the extent that their responses address these specific foreign
material control issues.
Page 12 of 16
Required Response
In accordance with 10 CFR 50.54(f), the PWR addressees are required to submit written
responses to this generic letter. This information is sought to verify licensees’ compliance with
the regulatory requirements listed in the Applicable Regulatory Requirements section of this
generic letter once their licensing basis has been updated to reflect the results of the
mechanistic analysis requested in this generic letter. This request is based on the identified
potential susceptibility of PWR recirculation sump screens to debris blockage during design
basis accidents requiring recirculation operation of ECCS and CSS and the potential for
additional adverse effects due to debris blockage of flowpaths necessary for ECCS and CSS
recirculation and containment drainage. The addressees have two options:
(1) Addressees may choose to submit written responses providing the information
requested above within the requested time period.
(2) Addressees who choose not to provide information requested or cannot meet the
requested completion dates are required to submit written responses within 30 days of
the date of this generic letter. The responses must address any alternative course of
action proposed, including the basis for the acceptability of the proposed alternative
course of action.
The required written responses should be addressed to the U.S. Nuclear Regulatory
Commission, ATTN: Document Control Desk, 11555 Rockville Pike, Rockville, Maryland
20852, under oath or affirmation under the provisions of Section 182a of the Atomic Energy Act
of 1954, as amended, and 10 CFR 50.54(f). In addition, a copy of a response should be
submitted to the appropriate regional administrator.
The NRC staff will review the responses to this generic letter and will notify affected addressees
if concerns are identified regarding compliance with NRC regulations. The staff may also
conduct inspections to determine addressees’ effectiveness in addressing the generic letter.
Reasons for Information Request
As discussed above, research and analysis suggest that (1) the potential for the failure of the
ECCS and CSS recirculation functions as a result of debris blockage is not adequately
addressed in most PWR licensees’ current safety analyses, and (2) the ECCS and CSS
recirculation functions at a significant number of operating PWRs could potentially become
degraded as a result of the effects of debris blockage or extended operation with debris-laden
fluids as identified in this generic letter. An ECCS that is incapable of providing long-term
reactor core cooling through recirculation operation would be in violation of 10 CFR 50.46. A
CSS that is incapable of functioning in recirculation mode may not comply with GDCs 38 and 41
or other plant-specific licensing requirements or safety analyses.Bulletin 2003-01 requested
information to verify addressees’ compliance with NRC regulations and to ensure that any
interim risks associated with post-accident debris blockage are minimized while evaluations to
determine compliance proceed. This generic letter is the follow-on generic communication to
Bulletin 2003-01 and requests information on the results of the evaluations referenced in the
bulletin. This information is sought to verify licensees’ compliance with the regulatory
requirements listed in the Applicable Regulatory Requirements section of this generic letter
once their licensing basis has been updated to reflect the results of the mechanistic analysis
requested in this generic letter. This request is based on the identified potential susceptibility of
Page 13 of 16
PWR recirculation sump screens to debris blockage during design basis accidents requiring
recirculation operation of the ECCS and CSS and the potential for additional adverse effects
due to debris blockage of flowpaths necessary for ECCS and CSS recirculation and
containment drainage.
The NRC staff will also use the requested information to (1) determine whether a sample
auditing approach is acceptable for verifying that addressees have resolved the concerns
identified in this generic letter, (2) assist in determining which addressees will be subject to the
proposed sample audits, (3) provide confidence that any nonaudited addressees have
addressed the concerns identified in this generic letter, and (4) assess the need for and guide
the development of any additional regulatory actions that may be necessary to address the
adequacy of the ECCS and CSS recirculation functions.
Related Generic Communications
• Bulletin 2003-01, “Potential Impact of Debris Blockage on Emergency Recirculation
During Design-Basis Accidents at Pressurized-Water Reactors,” June 9, 2003.
• Bulletin 96-03, “Potential Plugging of Emergency Core Cooling Suction Strainers by
Debris in Boiling-Water Reactors,” May 6, 1996.
• Bulletin 95-02, “Unexpected Clogging of a Residual Heat Removal (RHR) Pump Strainer
While Operating in the Suppression Pool Cooling Mode,” October 17, 1995.
• Bulletin 93-02, “Debris Plugging of Emergency Core Cooling Suction Strainers,” May 11,
1993.
• Bulletin 93-02, Supplement 1, “Debris Plugging of Emergency Core Cooling Suction
Strainers,” February 18, 1994.
• Generic Letter 98-04, “Potential for Degradation of the Emergency Core Cooling System
and the Containment Spray System After a Loss-of-Coolant Accident Because of
Construction and Protective Coating Deficiencies and Foreign Material in Containment,”
July 14, 1998.
• Generic Letter 97-04, “Assurance of Sufficient Net Positive Suction Head for Emergency
Core Cooling and Containment Heat Removal Pumps,” October 7, 1997.
• Generic Letter 85-22, “Potential For Loss of Post-LOCA Recirculation Capability Due to
Insulation Debris Blockage,” December 3, 1985.
• Generic Letter 91-18, Rev. 1, “Information to Licensees Regarding NRC Inspection
Manual Section on Resolution of Degraded and Nonconforming Conditions,” October 8,
1997.
• Information Notice 97-13, “Deficient Conditions Associated With Protective Coatings at
Nuclear Power Plants,” March 24, 1997.
• Information Notice 96-59, “Potential Degradation of Post Loss-of-Coolant Recirculation
Capability as a Result of Debris,” October 30, 1996.
Page 14 of 16
• Information Notice 96-55, “Inadequate Net Positive Suction Head of Emergency Core
Cooling and Containment Heat Removal Pumps Under Design Basis Accident
Conditions,” October 22, 1996.
• Information Notice 96-27, “Potential Clogging of High Pressure Safety Injection Throttle
Valves During Recirculation,” May 1, 1996.
• Information Notice 96-10, “Potential Blockage by Debris of Safety System Piping Which
Is Not Used During Normal Operation or Tested During Surveillances,” February 13,
1996.
• Information Notice 95-47, “Unexpected Opening of a Safety/Relief Valve and
Complications Involving Suppression Pool Cooling Strainer Blockage,” October 4, 1995.
• Information Notice 95-47, Revision 1, “Unexpected Opening of a Safety/Relief Valve and
Complications Involving Suppression Pool Cooling Strainer Blockage,” November 30,
1995.
• Information Notice 95-06, “Potential Blockage of Safety-Related Strainers by Material
Brought Inside Containment,” January 25, 1995.
• Information Notice 94-57, “Debris in Containment and the Residual Heat Removal
System,” August 12, 1994.
• Information Notice 93-34, “Potential for Loss of Emergency Cooling Function Due to a
Combination of Operational and Post-LOCA Debris in Containment,” April 26, 1993.
• Information Notice 93-34, Supplement 1, “Potential for Loss of Emergency Cooling
Function Due to a Combination of Operational and Post-LOCA Debris in Containment,”
May 6, 1993.
• Information Notice 92-85, “Potential Failures of Emergency Core Cooling Systems
Caused by Foreign Material Blockage,” December 23, 1992.
• Information Notice 92-71, “Partial Plugging of Suppression Pool Strainers at a Foreign
BWR,” September 30, 1992.
• Information Notice 89-79, “Degraded Coatings and Corrosion of Steel Containment
Vessels,” December 1, 1989.
• Information Notice 89-79, Supplement 1, “Degraded Coatings and Corrosion of Steel
Containment Vessels,” June 29, 1990.
• Information Notice 89-77, “Debris in Containment Emergency Sumps and Incorrect
Screen Configurations,” November 21, 1989.
• Information Notice 88-28, “Potential for Loss of Post-LOCA Recirculation Capability Due
to Insulation Debris Blockage,” May 19, 1988.
Page 15 of 16
Backfit Discussion
Under the provisions of Section 182a of the Atomic Energy Act of 1954, as amended,
10 CFR 50.109(a)(4)(i) and 10 CFR 50.54(f), this generic letter requests that addressees
evaluate their facilities to confirm compliance with the existing applicable regulatory
requirements as outlined in this generic letter. This generic letter also transmits an information
request for the purpose of verifying compliance with existing applicable regulatory
requirements. The staff has determined that, in light of the information identified during the
efforts to resolve GSI-191, the previous guidance used to develop most addressees’ current
licensing basis analyses does not adequately and completely model sump screen debris
blockage and related effects. Due to the deficiencies in the previous guidance, a potential
analytical error could have been introduced which results in ECCS and CSS performance that
does not conform with existing applicable regulatory requirements. In response, the staff
revised its guidance for determining the susceptibility of PWR recirculation sump screens to the
adverse effects of debris blockage during design basis accidents requiring recirculation
operation of the ECCS or CSS to ensure compliance with existing applicable regulatory
requirements. Thus, the information requested by this generic letter is considered a
compliance exception to the rule in accordance with 10 CFR 50.109(a)(4)(i).
Small Business Regulatory Enforcement Fairness Act
The NRC has determined that this generic letter is subject to the Small Business Regulatory
Enforcement Fairness Act of 1996. Office of Management and Budget (OMB) has declared the
letter not to be a major rule. Notification of the letter has been sent to Congress.
Federal Register Notification
A notice of opportunity for public comment on this generic letter was published in the
Federal Register (69 FR16980) on March 31, 2004. Comments were received from ten industry
groups, one non-profit organization, one private citizen, and the State of New Jersey. The staff
considered all comments that were received. The staff's evaluation of the comments is publicly
available through the NRC’s Agencywide Documents Access and Management System
(ADAMS) under Accession No. ML042260161.
Paperwork Reduction Act Statement
This generic letter contains information collections that are subject to the Paperwork Reduction
Act of 1995 (44 U.S.C. 3501 et seq.). These information collections were approved by the
Office of Management and Budget (OMB) under approval number 3150-0011, which expires on
February 28, 2007.
The burden to the public for these mandatory information collections is estimated to average
7000 hours0.081 days <br />1.944 hours <br />0.0116 weeks <br />0.00266 months <br /> per response, including the time for reviewing instructions, searching existing data
sources, gathering and maintaining the necessary data, and completing and reviewing the
information collections. The staff received two public comments on the estimated burden to the
public. In both comments, the burden was estimated to be between 5,000 and 10,000 hours0 days <br />0 hours <br />0 weeks <br />0 months <br />.
The staff solicited input from three addressees to better estimate the burden to the public.
Based on the public comments and the solicited input, the staff estimates the burden as shown
above. Send comments regarding this burden estimate or any other aspect of these
information collections, including suggestions for reducing the burden, to the Records and
Page 16 of 16
FOIA/Privacy Services Branch (T-5F52), U.S. Nuclear Regulatory Commission, Washington,
DC 20555-0001, or by Internet electronic mail to INFOCOLLECTS@NRC.GOV; and to the
Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202 (3150-0011), Office of
Management and Budget, Washington, DC 20503.
Public Protection Notification
The NRC may neither conduct nor sponsor, and an individual is not required to respond to, an
information collection unless the requesting document displays a currently valid OMB control
number.
If you have any questions about this matter, please contact the technical contacts or lead project
manager listed below.
/RA/
Bruce A. Boger, Director
Division of Inspection Program Management
Office of Nuclear Reactor Regulation
Technical Contacts: Ralph Architzel, NRR David Cullison, NRR
301-415-2804 301-415-1212
e-mail: rea@nrc.gov e-mail: dgc@nrc.gov
Lead Project Manager: Michael Webb, NRR
301-415-1347
e-mail: mkw@nrc.gov
Page 16 of 16
FOIA/Privacy Services Branch (T-5F52), U.S. Nuclear Regulatory Commission, Washington,
DC 20555-0001, or by Internet electronic mail to INFOCOLLECTS@NRC.GOV; and to the
Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202 (3150-0011), Office of
Management and Budget, Washington, DC 20503.
Public Protection Notification
The NRC may neither conduct nor sponsor, and an individual is not required to respond to, an
information collection unless the requesting document displays a currently valid OMB control
number.
If you have any questions about this matter, please contact the technical contacts or lead project
manager listed below.
/RA/
Bruce A. Boger, Director
Division of Inspection Program Management
Office of Nuclear Reactor Regulation
Technical Contacts: Ralph Architzel, NRR David Cullison, NRR
301-415-2804 301-415-1212
e-mail: rea@nrc.gov e-mail: dgc@nrc.gov
Lead Project Manager: Michael Webb, NRR
301-415-1347
e-mail: mkw@nrc.gov
DOCUMENT NAME: G:\DIPM\OES\GEN COM\GIS-191 GL post CRGR.wpd
ADAMS ACCESSION NUMBER: ML042360586
OFFICE SPLB:DSSA Tech Editor SPLB:DSSA BC:SPLB:DSSA D:DSSA D:DLPM
NAME DCCullison PKleene DLSolario JNHannon SCBlack LBMarsh
DATE 08/25/2004 08/27/2004 08/31/2004 08/31/2004 09/03/2004 08/25/2004
OFFICE OGC (SBREFA) OE OGC (NLO) IROB:DIPM A:SC:OES:IROB:DIPM A:C:IROB:DIPM
NAME TRothschild CNolan CMarco CDPetrone ACMcMurtray FMCostello
DATE 08/26/2004 08/24/2004 08/24/2004 09/07/2004 09/08/2004 09/08/2004
OFFICE A:C:IROB:DIPM ADPT
NAME BABoger BWSheron
DATE 09/10/2004 09/13/2004 / /2004
OFFICIAL RECORD COPY