ML20059L106
| ML20059L106 | |
| Person / Time | |
|---|---|
| Issue date: | 01/12/1994 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20059K171 | List: |
| References | |
| PROJECT-683A NUDOCS 9402030163 | |
| Download: ML20059L106 (13) | |
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ENCLOSURE F
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ORAFT SAFETY EVALUATION _BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO THE BABC0CK AND WILC0X OWNERS GROUP INTEGRATED PLANT ASSESSMENT SYSTEM / STRUCTURE SCREENING METHODOLOGY BABC0CK AND WILC0X OWNERS GROUP REPORT 47-1223141-00 PROJECT NUMBER 683 P
3 9402030163 940112 PDR TOPRP.EMVB C
CONTENTS Paae
1.0 INTRODUCTION
I 1.1 License Renewal Rule......................
I 1.2 IPA System / Structure Screening Methodology Report.......
I 2.0 SCOPE AND CONDUCT OF REVIEW.....................
2 3.0 EVALUATION OF BWOG SYSTEMS SCREENING METHODOLOGY..........
3 3.1 Safety-Related Systems and Structures (Subsection (1))
3 3.2 Nonsafety-Related Systems and Structures (Subsection (2))...
6 3.3 Systems and Structures Relied on in Safety Analyses or Plant Evaluations to Demonstrate Compliance with Commission Regulations (Subsection (3)).
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3.3.1 Fire Protection.
8 3.3.2 Environmental Qualification 8
3.3.3 Pressurized Thermal Shock 9
3.3.4 Anticipated Transients Without Scram...........
9 3.3.5 Station Blackout....................
10 3.4 Systems and Structures Subject to Technical Specification Operability Requirements (Subsection (4)).
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4.0 CONCLUSION
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11 5.0 OPEN ITEMS.............................
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DRAFT SAFEIY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO THE BABC0CK AND WILC0X 0WNERS GROUP INTEGRATED PLANT ASSESSMENT SYSTEM / STRUCTURE SCREENING METHODOLOGY RSBCOCK AND WILCOX OWNERS GROUP REPORT 47-1223141-00 PROJECT NUMBER 683
1.0 INTRODUCTION
1.1 License Renewal Rule As discussed in Title '~
F the Code of Federal Regulations (10 CFR) 50.51, the U.S. Nuclear Regulat r; Commission (NRC) issues nuclear power plant operating licenses for a fixed period of time not to exceed 40 years.
The Commission may, however, renew these licenses.
The license renewal rule, 10 CFR Part 54, published on December 13, 1991 (56 FR 64943), sets forth the requirements for the renewal of operating licenses for nuclear power plants.
The license renewal rule requires th applicants for license renesal perform an integrated plant assessment (IPA) dentify systems, structures, and components (SSCs) important to license renosal (ITLR).
The identification of systems and structures that are ITLR is the first step in the detailed IPA l
process discussed in 10 CFR 54.21.
The license renewal rule requires that renewal applicants evaluate those systems and structures that are designated as ITLR using the remaining requirements of 10 CFR 54.21.
1.2 1PA System / Structure Screenino Methodoloav Report The Babcock and Wilcox Owners Group (BWOG) is developing a ge, program for i
the renewal of operating licenses for Babcock and Wilcox (B&W) nuclear power plants.
The goal of the generic license renewal program is to streamline the license renewal process for B&W nuclear power plants by resolving generic pressurized-water reactor technical issues. The resolution of these technical issues should, according to the BWOG, minimize regulatory uncertainty and i
provide a high degree cf cenfidence that the NRC will approve future license renewal applications, j
By letter to tha NRC dated March 2, 1993, the BU3G submitted the report,
" Integrated Plant Assessment ;ystem/ Structure Screening Methodol-y '" (BWOG System Screaning Methodology).
This report discusses the BWOG's generic screening process that would, when supplemented with plant-specific information, result in the identification of all systems and structures that are ITLR.
The report defines the type of information and data sources that renewal applicants must use to implement this process for identifying those systems and structures that are ITLR.
The report also contains BWOG positions on the implementation of certain requirements of the license renewal rule. According to the BWOG, renewal applicants should use the methodology contained in its
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report as a guideline and, that because of the variability of documentation and information management systems, each renewal applicant must develop a plant-specific approach best suited to its needs. The bases for the plant-specific system and structure screening task will be the plant-specific documentation that supports each plant's current licensing basis (CLB).
l The staff concludes that license renewal applicants may choose to reference the BWOG System Screening Methodology report when developing their plant-specific license renewal applications; however, all applicants are responsible for the review of appropriate plant-specific documents and information to 1
ensure that the assumptions and criteria used as the basis for their evaluation are applicable.
2.0 SC0pE AND CONDUCT OF REVIEW The staff reviewed the BWOG system screening methodology to determine if the proposed approach adequately describes and contains sufficient justification for the methods and license renewal implementation positions that could ts used to identify systems and structures that are ITLR.
To review the BWOG methodology, the staff red the re 'irements specified in the license renewal rule and the guidance contained in the Statements of Consideration for 10 CFR Part 54, " Requirements for Renewal of Operating Licenses for Nuclear Power Plants."
The license renewal rule contains a definition for " systems, structures, and components (SSCs) important to license renewal" in 10 CFR 54.3.
The staff reviewed the BWOG system screening methodology to determine whether the proposed BWOG approach, if properly implemented, would provide reasonable assurance that a complete list of structures and systems would be identified for each of the four groups of SSCs in the definition.
These four groups l
of SSCs in the definition are safety-related SSCs (Subsection (1)), non-safety-related SSCs (Subsection (2)), SSCs relied on in safety analyses or plant evaluations to demonstrate compliance with Commission regulations j
(Subsection (3)), and SSCs suh 4 to technical specification operability staff's conclusions about the adequacy requirements (Subsection (4)).
o of the BWOG report pertain to u..wral compliance with the language of 10 CFR l
Part 54 and the intent of the Statements of Consideration for the license renewal rule.
Specific safety and technical assessments could not be supported by the level of detail in the submittal.
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The staff completed its initial review of the BW0G system screenir.g methodology and provided BWOG with requests for additional information (RAls).
BWOG responded to the staff's RAls oy letters dated May 19, July 22, and f
August 4, 1993.
In addition, the staff c:iscussed questions concerning the BWOG System Screening Methodology with BWOG representatives at public meetings r_
held on March 16, April 13, May 11, and June 7 and 8, 1993.
l The staff reviewed the BWOG responses to the RAls to determine if the response was acceptable or if additional information will be required. On pages 4 i
and 5 of this report, Table 1, " Status of Staff's Review of BWOG Responses to l
RAls," describes the sta+"s of t M staff's review of the responses to the RAI.
l The status of each RAI i-Jescribed as either open, confirmatory, or closed.
l Open means that the staff requires additional information before it can P
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determine the acceptability for the specific area of concern; confirmatory means that the staff has determined that the specific area of concerr. will be acceptable pending completion of an agreed upon revision to the document; and closed means that the BWOG response is acceptable and no further action is required.
For those RAls that are discussed in this report, the table cross-references the appropriate report section. The staff will require the BWOG to revise its system screening methodology, as necessary, to incorporate the satisfactory resolution of all confirmatory and open issues before publication of a final safety evaluation repcrt.
3.0 EVALUATION OF BWOG SYSTEMS SCREENING METHODOLOGY P"' ORT in Section 3.0 of the BWOG report, the approach uses F.a lcvels of screening to complete the IPA:
(1) system and structure screening and (2) component screening. The BWOG approach uses for sequential tasks to complete the systems and structures screening:
(1) define ITLR requirements to be used in the evaluation, (2) identify information sources, (3) evaluate plant systems and structures, and (4) document systems and structures that are determined to be ITLR.
3.1 SAfsty-Related Systes and Structures (Subsection (1))
The license renewai rule and the BWOG guideline specify that safety-related systems and structures are those relied upon to remain functional during and following design-basis events (DBEs).
In Section 3.1.1 of the BWOG System Screening Methodology, the BWOG states that DBEs include design-basis accidents, normal operation and anticipated operational transients, and external events.
Design-basis accidents are those accidents included in each plant's final safety analyses report (FSAR).
Normal operational transients, as defined in 10 CFR 50.49(b)(1), are bounded by the design-basis accidents and FSAR site-specific hazards analyses.
Site-specific hazards include external events and natural phencmena such as seismic events, hurricanes, tornadoes, floods, airplane crashes, and external plant fires.
In its RAI, the staff asked how internal flooding will be considered, what internal flooding sources will b.s considered, and will pipe or tank ruptures be considered even if they are outside the CLB (RAI 1.1.12).
In its response, the BWOG stated that if the plant-specific licensing basis includes an analysis of internal flooding, then it will be included in the plant-specific assessment.
The scope of ITLR review will be consistent with the CLB and will not consider any postulated events outside the CLB.
The BWO" committed to modify Section 3.1 of its report to clarify the position relc. 'd to the treatment of internal flooding considerations.
The staff co 'sudes that the treatment of internal flooding and pipe or tank ruptures is consistent with the principles of license renewal and is acceptable pending revision of the BWOG report.
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TABLE 1.
STATUS OF STAFF'S REVIEW OF BWOG RESPONSES TO RAIs DSER RAI NUMBER STATUS SECTION COMMENTS RAI 1.1.1 Closed NA Plant-specific review. Additional discussion during review of plant screening results.
RAI 1.1.2 Confirm-NA Clarified the scope of the report, atory RAI 1.1.3 Closed NA Response clarified the term " safe shutdown."
RAI 1.1.4 Closed NA Plant-specific review.
Additional discussions concerning format of displaying screening results may occur during review of plant-specific screening results.
RAI 1.1.5 Closed NA Clarified the terms " requirements" and
" criteria."
RAI 1.1.6 Confirm-NA Clarified use of current licensing basis atory in screening process.
RAI 1.1.7 Confirm-NA Clarified relationship between l
atory commitments and final safety analyses report (FSAR).
i RAI 1.1.8 Closed NA Clarified screening for non-safety-related systems.
RAI 1.1.9 Closed NA Listed " generic" design-basis events.
RAI 1.1.10 Closed NA Provided generic examples of safety-related SSCs.
RAI 1.1.11 Confirm-NA Clarified difference between " included atory in this assessment" and " included as ITLR."
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RAI 1.1.12 Confirm-3.1 Discussed how internal flooding will be atory considered.
RAI 1.1.13 Closed 3.1 Plant-specific verification.
RAI 1.1.14 Closed 3.2 Screening criteria for non-seismic Category I structures.
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TABLE 1.
(Cont.) STATUS OF STAFF'S REVIEW OF BWOG RESPONSES TO RAIs RAI NUMBER STATUS DSER COMMENTS SECTION RAI 1.1.15 Confirm-3.1 Clarified criteria for systems and atory structures that are required to remain functional during a seismic event.
RAI 1.1.16 Open 3.2 Impact of non-safety-related systems on operability of safety systems.
RAI 1.1.17 Closed NA Leak-before-break and pipe whip considerations.
RAI 1.1.18 Closed NA Clarified high energy line break (HELB) consideration and nonsafety systems.
RAI 1.1.19 Closed 3.2 Clarified use of cumulative usage factor (CUF).
RAI 1.1.20 Closed NA Clarified criteria for screening Category 1 and Category 2 environmental qualification equipment.
RAI 1.1.21 Closed NA Clarified screening criteria for long-lived cables and electrical systems.
RAI 1.1.22 Confirm-NA Clarified the acceptance criteria for atory pressurized thermal shock considerations.
RAI 1.1.23 Confirn-NA Clarified that all equipment needed to atory cope with and recover from a station blackout will be considered ITLR.
RAI 1.1.24 Closed 3.4 Technical specification limiting conditions of operation and process variables.
RAI 1.1.25 Closed NA Plant specific.
No intention of using NRC Interim Policy Statement for technical specifications.
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The BWOG report indicates that safe shutdown systems will remain functional after a seismic event.
In its RAI, the staff requested clarification about whether some safe shutdown systems and structures are required to remain functional during the seismic event (RAI 1.1.15).
The BWOG stated that some safety shutdown systems and structures are required to remain functional during and after a seismic event. Other systems and structures are required to remain functional only after a seismic event. The BWOG committed to modify Section 3.1.1.2 of its report to clarify that functionality requirements for equipment during and after a seismic event must be evaluated on a plant-t specific basis.
The BWOG approach will require a renewal applicant to classify systems and structures as ITLR if the system or structure must function during or following a seismic event.
The staff concludes that the BWOG screening criteria for equipment required to function during and after a seismic event are consistent with 10 CFR Part 54 and are acceptable pending revision of the BWOG report.
In the report, the BWOG states that normal operation and operational transients are bounded by the design-basis accidents contained in the plant-specific FSAR.
In its RAI, the staff requested that the BWOG provide the rationale for this conclusion (RAI 1.1.13).
In its response, the BWOG stated that design-basis accidents were selected, by definition, to bound conditions of normal operation and anticipated operational occurrences.
The BWOG Generic License Renewal Program will consider DBEs to be bounded by design-basis accidents (i.e., FSAR Chapter 14 or 15 accidents) and site-specific hazards (e.g., earthquake, flood) for the purpose of system and structure screening.
The staff concludes that the BWOG approach for considering DBEs is consistent with the staff discussions in the Statements of Consideration for 10 CFR Part 54 and is acceptable.
As noted in the BWOG report, each renewal applicant must use a plant-specific approach to implement the screening steps to account for the variability of documentation, information management systems, and requirements of the plant-specific licensing basis for identifying safety-related systems and structures that are ITLR.
The staff will review the implementation of these screening criteria during its review of a plant-specific license renewal application.
3.2 tip_n-Safety-Related Systems and Structures (Subsection (2))
The license renewal rule requires that all non-safety-related SSCs whose failure could directly prevent the satisfactory accomplishment of required safety functions be identified as ITLR.
In Section 3.1.? of the BWOG report, i
the BWOG states that non-safety-related SSCs whose failure could directly prevent a safety function from being accomplished will be identified as ITLR.
According to the BWOG, non-safety-related SSCs that can aiversely affect the operability
' safety-related SSCs during seismic events are assessed at most plants using a seismic 2 over 1 evaluation and walkdown. The supports and restraints required to ensure operability of safety-related SSCs will be identified as ITLR.
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l High-energy line break (HELB) evaluations include pipe whip and harsh environmental considerations resulting from HELBs both inside and outside containment.
Harsh environments resulting from the postulated rupture of high i
i energy lines will be evaluated as part of the Environmental Qualification screening in Section 3.3.2 of this report.
Pipe wl.ip concerns affecting the ancillary equipment that ensures the operability of safety-related SSCs required for safe shutdown will be addressed during component screening.
In its RA!, the staff requested additional information regarding how structures will be evaluated to determine if they are ITLR and how the BWOG intends to evaluate non-Seismic Category I structures (RAI 1.1.14).
In its r
response, the BWOG stated that all safety-related structures are ITLR and a non-Seismic Category I structure is ITLR if it meets one or more of the elements of the SSC ITLR definition of the rule. According to the BWOG, detailed criteria for screening non-Seismic Category 1 structures are anticipated to be developed during the plant-specific implementation process.
The staff concludes that the BWOG approach for screening non-Seismic Category 1 structures on a plant-specific basis is acceptable.
The staff will review the implementation of this screening criteria and the plant-specific detailed criteria during its review of a plant-specific license renewal application.
The staff determined that the screening criteria for evaluating the effect of non-safety-related systems on the functionality of safety-related systems remained unclear (RAI 1.1.16).
In its response, the BWOG stated that decision criteria for implementing this ITLR requirement are being developed as a part of the plant-specific implementetion process. The BWOG committed to revise Section 3.1.2 of its report to state that non-safety-related SSCs that can adversely affect safety-related SSCs during seismic events are addressed at most plants using either a seismic interaction study or a seismic 2 over 1 evaluation and walkdown.
The staff concludes that this response is acceptable for operability concerns associated with seismic events.
However, the staff is interested in understanding where the proposed system and structure screening process truncates consideration of non-safety related systems.
For example, if a non-safety-related system (System A) is required for satisfactory accomplishment of one of the functions listed in Subsection (1) of the definition of SSC ITLR, would another non-safety system i
that supports System A be identified as ITLR7 This is an open issue that must be rcsolved before the staff can make its final acceptability determination regarding the BWOG system and structure screening methodology for evaluating non-safety-related systems.
In response to a staff question regarding HELBs and whether additional pipe failures will be postulated for those piping systems where projected cumulative usage factor (CUF) will exceed 0.1 during the renewal period (RAI 1.1.19), the BWOG stated that use of a CUF of 0.1 will be considered only if the CUF is contained
.a the plant-specific CLB.
For those plants where the CUF of 0.1 is part of the CLB, CUF will be evaluated in the component evaluation process.
The results of the evaluations will be used to determine if additional SSCs are ITLR.
The staff concludes that this approach for considering CUFs is consistent with 10 CFR Part 54 and is Tcceptable.
The staff will review the implementation of these screening criteria during its review of a plant-specific license renewal application.
. 3.3 Systtms and Structures Relied on in Safety Analyses or Plant Evaluations to Demonstrate Compliante With Commission Regulations (Subsection (31)
The license renewal rule requires that all SSCs relied on in safety analyses or plant evaluations to demonstrate compliance with the Commission's regulations for fire protection, environmental qualification, pressurized thermal shock, anticipated transient without scram (ATWS), and station blackout be identified as ITLR.
In Section 3.1.3 of the BWOG report, the approach would require that all SSCs relied on to demonstrate compliance with the requirements for fire protection, environmental qualification, pressurized thermal shock, ATWS, and station blackout be identified as ITLR.
3.3.1 Fire Protection in Section 3.1.3.1 of its report, BWOG states that a fire protection plan has been implemented at each BWOG site in accordance with 10 CFR 50.48 and Appendix A to 10 CFR Part 50, General Design Criterion 3.
Currently, each operating BWOG plant complies with either Appendix R to 10 CFR Part 50 or NRC Branch Technical Position CMEB 9.5-1, " Guidelines For Fire Protection For Nuclear Power Plants," and the backfit provisions of Sections III. G, J, and 0 of Appendix R to 10 CFR Part 50.
The BWOG system screening methodology will require each utility to review the plant-specific documents in its CLB to determine required fire protection equipment and to document the source of the requirement.
An evaluation of the fire protection programs will be performed to identify all SSCs relied on to meet each plant's CLB commitments.
The BWOG approach would require'a renewal applicant to identify as ITLR any fire suppression or detection systems required for compliance with the 10 CFR 50.48.
For compliance with Appendix R to 10 CFR Part 50, all equipment required to bring the plant to a cold shutdown condition will be identified as ITLR.
The staff concludes the BWOG approach for screening fire protection systems and structures is consistent with the philosophy of the license renewal rule and is acceptable.
The staff will review the implementation of these screening criteria during its review of a plant-specific license renewal application.
3.3.2 Environmental Qualification ln Section 3.1.3.2 of its report, BWOG states that 10 CFR 50.49 identifies the requirements for the environmental qualification of electrical equipment.
Safety-related electrical equipment and non-safety-related electrical equipment whose failure could prevent accomplishment of safety functions, and post-accident monitoring equipment are included in the environmental qualification (EQ) program. The equipment in the EQ program should remain functional during and following DBEs. The BWOG approach would require a renewal applicant to use its plant-specific EQ master list and identify systems with components on the EQ master list as important to license renewal.
The staff concludes the BWOG approach for screening EQ-related systems and structures is consistent with the plant's CLB and with the philosophy of the license ranewal rule and therefore is acceptable.
The staff will review the implementation of these screening criteria during its review of a plant-specific license renewal application.
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3.3.3 Pressurized Thermal Shock In Section 3.1.3.3 of its report, the BWOG approach would require a license renewal applicant to perform or review a plant-specific analysis to ensure that neutron radiation damage to the vessel does not exceed the criteria defined in 10 CFR 5e.61. According to the BW0G, if these criteria are exceeded for any material in the belt line region, the licensee shall submit a list of any plant modifications to systems, equipment, and operation necessary to prevent potential failure of the reactor vessel.
The systems listed would be identified as ITLR. The staff concludes the BWOG approach for screening systems susceptible to pressurized thermal shock is consistent with each plant's CLB and with the philosophy of the license renewal rule and is acceptable.
The staff will review the implementation of this screening criteria during its review of a plant-specific license renewal application.
3.3.4 Anticipated Transients Without Scram In Section 3.1.3.4 of its report, BWOG states that all its plants are required by 10 CFR 50.62 to ensure that the following two systems and/or functions exist at each plant:
(1) System and/or equipment, from sensor output to final actuation device, that is diverse from the reactor trip system. and that will automatically initiate the auxiliary (or emergency) feedwater system and initiate a turbine trip under conditions indicative of an ATWS. This equipment must be designed to perform its function in a reliable manner and be indepen-dent (from sensor output to the final actuation device) from the existing reactor trip system.
(2) System and/or equipment that provides a diverse scram system from the sensor output to interruption of power to the control rods.
This scram system must be designed to perform its function in a reliable manner and be independent from the existing reactor trip system (from sensor output to interruption of power to the control rods).
The BWOG approach would require all renewal applicants to review their plant-specific ATWS analysis to identify all equipment relied on to comply with 10 CFR 50.62.
Renewal applicants will identify all systems required for ATWS implementation as ITLR.
The staff concludes the BWOG approach for screening ATWS-related systems and structures is consistent with each plant's CLB and the philosophy of the license renewal rule and is acceptable.
The staff will review the implementation of this screening criteria during its review of a plant-specific license renewal application.
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3.3.5 Station Blackout The loss of all offsite power sources and the failure of on-site ac emergency power supplies (except battery backed ac power) is considered a station blackout (SB0) as defined in 10 CFR 50.2.
SB0 requirements (10 CFR 50.63) can be satisfied by either installing an alternative ac power source or demon-strating by a coping analysis that the plant can be maintained in a safe shutdown condition.
The BWOG approach would require renewal applicants to review their licensing basis for SB0 and identify equipment required by either the coping analysis or the alternative ac power source option as ITLR equipment.
The BWOG approach would require renewal applicants to identify all equipment needed to cope with and recover from an SB0 as ITLR.
The staff concludes that the BWOG approach for screening SBO-related systems and structures is consistent with the philosophy of the license renewal rule and is acceptable.
The staff will review the implementation of this screening criteria during its review of a plant-specific license renewal application.
3.4 Systems and Structures Sub.iect to Technical Specification Operability Reauirements (Subsection (41)
The license renewal rule requires that all SSCs subject to operability requirements contained in the facility technical specification limiting conditions for operation (LCOs) be identified as ITLR.
In Section 3.1.4 of the BWOG report, the approach requires a renewal applicant to identify as ITLR all equipment covered by technical specification LCOs and required to be operable.
According to the BWOG, unless otherwise covered by an LC0 in the technical specifications, instrumentation to monitor process variables is not considered to be ITLR.
In its RAI, the staff requested the basis for the BWOG position on process variables and the decision criteria that will be used to determine when instrumentation would be identified as ITLR (RAI 1.1.24).
In its response, the BWOG stated that process variables are not SSCs and that there are two ways that process variables may be presented in technical specifications:
(1) the process variable can be associated with an instrument or (2) the process variable can be associated with a mechanical component.
Instrumen-tation associated with a technical specification process variable (i.e., level of the borated water storage tank (BWST)) will be considered ITLR if it meets one or more of the elements of the codified SSC ITLR definition.
Baron concentration would be an example of a process variable associated with a mechanical component and in this case the associated storage tank, the BWST, would be considered the ITLR component and would have been previously identified through the system and structure screening approach.
The staff concludes that the BWOG approach to evaluate process variables and identify SSCs subject to technical specification operability requirements is consistant with the requirements of the license renewal rule and is therefore acceptable.
Process variables are not SSCs and, as a result, systems, instruments, or other components associated with process variables will be identified as ITLR if they meet one of the four criteria contained in the definition of SSC ITLR.
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4.0 CONCLUSION
S On the basis of its evaluation of the BWOG System Screen ng Methodology, the 4
staff concludes:
With the exception of non-safety-related SSCs whose failure could prevent accomplishment of a safety function, the BWOG approach for screening systems and structures is consistent with the requirements of 10 CFR Part 54 and, if properly implemented on a plant-specific basis, should result in the identification of all systems and structures that are important to license renewal.
An applicant for license renewal that references the BWOG system screening methodology will be required to evaluate its CLB and develop its own implementation procedures.
The staff will review and audit the implementation of the plant-specific methodology during its review of a license renewal application.
5.0 OPEN ITEMS The following open issue pertains to the staff's review of the BWOG system screening methodology.
Screening requirements for non-safety-related SSCs whose failure could prevent accomplishment of a safety function.
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