ML17164A839
| ML17164A839 | |
| Person / Time | |
|---|---|
| Site: | Susquehanna  |
| Issue date: | 10/13/1998 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML17164A837 | List: |
| References | |
| NUDOCS 9810200368 | |
| Download: ML17164A839 (13) | |
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U S1AN OC ET NOS 50-387 3 8 By letter dated March 16, 1998, as supplemented by letters May 22, August 10, and September 17, 1998, Pennsylvania Power and Light Company (PPB L, the licensee) submitted a request for changes to the Susquehanna Steam Electric Station (SSES), Units 1 and 2, Technical Specifications (TSs). Technical details were provided in an earlier letter dated Februray 9, 1998.
PP8L is preparing to implement Hydrogen Water Chemistry (HWC) at SSES, Units 1 and 2, to enhance protection of the reactor vessel internals from intergranular stress corrosion cracking.
HWC operation willresult in increased main steam line (MSL) background radiation rates which are expected to exceed the monitor's existing trip setpoint.
To allow for plant operation with the HWC process, and to provide margin against inadvertent reactor scrams and steam line isolations that may result from MSL radiation transients during power operation, the trip setpoints and allowable values need to be increased.
By submittal dated March 16, 1998, the licensee requested approval of the following proposed amendments of the TSs that were in force at that time:
Revision of the MSL radiation monitoring instrumentation trip setpoints and allowable values in TS Table 2.2.1-1, "Reactor Protection System Instrumentation Setpoints," and Table 3.3.2-2, "Isolation Actuation Instrumentation Setpoints," for the SSES, Units 1 and 2.
Relocation of "Main Condenser Offgas Treatment System Explosive Gas Monitoring System," requirements and associated bases from TS Tables 3.3.7.11-1 and 4.3.7.11-1 to the Final Safety Analysis Report (FSAR) and procedures.
Relocation of Limiting Condition for Operation (LCO) and Surveillance Requirements for explosive gas concentration limits and the associated bases from TS.3.11.2.6/4.11.2.6 to a new section in Administrative Controls section of the TS, FSAR Section 16.3 (Technical Requirements Manual (TRM), and controlled documents.
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1 In addition, the licensee requested NRC's concurrence with the licensee's conclusion that the SSES offgas systems are designed to withstand the effects of a hydrogen explosion.
On February 9, 1998, the licensee submitted calculations to support its conclusion and provided additional information in letters dated May 22, 1998, and August 10, 1998.
However, the NRC issued the Improved Technical Specifications (ITS) before the NRC staff took actions to approve the proposed amendment mentioned above.
The ITS approved the relocation of the MSL radiation instrumentation trip setpoints and allowable values to the licensee's TRM and the relocation of the requirements for the explosive gas monitoring and requirements.
Therefore, the TS amendments as requested are no longer applicable because the ITS do not include these items.
By letter dated September 17, 1998, the licensee recognized the changes to its original proposal because of the issuance of the ITS and requested NRC authorization of changes to the FSAR to incorporate the increases in the MSL radiation monitor setpoints and allowable values and the change to the design basis of the offgas system to a detonation resistant design, because the licensee now considered the requested changes as unreviewed safety questions in accordance with 10 CFR 50.59.
2.0 EVALUATION 2.1 Main Steam Line Radiation Monitor Setpoints and Allowable Values The MSL radiation monitors measure the radiation level in the four MSLs downstream of the outboard main steam isolation valves.
Ifthe radiation levels at the MSLs reach the "trip setpoint,"
(1) the Reactor Protection System initiates a reactor scram, and (2) the Primary Containment and Reactor Vessel Isolation Control System initiates closure of the MSL isolation valves (MSIVs).
The licensee's submittal would raise (1) the "trip setpoint from 7.0 times full power background (FPB) to 15 times FPB, and (2) the "allowable value" from 8.4 FPB to 21 FPB. These higher values willhelp to prevent inadvertent reactor scrams and containment isolations when HWC is implemented.
The "allowable values" are specified to provide allowable ranges for instrument drift and uncertainty.
The values for "trip setpoint" and "allowable value" are expressed in terms of FPB at the time when hydrogen is not being injected (normal water chemistry), but they are applicable for the duration of normal water chemistry as well as HWC implementation. The licensee stated in its submittal that a trip setpoint of 15 times the FPB without hydrogen injection is expected to be approximately 3 times the background radiation when the hydrogen chemistry is implemented.
The licensee also stated that this setpoint is consistent with the trip setpoints at other BWRs that have implemented HWC.
In the evaluation of radiological consequences of postulated design basis accidents in the Susquehanna Safety Evaluation Report (NUREG-0776), the staff did not credit actions based on the MSL radiation monitor setpoints and found that the SSES, Units 1 and 2 meet the relevant dose criteria in 10 CFR Part 100 and GIX: 19 of Appendix A to 10 CFR Part 50.
In the event of
MSL break outside containment, the MSIVs are closed on high steam flow signal. The only other design basis accident that potentially releases a large enough amount of fission products into the MSLs to induce a trip would be a control rod drop accident (CRDA).
The licensee performed a calculation to determine the number of failed fuel rods that would be required to potentially induce the proposed MSL radiation monitor trip setpoint using measured nitrogen-16 dose rates at detectors and a 10-second fuel rod failure time (a burst release).
The licensee calculated that failure of 125 fuel rods would induce the proposed trip setpoint (15 times FPB). The staff reviewed the licensee's calculation and found it to be reasonable.
In NUREG-0776, the staff assumed that 770 fuel rods out of a total of 47,368 fuel rods willfail in the event of a postulated CRDA and concluded that the resulting radiological consequences at site boundaries and the control room are well within the acceptable dose criteria. The radiological consequences resulting from the failure of sufficient fuel rods to cause a trip at the proposed MSL radiation monitor setpoints are bounded by the CRDA consequences.
Therefore, the increased main steam radiation monitor setpoints do not affect the CRDA dose consequence analysis.
The proposed trip setpoints and allowable values were calculated in accordance with the licensee's setpoint methodology described in PP8L Design Guide No. JDG-03, "Instrumentation and Control Setpoint Calculation Methodology." The staff has previously accepted this setpoint methodology.
The proposed trip setpoint and allowable value changes are consistent with the licensee's setpoint methodology, and are, therefore, acceptable.
2.2 Change to the Design Basis of the Offgas System The current design basis for the SSES offgas system includes automatic isolation of the system when the hydrogen concentration is greater than 2% to prevent a hydrogen detonation.
In the May 22, 1998, response to the staff request for additional information, the licensee indicated that the offgas system was designed to the criteria in Branch Technical Position (BTP) ESTB 11-1, Revision 0, "Design Guidance for Radioactive Waste Management Systems Installed in Light-Water-Cooled Nuclear Power Plants."
However, the BTP does not address criteria for explosive mixtures in gaseous radioactive waste systems.
Current staff guidance in Standard Review Plan (SRP) Section 11.3, specifies that the offgas system should be provided with dual gas analyzers with automatic isolation provisions to prevent the buildup of hydrogen, or should be designed to withstand the effects of a hydrogen explosion.
Regulatory Guide (RG) 1.143, Design Guidance for Radioactive Waste Management Systems, Structures, and Components Installed in Light-Water-Cooled Nuclear Power Plants provides additional staff guidance pertaining to the design of the offgas system.
RG 1.143 references standard ANSI/ANS 55.4-1979, "Gaseous Radioactive Waste Processing Systems for Light-Water Reactor Plants." Appendix C of ANSI/ANS 55.4-1979 provides criteria for evaluating hydrogen detonations in these systems.
The licensee provided Calculation No. EC-072-1007, Revision 1, "Evaluation of Susquehanna Offgas System Pressure Integrity for Hydrogen Detonation," which was prepared by General Electric (GE), to demonstrate that the offgas system can withstand a hydrogen detonation.
The calculation is based on the methodology described in Appendix C of ANSI/ANS 55.4-1979.
With reference to ANSI/ANS 55.4-1979, RG 1.143 contains the following statement in the discussion section of the guide; "It is expected that these standards will be endorsed separately to be used
in conjunction with this guide or that reference to applicable sections may be used in future revisions to this guide." Therefore, the methodology in Appendix C was not explicitlyendorsed by RG 1.143.
Consequently, the staff reviewed the acceptability of the licensee's use of ANSI/ANS 55 4-1979, Appendix C methodology for the SSES offgas system.
BTP ETSB 11-1, Revision 0, contains a table which specifies the equipment design codes.
The BTP also specifies the seismic design criteria. The seismic design criteria described in BTP is the same as that contained in RG 1.143.
Section 2.1.3 of R.G. 1.143 only requires that the seismic design criteria apply to tank support elements and the building housing the tanks for systems that operate near ambient pressure and retain gases on charcoal adsorbers.
Therefore, the evaluation in Calculation EC-072-1007, Revision 1, did not consider a hydrogen detonation in combination with a seismic event for the offgas piping and equipment.
The staff agrees that this meets the criteria specified in RG 1.143.
The GE evaluation indicates that the detonation pressures are less than the design code allowable pressures for offgas system vessels and the criteria in ANSI/ANS 55.4-1979 for the piping. The design code for piping specified in both BTP ETSB 11-1 and RG 1.143 is ANSI B31.1, "Power Piping." ANSI B31.1 contains criteria for pressure design of piping.
The licensee contends that this criteria is not applicable for the hydrogen detonation analysis.
The staff agrees that an alternative criteria is appropriate for a hydrogen detonation event considering the low probability of the event and the short duration of the peak pressure pulse.
According to Appendix C of ANSI/ANS 55.4-1979, the peak pressure developed during the detonation is dependent on the initial operating pressure at the time of the ignition and system geometry in terms of length to diameter (UD) ratio. The initial operating pressures used for the calculation are taken directly from Susquehanna FSAR Table 11.3-8, and therefore, are acceptable.
For an UD less than seven, ANSI/ANS 55.4-1979 assumes the peak pressure is 17 times the initial pressures.
For UD greater than seven, it assumes the detonation can produce peak pressures as high as 170 times the initial pressure.
This increase is due to the phenomenon of pre-detonation run-up and due to the possible reflection of pressure waves that occur at piping discontinuities (ends, elbows, contractions, valves, etc.). SRP Section 11.3, states that for a system designed to withstand the effects of a hydrogen explosion, the design pressure of the system should be approximately 20 times the operating absolute pressure.
The staff determined the peak pressure model, described in ANSI/ANS 55.4-1979, Appendix C, being a refined model, meets the intent of the SRP.
Therefore, the peak pressure model is acceptable.
For Susquehanna, the factor of 170 was considered too conservative and was modified by GE according to test data that became available after ANSI/ANS 55 4-1979 was published.
The test data were provided by PP&L in a letter dated May 22, 1998, in response to the staff's request for additional information. The test data show that the factor of 170 applies to small diameter piping (8" or less).
For piping 8" diameter and larger the factor is reduced to 80. The staff reviewed the results of the test data and determined that the model used by the licensee to calculate the peak detonation pressures is an additional refinement of the original ANSI model, and therefore, is acceptable.
By letter dated May 22, 1998, in response to the staffs request for additional information (RAI),
PP8L addressed the concerns regarding the equipment survivability, monitoring and controlling the release of radioactivity, and operator actions following a postulated explosion of the offgas
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system.
PPBL stated that the offgas hydrogen analyzers, pre-treatment radiation monitors and other instrumentation, which are not safety-related, may fail following a detonation within the
'ffgas pressure boundary.
Failure of the equipment poses no personnel safety hazard.
As a backup to the offgas pre-treatment monitors, the Turbine Building stack monitor willalarm and prompt operator action under the existing procedures to prevent exceeding occupational and offsite dose requirements.
Plant-specific operator actions following an offgas detonation willbe specified in plant operating procedures prior to removing the offgas high hydrogen trip. Specific operator actions willdepend upon whether an offgas ignition occurs and whether the ignition results in a sustained combustion.
Operator actions following an ignition that willbe evaluated include I) shutdown and repair of failed components, or 2) addition of inert gas to dilute the mixture below the flammable limit. The NRC staff finds PP&L's response adequately addressed the post-detonation concerns raised in the RAI.
Appendix C of ANSI/ANS 55.4-1979 provides criteria for calculating the wall thickness of a pipe that is necessary to sustain the detonation pressure.
The Appendix C criteria specifies that the wall thickness calculation should be based on the material ultimate strength when considering one event for the life of the system or the material yield strength when considering ten or more events.
The criteria accounts for dynamic amplification due to the detonation pulse and contains a factor of safety of 1.15. The GE calculation indicates that the actual pipe wall thicknesses for all SSES offgas piping exceed the values calculated using material yield strengths.
GE used dynamic material properties in the pipe wall thickness calculations.
The staff questioned the applicability of these material properties to the piping systems at SSES.
The licensee replied that the values were derived from test values'reported in the technical literature by Costantino, Schultz, and by Randall and Ginsburgh.
The dynamic material strengths used in the GE calculation are much greater than the material strength values specified in ANSI 831.1.
The staff agrees that dynamic material properties are appropriate for a detonation loading.
However, the staff could not verify that the safety factor of 1.15 used in the wall thickness calculation is adequate to account for possible variations between test dynamic material properties obtained from the literature and the actual dynamic material properties of the SSES offgas piping. The licensee also discussed conservatism in the wall thickness calculations.
One conservatism is the assumption that the peak pressure extends over the entire length of the pipe run whereas the actual detonation pressure wave extends over a relatively short distance.
Another conservatism is the use of a dynamic load factor of 2 in the wall thickness calculation.
The staff agrees that a rigorous analysis would demonstrate that these assumptions in the wall thickness calculations are conservative.
In addition, the staff review of the GE evaluation indicates that the SSES piping would have adequate wall thickness ifthe code minimum ultimate tensile strength values specified in ANSI B31.1 were used in the GE evaluation.
Considering the factor of safety of 1.15 and the conservative assumptions in the thickness calculation, and considering that the piping would have adequate wall thickness ifthe design code minimum.
ultimate strength values were used in the evaluation as discussed above, the staff concludes that there is reasonable assurance that the SSES offgas piping willmaintain pressure integrity for the calculated detonation pressures.
The staff also questioned the licensee's evaluation of the offgas piping for only hoop membrane stresses.
The staff was concerned that the detonation could cause additional dynamic loads resulting from the transient pressure wave propagation through the system.
The licensee considered these loads insignificant because of the short duration of the detonation induced loading. The staff pointed out that a paper by P. M. Ordin, referenced in the licensee's submittal, indicated that the reaction loads from a hydrogen detonation experiment had caused failure in the exhaust duct. The failure occurred in a section of the exhaust duct adjacent to a bend.
The licensee indicated that the exhaust duct failure reported in the Ordin paper was thin walled matenal whereas the SSES piping was much thicker walled. Nonetheless, the staff requested that the licensee provide some assessment of the magnitude of these loads to assure the piping does not experience excessive bending stresses as a result of a hydrogen detonation.
The licensee provided an assessment performed by GE of the loads in an August 10, 1998 submittal.
The GE assessment involved an L-shaped model containing a pipe bend and anchored at both ends.
Although this is a relatively simple model, the staff considers it adequate to assess the potential forwave propagation forces in stiffsections of the piping. GE applied a force time history to the model and the resulting pipe bending stresses were within the ANSI B31.1 allowable limits. The staff considers the GE assessment adequate to address the, concern regarding excessive pipe bending stresses resulting. from pressure wave propagation through the system.
2.3 Conclusions Based on the above analysis, the NRC staff concludes that the licensee's request to increase the MSL radiation monitor "trip setpoint" and "allowable value" to 15.0 FPB and 21.0 FPB respectively to support the implementation of HWC is acceptable The staff reviewed the licensee's calculation of the peak hydrogen explosion pressure and its discussion of monitoring and controlling post-detonation radioactivity and found them acceptable.
The staff finds that the licensee's evaluation of the SSES offgas system components provides reasonable assurance the components can withstand a hydrogen detonation without pressure boundary failure.
3.0 Based upon the written notice of the proposed amendments, the Pennsylvania State official had no comments.
4.0 ENVIRONMENTALCONSIDERATION The amendments related to the MSL radiation monitor setpoints and allowable values change a requirement with respect to installation or use of a facilitycomponent located within the restricted area as defined in 10 CFR Part 20. The NRC staff has determined that the amendments involve no significant increase in the amounts, and no significant change in the types, of any efiluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure.
The Commission has previously issued a proposed finding that the amendments involve no significant hazards consideration, and there has been no public comment on such finding (63 FR 27764). Accordingly, the amendments meet eligibilitycriteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendments.
The above notice did not make a finding of no significant hazards for the offgas system design change.
Therefore, pursuant to 10 CFR 51.21, 51.32, and 51.35, an environmental assessment and finding of no significant impact have been prepared and published in the Zmipzal Eaghhr on October 13, 1998 (63 FR 54738). Accordingly, based upon the environmental assessment, the staff has determined that the issuance of these amendments willnot have a significant effect on the quality of the human environment.
5.0 The Commission has concluded, based on the considerations discussed above, that: (1) there is reasonable assurance that.the health and safety of the public willnot be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendments willnot be inimical to the common defense and security or to the health and safety of the public.
Principal Contributors:
J. Fair J. Lee C. Li B. Marcus S. Malur Date: October 13, 1998
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