ML20036B947
| ML20036B947 | |
| Person / Time | |
|---|---|
| Issue date: | 05/28/1993 |
| From: | Mayfield M NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| To: | Strosnider J Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 9306070220 | |
| Download: ML20036B947 (11) | |
Text
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'o UNITED STATES
[
k NUCLEAR REGULATORY COMMISSION t;
E WASHINGTON. D. C. 20555 s***e/
May 28, 1993 MEMORANDUM FOR:
Jack Strosnider, Chief Materials and Chemical Engineering Branch, DE/NRR FROM:
Michael Mayfield, Section Leader Fracture and Irradiation Section Materials Engineering Branch, DE/RES
SUBJECT:
TECHNICAL BASIS DOCUMENT FOR CODE CASE N-514, LTOP Pursuant to our earlier conversations, I requested a copy of the " official" technical basis document that was developed for ASME Code Case N-514, which addresses LTOP setpoints. Warren Bamford, in his capacity as the Chairman of the Section XI Subgroup on Evaluation-Standards, provided the enclosed Committee Correspondence which includes the requested technical basis document.
By copy of this memorandum, I am placing this information in the NRC's Public Document Room.
Michael Mayfield, Se tion eader Fracture and Irradiation Section Materials Engineering Branch, DE/RES
Enclosure:
As stated cc:
J. McKnight.(for PDR)-
010065 P
9306070220 930528 PDR ORG NRRB f0Y PDR I
l COMMITTEE CORRESPONDENCE committee:
Section XI We'tinghouse.Elec. Corp.
s address writer care of:
P.O. Box 355 Pittsburgh, PA 15230 Technical Basis for Code Case N514, LTOP date:
May 10, 1993
.Owen Hedden copy to:.
to:
Dr. Mich'ael Mayfield U.S. Nuclear-Regulatory Commission Mail Stop 217C NL/5 Washington, DC 20555
Dear Mike:
Per our discussion, I.am enclosing the technical basis for the rec'ently approved code case N514 on low temperature overpressure protection.for your use.
This code case was passed as a code change in essentially identical form; by the ASME Code Main Committee on May 7,1993, and subject to letter ballot and approval by the Board on Nuclear Codes and Standards, will. appear' in the.
1993 Addenda.
9
'9hW J
W. H. Bamford Chairman Section XI Subgroup on Evaluation - Standards rs i
Enclosure n
E.-S The American Societyof Mechanical Engineers 345 East 47th Street New York,' NY 10017.
Keep ASME Codes and Standards Department Informed i
BASES FOR ASME SECTION XI GUIDEUNES FOR LOW TEMPERARJN OVERPRESSUN PROTECTION CRITERLA 1.0 INTRODUCTICN AND BACKGROUND j
1.1 LTOP SERVICE EXPERIENCE in the late 1970's there were a total of approximately 30 events that produced pressure excursions above the technical specification pressure temperature (P/T) Ilmits while reactors were operating at low temperatures (1). The frequency of these overpressure events was high enough for the NRC to classify them as anticipated operational occurrences. Based on this classification PWR licensees implemented procedures to reduce the potential for overpressure events and installed equipment modifications to mitigate such events. The protection systems used to mitigate and reduce the potential for these events are termed low temperature overpressure protection (LTOP) systems.
Service experience during the period 1980 to the end of 1986 Indicates that LTOP events occur with frequency of about 0.1 events per operating year (2) -- 30 events at 55 Wes+Jnghouse and Combustion Engineering plants. This frequency was determined by the number of times that the LTOP system was challenged, e.g., the system pressure exceeded the LTOP set-point pressure. This service experience also indicates that LTOP events are isothermal, occur during heat-up, and that most events occur at temperatures greater then 100*F but less than 200*F (2).
An update of the service experience was performed in support of the WGOPC effort to develop guidelines for LTOP criteria. During the tin,e period 1986 through 1991, 16 challenges to the LTOP system occurred, producing an event frequently of approximately 0.06 events per operating year. However, several situations were identified where at least one of the two required channels of the LTOP system was not available. Further, other conditions were identified where the LTOP pressure relieving devices were found to be outside the specification, which could have resulted in higher then anticipated pressures being achieved had an LTOP evmt occurred.
in summary, service experience continues to indicate challenges to the LTOP systems, suggesting that a physical system to limit pressure to values near the technical specification P/T limit value is needed. Based on this experience, the philosophy adopted by the WGOPC in considering guidelines for LTOP limits was that administrative controls should be imposed to ensure that the P/T limits were not exceeded, and that physical protection system must provide adequate protection against failure of the reactor pressure vessel below the enable temperature where experience indcates the events occur.
1.2 CURRENT REGULATORY allDELINES AND OPERAllNG CONSIDERAllONS Current regulatory guidelines (2) require that a system be " designed and installed to prevent exceeding the applicable technical specifications and Appendix G limits for the reactor coolant system while operating at low temperatures." The technical specification and Appendix G limits, commonly termed the P/T Ilmits, are determined in accordance with 10CFR50, 1
Appendix G and Appendix G of Section XI (A,5). This physfcal protection system is termed the low temperature overpressure protection (LTCP) system.
The LTOP system can be characterized by two parameters: the enable temperature and the set-point pressure for the pressure relieving device. The LTOP system must be enabled at NOT + 90*F where RT or is the adjusted reference temperatures less than or equal to RT N
temperature, including margin (f), at the quarter thickness location. At temperatures greater than RTNOT + 90*F, LTOP protection need not be provided. The maximum allowable pressure is determined based on system-specific corTderations but is chosen so that the maximum pressure attained in the vessel will not exceed the Section XI, Appendix G P/T limit curve.
The pressure relieving devices used in LTOP systems are characterized as either fixed set-point relief valves or variable set-point PORVs. The variable set-point PORVs typically can be adjusted to " follow' the P/T limit curve during heat-up and cooldown while the LTOP system is enabled. However, fixed set-point systems have a constant set-point pressure below the enable temperature, and that pressure is determined for the lowest temperature at which the system can be pressurized, e.g., the minimum bolt-up temperature.
Figure 1 illustrates the relationship between the Pfr limit curve and the LTOP system set-point pressure and enable temperature for a fixed set-point system. Also illustrated are several operational constraints. For example, the maximum ' operating window' pressure is determined by subtracting from the LTOP set-point pressure a margin term introduced to avoid lifting the pressure relief valve and a margin introduced to compensate for pressure measuring system error or the gauge error. The minimum ' operating window
- is determined by adding to the minimum pump seal pressure a margin to account for the pressure difference between the seal and the pressure measuring device, and a margin to account for gauge error. In addition to these constraints on the pressure ' operating window *, the pressure must be high enough to preclude cavitation on the main coolant pump impeller as illustrated by the subcooling curve.
For an embrittled reactor pressure vessel, the LTOP system requirements can impose significant burden on plant operations in two ways.
First, with increasing levels of embrittlement, the allowable pressure for a given temperature is lowered -- a lower P/T limit curve. Second, the enable temperature increases. For fixed pressure set-point systems, this creates a narrow ' window' between the LTOP curve and the pump seal and subcoo!!ng curves (see Figure 1). The burden imposed by the LTOP system can have a significant economic impact by extending plant start-up time (slower heat-up), and restricting plant operation. Further, the narrow operating window has an adverse safety impact because it increases the likelihood of activating the pressure relieving device, and there is a possibility that once the device is activated it will fail to close, thereby creating a LOCA.
The Working Group on Operating Plant Criteria (WGOPC), which has responsibility for Appendix G to Section XI, has considered the burden and safety impact imposed by the LTOP criteria, and has developed Code guidelines for determining the LTOP set-point pressure and the required enabling temperature. These guidelines will relieve some operational restrictions, yet provide adequate margins against failure for the reactor pressure vessel. Further, by relieving the operational restrictions, these guidelines result in a reduced potential for activation of pressure relieving devices, thereby improving plant safety.
2 l
2.0 ASME CODE GUIDEUNES FOR LTOP LIMITS The philosophy adopted by the WGOPC in considering guidelines for LTOP limits was that administrative controls should be imposed to ensure that the technical' specification P/T limits were not exceeded, and that physical protection system must provide adequate protection against failure of the reactor pressure vessel below the enable temperature where experience indicates the events occur. The following guidelines implement this philosophy and have been developed for events that have been classified as Service Level A or B events.
LTOP systems shall be effective at coolant temperatures less than 200'F or at coolant temperatures corresponding to a reactor vessel metal temperature less than RTwor + 50*F, whichever is gre ater. 1,2 LTOP systems shall limit the maximum pressure in the vessel to 110% of the pressure determined to satisfy Appendix G of Section XI, Article G-2215.
The justification for the guidelines is presented in the following section.
3.0 BASIS FOR LTOP SYSTEM PRESSURE TEMPERATURE UMITS The following vessel dimensions and material propertjes were used in the LTOP evaluation:
Vessel inner radius:
86.9 inch, Vessel wall thickness:
8.9 inch, Surface Fluence:
2.3 x 1018 n/cm2, Chemistry Factor:
217 ' F,
initial RTNOT:
0.0*F, and RT or Margin Term:
66*F N
The vessel dimensions were selected to be typical of the population of U.S. commercial PWRs.
The fluence and chemistry factor conditions were selected because they represent the largest projected value of RT DT at end of design life for any of the U.S. plants. Also, at this value of N
RTNOT the allowable pressure is low enough, after accounting for various gage errors and margins, so that the operating window essentially will be closed for practical heat-up or cooldown conditions.
1 The coolant temperature is the bulk reactor coolant temperature.
2 The vessel metal temperature is the temperature at a distance one fourth of the vessel section thickness from the inside wetted surface in the vessel beltline region.
RTNDT s the highest i
adjusted reference temperature (for weld or base metal in the beltline region) at a dstance one fourth of the vessel section thickness from the vessel wetted inner surface as determined by Regulatory Guide 1.99, Rev. 2.
3 4
~
3.1 MARGIN EVALUATICN 3.1.1 Allowable Pressure Appendices G of 10CFR50 and Section XI of the ASME Code define the P/T limits for normal heat-up and cool down operation by using a factor of two on pressure, the ASME,Section XI. Appendix G reference stress intensity factor, K., and a postulated flaw having depth equal to one fourth i
the vessel wall thickness and length equal one-and-one-half times the wall thickness. To determine the effect of allowing the maximum LTOP pressure to be no more than 110% of the allowable pressure defined by ASME,Section XI, Appendix G the factor. on pressure was computed using the ASME reference toughness and postulated flaw size. This factor was then compared to the normal factors provided by Appendix G guidelines for normal heat-up and cooldown.
The basic Section XI, Appendix G criteria can be expressed as pR/t Mm *F+K,
< Kg.
(3-1) i where p = pressure, ksi, R = vessel mean radius, inch, t = base metal wall plus cladding thickness, inch, Mm = factor for membrane loading.
F = Code margin on pressure = 2.0, K,3 = stress intensity factor from thermal loading, ksi Vin, and i
Ki. = ASME reference stress intensity factor, ksi Vin.
Several different flaw location and heat-up and cooldown rates conditions were considered to evaluate the LTOP system maximum allowable pressure. These conditions included: (1) an inside surface flaw where the cooldown rate was 20*F/hr, (2) an inside surface flaw where K,t=0 (K,t ranges from 0 to a negative value for an inside surface flaw and heat-up condtjons; i
I K.t = 0 is used to obtain a conservative allowable pressure), and (3) an outside surface flaw l
with a 20*F/hr heat-up. The pressure temperature curves determined from ASME,Section XI, Appendix G for these flaw location and thermal conditions are presented in Figure 2 and indicate that the 20*F/hr cooldown is limiting overall. For heat-up conditions the 20*F/hr heat-up for an outside flaw is limiting up to about 190*F; at temperatures greater than 190*F tne inside flaw during heat-up is limiting.
4
To assess the recommended criteria it is convenient to express Eq. 3-1 as p + F = (K. - Ki,t) + (t/RMm)
(3-2) l i
To evaluate the LTOP pressure limit in the low temperature region the limiting condition shown in Figure 2 is considered first. For an inside surface flaw with depth equal to a quarter of the wall thickness the RTuoy, including margin, is 300*F, and Ks. at 100*F is 27.5 ksMin. For the indicated vessel dimensions Mm from Appendx G to Section XI is 2.81. And for a 20*F/hr cooldown the K.t s 4.4 ksi Vin. Using these values Eq. 3-2 becomes I i p F = 0.8 ksi.
(3-3)
The t. owable pressure at 100*F determined from ASME,Section XI, Appendix G is determined from E q. 3-3 using F= 2, or p = 0.4 ksi.
(3-4)
If the maximum pressure allowed by the LTOP system is 110% of the allowable pressure then from Eq. 3-4 the allowable LTOP pressure = 0.44 ksi, and from Eq. 3-3 the associated factor on pressure is 0.8/0.44 = 1.8.
To determine the implications of the 1.8 factor on pressure for LTOP limits, vessel integrity is assessed by the ratio of reference stress intensity factor to the applied stress intensity factor, or Matgin = K./(Kip + K )
(3-5) i it where Kip = pRMm/t.
From Eq. 3-1 the value of Kip associated with the allowable pressure determined from ASME,Section XI, Appendix G is Kip = (K. - K )/2 (3-6) i it Substituting Eq. 3-6 into Eq. 3-5 gives Margin = 2Kr./(K. + K ).
(3-7) it For events where there is no thermal stress (i.e., Kit = 0) the margin from Eq. 3-7 is equal to 2.
For events where there is thermal stress the margin from Eq. 3-5 will be less than 2. For 4.4 ksiVin. (obtained for an inside surface flaw and a 20*F/hr example, using Kit =
cooldown), and Ki = 27.5 ksiVin (obtained for a temperature = 100*F and RTNDT equal to 300*F) in Eq. 3-7 gives 5
0 Margin = 2(27.5)/(27.5 + 4.4) = 1.7 (3-8)
The results indicated by Eq. 3-7 for no thermal stress and Eq. 3-8 for a 20*F/hr cooldown show margins with respect to toughness between 1.7 and 2.0 are provided by ASME,Section XI, Appendix G.
Because service experience indicates low temperature overpressure events are essentially isothermal, the margin with respect to toughness for these events is within the range associated with normal heat-up and cooldown in the low temperature range.
Consequently, setting the LTOP allowable pressure at no greater than 110% of the Appendix G allowable provides essentially the same margin against vessel failure as is provided by ASME,Section XI. Appendix G for normal vessel heat-up and cooldown operation.
3.2.2 LTOP Enabling Temperature The LTOP enabling temperature assessment involved determining the temperature that would allow the pressure to reach 110% of the design pressure, or typically about 2,750 psi for PWRs, without initiation of a postulated quarter-thickness depth flaw having RTNOT at the tip of the flaw equal to 300*F. This assessrnent was performed for conditions simulating reactor heat-up (i.e. Kn = 0) consistent with service experience with these events; an evaluation for a 20F/hr cooldown also was included for reference. The results are presented in Figure 3 and indicate that pressure greater than 110% of design pressure is achieved at temperature equal to approximately RTNOT + 50*F.
Based on the results in Figure 3 and service experience that indicates that LTOP events occur below 200*F, the LTOP enabling temperature was selected as the greater of 200*F or RT or +
N 50*F.
This value provides protection by ensuring the LTOP limit is outside the temperature range where the events may occur and is protected from failure by the safety reflef valves.
4.0 iU utNCES 1.
NUREG 0224, ' Reactor Vessel Pressure Transient Protection For Pressurized Water Reactors', September 1978.
2.
NUREG 1326, ' Regulatory Analysis for Resolution of Generic issue 94, December 1989.
3.
Standard Review Plan 5.2.2, Rev. 2, ' Overpressure Protection'.
4.
10 CFR Part 50, Appendix G, ' Fracture Toughness Requirements *.
5.
ASME Eoiler and Pressure Vessel Code,Section XI, Appendix G, ' Fracture Toughness Criteria for Protection Against Failure'.
6.
Regulatory Guide 1.99, Rev. 2, " Radiation Embrittlement of Reactor Vesset Materials'.
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