ML26029A404
| ML26029A404 | |
| Person / Time | |
|---|---|
| Site: | Callaway  |
| Issue date: | 01/28/2026 |
| From: | Ameren Missouri, Union Electric Co, Westinghouse |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML26029A397 | List: |
| References | |
| ULNRC-06922 | |
| Download: ML26029A404 (0) | |
Text
ULNRC-06922 Page 1 of 11 Plant Specific Justification for Application of WCAP-15806-P-A, Revision 0, Westinghouse Control Rod Ejection Accident Analysis Methodology Using Multi-Dimensional Kinetics, November 2003, to Address RG-1.236 (Non-Proprietary)
(11 pages including this cover page)
August 2025 AXIOM, ADOPT', AP1000, and ZIRLO are trademarks or registered trademarks of Westinghouse Electric Company LLC, its affiliates and/or its subsidiaries in the United States of America and may be registered in other countries throughout the world. All rights reserved. Unauthorized use is strictly prohibited. Other names may be trademarks of their respective owners.
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ULNRC-06922 Page 2 of 11 Piant Specific Justification for Application of WCAP-15806-P-A Introduction WCAP-15806-P-A (Reference 1) is the general methodology document for carrying out 3D core kinetics for the rod ejection accident (3DRE). It was approved before RG 1.236 (Reference 2) was published.
The methodology is still applicable for the modeling of the accident and analyses can be used to show compliance with RG 1.236. The newer fuel products to be used for Callaway have been approved by the NRC and can also be used to show compliance with RG 1.236 as described in their topical reports.
The PADS5 code described in WCAP-17642-P-A (Reference 3) provides inputs to WCAP-15806-P-A and the translation of the failure limits to burnup dependence using its approved methods.
The following information shows how the guidance of RG 1.236 has been met.
RG 1.236 Section 1 of RG 1.236 states that it is applicable to the fuel type used for Callaway. In addition, the ADOPT topical (WCAP-18482-P-A, Reference 4) and AXIOM topical (WCAP-18546-P-A, Reference
- 5) discuss the applicability of RG 1.236 to those products.
Section 2.1 describes the analytical methods to be used and WCAP-15806-P-A is in compliance with the codes described and used for Callaway.
Section 2.2 describes some of the calculations that must be carried out, and these have been completed for Callaway and/or they are described in WCAP-15806-P-A. Calculations were carried out for beginning of cycle (BOC), middle of cycle (MOC), and end of cycle (EOC) and for a range of power levels. Some of the sensitivities described in this section were carried out as part of WCAP-15806-P-A.
Some burnup dependent parameters have also been combined to create a composite bounding case.
Section 2.3 describes how the failure limits should be used and this guidance is followed.
Section 2.4 describes the pressure surge calculation. The WCAP-15806-P-A methodology complies with Section 2.4 of the RG 1.236.
Section 3.1 describes the high temperature failure limits which are confirmed using the methodology in WCAP-15806-P-A. In particular, for prompt critical cases, Figure 1 is converted to a burnup dependent limit using steady state rod internal pressure data conservatively calculated by PADS and approved in WCAP-17642-P-A. The transient fission gas release (FGR) from Appendix B of RG 1.236 is used. The transient FGR is described later. Non-prompt events are checked to meet the failure limit by meeting the departure from nucleate boiling ratio (DNBR) limit.
Section 3.2 has the pellet clad mechanical interaction (PCM)) failure limits defined and uses the stress relief annealed (SRA) and recrystallized annealed (RXA) limits from this section and these are combined as described in WCAP-18546-P-A for the AXIOM cladding.
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ULNRC-06922 Page 3 of 11 Section 3.3 describes that molten fuel is assumed to fail. The burnup dependent PADS fuel meit equation from WCAP-17642-P-A, Revision 1, is used to confirm no fuel melt is predicted.
Section 4 describes the radiological consequences. These are met as the amount of fuel failed is less than the amount of failed fuel used in the dose analysis.
Section 5 describes the reactor coolant system (RCS) pressure limit and is confirmed to be met using the worst power trace from the WCAP-15806-P-A simulations.
Section 6 describes the coolability limits that are met. Pellet rim melt is also confirmed to be precluded.
Appendix B describes the transient FGR model which was used to adjust the steady state RIP for the high temperature failure criterion. The transient FGR is calculated from the most limiting enthalpy rise case and uses a conservative axially averaged enthalpy rise and conservatively assumes the burnup is always greater than [
}**. The Callaway analysis use of transient FGR is consistent with this section.
Appendix C describes the hydrogen uptake models. The PAD5 model was used as described in WCAP-17642-P-A, Revision 1 and WCAP-18546-P-A.
WCAP-15806-P-A This is a general methodology, and this report describes the codes and uncertainties to be used for general modeling of the rod ejection accident using 3D core kinetics and the linked SPNOVA/VIPREW codes. The limits used in this report are sample ones and are not approved for use. This topical report describes how to conservatively calculate the following core related parameters:
- 1. Peak fuel enthalpy
- 2. Enthalpy rise
- 3. DNBR
- 4. Fuel temperatures
- 5. Pressure surge Using the approved PADS methodology, conservative values of rod internal pressure (RIP) versus burnup can be calculated, and subtracting the system pressure, the clad delta-pressure can be calculated (adjusting RIP by the transient FGR). Then, Figure 1 of RG 1.236 can be translated to a burnup dependent peak enthalpy limit. The peak rod average burnup from ANC9 can then be used to determine the particular peak enthalpy limit for each fuel rod.
Using the approved PAD5 methodology for AXIOM, conservative values of hydriding versus burnup can be calculated to translate the AXIOM enthalpy rise limit to be against burnup. The peak rod average burnup from ANC9 can then be used to determine the particular peak enthalpy rise limit for each fuel rod.
WCAP-15806-P-A was used for AP1000 PWR and approved in WCAP-17524-P-A Rev. 1 (Reference 6). This application used the draft guide to RG 1.236 available at the time. The PCMI limit in the draft guide was similar to RG 1.236 but the draft guide had it based on oxide wall (O/W) ratio. The same method was used to generate this limit to be burnup based but used O/W ratio as the input instead of hydride.
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ULNRC-06922 Page 4 of 11 The DNBR, fuel temperature/melt, and pressure surge limits are used exactly as described in RG 1.236 and are compared to the values calculated using the methodology in WCAP-15806-P-A.
The code limitations are summarized in WCAP-15806-P-A, Appendix A and they are met.
The approach followed is compliant with this topical report.
APPLICATION This analysis is applicable to Callaway during the transition to and after the implementation of the 18-Month fuel cycle transition. The evaluation covers both currently used IFBA burnable absorbers and planned Gadolinia burnable absorbers. The core models used as input assume a
rodded operating history with insertion of the lead control bank to a position bounding of the typical bite position to conservatively account for rodded operation axial burnup effects and RCCA rod repositioning strategy.
The final safety analysis evaluations were performed with a moderator temperature coefficient (MTC) bias of [
}**.
The current minimum Bea limit of [
}** was used and
}** to account for rod worth uncertainty, resulting in an assumed value of
{
}***. This will guarantee a minimum of l
}** conservatism is applied to the best estimate core average Ber.
Evaluations were performed assuming a code input for the reactor trip at 0.3 seconds into the transient with 0.5 seconds trip time delay. This assumption was justified to be conservative for the final safety evaluation cases since the resulting excore power response exceeds the high fiux (high or low settings depending on the precondition power) trip setpoint and/or high positive nuclear flux rate reactor trip setpoint before 0.3 seconds into the transient.
The potential reactivity addition on rod ejection is dependent on maximum allowed bank insertion limits, which are dependent on power level. Potential reactivity additions are consequently greatest at zero power due to the greater insertion limits. However, key transient results such as peak fuel temperature and enthalpy and minimum ONBR are also dependent on the initial core conditions such as core power. Fuel rod properties such as thermal conductivity will also vary as a function of fuel burnup. Therefore, the accident was evaluated at several times in cycle life and assumed several different initial power levels to conservatively confirm the acceptance criteria.
The clad-pellet gap heat transfer coefficient varies with time. For the hot rod analysis, the initial value is adjusted to obtain steady-state upper or lower bound fuel temperatures versus linear heat rate.
For the hot rod enthalpy analyses, it is conservatively assumed that the hot spot is in departure from nucieate boiling (DNB) within 0.1 seconds from the beginning of the transient. Allowance is made for the zirconium-water reaction and for the variation of material properties. The legacy VANTAGE+ fuel was shown to be bounded by the ADOPT fuel with AXIOM cladding.
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ULNRC-06922 Enclosure § Page5 of 11 A detailed generic rod ejection over-pressurization analysis included conservative assumptions. This precludes the need for a plant-specific rod ejection over-pressurization analysis as long as the core power transient from the updated 3DRE evaluation remains bounded by the reference power transient used in the generic analysis. The limiting core power transient response was reviewed, and it was determined that a plant-specific over-pressurization analysis was not required.
While the ADOPT fuel and AXIOM cladding analyses bound the legacy ZIRLO-clad fuel, PADS fuel temperatures were only generated for AXIOM/ADOPT.
As such, this analysis only covers the AXIOM/ADOPT fuel product during the transition and equilibrium cycles.
RESULTS Several cases were determined to be limiting regarding the acceptance criteria. The most limiting high temperature cladding failure prompt-critical case was determined to be a
rod ejection event initiated from
[
}**
power with the axial power conservatively skewed in the positive direction. This power level is below the applicable RAOC AFD limits and is expected to bound higher initial power cases which use the current RAOC AFD limits within typical cycle-to-cycle variation. The axial power shape of the core was skewed conservatively even though there is no axial offset limit below 50% power level.
For the peak enthalpy case, the maximum value seen in the core is not the limiting result due to the burnup dependence of the limits.
The case with the maximum calculated peak enthalpy and the case closest to the burnup dependent limit are given for this reason.
All other cases used a conservative end of cycle limit for all the rods in the core.
A case initiated from [
}** power was run, and this case represents the prompt-critical breakpoint ($1 ejected rod worth) for the high temperature failure criterion in Figure 1 of RG 1.236.
This case was used to conservatively evaluate the most limiting conditions for the non-prompt critical DNB cases.
A case initiated from [
}** power was run, and this case represents the limiting maximum fuel temperatures conditions.
A case initiated from Hot Zero Power conditions was run, and this case represents the limiting PCMI cladding failure enthalpy conditions.
Table 1
provides a
timeline of the sequence of events for the limiting
[
F*
power case. Table 2 provides a summary of the results for all limiting cases.
Figures 1 through 4
provide plots of the limiting acceptance criteria as a function of time.
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ULNRC-06922 Page 6 of 11 Table 1
- Sequence of Events for 46.5% Power Case
[ Event oo, Time, seconds
_Controlrod ejection initiated 0.0
__Control rod exits the topofthe core 0.1 Peak nuclear power ee.
l 0.1.
Assumed trip signal on high flux l
0.3 Control rods start entering the core
__9.8.
Control rods fully inserted incore 4.2425 Table 2
- Limiting Results Case DNBR l
Maximum Enthalpy* Enthalpy Rise~
(cav/gram)
___ (cal/gram) l Limit
' EOC, 0% Power
- Denotes low burnup (1(170 cal/gram) and hic high burnup (100 cal/gram) limits Westinghouse Non-Proprietary Class 3 Fuel Centerline Temperature,
ULNRC-06922 Enclosure § Page 7 of 11 a
Figure 1
- Maximum Fuel Enthalpy Westinghouse Non-Proprietary Class 3
ULNKC-U6922 Page 8 of 11 l
iL Figure 2
- Minimum DNBR Westinghouse Non-Proprietary Class 3
ULNRKU-UVoyZz Page 9 of 11 l
Figure 3
- Maximum Fuel Centerline Temperature Westinghouse Non-Proprietary Class 3
ULNKU-Uoydd Page 10 of 11 Figure 4
- Maximum Fuel Enthalpy Rise Westinghouse Non-Proprietary Class 3
VULINNU-V0OysZs Page 11 of 11 CONCLUSIONS The 3DRE evaluations performed demonstrate that the acceptance criteria are met:
No fuel failures are predicted to occur due to PCMI.
No fuel failures due to the maximum calculated fuel enthalpy criterion.
No fuel failures due to DNB.
The peak transient enthalpy is less than the core coolability criterion. Fuel melting is not predicted to occur during the transient.
The RCS over-pressurization results remain bounded by the current analysis.
In summary, there are no additional restrictions on the use of ADOPT fuel and AXIOM cladding due to the rod ejection accident as the RG 1.236 failure limits and other associated limits have been met.
REFERENCES
- 1. WCAP-15806-P-A, Revision 0, Westinghouse Control Rod Ejection Accident Analysis Methodology Using Multi-Dimensional Kinetics, November 2003.
- 2. Regulatory Guide (RG) 1.236, Pressurized-Water Reactor Control Rod Ejection and Boiling-Water Reactor Control Rod Drop Accidents, June 2020.
- 3. WCAP-17642-P-A, Revision 1, Westinghouse Performance Analysis and Design Model (PADS), November 2017.
4, WCAP-18482-P-A, Revision 0, Westinghouse Advanced Doped Pellet Technology (ADOPT') Fuel, September 2022.
- 5. WCAP-18546-P-A, Revision 0, Westinghouse AXIOM Cladding for Use in Pressurized Water Reactor Fuel, March 2023.
- 6. WCAP-17524-P-A, Revision 1, AP1000 Core Reference Report, June 2015.
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