1. ECCS Operability Determination Jim Andrachek Fellow Engineer Westinghouse Electric Company

2. ECCS Operability Determination  WCAP-16793, Rev. 2  Fuel Assembly Testing Conservatisms  Use of the Testing Conservatisms to Support an Operability Determination  Example of Using the Testing Conservatisms to Support an Operability Determination  Summary and Conclusions

3. ECCS Operability Determination  WCAP-16793, Rev. 2 – Documents the Westinghouse (W) and AREVA fuel assembly (FA) testing that was performed to determine the LOCA generated fibrous debris that would be deposited on an individual FA to ensure adequate core cooling – A limiting value of 15 grams per fuel assembly (g/FA) was established for W and AREVA FAs

4. ECCS Operability Determination  WCAP-16793, Rev. 2 (cont.) – The 15 g/FA in-vessel fiber limit is very conservative and resulted from using bounding values for individual test parameters – The limit reflects the cumulative effect of multiple conservative test conditions and assumptions

5. ECCS Operability Determination  WCAP-16793, Rev. 2 (cont.) – A significantly higher in-vessel fiber limit would be determined if more realistic test conditions and assumptions were used

6. ECCS Operability Determination  FA Testing Conservatisms – Ambient Test Temperature The testing was performed at an ambient temperature (approximately 70°F) versus the actual sump fluid temperature A lower water viscosity results in a lower pressure drop through the debris bed

7. ECCS Operability Determination  FA Testing Conservatisms (cont.) – Constant Flow Rate The hot-leg testing used a constant flow rate of 44.7 gpm FA, and the available driving head calculations assumed both a water solid core and the shortest steam generator (SG) U-tubes

8. ECCS Operability Determination  FA Testing Conservatisms (cont.) – Constant Flow Rate (cont.) Plant specific ECCS flows and actual SG tube heights would provide additional driving head The driving head calculations assumed a water solid core and did not credit the increase in available driving head for a core with a large void fraction

9. ECCS Operability Determination  FA Testing Conservatisms (cont.) – Uniform Flow Following a LOCA, the FA-to-FA power difference will initially promote non-uniform flows, which will result in a non-uniform debris distribution throughout the core Only one FA needs to be open to remove core decay heat and ensure LTCC

10. ECCS Operability Determination  FA Testing Conservatisms (cont.) – Recirculating Debris Debris that enters the RCS but does not settle or is not captured in the core, would be carried out of the break into the sump where it could settle out or be rescreened before entering the ECCS again Any reduction in the amount of debris entering the RCS will result in a lower core ∆P promoting adequate flow through the core to ensure LTCC

11. ECCS Operability Determination  FA Testing Conservatisms (cont.) – Surrogate Chemical Effects The FA testing used aluminum oxyhroxide (AlOOH) as a surrogate representative of all chemical precipitate products Plant specific chemical products and precipitates would reduce the head loss due to chemical effects for many plants where ALOOH is not representative

12. ECCS Operability Determination  FA Testing Conservatisms (cont.) – Staging of Debris The NRC guidance on strainer head loss, which was adopted for the FA testing, indicates that the sequence of 100% particulate, followed by fiber may be overly conservative and offers alternative methods of debris introduction, including a series of tests using homogeneous debris addition

13. ECCS Operability Determination  FA Testing Conservatisms (cont.) – Limiting the Particulate to Fiber Ratio (p:f) It was determined that a p:f of 1:1 was limiting at the 44.7gpm limiting flow rate used in the hot-leg testing, resulting in a conservatively high head loss upon the introduction of ALOOH for the limiting hot-leg break As p:f increases to values greater than 1:1, the head loss due to the effect of ALOOH is reduced for hot-leg conditions

14. ECCS Operability Determination  FA Testing Conservatisms (cont.) – Absence of Boiling The FA testing assumed no boiling within the fuel assembly Industry data indicates that debris beds will not form in the presence of boiling Boiling is minimized in the event of a hot-leg break. In the event that a total blockage would occur at the core inlet, the coolant will begin to boil. The presence of boiling also provides a greater core void fraction than what has been assumed. This results in a greater available driving head, which, in turn, provides the capability to withstand a greater head loss due to debris

15. ECCS Operability Determination  FA Testing Conservatisms (cont.) – Absence of Boiling (cont.) At the onset of the event, there will be boiling in the core. This will prevent the buildup of debris on spacer grids throughout the core. The turbulent nature of the boiling higher in the core will cause the water-solid areas to be turbulent, thus minimizing the debris collection in those areas. The presence of boiling also provides a greater core void fraction than what has been assumed. This results in a greater available driving head, which, in turn, provides the capability to withstand a greater head loss due to debris

16. ECCS Operability Determination  FA Testing Conservatisms (cont.) – Alternate Flow Paths Many PWRs have core designs that provide alternate flow paths around the periphery (baffle) of the core Additionally, if the maximum debris bed resistance is reached at the core entrance, it would cause ECCS water to back up through and spill over the lowest SG tubes, water would drain through the hot- legs to the top of the core, providing an additional source of cooling

17. ECCS Operability Determination  FA Testing Conservatisms (cont.) – Alternate Flow Paths (cont.) The FA testing did not consider flow through the baffle region or possible spillover of the SG tubes or hot legs. For plants with “upflow” baffle geometries, some debris accumulation in the core inlet may divert flow into these regions, which will lead to debris and additional flow introduction higher in the core. These paths are available to provide flow to the core in the unlikely event the core inlet is completely blocked with debris

18. ECCS Operability Determination  Use of Testing Conservatisms to Support an Operability Determination – Tech Spec 3.5.2, “ECCS- Operating,” in NUREGs-1430, 1431, and 1432 requires 2 trains of ECCS to be operable – LCO 3.0.3 must be entered if 2 trains of ECCS are inoperable (i.e., less than 100% of the ECCS flow equivalent flow to 1 operable ECCS train)

19. ECCS Operability Determination  Use of Testing Conservatisms to Support an Operability Determination (cont.) – If a licensee incorporates the 15 g/FA fiber limit contained in WCAP-16793, Rev. 2 in its design and licensing basis, there can be circumstances where that limit may not be met: – Additional insulation is identified that was not included in the calculation that determined that the 15 g/FA fiber limit was met

20. ECCS Operability Determination  Use of Testing Conservatisms to Support an Operability Determination (cont.) – The amount of fiber that bypasses the screens and enters the core is higher than previously determined – Debris in containment is identified that would have resulted in exceeding the 15 g/FA fiber limit in the past (Past Operability/Reportability)

21. ECCS Operability Determination  Use of Testing Conservatisms to Support an Operability Determination (cont.) – If any of these circumstances occur, an Operability Determination (OD) must be made – The testing conservatisms can be used as the basis for an Immediate OD

22. ECCS Operability Determination  Use of Testing Conservatisms to Support an Operability Determination (cont.) – The quantification of the margins associated with the testing conservatisms can be used as the basis for a Prompt OD

23. ECCS Operability Determination  Example of Using the Testing Conservatisms to Support an Operability Determination – The ambient test temperature and constant flow rate conservatisms provide an additional [10] g/FA of margin that is applicable to all plants – Plants with alternate flow paths to cool the core can credit an additional [25] g/FA of margin

24. ECCS Operability Determination  Example of Using the Testing Conservatisms to Support an Operability Determination (cont.) – Plants that can confirm that they do not form chemical precipitates prior to hot leg switchover can credit an additional [30] g/FA of fiber margin

25. ECCS Operability Determination  Summary and Conclusions – If the 15 g/FA fiber limit is exceeded; There is at least [10] g/FA of margin that is applicable to all plants An additional [55] g/FA of margin is applicable to some of the plants as discussed above The maximum additional margin that can currently be demonstrated to be available is [65] g/FA

26. ECCS Operability Determination  Summary and Conclusions (cont.) – If the 15 g/FA fiber limit is exceeded; (cont.) These testing conservatisms would provide reasonable assurance that the ECCS would continue to perform its specified safety function

27. ECCS Operability Determination  Summary and Conclusions (cont.) – If the 15 g/FA fiber limit is exceeded; (cont.) The arguments in this template demonstrating additional margin provided by these testing conservatisms will only be used to support an OD, and not to revise the 15 g/FA fiber design and licensing basis limit –Revision of the fiber design and licensing basis limit will be part of the on-going PWROG closure program and will be based on further testing and analysis

28. Backup Slides (OD Examples)

29. Example #1: <10 g/FA fiber identified  Example – Plant at 15 g/FA fiber loading finds insulation yielding up to 10 g/FA more (total of 25 g/FA of bypassed fiber)  Credit 10 g/FA margin (total of 25 g/FA fiber supportable) – Combination of two generic credits 10 g/FA margin from conservative flows and temperatures Minimum of 7 g/FA margin from p:f ratio

30. Flow and Temperatures  <3 gpm needed to maintain LTCC  Sump begins near 200°F and slowly reduces; most plants stay above 130°F until HLSO  Tests CIB42,48 at Churchill show repeatable, acceptable ∆P at 50 g/FA and 15.5 gpm (vs. 44.7 gpm used in most tests) – 6-7 psi repeatable improvement from 44.7 gpm (CIB49-51)  AREVA and Westinghouse tests at CDI at 25 g/FA of fiber demonstrate acceptable ∆P at 130°F with flow rates exceeding 3 gpm

31. Particulate-to-Fiber Ratio (p:f)  Per NEI 04-07, Rev.0, Vol. 2, Section 3.5, plants can assume 200 lb latent debris (30 lb of fiber) – yields a 5.7:1 p:f ratio of available latent debris  Bypass of debris through strainer more likely to increase, not reduce, p:f ratio (particulate easier to bypass than fiber)  For a plant at ~15 g/FA bypassed fiber, this yields 85 g/FA bypassed particulate, so it takes discovery of 70 g/FA of additional fiber that could be bypassed to reach limiting 1:1 ratio  CIB35-39 demonstrated acceptable ∆P at 150g/FA at 5:1, marginal ∆P at 2:1, and limiting results at 1:1 – Interpolation suggests that ~3.8:1 yields acceptable results for all plants – With 85 g/FA particulate, 3.8:1 is over 22 g/FA total fiber, which yields a 7 g/FA generic margin available

32. Example #2: <35 g/FA fiber identified  Example – Upflow plant at 15 g/FA fiber loading finds insulation yielding up to 35 g/FA more (total of 50 g/FA of bypassed fiber)  Credit 10 g/FA margin from flow, temperature, and p:f  Credit additional 25 g/FA margin from alternate flow paths (total of 50 g/FA fiber)

33. Alternate Flow Paths  AREVA calculations supporting PA-SEE-0779 for upflow plants of B&W, CE, and Westinghouse design demonstrated that debris limits of at least 60 g/FA can be supported due to cooling effects of available alternate flow paths  Based on evaluations and recent STP results, hot leg breaks expected to be cooled by alternate flow paths and/or flow over SG tubes, regardless of debris – Leaves cold leg breaks, which deliver only 20-50% of the bypassed fiber to the core; therefore, the allowable fiber (25 g/FA) can be at least doubled (to 50 g/FA)

34. Available Margins Summary  15 g/FA limit  +10 g/FA additional margin from temperatures, flows, and p:f ratio  +25 g/FA additional margin available from alternate flow paths for all upflow plants  +30 g/FA additional margin available from chemical effects conservatism for plants who can demonstrate delay of chemical precipitation until after HLSO – Based on Churchill tests with acceptable ∆P at 65 g/FA  These margins, based on test results and solid engineering evaluation and judgment, allow plants to address operability determinations