1. Debris Effects in Long-Term Post-LOCA PWR Cooling
Graham WALLIS
American Nuclear Society Northeastern Meeting
October

2. Short-Term Cooling 1960 “No need for ECCS” 1971 Public Hearing
Creation of NRC
LOFT RELAP TRAC
10CFR Appendix K PCT<2200F
Realistic/uncertainty 95/95 confidence

3. Long-Term Cooling As important as short-term cooling Water sources:
Inside containment
Outside containment
Recirculation
Fukushima
External water source. Hundreds of storage tanks

4. GSI-191 1992 Barsebäck BWR event
GSI “Assessment of Debris Accumulation on PWR Sump Performance”
Utilities required to demonstrate effective long-term cooling
No predictive codes
Prototypical tests

5. Sump Strainers NRC allowed strainers to be 50% blocked
Some were “the size of a garbage can”. 10s of square feet of surface.
ACRS presentation “30-50 pickup loads of debris from a large break LOCA”.
Utilities have installed strainers with 1000s of square feet of surface

6. Functional Requirements
Protect downstream devices, particularly the core
Work for all LOCAs
Head Loss not to exceed allowable pump NPSH

7. Protecting the Core
Spacers and grids. Complex shapes with small spaces may trap particles and fibers.
Tests desirable before designing strainers.
Tests are still underway after installing strainers.

8. LOCA Debris Fiberglass. Wide range of lengths.
Particles. Paints, coatings, insulation (CalSil), latent debris.
Chemicals. Hot acidic jet. Long residence in sump at high pH. AlOOH.

9. Comparison with Short-term Cooling
Because of the variety of debris constituents the development of a knowledge base and predictive techniques differs from the short-term case in which the medium was water alone.
Relating head loss and bypass tests to reality is tenuous and risky.
Numerous surprises and anomalous results from tests.

10. Effects on Head Loss and Bypass
Fiberglass and CalSil prepared in blenders.
• Specifying amounts is inadequate. Size matters. More may be better.
• Size spectra. Micron-sized particles.

11. Some Effects
Flow history
Arrival sequence
Sump, Pumps
Surrogates

12. Tests Should be realistic. Uncertain what may be “conservative”.
Prototypical tests
Use of single module (fuel assembly) tests to predict multi-module (core) performance

13. Alternative approaches
Change injection location
Backflushing
Bypass or control rod flow paths
Other, such as removing all fiberglass and CalSil
Risk-inform using PRA?