Debris Effects in Long-Term Post-LOCA PWR Cooling

Graham WALLIS

American Nuclear Society Northeastern Meeting

October 23 2013

Short-Term Cooling

• 1960 “No need for ECCS”• 1971 Public Hearing • Creation of NRC• LOFT RELAP TRAC • 10CFR 50.46 Appendix K PCT<2200F• Realistic/uncertainty 95/95 confidence

Long-Term Cooling

• As important as short-term cooling• Water sources:

• Inside containment• Outside containment• Recirculation

• Fukushima • External water source. Hundreds of storage tanks

GSI-191

• 1992 Barsebäck BWR event• 1996 GSI-191 “Assessment of Debris

Accumulation on PWR Sump Performance”• Utilities required to demonstrate effective

long-term cooling• No predictive codes• Prototypical tests

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

Functional Requirements

• Protect downstream devices, particularly the core

• Work for all LOCAs

• Head Loss not to exceed allowable pump NPSH

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.

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.

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.

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.

Some Effects

• Flow history

• Arrival sequence

• Sump, Pumps

• Surrogates

Tests

• Should be realistic. Uncertain what may be “conservative”.

• Prototypical tests

• Use of single module (fuel assembly) tests to predict multi-module (core) performance

Alternative approaches

• Change injection location

• Backflushing

• Bypass or control rod flow paths

• Other, such as removing all fiberglass and CalSil

• Risk-inform using PRA?