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THE MEASURED HEAT RELEASE RATES OF TRANSIENT FUEL ITEMS FOUND IN NUCLEAR POWER PLANTS

Jason Floyd and Matthew DiDomizio, Jensen Hughes, USAKevin McGrattan and Matthew Bundy, National Institute of Standards and Technology, USA

Marko Randelovic and Ashley Lindeman, Electric Power Research Institute, USAInterflam 2019 – July 2019

Hot Gas Layer (HGL)Model (typically CFAST) to determine damage

Fire Modeling for PRA*

in a Nutshell1

Cable Trays (Targets)

Zone of Influence (ZOI)Maximum distance where plume temperature or radiative flux can cause damage

In ZOI = damaged

Not in ZOI or HGL = undamaged

In HGL = ?

Transient Fires in PRA2 PRA Realism3A transient fire is a fire that results from combustible materials not fixed in place like a motor, transformer, or electrical cabinet. A transient fire could occur anywhere that there is a surface where transient materials could be placed.

The current recommended approach on treating transient fires is contained in NUREG/CR-6850 and NUREG/CR-6850 Supplement 1. The recommendation is that transient fires are represented by a gamma distribution with a 75th percentile HRR of 142 kW and a 98th percentile HRR of 317 kW with a growth rate of 2 minutes (unconfined) or 8 minutes (confined – e.g. trash can).

In a PRA, transient fires are typically the number two or three source of fire risk in a nuclear power plant.

At first fire PRAs were used for NFPA 805, to demonstrate that a plant met an overall limit on core damage frequency. It was recognized that conservatisms in the PRAs existed, but they were tolerated since plants were still able to meet risk targets.

*PRA = probabilistic risk assessment

Now PRAs are being used for risk informed maintenance activities. All plants have technical specifications which define minimum amounts of safety equipment that must be operational. If a plant drops below this it must go offline. For some equipment a brief time period for repair is allowed. Being offline can mean $1,000,000 a day or more in lost revenue. Technical specification times originated long before PRAs and were largely based on expert judgement. A plant can get a longer time for repair if the PRA shows the risk is small. PRA conservatism makes this more difficult. This has resulted in a number of research efforts to improve realism in fire PRAs.

The Realism Issue4 Project Outline5 Ignition and Fuels6Phase 1: Burn fuel packages representative of transient fire events in the Fire Events Database (FEDB).

• 290 tests of 99 fuel package were done using a 100 kW calorimeter at Jensen Hughes in Baltimore, MD USA (EPRI funded tests) and the 1 and 3 MW calorimeters at the National Institute of Standards and Technology in Gaithersburg, MD USA (US Nuclear Regulatory Commission [NRC] funded tests). Fires burned without intervention until there was no visible flame.

• Ignition sources were selected to be representative of the strength and duration of those in FEDB events.

Phase 2 (Future Work): Using the FEDB transient fire events, weight the test data and generate new gamma distributions for HRR and ZOI plus guidance on how to model the time-dependence of a transient fire.

• Butane lighter – fast igniting fuels• Nylon wick – 6 cm x 1 cm nylon rope with 5 mL of heptane• Continuous flame – ~100 W propane flame from 0.5 cm tubing• Radiant panel – 15 cm x 25 cm panel at 30+ kW/m2 representing a

halogen work light.

Cotton rags, paper, cardboard boxes, duct tape, tarps, debris piles, plastic buckets, vacuums, trash bags, mop+bucket, rope, plastic chain, power cords, safety cones and chain stanchions, plastic tubing, canvas tool bag with tools, wood scaffolding and pallets, laptop computer, tablet computer. Flammable liquids in plastic bottles, oil absorbing pads, tablet computer, temperature ventilation ducts

The 317 kW 98th percentile heat release rate is based on a set of 27 experiments from the literature at the time NUREG/CR-6850 was being written. Some of the experiments contain fuel items not typically found in risk significant areas of a plant. As a whole, the set of experiments appear to be more severe than operational experience (all plants report fire events to a centralized database maintained by EPRI).

A typical cable tray arrangement is for trays to run 0.3 to 0.6 m above electrical cabinets that are 1.8 to 2.1 m tall. This is within the ZOI of a 317 kW fire.

This suggests that not only does the guidance lack realism (not matching operational experience) but it is also likely resulting in significant conservatisms in fire PRAs. For each fuel type there were multiple configurations and repeat tests.

Peak HRR and Total MJ7 Fire Characteristics8 Cardboard Boxes9

Trash Cans10 Plastic Cart w/ Laptop11 Rope/Hose12

Safety Cone/Stanchion13 Plastic Tarps14 Cotton Rags15

Medium Empty Box

15.2 m of 2.5 cm Nylon Rope

15.2 m of 1.9 cm Vinyl Water Hose

Fire Resistant Tarp

Non-Fire Resistant Tarp

Dry Cotton Rags

Cotton Rags w/5 mL Heptane Per Rag

Stack of Four 23 cm Tall Plastic Safety Cone

102 cm Tall Plastic Chain Stanchion

0.11 m3 Metal Trash Can

0.13 m3 PE Trash Can

Cans have PE bag, plastic water bottles (empty), paper

Laptop, 3-ring Binder w/paper, and printer on two-shelf plastic work cart

Small (0.006 m3) Medium (0.03 m3)

Large (0.2 m3)

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This test represents the worst transient fire event in the FEDB

Test data will be published as a joint EPRI/NRC report (NUREG)

This project was sponsored by the Electric Power Research Institute

© 2019 Jensen Hughes, Inc. All rights reserved.

Tests ≤ 400 kW Tests > 400 kW

Tests ≤ 100 MJ Tests > 100 MJ