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Probabilistic examination of an in-cell solvent fire at a reprocessing plant

Mr. Yoshikazu Tamauchi, Mr. Satoshi Segawa, Mr. Katsuyoshi Omori, Mr. Kunihiko Ogiya, Mr. Yoshiaki Hayashi, Mr. Kazumi Takebe and Dr. Shingo Matsuoka

Safety Technology Office, Japan Nuclear Fuel Limited

October 16-18, 2007

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Introduction

An in-cell solvent fire is a typical accident for assessment in any reprocessing plant

The purpose of this report is to reexaminethe fire from a probabilistic viewpoint quantifying

its occurrence frequency and consequence

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Contents

Safety measures and accident scenario of an in-cell solvent fire

Quantification of frequency

Quantification of consequence

Conclusion

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Safety measures and accident scenario of an in-cell solvent fire

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The cause of an in-cell solvent fire

Three conditions are necessary to occur simultaneously.

Organic solventleakage

Ignitionsource

In-cellfire

may occur Heat source

Piping rupture, etc…

Electrostatic spark, etc…

Decay heat, etc…

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The outline of safety measures

Leakage detector

SJP

Connected to ground

Temperature indicator

HW

Heat exchangerHW

SR

Fire damper

CO2

extinguisher system

CO2 Injectionnozzle

Firedetector

Main stack

Cell exhaust blower

Stripping pulsed column

Plutonium purification Column cell

From other cells

HW:Hot WaterSR: Stripping RegentSJP: Steam Jet Pump

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Discussion on accident scenario

Piping rupture, etc…Frequency of occurrence was set 1x10-3/yr

Electrostatic spark, etc…

Organic solventleakage

Ignitionsource

In-cellfire

may occur Heat source

1. Abnormal condition of process which heat up the org. solvent

2. Back flow from the steam jet pump during recovery

3. Decay heat

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Heat source (Hot water)

The possibility to heat solvent above its flashpoint is less than 1x10-3.

The flashpoint of 30% TBP/ n-dodecane is approx. 354 K

Temperature of hot-water is 353 K.

It is necessary to heat solvent over 354 K by the following simultaneous abnormalities.

Supplied hot water has to be over heated.Temperature controller fails.

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Heat source (Decay heat)

It has been confirmed by using FLUENT ( thermal hydraulic analysis program) that the temperature rise due to decay heat was only a few degrees

303.0 305.0 K303.5 304.0 304.5

Approx, 304K

Approx, 305 K

Overhead view of drip tray

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Hot water

Hot water

Org. solvent

Hot waterSteam

Striping regent

Heat source (Steam back flow from SJP)

Pipe choking and steam back flow. A probability was 1X10-2.

Approx, 463K

Organic solvent leakage

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Back flow from the steam jet pump during recovery

Organic solvent leakageInitiating Event

Safety function Eliminate a ignition source

An in-cell solvent fire breaking

Start of recovery using SJP

Choke of SJP (back-flow of steam)

Doesn’t reachits flashpoint

An in-cell firedoesn’t occur

Success

The outline of the accident scenario

It occurred

Failure

It didn’t occur

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Quantification of frequency

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Org.solvent is ignited at 100% probability after a year by electrostatic spark, etc.

The ignition probability is 100％

The duration of heated up solvent above its flashpoint is several hours

One year

Assumption of the ignition probability

The ignition probability of one hour is 1 / 8,760 = 1.2x10-4

The Ignition probability of several hours is 1x10-3

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○ Initiating event (leakage) : 1x10-3/yr○ Failure of heat source removal : 1x10-2

○ Failure of ignition source removal : 1x10-3

◎ Frequency of fire occurrence : 1x10-8/yr

The result of accident occurrence frequency

The frequency of occurrence of in-cell solvent fire

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Quantification of consequence

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Temperature rise analysis in the cell

Analysis model

To quantify the consequence, the behavior of fire was analyzed as a function of time by using FIRST program

The geometric data : based on the Rokkasho Reprocessing Plant

Large-scale fire Small-scale fire

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The result of temperature rise analysis

Temperature of the cell atmosphere(Large-scale fire)

300

350

400

450

500

550

600

650

700

0 50 100 150 200 250

Time (sec)

Tem

pera

ture

(K)

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The result of temperature rise analysis

The position of the boundary of hot and cool layer zones in the cell

0

5

10

15

20

0 50 100 150 200 250

Time (sec)

The p

osi

tion o

f hot

layer （

m)

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Amount of burnt solvent

Large-scale fire small-scale firebecause of lack of oxygen in the cell

Less than 500 liters

Therefore we considercombustion continues 3 hours by ventilation

The amount of organic solvent burnt will be limited by operator actions such as stoppage of ventilation, etc.

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The estimation of consequence

Amount of consumed solvent : 500 liters in 3 hours

There is no degrading of the filter function in case of the fire of 500 liters

The five-factor formula method is used for quantification

Its consequence is calculated as 0.1 mSv

(Considering the main stack release)

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Conclusion

The accident occurrence frequency

Less than 1x10-8/yr

The consequence of accidentApprox. 0.1mSv

There is a room for reconsideration about the importance of in-cell solvent fire

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Conclusion (continued)

• We hope that the reexamination is made not only for the fire described in this presentation but also for other postulated accidents, based on the recent knowledge accumulated in the world, and the results are reflected to a near future reprocessing plant after international discussion and agreement.