On-line Drying of Augmented Offgas

Charcoal

Ken Sejkora, Brad Barrus, Charles Minott, Phil Harizi, Paul McNulty

Entergy Nuclear Northeast – Pilgrim Station

Presented at the 12th Annual RETS-REMP WorkshopAtlantic City, NJ: 24-26 Jun 2002

Offgas Treatment System Diagram

Air Ejector

30-min Holdup

Main Stack

Recombiner 30-

min Holdup

Cooler Condenser & Moisture Separator

Charcoal Adsorbers

Augmented System

Offgas Condenser

PNPS AOG Charcoal System Design

Two trains of six series beds each; configurable as series or parallel operation; 12 beds total

3 tons charcoal/bed; 36 tons total Nominal flow rate = 12 scfm Design retention = 29 hours for Kr, 525 hours

(22 days) for Xe; Activity Reduction Factor = 185

Permanent fixture… not designed for removal or replacement of charcoal, no sample ports, minimal instrumentation/monitoring

Charcoal Retention Behavior

Lower is better! Lower humidity/moisture content

yields more available adsorption sites Lower flow rate yields more contact

time, longer retention time; inverse relationship… flow*2 = retention*0.5

Lower temperature yields better dynamics for retention on available sites

Noble Gas Retention Efficiency vs. Humidity

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Relative Humidity

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Charcoal Moisture Content vs. Humidity

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Noble Gas Retention Efficiency vs. Charcoal Moisture Content

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Moisture Content

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Observed AOG Retention

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AOG Inlet/Outlet Comparison 04-Sep-2000 Data, Flow = 37 scfm

NuclideInlet uCi/cc

Outlet uCi/cc

DF

Number of

Halflives

Holdup hours

Percent of

Design

Kr-85m 3.70E-4 2.41E-4 1.54 0.62 2.77 9.4%

Kr-87 2.16E-3 5.66E-4 3.82 1.93 2.46 8.3%

Kr-88 1.29E-3 7.20E-4 1.79 0.84 2.39 8.1%

Xe-133 1.38E-4 9.35E-5 1.48 0.56 70.7 13.3%

Xe-135 2.10E-3 1.35E-4 15.6 3.96 36.1 6.8%

Xe-138 2.67E-2 NDA -- -- -- --

PNPS CHALLENGES High moisture content: charcoal in vessels

was 22% moisture by weight… saturated!! High air inleakage: 32-62 scfm vs. design

value of 12 scfm Perception: tight fuel meant noble gas

releases were low… belief by some that AOG wasn’t needed for ALARA, as doses were already well below 10CFR50 Appendix I objectives

Challenges: Moisture Poorly instrumented lines… most points were

local readout only, no recording of flow, temperature, dew point, etc.

Charcoal was saturated, 22% moisture content… 15,800 lbs = 1,900 gal water

High dew point (50+ deg.F), high humidity (70+%) air into charcoal… kept charcoal saturated

Automatic cooler condenser drain valve was not functioning, would not allow condenser to drain; poor condenser performance, plus carryover of liquid water into charcoal vessels

Challenges: Moisture (continued)

Opening cooler condenser drain valve improved condenser performance… reduced dew point from 50+ deg.F to <28 deg.F

Objective: Now that we had dry air going into charcoal, how do we remove existing 2,100 gallons of water in vessels?

Engineering solution: On-line drying, by heating room to 100+ deg.F, to raise temperature of beds… low dew point air flowing through beds will remove water

On-line Drying: HVAC Approach

Detailed engineering and 10CFR50.59 evaluation Concern for potential combustion of charcoal… limited

room temperature to maximum of 125 deg.F High room temperature raised charcoal temperature from

~77 deg.F to ~89 deg.F Concern that higher temperature would reduce retention

times by affecting dynamics… determined benefit of drying outweighed risk of poorer dynamics

Increased noble gas sampling from once/month to once/week to monitor for degraded dynamics… none seen

Commenced drying Sep 2000, secured drying in Nov 2001

Challenges: Inleakage Turnover of system engineers… lack of continuity Poorly instrumented lines… most points were

local readout only, no recording of flow, temperature, dew point, etc.

High dose rates in areas where leakage was suspected… downpower evolution, past efforts inconclusive

Air purge valve seat leakage: 10-15 scfm 2nd point feedwater heaters: 30 scfm; very

inaccessible, hard to locate Current air inleakage: 13-15 scfm

AOG Inlet/Outlet Comparison 29-Apr-2002 Data, Flow = 16 scfm

NuclideInlet uCi/cc

Outlet uCi/cc

DF

Number of

Halflives

Holdup hours

Percent of

Design

Kr-85m 9.21E-4 NDA -- -- -- --

Kr-87 4.26E-3 NDA -- -- -- --

Kr-88 3.30E-3 NDA -- -- -- --

Xe-133 3.26E-4MDC = 5.25E-6

62.1 5.96 750 141%

Xe-135 4.76E-3 NDA -- -- -- --

Xe-138 1.40E-2 NDA -- -- -- --

Post-treatment Process Radiation Monitor Response

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Challenges: Perception Even with diminished AOG performance,

maximum offsite noble gas dose was <5% of 10CFR50 Appendix I ALARA objectives

Education process… convey that AOG is not for ‘normal’ operations, but meant for minimizing dose impact in the event of fuel failure

“Sell” from standpoint of INPO comparisons for noble gas releases; increased failed fuel operating margin; increased NRC & INPO concern… it’s the right thing to do!

Challenges: Perception (continued)

Activity reduction factor vs. dose reduction factor Substandard retention can provide an

‘acceptable’ DF for short-lived noble gases which have low dose impact… activity reduction may look good, but minimal impact on dose

Reduction factors @10%: Activity Reduction = 8.4; Dose Reduction = 8.2

Reduction factors @100%: Activity Reduction = 190; Dose Reduction = 1,560

Summary Key to success was interdisciplinary team

approach… Engineering, Operations, Chemistry, Maintenance, etc. -- everyone contributed!

Think outside the box… On-line drying?? You’ve got to be kidding!

Attack all the problems, not just one or two Older plants with degradation of ancillary

systems… drain pots, loop seals, condenser cooler performance, etc.

Summary (continued)

Importance of recording instrumentation to aid in diagnosing performance, problems, and success of solutions… need to retrofit?

Dose reduction vs. activity reduction… what are you really going after? How do you measure success?