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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
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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
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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
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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
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Noble Gas Retention Efficiency vs. Humidity
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20%
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100%
0% 20% 40% 60% 80% 100%
Relative Humidity
Rete
nti
on E
ffici
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Charcoal Moisture Content vs. Humidity
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Relative Humidity
Mois
ture
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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 -- -- -- --
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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
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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
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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
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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
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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
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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 -- -- -- --
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Post-treatment Process Radiation Monitor Response
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5 0 0
1 0 0 0
1 5 0 0
2 0 0 0
2 5 0 0
3 0 0 0
3 5 0 0
4 0 0 0
19
97
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00
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D a te
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cp
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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!
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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
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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.
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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?