Quad Cities Unit One Cobalt-60 Experience Presenter:Kenneth Ohr Authors:Kenneth Ohr Radiological Engineering Manager - Quad Cities Ellen Anderson Director - Special Assignment - Exelon Nuclear Don Barker Radiation Protection Manager - Quad Cities

2 Quad Cities Nuclear Generating Station Two Unit GE-BWR III Cordova, Illinois United States On-line: 1973 Unit 1: 825 MWe Unit 2: 912 MWe (power uprated)

3 Where is Quad Cities? Quad Cities Station Cordova, Illinois United States 170 miles West of Chicago, Illinois on the Mississippi River Two (2) units of a total of eleven (11) operating in Illinois

4 Chemistry Background  As an older Boiling Water-type Reactor (BWR), Quad Cities has been engaged in evaluating chemical remedies to both:  Mitigate Inter-Granular Stress Corrosion Cracking (IGSCC), and  Reduce Source Term creation and transport  This quest for Optimal Water Chemistry has included  Hydrogen Addition  Depleted Zinc Oxide Injection  Noble Metals Chemical Addition

5 Chemistry Background  Hydrogen Addition and Hydrogen Water Chemistry  HWC protects stainless steel by scavenging oxygen, ultimately decreasing corrosion on the piping  Increases N-16 dose during power operation  High injection rates required due to need for hydrogen concentration throughout entire volume of water passing though Recirculation piping  Impact:  Protected core internals from IGSCC but only marginally protects Recirculation Piping  Increased dose rates in Steam-Affected areas 5x to 7x

6 Chemistry Background  Depleted Zinc Oxide (DZO) Injection  Reduces the amount of Co-60 incorporated into the primary system corrosion films  Reduces dose rates on primary system piping  Inhibits corrosion on stainless steel surfaces  Competes with cobalt for sites in corrosion films  Suppresses the release of established Co-60 from fuel cladding and in core cobalt-bearing materials  Impact:  Effective in reducing build-up of dose rates on Recirculation Piping (resulted in Lowest Dose Outage ever at Quad Cities)

7 Chemistry Background  Noble Metals Chemical Addition (NMCA)  Platinum and Rhodium are injected into the primary system where they deposit on the piping and other surfaces  Recirculation System piping and vessel internals are protected from inter-granular stress corrosion cracking (IGSCC)  Less hydrogen is required to be injected due to catalytic effect of Noble Metals on piping surfaces  Impact:  Full Protection of Recirculation Piping and core internals  Hydrogen injection flow rates reduced to one-fifth of former flow rates

8 Synergy or Disaster?  The Collective Impact  Each of these components affects the oxide layer on the fuel and other metal surfaces in and outside the core  Changes to any of these components affect both the composition of the oxide layer, and how tightly or loosely it is bound to the metal surfaces  Consequently, they also affect the concentration of isotopes.(Co-60) in reactor water

9 Q1R16 Refueling Outage Discovery  Q1R16 (Unit 1 Sixteenth Refuel)  Commenced at Midnight 14 October 2000  Radiological Conditions were NOT as expected...

10 As-Found Radiological Conditions  Reactor Water Chemistry  Spike in reactor water activity at shutdown: Co-60 in reactor water increases ~15 times normal  Reactor water Co-60 increased by a factor of 1000 during outage  Drywell Dose Rates  3 to 5 times expected values  Dose rates similar to what was seen pre-decon in Q1R15  Secondary (Steam Side) Dose Rates  2 to 5 times normal  Moisture separator dose rates elevated ~10 times normal

11 Unit One Drywell Dose Rates 3.4 mSv 3.0 mSv 2.6 mSv 4.2 mSv 2.0 mSv 1.0 mSv 1.2 mSv 1.8 mSv

12 Unit One Drywell Dose Rates 7.0 mSv 1.3 mSv 2.2 mSv 6.0 mSv 2.4 mSv 2.0 mSv 4.0 mSv

13 Unit One Drywell Dose Rates 3.0 mSv 4.0 mSv 2.8 mSv 8.0 mSv 1.4 mSv 1.0 mSv 1.4 mSv 2.0 mSv

14 Radiological Response  Increased dose rates that did not match any known model at that time led to several major actions to allow for completion of Refuel Outage activities:  Additional Lead Shielding (including shielding of Main Steam Lines - now a significant source)  Deferral of high dose work scope where prudent  Re-evaluation of all ALARA Planning and respiratory requirements / calculations  Increased radiological job coverage with augmented technician and management staffing  Daily Station ALARA Committee Meetings

15 Radiological Response  In parallel an expert team was formed to determine the cause of the unexpectedly high dose rates  Team included members from:  Quad Cities Generating Station  Exelon Station and Corporate RP/Chemistry  General Electric, and  Electric Power Research Institute (EPRI)  Three additional teams were formed to begin evaluation of other long term consequences

16 Root Cause  Root Cause:  Q1R16 High Shutdown Drywell Dose Rates occurred due to a Combination of the Fuel Crud Corrosion Layer Not Being Optimally Stabilized and the High Initial Co-60 Inventory in the Primary Coolant  Other possible Contributing Causes:  NMCA application during mid-cycle outage (no fuel removal)  Excessive hydrogen cycling

17 Root Cause Starting with...  Historically high Co-60 levels at Quad Cities Unit 1 Then...  DZO was not applied long enough prior to NMCA to allow the fuel deposit corrosion films to stabilize Further exacerbated because...  DZO was not injected at a high enough concentration to stabilize the fuel deposits.

18 Insoluble Cobalt-60 Concentrations

19 Soluble Cobalt-60 Concentrations

20 Zinc (DZO) Concentrations

21 Missed Indications May October 2000:  Several anomalies noted by Chemistry Department in reactor water chemistry  Insoluble Co-60 (and other similar isotopes) increased by a factor of 50 times normal  Soluble Co-60 increased by a factor of two  Zinc concentration in reactor water steadily decreased to 2 ppb despite no significant change to the input rate  Poor Communications between departments and lack of confirming indicator (i.e. actual dose rates) prevented early actions

22 Informing the Industry  SIL (Services Information Letter) 631 published by General Electric in November 2000  Lists 6 recommended actions to reduce the impact of corrosion product release and Drywell dose rate increases during operation and shutdown following NMCA applications.

23 Lessons Learned  Many technical and management Lessons have been Learned as a results of the Unit 1 high dose rates  Zinc (DZO) injection requirements captured in SIL 631  Details injection rates (5-10 ppb) and other chemistry parameters  Information now known to the industry as a result of the Quad Cities experience  Open communication between departments  Be aware of laboratory results versus real-world results  Trend and analyze actual response versus anticipate response

24 Extent of Condition  Time between Quad Cities implementation of NMCA and outage confirming dose rate anomalies was 510 days of continuous run on Unit 1  During this time, 12 other NMCA applications were performed in the United States  Latent issue of increased Source Term transport now seen on several other units:  Nine Mile Point Unit 1  LaSalle County Station Unit 1  Peach Bottom Units 1 and 2  But what about Quad Cities Unit 2?

25 Unit One versus Unit Two  Quad Cities Chemistry Timeline revisited...

26 U2 Zinc (DZO) Concentrations

27 Zinc (DZO) Comparison

28 Unit Two Impact  Many differences existed between Unit 1 and Unit 2  29 month DZO injection period prior to NMCA (versus 5 months on Unit One)  NMCA application at End-of-Cycle versus Mid-Cycle  Mid-course correction in DZO concentration on Unit 2  (39 weeks below 5 ppb goal versus 73 weeks)  Better understanding of Co-60 chemistry and impact  Contingency Planning  Contingency shielding packages pre-engineered  Dose “Action Levels” established in each affected area  Contingency scope reductions/ deferrals in place  Staffing and planning to support worst case

29 Unit Two As-Found Conditions  Increases noted in same locations as Unit 1 but to a lesser magnitude  Able to mitigate impact through planned contingency shielding packages in all areas except:  Drywell undervessel area  Refuel Floor Cavity post-draindown for reassembly  Even with mitigation of most of the impact, areas such as the Recirculation Piping discharge risers still doubled (2x) in dose rates

30 U1 (and U2) Recovery  Given the high dose rates and need to complete work in future outages, several long-term actions will take place on Unit 1:  Removal of remaining Stellite (cobalt)-bearing Control Rod Blades (U1 and U2)  Chemical Decontamination of Suction and Discharge Recirculation Piping (U1, U2 being evaluated)  Chemical Decontamination of Moisture Separators (U1)  Installation of Steam Dryer Modification to reduce moisture carry-over (U1 and U2)  Installation of permanent Drywell Shielding (U1 and U2)

31 Long-Term Chemistry Philosophy  Chemical Decontamination will strip both DZO and Noble Metals from Recirculation Piping  What should we do with this “blank slate?”  Issues we will be evaluating:  Impact of residual Noble Metals in vessel  Need to re-apply NMCA every 3 cycles  Appropriate DZO injection levels post-Decon  Increase in Hydrogen addition levels  Ensuring Piping and Core Internals IGSCC protection  Dose impact

What comes next will be the subject of a future story. Thank You. Comments?Questions?