Radiolysis in Reactor Coolant Systems

Transcript Radiolysis in Reactor Coolant Systems

Radiolysis in Reactor Coolant Systems - Field Application of Laboratory-scale Information

Craig R Stuart Component Life Technology Branch 2009 May 11

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Acknowledgements

• • John Elliot – co-investigator Glenn Glowa, Joanne Ball – radiolysis modelling and discussions • • Chemistry staff at CANDU utilities CANDU Owners Group and AECL for funding

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Bridging The Gap

Radiation Chemistry of Neutral Deoxygenated Water (Equally valid for light and heavy water)

• Initially the ionising radiation forms short-lived (~  s time scale) reactive radicals:

H 2 O -radn



e aq , ·H, ·OH, ·HO 2

, H 2 , H 2 O 2

• These species react to form steady-state concentrations of stable products:

e aq , ·H, ·OH, ·HO 2

, H 2 , H 2 O 2



H 2 , O 2 , H 2 O 2 UNRESTRICTED / ILLIMIT É

Event H 2 O Time scale per s ·H + ·OH H 2 O* H 2 + ·O H 2 O + + e H 2 O ·OH + H 3 O + Formation of molecular products in the spurs and diffusion of radicals out of spurs.

e aq e aq -, ·H, ·OH, H 2 , H 2 O 2 , H 3 O +

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Figure 4. Radiolysis of water.

10 16 10 14 10 13 10 7 5

Simplistic Radiation Chemistry View

• Reactor systems can be designed to operate in one of two steady-state modes – Net radiolytic breakdown of water H 2 O Radiolysis H 2 + O 2 + H 2 O 2 – Suppression of net radiolytic breakdown of water H 2 O + H 2 added Radiolysis H 2 O + H 2 added – Transitions between these states can lead to unanticipated system chemistry conditions

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CANDU Irradiated Systems

• Primary Heat Transport System – Add hydrogen to operate with no net radiolytic production of hydrogen and oxygen • • • Moderator – Net radiolytic breakdown of water, cover gas recombiners End Shield Cooling – Allow hydrogen to build-up, operate with no net radiolytic production of hydrogen and oxygen Liquid Zone Controls – Net radiolytic breakdown of water, cover gas recombiners

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Reactor Applications

• Understanding current chemistry specifications – Consider radiolysis effects when changing chemistry specifications – Modify chemistry in response to degradation mechanisms • Troubleshooting operating reactor chemistry – Radiolysis mechanisms key to interpreting changing reactor chemistry • Predicting the consequences of material ingress/addition – Often maintenance related • Predicting containment chemistry following loss of coolant

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Primary Heat Transport System

• Chemistry Objectives – Reducing conditions – Alkaline pH a – Low impurity concentrations • Operates with suppression of net radiolytic breakdown of water H 2 O + H 2 added Radiolysis H 2 O + H 2 added • Need to ensure sufficient added hydrogen in system to minimize net production of oxidizing species

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(~130



M H 2 )

3.0

2.0

Example of Critical Hydrogen Concentration (Non-Boiling) (Light Water, ~300 o C - U2 Loop NRU - 1995) (Elliot and Stuart - AECL Available Report: 153-127160-440-003)

Hydrogen (in-core) Oxygen (out-of-core) Oxygen (in-core) Hydrogen (out-of-core) 2000

(~ 60



M O 2 ) Hydrogen Added Quickly Hy dr by og oxy en Remov gen add ed ition S low ly

1500 1000 1.0

Critical Hydrogen Concentration (Non-Boiling)

0.0

15.0

15.1

Supressed Radiolysis 15.2

15.3

15.4

15.5

Time / hrs

15.6

15.7

500

Net Radiolysis

15.8

15.9

0 16.0

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Radiolysis Modelling

1.0E-04 8.0E-05 6.0E-05 4.0E-05 Hydrogen (Experimental) Hydrogen (Simulated) Oxygen (Experimental) Oxygen (Simulated) 1.0E-04 8.0E-05 6.0E-05 4.0E-05 2.0E-05 2.0E-05 Critical Hydrogen Concentration 0.0E+00 0 0.1

0.2

0.3

0.4

Time (h)

0.5

0.6

0.7

0.8

0.0E+00 • Radiolysis model based on rate constants and radiolytic yields measured at CRL and other laboratories •Low rate constant for key reaction required : • OH + H 2  • H + H 2 O

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Radiolysis Modelling

• Full details and results of reactor loop experiments are available – Elliot and Stuart, 153-127160-440-003 – Allow other organizations to use these data to assess models • AECL (Elliot) working with international collaborators to produce up to date summary of radiolysis parameters – The Reaction Set, Rate Constants and g-Values for the Simulation of the Radiolysis of Light Water over the Range 0 ° to 350°C Based on Information Available in 2008.

– Available this summer!

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Case Study

• • Issue: cracking of carbon steel outlet feeder piping Possible chemistry influences – Radiolytic formation of oxidizing species during normal operation – Radiolytic removal of oxygen, from air ingress during maintenance outage, prior to start-up

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Chemistry Considerations – Normal Operation

• Crack propagation rate dependent upon Electrochemical Corrosion Potential (ECP) of carbon steel surfaces • In addition to oxygen, hydrogen peroxide concentration is very important in determining ECP • Modelling in-core experiments predicts peroxide produced at higher H 2 concentrations than O 2 • Result: lower dissolved deuterium concentration limit raised for affected plant

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Oxygen removal during start-up

• Oxygen (from air) can enter system during maintenance outages – Could lead to initiation of cracks during start-up thermal transient • Several methods of oxygen removal: – Hydrogen addition – Hydrazine addition – System degassing (very slow) – Corrosion of system materials (not a beneficial means)

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Hydrogen addition to remove oxygen during start-up

• Hydrogen added to remove oxygen prior to start-up – Radiolysis modelling determined that radiation fields during plant shutdown sufficient to recombine hydrogen and oxygen – Used existing reactor systems – Additional online chemistry monitoring installed – Process did not add to critical path time to reactor start-up • Hydrazine addition would have achieved same goal – No procedures for addition available – Addition of new chemicals to systems requires additional approvals

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Start-up - O

removal prior to warm-up

8 Dissolved oxygen / mg.kg

-1 Dissolved deuterium / mL.kg

-1 6 4 2 0 06:00 08:00 10:00 Time / hours 12:00 14:00 • Dissolved deuterium peaks correspond to hydrogen additions • Hydrogen addition facilitates oxygen removal

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Summary

• Laboratory-scale radiolysis information is actively being applied to improving CANDU reactor chemistry – Modelling of whole systems – Application of mechanistic information • More high temperature rate constant and yields data needed to refine models – Especially with move to higher temperature reactors (SCW) • More information regarding radiolytic breakdown mechanisms useful in responding to changes in reactor chemistry.

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