Interim Storage and Surface Cleanliness Kevin Christie, AMEC

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Interim Storage and Surface Cleanliness Kevin
Christie, AMEC
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Interim Storage of Stainless Steel Waste
ContainersAn Achilles Heel?

• Following the NWAT EA report (last lecture) the
  Independent Newspaper filed a story headlined
• Nuclear waste containers likely to fail, warns
  devastating report
• The document effectively destroys Britains
  already shaky disposal plans
• ..many containers used to store the waste are
  made of second-rate materials, are handled
  carelessly, and are liable to corrode.
• The failure of just one container could prove
  catastrophic
• 40 of the containers could fail within 1,000
  years and that under certain scenarios this
  timescale could shrink to less than 200 years

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And now for reality?

• For those of you that have not yet resigned or
  retired before the catastrophe it might be
  useful to look at the issues.
• Originally the concept for the containers was to
  be stored for as little as 50 years in Interim
  store before being transported to the repository
  and when the chamber is full, backfilled with
  NRVB grout.
• Now the Waste packages may be expected to last
  500 years
• 150 years on licensed sites 50 of repository
  emplacement and up to 300 years of retrievable
  storage
• How can we justify this period on the basis of
  our short term knowledge of stainless steels and
  their potential weakness of ASCC?

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Waste Packages

• Over the last 20 years tens of thousands of
  packages have been produced, most are stored in
  Purpose Built stores mainly at Sellafield.
• Some other packages are stored at Dounreay and
  Harwell, as well as many Magnox sites.
• Not all packages are stored in pristine
  conditions, some older packages were originally
  stored in less than ideal conditions.
• The majority of packages are made from 316L and
  304L stainless steel.
• Stainless steel can suffer from a number of
  localised corrosion mechanisms, many of which can
  be triggered during these interim storage periods

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Corrosion issues for packagesExternal

• Pitting
• Atmospheric Stress Corrosion Cracking (ASCC)
• Galvanic effects
• Microbially Induced Corrosion
• Radiolytic effects (hydrogen peroxide)
• General corrosion

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Interim Storage Environments

• In the 1990s Nirex issued Guidance on the
  Storage environments and the Monitoring of
  packages (WPS 630 and 640)
• These assume that the packages are initially
  totally clean and that the environment does not
  provide any corrodants.
• This can be achieved by air control (HVAC) and
  filtering but in reality is not controlled as
  packages are not checked for cleanliness prior to
  storage.
• There is a dependence on the avoidance of
  condensation as a means of corrosion control
• Most stores are built from concrete dust and
  potential ionic salt issues

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ASCCA real threat to stainless steel?

• Needs chloride, stress and some moisture
• AMEC predecessor NNC began work on this issue in
  the early 1960s when stainless steel around
  refuelling ponds were affected by chloride.
• Structural steels were showing corrosion by
  chlorides in conditions were there was little
  environmental control. At low temps (30C) steel
  surfaces were wetting at lower than expected
  dewpoints. Ie the salts were absorbing moisture
  and allowing this wetting.
• Wetting was expected at 65 RH in the presence of
  chloride (as sodium salt) but with other salts
  such as ammonium chloride and ferric chloride
  wetting occurred at 50 and 35 respectively.

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Nuclear Industry Guidelines

• In the late 1980s a test programme was
  undertaken on 304L stainless steel, sensitised
  and unsensitised at a variety of stress levels,
  RHs, temperatures and salt types and
  contamination levels.
• In excess of 1000 tests were carried out at salt
  levels ranging from zero to 100µg.cm-2 using
  sodium chloride or sea salt.
• Sea salt produced ASCC at lower RHs and at lower
  stresses than NaCl. This led to the Sizewell B
  guidelines for metal surface cleanliness.
• Templt50C chloride level lt0.1 µg.cm-2
• Temp at ambient chloride level lt1 µg.cm-2
• Low RHs (typically less than 50) chloride level
  5 µg.cm-2

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Other Nuclear Guidelines

• UK SGHWR chloride limit of 0.8 µg.cm-2
• UK AGR on stainless steel total salt level 1.0
  µg.cm-2 of which only a portion would be chloride
• Rolls Royces limit for maritime PWRs is 0.2
  µg.cm-2 chloride
• Westinghouse PWR stainless steel limit is 0.015
  µg.cm-2 chloride
• Japanese reactors found to have ASCC on stressed
  components at chloride level of 0.8 µg.cm-2
• South African reactor found to have ASCC on
  stainless steel parts
• Chloride and other salts come from rain, sea
  mist, pollution and towns water

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Other Sources of Chloride Contamination

• Other sources include human sweat, urine, finger
  grease, foods beverages. Finger grease is
  difficult to remove with water.
• Fabrication chemicals such as cutting fluids,
  couplants, fluxes and dye penetrant chemicals
• Marking inks, felt pens, tape adhesive
• Adhesives, plastics, rubbers which contain
  combined chlorides capable of release by heat,
  fire or radiation.
• Use of non-stainless wire brushes or moving
  equipment such as mild steel forks on lift trucks
  can result in iron pickup-causing staining and
  localised attack

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Typical Measured Values
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Important factors in controlling surface
cleanliness and ASCC

• Relative Humidity
• Temperature
• Salt concentration and salt type
• RH determines the wetting range of the particular
  salt
• NaCl wets at gt65
• other salts such as ferric chloride wet at 35
• Problem arises when mixtures of salts are present
• Salt mixtures may wet at lower RHs than believed
  leading to possibility of early corrosion

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Factors 2

• Temperature
• Causes increase in likelihood of ASCC for any RH
  in range 50-70 RH
• Salt concentration
• Amec have produced ASCC at 50 RH at Cl conc. of
  10µg.cm-2 and above providing other salts were
  present
• Salt types
• Much of the published literature uses pure sodium
  chloride or other single species chloride

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Interim StorageConditions and potential issues

• Many stores are new and purpose built for 100
  year life (after which time significant
  refurbishment of HVAC and other systems will be
  required)
• Older stores may not meet current best practice,
  lack of HVAC, dust, dirt, even the odd pigeon and
  its guano
• Even if the store is perfect, were the packages
  clean before they were put in? Dust and dirt
  will land on the packages (they are not dust free
  stores) entrained salts will build up and if
  wetting occurs then corrosion can initiate.
• Even where the packages are clean, the store well
  monitored, the packages are still packed in such
  a way as to make it difficult to have consistent
  RH and airflow everywhere within the store.

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Interim StorageRequirements

• Good control of temp, RH and package cleanliness
• Good design of stores
• Requirements for package monitoring
• Removal of potential problems heat tint,
  mechanical damage from handling ops
• Regular inspection and maintenance programme
• Cleaning of packages before placing in stores
  even after minor transport moves
• Prevention of pitting and other localised attack
  which may provide initiation sites for ASCC
• Setting up of a database of unacceptable products
  to be kept away from packages

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An Example in South AfricaPWR pipes

• Externally initiated ASCC transgranular in nature
  on external 304L stainless steel plate and
  pipe-operating temperatures between 20 and 25C
• Sea salt brought in as spray as building had roof
  to protect workers from sun and rain but no sides
  so no rain wash possible.

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South African ASCC 2Transgranular cracking- note
starting from pit
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South African ASCC 3Replica of cracks
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South African ASCC 4Pump casting
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Conclusions

• 304L is particularly sensitive to ASCC since the
  lack of molybdenum makes it prone to pitting.
• Better control of environmental conditions within
  Stores during interim storage
• Corrosion monitoring systems either on packages
  or on simulated electrode systems to monitor when
  conditions are out of tolerance
• Is 304L or even 316L stainless steel the most
  appropriate alloy for package construction?
  Bearing in mind the tens of thousands of packages
  already filled and in store
• Mandatory storage conditions
• Regular cleaning of packages

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And now for the advert!!Salt Contamination Meter

• SCM400 Features
• 2 minute sampling time
• Range from 0.1-20 µg.cm-2
• No hazardous chemicals
• Simple to use
• Direct readout - no calculations to do
• Suitable for metallic and non-metallic surfaces
• Use on curved, horizontal and vertical surfaces
• Battery powered
• Water resistant