michele laraia, former iaea decommissioning unit leader ... · pdf filefe -55, co 60, ni 59,...
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Michele Laraia, former IAEA Decommissioning Unit Leader,
consultant
Review purpose and content of characterization survey
Review recommended characterization plans and reports
Review characterization design / methods / techniques
Review those areas requiring special attention
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Radiological Characterization for decommissioning purposes provides a database of information on a facility including:
Quantity and type of radionuclides Distribution of contaminants in large areas and by
specific room, elevation, etc. Physical and chemical state of contaminants
Characterization assists in : Estimating waste volumes requiring disposal Technologies for performing decommissioning Estimating worker doses (ALARA) Identifying safety requirements (radiological and
non-radiological) for workers3
• Radiological Characterization involves: Reviewing existing data and information Performing calculations In-Situ measurements Sampling and lab analysis Documentation and data analysis (incl. statistical
treatment)
• Non-radiological characterization (chemical and structural) may also be an important element in decommissioning
• Characterization must be completed before the d. planning process is finalized
• Characterization is not necessarily straightforward (e.g. hidden components, legacy waste)
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Typical elements of physical (structural) characterization include among others:
• Long term deterioration
• Load bearing
• Acquisition of undocumented or wronglydescribed detail
• Poorly accessible areas and remote characterization
• Access and egress routes
• Slippery surfaces
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1st stage) Perform a review of available historical information
This is a critical aspect of the planning for decommissioning
Operations documentation
This aspect only reviews readily available information
Assists in planning overall decommissioning approach
Can help to reduce the cost in preparing for and performing detailed characterization
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2nd stage) Perform a more detailed characterization. The Characterization Plan
Fill in gaps from previous data reviews
Calculations of induced activity
Taking samples, conducting inspections, performing analyses
Serves as technical basis for developing work sequences including man power and radiation exposures
If immediate dismantlement is planned, a more intensive characterization effort will be required then if deferred dismantlement is planned
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Pre
Decomm
Early
Decomm
Mid
Decomm
Late
Decomm
CharacterizationSurveys
OperationalSurveys
Final Surveys
Record Keeping
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• Use of competent personnel
• Some degree of “unknown” (records, access to areas)
• Accuracy of required measurements
• High radiation doses possible
• Remote systems e.g. gamma cameras
Techniques used should not
endanger safety and long-term
integrity of structures
ISSUES
Review Historical Information Records/recollections of past spills Process upsets/unusual occurrences Previous radiological and chemical survey records Occupational exposure records Structural condition of facility “As built” drawings of facility Availability of reactor staff to assist
Don’t overlook chemical contaminants Heavy metals, polychlorinated bi-phenyls
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• Once a review of historical information has been performed, a Characterization Plan should be developed to obtain required information to close any data gaps
• Defines the quality and type of data necessary to achieve the characterization objectives
• The characterization objective defines the types of and sensitivities for the measurements that will be needed
• The Characterization Plan is an essential component of the Decommissioning Plan
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• Facility history and photos
• Types, numbers, sizes, locations and analyses of samples required
• Instrumentation required
• Data reduction, validation and reporting
• Quality assurance requirements
• Methods to be used in collecting samples and performing analysis
• Radiation protection and other hazard controls
• Methods to be used for sample disposal
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• Sampling performed at different heights depending on core size and location
• Aimed to validate theoretical activation models
• Statistically significant samples
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Astra reactor,
Seibesdorf, Austria
3 types of characterization data
1. Calculated data-radioactive content of various structural materials
2. In-situ measurement data-manual or remote measurements of dose rates and/or contamination levels
3. Sampling & Analytical data-detailed information on types and amounts of radioisotopes present (Expensive, but precise)
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Neutron activated materials In or near the reactor core
Biological shield and reactor materials
Accelerators can also cause activation
Contaminated materials - loose or fixed Corrosion products
Circuit leakage, repair activities
Fuel breaks, fuel discharges
Inventory should include a detailed listing for each component including radionuclide type and chemical form, component weight and dimensions
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Reactor type, design, power level and shutdown period
Composition of construction materials, including trace elements
Operational parameters (water chemistry)
Unplanned events (fuel failures resulting in fission products and maybe alpha emitters in primary coolant)
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59Co(n,)60Co
• Neutron-induced activity calculations will need to be supplemented with actual field sampling (see slide showing concrete drilling at Astra reactor)
• Material samples: coupons for embrittlement purposes etc.
• Actual samples collected may not be representative of entire structure
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• Spatial and energy distribution of the neutrons flux ANISN
MCBEND
(many more)……………
• Spatial distribution of neutron induced radioactivity in all reactor construction materials
ORIGEN
FISPACT
(many more)………
Codes to calculate dose rates (e.g. MICROSHIELD)
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Steels, zircaloy
Fe-55, Co-60, Ni-59, Ni-63 and Nb-94
Graphite
H-3, C-14, Cl-36, Ca-41, Fe-55, Co-60, Eu-152 and Eu-154
Concrete
H-3, C-14, Cl-36, Eu-152 and Eu-154
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Typical uncertainties are due to:
Methodologies used e.g. 1-D, 2-D or 3-D models?
Geometry and modelling simplifications
Materials composition errors
Streaming effects
Inaccuracies
in cross-section data
Agreement to ~ 2 in fuelled core region
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Three types of in-situ measurements are typical of pre-decommissioning characterization
Dose rate measurements
Contamination measurements
Spectrometry measurements - relative radionuclide distribution
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Dose rate measurements - made at fixed, convenient distances from either internal or external contamination. Gross radiation readings will not indicate nature and quantity of isotopes. Accuracy is highly variable
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Fixed contamination (or loose and fixed contamination) can be measured by using one of 2 techniques. A stationary detector used at a fixed point for a fixed period will give a numeric result. The second method entails scanning a surface. Systematically move the detector across a surface at a speed sufficiently slow enough to allow detection of a change in radiation field. Limiting factors are: sensitivities, radionuclides involved and instrument resolution time
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In-situ measurements (ctd.)
Spectrometry measurements: ISOCS, Canberra
Must be suited to the energy levels of radiation and resolution and accuracy needed
Wide variety of types are available - gas-filled detectors, scintillation detectors and solid state detectors
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Victoreen, a common Ion Chamber
radiation survey instrument
Ludlum 3
Type: Normally equipped with a Geiger-
Mueller 44-9 (pancake) probe
Detects: Beta, gamma
Ludlum 2224
Type: Plastic
scintillation for beta
detection that has a
[ZnS (Ag)] coating for
alpha detection
Detects: Alpha and Beta
Ludlum 16 with a 44-3 thin
window NaI probe
Liquid Scintillation (LSC)
Dead or low batteries - erratic or no detection
Calibration has changed - may read high or low
Defective cable or other problems
Poor survey technique
angle of probe to source - only detects part
to far from source - radiation absorbed by air
survey too fast – only detects part
You must use them correctly if you expect them to work for you
In the short and medium term:
H-3, Fe-55, Ni-63, Co-60, Sr-90 and Cs-137 dominate
In longer term:
Actinides dominate, U, Pu, Am
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Used for difficult to measure nuclides e.g. H-3, Fe-55, C-14 Expensive and time consuming Sampling programme required (statistics?) Specialist (one-off) sampling tools required Requires equipped radiochemical laboratory Requires qualified personnel
‘Fingerprinting’ for hard-to-detect nuclides (next slide)
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Scaling Factors
Direct measurements for some strong gamma emitters can assist in developing scaling factors to allow correlation of less intensive fingerprinting of hard to detect radionuclides
Can reduce the number of samples required
Beware: scaling factors highly variable from plant to plant and even within same plant
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Biased sampling
Use systematic approach - grid for reference measurements.
Finding or defining contamination or activation that is known to exist or likely to be present
Used for areas where contamination or activation is likely
Unbiased sampling
Areas expected to have little or no surface contamination.
Areas are expected to be homogeneous in degree and characterization of contamination
The facility is divided into discrete sampling areas and survey units. These sampling areas are then compared to background populations to determine impacts (if any)
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• Determine background for each different type of material or area
• Areas used for this activity should be representative of general area
• Problem areas Heavy concrete Areas with elevated natural radioactivity (e.g. due to
poor ventilation) External sources of radiation
• Corrective Actions Use correction factors for elevated activity Use shielding
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Generally, QA for characterization is a part of the larger decommissioning project QA Programme
Key aspects include: Personnel
Instruments
Methods
Data/Documentation
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Surface soil sampling
Sub-surface soil sampling
Large area detectors
Groundwater sampling
Airborne particulate sampling
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Soil is
contaminated
Hazardous
materials
• Asbestos• PCB’s• Mercury• Hazardous chemicals• Locations, quantities
and physical forms
Non-radioactive but conventionally hazardous materials need to be considered
Removal of asbestos at Trino NPP, Italy
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Purpose
Facility Description and Operational History
Quality Assurance - organization, instruments, procedures, methodology, and data quality objectives
Instrumentation - types, sensitivities
Release Guidelines
Results - radioactivity, hazardous materials, inventory, unexpected
Appendices - references, tables, figures, maps, calibration, and analytical results
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Inadequate documentation and planning for characterization
Not accounting for natural radioactivitybackground of construction materials
Not using techniques with sensitivities capable of achieving release criteria activity levels
Poorly documented construction activitiesduring periods of inactivity at a site/facility
Use of soils containing radioactivity as fill material around and within sewers and other underground utilities
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Characterization is an on-going process that must be started early and continued throughout the project lifecycle
(TLG’s slogan) There are three important tasks in decommissioning: characterize, characterize, characterize (but not too much…)
Objectives of characterization activities need to be clearly defined and the work well planned
It is important to have access to past records and these must be carefully reviewed to: direct further characterization efforts and reduce costs
Characterization is an essential step in estimating waste volumes and treatment/disposal options
The process is an iterative one-as more data becomes available, you must reassess the situation
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1. Manual for Conducting Radiological Surveys in Support of License Termination, NUREG/CR-5849, June 1992
2. Multi-Agency Radiation Survey and Site Investigation Manual (MARSSIM), NUREG-1575, December 1997
3. EPRI Report 1009410, Capturing Historical Knowledge for Decommissioning of Nuclear Power Plants. March 2004
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Questions?