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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?