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Assessment of radionuclide content in waste barrels using the Canberra In-Situ Object Counting System (ISOCS)

Persson, Marcus (2015) MSFT01 20151
Medical Physics Programme
Medical Radiation Physics, Lund
Abstract
In order to store radioactive waste its contents need to be known. Determining radioactive waste can be achieved by different means. The most common method is using a detector to measure the gamma photons produced by the radionuclides in the waste. A widely used method is Segmented Gamma Scanning, in which the waste package is rotated in front of a detector and then raised to the next vertical segment etc. until the top of the package is reached. Commercially available waste assay systems from Canberra, ISOCS™, and ORTEC®, ISOTOPIC, exist in which the geometry of the measurement can be modelled in its entirety. Studies are inconclusive as to which accuracy can be achieved.

This work aimed to determine how well Canberra’s ISOCS™ could... (More)
In order to store radioactive waste its contents need to be known. Determining radioactive waste can be achieved by different means. The most common method is using a detector to measure the gamma photons produced by the radionuclides in the waste. A widely used method is Segmented Gamma Scanning, in which the waste package is rotated in front of a detector and then raised to the next vertical segment etc. until the top of the package is reached. Commercially available waste assay systems from Canberra, ISOCS™, and ORTEC®, ISOTOPIC, exist in which the geometry of the measurement can be modelled in its entirety. Studies are inconclusive as to which accuracy can be achieved.

This work aimed to determine how well Canberra’s ISOCS™ could estimate the activity content in a waste barrel with different radionuclide content and waste matrices and to estimate the accuracy of this estimated activity. Also if a NaI(Tl) detector could be used to gain additional information about the measured geometry.

Three different measurement setups were performed at Studsvik Nuclear AB. In setup 1 liquid sources of 111In and 131I and a point source of 134Cs were measured placed in the radial centre as well as in the radial periphery of a barrel filled with water. Homogeneous activity distributions of 111In and 131I were also measured. In setup 2 point sources of 57Co, 60Co, 133Ba and 137Cs were measured in the radial centre and the radial periphery of a barrel filled with water. All point sources were also measured at a different vertical position in the barrel. 57Co and 133Ba were measured individually. In setup 3 a cylindrical liquid source of 18F were measured in the radial centre and radial periphery of a barrel filled with water and homogeneously distributed in the water.

The conclusion is that ISOCS™ can estimate the activity well when the correspondence between the modelled and measured geometry is very good, e.g. sources free in air or homogeneous activity distributions. When the matrices are more complex the correspondence is worse but whether this is due to the software being sensitive for mismatches between the model and reality or because of uncertainties in the experimental setup or a combination of both is not clear. ISOCS™ underestimates the activity content by 60 % for 511 keV when
applying a homogeneous activity distribution for a heterogeneous matrix with the source in the centre of the barrel. A NaI(Tl) detector can be used to assess large inhomogeneities in the activity and to apply an accuracy to the activity estimation. (Less)
Popular Abstract (Swedish)
Drift av kärnkraftverk och andra kärntekniska anläggningar resulterar i radioaktivt avfall. Att mäta och karakterisera detta avfall finns det ett stort behov av då en tunna med radioaktivt avfall hamnar i olika förvar beroende på hur högaktivt och långlivat innehållet är. Visar det sig att avfallet är tillräckligt lågaktivt kan innehållet friklassas, d.v.s. att hantering/användning av materialet kan ske utan fortsatt kontroll ur strålskyddssynpunkt. För mätning och karakterisering av radioaktivt avfall finns det olika system på marknaden där ett av de vanligast förekommande systemen är ISOCS™ som tillverkas av Canberra, där man i programmet modellerar sin mätuppställning. I detta systemet används en strålningsdetektor, oftast en... (More)
Drift av kärnkraftverk och andra kärntekniska anläggningar resulterar i radioaktivt avfall. Att mäta och karakterisera detta avfall finns det ett stort behov av då en tunna med radioaktivt avfall hamnar i olika förvar beroende på hur högaktivt och långlivat innehållet är. Visar det sig att avfallet är tillräckligt lågaktivt kan innehållet friklassas, d.v.s. att hantering/användning av materialet kan ske utan fortsatt kontroll ur strålskyddssynpunkt. För mätning och karakterisering av radioaktivt avfall finns det olika system på marknaden där ett av de vanligast förekommande systemen är ISOCS™ som tillverkas av Canberra, där man i programmet modellerar sin mätuppställning. I detta systemet används en strålningsdetektor, oftast en halvledardetektor av germanium, HPGe (High-Purity Germanium) med hög energiupplösning, som är grundligt karakteriserad av tillverkaren och kalibrerad för olika mätsituationer. Studsvik Nuclear AB utanför Nyköping har två ISOCS™ system som används för just karakterisering av radioaktivt avfall och friklassning av material.

För att ta reda på hur väl ISOCS™ uppskattar aktiviteten i radioaktivt avfall har mätningar med olika strålkällor med känt aktivitetsinnehåll och för olika mätuppställningar utförts. Strålkällorna har varit placerade på olika ställen i en vattenfylld tunna. I andra experiment har radioaktive ämnen spätts ut i vatten för att se hur väl ISOCS™ klarar av denna mätsituation. Till alla mätningar har ytterligare en strålningsdetektor använts i syfte att kunna ta reda på information om den aktuella mätuppställningen som inte går att ta reda på med HPGe-detektorn.

Resultaten visade att ISOCS™ klarar väl av att uppskatta det radioaktiva innehållet i tunnan när aktiviteten är utspädd i vattnet eller när de radioaktiva ämnena var placerade fritt i luft framför detektorn. När ämnena var placerade centralt i tunnan eller i periferin av tunnan så blev överrensstämmelsen sämre. Detta kan bero på systemet eller på grund av att det inte går att uppnå perfekt överrensstämmelse mellan den modellerade mätuppställningen och den praktiska mätuppställningen. Den andra strålningsdetektorn kunde användas för att få reda på mer information om aktivitetsfördelningen i tunnan och öppnar därmed möjlighet till en bättre modellering och förbättrad mätnoggrannhet. (Less)
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author
Persson, Marcus
supervisor
organization
course
MSFT01 20151
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
8033398
date added to LUP
2015-09-29 22:04:20
date last changed
2017-01-09 16:31:15
@misc{8033398,
  abstract     = {In order to store radioactive waste its contents need to be known. Determining radioactive waste can be achieved by different means. The most common method is using a detector to measure the gamma photons produced by the radionuclides in the waste. A widely used method is Segmented Gamma Scanning, in which the waste package is rotated in front of a detector and then raised to the next vertical segment etc. until the top of the package is reached. Commercially available waste assay systems from Canberra, ISOCS™, and ORTEC®, ISOTOPIC, exist in which the geometry of the measurement can be modelled in its entirety. Studies are inconclusive as to which accuracy can be achieved.

This work aimed to determine how well Canberra’s ISOCS™ could estimate the activity content in a waste barrel with different radionuclide content and waste matrices and to estimate the accuracy of this estimated activity. Also if a NaI(Tl) detector could be used to gain additional information about the measured geometry.

Three different measurement setups were performed at Studsvik Nuclear AB. In setup 1 liquid sources of 111In and 131I and a point source of 134Cs were measured placed in the radial centre as well as in the radial periphery of a barrel filled with water. Homogeneous activity distributions of 111In and 131I were also measured. In setup 2 point sources of 57Co, 60Co, 133Ba and 137Cs were measured in the radial centre and the radial periphery of a barrel filled with water. All point sources were also measured at a different vertical position in the barrel. 57Co and 133Ba were measured individually. In setup 3 a cylindrical liquid source of 18F were measured in the radial centre and radial periphery of a barrel filled with water and homogeneously distributed in the water.

The conclusion is that ISOCS™ can estimate the activity well when the correspondence between the modelled and measured geometry is very good, e.g. sources free in air or homogeneous activity distributions. When the matrices are more complex the correspondence is worse but whether this is due to the software being sensitive for mismatches between the model and reality or because of uncertainties in the experimental setup or a combination of both is not clear. ISOCS™ underestimates the activity content by 60 % for 511 keV when
applying a homogeneous activity distribution for a heterogeneous matrix with the source in the centre of the barrel. A NaI(Tl) detector can be used to assess large inhomogeneities in the activity and to apply an accuracy to the activity estimation.},
  author       = {Persson, Marcus},
  language     = {eng},
  note         = {Student Paper},
  title        = {Assessment of radionuclide content in waste barrels using the Canberra In-Situ Object Counting System (ISOCS)},
  year         = {2015},
}