Nondestructive Fission Gas Measurements by Means of Gamma Spectroscopy and Gamma Tomography

Scott Holcombe

Licentiate Thesis

Department of Physics and Astronomy Division of Applied Nuclear Physics

2012

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Abstract There is a continuous need in the nuclear industry to characterize irradiated nuclear fuel rods and assemblies, both for fuel performance and for safeguards purposes, and consequently there are various destructive and nondestructive measurement techniques available to meet this need. Gamma spectroscopy is one such nondestructive technique, which has been extensively used for a variety of fuel characterization applications. Furthermore, gamma tomography – a combination of gamma spectroscopic measurements and tomographic reconstruction – has in recent years been demonstrated as an efficient technique for characterization of irradiated nuclear fuel assemblies on a rod-by-rod basis without the need to dismantle the fuel. This thesis comprises four scientific papers in which novel applications of these two techniques have been developed and evaluated.

The major part of this work has been performed at the Halden Boiling Water Reactor (HBWR), where a gamma tomography measurement system is currently under construction, as presented in this thesis. The methods and evaluations presented in this work are based on the conditions at the HBWR.

Based on gamma spectroscopy, a novel nondestructive method for determining fission gas release which occurs over short irradiation sequences has been developed, comprising the measurement and analysis of short lived isotopes in individual fuel rods. The method has been demonstrated based on gamma-ray spectra recorded from an experimental fuel rod irradiated in the HBWR.

Based on gamma tomography, a novel method for identifying failed fuel rods within a nuclear fuel assembly has also been developed. The method comprises the measurement of gamma rays emitted in the decay of selected fission gas isotopes in the gas plenum region of a fuel assembly, tomographic image reconstruction of the internal source distribution and subsequent analysis of the resulting image in order to determine if any of the fuel rods in the assembly has unexpectedly low activity, indicating that it is a leaking fuel rod. Simulation studies performed for HBWR fuel show highly promising results for gamma rays emitted in the decay of two selected fission gas isotopes.

The methods will be further investigated at the HBWR, by performing dedicated gamma spectroscopy measurements and by using the tomographic measurement system currently under construction.

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List of Papers Paper I

Feasibility of Performing Pool-Side Fission Gas Release Measurements on Fuel Rods with Short Decay Time

S. Holcombe, K. Eitrheim, S. Jacobsson Svärd, L. Hallstadius, C. Willman Proceedings of 2011Water Reactor Fuel Performance Meeting / LWR/Top Fuel, Chengdu, China, September 11-14, 2011 (peer-reviewed conference paper)

My contribution: I performed simulations to determine the amount of different fission gas isotopes present in irradiated fuel and selected three isotopes for further evaluation. I calculated the expected count rate at the detector for these gamma rays for an assumed measurement geometry. I analyzed experimental gamma spectra from irradiated fuel rods and determined that they agreed with the calculations. I concluded that three gamma rays are measureable within a few days or weeks after the end of irradiation; the 81 keV peak from 133Xe, the 233 keV peak from 133mXe and the 250 keV peak from 135Xe. I was the main author of the paper and presented it at the TopFuel 2011 Conference.

Copyright 2011 by the Chinese Nuclear Society, Beijing, China.

Paper II Advanced Fuel Assembly Characterization Capabilities Based on Gamma Tomography at the Halden Boiling Water Reactor

S. Holcombe, K. Eitrheim, S. Jacobsson Svärd, L. Hallstadius, C. Willman Proceedings PHYSOR 2012, Knoxville, Tennessee, USA, April 15-20, 2012 (peer-reviewed conference paper)

My contribution: I reviewed the design of the gamma tomography device previously developed by Uppsala University and interfaced with the design team at the Halden Boiling Water Reactor to set the target design parameters of the gamma tomography device being designed for use at this reactor. I was the main author of the paper and presented it at the PHYSOR 2012 Conference.

Copyright 2012 by the American Nuclear Society, La Grange Park, Illinois, U.S.A.

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List of Papers (continued) Paper III

A Method for Analyzing Fission Gas Release Based on Gamma-ray Measurements of Fuel Rods with Short Decay Time

S. Holcombe, S. Jacobsson Svärd, K. Eitrheim, L. Hallstadius, C. Willman

My contribution: I developed the expression for calculating percent fission gas release in fuel rods with short decay time based on the measurement of isotopes I selected in Paper I. I performed the feasibility demonstration based on data from gamma spectroscopy measurements of an experimental fuel rod irradiated in the Halden Boiling Water Reactor. I concluded that the method is feasible for determining the fraction of fission gas release which occurs over short periods of irradiation. I was the main author of the paper.

This paper has been submitted to Nuclear Technology and is presented here in manuscript form.

Paper VI Feasibility of Identifying Leaking Fuel Rods Using Gamma Tomography

S. Holcombe, S. Jacobsson Svärd, K. Eitrheim, L. Hallstadius, C. Willman

My contribution: I performed simulations representing gamma tomography measurements of fission gasses, of which two were identified in Paper I. The simulations were made for an example Halden Boiling Water Reactor fuel assembly in the gamma tomography system currently under development, as described in Paper II. I performed tomographic reconstructions of the simulated measurement data and evaluated the resulting images to determine whether any of the three evaluated source gamma-ray energies was suitable for discerning a leaking rod by means of detecting low fission gas content. I concluded that the lowest gamma-ray energy was unsuitable. The two with higher energies were found suitable, but it is important to make efforts to maximize the count rate, such that the time required for measurement may be kept reasonably short. I was the main author of the paper.

This paper has been submitted to Annals of Nuclear Energy and is presented here in manuscript form.

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Table of Contents

1 Introduction ............................................................................................................................. 1 2 Nuclear Fuel ............................................................................................................................ 3 3 Gamma Spectroscopy on Nuclear Fuel ................................................................................. 4

3.1 Established Gamma-ray Spectroscopy Applications ..................................................... 5 3.2 Proposed Novel Application for Gamma-ray Spectroscopy ......................................... 5

4 Gamma Tomography on Nuclear Fuel .................................................................................. 7 4.1 Gamma Tomography Measurement Devices ................................................................. 7 4.2 Previously Demonstrated Applications of Gamma Tomography on Nuclear Fuel

Assemblies........................................................................................................................ 8 4.3 Proposed Novel Application of Gamma Tomography .................................................. 9

5 Outlook .................................................................................................................................. 11 6 Acknowledgements ............................................................................................................... 12 7 References ............................................................................................................................. 13

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1 Introduction There are over 400 operating nuclear power plants in the world [1], which supply around 13% of the world’s electricity [2]. The vast majority of these are Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs). The nuclear fuel assemblies powering these reactors must tolerate intense neutron irradiation, high temperatures and pressures, and aggressive chemical environments. The fuel must perform reliably in this challenging environment, within safe limits, during normal operation and accident conditions. Ensuring that nuclear fuel behaves as designed is achieved through modeling, simulations, post-irradiation examinations (PIE), and experiments conducted in commercial as well as research reactors. Some parameters of importance with regard to fuel performance are:

• Cladding integrity – Maintaining the integrity of the fuel rod cladding is important since it is a barrier to the release of radioactive fission products into the primary loop, keeping doses to plant workers at a low level.

• Burnup and Power distribution – These parameters are a function of the neutron flux, and have implications for the safe operation of the fuel, i.e. safety margins are, in part, based upon the burnup of the fuel and the rod-by-rod power distribution.

• Fission gas release (FGR) – Fission gasses released from the fuel pellets into the free volume within a fuel rod contribute to increasing pressure within the fuel rod and lead to reduction in the thermal conductivity of the fill gas. Fission gas release must be accurately taken into account to ensure the fuel operates within the design limits.

There are many nondestructive and destructive methods available to characterize nuclear fuel in spent fuel pools and in hot laboratories. Destructive methods normally require expensive transport of the fuel to hot-laboratories for measurement and special disposal considerations. Nondestructive methods, on the other hand, have the advantage that fuel may be characterized repeatedly during its lifetime, without any consequences for disposal. Furthermore, the relative cost-effectiveness of nondestructive methods implies that a larger number of fuel assemblies may be characterized.

In addition to characterization in terms of fuel performance, nondestructive measurements are also important to control the non-diversion of fissile material – part of the international efforts to ensure non-proliferation of nuclear weapons – also known as nuclear safeguards.

The detection of gamma rays emitted by fission products contained in nuclear fuel has long been used as a nondestructive means to characterize irradiated fuel and to perform safeguards verification measurements. Gamma spectroscopy is a well-known and established method for recording and analyzing the emitted gamma rays, which depending on their emitting isotopes, reveal a variety of information about the fuel. Gamma spectroscopy may be applied to either individual fuel rods or to entire assemblies. Recently, gamma spectroscopy has been combined with tomographic reconstruction techniques to allow for characterizing the gamma-ray source in individual fuel rods without the need to remove them from a fuel assembly for individual measurements.

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This thesis comprises four scientific papers, contributing to the ongoing efforts to ensure safe and economical operation of nuclear fuel. Paper I describes the feasibility of analyzing short-lived fission gasses using gamma-spectroscopy measurements at short cooling times and Paper III covers a novel method of determining percent fission gas release based on these isotopes. Paper II describes a tomographic measurement device under development at the Halden Boiling Water Reactor (HBWR), and Paper IV covers a novel method for identifying failed fuel rods based on tomographic fission gas measurements.

The work has been performed at the HBWR, and accordingly, the studies have based on the conditions there.

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2 Nuclear Fuel Nuclear fuel assemblies in commercial water-cooled reactors have many similar design features regardless of the type of reactor. The fuel consists of ceramic pellets, made of either UO2 or a mixture of UO2 and PuO2 (so called MOX fuel), which are contained in metal tubes that are welded shut on both ends. The metal tubes are generally an alloy of Zirconium and are referred to as cladding tubes. The sealed cladding tubes containing the fuel are referred to as fuel rods. The fuel rods are bundled together into a fuel assembly containing from ~100 to ~300 fuel rods, depending on the fuel type. The rod-to-rod spacing is maintained by metal spacer grids. At the lower and upper ends of each fuel assembly is a lower and upper tie-plate or nozzle. Figure 1 shows a BWR, PWR, and HBWR fuel assembly where many of the sub-components are visible.

Figure 1, BWR (left) and PWR (middle) and HBWR (right) fuel assemblies showing some of the fuel assembly sub-components. The assemblies are not illustrated to scale relative to each other. The images are reproduced by courtesy of Westinghouse Electric Sweden AB, and the Institute

for Energy Technology – OECD Halden Reactor Project.

Fuel assemblies are designed such that individual fuel rods may be removed to allow for inspection or for removal and/or replacement of failed fuel rods. In some fuel assembly designs some fuel rods are part of the structure of the assembly, and are much more difficult to remove. The process of removing fuel rods from a fuel assembly is often time consuming and it involves the risk of inflicting handling damage to the fuel. Removing fuel rods from an assembly may also raise safeguards concerns. In spite of these issues plant operators often find that the benefits associated with the dismantling of fuel assemblies for measurement outweigh the risks and individual fuel rods are removed in order to perform destructive or nondestructive PIE.

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3 Gamma Spectroscopy on Nuclear Fuel Gamma spectroscopy is a well-established and widely used technique for characterizing nuclear fuel, where the analysis of isotope-specific gamma-rays allows investigation of the fuel properties to which the emitting isotopes correspond. Specifically, the content and distribution of the selected isotopes within the fuel is determined and correlated to the respective fuel property. Gamma-ray spectra recorded from fuel rods with approximately 3 days and 1 year cooling time respectively are shown in Figure 2. Each of the peaks seen in the spectra corresponds to the characteristic gamma-rays emitted in the decay of some fission product.

Figure 2, Gamma-ray spectra recorded using an HPGe detector. The top spectrum is from a fuel rod with approximately 3 days cooling time, and the bottom spectrum is from a fuel rod

with approximately 1 year cooling time. Although not indicated in these spectra, the gamma-ray emission from the former is much stronger than from the latter.

As seen in Figure 2, the gamma-ray spectrum after a short cooling time contains significantly more gamma peaks than the spectrum recorded after a longer cooling time. At short cooling times, the strong gamma-ray emission from short-lived isotopes may hinder the analysis of gamma peaks from long-lived isotopes. As a result, one must often wait for the short-lived isotopes to decay before measuring and analyzing long-lived isotopes.

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3.1 Established Gamma-ray Spectroscopy Applications

Some fuel properties for which gamma spectroscopy is an established technique of investigation are listed in Table 1 along with the isotope which corresponds to the fuel property and the energy of the gamma rays emitted in the decay of the respective isotope(s). References documenting the investigation of each of the properties are also listed.

Table 1, Common fuel properties investigated using gamma spectroscopy, along with the corresponding radioactive isotope, its half-life and energies of emitted gamma rays.

Fuel Property Isotope Half-life Gamma-ray energy [keV]

Reference(s)

Power distribution* 140Ba/140La 12.75 days 1596 [3], [4], [5] Burnup* 137Cs 30.08 years 662 [6] Integrated Fission Gas Release

85Kr 10.75 years 514 [7], [8], [9], [10], [11], [12]

Irradiation history* 137Cs 134Cs

30.08 years 2.07 years

662 605, 796

[13]

Initial Composition (UO2 vs. MOX)*

134Cs 154Eu

2.07 years 1.6 years

605, 796 1275

[14]

*Characterization measurements are also used as a means to perform verification of the fuel for nuclear safeguards purposes.

All of these measurements, with the exception of integrated fission gas release, are performed on either individual fuel rods or entire fuel assemblies. Integrated fission gas release measurements are only performed on individual fuel rods. Measurements performed for safeguards verification are typically performed on entire assemblies since dismantling fuel to measured individual fuel rods may itself raise safeguards concerns.

3.2 Proposed Novel Application for Gamma-ray Spectroscopy

Papers I and III explore a novel application of gamma-ray spectroscopy, whereby the release of fission gasses which has occurred over short irradiation sequences may be determined for the purpose of enhancing knowledge of the mechanisms and timescales of the FGR phenomenon. Determining FGR, which over short periods of irradiation, is of particular interest in research reactor settings such as at the HBWR, where experiments often involve short irradiation cycles where fuel is subjected to intentional transients during which the fuel behavior is investigated.

Currently, pressure sensors are used in selected specially-built fuel rods at the HBWR to monitor the real-time fuel rod internal pressure. Measurements using this method are limited to those experimental fuel rods in research reactors that are equipped with pressure sensors. An alternative, generally-applicable nondestructive method is desired in order to allow FGR characterization of fuel which is not equipped with pressure sensors or in fuel rods in which the pressure sensors have become unreliable.

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In Paper I the decay properties of the fission gas isotopes most abundant in the fuel at the end of a one-year cycle were evaluated to determine if any of these isotopes emitted gamma rays at suitable energies, and had suitable half-lives, such that they could be measured after short decay times. Three isotopes were identified and further evaluated by calculating the expected count rate at the detector for five of their emitted gamma-ray energies, based on MCNP [15] simulations of the gamma-ray transport through the fuel to the detector setup. The calculations were also verified by analyzing experimental gamma spectroscopy measurements from two fuel rods which had cooled for two days and twenty-three days, respectively. Three gamma-ray energies were identified as feasible for measurement after short decay time. The isotopes along with their corresponding half-lives, gamma-ray energies, and branching ratios are listed in Table 2.

Table 2, Short-lived isotopes identified in Paper I as potentially useful for determining fission gas release in fuel rods with short decay time.

Isotope Half-life γ energy [keV]

Branching ratio [% decays]

133Xe 5.24 days 81 38 133mXe 2.19 days 233 10

135Xe 9.14 hours 250 90 In Paper III a method, based on the feasibility demonstrated in Paper I, was developed for quantifying %FGR which occurs over short time periods. Specifically, Paper III focused on measuring the 81 keV gamma rays emitted in the decay of 133Xe present in the plenum of the fuel rod and comparing that to the amount produced in the fuel. The method was demonstrated using measurement data from experimental fuel irradiated in the HBWR. However, because the data had not initially been collected for this purpose, the uncertainty in the resulting %FGR was larger than normally expected. The major sources of uncertainty stem from incomplete knowledge of the physical arrangement of the measurement set-up, and from a lack of characterization of the relative detector efficiency at 81 keV and 1596 keV.

While this method is generally applicable, i.e. it may be applied to any fuel rod with a gas plenum, at commercial or research facilities, it is expected to be applied primarily at research facilities where the power history is such that the FGR behavior may be of interest over short time periods, and where it is possible to perform measurements before the short-lived isotopes have decayed away.

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4 Gamma Tomography on Nuclear Fuel Gamma tomography is a technique for characterizing nuclear fuel based on gamma spectroscopy measurements and tomographic reconstruction techniques. Here, Single Photon Emission Computed Tomography (SPECT) is used, which involves two basic steps: 1) recording the gamma radiation field surrounding the fuel assembly using gamma spectroscopy, and 2) reconstructing the assembly’s internal source distribution using tomographic techniques. A major advantage of gamma tomography is that all fuel rods in a fuel assembly may be characterized simultaneously without the need to dismantle the fuel.

4.1 Gamma Tomography Measurement Devices

Two gamma tomography devices have been previously developed and demonstrated for use on irradiated commercial fuel assemblies. The first device was developed for fuel performance investigations [16], and the second was developed for safeguards purposes [17]. Both of these devices were designed for characterization of commercial fuel assemblies, underwater, in spent fuel storage pools at nuclear power plants or interim storage facilities. The measurement sequence for each of these devices was/is based on rotating a set of collimated detectors around the fuel at a selected axial location. Figure 3 shows a top, schematic view of the first of these two gamma tomography devices.

Figure 3, Top schematic view of the fuel performance gamma tomography device in [16], showing how the collimated detectors rotated around the fuel assembly which was placed in a

water filled channel in the center of the device.

As described in Paper II, there is currently a novel gamma tomography system under development at the HBWR. Unlike the two previously-mentioned gamma tomography measurement devices, the measurement system at the HBWR is designed to rotate the fuel in

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front of the detector, and to measure the fuel in air inside a shielded enclosure instead of underwater. Figure 4 shows a side cutaway conceptual view of the HBWR gamma tomography system, which is now under construction. In this device the fuel is lifted and rotated while the detector is translated from side-to-side.

Figure 4, Side cutaway conceptual view of the HBWR gamma tomography measurement system.

The HBWR gamma tomography system will be used for investigating fuel performance parameters such as power and burnup distributions for various experimental fuel assemblies irradiated in the HBWR. In addition, the system will be used as an experimental platform for investigating further applications and refinements of the gamma tomography technique for nuclear fuel assemblies, including FGR and leaker rod identification (see section 4.3).

4.2 Previously Demonstrated Applications of Gamma Tomography on Nuclear Fuel Assemblies

Gamma tomography is based on gamma spectroscopy measurements, and it follows that many applications of gamma tomography are similar to the applications of gamma spectroscopy, but with gamma tomography there are additional capabilities to study spatial dependencies of the fuel properties under investigation. In particular, gamma tomography can be used to characterize all fuel rods in an assembly simultaneously, without the need to dismantle the fuel.

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There are numerous examples of gamma tomography measurements on individual fuel rods, however, the technique has seldom been applied on whole fuel assemblies. Some examples of demonstrated applications on fuel assemblies are the following:

• Determination of rod-by-rod power distribution in irradiated BWR fuel assemblies at the Forsmark Nuclear Power Plant in Sweden [18],

• Determination of rod-by-rod burnup using an experimental gamma tomography measurement set-up at the CLAB facility in Sweden [19],

• Safeguards partial defect verification demonstrated using a laboratory mock fuel assembly at Uppsala University in Sweden [19], and using a gamma tomography device for of commercial nuclear fuel developed for the IAEA [17].

4.3 Proposed Novel Application of Gamma Tomography

In the event that fuel cladding fails during reactor operation, the operator has to identify the assembly containing the failed rod in order to remove it from the core. It is also of interest from an operator’s perspective to identify the failed rod(s) within the assembly to be able to replace them and reintroduce the assembly into the core for further irradiation. At present, reliable methods exist for locating the failed assembly; however, the currently used methods for determining which rod is failed in the assembly are not always adequate [20]. As an alternative, gamma tomography has been suggested as a method for identifying leaking rods [21].

In Paper IV, a simulation study was performed to investigate the feasibility of the novel application of gamma tomography measurements for locating failed rods within a failed fuel assembly. The application is specifically based on measuring gamma rays emitted in the decay of a fission gas isotope in the gas plenum region of a fuel assembly and where a leaking rod is expected to be identified in the reconstructed image by its relative low activity. Simulations were performed based on the gamma rays emitted in the decay of 133Xe, 135Xe, and 85Kr, in order to assess each isotope’s suitability for measurement. These isotopes are listed in Table 3 along with their respective half-lives, emitted gamma-ray energies, and linear attenuation coefficients for various materials present in nuclear fuel and/or in the measurement setup.

Table 3, Fission gas isotopes used for simulation studies of gamma tomography as a leaker rod identification method, listed along with their half-lives, gamma-ray energies, and linear

attenuation coefficients for selected materials.

Isotopes Half-life Gamma-ray energy [keV]

Linear attenuation coefficients [cm-1] ‡

Water Stainless

Steel Zircaloy Air

(1 atm) Helium (30 atm)

133Xe 5.24 days 81 0.183 4.538 11.01 0.00020 0.00022 135Xe 9.14 hours 250 0.127 0.976 1.068 0.00014 0.00016

85Kr 10.75 years 514 0.096 0.659 0.560 0.00011 0.00012 ‡ from National Institute of Standards and Technology, XCOM, Photon Cross Sections Database

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The results of these simulations indicate that the method is feasible for leaker rod identification when measuring the 250 keV gamma rays emitted in the decay of 135Xe or the 514 keV gamma rays emitted in the decay of 85Kr. As an example, a reconstructed image using the 250 keV energy gamma rays is presented in Figure 5, in which one failed rod that was simulated with a fission gas content of 10% of the intact fuel rods may be discerned. In the reconstructed images obtained based on simulations of the 81 keV gamma-ray energy, the failed rod was not discernible, even when materials with the minimum attenuation were used in the simulations.

Figure 5, Reconstructed image showing feasibility of leaker rod identification using gamma tomography in a simulated measurement of the 250 keV gamma rays emitted by 135Xe.

While this application of gamma tomography was determined to be theoretically possible, the measurement of these particular gamma rays is however challenging. The short half-life of 135Xe means that it must be measured within the first days after the fuel is removed from the reactor, and the emission properties of the 514 keV gamma rays mean that measurements based on 85Kr must be carried out after the fuel has been allowed to cool for at least six months. Additionally, measurements based on 85Kr may be problematic because of the expected low emission rate of the 514 keV gamma rays and the presence of the interfering positron annihilation peak at 511 keV. Furthermore, the study showed that the expected measurement time may be unacceptably long if efforts are not made to maximize count rates.

The simulation study was based on a uniform rod-by-rod fission gas content distribution, except for the failed rod which contained ten times less fission gas than an intact rod. This uniform fission gas distribution is not expected in practice since the rod-by-rod fission gas concentration is known to vary widely from rod-to-rod, which may lead to a difficult-to-interpret image, i.e. rods with low fission gas release may be indistinguishable from leaking rods. Due to this, it is suggested that the reconstructed data be compared to the predicted rod-by-rod fission gas content distribution as calculated by e.g. the STAV code [22].

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5 Outlook The novel gamma spectroscopic method of determining %FGR and the novel gamma tomographic method for locating leaking rods in a fuel assembly will be further investigated and benchmarked using the tomographic device currently under construction at the HBWR - a unique opportunity to develop and refine these methods. Experimental fuel irradiated in the HBWR is intentionally subjected to a variety of operating conditions, is available for characterization a short time after the end of irradiation, and is furthermore, extensively characterized using calibrated in-core measurements as well as calibrated destructive and nondestructive methods during PIE. These conditions offer a unique opportunity to obtain experimental data for further investigation and benchmarking of the novel methods presented in this thesis.

The HBWR gamma tomography equipment is expected to perform the first test measurements in early 2013. Extensive measurement campaigns are planned in which applications of the gamma tomography technique will be investigated and evaluated, including the novel method of leaker rod identification. Measurements will be performed in a well-controlled laboratory setting where there is access to a wide variety of experimental and standard fuel assemblies and where cooling times range from hours to decades. There is expected to be high accuracy in the measurements using this device since it has been designed with favorable materials with regard to gamma-ray attenuation and since the measurements will be performed in air, allowing the use of high resolution HPGe detectors.

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6 Acknowledgements This work has been conducted as part of an Industrial PhD project originally set up between Westinghouse Electric Sweden AB, and Uppsala University, but which soon expanded to include the OECD Halden Reactor Project, in Halden, Norway, where the majority of the work is now performed. There are many, many, people who have contributed to or supported me in this work.

I would like to thank all those in the Uppsala University Department of Applied Nuclear Physics – past and present – who laid the foundation for the work I am doing now. You have all produced a lot of interesting reading material. I would especially like to thank my advisor, Staffan, for always guiding me in the right direction and for providing specific and challenging comments and questions that always seem to improve my work.

From Westinghouse, I would like to thank my colleagues in the Materials group (BTM). I would especially like to thank Lars Hallstadius, Christofer Willman, and Ben Clever who were instrumental in creating this Industrial PhD project. Thanks also go to Lars and Christofer for all of your valuable input into the development and evaluation of these methods and for your continued support of this project.

At the OECD Halden Reactor Project, there are many who deserve thanks. Thanks go to Margaret McGrath and Carlo Vitanza for being interested in this project and offering the exciting opportunity to conduct this research at the HBWR. Many thanks go to Knut Eitrheim and Terje Tverberg who have answered countless requests for experimental data. Thanks to Knut for guiding the gamma tomography project at the HBWR. I would also like to thank everyone on the design team (Cato Edvardsen, Morten Limi, Jan Liverud, Hans-Martin Solheim and Vidar Torp) for their continued hard work and creative solutions.

I would like to thank the Swedish Research Council (Vetenskapsrådet), for selecting and partially funding this Industrial PhD project.

I would like to thank my wife, Heather, who has supported me throughout this work, and who is always understanding of my hectic schedule. Thank you!

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7 References [1] “Nuclear Power Reactors in the World 2012 Edition,” International Atomic Energy

Agency, Vienna, Austria [2] “Key World Energy Statistics 2011,” International Energy Agency, Paris, France [3] I. Matsson, B. Grapengiesser, B. Andersson, “LOKET – A Gamma-ray Spectroscopy

System for In-pool Measurements of Thermal Power Distribution in Nuclear fuel,” Nucl. Instrum. Meth. A, vol. 569, issue 3, pp. 872-882, (2006)

[4] B. Andersson, B. Grapengiesser, “Measurements on Fuel Assemblies from Cofrentes for Determination of Relative Power Distribtion,” Westinghouse Report, BTM 02-169 (2002)

[5] S. Holcombe, “Measurements on Fuel Assemblies and Fuel Rods from Oskarshamn 2 for Determination of Relative Power Distribution, August 2007”, Westinghouse Report, BTM 08-0248 (2008)

[6] A. Bäcklin, A. Håkansson, P. Björkholm, A. Dyring, K-G. Görsten, “A Gamma-Spectroscopy System for Burnup Measurements of Nuclear Fuel,” 13th ESARDA Symposium of Safeguards and Nuclear Material Management, Avignon, France, May (1991)

[7] I. Matsson, P. Jansson, B. Grapengiesser, A. Håkansson, A. Bäcklin, “Fission Gas Release Determination Using an Anti-Compton Shield Detector,” Nuclear Technology, vol. 122, pp. 276-283 (1998)

[8] B. Grapengiesser, L. Hallstadius, “Non-destructive Pool-side Methods for Fission gas Release Determination and Fuel Column Characterisation of Intact fuel Rods,” Ninth International Conference on Nondestructive Evaluation in the Nuclear Industry, Tokyo, Japan, April 25-28, (1998)

[9] T.R. Blair, L.F. Van Swam, “Nondestructive Determination of Fission Gas Release at Poolside,” Transactions of the American Nuclear Society, vol. 44, pp. 237-238 (1983)

[10] P.M. O’Leary, D.R. Packard, “Nondestructive Fission Gas Release Measurement and Analysis,” Transactions of the American Nuclear Society, San Diego, California, June 20-24, (1993)

[11] S. Holcombe, C. Willman, A. Knuutila, K. Ranta-Puska, “Experimental Fission Gas Release Determination at High Burnup by Means of Gamma Measurements on Fuel Rods in OL2,” Proceedings of Top Fuel 2009, Paris, France, September 6-10, (2009)

[12] S. Holcombe, “FPP-2 Fission Gas Release Measurements.” Westinghouse Report BTM 08-1260 rev. 0, (2008)

[13] C. Willman, A. Håkansson, O. Osifo, A. Bäcklin, S. Jacobsson Svärd, “Nondestructive Assay of Spent Nuclear Fuel with Gamma-ray Spectroscopy,” Annals of Nuclear Energy, 33(5):427-438 (2006)

[14] C. Willman, A. Håkansson, O. Osifo, A. Bäcklin, S. Jacobsson Svärd, “A Nondestructive Method for Discriminating MOX Fuel from LEU Fuel for Safeguards Purposes,” Annals of Nuclear Energy, 33(9):766-773 (2006)

[15] D. B. Pelowitz, “MCNPX User’s Manual Version 2.5.0,” (2005) [16] P. Jansson, S. Jacobsson Svärd, A. Håkansson, A. Bäcklin, “A Device for Non-

destructive Experimental Determination of the Power Distribution in a Nuclear Fuel Assembly,” Nuclear Science and Engineering, vol. 152, nr. 1, pp. 76-86, (2006)

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[17] F. Lévai, S. Desi, S. Czifrus, S. Feher, M. Tarvainen, T. Honkamaa, J. Saarinen, M. Larsson, A. Rialhe, R.K. Arlt, “Feasibility of Gamma Emission Tomography for Partial Defect Verification of Spent LWR Fuel Assemblies,” STUK report STUK-YTO-TR 189 (2002)

[18] S. Jacobsson Svärd, A. Håkansson, A. Bäcklin, O. Osifo, C. Willman, P. Jansson, “Non-destructive Experimental Determination of the Pin-power Distribution in Nuclear Fuel Assemblies,” Nuclear Technology, vol. 151, nr. 1, pp. 70-76, (2005)

[19] S. Jacobsson Svärd, A. Håkansson, A. Bäcklin, P. Jansson, O. Osifo, C. Willman, “Tomography for Partial Defect Verification – Experiences from Measurements Using Different Devices,” Esarda Bulletin, ISSN 0392-3029, vol. 33, pp. 15-25, (2006)

[20] “Review of Fuel Failures in Water Cooled Reactors,” IAEA Nuclear Energy Series, No. NF-T-2.1, International Atomic Energy Agency, Vienna Austria, (2010)

[21] Inventors: Y. Enokido, A. Kuniaki, H. Hiroi, “Method for Detecting Failure of Nuclear Reactor Fuel,” European Patent Application, No. 0 684 611 A1, (1995)

[22] G. Zhou, “High Burnup Fuel Behavior Modeling,” ESB International Seminar – Integral Advanced Fuel Modeling, Zurzach, Switzerland, January 10-11, (2008)

1 Introduction2 Nuclear Fuel 3 Gamma Spectroscopy on Nuclear Fuel3.1 Established Gamma-ray Spectroscopy Applications3.2 Proposed Novel Application for Gamma-ray Spectroscopy

4 Gamma Tomography on Nuclear Fuel4.1 Gamma Tomography Measurement Devices4.2 Previously Demonstrated Applications of Gamma Tomography on Nuclear Fuel Assemblies4.3 Proposed Novel Application of Gamma Tomography

5 Outlook6 Acknowledgements7 ReferencesArticle-FGR-Final - Formatted for Lic_.pdf1 INTRODUCTION:1.1 FISSION GAS RELEASE1.2 THE NEED FOR A NOVEL EXPERIMENTAL METHOD OF DETERMINING FISSION GAS RELEASE OVER SHORT TIME PERIODS

2 AVAILABLE GAMMA-SPECTROSCOPY-BASED METHOD FOR MEASURING INTEGRATED FISSION GAS RELEASE3 THE PROPOSED METHOD 3.1 ISOTOPE SELECTION3.2 CONSIDERATIONS DUE TO IRREGULAR IRRADIATION3.3 MEASURING GAS CONTENT IN THE PLENUM3.4 RELATING MEASURED GAS CONTENT TO FISSION PRODUCTS IN THE FUEL3.5 RESULTING EXPRESSION FOR CALCULATING %FGR BASED ON MEASUREMENTS AND CORE PHYSICS CODE CALCULATIONS

4 FEASIBILITY DEMONSTRATION5 CONCLUSIONS AND OUTLOOK 6 REFERENCES

leaker id final.pdf1 INTRODUCTION:1.1 LEAKING FUEL1.2 EXISTING METHODS FOR LEAKER ROD IDENTIFICATION

2 GAMMA TOMOGRAPHY FOR LEAKER ROD IDENTIFICATION2.1 THE GAMMA TOMOGRAPHIC METHOD2.2 FISSION GAS ISOTOPES AVAILABLE FOR LEAKER ROD IDENTIFICATION USING GAMMA TOMOGRAPHY

3 SIMULATION STUDIES3.1 FUEL MODEL AND SOURCE3.2 MODELLED INSTRUMENTATION3.3 SIMULATED MEASUREMENT SEQUENCE3.4 SIMULATED MEASUREMENTS AND TOMOGRAPHIC IMAGE RECONSTRUCTIONS

4 CONSIDERATIONS FOR THE PRACTICAL APPLICATION OF THE METHOD4.1 AVAILABILITY OF FUEL AT SHORT COOLING TIMES4.2 EXPERIMENTAL CHALLENGES FOR 85Kr MEASUREMENTS4.3 RODS WITH LOW FISSION GAS CONTENTS4.4 TIME REQUIRED FOR MEASUREMENT

5 CONCLUSIONS, DISCUSSION AND OUTLOOK6 REFERENCES