Neutron radiography and tomography applied to fuel degradation during ramp tests and loss of coolant accident tests in a research reactor
Introduction
Norway was among the first countries to put a nuclear research reactor in operation. The Halden Reactor Project (HRP) was affiliated to OECD NEA and the first agreement was signed in 1958. Customers in the HRP are nuclear utilities, vendors, licensing authorities and R&D centers. The project contributes to improve the safety at nuclear plants around the world through investigation of fuels and irradiated reactor materials. The neutron radiography facility at the IFE JEEPII (Joint European Energy Pile) heavy water research reactor facility located at Kjeller, Oslo region, was constructed and built during 1972–1974.
Neutron radiography is a powerful non-destructive material examination technique (Lehmann et al., 2003, Jenssen and Oberländer, 2002). The high attenuation of neutrons in hydrogenous materials, with a high penetration for heavy metals, makes neutron radiography a complementary technique to X-ray imaging. Unlike X-rays and γ-rays, neutron interaction is characterized by nuclear rather than electronics of the medium through which it passes. The development of neutron radiographic relevance had to wait for the advent of sufficiently intense neutron beams which became available with the development of research reactors in the 1950s. Today, also spallation initiated neutron sources (SINQ) are utilized in neutron radiography with excellent examination results, e.g. at PSI in Switzerland (Lehmann et al., 2011).
Neutron radiography of irradiated fuel rods is extensively used for image acquisition and qualitative analysis of cladding with respect to hydrogen up-take, secondary fuel degradation experiments and reactor power ramp experiments performed under the HRP. Pellet cladding interaction (PCI) failures and clad hydriding are well documented from neutron radiographs. Some examples from ordinary neutron radiography are discussed in the paper. Neutron radiography is applied on experimental boron carbide (B4C) control rods and under re-fabrication of high burn-up fuel rods which are modified with new instrumentation for further testing in the HBWR. Design basic (DB) LOCA experiments are performed for many years in the HBWR (Oberländer et al., 2008, Kolstad et al., 2011). Neutron radiography is a very useful tool utilized under re-fabrication of irradiated fuel rods and interpretation of examination results, e.g. fuel fragmentation, relocation and disposal under LOCA examination.
Tomography utilizing neutrons has developed over the last 10–15 years as an interesting expansion of the traditional two-dimensional neutron radiography. The meaning of computerized tomography (CT) is the reconstruction of a function from its line or plane integrals, irrespective of the field where this technique is applied. Neutron tomography provides three-dimensional spatially resolved images which normally display the attenuation coefficient distribution over the sample volume. The reconstruction of image cross-sections is based on a set of radiographs (projections) obtained at different equidistant angles spread 180° around the sample. The reconstruction of the sample attenuation coefficient acquired from a fuel rod irradiated in the Halden Reactor in a particular LOCA test is demonstrated with the Simultaneous Algebraic Reconstruction Technique (SART) using the open source ASTRA toolbox developed at the Antwerp University (Andersen and Kak, 1984). Also, another tomography method based on Chebyshev moments, which is very promising, is presented in the paper. The motivation for applying CT in neutron radiography in examination of irradiated fuel rods is to obtain fuel rod macrographs without cutting the rod for destructive metallography and ceramography examinations.
Section snippets
Source, principle and techniques utilized in neutron radiography
Two neutron radiography (Harms and Wyman, 1986) techniques are utilized at the Institute for Energy Technology under examination of irradiated experimental rods with UO2 or MOX fuels, zircaloy cladding and instrumentation devices. Both techniques allow a collimated thermal neutron beam to be used and enable neutron radiography of very active samples. The traditional method uses an activation transfer technique, utilizing a dysprosium foil and X-ray film. The other method uses a special solid
Results
Reactor in-pile results obtained from LOCA-, ramp- and fuel degradation testing are presented.
Conclusion and neutron radiography perspectives
Neutron radiography is an excellent non-destructive method for examination of irradiated experimental fuel rods, e.g. LOCA testing, fuel degradation, power ramp tests and re-fabrication and instrumentation situations. Neutron radiography is a powerful method particularly under examination of fuel rod failure analysis. However, the best achievable spatial resolution for traditional film techniques is limited to about 40–50 μm which restricts the detail observations of neutron radiographs and
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