|NDT.net - August 2002, Vol. 7 No.08|
3nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, Nov 14-16, 2001, Seville Spain
Thermal and sub-thermal neutrons are invaluable tools for investigations in fundamental, engineering and medical science. These neutrons have unique physical properties, i.e., a magnetic moment, very suitable wavelength, no electric charge, which enables the experimental scientist to obtain information, which is simply not available otherwise. In addition, most applications of neutrons are non-destructive, which is mainly due to the particular type of neutron-matter interaction, which allows neutrons to penetrate deeply into most materials.
In the engineering field neutrons are nowadays mostly applied for measuring residual stresses. Nevertheless texture, and microstructure investigations are common practice and defects analysis methods are progressing.
In this paper the unique features of neutrons are described and the borderline towards similar experimental tools (like X-rays) is drawn. A number of applications of neutrons are shown and a brief overview of the installations available at the High Flux Reactor (HFR) of the European Commission is given.
The High Flux Reactor is located in Petten (The Netherlands). It is managed by the European Commission and operated under contract by the Nuclear Research and Consultancy Group (NRG, NL).
In this reactor 12 beam tubes are available for fundamental, engineering and medical research. Apart from those used for medical purposes, the neutron beams are used for various types of non-destructive investigations on materials and components.
The purpose of this paper is to give a general idea about the relevance of neutron techniques for such measurements and illustrate this with some related test data obtained at the HFR. This brief overview can certainly not present exhaustively the possible neutron applications in the NDE area.
The arrangement of the 12 horizontal beam tubes is presented in Table 1., while a schematic presentation of the corresponding experimental facilities is depicted in Fig. 1. The fluence rates available at the entrance windows of the beam tubes, are: thermal fluence rate 9-10 x 1013 and fast fluence rate 3-10 x 1012 cm-2s-1. The beam tubes HB11 an HB12 form the "large neutron facility" currently dedicated to medical research using neutrons.
|HB 1||Triple-axis spectrometer / Diffuse-scattering diffractometer|
|HB 2||Nuclear polarisation facility|
|HB 3a||Triple-Axis Spectrometer (TAS)|
|HB 3b||Small Angle Neutron Scattering facility (SANS)|
|HB 4||Residual stress diffractometer /
Large Component Neutron Diffraction Facility (LCNDF)
|HB 5||Combined powder and stress diffractometer|
|HB 7||Facility for capture gamma-ray analysis|
|HB 8||Neutron-radiography facility|
|HB 9a||Four circle diffractometer|
|HB 9b||Activation analysis facility|
|HB 11||Boron Neutron Capture Therapy facility (BNCT)|
|HB 12||Filtered neutron facility|
|Table 1: Arrangement of the twelve horizontal beam tubes.|
|Fig 1: Arrangement of the horizontal beam tubes HB1 to HB12.|
Neutrons have specific characteristics explaining their relevance to non-destructive testing techniques and their ability to provide information from inside of material specimens.
Due to the aforementioned relatively weak neutron-matter interactions one finds three significant differences between neutrons and X-rays in testing of material specimens:
|Fig 2: Penetration depths for neutrons, X-rays and electrons as function of atomic number.|
During the conception and the fabrication of materials, certain levels of chemical impurity are not admissible. Activation analysis is a well-established non-destructive method in science and technology to detect even minimum concentrations of elements. Its applications reach from environmental and multi-element analysis, medicine, production control, and wear analysis (by tracer methods) to geo-chemistry and archaeology. In a large number of cases the purity of materials can be checked with a maximum degree of sensitivity.
This method facilitates detecting of about 60 chemical elements to an accuracy of one nanogramme per gramme. The method is applied quickly, easy to handle and highly sensitive. However, it only provides information on the global quantity of elements inside the material, but nothing on the their distribution and chemical form. A related technique is used for the measurement of boron concentration in the blood of patients at HFR/HB7 in support of a clinical trial on cancer treatment performed at the HFR beam tube facilities.
X-ray and neutron radiography are well-established methods for generating shadow pictures of the interior of macroscopic bodies. The beam attenuation (absorption or scattering) within the sample is recorded on a two-dimensional detector, and objects up to several metres in size can be examined this way.
|Area or topic of application||Objects||Purpose of inspection, Check|
|Aircraft & helicopter maintenance, turbine manufacturing||Aluminium, honeycomb & composite structures, turbine blades||Corrosion, moisture, adhesive defects, QC on remainders of materials used for cores|
|Aerospace industry & research||Pyrotechnique devices (actuators, cable cutters), mechanical and electronic components||Assembly control, QC of explosive charges, function, seals, isolation, lubrication|
|Automobile industry & Research||Operating combustion engine, airbag charge||Study of fluid flow, lubrication, QC gas pile|
|Chemical & petrochemical industry & research||Mechanical components & structures, two phase processes||Hydriding of steel, QC of sealing, visualise two phases|
|Materials sciences, ceramics & composite R&D||Metallurgy samples, high- tech ceramics, composite structures||Alloy distribution, QC for cracks, inclusion, density, bonding & porosity|
|Civil engineering||Concrete samples, reinforced concrete, concrete with plastic coated reinforcement||Water permeability, aging of concrete, behaviour of steel in reinforced concrete|
|Heat transfer||Heat pipes, two phases in steel pipe (e.g. gas&water, molten metals, salts)||Visualisation of two phases and behaviour|
|Defence industry & ordnance||Explosive, ignitors||QC explosive charges & mechanical structures|
|Table 2: Survey of main technical and industrial applications of neutron radiography.|
The linear attenuation coefficient of X-rays increases with the third to fourth power of the atomic number, whereas the attenuation of thermal and sub-thermal neutrons is specific to the isotope and varies by more than three orders of magnitude throughout the periodic system of elements (see Fig. 3). It may vary strongly even for different isotopes of the same element. Metals like aluminium, iron or lead are easily penetrated by neutrons, whereas the attenuation for hydrogen is 400 times stronger for neutrons than for X-rays, and organic compounds like oil or plastics are easily detected as "black" areas in neutron images but they are virtually invisible for X-rays. With their different properties, combined X-ray and neutron radiography will become a valuable tool for non-destructive material testing. In fact the mass attenuation coefficients are also a function of the energy of the neutrons.
|Fig 3: Thickness of material necessary to decrease the neutron flux by 50%.|
|Fig 4: Mass attenuation coefficients for X-rays and neutrons of different energies (e.g. epithermal, thermal and sub-thermal).|
Figure 4 gives a comparison of neutron and X-ray mass attenuation coefficients for sub-thermal, thermal and epi-thermal neutrons. Thus X-ray and neutron radiography are complementary techniques and neutrons are to be preferred in cases, where fluids, oil traces, adhesive taps presence, corrosion before thickness modification (hydrogen in aluminium), organic seals of organic explosive continuities, etc., shall be detected. Table 2. (extracted from ) presents some applications.
Most of what we know about the crystal structure of matter is derived from Bragg diffraction performed either using X-rays or neutrons. Both types of radiation can be tuned to have the appropriate wavelength, comparable to typical lattice spacing. From careful analysis of diffraction patterns, consisting sometimes of thousands of diffraction peaks generated in different directions, structures of high complexity can be analysed. Certainly, X-ray diffraction is the predominantly applied method, and synchrotron beams with their outstanding intensities for X-rays open new areas of complexity still to be studied. However, due to their specific properties, neutrons remain unalienable in diffractometry: unlike X-rays, neutrons provide information from the interior of material specimens. Furthermore, the simple neutron-atom interaction (neutrons see the point-like nucleus, X-rays see the extended electron cloud around the nucleus) renders neutron data to be easily interpreted.
Due to their high penetration into many materials of technological importance neutrons facilitate non-destructive examination of texture, crystal orientation and distribution of phases. Three-dimensional patterns of residual stress in railway rails, turbine blades, welds or in magnetic metal sheets yield crucial information related to their performance.
|Fig 5: New neutron beams diaphragm system installed at HB4/LCNDF.|
Two neutron diffractometers are installed at the HFR (HB4 and HB5) for residual stress measurements. During the last years a number of significant results have been obtained on these facilities [4 – 10]. Fig. 5 shows the installation at HB4 upgraded in 2000, that facilitates stress measurements in industrial components of up to 1000 kg. Figure 6 shows results of a large round-robin test  (HFR results included).
|Fig 6: Neutron diffraction round robin, measurements of strain in an interference fit (Al ring& plug) .|
Small angle neutron scattering provides for detection of defects, precipitates, voids etc. The HFR/HB3b SANS facility has been successfully used for such investigations in the past. Processes like irradiation can usually introduce such defects. A large number of experimental campaigns to study the effects of long term irradiation on structural materials are performed at the HFR. Recently the SANS instrument has been upgraded and defects investigations are about to be commenced.
Other applications of thermal neutrons for NDE are:
Neutron beam based techniques are by definition, non-destructive methods facilitating a large variety of analyses on material specimens. The main limitation of these methods is that the specimens have to be tested at large scale facilities like reactors and spallation sources. On site investigations are not possible. For this reason techniques based on transportable equipment like ultrasonic or sometimes X-ray methods are more flexible in use. Nevertheless, due to the properties of the neutrons these techniques provide unique information that is very often not available otherwise. In particular through-thickness of material stresses, micro-structural analysis, internal stresses or tracing of elements are only possible with such specialised neutron techniques.
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