Understanding Neutron Radiography Reading 2016-III-NRT-A

Understanding Neutron R adiography NDTH book 4-CH P16 R eading 2016-3– NR T My ASNT Level III, Pre-Exam Preparatory Self Study Notes 4thAugust 2016 Charlie Chong/ Fion Zhang

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The Magical Book of Neutron Radiography Charlie Chong/ Fion Zhang

数字签名者：Fion Zhang DN：cn=Fion Zhang, o=Technical, ou=Academic, email=fion_zhang @qq.com, c=CN 日期：2016.08.04 06:01:42 +08'00' Charlie Chong/ Fion Zhang

ASNT Certification Guide NDT Level III / PdM Level III NR - Neutron Radiographic Testing Length: 4 hours Questions: 135 1. Principles/Theory • Nature of penetrating radiation • Interaction between penetrating radiation and matter • Neutron radiography imaging • Radiometry 2. Equipment/Materials • Sources of neutrons • Radiation detectors • Non-imaging devices Charlie Chong/ Fion Zhang

3. Techniques/Calibrations • Electron emission radiography • Blocking and filtering • Micro-radiography • Multifilm technique • Laminography (tomography) • Enlargement and projection • Control of diffraction effects • Stereoradiography • Panoramic exposures • Triangulation methods • Gaging • Autoradiography • Real time imaging • Flash Radiography • Image analysis techniques • In-motion radiography • Fluoroscopy Charlie Chong/ Fion Zhang

4. Interpretation/Evaluation • Image-object relationships • Material considerations • Codes, standards, and specifications 5. Procedures • Imaging considerations • Film processing • Viewing of radiographs • Judging radiographic quality 6. Safety and Health • Exposure hazards • Methods of controlling radiation exposure • Operation and emergency procedures Reference Catalog Number NDT Handbook, Third Edition: Volume 4, Radiographic Testing 144 ASM Handbook Vol. 17, NDE and QC 105 Charlie Chong/ Fion Zhang

Fion Zhang at Copenhagen Harbor 4thAugust 2016 Charlie Chong/ Fion Zhang

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Chapter 16 Neutron R adiography Charlie Chong/ Fion Zhang

PAR T 1. Applications of Neutron R adiography Neutron radiation is similar to X-radiation. The radiation can originate from an effective point source or can bec ollimated to shine through an object in a coherent beam. The pattern of penetrating radiation can then be studied to reveal clues about the internals of the object. The information conveyed can be very different from that obtainable with X-rays. Whereas X-rays are attenuated by dense metals more than by hydrocarbons,neutrons are attenuated more by hydrocarbons than by most metals. The difference can mean much more than the reversal of a positive image to a negative image. Neutrons, for example, can reveal details within high density surroundings that cannot be revealed by other Charlie Chong/ Fion Zhang

A typical application for neutron radiography is shown in the images of a pyrotechnic device (Fig. 1), where the small explosive charge is encased in metal. Other applications include inspection of explosive cords used in pilot ejector mechanisms; inspection of gaskets, seals and O-rings inside metallic valves; confirmation that coolant channels in jet engine turbine blades are free of blockage; studies of coking in jet engine fuel nozzles; and screening of aircraft panels to detect low level moisture or early stage corrosion in aluminum honeycomb (Fig. 2). Charlie Chong/ Fion Zhang

FIGURE 1. Electric bridge wire squid: (a) drawing and (b) neutron radiograph of part as aid to interpretation; (c) helium-3 gaseous penetrant applied to serviceable unit; (d) penetrant applied to dysfunctional unit. Charlie Chong/ Fion Zhang

User’s Guide Unlike many other forms of nondestructive testing, neutron radiography is not a do-it-yourself technique. There have been neutron radiography service centers in the United States since 1968. To try out neutron radiography on an object of interest, it is simply necessary to locate the services currently available and, if agreed, mail your item to them. Typically, the neutron radiograph and your item will be mailed back within a day or two. The cost could be less than 1 or 2 h of an engineer’s time. If assistance is required to interpret the findings, this too may be requested ona service basis, as may referrals to more specialized neutron radiographic techniques. The providers of neutron radiography services use equipment and expertise that is highly specialized. Even though one or more neutron radiography service centers have been operating successfully for over 30 years, there has been no in- house neutron radiography available at any general service, commercial nondestructive testing center. Charlie Chong/ Fion Zhang

FIGURE 2. Comparison of neutron radiographs of moisture globules in aluminum honeycomb panel, later dried: (a) before processing; (b) after processing. (a) before processing Charlie Chong/ Fion Zhang

(b) after processing. Charlie Chong/ Fion Zhang

The interested user is therefore advised to seek a supplier of neutron radiographic services using leads such as society directories or the published literature. Because neutrons are fundamentally different from X-rays, any object that is a candidate for inspection by X-radiography could also be a candidate for neutron radiography. If X-rays cannot give sufficient information, then trials with neutron techniques may be prudent. The most frequently successful complement to X-radiography is static radiography with thermal neutrons. This approach is reviewed next. Then more specialized neutron radiology techniques are reviewed, such as neutron computed tomography, dynamic neutron imaging, high frame rate neutron imaging, neutron induced autoradiography and neutron gaging. For each of the neutron radiology techniques different neutron energies may be selected. The user should be aware that many of the specialized services are only available at one or two centers worldwide. It is therefore important to shop in the global market and to take advantage of the excellent communications existing between neutron radiography centers in various countries. Charlie Chong/ Fion Zhang

PAR T 2. Static R adiography with Thermal Neutrons Charlie Chong/ Fion Zhang

Neutron Energy Thermal energy neutrons are those that have collided repeatedly with a moderator material, typically graphite or water, such that they reach an equilibrium energy with the thermal energy of the moderator nuclei. The attenuation coefficients for thermal neutrons differ from material to material in a way that is different from X-rays as shown in Table 1. As a consequence, a high degree of contrast between the elements in an object is possible. In addition, thermal neutrons are relatively easy to obtain and easy to detect Charlie Chong/ Fion Zhang

TABLE 1. Comparison of X-ray and thermal neutron attenuation. Charlie Chong/ Fion Zhang

Neutron Collimation Because the source of thermal neutrons is a dispersed moderator volume, rather than a point source, it is necessary to use a collimator between the source and the object. In preference to a single tube parallel sided collimator or a multiple slit collimator, the most frequently used design uses divergent beam geometry.16 The collimator may be used to extract a beam in any one of a variety of different geometries including horizontal or vertical, radial or tangential to the source. A collimator that is tangential to the source can provide a thermal neutron beam relatively free of fast neutron and gamma ray contamination. An incidental consequence of the divergent collimator principal is that even very large objects can be radiographed using an array of side-by-side films (Fig. 3). Charlie Chong/ Fion Zhang

FIGURE 3. Radiographs of full size motorcycle: (a) neutron radiograph; (b) x- radiograph. Charlie Chong/ Fion Zhang

FIGURE 3. Radiographs of full size motorcycle: (a) neutron radiograph; (b) x-radiograph. Charlie Chong/ Fion Zhang

Neutron Imaging Collimation Ratio The collimation ratio is the ratio L·D–1of the collimator length L to aperture diameter D. This ratio helps to predict image sharpness. Imaging Processes For static thermal neutron radiography of nonradioactive objects, two important imaging processes are 1. the gadolinium converter with single emulsion X-ray film and 2. the neutron sensitive storage phosphor (neutron imaging plate). For static neutron radiography of radioactive objects, additional imaging processes are (transfer methods) 1. dysprosium foil activation transfer to film, 2. Indium foil activation transfer to film and 3. Track etch imaging using a boron converter and cellulose nitrate film. Charlie Chong/ Fion Zhang

The established direct imaging technique uses thin gadolinium layer vapor deposited on a solid converter screen, which is held flat against a single emulsion film inside a vacuum cassette of thin aluminum construction. An exposure of 109neutrons per square centimeter can give a high resolution, high contrast radiograph if careful dust free film darkroom procedures are used. Neutron sensitive imaging plates consist of a thin phosphor layer containing a mixture of storage phosphor, neutron converter and organic binder. Following the neutron exposure stage is the information readout phase, in which the plate is scanned by a thin laser beam stimulating the emission of a pattern of light. Merits of this neutron imaging technique include five decades of linearity, wide dynamic range, direct availability of digital data for processing converter efficiencies of 30 to 40 percent, and spatial resolution acceptable for some applications Charlie Chong/ Fion Zhang

For neutron radiography of highly radioactive objects, dysprosium and indium foil activation transfer to film and track etch imaging each offer complete discrimination against gamma ray fogging. Examples of nuclear fuel neutron radiography are shown in Fig. 4. Dysprosium transfer can be combinedwith a cadmium indium foil sandwich for dual energy radiography. Alternative track etch techniques have been developed to yield more precise dimensional measurements. Charlie Chong/ Fion Zhang

FIGURE 4. Neutron radiographs of nuclear fuel: (a) longitudinal cracks in pellets; (b) missing chips in compacted fuels; (c) inclusions of plutonium in pellets; (d) accumulation of plutonium in central void; (e) deformed cladding; (f) hydrides in cladding. (a) longitudinal cracks in pellets; Charlie Chong/ Fion Zhang

(b) missing chips in compacted fuels; (c) inclusions of plutonium in pellets; Charlie Chong/ Fion Zhang

(d) accumulation of plutonium in central void; (e) deformed cladding; Charlie Chong/ Fion Zhang

(f) hydrides in cladding. Charlie Chong/ Fion Zhang

Image Quality Indicators For any nondestructive system, the best measure of quality is to compare the image of the test object with an image of a similar object that contains a known artificial discontinuity, a defect standard, or reference standard. However, neutron radiography has the same problems as other nondestructive testing methods: the quantity of reference standards required is too large to obtain and maintain. In lieu of a reference standard, neutron radiographers have chosen to fabricate a resolution indicator that emulates the worst case scenario with gaps placed between and holes placed beneath different plastic thicknesses. For defining the neutron beam characteristics a beam purity indicator has been devised to accompany the sensitivity indicator. The image quality indicator system of ASTM International has become the primary or alternate system for most manufacturing specifications on an international basis. The no umbra device, a device to measure resolution, is described in ASTM E 803-91 and can be used to determine the collimation ratio L·D–1of the neutron radiography facility. Charlie Chong/ Fion Zhang

Nuclear Reactor Systems A nuclear reactor system operated for over 30 years solely to provide a commercial neutron radiographic service is illustrated in Fig. 5. The reactor core, positioned underground in a tank of water, is only about 0.38 m (15 in.) in diameter and operates at 250 kW power. The tangential beam tube is orientated vertically with air displaced by helium. Parts for neutron radiography can therefore be supported on horizontal trays. Usually the neutron imaging uses a gadolinium converter with fine grain radiographic film and the exposure time at a selected collimation is typically about 2 min. Charlie Chong/ Fion Zhang

FIGURE 5. Representative neutron radiographic service center for nonnuclear applications. Charlie Chong/ Fion Zhang

Another reactor that has provided neutron radiography services since 1968 is illustrated in Fig. 6. It is above ground and the fuel of the 100 kW core is arranged in an annulus with a moderator region in the center. Two horizontal beams are extracted from the central moderator, one for direct film neutron radiography of nonradioactive objects, the other for dysprosium activation transfer neutron radiography of radioactive nuclear fuel. Charlie Chong/ Fion Zhang

FIGURE 6. Representative neutron radiographic service center for nuclear and nonnuclear applications. Charlie Chong/ Fion Zhang

Another service for static neutron radiography of radioactive nuclear fuel has been provided by a 250 kW nuclear reactor installed in a hot cell complex (Fig. 7). Also several university reactors in the United States have been equipped for neutron radiography. Worldwide, over fifty nuclear reactors have contributed to development of this field. Charlie Chong/ Fion Zhang

FIGURE 7. Hot cell fuel inspection system. Charlie Chong/ Fion Zhang

Accelerator Based Systems An initial user of neutron radiography need not, in general, be concerned with accelerator source options unless there is an established need either for an in-house system or for a transportable system. Almost all neutron radiography service providers use a nuclear reactor source. One exception has been the powerful spallation type accelerator in Switzerland; the accelerator is a multipurpose facility comparable in complexity and cost to a research reactor. An in-house system that was operated successfully for over 15 years at the United States Department of Energy’s Pantex Plant used a van de graaff accelerator. The operation of this machine, which accelerates over 200 μA of deuterons at 3 MeV into a beryllium target, is illustrated in Fig. 8. Charlie Chong/ Fion Zhang

The system provided a peak thermal neutron flux of about 109neutrons per square centimeter second, two orders of magnitude less than the reactor systems described above but sufficient for low throughput work using 2 h exposure times and a relatively low beam collimation ratio. Cyclotrons and radio frequency quadrupole accelerators are other candidates for a potential custom designed in-house neutron radiographic system. Neutron radiographic performance data have been reported for designs with a variety of sizes, neutron yields and costs. For transportable systems much of the development work has used sealed tube acceleration of deuterium tritium mixtures. This can consist of a source head that is maneuverable with long high tension cable linking it to the high voltage power supply and control unit as illustrated (Fig. 9). The particular type shown yields a peak thermal neutron flux of about 108neutrons per square centimeter second with a tube operation half life of about 200 h. Charlie Chong/ Fion Zhang

FIGURE 8. Cross section showing van de graaff principle. Charlie Chong/ Fion Zhang

FIGURE 9. Components of mobile deuterium tritium neutron radiographic system: (a) deuterium tritium source head, typically on 6 m (20 ft) cables; (b) cooling unit (left) and power supply; (c) control unit. Charlie Chong/ Fion Zhang

High Intensity Californium-252 Systems Of the many radioactive neutron sources, such as polonium-210 beryllium and americium-244 beryllium, one has dominated interest for neutron radiography: californium-252. This transplutonic isotope is produced as a byproduct of basic research programs. In the United States, some government centers have been able to obtain the source on a low cost loan basis from the Department of Energy. The isotope yields neutrons by spontaneous fission at a rateof 2 × 109 neutrons per second per milligramand has a half life of 2.5 years. Charlie Chong/ Fion Zhang

Understanding Neutron Radiography Reading 2016-III-NRT-A