Neutron Imaging for Aerospace Applications

Abstract

Neutron imaging plays a major role in non-destructive testing of components used in aircraft and space launch vehicles, especially turbine blades and explosive-filled pyrotechnic devices. We discuss the particular features or information extracted using neutron imaging which help flaw detection in such components with special focus on applications of neutron imaging in the Indian space programme. In this context, three different neutron imaging facilities, namely reactor-based neutron imaging facility, electron Linac/photoneutron-based neutron imaging facility and D-T neutron-based imaging facility, have been discussed. All three facilities have been used during different stages of the Indian space programme, and a comparison of their features and utilities has been discussed.

8.1 Introduction

In the aerospace sector, neutron imaging has become vitally important. Various critical components used in aircraft and spacecraft such as turbine blades, components used in stage separation mechanisms, cockpit ejection device and explosive stimuli transfer mechanisms must be carefully tested to meet the optimal design standards as any flaw or defect in the samples can lead to mission failure. Neutron radiography (NR) has become a mandatory inspection technique for aerospace applications in recent years. It has become an indispensable technique in the field of quality control and inspection of explosive-based mechanical devices used in aerospace industry.

Pyrotechnic devices or energetic devices (explosives) are used to power the ejection mechanism in fighter plane. It uses a series of small explosive charges that explode in tandem to actuate the pilot ejection mechanism. Similarly, in space launch vehicles used to inject satellites into orbit, pyrotechnic devices (or pyro devices, in short) containing explosive charges are used for various mechanisms such as solid rocket motor ignition, stage separation, bolt cutters and payload faring separation system.

Another area of application of neutron imaging in aerospace sector is identifying flaws in turbine blades. One of the largest industrial users of neutron imaging is the turbine blade manufacturing industry, especially in the USA, where most of the turbine blades used in jet engines are manufactured.

A vital aspect of these applications is an easy access to neutron imaging. Though reactor-based imaging facilities still remain the first choice, many a times the samples need to be tested on-site at the manufacturing plant. Hence, some of these applications use non-reactor-based neutron imaging facilities as well based on D-T neutron source or electron Linac-based photoneutron sources.

8.2 Turbine Blades

Turbine blades are cast in ceramic moulds out of strong and lightweight metal, such as Ti–Al alloys, that has a melting point lower than the operating temperature of the engine. The turbine blades need to be properly cooled as they are subjected to lot of heat and stress. Flaw in them might result in engine failure and potentially lead to plane crash and loss of life. Hence, turbine blades have to undergo rigorous quality checks due to their high cost of failure.

To cool the turbine blades, airflow is provided through channels running through the internal structure of the blades. This prevents the blades from melting or failing during operation. Due to their design and manufacturing process, it is possible that some of the remnants of ceramic moulds find their way into the air-cooling channels and get trapped. These remnants impede and/or redirect the air flow through the channels, thereby creating hot spots and inducing stress in the channels leading to mechanical failure. Such defects need to be identified during the manufacturing process. A rigorous inspection and regulation mechanism is required to ensure that turbine blades suffering from these flaws are not installed in jet engines.

Neutron imaging is an effective and reliable NDT method for detecting low Z elements which in this case are ceramic remnants in turbine blades. It is superior to X-ray imaging due to its high contrast in neutron attenuations between ceramic remnants and the blade.

Another advantage of neutron imaging is in the use of gadolinium as a contrast agent. Gadolinium has a high thermal neutron cross section and can be efficiently utilized in improving contrast in neutron imaging. If a turbine blade is immersed or washed in liquid gadolinium solution, the gadolinium settles inside cracks and pores in it. In particular, ceramic remnants are quite porous and soak up the gadolinium solution. This process of immersing a component in gadolinium is known as gadolinium tagging. During neutron imaging, gadolinium atoms absorb neutron due to its high capture cross section and the region containing gadolinium atoms shows up as more opaque in the radiograph, thereby increasing the image contrast. Very small remnants of ceramic, which miss detection in normal neutron imaging, show up clearly with gadolinium tagging. Figure 8.1 shows the neutron radiograph of turbine blades before and after gadolinium tagging. It can be seen that the neutron radiograph of the turbine blade shows the presence of tiny ceramic blockages in the air channels when tagged with gadolinium.

Fig. 8.1

Neutron radiograph of turbine blades inspected with neutrons (left) and after gadolinium tagging (right) [1]

8.3 Pyrotechnic Devices or Energetic Devices

Pyro devices generally use solid, hydrogen-rich chemical explosive encased in denser metal shells or enclosures (Fig. 8.2). Pyro devices are mission critical mechanical devices actuated with the help of explosives. These devices perform a mechanical action such as cable cutting and bolt cutting which will aid in various flight operations such as stage separation, satellite deployment and heat shield separation with the help of explosive loaded cartridges. When initiated by a suitable current in the circuit, the explosive train gets activated which results in the release of large volume of gases in a short time or produces a sudden shock that makes a piston and anvil mechanism to perform the desired mechanical action. The reliability of these devices is very high, and mostly, they are positioned with sufficient redundancy so that if one pyro device fails, the other circuit will complete the activity. Pyro devices play a major role in liquid engines as well as cryo engines, wherein the flow of fuel and oxidizer has to be regulated as and when required. Normally closed or opened pyro valves are used in such scenarios to ensure the flow gets controlled as per programme.

Fig. 8.2

Typical pyro devices used in space launch vehicles

Flaws or defects in pyro devices, such as voids, cracks, gaps and inclusions, can denote breakdowns in the chemical composition that can lead to misfires. X-ray imaging does not work well on these devices because X-rays will not be attenuated by low Z explosive material inside the metallic enclosure and will be unable to create sufficient contrast towards flaw detection. However, the high attenuation of neutrons by hydrogen in the explosives makes neutron radiography a suitable candidate for imaging such devices.

8.3.1 Quality Analysis

Pyro devices are one-shot devices. This means that the pyro device to be used in launch vehicle or satellite cannot be tested on ground before use. Hence, pyro devices are produced in batches in sufficient numbers and few of them are identified for ground tests to ensure their performance. Few numbers from each batch will be stored so that by any chance the pyro device used in flight has reached its permitted lifetime, and the one from the same batch kept on ground will be tested and provide life extension. Because of this critical nature, quality of these products is to be ensured with utmost care. Various quality checks are carried out, such as visual inspection and recording of the process sequence, loading of explosives, etc., by quality control (QC) personnel to ensure that the pyro device is fabricated without any flaws. All these pyro devices undergo X-ray radiography to confirm the general assembly of the internal parts as well as to check for any material defects. The devices in their cartridge level (the item loaded with explosive chain and initiators) undergo neutron radiography to ensure the presence of explosive charge as well as its filling uniformity.

8.3.2 X-ray Versus Neutron Radiography of Pyro Devices

X-ray radiography is carried out in pyro elements to confirm general assembly, presence of voids in glass seal, cracks in ceramic cup, bend in terminal pins, damages to shear pins, etc. The pyro charges that are filled in pyro devices are mostly RDX, PETN, HMX, HNX, B/KNO₃, lead azide, zirconium potassium nitrate, etc. Some explosives such as lead azide and zirconium/potassium nitrate are detectable using X-ray radiography which confirms its presence during structural inspection itself. However, to detect the presence of explosives such as RDX, PETN, HMX, HNX and B/KNO₃, it is essential to carry out neutron radiography. Since most of these explosives are hydrogenous in nature or contains boron in it which are highly neutron attenuating in nature, NR will be able to detect their presence quite easily.

Figure 8.3 shows X-ray and neutron radiographs of two pyro devices where the explosive charge is located at the bottom. The explosive part at the bottom (white region) is clearly observed in neutron radiograph whereas the metallic parts are highlighted in the X-ray radiograph.

Fig. 8.3

(Left) X-ray and (Right) neutron radiograph of two pyro devices

8.4 Neutron Imaging for Indian Space Programme

Neutron imaging has been extensively used by Indian Space Research Organisation (ISRO) for imaging and quality assurance of explosive laden pyrotechnic devices used in Indian space programme. In the subsequent sections, we shall discuss both reactor and non-reactor facilities used by ISRO as examples of neutron radiography for space applications.

NR of pyro devices for launch vehicle application started during the initial days at Apsara reactor at BARC. The first NR facility in Indian Space Research Organisation (ISRO) was developed at Satish Dhawan Space Centre, SHAR (Sriharikota Range), Sriharikota [2], using 15 MeV electron Linac. Subsequently, neutron imaging activity was shifted to reactor-based neutron imaging at KAMINI (KAlpakkam MINI) reactor facility of IGCAR Kalpakkam, India. In recent years, due to increased production of pyros to meet the tight launch schedules as well as difficulty in transportation of explosive loaded pyro devices to IGCAR each time, there was a strong requirement to have an in-house neutron radiography facility. This led to setting up a neutron radiography facility using a deuterium–tritium-based neutron generator with the technical support of BARC, Mumbai.

In short, three different modes of neutron sources—namely reactor neutron source, electron Linac-based photoneutron source and D-T neutron source—have been used for neutron imaging of pyro devices. The subsequent discussion will highlight the features of each of these facilities which will serve as representative examples of different neutron imaging sources which can be used for space application.

8.4.1 Electron Linac-Based Neutron Imaging

The neutron imaging facility at SHAR consists of a 15 MeV electron Linac with tungsten target which produces an unflattened X-ray output of 6000 rads. There are four sub-systems, namely (i) target assembly, (ii) moderator, (iii) collimator and (iv) shielding assembly. The target assembly consists of a number of beryllium (Be), uranium (U) and polyethylene discs. Photoneutrons are produced in Be target via (γ-n) reaction. The moderator is a polyethylene cylinder surrounding the target assembly to thermalize the neutrons produced in the target assembly. The thermal neutron beam is extracted from the central isotropic high flux region through a cadmium-lined aluminium divergent collimator. The schematic of the system is shown in Fig. 8.4 [2], and main characteristics of NR facility are given in Table 8.1.

Fig. 8.4

Neutron radiography facility using 15 MeV Linac system at SDSC, SHAR. Reproduced from [2] with permission from Elsevier

Table 8.1 Major characteristics of the NR facility at SHAR [2]

Neutron radiography is carried out using dysprosium foils of optimum thickness (150 µm) with irradiation time of 60 min and transfer time of 12 h. Figure 8.5 shows some sample NR images taken in the facility.

Fig. 8.5

Typical NR image of pyro device tested at SDSC, SHAR. Reproduced from [2] with permission from Elsevier

8.4.2 Reactor-Based Neutron Imaging

Reactor-based neutron radiography for space application was carried out at KAMINI reactor facility of IGCAR Kalpakkam. KAMINI reactor is a ²³³U fuelled, demineralized light water moderated and cooled, beryllium oxide reflected, low-power (30 kW) nuclear research reactor with a thermal neutron flux of 10¹² n/cm² s at core centre. It has facilities for carrying out neutron radiography, neutron activation analysis and neutron shielding experiments. The L/D of the collimator is about 160, and the aperture size is 220 mm × 70 mm. Thermal neutron flux at the outer end of the beam tube is ~10⁶—10⁷ n/cm² s. The availability of high neutron flux coupled with good collimated beam provides high-quality radiographs with short exposure time [3]. Figure 8.6 shows the radiography beam port at KAMINI reactor and the assembled pyro device kept for NR.

Fig. 8.6

a Radiography beam port at KAMINI reactor and b assembled pyro device for NR

Neutron radiography of pyro devices is carried out by transporting the pyro devices from VSSC to IGCAR Kalpakkam after completion of X-ray radiography. The pyro devices are loaded in special fixtures with provisions to position the dysprosium foils behind the object. Figure 8.7 shows the mechanism for assembly of multiple pyro devices on a fixture for imaging. The fixture is designed in such a way as to hold the pyro devices strongly without chances of falling during handling of the same. A stepper motor-controlled carriage drive and indexing mechanism (Fig. 8.8) is provided for neutron radiography. It has provisions for moving the carriage up/down and for lifting the indexing mechanism. Limit switches provided in the carriage drive mechanism indicate the position of the carriage inside the guide tube. The cassette drive mechanism has provisions for arranging ten cassettes and indexing is done remotely from the control room. Indexing is provided to bring the pyro devices to be tested in front of the neutron beam each time.

Fig. 8.7

Assembly of pyro devices for NR

Fig. 8.8

Reproduced from [4] with permission from AIP Publishing

Cassette drive mechanism used in NR facility at IGCAR.

Neutron radiography is carried out on film by transfer technique (Fig. 8.9). The dysprosium sheet is positioned behind the sample in such a way that the foils could be taken out easily without any damage and could be transferred to the dark room in suitable containers where it will be placed in between radiographic films tightly and allowed to expose the films for more than 6 h. The films are later processed in the dark room and interpreted.

Fig. 8.9

Transfer method for neutron radiography

Few of the images of inert pyro cartridges tested at KAMINI reactor are shown in Figs. 8.10, 8.11 and 8.12. The encircled regions highlight the presence of explosives (white region).

Fig. 8.10

NR images (negative image) of pyro devices at IGCAR Kalpakkam

Fig. 8.11

NR images (negative image) of cartridge at IGCAR Kalpakkam

Fig. 8.12

(Top) Photograph and (Bottom) NR image (negative image) of cable cutter with cartridge assembly at IGCAR Kalpakkam

8.4.3 D-T Neutron Source-Based Neutron Imaging

A D-T neutron source-based thermal neutron imaging system was designed and developed by Bhabha Atomic Research Centre (BARC), and the system was installed at NR Facility, Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, India [3, 5, 6].

A D-T generator is used as neutron source. It emits 14.1 MeV neutrons by means of following nuclear fusion reaction

$${}_{1}{}^{3}H+{}_{1}{}^{2}H\underset{ }{\to }{}_{2}{}^{4}He+{}_{0}{}^{1}n \left(14.1 \mathrm{MeV}\right)$$

The fast neutrons produced by the source are moderated and collimated using a moderator–collimator assembly and an output of 1 × 10⁴ n cm⁻² s⁻¹ thermal neutron flux is obtained at the sample plane which is used for neutron radiography. Since the thermal neutron flux output is low, dysprosium foil-based transfer technique cannot be used towards optimum use of the neutron generator. Instead, a 16-bit cooled intensified CCD camera-based neutron imaging system is used for capturing the NR image. It uses a neutron scintillator screen [⁶LiF: ZnS(Ag) (0.4 mm thick)]. This camera is well suited for capturing the image within few minutes. Figure 8.13 shows the neutron radiography assembly.

Fig. 8.13

Photograph of neutron radiography assembly at NR facility, VSSC

The NR beam output is fixed, and hence, a sample manipulator was designed in such a way that the object will be in line with the beam path. A steel base plate moves over an aluminium support structure using rack and pinion mechanism. The plate has provisions to hold multiple fixtures separated by a fixed distance. The support structure is wide enough to provide stability to the plate during motion. Vibration isolator has been provided to the support structure to prevent vibration of the fixtures while in motion. The L-shaped plate to hold fixture containing pyro devices is made of aluminium plates of thickness 30 mm. The fixture made of aluminium plates of thickness 3 mm is attached to the top of this L-shaped plate and is designed in such a way that the object is just 3 mm from the imaging plane. The camera base plate is designed to move up to 0.5 mm distance from the fixture. This design has been incorporated to reduce the geometric unsharpness.

The pyro devices are assembled in a cassette which has removable plates containing holes to assemble the devices. 12 such cassettes are fixed to the assembly frames on the manipulator. When the fixture is in motion, it is ensured that no vibration occurs that might cause fall of pyro devices. Nearly 100 components can be mounted over 12 fixture stations and imaged in a single run without regular user intervention to mount samples. Figure 8.14 shows the fixture which is designed to easily assemble the plates containing pyro devices.

Fig. 8.14

Fixture to hold pyro devices

The intensified CCD camera is an integral part of the system and is incorporated in the manipulator assembly. This requires a similar base plate movement on guided rails mounted on an aluminium support structure holding the camera. Since L-shaped plate was used to hold fixture, the base plate of camera was designed so as to include the distance that the camera has to move in the forward direction. The system was adjusted in such a way that a gap of just 0.5 mm only will exist in between imaging plane and the object holding fixture. Once the manipulator system is in motion, the camera shall be retracted remotely to prevent camera and the pyro devices from touching each other accidently.

The manipulator is controlled using servo motors which are provided with programmable human–machine interface (HMI) stations in control room and NR hall. They provide manual and automatic movements to the manipulator system and the camera. The HMI station inside the hall helps in checking the fine gap while system moves, while the HMI station outside helps in controlling the movement of fixtures remotely from the control room. Thus, it is possible to move each station to come in line with the neutron beam and ICCD camera one by one after each exposure time period is over. Each time, image shall be captured, interpreted and archived. Provisions to finely adjust the position of fixtures and to bring back already radiographed station again for radiography purpose so as to clear any doubts in interpretation shall also be done manually from the control room.

Image Quality Requirements

ASTM E 545 describes the image quality requirements in neutron radiography; however, it is for direct NR technique by using gadolinium (Gd) screen. The standard uses a beam purity indicator (BPI) and a sensitivity indicator (SI) as reference standard to classify a neutron radiograph category. As per the standard, the radiograph shall be of category I or II. However, achieving such high-quality radiographs with accelerator-based neutron source is very difficult.

However, para 5.2 of ASTM E 545 states, “The only truly valid sensitivity indicator is a reference standard part”. A reference standard part is a material or component that is similar to the object being neutron radiographed except with a known standard discontinuity, inclusion, omission or flaw. The sensitivity indicators were designed to substitute for the reference standard such that achieving/detecting a standard discontinuity is adequate to qualify a neutron radiograph. The sensitivity indicators were designed to substitute for the reference standard and provide qualitative information on hole and gap sensitivity which can be interpreted as if one is able to generate a reference standard with intended defects and the defects thus generated are captured by the NR technique. Then, it is not essential to achieve category I or category II.

The qualification of image is based on detecting the intentionally created defect in the reference standard part. For each item that is to be radiographed at NR facility, a reference standard part is prepared and it is radiographed along with normal part which is identical. All the geometrical, neutron tube and image acquisition parameters were varied to get the optimum image which can clearly discern the intentionally created defect in reference standard part. Some of the images taken at NR facility are shown in Fig. 8.15.

Fig. 8.15

NR images (negative image) of a cryo cartridge, b bolt cutter, c explosive transfer assembly tip, d explosive manifold leads

8.5 Comparison of the NR Facilities Used in Indian Space Applications

A comparison of the three neutron radiography facilities used for aerospace applications is shown below.

S. No.	Parameter	IGCAR facility	SHAR facility	VSSC facility
1	Neutron source	Research reactor (KAMINI)	Electron Linac -based photoneutron (15 MeV Linac-Be9)	D-T neutron generator
2	Source neutron flux available	1012 n/cm2 s	1011 n/cm2 s	1010 n/s (14.1 MeV neutron yield)
3	Thermal neutron flux at imaging plane	106–107 n/cm2 s	105–106 n/cm2 s	1 × 104 n/cm2 s
4	L/D ratio	160	18	35
5	Imaging area (sq. mm.)	220 × 70	225 × 225	100 × 100
6	Image capturing mode	Indirect method using dysprosium sheet and X-ray film	Indirect method using dysprosium sheet and X-ray film	Direct method using cooled intensified CCD camera (digital image)
7	Exposure time/frame using the detector mentioned in Sl. no. 6	10–15 min	30–90 min	5 min

8.6 Summary

Neutron imaging in aerospace industry has become a major non-destructive technique for testing of aerospace components such as turbine blades and pyro devices. This chapter emphasized the application of neutron imaging in testing of pyro devices used in space launch vehicles. As an example, we have discussed neutron imaging in the Indian space programme where three different facilities have been used over the years. These include reactor-based neutron imaging, electron Linac-based photoneutron imaging and D-T generator-based thermal neutron imaging. This shows the potential of both reactor- and non-reactor-based imaging facilities for such applications.