B A RC/2004/E/017 5
THE DESIGN AND FABRICATION OF AN OPTICAL PERISCOPE FOR CORE VIEWING OF FAST BREEDER TEST REACTOR (FBTR) by N C Das, San|iva Kumar. O.V Udupa and R P Shukla Spectroscopy Division and AM Kadu and R.K.Modi Division of Remote Handling and Robotics IN0501613
ÏTTTïï VWT Government of India
«mur mmuj 3TJWIR Bhabha Atomic Research Centre ggf Mumbai - 400 085, wrw India 2004 BAR C/2004/E/017
GOVERNMENT OF INDIA ATOMIC ENERGY COMMISSION
THE DESIGN AND FABRICATION OF AN OPTICAL PERISCOPE FOR CORE VIEWING OF FAST BREEDER TEST REACTOR (FBTR) by N.C. Das, Sanjiva Kumar, D.V. Udupaand R.P. Shukla Spectroscopy Division and A.M. Kadu and R. K Modi Division of Remote Handling and Robotics
BHABHA ATOMIC RESEARCH CENTRE MUMBAI, INDIA 2004 BARC72OO4/E/0I7
BIBLIOGRAPHIC DESCRIPTION SHEET FOR TECHNICAL REPORT (•9 per IS : 9400 - 19H0)
01 Security classification : Unclassified
02 Distribution : External
03 Report status : New
04 Series : BARC External
05 Report type : Technical Report
06 Report No. : BARC/2004/E/017
07 Part No. or Volume No. :
08 Contract No. :
10 Title and subtitle : The design and fabrication of an optical periscope for core viewing of fast breeder test reactor (FBTR)
I! Collation : 37 p., 13 figs.
13 Project No.
20 Personal author(s) : 1 ) N.C. Das; Sanjiva Kumar; D.V. Udupa; R.P Shukla 2) A.M. Kadu; R.K Modi
21 Affiliation of author(s) : 1 ) Spectroscopy Division, Bhabha Atomic Research Centre, Mumbai 2) Division of Remote Handling and Robotics, Bhabha Atomic Research Centre, Mumbai
22 Corporate author(s) : Bhabha Atomic Research Centre, Mumbai-400 085
23 Originating unit : Spectroscopy Division, BARC, Mumbai
24 Sponsor(s) Name : Department of Atomic Energy
Type Government
Contd... BAR(72004/H/O17
10 Date of submission July 2004
31 Publication/Issue date August 2004
40 publisher/Distributor : Head, Scientific Information Resource Division, Bhabha Atomic Research Centre, Mumbai
42 Form of distribution Hard copy
50 Language of text : English
51 Language of summary : English, Hindi
52 No. of references : 10 refs.
53 Gives data on :
60 Abstract A FBTR (Fast Breeder Test Reactor) periscope has been designed and fabricated indigenously for viewing and photography/video recording the objects in the reactor core The jn-t iscope consists of a scanning prism mechanism, zoom lens objective, a system of relay lenses and ;m eyepiece sub-assembly for viewing the objects. The objective of the periscope is a zoom lens system lor obtaining a continuously varying magnification from 2X to 5X. Zoom lens objective system has a variable focal length from 100 mm to 250 mm with an aperture varying from 10 mm to 25 mm respectively. This covers a semi-field angle of 3° for the objective tens of focal length of 250 mm and 4° for the objective ni focal length of 100 mm. Two prisms of 45° -90° -45° types are used for scanning the object space in vertical direction. One prism is fixed, whereas the prism facing the object can be rotated about the horizontal axis through an angle of 110°. The rotation of the entire periscope assembly along the vertical axis scans the object space on the horizontal plane. The combination of these two rotations is used to scan the field of interest. It may be noted here that it is absolutely essential to introduce a Pcchan prism before each eyepiece. Pechan prism is used for the rotation of the image, which is produced due to the rotation of the scanning prisms. The measured value of the linear resolution of the instrument is 0.7 mm at an object distance of 2.5 meter from the zoom lens objective system. The periscope has two arm labeled 1 and II. The arm 1 is used for visual inspection, while the arm 11 is used for video recording/photography. The periscope will be used as an in-service instrument for Fast Breeder Test Reactor, IGCAR, Kalpakkam.
70. Keywords/Descriptors PERISCOPES; KALPAKKAM LMFBR REACTOR; SPECIFICATIONS; REACTOR CORES; INSPECTION; SHIELDING; IMAGE SCANNERS; IN-SERVICE INSPECTION
71 INIS Subject Category : S21
99 Supplementary elements : ère fti^W (FBTR) ^^^BF^^
UKKH
W fo^ZT (FBTR) % ^ ^ptf ^ ^ 3ÄT tg W MIWÏÏ ^T «IWW 3ÄT foW ftF^ÎT W ft I W TOTTöt ç^T fepr sfa ^Rpn ^ ^ ^ ^ ^ ^-^eepr £ ft^r i ' ^r i^TTcfi^ tforft^rfr # ^feïï^ft 100 mm 3 250 mmc^ftpRî? fc ^ftl^uj -3R u^i iu tft 10 mm ^ 25 mm «l«ci<1l £ 1250 mm 3ÄT 100 % MÎV^ %feïïT aré-tf^^mSWT: 3°3ft7 4°fr l ^aiïtîT feïï ^ 45°-90°-45° îRFiïT % firt ^r îrôïï feiï no0 «ït ^«bdi 11M^ mwhfer^^kïïfr MIO^T%^Hwh-i^«fafervffnrrlt' i^rr^r £ 2.5 ^ f^RT ^ % fi^ W 4 ABSTRACT A FBTR (Fast Breeder Test Reactor) periscope has been designed and fabricated indigenously for viewing and photography/ video recording the objects in the reactor core. The periscope consists of a scanning prism mechanism, zoom lens objective, a system of relay lenses and an eyepiece sub-assembly for viewing the objects. The objective of the periscope is a zoom lens system for obtaining a continuously varying magnification from 2X to 5X. Zoom lens objective system has a variable focal length from 100 mm to 250 mm with an aperture varying from 10 mm to 25 mm respectively. This covers a semi-field angle of 3° for the objective lens of focal length of 250 mm and 4° for the objective of focal length of 100 mm. Two prisms of 45°-90o-45° types are used for scanning the object space in vertical direction. One prism is fixed, whereas the prism facing the object can be rotated about the horizontal axis through an angle of 110°. The rotation of the entire periscope assembly along the vertical axis scans the object space on the horizontal plane. The combination of these two rotations is used to scan the field of interest. It may be noted here that it is absolutely essential to introduce a Pechan prism before each eyepiece. Pechan prism is used for the rotation of the image, which is produced due to the rotation of the scanning prisms. The measured value of the linear resolution of the instrument is 0.7 mm at an object distance of 2.5 meter from the zoom lens objective system. The periscope has two arm labeled I and II. The arm I is used for visual inspection, while the arm II is used for video recording/photography. The periscope will be used as an in-service instrument for Fast Breeder Test Reactor, IGCAR, KaJpakkam. The Design and Fabrication of an Optical Periscope for Core Viewing of Fast Breeder Test Reactor (FBTR) 1. Introduction Periscopes are quite well known optical device for military f I ] and nuclear reactor applications. It is just a telescope having a bent optical path and a limited field of view [2]. These two things make the periscope a complicated optical instrument from the design point of view. Moreover, the final image formed by the periscope must be erect and non-reversed. The periscope may be defined as an instrument in which the general direction of the rays is not in a straight line but is deflected one or more times with the purpose of providing the observer a view from a position in which he cannot put his head. Thus the instrument has a decided offset between the line of sight and viewing (eyepiece) axis. By reason of this offset, observation can be done from confined locations, e.g., from the interiors of armored vehicles, submarine or aircraft. The simplest form of the periscope would be a one or more mirror. The requirements of brightness, image orientation, field of view, and maneuverability have led the periscope to consist of several image-forming optical components. A periscope is supposed to contain a number of lenses in the straight line but they are equipped with a movable prism or mirror mounted in front of the objective lens. By rotating the entire periscope on its optical axis and also moving the prism or mirror, the operator can survey a large volume of space. The periscope under the current project is an in-service inspection instrument and is to be used for visual examination of FBTR (Fast Breeder Test Reactor) main vessel internals in the cover gas space. Inspection of the main vessel is carried out in shutdown condition, in the presence of cover gas, Argon. The periscope has three main parts as objective sub- 1 assembly, main tube sub-assembly and eyepiece module; Main tube sub-assembly contains an image canal to transfer the image of the target area under inspection, to the eyepiece for viewing by the operator. The periscope is provided with two arms, axial and rotational movement system to cover the area of interest for visual examination. Z Design Parameters for the Optical Layout of the Periscope For designing a periscope for viewing in the irradiated or unreachable areas, following requirements [3] must be satisfied: (1) The periscope must move the eye to the other side of a radiation shield. (2) It must enable the eye to scan the area at the other side of the shield. (3) It must, by reason of its construction, its shape, or the aperture through which it is introduced, protect the observer from radiation. (4) The optical parts of the instrument must be protected from coloration by radiation or must be of materials not so colorable. (5) Final image formed by the periscope should be erect and non-reversed. Problems for constructing such kind of instruments come from both kinds of requirements involving optical principles and mechanical construction. Several other optical parameters, which should also be considered while designing the periscope, are as follows: 1. Optical Aberrations and Brightness of Images Perfect imagery is not possible due the diffraction limited image formation. So aberrations of the optical components have to be minimized for getting a high quality image. Moreover, amount of light reaching the eye of the observer should be quite 2 sufficient for the required resolution. Additionally, the size of the entrance pupil should also be larger than the size of exit pupil. 2. Field of View, Resolution and Magnification The magnification of a periscope varies with distance of the object. So the magnification should be defined for a particular object distance. It is also not possible to have both high magnification and large field of view. Field of view is limited by the size of entrance aperture, field lens after the objective and the magnifying power of the eyepiece. High magnification and best definition of the image are needed only when measurement is required. It is also important to state here that increased magnification does not always mean higher resolution. The actual ability of an optical instrument to resolve two close objects depends not only on the perfection of design and construction but also on the laws of physical optics. The limit of resolution is set by residual aberrations, imperfections in lens manufacture, imperfection in lens adjustment, and departures from tolerable focal ranges. Considering the above points for the design of optical layout of the periscope, following design strategy has been adopted: In the initial design, the clear apertures (sizes) of the optical components and their positions in the layout were determined by the method of ray tracing. An axial ray is traced using paraxial ray trace equations [4] to determine the lens positions for the required final image position. A principal paraxial ray is traced for determining the clear apertures of the optical components. Several combinations of relay lens focal lengths were considered. From practical considerations, such as the lens diameters are less than 45 mm and the feasibility of lens 3 alignment in the assembly, the relay lens focal tenglhs were chosen to be 500 mm for the periscope The numbers of optical elements were kept to a minimum possible, keeping in mind that the relay lenses can be changed only in a group of four for retaining the erectness of the relayed image. The minimum number of components will reduce loss of light (due to absorption, surface reflections etc.) and also reduce the overall work in optical alignment. To keep the image non-reversed, even numbers of reflections have been taken. After getting the optical layout, every component of the system is designed individually to compensate the optical aberrations. 3. Optical Layout of the Periscope Fig. 1 shows a schematic optical layout of the FBTR periscope. The periscope has two image observation / recording arms, labeled I and 11. The purpose of the periscope is to facilitate the viewing and video recording of the objects and the main vessel internals. The arm I is used for visual inspection, while the arm II is used for video recording/ still- photography. FBTR periscope is having a length of about 5.4 m. Distance mentioned is the distances of the folding mirror M2 to the bottom of the periscope i.e. the viewing plane of the objective sub-assembly. The periscope will consist of the following parts in the optical layout: 3.1. Scanning Prism Mechanism o Two prisms (?u Pi) of 45°-90°-45 [5] types are used for scanning the object space in vertical direction. The prism P2 is fixed, whereas the prism Pi can be rotated on the horizontal axis as shown in Fig. 2. The axis of rotation is kept parallel to the refracting edge. The rotation of prism Pi scans the object space in vertical plane. Such kind of anangement for the scanning system alone will give an erect image at all times. The 4 rotation of the entire periscope assembly along the vertical axis scans the object space in the horizontal plane. The combination of these two rotations is used to scan the entire inspection field. The turning over of the image also takes place due to the rotation of scanning prisms. The image rotation can be compensated by using a special type of prism such as Pechan (Schmidt) or Dove prism elsewhere in the optical path of the system. 3.2. Zoom Lens Objective The objective lens L] of the periscope is a zoom lens system [6] for obtaining a continuously varying magnification from 2X to 5X. Zoom lens objective system has a variable focal length from 100 mm to 250 mm with an aperture varying from 10 mm to 25 mm respectively. This covers a semi- field angle of 3° for 250 mm focal length and 4 for 100 mm focal length of the objective system. The zoom lens objective has been designed for viewing the objects kept at a distance in the range of I 5 m to 3m from the objective lens. The zoom lens system is an "optically compensated" type and consists of three doublet lenses. In an optical compensated zoom lens system, the image shift due the movement of one lens element is optically compensated by moving another lens element by the same amount. The first lens L^A) and the third lens L((C) are movable and they are mechanically coupled so that they can be moved together by an equal amount. The central lens Lj(B) is a fixed lens. Moving the coupled lenses along the optical axis changes the magnification. It is found that the zooming system can be used for resolving [7] objects with a linear resolution of 0.2 mm when viewed with an eyepiece of focal length 50 mm. The schematic diagram of the zoom lens assembly is shown in Fig. 3. 5 3.3 Relay Lens System The relay lens system is for the purpose of relaying the primary image formed by the lens L, to the object plane of the EP for viewing/recording. The EP is at the distance of about 5.4 m from the scanning prism. The relay lens system consists of the following parts: (a) Field Lens L2 and U Field lenses are used for preventing the peripheral rays from the image from escaping the system. The field lens L2 is placed on the image plane of Li. The field lens L2 makes the rays felling on it at the edges to fell on the lens L3 thus increasing the field to the maximum possible extent, without contributing to the focal power. The field lens L* is kept at the back focal plane of the relay lens L», maximizing the field seen by the eyepiece. (b) Relay Lenses L3 to L* An image-re laying lens is made using a set of two lenses, having the same focal length. The image fells on the focal plane of the first lens, which col limâtes the beam. I The second lens focuses the beam on its focal plane. The image is thus relayed from the focal plane of the first lens to the focal plane of the second lens with a magnification factor m = - 1. The relayed image is thus unmagnified but inverted with respect to the original image. The distance between the two lenses is decided by the image size, since off-axis rays must not escape the relay system. For a practical lens diameter of 45 mm, the maximum distance between the two lenses of 1 m is found appropriate in our case. Referring Fig. 1 the original image due to zoom lens is inverted. The relay set L3-L4 erects the image while relaying it to a distance of 2.3 meter. The next set LrU relays further to a distance of 4.3 meters while inverting the image. The third set Lt-L* relays 6 the image to a distance of 5.4 meters while again erecting it. All the lenses L3 to Lg are achromatic air-separated doublets. 3.4. Shield Plug Sub-Assembly Two prisms P3 and P4 are introduced into the collimated light path between the relay lenses L5 and L6- The purpose of these prisms is to prevent the radiation from the object space to the observer. Fig. 4 shows a schematic diagram of the shield plug sub- assembly consisting of two offset prisms. Light is deflected by the prism P3 while high energy radiation is not reflected by the prism P». Thus high-energy radiation is blocked by the lead filled plugs inserted in this assembly as per the diagram. 3.5. Plane Mirrors After the focusing lens L8, two highly reflecting plane mirrors M» and M > (size 50 mm X 50 tnm X 3 mm) are mounted at an angle of 45° with respect lo the optical axis The mirrors have been mounted at two different locations separated by a distance of 100 mm. The function of each mirror is to deflect the light path by 90 deg. towards the eyepiece sub-assembly. The image formed by the instrument is focused in the plane of a field lend L9 after reflecting from a plane mirror (say Mi). The mirrors have been mounted such that they can be removed/ inserted in the optical path of the system as per the requirement of the operator. It may be pointed out here that the alignment of the minor with the eyepiece assembly is not disturbed during the movement process. 3.6. Correcting Pechan Prism P6 While scanning the inspection field, as the prism Pi is rotated, the image also turns over, and therefore objects other than those directly in front of the periscope will appear to be lying on their sides or at an unusual angle. In many cases it can not be 7 tolerated. Hence a compensating device known as a Pechan prism [8] must be inserted at a convenient place in the optical path. This mechanism is expensive and must be highly precise. Pechan prism (Fig. 5) is a high precision optical device made up of two air-spaced prisms P5(l) and Ps(2). A Pechan prism has the property of introducing lateral inversion without changing the deviation of the beam and can be used in convergent, divergent and parallel light. The image can also be rotated by rotating the Pechan prism with respect to the optical axis, thus providing a rotation correction to the image. The Pechan prism P5 is thus used for correcting the lateral inversion of the image introduced by the mirror Mi or M2 and also used for rotation correction of the image, which is produced due to the rotation of the scanning prisms Pi and P2. Thus, this feature is desirable for reading any marking on fuel assembly. 3.7. Eyepiece The eyepiece used here is of symmetrical type and consists of two identical achromatic doublets having a focal length of 100 mm. The achromatic doublet lenses have been mounted to give a focal length of 50 mm. Such kind of eyepieces [9] has economic advantages that these can be manufactured with a single pair of radius tools. The image formed by the periscope after focusing lens is made to fell on the field lens L9 which lies in the focal plane of the eyepiece. The image formed in the focal plane of the eyepiece is of unity magnification. This image acts as the object for the eyepiece and final image is formed at infinity. The eye is placed after the eyepiece and image can be viewed with relaxation. It may be mentioned here that the eyepiece has monocular vision. g 4. Mechanical Design Considerations for the Construction of the Periscope The mechanical design of FBTR periscope is determined by the optical layout (refer Fig. I) of the periscope. The main requirements are the optical alignment of the optical elements of the periscope, the movement of scanning prism, zoom lens movement, radiation shielding and Pechan prism rotation. The scanning prisms P| and P2 have a very small gap of 5 mm between them and require a rotary motion. There is very little space to put ball bearings, so we have designed a smalt bush bearing with a clear hole of size matching the clear apertures of prisms. The rotation of prisms is achieved by fixing a bronze pinion (gear) to the prism and gearing. The rotation of the gear is achieved by a slider crank mechanism. Zoom lens has three lenses l>i(A), Li(B) and ï-i(C). The lens 1.i(B) is fixed and lenses Li(A), Li(C) have to be moved. This is achieved by three guide rods of SS 304 fixed to the lens mounts of L|(A) and Li(C); and these guide rods move in the holes fabricated in the lens mount of Lj(B). The movement is achieved by a screw and nut I arrangement. The alignment of the relay lenses and prisms is achieved by hav ing a modular design of all optical mounts and fixing with screws and putting dowels. The whole optical assembly is inserted in a honed S. S. tube, which has I. D. very close (a gap of 0.2 mm) to O. D. of optical modules. The drives for scanning prism and zoom lens are achieved by the drive rods connected by universal couplings. The drive rods can be rotated by the knobs provided at the top of the periscope near the focusing lens assembly. Two tie rods are also used to carry the loads (weight) of shielding plugs. 9 The periscope assembly is a leak tight assembly. It is to be filled with Argon gas during operation and maintained at a pressure of 7 psig (pounds per square inch gage). The design of main tube assembly, window assembly and mirror assembly is based on the requirement of leak tightness. We have used silicon material '0' rings between mating components of different assemblies. The assembly has been tested for leak tightness at 7 psig pressure. The Pechan prism and eyepiece module is designed such that the Pechan prism can be rotated using ball bearings and can be aligned with mirror assembly and eyepiece assembly. Fig. 6 shows the mechanical design assembly of FBTR (Fast Breeder Test Reactor) periscope. Freedom from body strain is important in the positioning of the observer. The head should be in a comfortable position relative to the standing or sitting position of the body to prevent eyestrain, ln our instrument, which will scan a large area both vertically and horizontally, the eyepiece has been turned through 90 deg from a vertical to a horizontal position. The assembly and its controls have been so designed that it can be operated by minimum number of trained personnel, preferably one. For the sake of simplicity and directness of operation, manual control has been preferred. Due to radioactivity, it becomes important to consider the materials, which have small capture cross-sections. Generally aluminum alloys, hardened steel and brass have been used for the construction of FBTR periscope. 5. Radiation Shielding (a) Selection oi Radiation Resistant Optical Glass Radiations from radioactive substances color glass by producing absorption bands lying in the visible spectrum region. The character and location of these bands depend on 10 the composition of the glass. Due to gamma and neutron radiations, conventional optical glass elements become darkened and even opaque to light when they are exposed to ionizing radiation. The prisms and all the lenses in the objective and relay system are therefore made from radiation-stabilized glasses, which are resistant to the darkening when exposed to ionizing radiation. Even though the periscope is to be used in shut down condition, when the radiation level is low, we have chosen radiation resistant glass for the optical components since there is large number of lenses. Even a small amount of darkening in each element will have a cumulative effect on overall darkening which can spoil the image visibility, (b) Baffling to Prevent Coloration High-energy radiation is not reflected or refracted in the same manner as the light by ordinary optical means. In consequence, if space permits, light rays from an object may be reflected into an optical instrument by mirrors or right angle prisms that only partially scatter high-energy radiation. This is one type of baffling. On the other hand, the light rays themselves may be made to travel a straight path through the instrument, and the optical parts may be surrounded by materials which are most effective in absorbing the high-energy radiation. This is another type of baffling. 6. Details of Optical Components The glass material used for the lenses is chosen to be radiation-stabilized glasses, which can withstand radiation doses of up to 10* radon without significant darkening. The cementing of the optical elements has been avoided in order to eliminate any chances of darkening of the cementing material in the presence of ionizing radiation. The achromatic doublet lenses were designed using a paraxial ray tracing approach. The glass 11 material used for designing the zoom lens system, field lenses and achromatic doublet relay tenses has been chosen to be radiation resistant crown (BK7G25, 520630) and flint (F2G20,621366) glass [10], 7. Assembly, Alignment and Testing The entire periscope was assembled as per the mechanical design and optical layout. As the periscope consists of several modules to be coupled in series, each part was first tested individually for horizontal alignment of optical components. A halogen lamp (500-Wan) was chosen to illuminate the object space for alignment and testing purpose. A simple method using He-Ne laser has been utilized for aligning the optical components in the periscope tube. The laser beam from He-Ne laser (5 raW) was expanded using a microscope objective (10 X) placed at a distance of 2.5 meter from the first prism Pt of the scanning system. The focused spot due to microscope objective served as a bright point object for the periscope system. The assembly and alignment was done for scanning prism system, zoom lens sub-assembly, relay lens assembly and finally for the eyepiece sub-assembly. i The zoom lens sub-assembly was separately tested for focal plane determination. Fig. 7 shows a schematic optical arrangement for focal plane determination of the zoom lens system. Average distance of the focal plane of zoom system from the central lens element Li(B) was found to be 215 mm with an accuracy of ± 6 mm; the signs '+' and indicating for 2X and 5X respectively. A ground glass plate was used as the image- receiving screen. A Pechan prism has been introduced as per design in the eyepiece assembly for image rotation correction. The optical alignment of the Pechan prism is a very critical 12 process. The alignment for the Pechan prism was done such that the rotation axis is parallel to and coincident with the entering and emerging rays. Any kind of misalignment shows the field of view of the system to move about in a circle. It may be noted down here that the prism was given translation as well as tilt with respect to the rotation axis to minimize the rotation of the field of view in the circular way. The Pechan prism is then locked in this optimum position. The eyepiece sub-assembly was also separately aligned and tested for image rotation. Two numbers of eyepiece sub-assemblies have been assembled and aligned for the purpose of visual inspection and photographic/ video recording. The complete periscope was assembled, aligned and tested in a horizontal position. Fig. 8 shows the photograph of the assembled FBTR periscope. Fig. 9 and Fig. 10 show the photographs of different modules of the assembled periscope. The observed field of view of the instrument is found to be 3° to 4° for a magnifying power range of 5X to 2X respectively. The resolution of the instrument was observed to be 1.6 mm and 0.7 mm for an object placed at a distance of 2.5 meter from the scanning prism objective for a magnifying power of 2X and 5X respectively. The resolution test was performed using several line targets consisting of 50/50 bar-to-space ratio. The illumination level at the object plane was maintained as 1000 lux as per the requirement of the user. A rectangular aluminum strip was bent in a circular manner with a radius of curvature of 2.5 meter for checking the total angle scanned by the periscope. Total angle scanned by the periscope was found to be 110°. The object was also printed with vertical lines for testing the rotational behavior of the Pechan prism. As the object was scanned by moving the prism Pi through an angle of 90°, image of the vertical 13 straight lines is rotated by 90° and becomes horizontal in an anticlockwise direction. Now comes the Pechan prism to play a very important role for correction to this image rotation. Rotation of Pechan prism by 45° in the clockwise direction makes the image of straight vertical lines again vertical. Calibrations for the several assemblies such as zooming sub-assembly (2X-5X), focusing lens L» and scanning prism mechanism has also been done to ease the operator. Each mechanism was controlled by a rotating knob at eyepiece side. These knobs are connected to their respective mechanisms by some guiding drive rods / tie rods. Rotation of these knobs can control the movement of focusing lens Lg, scanning mechanism and zooming process. Each knob has been calibrated in terms of number of its rotations. The controlling knobs have also been arrow marked for seeking the desired directions of rotation to ease the operator. The scanning mechanism, zooming procedure and focusing lens movement have also been limited / locked to avoid the unnecessary movement of these mechanism and to ease the operation of the periscope. Th^ rotation of the Pechan prism sub-assembly was also limited for the useful viewing in the object space. The assembled periscope was finally inserted into a well-machined SS 304 tube (length = 4800 mm, ID = 94.4 mm, OD = 101.6 mm). The periscope assembly was tightened with the outer SS 304 tube at the flange carrying the relay lens L7. A transparent glass (WG9 G9/520617) window was also mounted on the bottom of the outer tube in such a way that it should enable the operator to scan with a range of-10 degree to 100 degree. The glass window assembly was welded at an angle of 45° with respect to the optical axis. The window serves the purpose of sealing the instrument with 14 the object space (reactor core). The tube containing the periscope is also sealed at another end at the location of a flange of L7 module. The periscope is to be operated in an atmosphere of Argon gas. 8. Results and Discussions Various line targets having line spacing of 0.5 mm to 2.0 mm with an increment of 0.1 mm were placed at a distance of 2.5 m from the scanning objective prism. These targets were illuminated using a halogen light source of 500 W. Images were viewed through eyepiece and the photographs (Fig. 11) were taken using a digital camera (make Kodak DC3200). Photographs of several line targets were taken through the FBTR periscope to check the practical resolution of the instrument. The resolution of the instrument was observed to be 1.6 mm and 0.7 mm for an object placed at a distance of 2.5 meter from the scanning prism objective for a magnifying power of 2X and 5X respectively. These values of resolution are applicable only when object surfaces are perpendicular to the line of sight and are not applicable when viewing the surfaces which are inclined to the line of sight. It may also be noted down here that the resolution of the instrument depends on the illumination level in the object space. Hence a proper illuminator is required to illuminate the main vessel internals so that the objects in the reactor core can be viewed using the periscope without compromising for the desired resolution. The illuminator to be used in the reactor core consists of a mercuric halide lamp (400 W), ellipsoidal reflector, condenser lens and a relay lens system. Total angle scanned through the scanning prism was found to be 110°. The scanning system can scan 10 degree below the vertical direction and 10 degree above the 15 horizontal direction One lotation of scanning knob covers an angle of about 11.5 degree. Scanning is done by rotating the scanning knob in anticlockwise direction to capture the view from -10° to 100°. Rotation of the zooming knob in anticlockwise direction indicates the magnification varying from 2X to 5X. Total number of rotations in the zooming process is 40. The total number of rotations for getting the image shifted from Eyepiece 1 to Eyepiece II is 50 in clockwise direction. Fig. 12-Fig. 13 shows the typical photographs of several other target objects such metallic rods, threaded objects, screws and some other coarse objects as viewed through the eyepiece of the periscope. The photographs clearly show the finer details of the objects such as threading, cracks, deformation and scratches etc. 9. Operating Procedure for the Periscope The periscope under the project is an in-service inspection instrument and it is to be used for visual inspection of FBTR (Fast Breeder Test Reactor) main vessel internals in the cover gas space. Inspection of the main vessel is carried out in shutdown condition, in the presence of cover gas. Argon. The periscope is to be inserted in the reactor core through an opening provided in Large Rotating Plug (LRP). This operation is done in a leak-tight manner. During normal reactor operation, a shield plug covers this opening. The illuminator is put on and the eyepiece is adjusted manually to focus the image by observing in the arm I. The required area is scanned by rotating the entire periscope assembly about a vertical axis and the scanning prism P; about the horizontal axis. 16 The object space is first viewed in 2X zooming mode to enable the operator to get a larger field of view. In addition to horizontal viewing, the objective prism is capable of swiveling by 110 degree, about the horizontal axis, to change the direction of vision, in a vertical plane. In order to get the finer details of a small region of interest, the objective can be manually zoomed up to 3X, 4X or 5X as per the satisfaction of the operator. The resulting rotation of the image is adjusted by rotating the Pechan prism Ps manually. Periscope can be set for image recording in Arm II by moving the focusing lens L* by 100 mm towards the mirror M2. The image once adjusted can then be recorded/photographed in the arm II, by removing mirror M| and inserting mirror M2 in the optical path. The lens L« is moved to a new position (as shown in dotted lines in Fig. 1 ) for proper focusing and the image is recorded. 10. Summary of the Parameters of the FBTR Periscope ® The length of the periscope: 5.4 m 0 Resolution achieved for an object placed at a distance of 2.5 m for 5X zooming - 0.7 mm 0 Resolution achieved for an object placed at a distance of 2.5 m for 2X zooming = 1.6 mm ® Total angle scanned in vertical plane = 110n ® Rotation of Pechan prism required for correction to image rotation for the entire scanning range = 55° 17 # Field of view for varying zooming power for an object distance of about 2m ^\Zooming Power 2 X 3 X 4 X 5 X Without scanning 460 mm 375 mm 255 mm 203 mm Prisms With Scanning Prisms 280 mm 270 mm 250 mm 200 mm 11. Conclusion A FBTR periscope has been designed and developed indigenously. The instrument is found to be useful for visual inspection and photography/ video recording of the objects and the main vessel internals in the reactor core. The periscope will be installed at the Fast Breeder Test Reactor, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam. 12. Acknowledgments We would like to thank Shri T. N. Desai and Shri S. J. Shinde, Centre for Design and Manufacture for providing us all kinds of help and support for the assembly of the periscope. 18 References 1. Francis B Patrick, "Military Optical Instruments" in Applied Optics and Optical Engineering, Vol. V Part II, R. Kingslake. ed. Academic Press, New York, 1969, P. 209-214. 2. M Bom and E Wolf, "Principles of Optics", Pergamon Press London, New York (1980), 6th Ed., P.243 - 245 3. George S. Monk and W. H. McCorkle, "Optical Instrumentation", McGraw-Hill Book Company, Inc., New York. 1954, P. 37-41 4. Kingslake R, "Lens Design Fundamentals", Academic Press, London. 1978, P 61-66 5. R. E. Hopkins, "Mirror and Prism Systems" in Applied Optics and Optical Engineering. Vol. Ill, R. Kingslake, ed. Academic Press, New York, 1965. P. 296- 297. 6. N. C. Pas, D. V. IJdupa. and R. P Shukla., "Design and Development of;« Zoom Lens Objective for the Fast Breeder Test Reactor", BARC Report, 17030. 2001 7. N. C Das. D. V. Udupa. and R. P. Shukla., "Design and Development of a Zoom Lens Objective for the Fast Breeder Test Reactor", BARC Report. E/030, 2003. P. 12 8. R. E. Hopkins, "Mirror and Prism Systems" in Applied Optics and Optical Engineering, Vol. Ill, R. Kingslake, ed. Academic Press, New York, 1965. P. 307. 9. Seymour Rosin, "Eyepieces and Magnifiers" in Applied Optics and Optical Engineering, Vol.Ill, R. Kingstake, ed. Academic Press, New York. 1965, P. 344. 10. SCHOTT-MAINZ, JENA1* GLAS: Schott & Gen., Mainz, Verkauf Optisches Glas. Postfach 2480, West Germany. 19 Arm II for Video recording/ Photography Zoom movement 7 P 2 L LiC L1i A 1B L2 l l l l l l 8 8 Pi L3 4 5 6 7 P3 P Pechan Prism Scanning Prisms Focusing Lens L .EP Arm I for Visual Examination Fig. 1. Schematic optical layout of a 5.4 meter long Fast Breeder Test reactor (FBTR) periscope. Lenses L1A, L1B and L1C form the zoom lens objective. Fig. 2. Schematic diagram of scanning prisms showing the rotation of P1 for vertical scanning. 21 Focal length = 250 Image plane 215 ïa| Focal length = 100 1 L1(A) L1(C) L1(B) ft] r\v7 (Fixed) nr\ WU Jl\ 71 Image plane ^ 215 Fig. 3. Schematic optical layout of the designed zoom lens system. The lens L1(A) and L1(C) are coupled together with a separation of 83.5 mm. The middle lens L1(B) is fixed. The top layout shows the position of the lens L1 (A) and L1(C) to obtain a focal length F = 250 mm. The bottom layout shows the position of the lens L1(A) and L1(C) to obtain a focal length F = 100 mm. 22 Offset Prisms S. S. Tube Fig.4. A schematic arrangement for the baffling/ shielding of radiation using two offset prisms and lead plugs. The arrangement also prevents the radiation to reach the observer. 23 Fig. 5.: Schematic diagram of the Pechan prism P5. The Pechan prism is a combination of two prisms P5 (1) and P5 (2). 24 Zoom Lens Objective Glass Screen Eye Fig. 7. Schematic optical diagram showing the experimental arrangement for measuring the image distance from the central zoom element. The target object is kept at a distance of 2 meter from the zoom lens. 26 Arm I Zoom Lens Relay Lens Tube Objective Eyepiece Module Arm II Fig. 8. The photograph of the assembled Fast Breeder Test Reactor (FBTR) periscope. Eyepiece I (a) Field Lens L Zoom Lens Objective Scanning Prism Pi Prism P2 (b) Fig. 9. Photographs of (a) the eyepiece module and (b) the zoom lens objective sub-assembly. Page 1 of 1 file://D:\REPORTS\E017\fig10.jpg 9/30/2005 Line thickness = 0.8 mm Line spacing = 1.6 mm Line thickness = 0.5 mm (a) Line spacing = 1.0 mm Line thickness = 0.6 mm Line spacing = 2.1 mm Line thickness = 0.4 mm Line spacing = 1.0 mm (b) Fig. 11. Photographs of two line targets as viewed through the FBTR periscope. The photographs were taken using a digital camera. 30 (b) Fig. 13. Photographs of (a) an object having a slit and (b) an aluminum rod as viewed through the FBTR periscope. 32