Division VI, Paper ID 175
DESIGN AND DEVELOPMENT OF RABITS (RUPTURE AND
BALLOONING IN TUBES) FACILITY
Rosy Sarkar1, R. Suresh Kumar, S. Jalaldeen, K. Velusamy, P. Selvaraj and P. Chellapandi
Reactor Design Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu – 603102
ABSTRACT
Fast Breeder Reactor (FBR) fuel pins are subjected to very high heat flux, of the order of 2 MW/m2. The fuel clad experiences a temperature ranging from 700 °C to 1200°C and a pressure as high as 60 – 100 bars. Under extreme loading conditions, the fuel clad tends to develop considerable plastic strains in the form of localized bulging, eventually leading to rupture. This phenomenon of bulging is referred to as ‘ballooning’ of the clad. In an FBR fuel pin bundle, the gap provided between the pins for the coolant (liquid sodium) flow is very small, with a hydraulic diameter of ~3 mm. The fuel clad ballooning can lead to flow starvation to the bundle and the consequent damage to fuel pins, causing severe damage to the core. Hence, there is a need to understand structural instabilities in fuel clad. Towards this, a multiple fuel pin test facility, known as “Rupture And Ballooning In TubeS” (RABITS) has been developed to study the Ballooning in fuel clads caused due to various combinations of temperature, pressure and loading rates. The salient features of this facility is simultaneous testing of multiple fuel clads yet having independent control of temperature and pressure in each specimen, online monitoring of the specimens and radiological protective shielded room. Large number of experiments have been planned to identify the zones where the clad would either rupture or undergo ballooning, to obtain the whole deformation properties, multi-axial creep curves and to conduct further research activities using the RABITS facility.
INTRODUCTION
Fuel clad behaviour under extreme loading conditions, within the regime of higher thermal and mechanical loadings, tends to develop considerable plastic strains in the form of localized bulging, eventually leading to rupture. This phenomenon of bulging is referred to as ballooning of the clad.
The gap provided for coolant flow is very less in the design of FBR. The fuel clad ballooning can lead to severe damage to the reactor core due to improper cooling. Hence proper quantification of clad tube ballooning is necessary to ascertain the safety margin available in the clad tube designing under the various extreme operating conditions.
By considering the quantum of the experimental works and the importance of the phenomena, a multiple fuel pin test facility has been designed. It is known as Rupture And Ballooning In TubeS (RABITS). Towards deriving proper guideline for the integrity, many simulated experiments have to be carried out to understand the ballooning phenomenon and the associated high temperature structural behaviour.
• To obtain the whole deformation properties at various time intervals which will provide important data for the development of constitutive models for the material.
• To identify the zones where the clad would either rupture or would undergo ballooning.
• Experiments are to be conducted at various heating rates and the effect of ballooning at various combinations of heating rate, pressure and temperature is to be studied.
• Muti-axial creep curves at high temperatures at various stresses can be obtained and hence the constitutive equations can be derived.
• Repeatability of the tests is necessary to gain confidence in the results.
This report covers the salient features associated in the design and development of the RABITS facility. It also covers the details about the sophisticated instruments deployed in the facility to capture the online deformation of the high temperature specimens. The salient features of this unique facility are presented in this report.
THE RABITS FACILITY
RABITS facility has been developed to study the Rupture and Ballooning in clad Tubes. The photograph of the facility is shown in Figure.1.
Figure.1: Photograph of the RABITS test Facility
The salient features of this integrated high temperature experimental facility are
• Simultaneous testing of multiple fuel clads yet having independent control • Online monitoring of the specimens
• Design of properly shielded X-Ray room
SIMULTANEOUS TESTING OF MULTIPLE FUEL CLADS
In order to conduct more number of experiments simultaneously, the facility is designed such that six specimens can be tested at a time. Towards this, elegant fabricated cabinet enclosure has been constructed. The cabinet front has separate shutters for each section under each furnace. Each section consists of a pressure chamber (provided with pressure gauge) hung from the top of the cabinet. The shutters have perspex windows for observation of the pressure gauge while under test.The specimens are welded to the pressure chamber and is arranged such that it passes through the center of the furnace. Each chamber can be controlled independently to set different temperature and pressure so that many parallel experiments can be carried out using the same facility. The schematic of the details of the RABITS facility are given in Figure.2.
Each furnace is cylindrical and vertical model, split type single zone furnace with maximum operating temperature of 1100 0C and accuracy +/- 1 0C. The inner chamber is formed to withstand a temperature up to 1150 0C. The inner chamber has 75 mm diameter and 150 mm height with 75 mm hot zone length. There is a heating element evenly distributed round the inner chamber. High quality ceramic fiber insulation is provided all round the sides of the inner chamber to have skin temperature not exceeding 50 0C at maximum operating temperature. The six sets of furnaces are mounted on the top of the cabinet. The cabinet has two control panels one each mounted on each side of the cabinet. Each control panel houses three sets of furnace control and indicating instruments which independently controls each furnace.
ONLINE MONITORING OF THE SPECIMENS
High sensitivity X-Ray machine is used for online monitoring of the specimens enclosed inside the furnaces. Also, to enable simultaneous testing of the specimens, a trolley is designed to hold the X-Ray source and travel to the desired location which is controlled remotely from the control room. The details are given below.
X-Ray Machine
A constant potential 225 kV X-Ray machine with metal ceramic X-Ray tube and 40° x 30° beam angle has been procured. The voltage range is 10 kV - 225 kV and current range is 0-15 mA. It consists of 4 variable focal spots 0.25 mm/ 0.3 mm/ 0.5 mm and 0.8 mm and max power outputs of 290 W/ 540 W/ 1020 W and 1600 W respectively. The Variofocus X-Ray system with focal spot technology offers extraordinarily small in-performance variable focal spot. Compared to conventional X-Ray tubes, variofocus focal spot is less than half the size and considerably more symmetrical, providing the power of high-power X-Ray tubes. The drawing of the X-Ray tube is shown in Figure.3.
The control unit has provision to record exposure parameters, facility for recalling the exposures; automatic warm-up and provision for auto trips due to high target loading or when the temperature of the target/oil exceeds prescribed limits.
Benefits in comparison to conventional X-Ray:
• Better detail visibility due to a minimum effective focal spot of 250 microns.
• Reduced inspection time, because the user can use an optimized focal spot with more power. • Circular shape focal spot for the same image quality in and perpendicular to the tube axis.
Unique benefits:
• Magnification up to 10
• Focal spot size can be optimized for the application and provide maximum detail visibility Figure.3: Constant Potential 225 kV X-Ray
• Minimum effective focal spot of 250 Microns and 292 W power • Reduced inspection time due to optimized focal spot size • Circular Focal spot for best image quality
• Closed Design in a standard 225kV housing with standard H.V.cable
Remote controlled Mobile Support for X-Ray Source
X-Ray source has been supported on a mobile platform. This mobile platform can be controlled precisely from the remote control room. Towards ensuring the precise control appropriate drive motor mechanism is attached with the mobile platform. With this, the X-Ray source can be comfortably placed on the mobile platform and it can be used for taking the images of multiple specimens without having any physical intervention. There is a provision for height adjustment of the X-Ray source up to 75 mm. The remote controlled panel connected with this mechanism will have the provision to move the X-Ray source in a forward and reverse direction. The camera and the Laser pointer mounted along with the mobile platform helps in online monitoring of the X-Ray source with reference to the specimen. The conceptual sketch of this mechanism is shown in Figure.4, and the photograph of this arrangement is shown in Figure.1.
RADIOLOGICAL PROTECTIVE SHIELDED ROOM
Dedicated X-Ray unit operating facility is constructed at SML for the RABITS facility. The design of the room is made such that proper shielding is provided.
Shield design calculation for the X-Rays has been done using three dimensional code MCNP4C3 (Rajeev and Sen (2013)). The data used in the calculation has been taken from “Calculated Radioscopy System Performance Parameters” provided by M/s Blue Star Ltd with 100% detector response. Shield calculations have been done for the gamma rays dose rate using normal concrete of mass density 2.4 g/cc. The shield design dose rate criteria has been taken as 1 μSv/h. A safety factor two has been included for shield calculation.
It is recommended from the conclusions that normal concrete (2.4 g/cc) of thickness 65 cm in the beam facing side, 35 cm along the other three sides and 25 cm concrete equivalent shield on the roof of the installation hall is required to satisfy the dose criterion of 1 μSv/h as per the given dimensions of the set up. Shielding adequacy has been verified and the clearance is received for the reconstruction of the lab. The final layout of the room is shown in Figure.5.
The opening for the shielded room is provided using sliding door arrangement. The sliding door concept is selected from the space consideration point of view. The thickness of the sliding door meets the shielding requirements. The dimension of the opening is 1800 mm X 2500 mm. The door is in two pieces of size 1050 x 2500x 110 mm and they are latched together to form 2000x 2500x 110 mm. The latch is mechanical / fastener types, so as to slide open the door separately. The Indian Standard Manufactured Beam (ISBM) is used to guide the wheels for sliding the door. The required mechanical stoppers are provided at the ends. The sliding of the door is manual. The layout of the sliding door is shown in Figure.6.
EXPERIMENTS PLANNED
S.no Pressure
(bars)
Temperature
(0C) Heating Rate Purpose
1 100 950 40 K/min Multi-axial creep curve -1a(repeat)
2 80 950 40 K/min Multi-axial creep curve -2
b
(or Ballooning Curve)
3 60 950 40 K/min Multi-axial creep curve -3
(or Ballooning Curve)
4 60 970 40 K/min Category-1 event c(acceleratedd)
5 60 800 40 K/min Category-2 event
6 60 900 40 K/min Category-3 event
7 60 1200 40 K/min Category-4 evente
8 60 1200 100 K/min Category-4 event
Table 1: Experiments planned using RABITS Facility
a
Multi-axial Creep Curve is obtained at 100 bars pressure and 950 0C using High Temperature Clad Tube Test Facility (Sarkar et al. (August 2013)). The same experiment is planned using RABITS Facility to verify the results and to confirm the scattering.
b Few more tests are planned keeping the temperature constant and varying the pressure. From this we can
get the Norton’s Creep Power Law constants and compare the uni-axial data available with these multi-axial data.
c Design Basis Events in PFBR are classified into four categories of events based on the frequency of
occurrences. Category-1 event is the normal operating condition of the reactor. The pressure in the fuel clad in the reactor is 60 bars and the temperature of the clad during normal operation is 700 0C.
d To carry out the experiment in a short time accelerated test is performed. Larson Miller Parameter is
used to arrive at the set temperature 970 0C (Sarkar et al. (2013))
e
The temperature at category-4 event is 1200 0C. The actual heating rate during an enveloping event of category-4 event, i.e. primary pipe rupture is 100 K/s. It is planned to conduct tests at different heating rates to find the heating rates at which the clad would either rupture or balloon. Rupture during category-4 event is allowed. But ballooning would block the coolant flow passage and cause severe damage to the core.
CONCLUSION
Under extreme loading conditions, the fuel clad tend to develop considerable plastic strains in the form of localized bulging, eventualy leading to ruputre. This phenomenon of bulging, referred to as ballooning of the clad can lead to severe damage to the reactor core due to improper cooling. Hence RABITS facility is designed and developed indigenously for the quantification of clad tube ballooning under various extreme operating conditions. The salient features of this facility is simultaneous testing of multiple fuel clads yet having independent control of temperature and pressure in each specimen, online monitoring of the specimens and radiological protective shielded room. Large number of experiments have been planned to identify the zones where the clad would either rupture or undergo ballooning; to obtain the whole deformation properties, multi-axial creep curves etc.
REFERENCES
Rajeev R. Prasad and Sen Sujoy, (2013)“Calculations and Design of Shield for 225 keV, 10 mA X-Rays RABITS Facility”, Design Note No. RSSD/RSS/310.
Sarkar Rosy, Suresh Kumar R, Jalaldeen S, Selvaraj P, Puthiyavinayagam P and Chellapandi P, (August 2013), “Experimental Assessment of Clad Deformation from Coolability consideration at High Temperature in Fast Reactors”, Transactions, SMiRT-22, San Fransisco, California, USA, August 18-23,.
