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INTRODUCTION
The Intel 8051 is a Harvard architecture, single chip microcontroller (µC) which was developed by Intel in 1980 for use in embedded systems. Intel's original versions were popular in the 1980s and early 1990s, but has today largely been superseded by a vast range of faster and/or functionally enhanced 8051-compatible devices manufactured by more than 20 independent manufacturers including Atmel, Maxim Integrated Products, Silicon Laboratories, Texas Instruments and Cypress Semiconductor. This project involves the use of Atmel’s AT89C51RD2 microcontroller, which is a modified, and enhanced version of 80C51 and it is 80C51 instruction compatible also.
Sodium Coolant:
Sodium, because of its good heat transfer and nuclear properties, is used as a coolant in fast reactors. It melts at 980C and boils at 8830C (under atmospheric pressure). While the physical uses of sodium are many, the most important one is its application as heat transfer fluid / medium in fast reactors. Sodium is circulated in liquid condition at an elevated temperature in various circuits in the reactor at the temperature range of 180 0C to 550 0C. To keep the sodium in molten state in the pipelines heaters are provided along the sodium pipelines. Since sodium is in solid state at room temperature, it is always essential to pre-heat the sodium system to the desired temperature condition. For pre-heating the pipelines and other components, electrical cable heaters, u-type immersion heaters etc. are used.
Sodium Coolant
This reactor is sodium cooled with a closed fuel cycle. It can be used to manage high-level wastes, particularly plutonium and other actinides. Its safety features include a long thermal response time, a large margin between the coolant (sodium) working temperature and its boiling point, and a primary system that operates near atmospheric pressures. There's also an intermediate sodium system between the radioactive sodium in the primary system and water and steam in the power plant.
Intel 8051 MICROCONTROLLER:
Intel's original 8051 family was developed using NMOS technology, but later versions, identified by a letter C in their name (e.g., 80C51) used Complementary Metal Oxide Semiconductor (CMOS) technology and were less power-hungry than their NMOS predecessors. This made them more suitable for battery-powered devices. It's also low power consumption max 5V.
8051 is an 8-bit microcontroller and it has 128 bytes of on chip RAM, 4KB of on chip ROM, two 16-bit timers/counters, 8 bit data bus, 16 bit address bus, 5 interrupt sources and four 8-bit ports of which one is a serial port.
AT89C51 is an 8-bit microcontroller, which is an enhanced version of 80C51 and has an internal flash memory of 64 KB, which can be programmed either in parallel mode or serial mode with the ISP capability. It has an extended data memory of 1 KB. It is used for accessing the logical data from EPROM and detecting the status of the various parameters involved. In addition, the AT89C51 has a Programmable Counter Array, an XRAM of 1792 bytes, a Hardware Watchdog Timer, SPI interface, keyboard and a more versatile serial channel that facilitates multiprocessor communication (UART). A key feature of the AT89C51RD2 is its X2 mode option. One can choose to run the application with the conventional 80C51 clock rate (12 clocks per machine cycle) or select the X2 mode (6 clocks per machine cycle). Another way to benefit from this feature is to keep the same performance, reducing the clock frequency by half, thus dramatically reducing the Electro Magnetic Interference (EMI). One can also save power consumption while keeping same CPU power.
The reset input is the RST pin, which is the input to a Schmitt Trigger. A reset is accomplished by holding the RST pin high for at least two machine cycles (24 oscillator periods), while the oscillator is running. The CPU responds by generating an internal reset.
The internal flash memory is used during testing and debugging. After testing and debugging the Program code is burnt into the External EPROM. Identification of internal or external code access is done by connecting the EA\ pin to high for internal access and low for external access. The baud rate in the AT89C51 in the microcontroller is programmable.
2.2 TEMPERATURE SENSING
Thermocouple is used for sensing the variation of temperature of the liquid sodium in FBTR.
The thermocouple basically used for this purpose is the K-type thermocouple. It is known as "general purpose” thermocouple. It is low cost and popular. It can be used in the range of -200 to +1200 °C. The characteristic of the thermocouple undergoes a steep change, when a magnetic material reaches its Curie point. This occurs for this thermocouple at 354°C. The material, which is used in the K -type thermocouple {Chromel (Ni-Cr alloy) / alumel (Ni-Al alloy)} is not affected by the nuclear radiation.
When compared to the other thermocouples, the cost, temperature range, accuracy, stability and operational life expectancy are found to be very good. Hence, the K type thermocouple is chosen. The sensitivity of the thermocouple is about 41 µV/°C.
The Remote Terminal Unit is used in the remote field for the sodium temperature control in the pipelines, which is used as a coolant in Fast Breeder Reactors. The RTU is used to get information about the sodium temperature in the pipelines, processes and indicate the condition in the control room.
The Remote Terminal Unit consists of a FRC Header to receive the voltage signals from the Sensor. A K-type thermocouple senses the heat input from the pipelines and converts the same into electrical output. The output of the thermocouple is given to the cold junction compensation for balancing the temperature followed by a signal conditioning unit and isolation amplifier, which provide channel-to-channel isolation & ground loop elimination from the field signals. It is followed by a multiplexer to select the channels. The output of the multiplexer is given to the amplifier circuit followed by ADC, which converts the analog voltages into digital form. The output of the multiplexer forms the address lines of an EPROM. The logical output is accessed by the microcontroller and indicates the sodium temperature in the pipelines.
2.3 LIQUID SODIUM LEAK DETECTION
Sodium leak results in the formation of fire and smoke thus affecting visibility. Different types of leak detectors used in reactors to avoid the sodium leaks which can detect as low as a nano gram of sodium in a cubic cm of the carrier gas. Some of them are
2.3.1 Spark plug type leak detectors
SPLD works on the principle of high electrical conductivity of sodium and used for detecting sodium leak in enclosed space. It is mounted on a small pocket in to which the sodium collects and causes electrical short between its installed central conductor and the body. It is used in the double enveloped primary sodium piping where interspace is filled with nitrogen. In sodium facilities, leak collection trays are provided below all major capacities. Such trays will have slope to enable collection of leaked sodium. SPLDs are provided in such systems to alert operator in case of leak.
2.3.2 Wire type leak detectors
WLDs consist of metallic wire (Ni/SS) insulated with ceramic beads, which is laid over the surface of the pipe all along its length, and dish end of the sodium vessels. The exposed portion of the wire is in close proximity to the pipe surface. If any sodium leaks from the pipe and contact the wire, it bridges the small gap between the wire and the pipe surface, causing earthing and gives sodium leak alarm. In order to make the leak detection system more effective the ceramic insulted wire is laid underneath the horizontal pipelines and wound helically around vertical pipes.
2.3.3 Mutual inductance type leak detectors
Mutual inductance type level probes are used for continuous as well as discrete sodium level measurements in the reactor level and other sodium tanks.
For early detection of sodium leaks, wire type leak detectors for single walled pipes, spark plug leak detectors/ mutual inductance type leak detectors for double walled pipes and vessels and sodium ionization detectors in closed cells and cabins are used. Leak-before-break enables early detection before any leak escalation. In the event of any leak, sodium level in the reactor stabilizes above the outlet pipes and circulation by pumps is maintained.
Working of SPLD
The sensing mechanism for a single channel comprises of three resistors, cables and detectors. The two terminals of the switch remains open for healthy condition and get shorted when the sodium leak is detected. Fixed resistors of 100 ohms and 470 ohms are connected to the cables in the field.
SPLD CIRCUIT DIAGRAM
The leak detector should provide the four conditions at the input of the signal conditioning circuit.
They are
• When there is no leak (Healthy Condition), the total resistance will be 570 ohm and the voltage at the terminals greater than 3.6V.
• When there is leak, the resistance will be 1000 ohm and the voltage at the terminals is greater than 0.65 V.
• When there is Cable short, the resistance will be 50 ohm (approx) and the input potential is measured to be approximately 0V.
• When there is Cable open, the resistance will be more than 10 K ohm and the input potential is measured to be above 5.6 V.
2.3.5 Remote Terminal Unit
The Remote Terminal Unit is used in the remote field for the detection of leakage of sodium in the pipelines, which is used as a coolant. The RTU is used to get information from the Leak Detector, process and indicate the condition whether there is a leakage or not and whether the Leak Detector is healthy or not to the control room.
DESIGN AND PCB LAYOUT
3.1 ELECTRONIC DESIGN
OrCAD is a proprietary software tool suite used primarily for electronic design automation. The software is used mainly to create electronic prints for manufacturing of printed circuit boards, by electronic design engineers and electronic technicians to manufacture electronic schematics and diagrams, and for their simulation.
OrCAD Capture CIS is a software tool used for circuit schematic capture. It is part of the OrCAD circuit design suite.
Capture CIS is nearly identical to the similar OrCAD tool, Capture. The difference between the two tools comes in the addition of the component information system, or CIS. The CIS links component information, such as printed circuit board package footprint data or simulation behavior data, with the circuit symbol in the schematic. When exported to other tools in the OrCAD design suite, the data stored in the CIS is also transferred to the other tool. Thus, when a design engineer exports a schematic to the circuit board layout utility, the majority of the circuit elements have footprints linked to them. This saves time for the design engineer.
Capture also has the ability to export netlists to several different circuit board layout utilities, such as OrCAD Layout, Allegro, and others. When combined with the CIS, circuit board footprints are linked to this netlist. This, combined with the pin to pin interconnect description of the netlist, will open the correct part footprints, and, if the CIS data that CIS exported is correct, will connect all of the pads together with representative lines. This feature makes the circuit board design process easier for the design engineer.
3.2 SCHEMATIC REPRESENTATION
Using ORCAD–EDA (Electronic Design Automation package) schematic entry tool, the schematic diagram was drawn using CAPTURE CISin the ORCAD package. The IC’s and components are available in the library and wire connections are given. Connectors are used to connect devices in the page and between other pages. After drawing the schematic diagram net list is generated (which gives the pin to pin connection of different ICs) in order to verify the connections. From the schematic diagram, net list (*.net) file is generated which gives the interconnection between components present in the circuit diagram.
PCB LAYOUT
Component Placement
One can place components in Layout either manually or use the interactive and Auto placement utilities.
Board Routing
With Layout, we can route our board manually, or we can use Layout’s interactive and automatic routing tools (available in Layout Plus and Layout only). First the .max file is created from the .mnl file, which is created in the Capture CIS. Before creating the .max file .net, the netlist file is created in the Capture CIS and the Footprints available in the Libraries are linked with the components. If for an IC, the footprint is not available in the libraries, it is created from the datasheet of the IC and then linked. After the .max file is created, the board outline is drawn for the needed dimensions. The IC’s are then placed inside the board one by one according to our own design and to minimize connections. The copper track widths are specified. For power supply lines the track width is given as 20 mil and for other lines the width is given as 12 mil.
Then routing is done using SMART ROUTE which auto routes the board. Smart routing is preferred to Auto routing as it takes less time and also the vias appearing are less.
Gerber File
It is a file format that can be read by Gerber and other photo plotter systems that require separately or previously defined aperture lists. Gerber photo plotting is a method of transferring board information to film. From the layout, Gerber file was generated for PCB fabrication.
Pspice Express gives the tools need to take programmable logic device project through each phase of the design flow. It provides design entry capabilities (establishment and control of design hierarchy, schematic and VHDL model development, and design rule checking) as well as simulation and debugging capabilities.