Going Nuclear! Circuitry of a Reactor
In this article, Srikar from the Dept. of EEE, IITG, recounts his experience as a trainee at India’s largest nuclear power plant!
My parents are scientists in India’s largest nuclear power plant at Kudankulam. So, when the officials of the plant released a notice inviting engineering students across India for a training program, my dad forwarded it to me, and after finishing all the official things with the department, my training began during the summer break after my first year of college.
The first day of training kicked off with a lecture on the history of nuclear power in India, the construction of KKNPP, the working of the plant — fuel assembly and fission. They briefed me about the structure of the reactor and various other important systems and their design concepts. It felt as though they were revealing all their secrets about the power plant and when they taught me a few code terms (which are used in the plant during daily operation and during emergency), I started feeling all important!
Whenever the lecturer said, “This plant is the safest in the country”, I felt proud, sitting inside the plant premises and learning all the things that made this power plant so special.
A brief emergency training and then a lecture about Industrial Safety followed afterwards. This was all about how one can be quick and decisive during the time of emergency in an industry and what all precautions one must take to be alive. Before I knew it, my first day at training was over!
Being an undergrad in EEE, I was redirected to the Electrical Maintenance Unit (Electrical Section) of the plant from my second day. This section was responsible for the working, testing and maintenance of all the electrical equipment in the power plant i.e. transformers, switch gears, generators etc. I was told to meet a senior scientist whose office was in the operating island i.e. the place where the reactors are and the plant is operating. The most exciting part about going into the operating island is that one is supposed to wear a safety helmet whenever one enters the area. Moreover, to enter the area, one has to use an RFID card to open the gates. (I was given a temporary card :P)
The senior scientist I met was one of the most important people in the section. Officially, the rest of my training took place under his esteemed guidance.
The Electrical Section divided their work into different units and each unit was headed by scientists. Every morning, I reported to my guide. He would brief me about the topic I was going to learn that day and about the work I would be allowed to see. He then directed me to the unit heads, who would engage me with theoretical concepts related to their unit.
Post-lunch time was dedicated to a practical session where I would go to the site and watch people work, test, and fix all the electrical equipment pertaining to that unit. I shall explain only a fraction of details regarding the different units without boring you. (As if I didn’t do that already :D)
The Generator:
The most important equipment, because the power is generated due to this. All of us know the working principle of a generator. The connection of the stator winding is double star with a Brushless excitation system. DM water is used for cooling the armature and hydrogen acts as a gas cooler for field winding.
An automatic voltage regulator (AVR) is an electronic device for automatically maintaining generator output terminal voltage at a set value under varying load and operating temperature. It controls output by sensing the voltage Vout and comparing it to a stable reference. The error signal is then used to adjust an average value of the field current. This is done with the help of a thyristor. It acts like a variable diode. Based on the gate impulse given to it, the voltage drop across it will change thereby changing the exciter voltage.
I was taken to the generator room where we needed ear plugs to protect our ears. Yes! The whole room was so loud that no one could hear anything but the noise. It was a huge room with dense lighting. I could see workers testing the generators performance by measuring different parameters. They tried explaining things to me but of course I heard nothing!
The Transformers:
A static device used in transmission and distribution. Again, everyone knows how they work. A new fact I learnt is that for bigger transformers, there is a tertiary winding apart from the ones we know, which is used for stabilizing and for suppressing the harmonics in case of star-star connection.
The transformer consists of a tank usually filled with oil that acts as a coolant. Inside, there is the core and winding. There is a conservator which acts as an expansion tank designed to receive the transformer oil flow as it expands or contracts due to thermal cycling. There is an On Load Tap Changer for adjusting secondary voltages in case of primary voltage variation.
Buchholz relay is probably the most important part of an industrial transformer. It is mounted between the Main Tank and the Conservator, and collects the escaping gases. In case of severe fault i.e. short circuit, it acts as a sudden pressure relay.
Gas Insulated Switchyard:
There are 2 switchyards at KKNPP; 400kV and 220 kV. They are connected by the Interconnecting Transformer. Normally the generated power is evacuated through 400 kV transmission lines. However, since 400 kV and 220 kV systems are interconnected, power can flow through 220 kV lines depending on generation/load scenario.
The electrical power generated (24 kV, three phase, 50 Hz) by the turbo-generator, is stepped up through the 24/400 kV generator transformer and is evacuated through six numbers of 400 kV transmission lines. As a reserve source of power supply, the power plant is connected to two 220 kV substations through two numbers of 220 kV transmission lines.
The specialty of this switchyard is the insulation of the switch gears by SF6 gas. Advantages include easier erection and extension, smaller space, less maintenance and safer for the longer run.
There are other units in the Electrical section which I have omitted here.
After my study in the Electrical section, I was given a chance to visit the Control Maintenance Unit (Control — Instrumentation Section). Here, I learnt how different control systems in the plant work to ensure efficient and safe operation. I will briefly explain a few of them.
Rod control and Indication system:
For a nuclear reaction to be feasible, the reaction should always be controlled. To control the reaction, neutron absorbers are used. One such absorber is boron. It is used both in solid (control rod) and homogenous liquid (borated water) form. Based on the requirement, the amount of boron in water is increased or decreased and the reaction is moderated.
Control Rods also perform similar action. They are neutron absorbers, made of Boron Carbide (B4C) in stainless steel tubes. They move in and out of the core inside fuel assemblies for control of the reaction. Their movement is controlled by 3 electromagnets — pulling, locking, and fixing — which lock the rod at a fixed position above the fuel assembly. Based on the response from the neutron flux detector, the electromagnets energize and de-energize to bring the rod to a particular position.
Automatic Radiation Monitoring System:
Radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. Radiation measurement is highly important when a nuclear power plant is under operation to protect the living beings and environment from radiation, and to search for sources emitting unwanted radiation.
The ARMS monitors the following — Process radiation, Healthiness of fuel element, Contamination control, Environmental radiation and Effluent discharge. There are three types of detectors that are most commonly used, depending on the specific needs of the device. These are: Gas-Filled Detectors, Scintillators, and Solid State detectors.
Monitoring, Control and Diagnostic System:
The MCDS is an integrated automated system for reactor core and equipment of the primary and secondary circuits located within the containment. It performs control functions, information functions like measurement, processing, and recording of sensor response corresponding to the reactor; and auxiliary functions like diagnosis of hardware, software and information exchange.
It has five subsystems: In-Core Instrumentation System, Leak Monitoring System, Loose Parts Monitoring System and Vibration Monitoring System.
Engineering Safety Feature Actuation System:
Being India’s biggest nuclear reactor, safety is given the most importance in the plant; more than the normal operation and systems associated with it. Taking lessons from historic nuclear accidents like Fukushima and Chernobyl, the safety at KK is what makes it special.
The ESFAS refers to the control system which actuates the safety systems of the plant. The main purpose is to prevent or mitigate damage to the core and reactor coolant systems, ensure proper containment and automatic reactor shutdown during anticipated transients without scram. The safety systems are designed such that they serve reliability, redundancy, diversity and fail safe design. It consists of protection systems, isolating systems and other support systems.
I would like to end my technical discussion here. I fear it might have been too much already! So, that’s how my training went; studying different systems, learning new concepts and exploring the power plant by foot in hot sunlight!
It is important to draw wisdom from different places. Interacting with all people in the power plant including technicians, workers, and scientists helped me gain a lot of knowledge — both technical and non-technical. Each day was new and my curiosity developed over time. I guess I also learnt to extract information out of people even when they dismiss me or are disinterested in me. :D Overall, it felt like a mini working experience in an industry. I made a report of all that I learned during training, titled “Nuclear Power Systems & Plant Automation”.
A year later, a friend of mine asked me what my motivation behind attending this program was. To tell the truth, I never actually wondered about this. It was then I looked inward and asked myself the question. I realized that one must have the will to try out every opportunity that knocks at the door. Being open to knowledge from every possible place makes us a better engineer or a better scientist. Making up one’s mind to strictly pursue a domain and leaving out the rest thinking, they are either useless or difficult will just make us rigid and stale. Destiny is a funny thing. One never knows how things are going to work. But if we have an open mind and open heart, I believe we can find out the surprise waiting for us!
