Koodankulam Nuclear Plant : Risk Analysis and What to expect – Part 2

(An engineer by profession, Sasikumar holds double masters degree in Engineering from India and Business Administration from USA.)

Read Part 1 of this article here

As an Engineer I am cognizant of the sentiments around the efforts – millions of man-hours and tax payer money that have been spent till date towards the construction of Koodankulam power plant. Also concur to the philosophy that power/electricity is core to the growth of human civilization and one has to take some level of risk to achieve growth. Let me remind you that views expressed in this series of articles are purely based on my Engineering judgment and are voluntary without any influence of position, money or any other factors.

Let me try to explain some basic concepts of a nuclear power plant first. In a normal working scenario, a nuclear power plant operates on the following mode: Nuclear fuel rod dissipates huge quantity of heat and will need to be cooled continuously by the Primary Circuit. In order to avoid water becoming steam, the Primary Circuit (violet color pipe line) will be maintained at higher pressure in the PWR system. The heat of the Primary Circuit is transferred continuously to the Secondary Circuit (red and dark blue pipe line) in the Steam Generator equipment.

In simple terms, Steam Generator is nothing but a heat ex-changer which transfers heat from Primary Circuit to the Secondary Circuit – i.e one side of the steam generator is filled with high pressure water and other side is filled with steam (actually feedwater enters from this side and becomes steam).

The Secondary Circuit steam will be used to rotate the turbine (coupled with generator) to generate electricity. Thus Thermal Energy (heat) will be converted to Mechanical Energy (rotation) and then to Electrical Energy (electricity).

After most of the energy is spent for rotating the turbine, Secondary Circuit Fluid remains yet as steam but at low pressure and temperature. In order to pump the low pressure steam back to the Steam Generator to continue the cycle, it has to be converted into water first. Typically it will be cooled in the Condenser equipment by the Cooling Water system (light blue pipe line) during when actual phase change occurs (steam to water). Again condenser is nothing but a heat ex-changer which transfers heat from Secondary Circuit (low pressure steam) to the Cooling Water system.

Temperature of the cooling water system is cooled continuously by the cooling tower equipment with the support of huge fans or in some cases with natural circulation. Failure of any one of the above circuit/system will result in a cascading effect and result to overheating of the nuclear fuel rod in the reactor vessel and will initiate eventual Fuel Rod Meltdown, if unattended.

Approximately, the size of the primary circuit pipeline is ~1.5 to 2 ft in diameter and cooling water system pipe line is ~6 to 9 ft in diameter. These pipe sizes vary based on the capacity of the nuclear plant. Enormous quantity of water will need to be circulated in these circuits to keep the plant running to produce electricity and avoid Fuel Rod Meltdown.

In the eventuality of failure of any one of the circuit, the plant should be designed to initiate emergency shutdown procedures immediately. Nuclear chain reaction should be stopped and fuel rods should be quenched in the under water storage tank located in the containment vessel. In classic circumstances these procedures will be executed by the plant operators. Plant management teams will be well trained to execute these tasks instantaneously.

However to keep each circuit in operational condition and continue with the cooling process, it is essential to run several pumps with an uninterrupted supply of electricity. Typically nuclear plants will be designed to have electricity supply from a Power Grid with a backup power supply via Diesel Generators in case the power grid fails. Also few hours worth of battery power will be available to keep the system running in case Diesel Generator and external Power Grid fails to kick in.

Usually the fuel rods need many hours of continuous cooling for a safe shutdown. Each plant will be designed with emergency core cooling system consisting of a series of systems that are designed to safely shut down the nuclear reactor during unforeseen accidents. These core cooling systems are high pressure Coolant Injection system – series of pumps that have enough pressure to inject coolant into the reactor vessel while it is pressurized. Depressurization system – has several valves which open to vent steam several feet under a large pool of water. This will depressurize the reactor and enable low pressure Coolant Injection to perform. Low pressure Coolant Injection system – consists of several pumps which inject additional coolant into the reactor once it is depressurized. Also some the plants may be equipped with Core Spray system and Primary Containment Cooling system.

In the Fukushima disaster several of these cooling systems failed simultaneously due to flooding and impact of tsunami waves. Reactors were left with only few hours of battery power and it was not enough to cool the fuel rod continuously for an extended period of time. This lead the Fukushima plant into the eventual fate of complete fuel rod meltdown and subsequent hydrogen explosion.

I am certain that the Koodankulam would be equipped with some of the above cooling and backup power supply system already. However every system has some limitation and are designed to handle only certain degree of disaster situations.

All the pumps and electricity based emergency cooling systems are prone to fail if the accident happens beyond the assumed design basis. At present most leading nuclear technology companies are promoting Passive Core Cooling system to mitigate the risks associated with emergency conditions. In this advanced Passive Core Cooling technology system, most of the safe shutdown and emergency cooling systems works on gravity based concept and does not require any power supply or pumps.

Passive Core Cooling technology is in the evolving stage though and is yet to get final approval from the US NRC (Nuclear Regulatory Commission).

Passive Core Cooling technology is in the evolving stage though and is yet to get final approval from the US NRC (Nuclear Regulatory Commission). However China is presently building few nuclear plants based on Passive Core Cooling system with the assistance of commercial US Nuclear companies.

Coming back to Koodankulam plant, given the fact that the construction of the plant has been in progress since 1997 it is using a 15 years old technology. Hence, I doubt the plant would be equipped with all the latest Passive Core Cooling system and latest back- up mechanism(s) to handle a worst case disaster scenario such as a tsunami.

I will further explore the Passive Core Cooling system and its limitations in the next episode.




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