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

Please also read Part 1, Part 2  and Part 4 of this article.

Passive core cooling system is the latest buzzword used by the top nuclear companies. This is the newest technique and part of the western nation’s third generation nuclear reactors. The larger VVER-1000 was developed after 1975 and falls under the second generation nuclear reactors. The attached drawing shows the cross-sectional view of the VVER-1000 containment.

In passive core cooling system nuclear power plant, apart from the regular pump-based cooling system, there will be a huge water tank located on the containment shell. In an emergency situation, if the pump-based cooling system fails to start or proves inadequate due to other external factors, as a last defense, these gravity-based cooling systems will supply water to the reactor vessel and initiate cooling process to prevent the fuel rod meltdown. These new systems have been designed with lesser number of valves, pumps, pipe connections and built with latest controls and minimal human interventions.

It is nothing but a tank designed to hold some fixed quantity of water. Nuclear chain reaction must be stopped as a mandatory first step during emergency shutdown condition in order to get the desirable results by deploying passive core cooling system.

Koodankulam power plant has been built based on the VVER-1000 technology. Though few documents claim some level of passive core cooling system has been added to ES-92 version of the VVER-1000 and customized specifically for the Koodankulam power plant, the extent to which all those systems are added and its operational efficiency are still debatable. The VVER-1200 is a third generation nuclear reactor offered by Russia that gives power output of 1200MWe with the entire passive core cooling safety features. Recently in the year 2010, oldest VVER-1000 reactor at Novovoronezh was shut down for modernization to add protection and emergency safety systems.

As we know, day-to-day technology is changing and human civilization is moving ahead for betterment. Let us compare an analogy of a customer who placed an order for a Hindustan Motors Ambassador Car in 1995 which was a top brand car available at that time in the Indian market. However due to unforeseen reasons, the manufacturing company was not able to deliver it immediately and they took 15 years to build and in the year 2011, if they provide the same Ambassador car to a consumer, would the customer feel happy about the car,its performance and safety features?

Nuclear containment shell has been designed to hold numerous critical components and systems. Generally it may not have enough space to accommodate the additional systems and piping. In the original design itself, all the components are arranged precisely with very little space. Though in today’s world passive core cooling system offers some additional safety features, it still has a minor flaw of possible rusting associated with the containment structure liner. In case the dome rusts through the shell and if left unattended, there will be some possibility of expulsion of radioactive contaminants to the atmosphere. Again these new technologies do not take into consideration their design basis to handle the tsunami type scenarios in which multiple systems can fail simultaneously as it happened in the Fukushima disaster.

Typically containment shell is ~115-160 ft diameter and ~225-350ft height. These sizes vary based on the generating capacity of the nuclear power plant and its inner arrangement of steam generators and nuclear reactor core. It is factual that nuclear reactor protected with few centimeters’ thick iron shell first and several inches thick concrete shell on top of it is capable of withstanding the impact of light air planes / other moving objects. Yet, there are several soft pockets in and around the nuclear containment shell.

Generally ~15-25+ pipe line penetrations are made in the nuclear containment shell in order to allow process fluid to move from containment to turbine building and back. Additionally few penetrations for compressed air system, instrumentation & control system and power supply cable connections are included. During design stage, it is part of the engineering team to analyze the risk factors associated with the failure of these penetration points. Standard assumptions during the Pipe Rupture Analysis are break will happen in these penetrations. Classically also, nuclear reactors in many designs are located at lower level in the containment shell.

In the event of containment being exposed to tsunami waves, these penetrations may fail due to the impact of debris and other floating materials. In such a scenario, possibility of flooding of the lower part of the containment and failure of critical components and systems are real.

The sole purpose of writing these articles is to educate the scientific theory behind the nuclear power plant, analyze the various systems and identify the weak points in and around the plant, establish proper safety features in place to counter those soft pockets. In case, if safety features are not in place, analyze the possible risk scenarios and its effects. As an Engineer, I love to keep the plant running like many Technocrats who dreams for it. However it is important to ask questions – How and Why?

It is all about Benefits Vs Risks.

Our entire goal is to make a stronger India and a safer environment. Raising the safety concerns is part of the patriotic responsibility of each civilian who cares for his/her country.

I will further explore the process of storing nuclear spent fuel in the next episode.