Most of us, glued to the TVs, watching the catastrophe in Japan are yet to come out of the shock. Nature has been known for its ruthlessness but this is by far the most brutal natural devastations on a country which had full knowledge of such eventualities. Despite all the preparedness and the much talked about resilience of the Japanese people a devastation of this magnitude can leave a country crippled for sometime. My hearts are with the separated families which are scanning through huge lists of missing people to identify their close ones.
The latest on TV is about an imminent ‘meltdown’ at the nuclear installations. TV stations in India have already sought the opinion of experts in India on what this meltdown is all about.
I found three distinct meanings for ‘Meltdown’:
1) The one that we are now talking about is the ‘accidental melting of the core of a nuclear reactor’
2) ‘A rapid or disastrous decline or collapse’
3) ‘A breakdown of self-control (as from fatigue or overstimulation)’. (Source: Merriam-Webster on-line dictionary ).
Coming to the nuclear meltdown in Japan the term has been explained by Wikipedia:
“A nuclear meltdown is an informal term for a severe nuclear reactor accident that results in core damage from overheating. The term is not officially defined by the International or by the U.S. Nuclear Regulatory Commission.
A meltdown occurs when a severe failure of a nuclear power plant system prevents proper cooling of the reactor core, to the extent that the nuclear fuel assemblies overheat and melt. A meltdown is considered very serious because of the potential that radioactive materials could be released into the environment. A core meltdown will also render the reactor unstable until it is repaired. The scrapping and disposal of the reactor core will incur substantial costs for the operator.”
Yet another website explains vividly the sequences in a rather layman’s language ‘Meltdown-What it is and is not?’ :
“The term - Meltdown - refers to melting of the fuel in the reactor. Unfortunately, the term has been loosely applied to refer to any case of fuel melting, however minor. Only in several events - Three Mile Island 2 and Chernobyl - has there been significant fuel melting and only in the case of Chernobyl were there significant offsite releases.
Overheating of the fuel typically can be caused only if there is an inability to remove heat from the fuel. Two situations are the only likely causes:
* Loss of coolant in the reactor cooling system followed by a failure of the emergency core cooling systems to operate
* Failure of the reactor protection system to shutdown the reactor down when required for a major fault
Such conditions are considered to be outside the design basis for nuclear plants and are referred to as Class 9 accidents. The design of the plants is intended to assure that such conditions do not occur - due to the redundancy and diversity of the reactor protection, emergency core cooling, and containment isolation systems, as well as the containment structure itself. In spite of this, the US Nuclear Regulatory Commission is requiring licensees to develop procedures for such cases. The procedures are referred to as Severe Accident Guidelines.
If a meltdown does occur, a release of radioactive materials to the environment can occur ONLY IF there is also a major failure of the containment structure. For this to occur, the following would also have to happen:
* Overpressure of the containment
* Failure of the containment isolation systems, lines, and valves to close.
Potential causes of containment overpressure are:
* Steam explosion in the reactor vessel or a dropping of at least 20% of the fuel mass of a molten core
* Generation of gases either due to hydrogen generated from a chemical reaction between Zircaloy (used in the fuel cladding) and steam at temperatures above 3400F or due to carbon dioxide generated from interaction of molten core material with the concrete structures under the reactor.
* Heating of the containment atmosphere due to a failure of the containment cooling and spray systems.
For there to be a meltdown with releases offsite, the following sequence would have to occur:
- Failure of the reactor to shutdown when required such that it continues to produce heat at a high rate OR a major amount of coolant is lost from the reactor cooling system,
- Diverse and redundant high and low pressure emergency cooling systems are unable to provide cooling to the reactor cooling system,
- Fuel melting starts and blockage of flow channels occurs in the reactor such that cooling cannot be provided,
- Diverse and redundant containment cooling and spray systems are unable to provide cooling to the containment atmosphere,
- Redundant Hydrogen recombiners will not operate,
- Containment isolation system and associated valves do not close as required,
- Specialized high efficiency particulate, absolute, and charcoal filters do not function as required.
The design of the plant systems is intended to reduce the likelihood of such an event occurring (e.g. once in 250 years for the 400+ reactors with current designs). It is impossible to say, with 100% certainty, that a fuel melting event will not occur. The redundancy and diversity of plant design, NRC regulations, plant Technical Specifications, plant operating procedures and operator training and qualification provide the defense in depth.”
Having now come to know what is Meltdown my worries are about the meltdown of the morale of the people who are otherwise known for their resilience and perseverance. Let us all hope that the Japanese will set an example for a second resurgence, the first being the twin city bombings after the world war.
P.Uday Shankar
Coimbatore-India.
