EARTHLIFE AFRICA are failing to articulate a coherent response to last week’s accident at Koeberg. Despite indications of a potential class 9 nuclear emergency and radioactive accident at the plant, after the system “scrammed”, the group seems to have bought into the Atomic Energy Corporation’s (AEC) spin on the near-disaster. The AEC has downplayed the accident, from an all-out scram, to a minor refueling incident, to problems with an external electrical connection.
If the backup and secondary systems had failed, the reactor would have been unable to switch-off, triggering a meltdown that could have disasterous consequences. Instead of examining the design flaws and possiblity of contamination within the plant structure, Earthlife has focused on the secondary “refueling incident”.
The term – Meltdown – refers to melting of the fuel in the reactor. According to the nucleartourist website, the term has been loosely applied to refer to any case of fuel melting, however minor. The site claims that “only in several events” such as Three Mile Island 2 and Chernobyl, ” has there been significant fuel melting and only in the case of Chernobyl were there significant offsite releases.”
Such understatement and disinformation is to be expected from the nuclear industry. In the interests of informed debate, I republish nuclear tourist’s, description of a meltdown, please bear in mind that it carries the same design-hubris that launched the failed Space Shuttle and created other technology responsible for catastrophic human errors:
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:
1. 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,
2. Diverse and redundant high and low pressure emergency cooling systems are unable to provide cooling to the reactor cooling system,
3. Fuel melting starts and blockage of flow channels occurs in the reactor such that cooling cannot be provided,
4. Diverse and redundant containment cooling and spray systems are unable to provide cooling to the containment atmosphere,
5. Redundant Hydrogen recombiners will not operate,
6. Containment isolation system and associated valves do not close as required,
7. 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.