Techniques de découpage télé-opérées dans le domaine du déclassement nucléaire

Abstract

Remote disassembly of radiologically burdened large components is among the most sophisticated and complex activities in the dismantling of nuclear installations. High local dose rates and contamination levels in combination with poor accessibility and complex geometries of parts imply major challenges.In many cases the design of nuclear reactors makes use of the shielding effect of water during operation. Hence, also in the later nuclear decommissioning it is possible to make benefit of this effect by dismantling/cutting of equipment under water. In the opposite, in some other cases of nuclear reactor design no shielding water is used during operation, and thus, also decommissioning activities have to focus exclusively on dry dismantling techniques to avoid, amongst others, the production of wet secondary waste.These and several other parameters entail the application of different cutting technologies thoroughly adjusted to the particular requirements.This article describes three thermal and one nonthermal cutting technology which have been effectively applied in nuclear decommissioning projects. The thermal cutting technologies addressed are “Autogenous Flame Cutting”, “Plasma Arc Cutting” and “Contact Arc Metal Cutting”. The non-thermal cutting technology addressed is “High Pressure Water Jet Cutting”.With the exception of Contact Arc Metal Cutting, all of these technologies are well known and prooven in the field of manufacturing. Nevertheless, to cope with the challenges in the field of nuclear decommissioning, it is also very important to combine the cutting technology/cutting equipment correctly with the remote controlled handling systems.At first the basic functioning will be described for understanding of the technologies themselves. The functioning accounts different fields of application and the particular technological limits. Adaptations for nuclear decommissioning requirements will be described presenting the most important modifications.At second applications and gained experience in particular nuclear decommissioning projects will be present by pictures and/or short videos.As conclusion details and data of all addressed cutting technologies will be presented in a matrix, giving a hand to concerned engineers in the selection of best matching technologies for particular nuclear decommissioning activities.<b>Short description of the different cutting technologies<b/>Autogenous Flame CuttingThe Autogenous Flame Cutting Technology has been introduced more than 100 years ago and has been developed to a standard method for the cutting of mild steel. A high pressure oxygen jet oxidises the material in a small kerf. Nevertheless, not all metal types are appropriate for the autogenous cutting process.Which are the limiting factors of the Autogenous Flame Cutting Technology? How can normally non-suitable material be cut anyhow? What is the maximum achievable cutting speed? Which peripheral equipment is necessary?Plasma Arc CuttingIn the nineteen-fifties the Plasma Arc Cutting Technology has been introduced as a complementary technology, to cope also with material unsuitable for flame cutting (for example stainless steel, copper, etc.).In contrast to Autogenous Flame Cutting, the Plasma Arc Cutting uses an electronically induced plasma arc to melt a kerf in the material. Inside the plasma arc temperatures up to 30.000 Kelvin exist. In theory, this temperature is sufficient to cut all materials. But where are the limits in practise?For nuclear decommissioning the Under-Water Plasma Arc Cutting Technology is important. Special equipment allows cutting under water, reducing the radiation impact to the concerned staff. Also the release of dust and particles to the air is reduced by 99.4 %.Contact Arc Metal CuttingContact Arc Metal Cutting is a new thermal cutting technology for under-water cutting jobs. In opposite to the previously introduced cutting technologies, here a carbon blade cuts the components. An electric arc between cutting blade and component melts the kerf into the material like a hot knife into a piece of butter.With the Contact Arc Cutting Technology it is now possible to cut also complex structures (structures with hollows and undercuts).How exact is the technology? Which applications are possible and where are the limits? The article will answer this and other questions.Water Jet Cutting(High Pressure) Water Jet Cutting can be distinguished in Pure Water Jet Cutting and Water Jet Cutting with additional abrasive particles. Both cutting principles use high pressure pumps to create the required high water pressure. In a special nozzle the water pressure is transformed into a high speed water jet (achieving speeds of several hundred meters per second). Nevertheless, cutting of metal or concrete requires the adding of abrasive particles to the water jet.There are two different technical solutions of Water Jet Cutting with additional abrasive particles. The Water Abrasive Injection Jet Cutting technology and the Water Abrasive Suspension Jet Cutting technology.The technological differences between the two technologies will be explained. Advantages for nuclear decommissioning projects and required peripheric equipment are explained in the article.