The treatment of scrap material from nuclear decommissioning

The number of nuclear power stations that have been in use for 40 years will reach a peak in 2015 (Ref.: IAEA, Nuclear Power Reactors in the World, April 1999). Levels will remain high between 2015 and 2025, increasing the number of decommissioning projects, something also influenced by national policy decisions and economic factors.

The primary objective of dismantling is to restore the site, as far as possible, to a condition that does not present any radiologic danger, but there different are a number of stages to the process. Storage under surveillance with minimal decontamination, consisting principally of the withdrawal of radioactive fuel. Limited dismantling with partial restitution of the site. Parts of the radioactive primary circuit can be removed. The entire site is then dismantled.

During phase 3, or even phase 2 of the dismantling stage, it is often economically advantageous to reduce the volume of radioactive waste being stored. For scrap that is weakly or moderately radioactive, compaction can reduce the volumes that need to be transported and stored. The majority of scrap metal obtained in dismantling is either slightly, or very weakly radioactive.

Densification of radioactive scrap

Storage is optimised using standard containers, meaning there is an economic imperative to use a standard gauge when processing. In the context of scrap obtained during dismantling, triple compression presses can partly meet this need, however hydraulic shears offer an essential flexibility that allows treatment of a wider range of scrap (piping, structural and support material, diffuser structural elements, etc …).

nuclear-shear-1     nuclear-shear-container     nuclear-shear-4

The main issues to be considered in the context of treating material using shears are:

…Logistical considerations

Handling methods for unprocessed scrap, or scrap that has already been sheared, should be designed to limit the risk of dust release. An automatic loading bucket and automatic container filling station, combined with a dust extraction system can partially address this problem (see photograph above).

… Optimising densification

The sheared product density that can be achieved, between 0.4 and 0.7 t/m3, does not in itself allow optimum storage. The dimensions of the containers used for storage are critical to the extent that they affect the length and width of cut material (examples of two types of container are shown in the photographs). The cutting length is easily adjustable as a result of indexing of the pusher in CIV-type press-wing shears or CIB-type lateral-box shears. Scrap is loaded into boxes in the form of logs, before being sheared at a fixed width of typically 800, 1000 or 1500 mm. Addition of a cupola system at the shearing head reduces the width to between 200 and 500mm, depending on the type of shears. This is essential for the optimum filling of containers, such as the one presented in the example here.

 nuclear-shear-cupola1     nuclear-shear-cupola2     nuclear-shear-6

 … Type of scrap

The following photographs show different types of weakly radioactive scrap. For a company processing scrap materials, the type and design of hydraulic shears bears a direct relation to both hourly productivity and the type of scrap it is possible to treat. In the context of the processing of radioactive scrap, the types of material to be processed and issues related to handling are the key factors when it comes to selecting the most suitable hydraulic shears.

nuclear-scrap1     nuclear-scrap2

…Safety considerations

Safety requirements must be taken into account right from the beginning, in the pre-project phase. Among other things, they need to address:

>Radioactivity-related risks - The equipment used to feed the shears can be equipped with remote control. To limit the impact of radioactive dust where it is not possible to site the controls remotely, a specially adapted suction system, combined with a pressurised operator-cab can be used (a sample extraction system is shown in blue in Figure 1). Where hydraulic shears are subject to significant vibration, pneumatic rubber insulators can be used to provide vibration insulation (Figure 3).
Fire risks for hydraulic power units & electrical cabinets - As a result of upstream sorting, the fire risk associated with the scrap is very limited. In addition to various fire prevention systems (Figure 2), flame-resistant hydraulic oil can be used.
>The risks present in hazardous areas- Different safety zones can be defined according to the risks of radioactivity, modes of operation employed, and requirements for preventive and corrective maintenance. The perimeters of these areas can be controlled by gates with secure access or anti-personnel detection.
>The risks associated with contaminated fluids - All hydraulic hoses are equipped with an anti-whiplash system. A holding tank is used to collect all fluids from containers not detected as full during upstream sorting, as well as those arising from hydraulic leaks.

Since the first hydraulic shears were installed in 1997, at EDF Group’s Socodei site at Marcoule in France, Danieli Henschel has been able to draw on the benefit of experience gained from a number of reference sites, such as CERN in Geneva and RosRAO in Murmansk.

Related topics:
DISMANTLING OF FORMER RUSSIAN NUCLEAR SUBMARINES: DANIELI HENSCHEL KNOW-HOW FOR A CONTRACT UNDER HIGH CONTROL!
www.hna.de video

 

nuclear-shear-7

Système anti coup de fouet sur flexibles

nuclear-suction

Vue 1 – Système d’aspiration (en bleu)

nuclear-fire-protect

Vue 2 – Système de détection incendie

nuclear-pneumatic-isolator

Vue 3 – Isolateurs pneumatiques caoutchoucs

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