DISMANTLING OF THE PHOSPHORIC ACID PLANT OF HYDRO AGRI ROTTERDAM BV

3.1. Plan of action

Problems can occur with the maintenance and dismantling of process installations in which NORM deposits exist as a result of the use of radioactive raw materials. Exposure to these radioactive substances can lead to health risks for the personnel involved as a result of inhalation, ingestion, and external exposure. Therefore, in the Netherlands a licence to perform these work activities is required in accordance with the Radiation Protection Ordinance.

Because of the need for specific knowledge of the process and the risks involved with these NORM contaminated installations, such installations are generally dismantled by the personnel of the owner. Permission is required from the Dutch authorities before dismantling can take place. The company that carries out the dismantling has to have a licence. The licence can be revoked only after the completion of the dismantling and fulfilment of all the obligations in the licence.

Before the dismantling takes place, a plan of action has to be drawn up with a description of the most important health and safety aspects and the impact on the environment, taking economic aspects into account. Such a plan of action contains the following:

• The necessary preparations (flushing, termination of operation);

• The organization of health and safety, and responsibilities;

• The method of disposal of radioactively contaminated installation components and radioactive substances;

• The method of dismantling of the installation;

• The method of use of technical equipment in the dismantling of the installation;

• The method in which the (radioactive) waste is treated;

• The disposal of the waste.

The plan of action must contain a risk inventory in which the exposure of workers and the public are estimated. Moreover, a description of how this exposure is to be kept as low as reasonably achievable must be given. During the dismantling, the plan of action has to be adjusted in the event that the information used in drawing up the original plan is superseded.

3.2. The final report as a dossier

At the end of the dismantling, a report with the following contents has to be presented:

• Historical information and references;

• A summary of the whole project;

• The organization with respect to responsibilities and supervision;

• The applicable licences;

• The results of the preliminary inventory;

• An overview of the contaminated installation components identified;

• An overview of the methods applied for treating these components;

• The plan of action for each method;

• An overview of the ultimate release of contaminated components;

• An overview of the release of equipment used;

• An overview of the tools and aids applied and released;

• A calculation of the dose received by workers and the public;

• An overview of the quantities and temporary storage of the (radioactive) waste produced;

• The transfer of radioactive substances to third parties for the purpose of further processing.

The report must be available for at least 15 years. The document may be used in evidence.

3.3. Technical approach

When dismantling a NORM-contaminated installation, it is advisable to start by conducting a radiation survey. During the survey the whole installation has to be measured and all the radiation levels have to be recorded and collated. With this information, the sources of the highest radiation levels, and the possibility of such levels limiting the radiation survey of the other parts of the installation, are identified.

The installation components that can be either dismantled or decontaminated elsewhere can now be selected. Next, a plan is drawn up to remove or clean objects with the highest radiation levels. As a result, the radiation level in a certain area is lowered and other objects with enhanced radiation levels can be identified in a second survey. With the information from this second survey, the objects with the highest radiation levels can again be removed, and ‘new’ contaminated objects become ‘visible’.

Radioactively contaminated metal objects that cannot be decontaminated properly can be transferred to a metal foundry that has a licence to melt such objects. In the Netherlands, the export of such objects for this purpose is permitted. On-site decontamination is the best option for objects that can be decontaminated or that cannot be transferred to a metal foundry.

It is known from experience that the radioactive deposits that occur with the production of phosphoric acid can generally be removed using high pressure or ultra high pressure (>2500 bar) water jetting. In general, the radioactive substances (scales) released are not very soluble in water.

During decontamination with water, particles with diameters from 0.1 µm to several millimetres are generated. The activity concentration as a function of the diameter is generally constant. This means that the used water can be cleaned with available separation techniques.

In most cases, the water remains contaminated with non-radioactive components. Because of this, discharge of the used water into surface water bodies is not permitted in the Netherlands.

Installation components that come into contact with acid are normally protected by a coating or rubber lining. As a result of the contact with the acid, radioactive deposits occur in the top layer of the coating and the rubber. During the dismantling project, the rubber also has to be decontaminated with high pressure water jetting. Using this method, a long rubber belt from the belt filter system, including the accompanying transportation reel, could be treated.

The radioactive contamination could be ‘milled’ away by applying a suitable combination of nozzles, pressure and water quantity.

The reactor vessels and acid storage tanks also have protective layers. The reactor vessels had a layer consisting of rubber on carbon stones that were glued to each other. The acid storage tanks had a rubber coating. Because of the long period for which the vessels and tanks had been used, the surfaces had become contaminated with deposits containing ²²⁶Ra.

With the use of a robot, the surfaces were decontaminated with high pressure water jets. The robot could be moved horizontally and vertically across the surfaces to which it was attached by producing a vacuum that was used also to drain away the water with the radioactive particles to a storage tank. The radioactive particles were removed from the water afterwards.

A mist occurs when using water at high pressure. This mist was removed through a filter system by a vacuum installation. Discharge of the water–air mixture was done using a demister and an absolute filter. Because of these measures, discharge of radioactivity to the environment was very limited.

4. RESULTS

4.1. Exposure of workers and members of the public

The following doses received by workers were determined:

• Wet decontamination workers: 1.2 mSv

• Dry decontamination workers: less than the minimum detectable level (MDL)

• Dismantling/construction workers and assistants: <1.7 mSv

• Supervisors: <MDL

The maximum dose received by a member of the public outside the plant site was determined to be <0.4 µSv (corresponding to a risk of <10^-8)

4.2. Work activities associated with the removal and disposal of radioactive substances 4.2.1. Wet decontamination

The following objects could be decontaminated using high pressure water jetting:

• Reactor vessel (transformation);

• Reactor ‘defluorizing’;

• Storage tank for soft acid;

• Stainless steel frame and transport reels of a belt filter system;

• Rubber belt of the belt filter, with the exemption of the centre strip of 10 cm;

• Buffer tank (spilled acids);

• Seal tanks and table filter;

• Remaining part of the tilting-pan-filter;

• Metal constructions, machine frames and smooth parts of the installation;

• Ceramic floor tiles;

• Drains and gutters;

• Carbon (stones) from the coating of the reactor;

• Some drain wells.

4.2.2. Dry decontamination

The decontamination of filter plates, funnels and a 20 000 kg table filter was attempted using stainless steel grit blasting. The results were disappointing because of distortion of the objects, and the decontamination of these objects by grit blasting has now been abandoned.

4.3. Other work activities

Other work activities involved the dismantling, transport, storage, etc. of pipes, pumps and funnels.

5. TREATMENT AND TRANSFER OF RADIOACTIVELY CONTAMINATED OBJECTS