Fundamental Concepts

METHODS FOR THE DECONTAMINATION OF PLANT AND EQUIPMENT

II.1. INTRODUCTION

Various decontamination methods are being applied on and off the site, the choice of method depending on the type and size of the components and the characteristics of the contaminating substance. Methods range from removal of bulk sludge from vessels (Fig. 34) followed by rinsing with water to the application of chemical or mechanical abrasive techniques. The methods of specific operational importance are described briefly below and summarized in Table VI.

FIG. 34. Removal of bulk sludge from a vessel (courtesy: Atomic Energy Commission of Syria).

II.2. IN SITU CHEMICAL DESCALING

Chemical methods are applied and are being developed further for downhole scale removal and scale prevention [53, 54, 101–103]. If scale prevention has failed and the extent of scaling interferes with production and/or safety, chemical methods are also applied for removal of scale from the production system. The chemicals used are based on mixtures of acids or on combinations of acids and complexing agents. Usually, the primary reason for in situ descaling is to restore or maintain the production rate rather than to remove radioactive contamination. Nevertheless, effective prevention of scaling causes radionuclides mobilized from the reservoir to be carried by the produced water through the production system rather than being deposited.

Chemical removal of scale also removes the radionuclides contained in the TABLE VI. NORM DECONTAMINATION METHODS

Method Comment

Manual removal or cleaning

Does not involve any machinery and may be as simple as hand washing or shovelling. Commonly used for removing sand and sludge from topside equipment.

Mechanical removal by drilling or reaming

Commonly used to remove scale (hard deposits) from tubulars and other types of surface contaminated equipment.

Wet drilling processes should be used to reduce/prevent the generation of radioactive dust. Should be enclosed to contain the contaminants and wash water should be filtered to remove scale.

HPWJ Commonly used to remove scale (hard deposits) from tubulars and other types of surface contaminated equipment. Provides effective scale removal and reduces dust generation by keeping the material wet. Should be enclosed and wash water contained, recirculated or filtered to remove scale.

Vacuuming Can be wet or dry processes to remove loose particle contamination.

Chemical cleaning Using commercially available scale dissolvers. Chemicals are also used to remove thin films of NORM from gas plant equipment.

Melting Equipment melting as scrap metal. Most of the NORM ends up in the slag, but volatile radionuclides end up in the off-gas dust and fume.

deposits, resulting in a liquid stream containing the radionuclides from the dissolved scale.

II.3. ABRASIVE METHODS

Dry and wet abrasive methods employing hand-held devices can be applied to remove scale from easily accessible surfaces of components. Dry gritting, milling, grinding and polishing are normally to be avoided because of the risk of spreading radioactive contamination in the air. With wet abrasive methods, this risk is reduced considerably. Consequently, the application of dry abrasive methods needs protective measures for workers and the environment, which can in practice only be provided by specialized companies or organiza-tions.

HPWJ has been shown to be effective for the decontamination of components from oil and gas production (Figs 35 and 36). Water pressures of 10–250 MPa are used, which necessitate the use of special pumps and safety measures. In principle it can be applied on the site and offshore as well as onshore, but its effective and radiologically safe application needs special expertise and provisions to obtain the correct impact of the jet, to contain the recoiling mist and to collect and dispose of the water as well as the scale. HPWJ is usually applied at a limited number of specialized establishments and service companies that are authorized to operate decontamination facilities [55].

Decontamination of tubulars is carried out with the aid of long HPWJ lances fitted with special nozzles that are moved through the whole length of a tubular while the water with the scale is collected at the open ends (Fig. 37). It is relatively easy to contain the recoiling water from tubulars. The application of HPWJ to the outer surfaces of components is strongly complicated by the mist produced by the impact of the jet. In the open air this will cause the spread of the radioactive contamination removed from the object and in enclosed spaces it greatly reduces visibility.

II.4. CHEMICAL DECONTAMINATION

Chemical methods have to be applied when the surfaces to be decontam-inated are not accessible for mechanical treatment, when mechanical treatment would cause unacceptable damage to the components being refurbished, and when the contaminating material is not amenable to mechanical removal.

Usually, components have to be degreased by organic or hot alkaline solvents prior to chemical decontamination. The chemicals used are acids, alkalis and

complexing agents, which are usually applied in agitated baths. Chemical decontamination results in a liquid waste stream containing the dissolving and complexing chemicals, and the matrix and radionuclides of the contaminating material. In many cases, some dissolution of the metal of the component being decontaminated cannot be avoided.

FIG. 35. Workers with personal protective equipment and HPWJ lance (courtesy: Atomic Energy Commission of Syria).

II.5. MELTING

The melting of metallic components contaminated with NORM will separate the metals from the NORM nuclides. The latter end up in the slag or in the off-gas dust and fume. Decontamination by melting is being applied at dedicated melting facilities, but only on a small scale. The typical processes involved in the melting of scrap steel are as follows:

FIG. 36. Workers using HPWJ lance (overhead extractor removes airborne contamination) (courtesy: Atomic Energy Commission of Syria).

(a) Transport to the recycling facility by road, rail or sea, using cranes to load and offload;

(b) Segmentation, by mechanical or thermal means, into sizes suitable for melting;

(c) Loading by crane or conveyor into an electric arc or induction furnace together with iron, fluxes and coke;

(d) Casting of the molten product steel into ingots and mechanized removal of the slag for disposal or reuse;

(e) Recovery and disposal of dust from the off-gas filters.

FIG. 37. Facility for tubular decontamination by HPWJ (courtesy: Atomic Energy Commission of Syria).