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UNDERWATER PILOT DISMANTLING AT COGEMA, LA HAGUE USING DIVERS TO INSTALL REMOTELY CONTROLLED EQUIPMENT

Objective

The objective of this project is to demonstrate the radiological, technical and economic feasibility of using divers to install remotely controlled cutting and decontamination equipment in a nuclear environment.

The demonstration consists in the underwater dismantling by plasma arc or mechanical tools of a number of stainless steel anti-shock mattresses installed in the bottom of the Pond NO 900 at COGEMA, La Hague after initial decontamination. The specific radioactive inventory is in the order of 3.7 E3 Bq/1, specific contamination is about 200 Bq/cm{2} beta and the dose rate is estimated in the range of 1-30 mSv/h. The contractual work will consist of the preparation of equipment and procedures followed by an industrial-scale dismantling operation.

The project will provide the information required to judge whether the use of divers represents a technical and economically viable alternative to either 'dry' manual or telemanipulation techniques for the decommissioning of nuclear installations and will permit a definition of the conditions in which the use of divers would represent the optimal solution. The project will also result in a validation of diving equipment and procedures for nuclear environments.

The results of the study should be used for both decommissioning and maintenance operations for nuclear power generation and fuel reprocessing plants. The techniques to be demonstrated could be used for installations which are normally immersed or which could be immersed specifically to permit the use of divers during maintenance or decommissioning. Typical decommissioning tasks would include decontamination, dismantling and assistance during removal.

Water is an effective biological shield and as such, intervention under water will permit a reduction in dose uptake when compared to manual intervention in an air atmosphere. Real time monitoring systems will ensure that the diver is subjected neither to contamiantion nor to excessive dose rates.

The project will be carried out by COMEX, NUCLEAIRE (CxN), COGEMA and DIAMANT BOART and will produce specific data on subjects such as dose levels (individual and total) and times and costs for specific tasks, which can be used in the planning of larger-scale decommissioning operations.
Modifications to the diving helmet have been completed. The hard hat helmet equipped with the semiclosed gas circuit has been tested in real conditions during diving operations on several nuclear sites (EdF-CEA), totalling more than 40 hours of use, and all the results obtained meet the specifications in terms of security, efficiency and comfort.

Main advantages of the system include:
total absence of risk of water ingress in the gas circuit and consequently, absence of risk of internal contamination for the diver. The air is rejected on the surfaceand not in the water through a relief valve located on the helmet shell; the air is delivered through a pressure regulator piloted by the water pressure, avoiding the noise generated by the ordinary free flow system which disturbs communications and fatigues the diver after a long time period. It furthermore allows the diver to control more precisely his buoyancy when working in a lying position on the pool bottom.

Helmet main characteristics: hard shell made of woven fibre glass and polyester resin; all metallic parts are made of stainless steel and chrome plated brass; equipped with two earphones and one microphone; weight in air: 10.6 kg; buoyancy in water: slightly negative.

All the specifications of the dose monitoring system have been defined. The detector has a response time that is now sufficiently rapid during exposure to measure low dose rates (2 seconds at 200 uGy/h). Range of measurement: 0.01 Mgy/h to 50 Gy/h; energy response: 60 KeV to 1.2 MeV; size of detector: 20 mm diameter x 85 mm length.

A cable-cutting machine able to test the various cables and cutting modes on a mock-up representing the structure to be cut was fabricated.

A decontamination machine used on pool side walls and bottom has been developed and constructed. The machine is operated by a diver, but can also be installed on a remotely controlled vehicle. The basic element is a rotating arm contained in a square metallic housin g which is moved across the wall to be cleaned. At each end of the rotating arm, two high pressure jetting nozzles directed toward the wall remove the thin layer of radioactive deposits, while be reaction two associated nozzles provide the rotation of the arm. The inside of the box is kept depressurised by means of a pump, avoiding deposits from being dispersed in the surrounding water. The whole system is supplied with energy from a high pressure water line at 350 bars.
Work Programme

1. Preparatory work
1.1 Modification of diver equipment, including safer leak-tightness and provision of real-time dose rate measurement with on-line calculation of integrated job dose at exposed parts of the diver, review and upgrade of existing procedures and regulations for divers in ionizing environment.
1.2 Implementation of comparative underwater cutting tests on flat plates with plasma arc and mechanical sawing, including the assessment of systems for collection of cutting waste and for an easier decontamination (CxN).
1.3 Assessment of cable sawing for metallic structures, also aiming at an application to components of the KRB-A reactor (CxN, Diamond Boart).
1.4 Development and testing of systems for the decontamination/cleaning of walls and bottoms of the storage pond by jetting or brushing (CxN).
1.5 Preparation of documents for the licensing and operation of all equipment at the storage pond for approval by COGEMA (CxN, COGEMA).

2. Execution of the dismantling work including the installation of the equipment, radiological mapping and pre-decontamination of the pool, and the conditioning and removal of dismantling waste and final decontamination of the pond (CxN, COGEMA).

3. Evaluation of results obtained with equipment for divers and for applied tools and procedures (all).

4. Generation of specific data on costs, radioactive job doses, working time and secondary waste arising from the execution of items 1, 2, and 3 (all).

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Comex technologie SA
Address
Boulevard Des Océans
13275 Marseille
France

Participants (2)

Compagnie Générale des Matières Nucléaires (COGEMA)
France
Address
4 Rue Paul Dautier
78141 Vélizy-villacoublay
Diamant Boart SA
Belgium
Address
74,Avenue De Pont De Luttre
1170 Bruxelles