DESCRIPTION OF DISMANTLING METHODS SELECTION 1. Early dismantling operations to facilitate reactor dismantling

After preliminary EDF studies, it was proposed to extract the steel assemblies and dismantle all the removable components crossing the slab in order to facilitate dismantling of the reactor block. The presence of retentions in the base of steel assemblies and under the components, could exclude underwater dismantling, for reasons of acceptability in terms of safety.

The components will be replaced by plugs:

— Restoring the tightness of the slab containment barrier;

— Ensuring biological shielding of the slab zone after sodium draining.

Before beginning reactor dismantling:

— All its components will have been removed from the slab (except for the core cover plug and the rotating plugs);

— The steel assemblies will be extracted from the vessel.

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2.2. Earliest possible reduction of risks The sources of risks are:

— The presence of intense radioactive sources mainly consisting of certain radioactive reactor structures;

— The presence of metallic sodium on the internal structures.

As far as possible, the following risk sources will be eliminated at the earliest possible date:

— Metallic sodium;

— Radioactive sources.

2.2.1. Presence of intense radioactive sources

The presence of very radioactive structures is the source of difficulties for dismantling operations:

— Need to carry out certain operations by remote means;

— Risk of personnel radiation;

— Waste from dismantling the reactor structures attributed to different radiological categories;

— Difficulties of handling and packaging the most radioactive waste.

The result was to make recommendations for the elimination of the highly irradiated structures as soon as possible, i.e. right at the start of the dismantling operation (insofar as this is technically feasible), that is to say, to implement provisions aimed at assured reductions of the ambient dose rate. Therefore, the recommendations are:

— The most irradiant structures will be eliminated as early as possible;

— In view of the radiological conditions in the vessel after sodium draining, the operations will be performed by remotely operated equipment, at least until the environmental conditions authorize direct human interventions.

2.2.2. Presence of metallic sodium on the internal structures and the main reactor vessel 2.2.2.1. Reduction of metallic sodium related risk

Metallic sodium bas certain disadvantages: it is highly reactive with water and with other bodies, reaction products such as caustic soda and hydrogen can themselves be sources of risks. There are several methods of neutralization:

— Neutralization by oxidation;

— Neutralization by reaction with liquid water or a body associated with water;

— Neutralization by a mixture of carbon dioxide and steam that leads to the formation of sodium carbonate.

The carbonates obtained can be placed in a water solution during a washing operation. The process of carbonation by a mixture of nitrogen/carbon dioxide/steam is well documented, as well as the influence of the various parameters ([CO₂], [H₂O], T, treatment gas flow) on the nature of the bodies obtained, and on the kinetics: these parameters and their impact was determined by the ‘CARNAC’

tests carried out by the CEA at Cadarache;

— EDF have a feed back of SPX drum tank treatment by carbonation process;

— Exchanges with EBR II on Laboratory test, secondary circuits and primary vessel carbonation treatment.

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After draining the reactor block, the residual metallic sodium is neutralized by carbonation. Total neutralization of the sodium cannot be guaranteed: metallic sodium may remain under the layer of carbonate and has to be taken into account for dismantling operations.

Carbonation of the sodium films should not present any particular problem.

Thickness of the sodium that can be ‘carbonated in a reasonable time (a few months)’ probably has limits: it will be difficult to neutralize the sodium on all the retention thickness if this exceeds a few centimeters due to the formation of a layer of carbonates covering the metallic sodium and preventing the uniform diffusion of the carbonation gas. In order to neutralize the largest possible quantity of sodium:

— The treatment gas must be able to reach the metallic sodium retentions;

— The treatment gas must be distributed as uniformly as possible in the vessel;

— There is as little retention as possible and these are as thin as possible.

After draining of the reactor block and before the residual sodium carbonation operation, the retentions will be treated in order to eliminate them or reduce the thickness.

2.2.2.2. Treatment of retentions

During 2000, it was proposed to treat (by drilling or siphoning) the five main retentions before draining the vessel, and – at the end of 2001 – to withdraw the steel assemblies and all the removable components crossing the slab.

Given that the removal of the primary pumps win give access to certain retentions, it appears convenient to examine the merits of treating them. In particular, the ‘feasibility of demonstrating safety’ aspect will be incorporated in the reflection as well as the technical and the cost aspects. The retentions will be treated as thoroughly as technical and economic considerations permit, taking into account the method of dismantling adopted and the associated safety studies.

2.3. Best achievable containment

During reactor vessel dismantling operations, the containment boundary will consist of the main vessel and the underside of the slab. The containment would then consist of the plugs sealing the penetrations left after extraction of the removable components, or airlocks for the transfers into and out of the vessel.

2.4. The ALARA approach

Dismantling operations will be performed using remotely operated equipment due to the dose rates encountered in the vessel. Direct human interventions will only be considered after withdrawal of the most irradiating structures or after implementation of measures to reduce radiation (removable protections or immersion of structures under water). The dosimetry targets will be defined and may require reviewing of the equipment used. In the field of radiation protection, all the dismantling operations will be carried out based on an ALARA approach.

3. ORIENTATIONS

The following decisions have been taken based on the ideas put forward and the dismantling methods selected.

3.1. Rejection of dismantling by the bottom

This would be a delicate operation as the components are situated between 2 and 20 m above floor level. If dismantling via the bottom is decided, i.e. transfer of the containment to the wall of the vessel pit. This increases the risk of carbonate dispersal to very probable when dismantling with sodium carbonate and completely excludes underwater dismantling. Lastly, dismantling by the bottom will not eliminate the most irradiating structures at the earliest stage, with the result that the operations become more delicate to execute. It was thus decided to reject this method.

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3.2. Operations prior to the two dismantling methods envisaged

Whichever scenario is adopted out of the two dismantling methods considered, the operations will have to be preceded by carbonation of the sodium remaining in the vessel. This sodium neutralization phase is necessary to reduce the risk.

This phase consists of several operations which could be as follows:

— Inspection of the vessel internal structures after the draining operation;

— Carbonation to optimize the parameters based on preset targets (nature of the carbonates required, carbonation time limit, etc);

— Inspection of the vessel internals in order to check the efficiency of this operation;

— Additional treatments if necessary, according to the selected dismantling scenario.

Whichever scenario is envisaged it may be technically preferable to perform thorough carbonation. It will be remembered that:

— In the case of the underwater dismantling scenario, the reduced quantities of sodium remaining under the carbonate will facilitate filling of the vessel with water and reduce the safety constraints;

— In the case of the dry dismantling scenario, any reduction in the quantity of metallic sodium remaining under the carbonates will reduce the precautions to be taken during dismantling operations.

3.2.1. Treatment of the retentions

After draining of the reactor vessel ready for sodium treatment, elimination of the steel assemblies and the components crossing the slab, and without other actions than the elimination of the five main retentions, the estimated quantities of sodium are:

Wetted surfaces (films) ≈ 1.3 m³ Retentions ≈ 1.3 m³

The first observation is that the sodium coating will be completely eliminated during the carbonation operation, provided that the carbonation fluid is circu1ated uniformly in the vessel. Removing the reactor coolant pumps provides the opportunity for treating certain retentions which would not be accessible otherwise. If we only treat the retentions that are directly accessible after withdrawal of the reactor coolant pumps, it is estimated that a further 600 L of sodium could be eliminated. This would leave only approximately 700 L of sodium in retentions in the vessel (before carbonation).

Retentions ≈ 0.7 m³

At present, we are able to guarantee that 10 mm of sodium would be neutralized by carbonation (provided that it takes place in the best possible conditions, i.e the retentions are accessible to the treatment gas and this gas can be renewed). However, the CEA glovebox tests have shown that several tens of millimeters of sodium can be at least carbonated in laboratory conditions. Therefore we have set a target which is considered ‘reasonably achievable’ and which is to carbonate 20 mm of sodium.

On the basis of these assumptions, between 100 and 200 L of metallic sodium will be left under the carbonates in the vessel. Taking into account the residual retentions, it seems at present that the underwater dismantling scenario can be applied.

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FIG. 1. View of the reactor.

FIG. 2.Dismantling methods envisaged.

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DECOMMISSIONING OF THE RAPSODIE FAST REACTOR:

DEVELOPING A STRATEGY J-M. GOUBOT, J. FONTAINE

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