Dr. W. Hawickhorst, R. Finkbeiner, G. Gestermann
GNS Gesellschaft für Nuklear-Service mbH,
Hollestr. 7 A, D-45127 Essen, Germany)

Today activated and heavily contaminated core components can be conditioned in two ways:

1. by underwater cutting in the power station, or
2. by cutting and high pressure compaction in a hot cell.

Whereas the procedure mentioned under a) has been successfully practised for many years, way b) is the result of a new development.

So far, GNS has performed more than 30 conditioning campaign of activated core components by underwater treatment in 17 of the German nuclear power stations, comprising a total quantity of more than 185 tons. About 750 cast iron waste containers of the MOSAIK type, qualified as type B cask for transport, and meeting the stringent criteria for interim storage, have been loaded.

The components treated were mainly water channels, control and absorber elements, in-core instrumentation probes, neutron sources, and a variety of small parts from the reactor cores.

The goal of future treatment of most of these components in a hot cell is to achieve optimised volume reduction by additional high-pressure compaction after previous cutting. The hot cell became operational in September 1997, and so far water channels from a German BWR have been treated.

In order to illustrate the underwater conditioning of activated core components, a campaign performed recently in a German BWR is described.

The components to be treated were absorber elements, in-core instrumentation probes, neutron sources and a number of smaller parts, in total representing a quantity of 2,2 tons, which contained an activity of 2 E15 Bq, the main nuclide being Co-60.

The equipment needed for underwater cutting was transferred into a separate pond adjacent to the main fuel pool by special tools, designed to stringent safety standards as required by German atomic law. The installation of the necessary process and support equipment is illustrated in Fig. 1

In the absorber elements and neutron sources, a high Tritium content had been expected, so special precautions had to be foreseen for it’s collection, balancing and release.

During actual cutting work on the absorber elements, an unexpectedly high gas release, resulting from a gas pressure of over 50 bar in the absorber rods (determined by back-analysis) was observed. The gas was collected by a special device and, after measuring the activity inventory was released through the off-gas system of the power station.

After cutting the absorber rods, the absorber element head pieces were compacted by a compaction device installed in the cutting device.

No Tritium has been found during cutting of the neutron sources.

The incore instrumentation probes consist of single tubular rods into which encapsulated cobalt rods are inserted in predetermined positions. Cutting of the tubular rods had to be done in a way to avoid destruction of the cobalt rods, and by this to avoid cobalt contamination of the fuel pool water. For that purpose, a special electronic length control device had to be used by which cutting the tubular rods in positions between the positions of the cobalt rods could be assured.

After cutting, the pieces were manually inserted into cylindrical stainless steel canisters by special underwater gripping tools, and after loading, these canisters were inserted into shielded cast iron containers of the MOSAIK type. Thereafter the container lid was placed on the container.

As a next step the MOSAIK containers were taken out of the pond, and the lids screwed on to the containers. After decontamination of the external surfaces, the containers were dewatered by means of a special turning device, and thereafter the internals were vacuum dried.

Finally the waste containers were tested for residual humidity of the internals, and for leak-tightness in order to meet the criteria for transport as a type B package.

After determination of the activity inventory, the surface dose rate and the dose rate in 1 meter distance, and after appropriate documentation, the waste packages were shipped to an interim storage facility.

The alternative way to the previously described underwater cutting technique is treating the core components in a hot cell, outside of the power station. By this method, an optimised volume reduction of the waste product can be achieved. This method is a recent development, jointly undertaken by GNS and the Karlsruhe Research Center (FZK). The hot cell facility has been put into active operation in September 1997. In order to transfer the material to the hot cell, special transport equipment is needed.

So, the system consists of two essential parts:

1. A transport cask denominated „MOSAIK 80T"

This cask can accommodate the non-dissembled parts to be treated. The cask is loaded in the fuel pond and prior to shipment to the hot cell its content is dried.

Six of these casks are available for the time being.



Figure 3. MOSAIK 80T/66

2. Hot cell for treatment of the core components

The first application of this system is for treatment of the water channels from BWRs.

Loading of the transport casks in the nuclear power station is done in the same way as for transport of spent fuel elements. The water channels are taken out from their position in the fuel pool by the refuelling machine of the power station, and are inserted into their position of the MOSAIK 80t cask basket. One such cask can accommodate 66 water channels.

After the lid is placed onto the cask body, the cask is taken out of the pool, dewatered, the external surfaces decontaminated, and the internals are vacuum dried in order to avoid water intake into the hot cell.

For transport, the cask is positioned onto a transport frame with integrated impact limiters, positioned on the transport vehicle. Shipment for convenience is done by rail, if the power station is connected to the railway system. Otherwise a heavy load road transport would be possible.

Upon arrival, the transport cask is locked to the hot cell in horizontal position, and the lid of the transport cask is removed by a remotely operated device in the hot cell. Then the basked loaded with the parts to be treated is removed from the cask into the entrance area of the hot cell, and is positioned onto a transfer trolley. This trolley is moving the basket into the operating area of the hot cell, where the water channels are taken out by a heavy load manipulator. The manipulator is loading the water channels into the shear for cutting, and the pieces are transferred into a compactable drum. After filling, the drum is closed and positioned into the high-pressure compactor, where it is compacted; thereafter the compacted pellets are loaded into 200- or 400-liter-drums, which are then positioned in shielded cast iron waste containers.

The high-pressure compactor is equipped with special tools in order to adjust the pellet diameter to the diameter of the internal shielding of the waste container, as required by the activity content of the compacted material.

So far good results have been obtained in cutting of water channels, and it can be expected that after high-pressure compaction a density of the waste product of more than 3,5 g/cm³ can be reached. By that, a considerable volume reduction compared to that of underwater cutting can be realised.

BACK