The first Czecho-slovak nuclear power plant A-1 was shut down after an accident in 1977 and it is now under decommissioning. In spite of that, some technological systems in nuclear power plant must remain in operation. The planned service life of these systems has already been exceeded and their technical design is not suitable for present requirements. These operated technological systems must be repaired and reconstructed.

The special drainage system is one of the technological systems that must be in operation during the decommissioning process. After twenty-nine-year’-s of operation it became necessary to carry out the repairs, adaptations and replacements of some parts of this system with the aim to ensure more reliable operation and higher safety.

Radioactive sludge from the bottom of the collecting tank and drain pit was pumped out using an ejector and it was caught in drums. The fixed contamination on the inner surface of the collecting tank was removed by the chemical loop decontamination method using an external circulating decontamination device.

The surface of the drain pit was decontaminated with evenly spread decontamination gel that disrupted the fixed contamination. The contamination was then washed down using a water jet system with attached rotary brushes.

At NPP A-1, an immobilisation method for the treatment of radioactive sludge and solid particles based on the addition of silicates was successfully used by AllDeco for the first time. This sludge is often sticky with the high tendency to settle out the liquid and has a specific activity three orders of magnitude higher than are the limits for the Conditioning Centre (with high content of alpha radionuclides) and because it is not sound and effective to treat it by technologies generally used for another type of wastes. The immobilisation of the sludge was performed at room temperature. The resulting solid product has suitable properties for temporary storage. Procedures for the verification of the treatment of further sludge from NPP A-1 have been started.

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