The present work concerns the "hard decontamination" of a tube bundle installed in a steam generator (SG) removed earlier from the DAMPIERRE 900 MWe PWR, in order to reduce significantly the radioactive dose rate before the final dismantling of the steam generator.
The SG in question is one of the original SGs designed and built by Framatome. They were removed from service in 1990 because of the large number of plugged tubes (approximately 10%) and the numerous primary-to-secondary leaks apparently due to stress corrosion cracking attack (SCCA).
The SGs have been stored since 1990 in a building designed for this purpose and the radioactivity present in each SG, at this time, is estimated to be about 100 Ci (essentially Cobalt 60). The dose rate on the outer surface of the dry SG varies from 0.05 mSv/h to 0.5 mSv/h.
The proposed decontamination process provides using a combination of nitric acid and Cerium nitrate with regeneration of the Cerium (Cerium 3+, Cerium 4+) by injecting ozone during the decontamination operation. The decontamination operation is performed at low pH and at ambient temperature.
After neutralization and precipitation of the decontamination solution containing the removed activity, the residue will be dryed in order to fix it in solid form for storage and disposal.
The objectives of the method selected to decontaminate the SG bundle made of Inconel 600 material are:
1. obtaining a high decontamination factor DF equal to approximately 1000.
2. decontamination at atmospheric pressure and at a temperature of less than 60 C because of doubts concerning tube integrity and the risk of leakage into the secondary side.
3. minimize the volume of generated secondary wastes.
4. filtration of sludge and residues generated by the decontamination process, if possible, using a standard process.
The programme will be implemented jointly between: Framatome (FRA), Electricite de France (EdF) as partners with the assistance of the plant owner Centrale Nucleaire de Dampierre (D) and KWO, owner of Obrigheim NPP which will provide some participation.
A decontamination process was applied to a regenerative heat exchanger removed from a nuclear power unit at Dampierre. The main parameters for the decontamination operation were as follows: nitric acid: Ph = 0.6 - 0.25 to 0.50 mol/l; caesium nitrate: 8 to 12 g/l; ozone: 7 to 8 ppm; fluid circulation speed: approximately from 2.5 cm/s to cm/s.
The results obtained after about 36 hours of decontamination enabled observing: residual activity inside the steam generator (SG) tubes equal to 1 to 3 Becquerels per square centimetre, and residual activity outside the SG tubes equal to 3 to 5 Becquerels per square centimetre, with a hot spot at 20 Becquerels per square centimetre.
The aspect of the stainless steel tubes after decontamination was perfectly clean. The volume of the effluents produced, including the rinsing effluent, was 3.5 cubic metres.
On the internal surface of 116 tubes pulled, the thickness of the deposited oxide layer varied from 0 to 5 um. The most frequent value was 1 um, and the average was 1.18 um. The oxides were sampled by dissolving the base metal in a bromine methanol solution, followed by filtering of this solution. The mass of oxide recovered per unit of surface area was highly variable, with an average of 4 grammes per square metre. On the average, the oxide deposited on the internal surface of the SG tubes was composed of 50% chromium oxide, 30% iron oxide and 20% nickel oxide.
The modifications carried out on the existing decontamination loop were as follows: installation of two 40 cubic metre pumps operating in tandem; modification of the instrumentation and control system to take into account both pumps; modification of the nozzles diameter from 50 to 80 mm; replacement of the 50 mm diameter piping by the 80 mm diameter piping; installations of a 4 kW cooler in the 4.6 cubic metre tank (added in series to the existing 3 kW cooler); making the tank ozone tight; adding a 30 kW cooler to the 1 cubic metre tank; adding a fluid flow reverser, fitted with a bypass; adding sleeves for connecting the pre-embedded filtering system; adding a peristatic pump to transfer the effluents; adding a test cell containing a sample of Dampierre SG tube, equipped with a delta P indicator; adapting the counter to measure the volumes of air and liquid passing in both directions through the piping.
1. Definition of the decontamination and of the waste treatment process.
1.1. Definition of the decontamination process based on the existing oxide layer thickness and the needed time to remove this oxide layer to obtain a DF of approximately 1000 (FRA).
1.2. Implementation of corrosion tests and laboratory studies for the assessment of the corrosion resistance of the equipment in the decontamination loop (EdF).
1.3. Establishing of the procedure for the in-situ decontamination (FRA and EdF).
1.4. Definition of the conditioning of effluents and waste treatment arising from the decontamination of the SG bundle and of shielding and protection requirements (EdF and D).
2. Procurement and adaptation of equipment.
2.1. Adaptation of existing equipment formerly used with an AP Citrox process (EdF).
2.2. Fabrication of the additional decontamination equipment, taking into account the characteristics of the provided decontamination unit, followed by commissioning testing (EdF, FRA).
3. Decontamination operations and liquid waste treatment.
3.1. Preparatory operations on site, including preparation and qualification of operating specifications and procedures (FRA, D).
3.2. Implementation of the decontamination operation (FRA).
3.3. Liquid waste treatment and conditioning (FRA, EdF).
4. Analysis of results including the estimation of the residual activity and the definition of an industrial-scale decontamination procedure (all).
5. Generation of specific data: Specific data on costs, worker exposure, working time and waste arisings will be derived from the execution of items 2. and 3. (all).
Funding SchemeCSC - Cost-sharing contracts