Chemical cleaning of fuel assemblies30 August 2001
The first ever chemical cleaning of used fuel assemblies was carried out at Paks 2. The unit is now running on full power with the cleaned and refuelled fuel assemblies. By GüNTER MARQUARDT & JANOS SCHUNK
A total of 149 fuel assemblies (FAs) and 21 control rods at Hungary’s Paks 2 (VVER 440) were successfully cleaned using the well-known chemical decontamination process HP/CORD UV. This process – chemical oxidation reduction decontamination (CORD) using permanganic acid (HP) and ultra-violet light (UV) – has been used about 300 times for decontamination of components, subsystems and full systems, but never before for fuel assemblies. As a result, premature scrapping of the FAs was not necessary.
The FAs of the Russian manufacturer MSZ that needed cleaning had spent between 0.5 and 2.5 years in the reactor core. The chemical cleaning was required because of corrosion products that were deposited in the FAs during power operation. This caused a restriction of the coolant flow through the FAs. As a consequence, the differential pressure drop increased and temperature asymmetry was found in the reactor core during normal operation.
Based on differential pressure measurements that were carried out by Paks using a PAFLO measuring device, it was possible to determine the extent of contamination of the FAs. The FAs with too high differential pressure could therefore be separated and then subjected to the chemical cleaning process.
MSZ examined and certified the general suitability and acceptability of the CORD process prior to the start of the cleaning. Seven fuel assemblies were subjected to a preliminary cleaning test after approval by the Nuclear Safety Inspectorate of the Hungarian Atomic Energy Authority. Based on the results obtained, the Authority and MSZ agreed to the cleaning of the remaining 163 fuel assemblies.
HP/CORD UV process
The HP/CORD UV cleaning process was carried out in connection with Framatome ANP’s mobile decontamination system AMDA (Automated Mobile Decontamination Appliance).
AMDA was used to establish the required flow rate through the cleaning loop and FAs. A special cleaning tank for insertion of the FAs was designed. Seven fuel assemblies could be cleaned simultaneously, and the amount of cleaning could be controlled by carrying out differential pressure measurements during the cleaning process.
HP/CORD UV can dissolve corrosion products deposited on stainless steel surfaces. The advantages of the CORD technology, for HP/CORD UV and all members of the “CORD family”, are:
•Oxidation with permanganic acid – the first step in a cleaning cycle (oxidation-cleaning-decompostion).
•Reduction/decontamination with cleaning chemical.
•Entire decontamination is done with only one fill of water – no intermediate rinse is necessary.
•Regenerative process. The cleaning solution is regenerated on interaction with ion exchange resins.
•Complete oxidative, in-situ decomposition of the decontamination acid to carbon dioxide at the end of the decontamination process. The CORD cleaning chemical, oxalic acid, is decomposed to water and carbon dioxide by UV light at the end of the process. Removal of the used cleaning chemical by ion exchange resins – which have to be disposed as radioactive waste – is therefore unnecessary. In comparison with other cleaning processes, HP/CORD UV saves on final storage costs.
•Chelate-free waste (no decontamination acid in waste).
•Little waste generated.
For the fuel assembly cleaning, the circuit is filled with boric acid solution (>12g/kg) and heated up to 92.5°C. After performing the basic differential pressure measurements the CORD chemicals are injected into the system. The boric acid solution – needed for neutron flux control – remains during the process and is chemically neutral. Dissolved ions of activity and corrosion products (Fe, Cr, Ni), as well as the manganese ions from the oxidation stage, are continuously removed by a bypass cleanup through ion exchanger resins throughout the cleaning step. At the end of each step, the cleaning chemical is decomposed to water and CO2 by an ultraviolet light source in the presence of peroxide, and differential pressure measurements on each FA are performed.
The water quality after completion of the cleaning is comparable to before cleaning, allowing for the cleaning circuit/ cleaning tank to be opened into pool number 1, which opens to the spent fuel storage pool.
The oxidation-reduction-cleaning–decomposition steps can be repeated as many times as required. Due to the structure of the top oxide layer, the first cycle is always carried out without the initial oxidation step. After the first cleaning cycle a second – and, if necessary, a third – cleaning cycle with oxidation by permanganic acid may be carried out. This oxidises the chromium in the Fe-Cr-Ni oxide structure, enabling any remaining oxides to be dissolved.
For the most blocked FAs up to three CORD UV cycles were necessary for sufficient cleaning; less blocked FAs were cleaned with one cycle.
Assessment of process
Assessment of the cleaning effectiveness was done by differential pressure measurements inside the cleaning tank before and after application of the chemical cleaning process. This was particularly important since the conditions did not permit the use of separate measuring equipment in parallel to the cleaning equipment. Based on these measurements and calculations, it was possible to measure the extent of cleaning for all 170 FAs.
Another important requirement was to monitor the integrity of FAs during the chemical cleaning process. This was done by continuous online monitoring of the Krypton-85 isotope in the non-pressurised surge tank of the AMDA cleaning system. The fission product Krypton-85 is contained inside the fuel rod cladding and would penetrate into the cleaning solvent in case of a fuel rod defect.
The AMDA cleaning equipment was positioned on the reactor 2 panel while the controls were positioned one level below in order to minimise personnel dose exposure. The cleaning tank was submerged into pool number 1, connected to the AMDA with hoses and finally covered with boric acid solution to a height of 14m. It was placed in such a way that the individual tank positions could be approached with the refuelling machine while in automatic mode.
This first ever fuel assembly chemical cleaning produced the following results:
•The flow-restricting corrosion product deposits were completely removed.
•Continuous Kr-85 monitoring demonstrated that the CORD process did not influence the tightness of any of the 170 FAs.
•Pressure losses of the cleaned FAs were identical to the new ones.
•Over 12kg of iron was removed from the FAs.
These results were approved by MSZ, who observed the cleaning process and also acted as a supervising authority.
All corrosion products dissolved during the cleaning process, and the manganese ions from the permanganic acid oxidation step were transferred onto ion exchanger resins. Due to the fact that the cleaning chemicals were decomposed to CO2 and water, only 914 litres of resins were produced as radioactive waste. The resins were flushed out to the spent resin tank of the plant for further processing.
Because of the successful chemical cleaning, the fuel assemblies could be returned to the fuel cycle. During the March 2001 outage, unit 2 was partially refuelled with the cleaned FAs and operation restarted. No hydraulic, pressure and temperature anomalies have been detected in the reactor core.