DECONTAMINATION OF TUBES

Number of nuclear power plants, especially of the older design, commonly employ tube-type heat exchangers as a way of transferring or collecting heat energy to and from the circulating water. Usually tubes of these heat exchangers are made of rather hard alloys ensuring a stable shape of the tube during the whole period of a power plant operation. However, during years of operation, tubes in such systems tend to absorb radiation to both inner and outer surfaces. Radioactive particles get trapped in a shallow layer of an oxide film, which is formed on the surface. Such contamination is difficult to neutralize and fully clean the metal alloy in form of a tube of considerable length. The special process and technology are created and installed to solve the problem of decontamination of more than 2000 tons of hard copper alloy (Cu 90,6% – 93.7%, Ni+Co 5.0 – 6.5%) tubes from the heat exchangers of both units of Ignalina NPP, which decommissions two RBMK-1500 reactors.

DECONTAMINATION PROCESS

The process and technology fully decontaminate tubes of 28 mm diameter and 1 mm of thickness which are retrieved from the heat exchanger of the dismantled power plant. During 21 years of the use tubes have the contamination of Cs-137 (surface ~ 0.8 Bq/cm2) and Co-60 (surface ~ 0.4 Bq/cm2).

The entire process consists of several main stages (in accordance with the schematic drawing):

1. The tubes are cut to the pieces of 6 meters in length. It could be admitted, that if required by a specific task of decontamination, tubes also could be cut to any other length and could vary in diameter significantly too.
2. Longitudinal cutting is performed from one side of the tube by pushing it to the cutting blade. The blade is made of special hard alloy of increased hardness and durability. In case of bigger diameter of tubes, the cutting could be performed from two sides of the tube. It is very important to notice that cutting is done without any shavings or dust. This not only prevents a loss of metal but also ensures that the contaminated dust or shavings are not spread. Laser, plasma or waterjet cutting could be employed depending on specific characteristics of tubes and other factors of an exact decommissioning project.
3. After the cutting the tube is pushed and gradually splitted as well as flattened by multiple splitting rolls into completely flat metal strip.
4. The metal strips are loaded vertically to the special cassette in the specific distance from each other allowing enough space for effective cleaning. The strips can be loaded to the cassette in levels.
5. Decontamination is done in special bathes arranged in specific order. The loaded cassette is submerged with the lifting means in three bathes: soaking, decontaminating and rinsing. Soaking and rinsing bathes are filled with water for this specific task. The decontamination bath is filled with a nitric acid solution of 0,5-1,5%. The bath itself is a closed tank where ultrasonic transducers are mounted on one side-wall of it. The ultrasonic waves are generated with wave vectors oriented longitudinally along the planes of the metal strips. These ultrasonic wave generators with a certain power and frequency should be selected based on the capacity of the bath and desired process duration. During the ultrasonic cleaning, the contaminated residue separates from the surface of the strips. It descends to the bottom part of the second bath, whereas vapour of the acidic solution rises to the top part of the decontamination bath. Thus, the cleaning is both mechanical a chemical.
6. The re-circulatory filtering system collects particles from the contaminated second solution from the bottom of the second bath by pumping and filtering the liquid throughout the special filters. The purified acidic solution is then transferred back to the top of the second bath through the circulation loop. Nonetheless, the contaminated vapour is directed to the vapour neutralising means, where it is further condensed, neutralized and accordingly filtered. Vapour suction means are arranged to pump out the contaminated vapour from the second bath, emerging during the cleaning process. The vapour neutralising means comprise a cyclone vacuum suction arrangement, which provides a spray of alkaline solution, which reacts with acidic vapour and neutralizes it. For managing the removed radioactive residue, the bath has a conical bottom – a narrowing part at the bottom, which is connected to a recirculating drainage system. The drainage system comprises one or more filtering arrangements, which completely collect residue with radioactive particles. The filters are arranged in a double-wall stainless steel capsule – container, which can be removed when full, sealed and transported for storage in a repository.

7. All used and remaining liquids are treated and solidified if can’t be used in this or other decontamination projects. It’s very important to mention that these liquids should be properly managed to a stable form to be disposed in repositories.

8. During the next stage the cassette is rotated in the rinsing bath and thoroughly cleaned with high-pressure water stream. In this washing step, both evaporated acidic solution, and radioactive debris are also properly treated.

9. After the drying of the metal strips with air stream generator, there is complete and rigorous checking of radioactivity levels. It must be admitted that because of the fully flat shape of the strip, it is possible to perform the check for Beta radiation in the closest proximity of the decontaminated surface as required by the safety standards. If any radiation is detected the metal strips goes to the repetition of decontamination process.Separate radiation checks can be done at other steps of the process as well.

10. After the completion of the entire process, decontaminated metal strips are ready to be transported out of the plant. Also, it should to be noted that the flat form of the strips is the most efficient for transportation and storage.

EFFICIENCY OF THE SOLUTION

The proposed solution proved to be very efficient as 3-4 tons of tubes can be decontaminated per day. The contractor decided to do speed up the project and reached the output of 10 tons in two shifts. The cleaned metal strips have been supplied to some big names from the recycling sector in Western Europe for the industrial re-use and buyers were satisfied with the quality of the received material.  The technology itself and its separate parts are rather easy to manufacture and operate. It has necessary flexibility to be adjusted for a specific area within any power plant. There are some unique features and solutions like clean longitudinal cutting and complex treatment of the contaminated material, which makes it highly efficient in many aspects of safe, cost-effective and timely decommissioning.

APPLICATION IN OTHER PROJECTS

Every nuclear power plant must address the tasks of efficiency and safety in decommissioning projects of various systems of contaminated tubes. In this example, the proposed solution and technology fully solve the task of decontamination of the tubes from the heat exchanger. Because of a possibility for more wide and universal application of the technology for tubes of various length and diameters, it can be applied for many systems employing tubes made from variety of different metals and their alloys. It also reduces a man power required for specific decommissioning projects and can be fully automated further increasing the efficiency in almost all stages. All these features ensure that the solution and technology proposed could be considered by a broad number of power plants and some other industrial entities under decommissioning.