Decontaminating tubes in ten steps

NUCLEAR POWER PLANTS COMMONLY EMPLOY tube-type heat exchangers as a way of transferring heat to and from the circulating water. Usually the heat exchangers tubes are made of hard alloys, ensuring the tube shape remains stable during the whole period of operation. However, during operation, tubes tend to absorb radiation to both inner and outer surfaces. Radioactive particles get trapped in a shallow layer of oxide film formed on the surface. It is difficult to neutralise such contamination and fully clean the metal alloy in a tube of considerable length. A special process and technology has been created and installed to manage 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 the two RBMK-1500 reactors being decommissioned at Ignalina.

Process overview

The process will fully decontaminate tubes of 28mm diameter and 1mm thickness removed from the heat exchanger of the dismantled power plant. After 21 years of use, the tubes have contamination by Cs-137 (surface ~ 0.8Bq/cm2) and Co-60 (surface ~ 0.4Bq/cm2).

The decontamination technology is shown in Figure 1 and the process is described in Figure 2. It has several stages:

1. The tubes are cut into 6m-long pieces. As required by the decontamination process, the tubes could be cut to any length and could also vary in diameter.

2. Longitudinal cutting is performed from one side of the tube by pushing it to the cutting blade. The blade is made of an alloy of increased hardness and durability. Larger-diameter tubes could be cut from both sides of the tube. It is very important 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 the specific characteristics of the tubes and other factors.

3. After it is cut the tube is split. It is rolled consecutively until it is a completely flat metal strip.

4. The metal strips are loaded vertically into the cassette at a distance from each other that allows space for effective cleaning.

5. Decontamination is done in a succession of baths. The loaded cassette is submerged in three baths: soaking, decontaminating and rinsing. The soaking and rinsing baths are filled with water. The decontamination bath is filled with a 0.5-1.5% solution of nitric acid. The bath is a closed tank with ultrasonic transducers mounted on one side-wall. The ultrasonic waves are generated with wave vectors oriented longitudinally along the planes of the metal strips. The power and frequency should be selected based on the capacity of the bath and the process duration. During ultrasonic cleaning, the contaminated residue separates from the surface of the strips. It falls to the bottom of the second bath, while vapour of the acidic solution rises to the top part of the decontamination bath. Thus, the cleaning is both mechanical and chemical.

6. The re-circulatory filtering system collects particles from the contaminated second solution at the bottom of the second bath by pumping and filtering the liquid through filters. The purified acidic solution is then transferred back to the top of the second bath through a circulation loop. Meanwhile, the contaminated vapour is directed to the vapour neutralising system, where it is condensed, neutralised and filtered. Vapour suction is used to pump out the contaminated vapour from the second bath, emerging during the cleaning process. The vapour is neutralised via a cyclone vacuum suction arrangement, in which a spray of alkaline solution reacts with the acidic vapour and neutralises it. To manage the radioactive residue, the bath narrows at the bottom, and is connected to a recirculating drainage system. The drainage system comprises one or more filtering arrangements, which completely collect the residue and its radioactive particles. The filters are arranged in a double-wall stainless steel container, which can be removed when full, sealed and transported for storage in a repository.

All used and remaining liquids are treated and solidified if they cannot be reused in this or other decontamination projects. It is very important to mention that these liquids should be properly managed and converted to a stable form to be disposed of in repositories.

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

After the metal strips are dried with an air stream generator, there is complete and rigorous checking of radioactivity levels. Because the strips have been completely flattened, it is possible to perform checks for beta radiation close to the decontaminated surface, as required by safety standards. If any radiation is detected the metal strip goes through a repetition of decontamination process. Separate radiation checks can be done at other steps of the process.

After the process is complete, the decontaminated metal strips are ready to be transported out of the plant. It should to be noted that the flattened strip is an efficient form for transportation and storage.

Results

After a trial at Ignalina, the proposed solution proved to be very efficient, decontaminating 3-4t of tubes per day. The contractor decided to speed up the project and reached the output of 10t in two shifts. All the tubes were processed in 12 months in 2015-2016. The cleaned metal strips have been supplied to some big names from the recycling sector in Western Europe for the industrial reuse and they are satisfied with the quality of the material. The technology and its separate parts are easy to manufacture and operate and can be easily adjusted for a specific area within any power plant. The clean longitudinal cutting and complex treatment of the contaminated material, makes it efficient in of safe, cost-effective and timely decommissioning.

Future applications

Every nuclear power plant must efficiently and safely decommission various systems of contaminated tubes. In this example, the proposed solution and technology fully decontaminate the heat exchanger tubes.

The technology can be used for tubes of various length and diameters, so it can be applied for many systems and for tubes made from variety of different metals and their alloys. It also reduces the personnel required for specific decommissioning projects and almost all stages can be fully automated, further increasing the efficiency. The solution and technology could be used by a large number of power plants and some other industrial entities being decommissioned.  

Author information: Vytautas Daniulaitis, CEO and founder of Aksonas Ltd; Peter Ziv, Technical director of the project