Nuclear Decommissioning activities generate a huge variety and quantity of radioactive metallic waste that has to be properly managed. Most of them are only superficially contaminated. The superficial contamination usually involves a thickness of about tens of µm and is due to the gathering of radionuclides such as Fe-55, Co-60, Sr-90, Ni-63, Ni-59, Cs-137, inside the pores as iron’s contaminants in the metallic form (Me) or as metal oxide (Me₂O_x). Developing decontamination technology for such metallic waste enables reuse of the biggest part of the material in the industry, after a suitable treatment that has to enable the waste volume reduction and to minimize the generation of secondary waste. The decontamination of the metallic scrap is based on the dissolution of the contaminants in a suitable solution, subsequently recovered as insoluble salts. Contaminants as metals and metal oxides could be dissolved by using a suitable acid:

• Metal + acid salt + hydrogen
• Metal oxide + acid salt + water

H⁺ coming from the acid could be a good oxidant for alkali (Ca, Sr, Ba, …) and transition metals (Fe, Co, Ni, …), as shown in Table 1.

Table 1. Standard Reduction Potentials at 25°C

In the pickling process applied in this exercise, an acid at high concentration has to be selected. The choice has to take into consideration the next step of precipitation, in which contaminants will be recovered in the form of insoluble salts.

Table 2. Salts solubility

High temperature favors the dissolution from a kinetic and thermodynamic point of view. The process is very slow at room temperature, a temperature around 50°C is high enough to speed up the dissolution. At this temperature the process could last several hours, even days in the case of disadvantageous surface/volume ratio. Too high temperatures however could lead to evaporation issues.

The so-obtained solution, called ferrous solution, has a dark green color, due to the presence of Fe(II), and also contains radioactive contaminants. At the end of the pickling, metal scraps contamination could be under the clearance level and after a proper radiometric measurement they can be released to be reused in the industry. The clearance level is the value below which any materials can be released from regulatory control with negligible risk from a radiation protection point of view. Remember that the clearance levels for the release of materials as scraps established in the Radiation Protection 122 document are 0.1 Bq/g for Co-60, 1 Bq/g for Cs-137, 1 Bq/g for Sr-90 and 100 Bq/g for Ni-63.

The lab has received contaminated pipes that have to be decontaminated. You can find sample information (sample size, contaminated layer, ..) in the sample data sheet. To do that, you need to dissolve the superficial layer with an acid.

You will need the proper amount of acid according to the layer to be removed, taking as iron target concentration 100 g/L, so remember to take note of the data necessary for calculations by the Store button on the balance. Pay attention to properly manipulate the contaminated sample and acids.

Then, prepare the experimental set-up: you will need to control temperature and evaporation.

Pay attention, you are working with radioactive materials!

In the first quest you have to weigh the sample. Please go to the Balance where you will find the container with the metal scraps to be decontaminated. The equipment consists of:

1. Plastic weighing boat
2. Stainless steel tweezers

Before proceeding with the experiment, you need to calculate the amount of acid at the computer and weigh it. To do so, you will need:

1. Bottle of acid
2. 0.5 L Beaker

Then, you have to prepare all the equipment needed for the pickling experiment. The equipment consists of:

1. Multi neck (3) reaction flask
2. Thermometer
3. Condenser
4. Thermostatic bath

Orient yourself in the labroom and find the proper desks, chemical cabinet, glassware cabinet, thermostatic bath and computer. When you became familiar with the environment, continue to fulfill the tasks indicated in the top-left corner and/or in the Quest journey [Q].