2.6.10 - Fe-55 and Ni-63 in Radioactive Waste

⁵⁵Fe and ⁶³Ni are both neutron activation products present in reactor derived solid low level wastes and effluents (i.e. the reactor core, the graphite and the construction materials like the concrete shield and the metal components). ⁶³Ni is considered as an important radionuclide for the nuclear waste repository. The separation of ⁶³Ni can be achieved by the formation of a Ni-DMG complex either on an Eichrom column or as batch method (see 2.3.8.). This step can be combined with ammonium hydroxide precipitation, organic extraction of the complex, chelation and anion exchange for its efficient separation from interfering radionuclides and matrices.

Due to the high concentration of Iron as a component of the steel, ⁵⁵Fe is a main contributor for nuclear waste in the first years after final reactor shut down. It must be completely separated from other radionuclides prior to its quantification. One of the most efficient methodologies for the purification of the ⁵⁵Fe is the chelation and anion exchange chromatography based on di-isobutyl-keton combined with solvent extraction.

Both ⁶³Ni and ⁵⁵Fe are separated and purified by different methodologies according to the nature of the waste sample. LSC is a suitable technique for its final measurement (see 2.3.7. and 2.3.8.). Chemical and color quenching effects are important and need to be taken into account.

The method presented below [Jäggi et al. 2009] describes the separation of ⁶³Ni from ⁶⁰Co and ⁵⁵Fe from a radioactive waste matrix. The authors applied a n-activated Fe tracer for the chemical yield determination. The procedure is done with two samples in parallel (spiked sample SS and control sample CS) for calculating the efficiency and chemical yield. It has been included in our Handbook as general example.

Materials and Equipment

• BIO-RAD AG1-X8 resin (Chloride form, 100-200 mesh)

• HNO₃ (65 %); HNO₃ (8 and 3 M)

• HCl (9M, 4M, 1M and 0.5 M); HCl concentrated (36-38 %)

• H₂O₂ (30 %)

• Ammonia (25 %)

• Ethanol (100 %)

• DMG

• Ni carrier

• Ascorbic acid

• Citric acid

• Microwave

• Teflon beakers

• Fiber filter

• Gelating cocktail

Procedure [Jäggi et al. 2009]

Dissolution using microwave:

1. Dry the sample at 40 ºC and glow at 500 ºC; add to a 2 g sample each in a Teflon beaker 8 mL HNO₃ (65 %) and 2 mL of H₂O₂ (30 %) for partial digestion in a microwave (during 2 h, 7 min at 1000 W heated to 140 ºC; 8 min at 1000 W heated to 210 ºC; 40 min at 600 W kept at 210 ºC)

2. Collect the cool sample, filter and wash with 8 M HNO_3; keep the filtrate for further chemical separation

Separation of Ni, Co and Fe:

1. 20 mL of the obtained filtrate are transferred into two 25 mL beakers (spiked sample SS and control sample CS); 100 µm of inactive Ni is added as carrier.

2. Evaporate the mixture to fume off nitrate, and twice evaporate again with 1 mL of conc. HCl to change to HCl medium

3. Dissolve the residue in 10 mL 9 M HCl and 0.5 mL H₂O₂ (30 %); simmer for 30 min (stirring) and cool down

4. Prepare the anion exchanger BIO-RAD 1-X8 column (2 g in distilled water) and condition by flushing with 10 mL 9 M HCl

5. Drain the solution through the column, rinse twice with 9 M HCl and collect the Ni/Cr fraction in a beaker for further Ni treatment

6. Strip Co twice with 6 mL 4 M HCl and prepare for γ-measurement

7. Elute Fe three times with 10 mL 1 M HCl into a 50 mL glass beaker

8. Evaporate the solution, dissolve with 10 mL 0.5 M HCl and add 20 mg of ascorbic acid to minimize color quenching

9. Transfer to a LSC vial, add 10 mL gelating cocktail and measure ⁵⁵Fe

Precipitation of Ni:

1. Dilute the Ni fraction with distilled water to 100 mL and add 1 g of citric acid (during stirring) to complex the Ca²⁺ for avoiding disturbance in the subsequent Ni precipitation

2. Add ammonia (25 %) until pH 10 (if residue is observed, leave the sample during 24 h and filtrate to continue the precipitation)

3. Add 25 ml of DMG (1 g DMG and ethanol (100 %) to 100 g) to precipitate Ni (during 30 min, stirring but not heating)

4. Filter the precipitate by using a fiber filter and rinse it with distilled water

5. Elute Ni into a 25 mL glass beaker by adding four times 2.5 mL of 3 M HNO₃

6. Evaporate to dryness, add 1 mL of conc. HCl (36-38 %) and evaporate again

7. Dissolve the residue by adding 10 mL of 0.5 M HCl

8. Transfer the solution into a LS vial, add 10 mL of gelating cocktail and measure Ni

Evaluation

Color quench correction is done with a series of solutions by increasing concentration of FeCl₃ (Fe III valent, yellow color) and NiCl₂ (Ni II valent, turquoise) in 10 mL of 0.5 M HCl according to the procedure described in chapter 2.1.1.

The calculation of the chemical yield is described below in more detail as general example.

1. The blank samples are measured and their counts are substracted to calculate R_N1, R_N1# and R_N2.

2. The activity A of the internal standardization process can be calculated by the radioactivity of the net counting rate R_N to the counting efficiency ε, which is the percentage of emission events that produce a detectable pulse of photons in cpm/dpm (counts per minute to disintegrations per minute).

3. Calculation of the efficiency ε₁ by using the spiked control sample activity A_1# (internal standardization)

R_N1# is the counting rate of the spiked control sample.

4. The correction factor for the chemical yield h is calculated assuming that control sample (CS) and spiked sample (SS) undergo the same chemical loss within a certain analytical uncertainty. R_N2 and A₂ are the counting rate and activity of the spiked sample. Both samples (spiked and control) are considered to be chemically equal, therefore in the equation it is assumed that ε₁ = ε_2. This is valid for quenching parameter (e.g. tSIE) being equal for the spiked and control sample.

5. The sample activity of the unspiked control sample A₁ is calculated by adding the chemical yield η in order to take into account the chemical loss of the tracer.

Lower Limit of Detection LLD: 50 mBq per sample (without preconcentration)

Jäggi M., Rüthi M. and Eikenberg J. 2009: Method for ⁵⁵Fe and ⁶³Ni determination by LSC in radioactive waste; in: J. Eikenberg et.al. “LSC2008 Advances in Liquid Scintillation Spectrometry”, pp 31-39, Radiocarbon 2009, Tucson