2.6.11 - Ca Isotopes in Biological Shield

Introduction

Bioshield concrete is an important structural component of a nuclear reactor, as it acts as a radiological shield. However, when a nuclear reactor is decommissioned, the bioshield concrete presents a significant waste to be treated. The quantification of the long lived activation products is important for the long-term safety of final repositories for nuclear waste.

Some of the most important radionuclides generated in concrete are the Calcium-Isotopes ⁴¹Ca (T_1/2 = 1.03 x 10⁵ a) and ⁴⁵Ca (T_1/2 = 163 d), both with a long biological half-life. They are produced by thermal neutron capture from stable ⁴⁰Ca and ⁴⁴Ca, respectively. ⁴⁵Ca as β-emitting radionuclide with E_max of 257 keV is easily measured by liquid scintillation. ⁴¹Ca decays by pure electron capture, emitting X-rays and Auger electrons with very low energies below 3.6 keV; hence its determination is more difficult due to the strong self-absorption.

Liquid scintillation counting represents an alternative methodology for ⁴⁵Ca quantification and ⁴¹Ca determination in bioshield concretes. As standard, IRMM 3701 material with a known ⁴¹Ca activity of about 6 Bq/g ⁴¹Ca, produced by the Institute for Reference Materials and Methods (Geel, Belgium) is employed for LSC calibration [Warwick et al. 2009].

For measuring details of the electron capture nuclide, especially applying TDCR, see 2.3.11. Detailed descriptions for sample preparation are outlined below.

A second procedure is described, which has been used by Xaolin [Xaolin 2002] for the analysis of concrete samples for the decommissioning of research reactors. It consists of an alkaline fusion for the decomposition of the sample followed by a group separation and further specific separation steps for individual radionuclides (fig. 31).

PROCEDURE 1: Ca-41 in concrete bioshield [Warwick 2009] 

Materials and Equipment

• Sr SPEC resin (EICHROM)

• ⁴¹Ca standard solution (≈ 6 Bq/g ⁴¹Ca; IRMM 3701), Institute for Reference Materials and Methods (Geel, Belgium)

• Lithium borate

• Fe carrier

• Ba carrier

• Conc. HNO₃; 8 M HNO₃

• Conc. HCl

• Saturated Na₂CO₃

• 4 % (NH₄)₂(COO)₂

• Acetate buffer

• 6 M acetic acid

• 1.5 M NaCrO₄

• Ammonia

• Gelating cocktail

Procedure 1 

Pre-concentration steps:

1. Mix the sample (dried ground concrete) with Lithium borate and fuse at 1100 ºC (dissolution of Ca), then dissolve the solid in 50 mL 8 M HNO₃

2. Add 20 mL 4 % (NH₄)₂-oxalate and adjust to pH 5 (precipitation of Ca-oxalate, ¹³³Ba, ²²⁶Ra)

3. Dissolve the precipitate in 5 mL conc. HCl; add 15 mL H₂O and 5 mL 4 % (NH₄)₂-oxalate and adjust the pH to 5 (precipitation of Ca-oxalate)

4. Ignite the precipitate at 500 ºC; dissolve the residue in 5 mL concentrate HCl and add 2 mL conc. HNO₃ and 10 mg of Fe carrier; adjust the pH to 5 to precipitate the Fe(OH)₃ (Ln(III), An(III, IV, V, VI))

5. Add 15 mL of saturated Na₂CO₃ to precipitate CaCO₃

6. Dissolve the CaCO₃ in 10 mL 8 M HNO₃

⁴¹Ca separation:

1. Wash the Sr resin column (4 x 0.5 cm) twice with 10 mL 8 M HNO₃

2. Drain the solution containing CaCO₃ through the column; ⁹⁰Sr is retained on the resin

3. Evaporate the eluent to dryness and dissolve the residue by adding 0.5 mL conc. HCl and 15 mL H₂O

4. Add 10 mg of Ba and 3 mL of acetate buffer; adjust the pH to 5 using 6 M acetic acid

5. Add 1 mL 1.5 M NaCrO₄ and warm up in order to precipitate BaCrO₄ (¹³³Ba, ²²⁶Ra)

6. Adjust the pH to 8 with ammonia; add 15 mL of saturated Na₂CO₃ to precipitate CaCO₃

7. Dissolve the precipitate in 1 mL of conc. HCl; warm up until a green color is observed and elute with 15 mL H₂O; add 10 mg of Fe and adjust to pH 7 with ammonia to precipitate Fe(OH)₃ (residual Cr(III))

8. Add 15 mL of saturated Na₂CO₃ to precipitate CaCO₃

9. Transfer the solid into a LS vial, dry and weigh for recovery

10. Dissolve CaCO₃ in 1 mL conc. HCl, add 3 mL of H₂O and 16 mL of gelating scintillation cocktail

Procedure 2 

Simultaneous determination of ⁴¹Ca in presence of ³⁶Cl, ⁵⁵Fe, ⁶³Ni and ¹²⁹I in concrete [Xaolin 2007]

Materials and Equipment

• Fe, Ni, Ca, Cl and I carriers

• 0.2 M HCl

• NaOH

• Gelating cocktail

Procedure

1. Add to the concrete sample Fe, Ni, Ca, Cl and I carriers and hold-back carriers, NaOH, NaCO₃, and fuse at 500 ºC

2. Leach with water, centrifuge and wash the precipitate with 0.2 M of Na₂CO₃ four times; the resulting supernatant contains Cl, I, Cs and K and can further be processed for ³⁶Cl and ¹²⁹I determination (see fig. 31, left side), while the precipitate contains metals, Ca and Sr.

3. Dissolve the precipitate with HCl; add NaOH until pH 9 and centrifuge the M(OH)x; the precipitate can further be used for the determination of ⁶³Ni and ⁵⁵Fe (see fig. 31, right side)

4. Add Na₂CO₃ to the solution containing Ca, Sr, and Ba, centrifuge and discard the supernatant

5. Dissolve the precipitate (which contains Ca, Ba, Sr, BaCO₃) in HCl and add NaOH to 0.5 M

6. Centrifuge and repeat the procedure 3 times and discard the supernatant containing Sr and Ba

7. Dissolve the ⁴¹Ca containing precipitate in HCl and use this solution for the LS measurement

Evaluation

LSC measurements were applied by the authors for ³⁶Cl, ¹²⁹I, ⁴¹Ca, ⁴⁵Ca, ⁶³Ni and ⁵⁵Fe.

For yield determination the stable Cl, I and Ca tracers have been evaluated by ICP-MS or alternatively ICP-AES.

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

Warwick P. 2009: Effective determination of long-lived nuclide Ca-41 in nuclear reactor bioshield concretes: Comparison of LSC and Accelerator Mass Spectrometry; Anal. Chem. 81 (2009) 1901/1906

Xaolin H. 2007: Radiochemical analysis of radionuclides difficult to measure for characterization of waste in decommissioning of nuclear facilities; J. Radioanal. Nucl. Chem. 273 (2007) 43-8