2.6.12 - Fe-55 and Ca-41 in Decommissioning Activities by TDCR-LSC

Introduction

In operating Nuclear Power Plants (NPP), γ-emitting radionuclides with relatively short half-lives are of major importance for radioactivity control.

In decommissioning activities, in contrast, long lived mostly pure α- and β-emitters as well as electron capture EC nuclides play a dominant role because of incorporation risk. They include the Pu-isotopes as α-emitters (239, 240, 242 and 244), the β-emitters ³H, ¹⁴C, ²⁴¹Pu, ⁶³Ni, ^89,90Sr and the EC nuclides ⁵⁵Fe and ⁴¹Ca beside others. The latter have to be analyzed for the characterization of radioactive waste. While for α-emitters mainly a-spectrometry is applied, for β-emitters, especially for the low energy ones, Liquid Scintillation is unalterable. LS spectrometry is also the only choice for EC nuclides.

For ⁸⁹Sr with E_βmax = 1.5 MeV, Cerenkov counting in LS counters is a suitable alternate, similar to ⁹⁰Sr when detected through the high energetic daughter ⁹⁰Y (E_βmax = 2.3 MeV) (see 2.3.5.).

A crucial step is the efficiency calibration because standard solutions are not always accessible. With the HIDEX 300SL instrumentation, the TDCR method for automatic quench correction has been introduced (see chapter 1.6.). The functionality as standard method has been proven for the low energetic β-emitters ³H, ²⁴¹Pu and ⁶³Ni in several publications. In recent years, TDCR for quantifying Cerenkov counting of ^89,90Sr and ⁹⁰Sr/⁹⁰Y has been suggested [Frenzel et al. 2013], see procedure 2.3.6. Several presentations on applications of the TDCR-LSC method have been presented at the recent Conferences LSC2013 in Barcelona and LSC 2017 in Copenhagen.

During the process of EC, a gap in the inner electron shell is created, which is filled up by the transition of outer electrons. The resulting characteristic X-rays are either emitted or are converted into Auger-electrons from the outer shell. These Auger-electrons as far as above 1 keV, activate the scintillator medium and create a low energy pulse. The low counting efficiencies between 15 and 45 % is subject to a high degree of quenching. Thus an efficient quench correction is precondition for the practical LS application.

We describe below a procedure for the determination of the EC nuclides ⁵⁵Fe (T_1/2 = 2.7 a) and ⁴¹Ca (T_1/2 = 103,000 a), which has been investigated in recent publications on the basis of TDCR with HIDEX 300SL [Oikari 2011; Hennig 2012].

Calcium is activated in the biological shield of NPPs by the reaction with neutrons according to ⁴⁰Ca(n,g)⁴¹Ca. ⁴¹Ca as EC nuclide emits Auger-electrons with a low 2.97 keV energy.

In the first step, the concrete matrix has to be dissolved and ⁴¹Ca purified by various chemical separation procedures. A certified ⁴¹Ca/⁴⁰Ca solution has been used as yield tracer. Inactive ⁴⁰Ca was quantified by classical analysis [Hennig 2012].

Materials and Equipment

• Gelating cocktail

• LSC equipment with TDCR facility e.g. HIDEX 300SL

For a detailed description see 2.3.10. procedure 1

Procedure

Radiochemical sample preparation for ⁵¹Ca [Warwick 2009]:

1. Microwave dissolution with HNO₃/HCl/HBF₄

2. Ca-oxalate precipitation (multiple)

3. Dissolution in aqua regia

4. Fe(OH)₃ scavenger purification leaving ⁴¹Ca with inactive Calcium in solution

5. CaCO₃ precipitation

6. BaCrO₄ purification step

7. Dissolution of CaCO₃ in HCl

Measurement:

1. The aqueous solution (⁴¹Ca or ⁵⁵Fe) is mixed with gelating cocktail.

2. The sample is measured in HIDEX 300SL using the TDCR mode.

3. The edited TDCR values are corrected and multiplied with the transition probability of the Auger-electrons (see formula below).

Evaluation

For continuous energy b-emitters, TDCR approximates counting efficiency usually within a few percent. For the mono-energetic Auger-electrons created by EC, the measured TDCR values have to be corrected by the derived formula from Poisson theory [Oikari 2012] and multiplied by the electron transition probability (0.607 for 5.19 keV Auger electrons) for ⁵⁵Fe and the detection efficiency for X rays.

In case of ⁵⁵Fe, the combined factor K is 0.869. Counting efficiencies of about 45 % for ⁵⁵Fe and some 20 % for ⁴¹Ca have been found.

For low level samples, additionally, the raw TDCR values have to be corrected for background.

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

Frenzel E., Kossert K., Oikari T., Otto R. and Wisser S. 2013: Neue Schnellmethode in der LSC-Messtechnik – Die Messung von ⁸⁹Sr/⁹⁰Sr und ⁹⁰Sr/⁹⁰Y mittels TDCR-Cerenkov-Zählung; Strahlenschutzpraxis 1/2013, pp. 26-32

Hennig H. 2012: Aktivitätsbestimmung von Beta- und Elektroneneinfang-Strahlern am Flüssigszintillationsmessgerät mittels TDCR-Methode; Bachelorarbeit DHBW, Karlsruhe 2012

Oikari T. 2012: TDCR and efficiency for monoenergetic electrons; Technical Note DOC 513-009, HIDEX Oy

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