# Assessment of the tritium analysis performance of a nonylphenol-ethoxylate-free liquid scintillator by interlaboratory comparison

Marielle Crozet, Véronique Labed, Celine Augeray, Cédric Rivier, Audrey Bacchetta, Lionel Bec-Espitalier, Nicolas Baglan, Eric Bohaud, Giacomo Canciani, Sabine Charbouillot, et al.

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## ► To cite this version:

Marielle Crozet, Véronique Labed, Celine Augeray, Cédric Rivier, Audrey Bacchetta, et al. Assessment of the tritium analysis performance of a nonylphenol-ethoxylate-free liquid scintillator by interlaboratory comparison. Environmental Technology and Innovation, 2026, 41, pp.104666. 10.1016/j.eti.2025.104666. hal-05441193

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Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
Assessment of the tritium analysis performance of a
nonylphenol-ethoxylate-free liquid scintillator by
interlaboratory comparison

Marielle Crozet<sup>a,*</sup>, Véronique Labed<sup>a</sup>, Céline Augeray<sup>b</sup>, Audrey Bacchetta<sup>c</sup>,
Lionel Bec-Espitalier<sup>d</sup>, Nicolas Baglan<sup>e</sup>, Eric Bohaud<sup>f</sup>, Giacomo Canciani<sup>a</sup>,
Sabine Charbouillot<sup>g</sup>, Christèle Colin<sup>h</sup>, Coryse Combes<sup>i</sup>, Théo Cordara<sup>j</sup>,
Benoît Daniel<sup>k</sup>, Nicolas Debain<sup>c</sup>, Natacha Einaudi<sup>l</sup>, Sandrine Fleury<sup>m</sup>,
Kevin Galliez<sup>m</sup>, Claire Gégout<sup>n</sup>, Héloïse Gervot<sup>o</sup>, Aurélie Gollion<sup>p</sup>, Linda Gravier<sup>i</sup>,
Hugo Jaegler<sup>b</sup>, Nathalie Jeanneret<sup>o</sup>, Laure Laîné<sup>q</sup>, Sophie Lamy-Hagelstein<sup>q</sup>,
Marielle Lecomte<sup>r</sup>, Pascale Lecuyer<sup>s</sup>, Myriam Le Ferrec<sup>t</sup>, Yvan Losset<sup>u</sup>,
Valérie Lourenço<sup>v</sup>, Rabha Mokrani-Kerdjoudj<sup>w</sup>, Daniel Orjollet<sup>x</sup>, Aurélie Ritt<sup>y</sup>,
Saroa Rozas<sup>z</sup>, Lionel Tenailleau<sup>aa</sup>, Alain Thoreux<sup>y</sup>, Cédric Rivier<sup>a</sup>

<footnote><sup>a</sup> CEA, DES, ISEC, DMRC, CETAMA, Univ. Montpellier, Marcoule, BP 17171, 30207 Bagnols-sur-Cèze Cedex, France</footnote>

<footnote><sup>b</sup> Autorité de sûreté nucléaire et de radioprotection (ASNR), DREE/SAME/LERCA, Le Vésinet F-78110, France</footnote>

<footnote><sup>c</sup> EDF - DISC - Direction Ingénierie et Supply Chain - Direction Technique, CNPE de Chinon BP 23, Avoine 37420, France</footnote>

<footnote><sup>d</sup> Orano Chimie Enrichissement, site du Tricastin, France</footnote>

<footnote><sup>e</sup> CEA Saclay DES/DDSD/UADS/SIAE, France, Palaiseau F-91120, France</footnote>

<footnote><sup>f</sup> Centre de stockage de l'Aube (CSA) laboratory, Aube, France</footnote>

<footnote><sup>g</sup> CEA, SPR, LMAR, Marcoule, BP 17171, 30207 Bagnols-sur-Cèze Cedex, France</footnote>

<footnote><sup>h</sup> Université Paris-Saclay, Service de Physico-Chimie, France</footnote>

<footnote><sup>i</sup> CEA Cadarache SA3E, LARC, Saint-Paul-lès-Durance 13108, France</footnote>

<footnote><sup>j</sup> CEA Cadarache SECC, LECD, Saint-Paul-lès-Durance 13108, France</footnote>

<footnote><sup>k</sup> Eurofins Eichrom Radioactivity laboratory - Bruz, France</footnote>

<footnote><sup>l</sup> CEA Cadarache SPR, LANSE, Saint-Paul-lès-Durance 13108, France</footnote>

<footnote><sup>m</sup> Autorité de sûreté nucléaire et de radioprotection (ASNR), DREE/SAME/LMN, Le Vésinet F-78110, France</footnote>

<footnote><sup>n</sup> CEA, SPR, LANSI, Fontenay-aux-Roses, France</footnote>

<footnote><sup>o</sup> LASEM Toulon, France</footnote>

<footnote><sup>p</sup> BCRM Cherbourg, Base navale, LASEM/LSR, France</footnote>

<footnote><sup>q</sup> Orano Cycle LH DSSEP/RE/L, France</footnote>

<footnote><sup>r</sup> Direction de la santé, SAR, Luxembourg</footnote>

<footnote><sup>s</sup> CEA Saclay LARP, Palaiseau F-91120, France</footnote>

<footnote><sup>t</sup> IMT Atlantique - Laboratoire Subatech / Smart, France</footnote>

<footnote><sup>u</sup> CEA, DAM, SCR, LMAR, Valduc, Is-sur-Tille F21120, France</footnote>

<footnote><sup>v</sup> Université Paris-Saclay, CEA, LIST, Laboratoire National Henri Becquerel (LNE-LNHB), Palaiseau F-91120, France</footnote>

<footnote><sup>w</sup> Autorité de sûreté nucléaire et de radioprotection (ASNR), LBMA, Le Vésinet F-78110, France</footnote>

<footnote><sup>x</sup> Autorité de sûreté nucléaire et de radioprotection (ASNR), DREE/STAAR LRTA, Cadarache, Saint-Paul-lès-Durance 13108, France</footnote>

<footnote><sup>y</sup> CEA, DAM, DIF, SRCE, LEXAN & LRBN, Île-de-France, France</footnote>

<footnote><sup>z</sup> University of the Basque Country - UPV/EHU, Spain</footnote>

<footnote><sup>aa</sup> BCRM Cherbourg - EAMEA/GEA, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: marielle.crozet@cea.fr (M. Crozet).</footnote>
ARTICLE INFO

Keywords:

Tritium ($^{3}$H)

Liquid scintillation counting (LSC)

Nonylphenol-ethoxylates (NPE)

Interlaboratory comparison (ILC)

ABSTRACT

With a view to using liquid scintillators compliant with the REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) regulations, some laboratories have expressed an interest in testing the ProSafe LT+, a liquid scintillator without nonylphenol-ethoxylates (NPE). In order to evaluate the impact of using this new liquid on the analysis results, the CETAMA consortium organised two interlaboratory comparisons. Two ranges of tritium activity were selected to meet the needs of environmental and waste monitoring. 23 laboratories took part in these exercises, comparing results obtained with ProSafe LT+ to those obtained with NPE-containing liquid scintillators. From the results it was seen that the background and the background noise obtained with the nonylphenol-ethoxylates-free liquid scintillator are not significantly different from that of the usual liquid scintillator, whether for the waste or environmental domain, over short (15 min) or long (200 min) counting times. Furthermore, the decision threshold and the detection limit values estimated with ProSafe LT+ were no different from those of usual scintillators. However, for the measurement of samples, and particularly for samples with a tritium activity above 15 Bq/kg, the ProSafe LT+ presented a reduced measurement quality compared to the nonylphenol-ethoxylates-containing LS i.e. the measurement uncertainty obtained with ProSafe LT+ is systematically higher. In light of the results obtained through this study, the authors recommend the use of the ProSafe LT+ with the proviso that the deterioration in measurement uncertainty be taken into account and found to be compatible with a laboratory's individual needs.

1. Introduction

Liquid scintillation counting (LSC) is a widely used technique for measuring radioactivity particularly in the case of low-energy beta emitters (Alliot et al., 2023). LSC requires liquid scintillators (LS), most of which contain nonylphenol-ethoxylates (NPE). NPE are surfactants, involved in facilitating the homogenization between a sample and a liquid scintillation cocktail. Furthermore, thanks to their aromatic nucleus, NPE facilitate the conversion of radioactive energy into light. However, NPE are known to be very toxic, leading to stringent regulations on their use. Indeed, under the REACH24 (Registration, Evaluation, Authorisation and Restriction of Chemicals) (European Commission, 2017) regulation NPE are classified as substances of very high concern (SVHC). In recent years, the increasing regulatory burden surrounding NPE has prompted a significant shift towards more environmentally friendly and safer LS. A particular attention has gone into the development of LS without NPE which are compliant with regulations such as the EEC directive 2003/53/EC and REACH24. Amongst such LS, the ProSafe LT+ and the ProSafe HC+ (Meridian Biotechnologies Ltd.) are LS dedicated to tritium ($^{3}$H) measurement and compliant with EU regulation. These new NPE-free LS have started to be used by the laboratories participating in the CETAMA (Commission d'établissement des méthodes d'analyses) consortium's working groups (WG) 14 (analysis of radionuclides in effluents and waste), 18 (analysis of radioactivity in water), and 31 (analysis of radioactivity in the environment). Following a request by these laboratories, the CETAMA consortium organised interlaboratory comparisons (ILCs) in order to provide a quantifiable understanding of the NPE-free LS' performance for measuring $^{3}$H. This study complements the study of the ProSafe LT+ performance for this same measurement carried out by the laboratory of Varlam et al. and published in 2019 (Varlam et al., 2019).

The CETAMA consortium has been interested in the NPE-free LS since 2019 (CETAMA, 2019) and organised an initial study (from 2019 to 2022) with the laboratories of the three WG mentioned above: this first study made it possible to select the conditions in particular sample/LS volume ratios wherein the two new NPE-free LS (ProSafe LT+ and ProSafe HC+) displayed a comparably good stability to the NPE-containing LS traditionally used by each laboratory. Following this study, ILCs have been organised from 2022 onwards, with the aim of estimating the ProSafe LT+ LS's performance in terms of (i) background noise, (ii) decision threshold, and detection limit as well as (iii) the method accuracy for the $^{3}$H measurement. In addition to a comparison based on a blank sample common to all the laboratories, two further comparisons were organised; one in a measurement range corresponding to environmental radioactivity laboratories of nuclear power plants or facilities, called "environment range" (IAEA, 2004) and the other at a higher measurement range corresponding to laboratories working on waste from the facilities, called "waste range". The work reported in this paper, carried out as part of the CETAMA consortium, presents the results of these three ILCs: on a blank sample, on samples in environment range and on samples in waste range.

2. Methods, materials and equipment

The 23 laboratories - specialized in environmental and/or waste analyses - participated in these ILC. These participants are listed in a table in the *supplementary material* (part 1).

$^{3}$H activity measurement is an ISO standardized method (ISO, 2019a) adopted at the French national level. As such, the materials and procedures used in these experiments are aligned with the recommendations provided in the ISO standard. The tested NPE-free LS was ProSafe LT+. ProSafe LT+ has a di-isopropylnaphtalene (DIN) base. The measurement of radionuclides such as $^{3}$H in aqueous phase assumes formation of a stable microemulsion, the compounds in the scintillation cocktails responsible for this being the
surfactants: alkyl phenol ethoxylates (included on the list of REACH SVHC control substances) or alcohol ethoxylates. ProSafe cocktails are based on alcohol ethoxylates and as such do not contain NPE's either as a base component or as a derivate component. In order to evaluate its performance for ³H measurement, it will be compared with the reference NPE-containing LS used by each participating laboratory for ³H measurement, called usual LS in the rest of this paper.

The distribution of the usual LS was as follows: 15 laboratories used the Ultima Gold LLT (Revvity) four laboratories used Ultima Gold (Revvity), one laboratory used Hionic-Fluor (Revvity), one laboratory used Insta-Gel Plus (Revvity), and one used Oxysolve-T (Zinsser Analytic). Following the results of preliminary studies, the ratio sample/LS was chosen equal to 10/10 with both LS types (NPE-containing and NPE-free). Liquid scintillator counters used in this study are Tri-Carb, Quantulus, Hidex or Aloka devices.

For the background, decision threshold, and detection limit studies, a blank solution was prepared for each of the two LS consisting of a mixture of laboratory reference water (water whose characteristics are known to the laboratory) and LS. The count rate of each of these two blanks (i.e. background) was measured over a period of 15 min (short time) and 200 min (long time) and repeated 30 times for each of the two durations.

For environmental and waste range studies, due to practical constraints (quantity to be produced, cost and transport), it was not possible to produce five CRMs, one for each level studied (three levels for the environmental range, three levels for the waste range, with the high level of the environmental domain being the same as the low level of the waste domain - the activity levels studied were chosen by the laboratories participating in the comparisons so as to be comparable to the levels of the samples they usually analyse). It was therefore decided to produce a single certified reference material (CRM) as a solution which was diluted by the participating laboratories to produce the samples to be measured. This CRM was produced by the LNE-LNHB (Laboratoire National Henri Becquerel, which is one of the national metrology institutes federated by the Laboratoire national de métrologie et d'essais (LNE) – the LNE delegated to the LNHB the role of national metrology laboratory in the field of ionizing radiation) with a ³H mass activity certified value A(³H)CRM equal to (2 507 ± 52) kBq/kg (k = 2) on the 17 October 2022. This solution was diluted by each laboratory in order to prepare working standard solutions using calibrated balances. The CETAMA organised these ILCs in accordance with the principles of standard ISO 17043:2023 (ISO, 2023): as the participating laboratories had to handle the samples before measurement (cf. (ISO, 2023) paragraph 7.3.1.3), the CETAMA provided appropriate instructions for preparing the samples to be measured through gravimetric dilutions as well as a results submission form which included the weighed masses and associated uncertainties.

For the waste range, three solutions for each of the three levels (high, medium and low) were prepared from the CRM, by a 25-fold dilution (i.e. 5\*5), by a 250-fold dilution (i.e. 5\*50) and by a 2500-fold dilution (i.e. 5\*25\*20) respectively. For the environmental range, three solutions for each of the three levels (high, medium and low) were prepared from the CRM, by a 2500-fold dilution (i.e. 5\*25\*20) – this level corresponds to the waste low level -, by a 25 000-fold dilution (i.e. 5\*25\*100\*2) and by a 166 667-fold dilution approximately (i.e. 5\*25\*100\*40/3) respectively (Fig. 1). Each prepared solution was counted three times (counting time: 200 min) for each LS (usual or ProSafe LT+) studied, for each of the two ranges, and for each level.

For each of the results returned by the laboratories, the CETAMA expressed the result in such a way that it could be compared to the

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Diagram of the preparation of working solutions for the two ranges (waste and environment) studied. The reference working solutions analysed in the laboratories correspond to the filled grey rectangles.</figcaption></figure>
CRM by calculating the dilution factor associated with each of the results and combining (by the JCGM100 (JCGM, 2008) approach) the measurement uncertainty with the weighing uncertainties of the dilutions provided by the laboratory, dilutions which had to be carried out gravimetrically by the participating laboratories (an example of measurement uncertainty estimation expressed at the CRM level for one of the L20 results is provided in this paper's [supplementary material - part 2]).

# 3. Results and discussion

## 3.1. Background, decision threshold & detection limit

The liquid scintillation cocktail plays a major role in obtaining a good detection limit. It has the following requirements for ³H measurement: long term stability of sample-cocktail mixture (due to the long measurement times at low levels of radioactivity) (Baglan et al., 2011), low background of the liquid scintillation cocktail, and good ³H efficiency counting.

### 3.1.1. Background

The aim was to assess the impact of the ProSafe LT+ LS on the blank count rate τ₀ (i.e. the background) in comparison with the

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Blank count rate measurement results for each laboratory for both counting times. The intervals correspond to ± the standard deviation of 30 measurement results of the blank count rate, s<sub>exp</sub>; cpm: counts per minute. Top: counting time: 15 min. Bottom: counting time: 200 min.</figcaption></figure>
usual LS. As discussed in Section 2, for each duration of 15 or 200 min, 30 blank count rates were measured: the mean (i.e. estimation of the background) and the standard deviation of these 30 measurement results of the blank count rate $s_{exp}(\tau_0)$ (i.e. estimation of the background noise $\sigma(\tau_0)$) were determined and are shown in Fig. 2.

It can be seen that the blank counting rate (i.e. estimation of the background) varied from 1 to 6 counts/minute with the same mean close to 2.4 counts/minute regardless of the counting time and regardless of the LS (the blank count rate measurement results distributions are available in supplementary material – part 3). These values are consistent with the values obtained by Gudelis and Gaigalaite in 2021 (Gudelis and Gaigalaite, 2021) for LS - containing either NPEs or other SVHC - Gold Star LT2 (0.95 cpm), OptiPhase HiSafe 2 (1.15 cpm), OptiPhase HiSafe 3 (0.99 cpm), and Ultima Gold LLT (1.37 cpm) for a counting time of 1500 min.

The background noise of the different LSs studied in the present series of experiments could be further compared through the use of mathematical models. Indeed, the blank count rate $\tau_0$ (cpm or counts per minute) can be expressed as (1)

$$ \tau_0 = \frac{N_0}{t_0} \qquad (1) $$

where $N_0$ is the number of counts and $t_0$ is the counting time (minutes). By applying the guide to the expression of uncertainty in measurement (GUM) (JCGM, 2008) and considering that the relative uncertainty associated with the measurement time $t_0$ is negligible compared to the relative uncertainty associated with the number of counts $N_0$, it is possible to obtain (2)

$$ \frac{\sigma^2(\tau_0)}{\tau_0^2} = \frac{\sigma^2(N_0)}{N_0^2} \qquad (2) $$

where $\sigma^2(x)$ is the variance of $x$, $x = \tau_0$ or $N_0$.

Assuming that the measured signal distribution corresponds solely to the phenomenon of radioactive decay (which follows a Poisson distribution), $\sigma^2(N_0) = N_0$, leading to (3) & (4)

$$ \frac{s_{Poisson}^2(\tau_0)}{\tau_0^2} = \frac{1}{N_0} = \frac{1}{\tau_0 t_0} \qquad (3) $$

Or

$$ s_{Poisson}(\tau_0) = \sqrt{\frac{\tau_0}{t_0}} \qquad (4) $$

For the same sample measured, the background noise decreases inversely proportionally to the square root of the counting time.

As such, it is possible to compare the values of the estimated background noise as calculated according to (4), ($s_{Poisson}(\tau_0)$, assuming that the signal follows a Poisson distribution and that the relative uncertainty associated to the measurement time is negligible) with those obtained experimentally ($s_{exp}(\tau_0)$) for both counting times. These values are shown in supplementary material – part 4. Overall, there is a good agreement between the experimental ($s_{exp}(\tau_0)$) and calculated values from $\tau_0$ and $t_0$ (4), $s_{Poisson}(\tau_0)$, particularly for a counting time of 200 min irrespective of the LS studied.

### 3.1.2. Decision threshold & detection limit

The decision threshold is defined by the IUPAC (International union of pure and applied chemistry) as the minimum significant value of an estimated net signal. The notion of ‘significant value’ is understood by comparing the estimated net signal with that of the blank (spurious signal). This comparison involves the risk $\alpha$, known as the false positive risk, corresponding to the risk of falsely declaring the measurand significant. By introducing the risk $\alpha$, the decision threshold can be defined as: ‘critical value for which the probability of obtaining a measurement value greater than this value, when the measurand is zero, is equal to $\alpha$’.

Although the VIM (international vocabulary of metrology) (Eurachem, 2011; JCGM, 2012) was drawn up by the Joint committee for guides in metrology (JCGM), which brings together the major international scientific and standards bodies such as the BIPM (Bureau international des poids et mesures), ISO (International organization for standardization), IUPAC, IEC (International Electrotechnical Commission), etc., the definition of the detection limit given by IUPAC is preferable to that of the VIM. The definition given in the VIM refers to the concept of a measured value, whereas the detection limit is not a specific measurement result but a characteristic of the analysis method. The following IUPAC definition is preferred here: ‘net true signal for which the probability that a measurement value is below the decision threshold is equal to $\beta$’. The $\beta$ risk is called the false negative risk, and corresponds to the risk of falsely declaring the measurand (in this case $^3$H mass activity) to be insignificant. The values of the decision threshold, $DT$, and the detection limit, $DL$, (Currie, 1968; Rivier and Crozet, 2014) for the $^3$H mass activity measurement can be written as (5)

$$ DT = z_{1-\alpha} \sigma_{0net} \text{ with } \sigma_{0net} = \sqrt{2} \sigma_O \text{ and } DL = DT + z_{1-\beta} \sigma_{DLnet} \qquad (5) $$

Where $z_{1-\alpha}$ and $z_{1-\beta}$ are the critical values of the reduced centered normal variable for probabilities $1-\alpha$ and $1-\beta$ respectively, $\sigma_{0net}$ and $\sigma_{DLnet}$ are the net $^3$H mass activity measurement standard uncertainty for an infinite number of measurements, at the blank and $DL$ levels respectively, $\sigma_O$ is the raw $^3$H mass activity measurement standard uncertainty for an infinite number of measurements at the blank level. Taking $\alpha = \beta$ and if the measurement uncertainty at the $DL$ level is not significantly different from that at the blank level (case of homoscedasticity), then we get (6)

$$ DL \approx 2DT \qquad (6) $$
In the absence of an infinite number of measurements and therefore $\sigma_O$ is not known but rather estimated by $s_O$, considering a number of degrees of freedom $\nu$. As such, assuming homoscedasticity is maintained and taking $\alpha=\beta$, the estimate of DL i.e. $\tilde{DL}$ becomes (7)

$$ \tilde{DL} \approx 2t_{1-\alpha,\nu} s_{0net} \quad \text{and with} \quad s_{0net} = \sqrt{2s_O} \qquad (7) $$

with $t_{1-\alpha,\nu}$: Student's t for probability 1-α with (α: one-sided risk) and number of degrees of freedom $\nu$, $s_O$ is the estimate of $\sigma_O$.

The DL and DT for the measurements performed by the laboratories with each LS in the present study were estimated through two different approaches, detailed as follows:

3.1.2.1. **Approach 1.** Each of the laboratories participating in this comparison made 30 measurements of a blank sample for each of the two selected counting times (15 and 200 min). The variance of the values of these 30 measurement results of the raw blank sample $s_{0exp}$ is an estimate of the variance of the blank-level measurement $\sigma_O$. Considering $\alpha=5\%$, (5) becomes (8)

$$ \tilde{DT}_1 \approx t_{1-5\%,29} \sqrt{2s_{0exp}} \qquad (8) $$

where $\tilde{DT}_1$ is the estimate of DT using approach 1 and $t_{1-5\%,29} = 1.699$ (one-sided α risk). The decision threshold can be expressed in signal units i.e. in counts per second (/s) (9).

$$ \tilde{DT}_{1/(s)} \approx t_{1-5\%,29} \sqrt{2s_{0\exp/(s)}} \qquad (9) $$

To express the decision threshold in the unit of the measurand, which is the net mass activity of $^3$H, $A(^3H)$ (or more simply in A in Bq/kg, it is necessary to write the relationship between the signal and the measurand (10))

$$ A = \frac{(\tau - \tau_0)}{\epsilon m} \qquad (10) $$

where $\tau$ is the counting rate for the sample (/s), $\epsilon$ is the counting efficiency of the measurement (unitless) and $m$ is the mass of the sample (kg).

GUM can be applied to (10), it comes (11)

$$ s_{rel}^2(A) = s_{rel}^2(\tau - \tau_0) + s_{rel}^2(\epsilon) + s_{rel}^2(m) \quad \text{and} \quad s(A) = A s_{rel}(A) \qquad (11) $$

where $s_{rel}^2(x)$ is the relative variance of $x$ and $s^2(x)$ its absolute variance.

For a blank sample, it comes logically for the net mass activity of $^3$H in the blank sample by applying (11) to the specific case where $A = A_0$, (12)

$$ A_0 = \frac{(\tau_0 - \tau_0)}{\epsilon m_0} = 0, \quad s_{rel}^2(A_0) = 2s_{rel}^2(\tau_0) + s_{rel}^2(\epsilon) + s_{rel}^2(m_0) \quad \text{and} \quad s(A_0) = A_0 s_{rel}(A_0) \qquad (12) $$

(8) can then be adapted for the expression of the decision threshold expressed in $^3$H mass activity (13)

$$ \tilde{DT}_{1(Bq/kg)} \approx t_{1-5\%,29} s(A_0) \qquad (13) $$

where $\tilde{DT}_1$ is the estimate of DT using approach 1.

Furthermore, considering that $\beta=5\%$ and the homoscedasticity of the measurement, (7) then becomes (14)

$$ \tilde{DL}_{1(Bq/kg)} \approx 2t_{1-5\%,29}s(A_0) \qquad (14) $$

where $\tilde{DL}_1$ is the estimate of DL using approach 1.

3.1.2.2. **Approach 2.** The second approach used by all the participants in the present study is based on the ISO 9698:2019 (ISO, 2019a) standard which refers to the ISO 11929-1:2019 (ISO, 2019b) standard and is generally used to determine DT and DL when measuring radioactivity. In this approach, the estimates of DT and DL for $\alpha=\beta=5\%$ are as follows (15) & (16)

$$ \tilde{DT}_{2/(s)} = \frac{1.645}{t_0} \sqrt{2N_0} \qquad (15) $$

and

$$ \tilde{DL}_{2(Bq/kg)} = \frac{2 \left( \frac{\tilde{DT}_{2/(s)}}{\epsilon m_0} \right) + \frac{1.645^2}{\epsilon m_0 t_0}}{1 - 1.645^2 (u_{rel}^2(\epsilon) + u_{rel}^2(m_0))} \qquad (16) $$

where $\tilde{DT}_{2/(s)}$ and $\tilde{DL}_{2(Bq/kg)}$ are respectively the estimates of the DT expressed in /s and DL expressed in Bq/kg by approach 2, $u_{rel}(x)$ is the relative standard uncertainty associated to $x$.
It is worth noting that the expression $\tilde{DT}_2 (s)$ implicitly assumes that the signal follows a Poisson distribution. This contradicts the assumption of normality (or Student for a finite number of measurements). However, this hypothesis is often used by the laboratories in nuclear measurement and in standards. Furthermore, (6) is no longer valid when we consider that the variable follows a Poisson distribution: by definition, this distribution is heteroscedastic since its variance is equal to its mean. Despite these remarks, we have kept the expressions of the (14) & (15) because they are generally used by radiometry measurement laboratories.

The decision threshold and detection limit values estimated using the two approaches are presented in Table 1. For both approaches, a relative standard uncertainty of the mass of the blank sample $u_{rel}(m_0)$ and of the efficiency $u_{rel}(\epsilon)$ has been chosen equal to 1 %.

It can be seen from Table 1 that for identical $\alpha$ and $\beta$ risks (i.e. 5 %), the decision threshold and detection limit estimated values are consistent between laboratories and between the two approaches (approaches 1 & 2). The few differences between these approaches are undoubtedly due to the additional assumption of a Poisson distribution for approach 2, which is not always true. Table 3 shows that there is no significant difference between the two LS. The decision threshold is of the order of 0.016 /s and 0.0046 /s for 15 and 200 min respectively, and the detection limit is 19 Bq/kg and 5.2 Bq/kg respectively, irrespective of the LS and the approach. It should be noted that the ratio of the values of the detection limit for 15 min and for 200 min i.e. 19/5.2 = 3.6 is in agreement with the square root of the counting times 200/15 which is 3.6. Furthermore, the detection limit values obtained (19 Bq/kg and 5.2 Bq/kg for 15 and 200 min respectively) are consistent with those measured by Gudelis and Gaigalaite (Gudelis and Gaigalaite, 2021) in 2021 with the LS Gold Star LT2 (called minimum detection activity (MDA) using the Currie criterion (Currie, 1968; Kirkpatrick et al., 2015) of 1.52 Bq/kg - assuming that the density of the Neris river water in which the measurements are taken is 1 - for a counting time of 1500 min, which corresponds to 15 Bq/kg (i.e. $1.52\sqrt{1500/15}$ ) and 4.1 Bq/kg (i.e. $1.52\sqrt{1500/200}$ ) for 15 and 200 min respectively.

## 3.2. Measurement performance

The proficiency of laboratories to measure $^3$H, the accuracy of the measurement method and the uncertainty associated with the measurement result were estimated with the usual LS and with ProSafe LT+ over two different measurement ranges and three levels per range (cf. Fig. 1): for the environment range, levels close to 15, 100 and 1 000 Bq/kg and for the waste range, levels close to 1 000, 10 000 and 100 000 Bq/kg. The measurement results, taking into account the mass dilutions carried out by each of the laboratories and the associated uncertainties, were expressed at the MRC activity level, in order to allow a direct comparison of the experimental values obtained at the six different measurement levels with the certified value.

Two approaches, for each LS and for each activity level, were used to process the results of the laboratories with distinct objectives. The first was to analyse the ILC to estimate the proficiency of laboratories to measure $^3$H mass activity, leading to performance scores (z-score and zeta-score). For this, the process followed is that presented in ISO 13528:2022 (ISO, 2022); the second approach used corresponds to the analysis of the results in order to estimate the accuracy of the method (i.e. liquid scintillation). Accuracy has two components, trueness and precision. Trueness and precision are estimated by processing laboratory results in accordance with ISO 5725-4:2020 (ISO, 2020) and ISO 5725-2:2019 (ISO, 2019c) respectively. Based on these two characteristics, the expected uncertainty of a result - for a $^3$H mass activity close to those of the ILC- can be estimated by applying ISO 21748:2017 (ISO, 2017).

### 3.2.1. Proficiency test

For each range (environment or higher activity), for each level, and for each LS, an aptitude test is carried out. As an example, the graph in Fig. 4 shows the laboratory results for laboratories working in the field of environmental measurement, for the new LS and for the 1000 Bq/kg level<fnref n="1" />. All laboratories (except laboratory L21 which provided only 2 results) reported 3 results which are called i, ii and iii in Fig. 3.

From the laboratory results, zeta and z scores are estimated. The z-score of a laboratory j is the number of standard deviations of participants' results between the result of laboratory j and the assigned value of the ILC (17) - In this document, the assigned value is the certified value and the standard deviation is the robust standard deviation calculated according to algorithm A of (ISO, 2022).

$$z_j = \frac{y_j - y_{CRM}}{s^*} \qquad (17)$$

where $z_j$ is the z-score of laboratoire j for $y_j$, $y_i$ is the result laboratory j result, $y_{CRM}$ is the assigned value of the ILC here the certified reference value and $s^*$ is the robust standard deviation of participants' results.

The zeta-score is the ratio of the difference between the laboratory j result and the assigned value of the ILC (here the certified value) and the uncertainty of this difference (18).

$$\zeta_j = \frac{y_j - y_{CRM}}{\sqrt{u_j^2 + u_{CRM}^2}} \qquad (18)$$

Where $\zeta_j$ is the zeta-score of laboratoire j for $y_j$, $u_j$ is the standard uncertainty associated to $y_j$ and $u_{CRM}$ is the standard uncertainty associated to $y_{CRM}$.

<footnote n="1">¹ Using the software Jmp ® 17.0.1, Statistical Discovery https://www.jmp.com</footnote>
<caption>Table 1</caption>

Estimation of decision threshold and detection limit for each laboratory for both counting times.

<table><thead><tr><th rowspan="3">counting time</th><th colspan="10">15 min</th><th colspan="10">20 min</th></tr><tr><th colspan="5">USUAL</th><th colspan="5">ProSafe LT+</th><th colspan="5">USUAL</th><th colspan="5">ProSafe LT+</th></tr><tr><th>1</th><th>2</th><th>1</th><th>2</th><th>1</th><th>2</th><th>1</th><th>2</th><th>1</th><th>2</th><th>1</th><th>2</th><th>1</th><th>2</th><th>1</th><th>2</th><th>1</th><th>2</th><th>1</th><th>2</th></tr><tr><th>Approach</th><th>DT<sub>1</sub><br>(/s)</th><th>DT<sub>2</sub><br>(/s)</th><th>DL<sub>1</sub><br>(Bq/<br>kg)</th><th>DL<sub>2</sub><br>(Bq/<br>kg)</th><th>DT<sub>1</sub><br>(/s)</th><th>SD<sub>inst.</sub><br>(/s)</th><th>DT<sub>2</sub><br>(/s)</th><th>DL<sub>1</sub><br>(Bq/<br>kg)</th><th>LD<sub>11929</sub><br>(Bq/kg)</th><th>DL<sub>2</sub><br>(Bq/<br>kg)</th><th>DT<sub>1</sub><br>(/s)</th><th>SD<sub>inst.</sub><br>(/s)</th><th>DT<sub>2</sub><br>(/s)</th><th>DL<sub>1</sub><br>(Bq/<br>kg)</th><th>DL<sub>2</sub><br>(Bq/<br>kg)</th><th>DT<sub>1</sub><br>(/s)</th><th>DT<sub>2</sub><br>(/s)</th><th>DL<sub>1</sub><br>(Bq/<br>kg)</th><th>DL<sub>2</sub><br>(Bq/<br>kg)</th></tr></thead><tbody><tr><td>L1 (HA)</td><td>0.020</td><td>0.018</td><td>25</td><td>24</td><td><strong>0.019</strong></td><td></td><td><strong>0.018</strong></td><td>27</td><td></td><td><strong>28</strong></td><td>0.0065</td><td></td><td>0.0048</td><td>8.2</td><td>6.1</td><td><strong>0.0056</strong></td><td><strong>0.0049</strong></td><td>8.2</td><td>7.2</td><td></td><td></td></tr><tr><td>L1 (env)</td><td>0.022</td><td>0.018</td><td>20</td><td>18</td><td><strong>0.016</strong></td><td></td><td><strong>0.016</strong></td><td>16</td><td></td><td><strong>18</strong></td><td>0.0054</td><td></td><td>0.0042</td><td>5.2</td><td>4.0</td><td><strong>0.0048</strong></td><td><strong>0.0044</strong></td><td>5.0</td><td>4.6</td><td></td><td></td></tr><tr><td>L3 (env)</td><td>0.015</td><td>0.013</td><td>15</td><td>15</td><td><strong>0.010</strong></td><td></td><td><strong>0.013</strong></td><td>11</td><td></td><td><strong>14</strong></td><td>0.0042</td><td></td><td>0.0038</td><td>3.8</td><td>3.9</td><td><strong>0.0039</strong></td><td><strong>0.0035</strong></td><td>3.9</td><td>3.6</td><td></td><td></td></tr><tr><td>L4 (env)</td><td>0.023</td><td>0.018</td><td>21</td><td>18</td><td><strong>0.018</strong></td><td></td><td><strong>0.018</strong></td><td>19</td><td></td><td><strong>20</strong></td><td>0.0064</td><td></td><td>0.0049</td><td>5.2</td><td>4.7</td><td><strong>0.0041</strong></td><td><strong>0.0051</strong></td><td>4.5</td><td>5.4</td><td></td><td></td></tr><tr><td>L5 (env)</td><td>0.019</td><td>0.013</td><td>18</td><td>15</td><td><strong>0.014</strong></td><td></td><td><strong>0.013</strong></td><td>16</td><td></td><td><strong>16</strong></td><td>0.0063</td><td></td><td>0.0036</td><td>4.1</td><td>3.6</td><td><strong>0.0040</strong></td><td><strong>0.0037</strong></td><td>4.5</td><td>4.1</td><td></td><td></td></tr><tr><td>L6 (env)</td><td>0.016</td><td>0.020</td><td>14</td><td>19</td><td><strong>0.020</strong></td><td></td><td><strong>0.022</strong></td><td>19</td><td></td><td><strong>22</strong></td><td>0.0037</td><td></td><td>0.0055</td><td>5.7</td><td>4.9</td><td><strong>0.0008</strong></td><td><strong>0.0057</strong></td><td>1.8</td><td>5.6</td><td></td><td></td></tr><tr><td>L8 (env)</td><td>0.017</td><td>0.015</td><td>16</td><td>16</td><td>-</td><td></td><td><strong>0.014</strong></td><td>-</td><td></td><td><strong>21</strong></td><td>0.0071</td><td></td><td>0.0040</td><td>6.0</td><td>4.0</td><td><strong>0.0039</strong></td><td><strong>0.0032</strong></td><td>5.4</td><td>4.5</td><td></td><td></td></tr><tr><td>L9 (env)</td><td>0.014</td><td>0.015</td><td>11</td><td>13</td><td><strong>0.012</strong></td><td></td><td><strong>0.013</strong></td><td>12</td><td></td><td><strong>13</strong></td><td>0.0044</td><td></td><td>0.0043</td><td>3.0</td><td>3.4</td><td><strong>0.0033</strong></td><td><strong>0.0034</strong></td><td>3.1</td><td>3.4</td><td></td><td></td></tr><tr><td>L10 (env)</td><td>0.014</td><td>0.012</td><td>16</td><td>15</td><td><strong>0.010</strong></td><td></td><td><strong>0.010</strong></td><td>13</td><td></td><td><strong>14</strong></td><td>0.0048</td><td></td><td>0.0032</td><td>7.9</td><td>3.7</td><td><strong>0.0033</strong></td><td><strong>0.0026</strong></td><td>5.0</td><td>4.1</td><td></td><td></td></tr><tr><td>L11 (env)</td><td>0.011</td><td>0.018</td><td>9</td><td>16</td><td><strong>0.0095</strong></td><td></td><td><strong>0.018</strong></td><td>9</td><td></td><td><strong>18</strong></td><td>0.0220</td><td></td><td>0.0048</td><td>3.8</td><td>4.1</td><td><strong>0.0039</strong></td><td><strong>0.0049</strong></td><td>3.9</td><td>4.7</td><td></td><td></td></tr><tr><td>L12 (env)</td><td>0.014</td><td>0.016</td><td>11</td><td>15</td><td><strong>0.020</strong></td><td></td><td><strong>0.017</strong></td><td>19</td><td></td><td><strong>17</strong></td><td>0.0089</td><td></td><td>0.0043</td><td>4.1</td><td>3.6</td><td><strong>0.0065</strong></td><td><strong>0.0050</strong></td><td>6.1</td><td>4.8</td><td></td><td></td></tr><tr><td>L13 (env)</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td></td><td>-</td><td>-</td><td></td><td><strong>31</strong></td><td>0.0089</td><td></td><td>0.0090</td><td>16</td><td>6.3</td><td><strong>0.0096</strong></td><td><strong>0.0089</strong></td><td>8.6</td><td>7.4</td><td></td><td></td></tr><tr><td>L14 (HA)</td><td>0.022</td><td>0.016</td><td>39</td><td>31</td><td><strong>0.022</strong></td><td></td><td><strong>0.015</strong></td><td>39</td><td></td><td><strong>24</strong></td><td>0.0040</td><td></td><td>-</td><td>-</td><td>-</td><td>-</td><td><strong>0.0065</strong></td><td><strong>0.0059</strong></td><td>6.9</td><td>6.1</td><td></td><td></td></tr><tr><td>L15 (HA)</td><td>0.017</td><td>0.020</td><td>14</td><td>17</td><td><strong>0.022</strong></td><td></td><td><strong>0.021</strong></td><td>22</td><td></td><td><strong>24</strong></td><td>0.0024</td><td></td><td>0.0055</td><td>7.4</td><td>4.6</td><td><strong>0.0065</strong></td><td><strong>0.0059</strong></td><td>6.9</td><td>6.1</td><td></td><td></td></tr><tr><td>L15 (env)</td><td>0.017</td><td>0.020</td><td>14</td><td>17</td><td><strong>0.022</strong></td><td></td><td><strong>0.021</strong></td><td>22</td><td></td><td><strong>16</strong></td><td>0.0056</td><td></td><td>0.0040</td><td>3.6</td><td>3.6</td><td><strong>0.0045</strong></td><td><strong>0.0040</strong></td><td>4.7</td><td>4.2</td><td></td><td></td></tr><tr><td>L17 (HA)</td><td>0.018</td><td>0.015</td><td>16</td><td>14</td><td><strong>0.015</strong></td><td></td><td><strong>0.015</strong></td><td>15</td><td></td><td><strong>12</strong></td><td>0.0026</td><td></td><td>0.0022</td><td>2.4</td><td>2.2</td><td><strong>0.0033</strong></td><td><strong>0.0022</strong></td><td>4.4</td><td>3.0</td><td></td><td></td></tr><tr><td>L18 (env)</td><td>0.007</td><td>0.008</td><td>7</td><td>9</td><td><strong>0.0063</strong></td><td></td><td><strong>0.0071</strong></td><td>8</td><td></td><td><strong>19</strong></td><td>-</td><td></td><td>-</td><td>-</td><td>-</td><td>-</td><td><strong>0.0031</strong></td><td><strong>0.0028</strong></td><td>3.2</td><td>3.0</td><td></td><td></td></tr><tr><td>L19 (HA)</td><td>0.035</td><td>0.018</td><td>34</td><td>19</td><td><strong>0.017</strong></td><td></td><td><strong>0.016</strong></td><td>19</td><td></td><td><strong>10</strong></td><td>0.0038</td><td></td><td>0.0029</td><td>2.3</td><td>2.5</td><td><strong>0.0067</strong></td><td><strong>0.0063</strong></td><td>9.3</td><td>8.8</td><td></td><td></td></tr><tr><td>L20 (env)</td><td>0.012</td><td>0.010</td><td>10</td><td>10</td><td><strong>0.011</strong></td><td></td><td><strong>0.010</strong></td><td>10</td><td></td><td><strong>34</strong></td><td>0.0067</td><td></td><td>0.0062</td><td>8.1</td><td>7.4</td><td><strong>0.0064</strong></td><td><strong>0.0063</strong></td><td>9.3</td><td>8.8</td><td></td><td></td></tr><tr><td>L21 (env)</td><td>0.029</td><td>0.023</td><td>34</td><td>28</td><td><strong>0.025</strong></td><td></td><td><strong>0.023</strong></td><td>34</td><td></td><td><strong>34</strong></td><td>0.0067</td><td></td><td>0.0062</td><td>8.1</td><td>7.4</td><td><strong>0.0064</strong></td><td><strong>0.0063</strong></td><td>9.3</td><td>8.8</td><td></td><td></td></tr></tbody></table>

env: measurement in environment range: HA: measurement in higher activity range

DT: detection threshold; DL: detection limit; LS: liquid scintillator
<figure><img src="image_5.png" /><figcaption>Fig. 3. Laboratory results and associated measurement uncertainty for measurement at level 1000 Bq/kg, with ProSafe LT+ in the environment range. Dots and intervals: lab results and measurement coverage intervals (k = 2); grey line and grey band: certified value and its coverage interval (k = 2); dotted line: lab results robust mean coverage interval (k = 2); the certified value and the robust mean are very close and cannot be distinguished from each other on the graph.</figcaption></figure>

If the score value is between -2 and 2, this score can be considered satisfactory, if it is between -2 and -3 or between +2 and +3, it is questionable, below -3 or above +3, it is unsatisfactory. The proportion of satisfactory, questionable and unsatisfactory scores measured for each LS for each level of the two ranges is given in supplementary material – part 5.

For the environment range, overall, the proportion of satisfactory z scores was slightly higher with the ProSafe LT+ LS (90 % compared with 84 % for the usual LS) and equivalent for the zeta score (80 % compared with 79 % for the usual LS). For the waste range, overall, the proportion of satisfactory z scores is higher with the new LS (97 % compared with 86 %), but the opposite is true for the zeta score (86 % compared with 96 %). It is not possible to say that the proficiency of laboratories in measuring ³H is affected by the new LS.

### 3.2.2. Measurement method accuracy

The accuracy of a method is characterised by its trueness and precision. The trueness (δ) is the closeness of agreement between the best estimate of the result obtained from a large number of results y<sub>ILC</sub> and a value considered to be the true value of the measurand, for example a certified value y<sub>CRM</sub>. A method is true when it is unbiased. To determine the trueness of a method, we need to estimate its bias and test whether or not it is significant by the calculation of the normalised difference E<sub>n</sub> (19).

$$E_n = \frac{y_{ILC} - y_{CRM}}{\sqrt{u_{ILC}^2 + u_{CRM}^2}} = \frac{\hat{\delta}}{u(\hat{\delta})} \qquad (19)$$

where y<sub>ILC</sub> is the best estimate value of the measurand by ILC and u<sub>ILC</sub> the standard uncertainty of y<sub>ILC</sub>, ̂δ is an estimate of the bias, and u(̂δ) is the standard uncertainty of ̂δ. When the normalised difference E<sub>n</sub> is between -2.0 and +2.0, there is no proven difference between y<sub>ILC</sub> and y<sub>CRM</sub>: the estimated bias ̂δ is not significant, the measurement method can be considered true. The risk associated with this conclusion is close to 5 %. If this is not the case, y<sub>ILC</sub> and y<sub>CRM</sub> cannot be considered to be identical and there is a reason to suspect that there is a bias; the method cannot be considered to be true.

The measured E<sub>n</sub> values are summarized in Table 2.

All the absolute values of E<sub>n</sub> measured are lower than 2 (Table 2), as such the method of measuring ³H by liquid scintillation can be considered true for both LS irrespective of the level of measurement over the two ranges studied.

The precision of the method is assessed by complying with the repeatability and reproducibility conditions. Repeatability conditions are met when measurements are made by the same operator, on the same instrument, using a single method, within a short period of time, in order to obtain measurements under conditions that are as similar as possible. In this paper, we talk about repeatability when the measurements have been made by the same laboratory (laboratory code to be precise). Reproducibility conditions are met when one of the following conditions changes: operator, instrument, variation in the application of the method, execution time or any other cause that introduces sources of variability.
<caption>Table 2</caption>

Measurement method trueness estimation for environment and waste ranges and for each measurement level - E<sub>n</sub> values.

<table><thead><tr><th rowspan="2"></th><th colspan="3">certified value (Bq/kg) y<sub>CRM</sub></th><th colspan="3">certified value standard uncertainty (Bq/kg) u<sub>CRM</sub></th><th colspan="3">26,000</th></tr><tr><th>Environment Range</th><th>delta estimate</th><th>delta standard uncertainty</th><th>normalised difference: En</th><th>Waste Range</th><th>delta estimate</th><th>delta standard uncertainty</th><th>normalised difference: En</th></tr><tr><th></th><th>measurement level</th><th>y<sub>ILC</sub> (Bq/kg)</th><th>u<sub>ILC</sub> (Bq/kg)</th><th></th><th>measurement level</th><th>y<sub>ILC</sub> (Bq/kg)</th><th>u<sub>ILC</sub> (Bq/kg)</th><th></th></tr></thead><tbody><tr><td>USUAL</td><td>15 Bq/kg</td><td>2,599,706</td><td>62,359</td><td>= y<sub>ILC</sub> - y<sub>CRM</sub></td><td>1,000 Bq/kg</td><td>2,513,586</td><td>27,116</td><td>= y<sub>ILC</sub> - y<sub>CRM</sub></td></tr><tr><td>LS</td><td>100 Bq/kg</td><td>2,585,626</td><td>37,450</td><td>= (u<sub>ILC</sub><sup>2</sup> + u<sub>CRM</sub><sup>2</sup>)<sup>0.5</sup> (Bq/kg)</td><td>10,000 Bq/kg</td><td>2,509,313</td><td>21,676</td><td>= (u<sub>ILC</sub><sup>2</sup> + u<sub>CRM</sub><sup>2</sup>)<sup>0.5</sup> (Bq/kg)</td></tr><tr><td></td><td>1,000 Bq/kg</td><td>2,512,726</td><td>19,064</td><td></td><td>100,000 Bq/kg</td><td>2,533,256</td><td>7,446</td><td></td></tr><tr><td>ProSafe</td><td>15 Bq/kg</td><td>2,627,931</td><td>75,539</td><td>1.4</td><td>1,000 Bq/kg</td><td>2,561,304</td><td>41,276</td><td>1.1</td></tr><tr><td>LT+</td><td>100 Bq/kg</td><td>2,593,985</td><td>47,480</td><td>1.6</td><td>10,000 Bq/kg</td><td>2,516,960</td><td>30,896</td><td>0.2</td></tr><tr><td></td><td>1,000 Bq/kg</td><td>2,504,235</td><td>36,525</td><td>-0.1</td><td>100,000 Bq/kg</td><td>2,528,216</td><td>27,945</td><td>0.6</td></tr></tbody></table>
From the results obtained during this ILC we can estimate by one factor analysis of variance (ANOVA)¹ the repeatability standard deviation, noted as $s_r$ and the reproducibility standard deviation, noted $s_R$. The formulas for calculating repeatability and reproducibility variances are given in Fig. 4.

For each ANOVA performed, the number of results $n_p$ and the number of laboratory codes $p$ used are provided in supplementary material – part 6. $s_r$ is an estimate of the dispersion of the results within the same laboratory whilst $s_R$ is an estimate of the dispersion of the results across all laboratory (codes) combined (Table 3).

As with most measurement methods, and for both LS, the relative standard uncertainty of repeatability and reproducibility of the ³H measurement by liquid scintillation decreases as the radioactivity in the measured sample increases: for the environment range, from 14 % to 1.8 % for the standard uncertainty of repeatability and from 17 % to 2.6 % for the standard uncertainty of reproducibility for the usual LS and from 13 % to 2.4 % for the standard uncertainty of repeatability and from 18 % to 6.2 % for the standard uncertainty of reproducibility for ProSafe LT+; for the waste range, from 1.7 % to 0.6 % for the standard uncertainty of repeatability and from 1.7 % to 0.8 % for the standard uncertainty of reproducibility for the usual LS and from 4.6 % to 1.2 % for the standard uncertainty of repeatability and from 6.8 % to 3.8 % for the standard uncertainty of reproducibility using ProSafe LT+.

For measurement levels above 15 Bq/kg, the precision with the usual LS is better than that with ProSafe LT+: there is therefore a systematic loss of precision with ProSafe LT+ of around 3–4 % for the standard reproducibility uncertainty. For the 15 Bq/kg level, the precision performances are equivalent with either LS: at this low level, the nature of the LS is no longer a major contributor to measurement uncertainty.

### 3.2.3. Measurement uncertainty

Based on ISO 21748:2017 (ISO, 2017), the trueness and the precision estimates obtained via an interlaboratory comparison can be used as a basis for evaluating the measurement uncertainty. The expression for the measurement result $y$ used is given by the mathematical model in (20), which includes the bias of the measurement method $\delta$, the bias from the laboratory $B$ and the random error $e$,

$$y = y_{CRM} + \delta + B + e \text{ and } u^2(B) + u^2(e) = s_R^2 \qquad (20)$$

As seen before, an ILC can be used to estimate the bias, denoted $\hat{\delta}$, and its associated standard uncertainty $u(\hat{\delta})$.

For a single result $y$, the measurement uncertainty $u(y)$ can be estimated from the following (21).

$$u^2(y) = u^2(\hat{\delta}) + s_R^2 \qquad (21)$$

The measurement uncertainty values obtained in this way are summarised in Table 4. It can be seen that whilst the estimates are consistent from one range to another, the measurement uncertainty obtained with the usual LS is always lower than that obtained with the ProSafe LT+ except for the lowest measurement level (15 Bq/kg) where they are equivalent. This is consistent with the fact that counting efficiency is lower for ProSafe LT+ as shown by Varlam et al. (Varlam et al., 2019).

These uncertainty estimates obtained for the ProSafe LT+ are consistent with those estimated by the National R&D Institute for Cryogenics and Isotopic Technologies (Varlam et al., 2019) in 2019 for the same LS (ProSafe LT+): 6 % ($k = 2$) for a measurement level of 300 000 Bq/kg (assuming a density of 1 for the water measured) and 30 % ($k = 2$) for a measurement level close to 6 Bq/kg.

<figure><img src="image_6.png" /><figcaption>Fig. 4. Repeatability and reproducibility variances formulas. where the code laboratory varies i from 1 to p, the replicate j from 1 to n, SS is the sum of square.</figcaption></figure>
<table><caption>**Table 3**
Measurement method precision (repeatability and reproductibility) estimation for both ranges and for each measurement level.</caption><thead><tr><th rowspan="2"></th><th colspan="3">Environment range</th><th colspan="3">Waste range</th></tr><tr><th>measurement level</th><th>relative repeatability standard<br>uncertainty: s<sub>i</sub> %</th><th>relative reproductibility standard<br>uncertainty: s<sub>R</sub> %</th><th>measurement level</th><th>relative repeatability standard<br>uncertainty: s<sub>i</sub> %</th><th>relative reproductibility standard<br>uncertainty: s<sub>R</sub> %</th></tr></thead><tbody><tr><td>Liquid<br>scintillator</td><td>Bq/kg</td><td></td><td></td><td>Bq/kg</td><td></td><td></td></tr><tr><td>Usual LS</td><td>15</td><td>14 %</td><td>17 %</td><td>1000</td><td>1.7 %</td><td>1.7 %</td></tr><tr><td><strong>ProSafe LT+</strong></td><td></td><td><strong>13 %</strong></td><td><strong>18 %</strong></td><td></td><td><strong>4.6 %</strong></td><td><strong>6.8 %</strong></td></tr><tr><td>Usual LS</td><td>100</td><td>3.9 %</td><td>4.9 %</td><td>10,000</td><td>1.0 %</td><td>1.3 %</td></tr><tr><td><strong>ProSafe LT+</strong></td><td></td><td><strong>5.0 %</strong></td><td><strong>9.4 %</strong></td><td></td><td><strong>1.3 %</strong></td><td><strong>4.3 %</strong></td></tr><tr><td>Usual LS</td><td>1000</td><td>1.8 %</td><td>2.6 %</td><td>100,000</td><td>0.6 %</td><td>0.8 %</td></tr><tr><td><strong>ProSafe LT+</strong></td><td></td><td><strong>2.4 %</strong></td><td><strong>6.2 %</strong></td><td></td><td><strong>1.2 %</strong></td><td><strong>3.8 %</strong></td></tr></tbody></table>
<table><caption>**Table 4**</caption><caption>Relative expanded (k = 2) measurement uncertainty estimates for both ranges, the different measurement levels and the two liquid scintillators.</caption><thead><tr><th colspan="2">³H measurement by liquid scintillation</th><th colspan="2">Environment range</th><th colspan="2">Waste range</th></tr></thead><tbody><tr><td>Measurement level (Bq/kg)</td><td></td><td>15</td><td>100</td><td>1,000</td><td>10,000</td></tr><tr><td rowspan="2">Relative measurement expanded uncertainty (k = 2)</td><td>Usual LS</td><td>36 %</td><td>11 %</td><td>7 %</td><td>4 %</td></tr><tr><td>ProSafe LT+</td><td>36 %</td><td>20 %</td><td>13 %</td><td>10 %</td></tr></tbody></table>

Following this evaluation of the performance of an NPE-free scintillating liquid for tritium measurement, some of the laboratories participating in the comparison now use ProSafe LT+ and report their measurement results with the performance determined here. Following the success of this work, the CETAMA consortium decided to organise the same type of interlaboratory comparisons to evaluate the performance of an NPE-free scintillating liquid for use in carbon-14 measurement.

## 4. Conclusion

The CETAMA consortium organised interlaboratory comparisons with 23 participants to study the performance of the new NPE (nonylphenol-ethoxylate)-free liquid scintillator (LS), ProSafe LT+ for tritium measurement. The background and its noise, the decision threshold and the detection limit estimates were not significantly different from that of the usual (NPE-containing) LS. When it comes to measuring samples, and particularly samples with tritium activity above 15 Bq/kg, the measurement uncertainty obtained with ProSafe LT+ is systematically higher. The authors recommend to use the ProSafe LT+ after checking that the deterioration in the measurement uncertainty is compatible with the laboratory's needs. The authors are well aware that the results presented in this manuscript are descriptive rather than explanatory: it would be interesting to follow up this work with a theoretical study of the various factors that could explain the observations described herein.

## CRediT authorship contribution statement

Céline Augeray: Investigation. Sophie Lamy-Hagelstein: Investigation. Audrey Bacchetta: Investigation. Marielle Lecomte: Investigation. Lionel Bec-Espitalier: Investigation. Nicolas Baglan: Investigation. Hugo Jaegler: Investigation. Nathalie Jean-neret: Investigation. Véronique Labed: Investigation. Laure Lainé: Investigation. Alain Thoreux: Investigation. Cédric Rivier: Writing – original draft, Supervision. Claire Gégout: Investigation. Daniel Orjollet: Investigation. Héloïse Gervot: Investigation. Aurélie Ritt: Investigation. Aurélie Gollion: Investigation. Saroa Rozas: Investigation. Linda Gravier: Investigation. Lionel Tenailleau: Investigation. Nicolas Debain: Investigation. Myriam Le Ferrec: Investigation. Natacha Einaudi: Investigation. Yvan Losset: Investigation. Sandrine Fleury: Investigation. Valérie Lourenço: Writing – original draft, Investigation. Kevin Galliez: Investigation. Rabha Mokrani-Kerdjoudj: Investigation. Théo Cordara: Investigation. Benoît Daniel: Investigation. Coryse Combes: Investigation. Marielle Crozet: Writing – original draft, Validation, Supervision, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Pascale Lecuyer: Investigation. Eric Bohaud: Investigation. Giacomo Canciani: Investigation. Sabine Charbouillot: Investigation. Christèle Colin: Investigation.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

The authors thank Hidex for the LS supply and Magali Bertaud, Valentin Blais, Francis Crumière, Thomas Grangeon, Emma Ligneul, Elodie Pouzol, Sandra Quell, Thibault Ringler, Julie Saint-Aubert, Alexandre Sement from laboratories taking part in interlaboratory comparisons.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.eti.2025.104666.

## Data availability

Data will be made available on request.
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