Estimation of the radiation dose of ¹⁰⁷Pd in palladium products and preliminary proposal of appropriate clearance level

ABSTRACT

The effective radiation dose of palladium under the assumption that it is contaminated with ¹⁰⁷Pd (1 Bq/g) is estimated through four exposure pathways: the inhalation of airborne particles emitted from an automobile exhaust control catalyst, ingestion via food or drinking water, ingestion of palladium in saliva after release from a dental application, and inhalation exposure of dust in the work place. The highest dose (3.17 × 10⁻⁹ μSv/y) was estimated in adult workers manufacturing palladium products. Considering variations in parameters and possible exposure pathways, it is concluded that this is the critical group for determining the clearance level of ¹⁰⁷Pd. The clearance level, which is defined as a ¹⁰⁷Pd concentration providing less than 10 μSv/y in any exposure pathway, is calculated as 3.16 × 10³ Bq/g.

KEYWORDS:

• Palladium
• Pd-107
• high-level radioactive waste
• HLW
• recycle
• reuse
• clearance level
• radiation dose

1. Introduction

The safe and efficient processing of high-level radioactive waste (HLW) from spent nuclear fuels used in light-water cooling reactors is an important issue in the nuclear industry. A research and development program regarding the partitioning and transmutation technology for reduction of HLW, and the resource recycling of useful elements from such waste, has recently been progressing in Japan [1–3]. Palladium is a precious metal of the platinum group, occurring rarely in nature, and has significant commercial importance. One ton of HLW contains approximately 1 kg of palladium, and it is one of the target elements to be recovered and recycled in this R&D program.

However, recovered palladium contains 17% ¹⁰⁷Pd, a long-lived fission product (LLFP) with a half-life of 6.5 × 10⁶ years [4]. Although ¹⁰⁷Pd can be removed from recovered palladium using a separation process such as odd-mass-selective ionization, a small amount of ¹⁰⁷Pd contamination is unavoidable [5]. Prior to releasing this element into the environment, it is therefore necessary to evaluate its radiation dose and determine the so-called clearance levels.

The lifecycle of palladium (namely, the material flow from production, commercial distribution, and utilization, to disposal or recycling), as well as its behavior in the natural environment have been previously reported [6]. The study identified four major routes through which humans may be exposed to palladium: (1) the inhalation of airborne particles containing palladium from automobile emission control catalysts, (2) ingestion of palladium in food or drinking water, (3) ingestion of palladium released from a dental application, and (4) inhalation of dust or aerosol in industrial or dental processes using palladium products. The first three routes are exposure situations for the public, and the last route is an inhalation exposure at working environment.

In the present study, effective radiation doses are estimated for each of the above routes using previously reported parameters. The variation and reliability of parameters are then validated and the clearance level of ¹⁰⁷Pd is determined with respect to releasing palladium products into the public environment.

2. Materials and methods

2.1. Inhalation of ¹⁰⁷Pd in particulate matter emitted from automotive exhaust catalysts

The necessary parameters for estimating radiation doses through this pathway are the concentration of palladium in the air, respiratory volume, duration of inhalation, and the dose coefficient (conversion factors, from inhaled ¹⁰⁷Pd activities to the effective dose). A calculus equation to estimate exposure doses is presented in Equation (A1)(A1) $E_{inh,Pub}=C_{dust}\cdot V_{Pub}\cdot t_{e,hour}\cdot C_{Pd}\cdot e_{inh,Pd}$(A1) .

Many studies have reported the concentrations of palladium in air, and reports written before 2004 were collected and reviewed by Ravindra et al. [7]. The Pd concentration in the air increased between 1970s and 1990s with the increase in the use of Pd as automobile catalysts, and were correlated with traffic conditions (e.g. the number of passing cars) [8–18]. For example, in an urban roadside area with heavy car traffic in Frankfurt, Germany, the air concentration of palladium was found to be 1.2–683 (43.9 on average) pg/m³, whereas it was found to be 0.2–10.9 (2.6 on average) pg/m³ in a rural area in Neuglobsow, Germany [17]. Previously reported palladium concentrations are summarized in Table 1. The measured value of 683 pg/m³ is the highest among previous data, and this value is used for conservative dose estimations.

Table 1. Previously reported atmospheric concentrations of Pd

City/country	Other specification	Pd concentration (pg/m^3)	References
California	–	< 0.06	Johnson et al. [8]
Ghent, Belgium and Milan, Italy	–	< 0.7	Schutyser et al. [9]
Caesarea, Israel	PM2.5	3.3	Gertler [10]
Czech Republic	Various observation sites	30–280	Vlašánkova et al. [11]
Berlin, Germany	Urban environment	0.2–14.6	Tilch et al. [12]
Rome, Italy	Urban sites (heavy traffic)	21.2–85.7	Petrucci et al. [13]
Göteborg, Sweden	Urban area, PM10	0.1–10	Rauch et al. [14]
Madrid, Spain	M-30 highway, PM10	5.1–32	Gómez et al. [15]
Vienna, Austria	Weekly averages, PM10	2.6	Kanitsar et al. [16]
	particle size < 30 µm	14.4
Frankfurt, Germany Neuglobsow	Urban road side	1.2–683 (av. 43.9)	Zereini et al. [17]
	Rural area	0.2–10.9 (av. 2.6)
Croatia	Urban road side	3.9–5.6	Rinkovec et al. [18]

The respiratory volume and dose coefficient for different age groups are those recommended by the International Commission of Radiation Protection (ICRP) [19,20]. Therefore, the nominal age groups were set as 3 months, 1, 5, 10, and 15 yrs of age, and adults, in accordance with ICRP publications [19]. As the target population is the public, the duration of inhalation and the period of activity were set as 24 h under rest conditions. The ranges of actual ages in the nominal age groups, dose coefficients, and respiratory volumes under rest conditions for each age group are presented in Table 2.

Table 2. Dose coefficient for ¹⁰⁷Pd recommended by ICRP for inhalation and ingestion, and respiration volume for each age group*

Inhalation	Dose coefficient (Sv/Bq) for each age group
Type of particle	3 months	1 yr	5 yrs	10 yrs	15 yrs	Adults
	< 1 years	1–< 2 years	2–< 7 years	7–12 years	12–17 years	17 years
F	2.60E–10	1.80E–10	8.20E–11	5.20E–11	3.10E–11	2.50E–11
M	6.50E–10	5.00E–10	2.60E–10	1.50E–10	1.00E–10	8.50E–11
S	2.20E–09	2.00E–09	1.30E–09	7.80E–10	6.20E–10	5.90E–10
Respiration volume m^3	2.86	5.16	8.72	15.3	20.1	22.2
Ingestion	Dose coefficient (Sv/Bq)
	3 months	1 yr	5 yrs	10 yrs	15 yrs	Adults
	1.40E–10	2.80E–10	1.40E–10	8.10E–11	4.60E–11	3.70E–11

* Values are those from ICRP publication 119.

2.2. Ingestion of ¹⁰⁷Pd contained in food and drinking water

The necessary parameters for estimating radiation doses from ¹⁰⁷Pd in food and water were as follows: (1) the concentration of palladium in food ingredients and drinking water, (2) the total amount ingested, and (3) the dose coefficients. Calculus equations to estimate exposure doses are presented in Equations (A2)(A2) $E_{ing,F}=I_F\cdot t_{e,day}\cdot C_{Pd}\cdot e_{ing,Pd}$(A2) and (A3)(A3) $E_{ing,W}=C_{water}\cdot I_W\cdot t_{e,day}\cdot C_{Pd}\cdot e_{ing,Pd}$(A3) for the food and drinking water, respectively.

Regarding the concentrations in food and daily uptake, only one total dietary study was available for palladium concentration in food, which was carried out by the Ministry of Agriculture, Fisheries, and Food in the UK, once in 1994 and again in 2006 [21]. In the study, multi-element analyses for 30 elements (including palladium) were conducted using ICP-MS in the major items of the national diet, which were combined into 20 groups of similar food categories. According to the report, there was little change in the concentration of palladium in food between 1994 and 2006, and the dietary exposure of the population was estimated to be 0.7 μg/d for palladium. The World Health Organization (WHO) further analyzed these data and reported that the population average and 97.5^th percentile of the total dietary intake for palladium were 1 and 2 μg/d for adults, respectively [22].

In the present study, we used a value of 2 µg/d for an estimation of the radiation dose in adults. No data were available regarding age differences relating to the daily intake of palladium, and the rates of change with age were assumed to be the same as the average intake of other minerals (Zn, Cu, and Fe). Table 3 shows the yearly intake of these minerals in Japan, and the ratio of intake for each age group to that of adults [23]. To compensate for the age difference, daily intakes of palladium in the 1- and 5-year age groups were assumed to be 61.7% that of adults, 95.7% for the 10-year age group, and 102.5% for the 15-year age group.

Table 3. Average intake of iron, zinc, and copper from food in Japan

		1–6 yrs	(%)	7–14 yrs	(%)	15–19 yrs	(%)	Above 20 yrs
Iron	mg	4.4	57.1	6.6	85.7	7.1	92.2	7.7
Zinc	mg	5.4	67.5	8.6	107.5	9.3	116.3	8
Copper	mg	0.69	60.5	1.07	93.9	1.13	99.1	1.14
Average % of adults	%		61.7		95.7		102.5	100

The daily intake of water is reported as 1.65 L/d for adults in the ICRP Reference Man [24]. For the other age groups, the amount are assumed to be 35%, 46%, 65%, and 89% of the adult intake for 1-, 5-, 10- and 15-year age groups, respectively, according to a report by the Food and Nutrition Board, Institute of Medicine of the National Academies [25]. The WHO repots the concentration of palladium in the drinking water to be within the range of 0–24 ng/L [22], and the highest value of 24 ng/L was used in the present study.

The dose was not estimated for the 3-month age group, because the food intake of this group is very different from that of the other age groups, and no data on the Pd concentrations in maternal milk have been reported.

2.3. Ingestion of dissolved ¹⁰⁷ Pd from dental applications

The use of gold-silver-palladium alloys in dentistry for application in both crowns and fillings is very common, particularly in Japan. There have been no comprehensive reviews conducted on the amount and dissolution ratios of palladium in the mouth. Therefore, in the present study, a survey on these points was conducted using mainly sporadically published research papers.

The dissolution ratios of palladium from palladium-containing dental restorations showed substantial individual variations depending on the dental condition, the material involved, and personal habits (e.g. gum chewing). Japanese researchers reported such ratios to be within the range of 0.00006–0.00059 ppm/d [26]. This means that a person using a dental crown made of 10 g of alloy (20% palladium) ingests 1.2 µg of palladium per day at maximum. The WHO reported a daily mean intake of <1.5–15 µg/d in adults, in consideration of the number of palladium-containing dental restorations and volume of saliva ingested [22]. In this study, as a conservative assumption, 15 µg/d is used for ICRP age groups of adults, and no estimations was made for the other younger age groups. A calculus equation to estimate the committed effective doses in this scenario is presented in Equation (A4)(A4) $E_{ing,D}=I_D\cdot t_{e,day}\cdot C_{Pd}\cdot e_{ing,Pd}$(A4) .

2.4. Inhalation of particulate matter-containing ¹⁰⁷Pd by industrial workers or dental technicians

The concentration of palladium in the air and dust, the work duration (in hours), and the dose coefficient for inhalation are parameters required for estimating the radiation doses. An equation to calculate the committed effective doses is presented in Equation (A5)(A5) $E_{inh,Work}=C_{dust}\cdot V_{work}\cdot t_{e,hour}\cdot C_{Pd}\cdot e_{inh,Pd}$(A5) .

The palladium concentrations in a variety of work places are summarized in Table 4. The concentrations are at relatively low levels, because ventilation is generally applied in the work place [8,22,27]. The occupational exposure limit for mineral dust (2 mg/m³), as regulated by the law in Japan, is used in the present study as the concentration of palladium in the air [28]. The other parameters applied are 2,400 h/y for the work duration, and 1.2 m³/h for the respiratory volume under labor conditions. The annual working time for full time workers in Japan is reported to be 1,972 h on average, and 2,400 h for 95.0^th percentile value [29].

Table 4. Air concentration of palladium in work place

Place	Concentration	Reference
Refinery	0.001–0.36 µg/m^3	Johnson et al.　[8]
Salt production	0.028 µg/m^3	Same as above　[8]
Production of compound	0.38–11.6 µg/m^3	Kielhorn et al.　[27]
Dental technician	3.5–15 µg/m^3 (0.5–3 µm in diameter) 0.7–1.1 mg/m^3 (total dust)	WHO　[22]
Occupational exposure limits	2 mg/m^3 (respirable particle size)	Japan Society for Occupational Health [28]

3. Results and discussion

3.1. Estimated radiation dose for each exposure route

The committed effective doses were calculated for the four exposure routes using the parameters described above. The specific activity was assumed to be 1 Bq/g. As the energy of gamma rays emitted from ¹⁰⁷Pd is very low, external exposure does not provide a significant radiation dose, and only an estimation of the internal dose is required. Table 5 shows the committed effective doses after inhaling of airborne palladium particles containing 1 Bq/g of ¹⁰⁷Pd for the different age groups. Estimated doses are not very large and are within the range of 1.38 × 10⁻¹⁶ to 3.27 × 10⁻¹⁵ Sv/y. A higher radiation dose was estimated to occur within the adult age group as compared with the other age groups, and for the class S particles (slow clearance from the respiratory tract). This finding may be attributable to the higher dose coefficient and larger respiratory volume in the adult age group.

Table 5. Committed effective doses by inhalation of airborne particles released from automotive emission control catalysts

		Effective dose (Sv/y) for each age group
		3 months	1 yr	5 yrs	10 yrs	15 yrs	Adults
Type	F	1.85E–16	2.32E–16	1.78E–16	1.98E–16	1.55E–16	1.38E–16
	M	4.63E–16	6.43E–16	5.65E–16	5.72E–16	5.01E–16	4.70E–16
	S	1.57E–15	2.57E–15	2.83E–15	2.98E–15	3.11E–15	3.27E–15

The estimated radiation doses from ingesting palladium in food and drinking water are presented in Table 6. The highest dose was estimated for the 1-year age group for both food and drinking water because the dose coefficient for ingestion is approximately 8 times higher in the younger age groups, whereas the total amount of Pd ingested in 1- and 5-year age groups is 61.7% that of adults [20,23]. The radiation doses of ¹⁰⁷Pd via ingestion from dissolved metal in dental appliances are also shown in Table 6. With an intake of palladium from dental appliances of 15 µg/person per day, the effective dose was calculated as 2.03 × 10⁻¹³ Sv/y for adults. Table 7 shows the radiation doses for inhalation of airborne dust in work areas where palladium metal alloys or chemical compounds are handled. In these cases, the doses were estimated only for adults (Table 7). Radiation doses were calculated not only for diameters of 1 µm but also for 5 µm, because it is a possible that the particle size in the working places may be larger than the particle size in a public space (1 µm in diameter), The effective doses were estimated to be relatively higher than those for the other exposure routes, namely, 3.17 × 10⁻⁹ Sv/y and 1.67 × 10⁻⁹ Sv/y for particles with diameters of 1 and 5 µm, respectively, This is because the concentration of palladium in the air within the work place was assumed to be 2 mg/m³, which is much higher than that in the atmosphere for the general public [28]. However, the actual air concentration may be lower, as indicated in Table 4.

Table 6. Committed effective doses for ingestion of ¹⁰⁷Pd in food and drinking water and for ingestion of dissolved ¹⁰⁷Pd from dental applications

	Effective dose (Sv/y) for each age group
	1 yr	5 yrs	10 yrs	15 yrs	Adults
Food	1.26E–13	6.31E–14	5.66E–14	3.44E–14	2.70E–14
Drinking water	1.42E–15	9.31E–16	7.61E–16	5.92E–16	5.35E–16
Dental applications	n. a.	n. a.	n. a.	n. a.	2.03E–13

Table 7. Committed effective doses for inhalation of airborne dust in the work place

		Effective dose (Sv/y) for adult age group
	Class	1 µm in diameter	5 µm in diameter
Occupational inhalation	F	1.50E–10	1.90E–10
	M	4.61E–10	3.00E–10
	S	3.17E–09	1.67E–09

3.2. Consideration of clearance level

The highest doses within the four exposure routes are summarized in Table 8. The doses for the public from inhalation of airborne particles originating from automobile exhaust catalysts are not very large, although the total anthropogenic emissions of palladium into the natural environment are the highest through this route [30]. Workers may receive a relatively high radiation dose if the concentration of dust in the air reaches the maximum level regulated by the law in Japan (2 mg/m³) [28], and all dust particles consist of palladium. However, it is necessary to note that the actual (measured) air concentration of palladium in different work areas is not very high, as indicated in Table 4.

Table 8. Highest effective doses for each of the four exposure pathways and concentrations of ¹⁰⁷Pd (Bq/g) providing a total exposure of 10 µSv/y

	Highest dose (Sv/y)	Concentration (Bq/g) which may give 10 µSv/y	Specification
Automobile catalyst	3.27E–15	3.06E+ 09	Adult, Class S
Food and drinking water	1.28E–13	7.84E+ 07	1 year age group
Dental appliance	2.03E–13	4.94E+ 07	Adult
Occupational inhalation	3.17E–09	3.16E+ 03	Adult, Class S

Materials used in a radiation-controlled area can be transferred to and used in a non-radiation-controlled area if they are not (or are only slightly) contaminated by radioactive isotopes. The radioactive concentration (Bq/g) for a specific radionuclide under which such materials can be used outside of a radiation-controlled area is called the clearance level. The clearance levels for solid materials released from nuclear reactor facilities were determined by the former Nuclear Safety Commission in 1999 in Japan, and the effective dose should not exceed 10 µSv/y for any type of exposure pathway. The concentrations of ¹⁰⁷Pd (Bq/g) that may result in an exposure level of 10 µSv/y are presented in Table 8. The lowest concentration was estimated within the route of occupation inhalation exposure (3,160 Bq/g), which means that if the palladium is contaminated with ¹⁰⁷Pd at this concentration, and the concentration of dust in the work environment is higher than 2 mg/m³, the workers will receive an annual effective radiation dose of 10 µSv.

A few other exposure pathways also exist, for example, from jewelry, electrical devices, and coins [28]. However, it is unlikely that more palladium is taken in from these sources than from the pathways evaluated herein. Therefore, the estimated concentration of 3,160 Bq/g may be a preliminary clearance level at present. Finally, it is important to note that all palladium considered in the dose estimation was assumed to be contaminated evenly by ¹⁰⁷Pd (1 Bq/g), which is a very conservative (safe) assumption. If palladium-containing ¹⁰⁷Pd at the proposed clearance level (3,160 Bq/g) was released into a public environment, it would take a long time until all the palladium used in the work place reaches the same concentration as the clearance level.

Acknowledgments

This work was supported by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).

Disclosure statement

No potential conflict of interest was reported by the authors.

Appendix

The committed effective dose per year for each evaluation pathway was calculated using the following equations.

1.1 Committed effective dose per year for the inhalation of aerosol released from automobile exhaust catalyst:

(A1) $E_{inh,Pub}=C_{dust}\cdot V_{Pub}\cdot t_{e,hour}\cdot C_{Pd}\cdot e_{inh,Pd}$(A1)

Here,

$E_{inh,Pub}$: committed effective dose per year,

$C_{dust}$: concentration of palladium in the air,

$V_{Pub}$: respiratory volume,

$t_{e,hour}$: duration of exposure,

$C_{Pd}$: activity concentration of ¹⁰⁷Pd

$e_{inh,Pd}$: dose coefficients for inhalation.

1.2 Committed effective dose per year for the ingestion of palladium in food and drinking water:

For food,

(A2) $E_{ing,F}=I_F\cdot t_{e,day}\cdot C_{Pd}\cdot e_{ing,Pd}$(A2)

Here,

$E_{ing,F}$: committed effective dose per year,

$I_F$: daily intake of palladium from food,

$t_{e,day}$: duration of exposure,

$C_{Pd}$: activity concentration of ¹⁰⁷Pd

$e_{ing,Pd}$: dose coefficients for ingestion.

For drinking water,

(A3) $E_{ing,W}=C_{water}\cdot I_W\cdot t_{e,day}\cdot C_{Pd}\cdot e_{ing,Pd}$(A3)

Here,

$E_{ing,W}$: committed effective dose per year,

$C_{water}$: concentration of palladium in the water,

$I_W$: daily intake of palladium from water,

$t_{e,day}$: duration of exposure,

$C_{Pd}$ activity concentration of ¹⁰⁷Pd,

$e_{ing,Pd}$: dose coefficients for ingestion.

1.3 Committed effective dose per year for the ingestion of dissolved palladium from dental applications:

(A4) $E_{ing,D}=I_D\cdot t_{e,day}\cdot C_{Pd}\cdot e_{ing,Pd}$(A4)

Here,

$E_{ing,D}$: committed effective dose per year,

$I_D$: daily intake of palladium from food,

$t_{e,day}$: duration of exposure,

$C_{Pd}$: activity concentration of ¹⁰⁷Pd

$e_{ing,Pd}$: dose coefficients for ingestion.

1.4 Committed effective dose per year for the inhalation of aerosol and dust by industrial workers and dental technicians:

(A5) $E_{inh,Work}=C_{dust}\cdot V_{work}\cdot t_{e,hour}\cdot C_{Pd}\cdot e_{inh,Pd}$(A5)

Here,

$E_{inh,Work}$: committed effective dose per year,

$C_{dust}$: concentration of palladium in the air,

$V_{Work}$: respiratory volume,

$t_{e,hour}$: duration of exposure,

$C_{Pd}$: activity concentration of ¹⁰⁷Pd

$e_{inh,Pd}$: dose coefficients for inhalation.