Dose Rates Comparative Study for Workers Involved in the Hot-Cells Clean-Up Activities of the VVR-S Nuclear Research Reactor under Decommissioning

Abstract

The present study consists in the assessment of the dose rates potentially received by the workers involved in Hot Cells decontamination from a VVR-S type Nuclear Research Reactor under decommissioning. Two exposure scenarios were considered: the dosimetrist performing contamination scanning measurements (H*(10) inside of the Hot Cell prior decontamination and the mechanical worker performing floor decontamination. The dose rates were calculated based on the floor hot spots activity concentration using a standard and a numerical method (RESRAD Build code) assuming that the highest radiological risks are from these surfaces. It was noticed that the external dose rate is relatively high both for the floor scanning and decontamination and the internal committed effective dose is relatively low for floor decontamination due to fact that the worker is equipped with a high filter efficiency mask. By comparing the two methods results it is noticed that the dose rate obtained using the numerical method is 32% lower than the dose rate evaluated with the standard method, due to model complexity.

1. Introduction

The VVR-S (water cooled and moderated) type, nuclear research reactor) from “Horia Hulubei” National Institute for R&D in Physics and Nuclear Engineering (IFIN-HH), Romania was operated between 1957 and 1997 without any radiological major incident. The reactor main purposes were the radioisotopes production for medical and industrial applications and the research in physics, biophysics, and biochemistry.

Due to the reactor’ components and systems aging the specialists and the Romanian Government decided to decommission the facility. Thus, the reactor decommissioning was performed between 2010 and 2020.

During the operation period, the radioisotopes production was performed using four Hot Cells located in the Reactor Building basement (see Figure 1). A detailed description of the Hot Cells design, purpose and usage is presented in paper [2].

Figure 1.

Reactor overview [1].

The reactor decommissioning process was split in 3 main phases. The Hot Cells were decommissioned in the 3-rd one. The main tasks performed during Hot Cells decommissioning consisted of waste evacuation, internal surfaces decontamination and internal components dismantling (equipment’s and stainless-steel lining). The wastes from the Hot Cells no. 3, 2 and 1 were remotely controlled evacuated using the mechanical arms and a trolley [2].

The wastes from the Hot Cell no. 4 were manually evacuated due to the mechanical arms malfunction. Also, the internal surfaces decontamination was performed manually. After the waste’s evacuation the Hot Cell internal surfaces were scanned to identify the contamination level [2].

The measurements revealed that the main contamination is located on the Hot Cell floor because of the irradiated substances spilling from vials or capsules during the operation period [2].

Prior the Hot Cells decontamination the dose rate equivalent was assessed by two different methods to prevent over exposure for the workers involved in this process.

2. Methods for dose assessment

Dose rate equivalent was calculated using a standard and a numerical calculation method, RESRAD-BUILD code modeling, based on in-situ activity concentration measurements. For this purpose, we considered the worst potentially over exposure scenario, the Hot Cell no. 4 decontaminations, because the process should be performed manually.

2.1 Standard method

To determine the potential exposure of the dosimetrist and mechanical worker during the Hot Cell decontamination a standard calculation method was used.

For dose rate calculation we assumed that the dosimetrist performs 5 minutes of the Ambiental Dose Equivalent H*(10) direct measurements inside of the Hot Cell, being positioned outside at about 70 cm from the hot area along the horizontal axis (see Figure 2) [2].

Figure 2.

Floor scanning [2].

For this purpose, a Thermo Scientific™ FH 40 G type, portable digital survey meter, with FHZ 612–10 gamma dose rate probe was used according with the specific procedure [3]. In order to determine the hot spots, the detector was placed less than 1 cm above the surface. Seven hot spots were identified on the Hot Cell no. 4 floor [2].

Parallel measurements were performed using dosimeters with thermo-luminescent detectors (TLDS), GR-200A type high sensitivity tissue equivalent [4].

The TLDS were placed on each hot spot for an hour (see Figure 3). A detailed description of the equipment used, and laboratories is presented in paper [2].

Figure 3.

TLD-s position [2].

The hot spots activities used for dose rate calculation were estimated by indirect measurement of the samples sampled on 100 cm² surface around each hot spot, using a gamma-ray spectrometry system with a GEM60P4–95 high-purity germanium coaxial detector (HPGe) [2].

The measurements were performed in compliance with EN/ISO IEC 17025:2005, according to the specific procedure [5, 6, 7, 8].

The penetrant dose rate for the workers was calculated using the activity concentration of the radionuclides of each hot spot, according to the methodology presented in paper [2]. For this purpose, we assumed that the sampling yield for activity measurement is 10% and the activity is concentrated in the hottest point (with a total activity equal with the sum of all hot spots) [2].

The internal committed effective dose, E (50), was calculated considering that it was concentrated in the air only 10⁻⁴ floor total activity and that the worker wore a mask with 99% filter retention efficiency [2].

For the mechanical worker dose calculation, we considered that the decontamination process was performed in three steps (4 minutes each).

First, the worker sprayed decontaminant (DeconGel type 1108), from 90 cm (on vertical axis) respective 45 cm (on horizontal axis) relative to the contaminated area, on the floor (see Figure 4a) [2].

Figure 4.

(a) Decomtaminat spraying [2]. (b) Decontaminant spreading [2]. (c) Gel film removal [2].

Then, the hydro gel coating was spread with a V-tooth trowel, from 40 cm (on vertical axis) and 45 cm (on the horizontal axis) relative to the contaminated area (see Figure 4b) [2]. Finally, the worker removed the gel film after drying it, from a similar position to the first step (see Figure 4c) [2].

2.2 Numerical method: RESRAD-BUILD code modeling

Considering that the mechanical worker exposure could be much higher than the dosimetrist, a supplementary assessment was performed using RESRAD Build computer code (3.5 version).

The code is specifically designed for radiation doses and risks estimation of RESidual RADioactive materials on radioactively contaminated sites [9].

The external radiation penetrating the walls, ceilings or floors is calculated based on the input parameters for shielding material type, thickness and density. Shielding material can be specified between each source-receptor pair. The internal exposure is calculated based on the air quality model that considers the air exchange between compartments (rooms) and with outdoor air [9]. The code takes into consideration following exposure pathways: the external exposure due to the source, materials deposited on the floor as well as air submersion, the internal exposure due to the airborne radioactive particulates’ inhalation, inhalation of the aerosol’s indoor radon progeny, the inadvertent ingestion of the radioactive material directly from the source or materials deposited on the surfaces (see Figure 5) [9].

Figure 5.

RESRAD Build code exposure pathway [9].

The total external dose at time t, over the exposure duration, ED, to a volume source containing radionuclide n in compartment i, $D_{iV}^n\left(t\right)$, was calculated using (Eq. (1)) [9]:

$$D_{iV}^n\left(t\right)=\left(\frac{ED}{365}\right)F_{\mathit{in}}·F_i·\overline{C_{sV}^n}·DC{F}_V^n·F_G^n$$E1

where:

ED is the exposure duration (days), total length of time considered by the dose assessment, including intervals during which receptors may be absent from the building or a contaminated indoor location; 365 is time conversion factor (days/year); Fin is the fraction of time spent indoors; Fi is the fraction of time spent in compartment i; $DC{F}_V^n$ is the FGR-12 dose conversion factor for infinite volume source [(mrem/yr)/(pCi/g)]; $F_G^n$ is the geometrical factor for finite area, source thickness, shielding, source material, and position of receptor relative to the source for radionuclide n; $\overline{C_{sV}^n}$is the average volume source concentration (pCi/g) of radionuclide n over the exposure duration, ED, starting at time t [9].

For the studied case (Hot Cell no. 4), the following input data and parameters was taken into consideration (see Figure 6 and Table 1): 1 compartment, the hot cell itself, the source, a 2 m radius circle located in the Hot Cell center. The RESRAD Build considers the area source as being a small thickness volume source (0.01 cm) with up to five layers any one contaminated [9]. Three distinct receptor (the mechanical workers) (x, y, z) positions relative to the source were also considered: R1 (1.45 m, 0.60 m, 0.90 m) – receptor 1 performing decontaminant spraying, R2 (0.55 cm, 0.60 cm, 0.45 cm) - receptor 2 spreading the decontaminant, R3 (0.55 m, 0.40 m, 0.90 m) – receptor 3 removing the gel film.

Figure 6.

RESRAD Build code input parameters on floor decontamination.

Parameter	Unit	Building renovation scenario	Remarks
Exposure duration	days (d)	30.00	Total length of time considered for dose assessment, including intervals during which receptors may be absent from the contaminated indoor location
Indoor fraction	dimensionless	0.003	Fraction of the exposure duration spent by one or more receptors inside a building [9] (working time/exposure duration)
Receptor location	m	R1 (1.45, 0.60, 0.90) R2 (0.55, 0.60, 0.45) R3 (0.55, 0.40, 0.90)	Position relative to source center
Receptor inhalation rate	m^3/d	28.8 [2]	For building renovation scenario, the breathing rate of moderate activity is 1.6 m^3/h in 24 hours, (breathing rate x 24 h) [10]
Receptor indirect ingestion rate	m^2/h	0.00001	Due to the mask wearing [2]
Source type -	—	Area	Source geometry
Direct ingestion rate	1/h (area) g/h (volume)	0.052	Calculated from the default ingestion rate of 1.1 x 10^−4 m^2/h in NUREG/CR-5512 building occupancy scenario [11]
Air release fraction	—	0.0001	Fraction of mechanically removed or eroded material that becomes airborne. For building renovation scenario, a smaller fraction is breathable [9]
Removable fraction	—	Not Required for our analysis	According with NUREG/CR-5512, 10% of the contamination is removable. For the volume source is null [12]
Time for source removal or source lifetime	d	Not Required for our analysis	Value for the building occupancy scenario is the most likely value from the parameter distribution [9]. The parameter is not required for the volume source.
Source erosion rate	cm/d	0	Due to very short time of decontamination process

Table 1.

Input parameters for case study.

The receptor inhalation rate is 28 m³/day (1.2 m³/h x 24 h) [2]; the receptor ingestion rate is 0 (the workers wearing respiratory mask) [2] and the air release fraction (fraction of mechanically removed material that becomes airborne) is 0.0001. The removable fraction and time for source removal (source lifetime) are not required for this kind of scenario - building renovation; the source erosion rate (cm/day) is 0 due to very short time of the decontamination [9].

We assume that is necessary to perform 3 decontamination cycle, 12 min. Each. The dose rate was calculated based on hottest spot activity (A₇) considering a 30-day exposure duration [13] and the indoor fraction (report between the working time and the exposure duration) of 0.003.

The assessment was performed: initially, after 7 days, 14 days, 21 days and 28 days respectively considering that the initial activity (Ai) decreased in time from 100–15%.

3. Results

The mean values of the Ambiental Dose Equivalent H*(10) are presented in Table 2 [2]. For the six hot spots the mean value, determined by floor contamination scanning is 15.6 mSv/h and respectively 28.6 mSv/h for TLDs measurements [2].

Hot spots	H^*(10)scan [mSv/h]	H^*(10)TLD [mSv/h]	Probe position above the hot spot [cm]	TLD position toward the hot spot [cm]
A1	23.00 ± 2.76	31.70 ± 4.08	1.22	1.04
A2	9.40 ± 1.13	31.80 ± 4.54	0.90	0.49
A3	17.00 ± 2.04	31.87 ± 4.71	0.83	0.61
A4	15.00 ± 1.08	41.40 ± 7.63	0.89	0.53
A5	18.00 ± 2.16	23.97 ± 2.37	0.57	0.50
A6	11.00 ± 1.32	10.70 ± 0.94	0.90	0.91
A7	400.00 ± 20.00	782.00 ± 6.07	6.49	4.64

Table 2.

Ambiental dose equivalent H*(10) [2].

The main risk for dosimetrist during contamination scanning is due to A₇ hot spot. For this hot spot, the Ambiental Dose Equivalent is about 26 (27 for TLDs) times higher than the mean value of the other six hot spots [2].

In the hottest point, A₇, the ⁶⁰Co activity (4.57E+08 Bq) is 3 orders of magnitude higher than the mean value of the other six hot spots (3.60 E+05 Bq) thus, the results for the Ambiental Dose Rate measurements are confirmed [2]. The activities of the ¹³⁴Cs, ¹³⁷Cs and ^108mAg are 3 and 2 times lower than ⁶⁰Co activity [2].

The penetrant dose rate for the workers was calculated by standard method assuming that the sampling yield for the activity measurement is 10% and that the entire activity is concentrated in the hottest point. The values are presented in Table 3. According to Romanian legislation the professional exposed dose limit is 20 mSv/year, and for 2000 working hours/year the dose rate is 10 μSv/hour [2].

Hot spot	${\dot{H}}_p$(10)scan [mSv/h]	${\dot{H}}_p$(10)decon. [mSv/h]	E (50) decon. [mSv]
A1	5.36E-03	1.12E-02	3.33E-06
A2	1.18E-03	2.45E-03	7.48E-07
A	2.09E-03	4.71E-03	1.14E-06
A4	2.41E-03	5.85E-03	1.16E-06
A5	1.19E-03	2.87E-03	5.74E-07
A6	2.45E-03	5.52E-03	8.55E-07
A7	3.37E+00	7.93E+00	1.62E-03
Total	3.39E+00	7.97E+00	1.63E-03

Table 3.

Dose rate assessed by standard method [2].

For the dosimetrist, the penetrant dose rate is 3.39 mSv/h, respectively 0.28 mSv for five minutes exposure and the risk are quite high. Consequently, the working time for future activities must not be longer than 5.9 hours/year [2]. For the mechanical worker, the penetrant dose rate is 7.97 mSv/h, 1.59 mSv for 12 minutes. The risk is also very high, consequently the working time must be less than 2.5 hours/year for future activities [2].

The internal committed effective dose E (50) was calculated for mechanical worker performing floor decontamination. For this purpose, we assume that inside of the hot cell is spread only the 10⁻⁴ of the total activity and the worker wearing a mask with filter (99% retention efficiency) [2]. The values are presented in Table 3. The internal irradiation could be due to the A₇ the hottest point presence (1.62 μSv). The dose is low enough and the internal irradiation does not affect the worker due to the high filter efficiency [2].

The dose rates for mechanical workers (receptors) who performed the floor decontamination, assessed using RESRAD Build code modeling, are presented in Table 4 as well as in the Figure 7.

Time [days]	Ai [%]	Ai	R1 [mSv/h]	R2 [mSv/h]	R3 [mSv/h]	Total [mSv/h]
0	100	4.57E+09	1.34E+00	2.86E+00	1.22E+00	5.42E+00
7	60	2.74E+09	8.04E-01	1.71E+00	7.30E-01	3.24E+00
14	40	1.10E+09	3.20E-01	6.80E-01	2.91E-01	1.29E+00
21	25	2.74E+08	7.97E-02	1.70E-01	7.25E-02	3.22E-01
28	15	4.11E+07	1.20E-02	2.54E-02	1.09E-02	4.82E-02

Table 4.

Dose rate assessed by numerical method.

Figure 7.

RESRAD Build dose rate.

At the inception of the decontamination process, the highest potential dose rate was received by the receptor 2 (R2) who spread the decontaminant on the floor. A similar behavior could be noticed for receptor 1 (R1) performing decontaminant spraying and receptor 3 (R3) performing decontaminant coating gel removal.

The potential dose received by R2 receptor is 2.1 times higher than the dose received by R1 and respectively 2.3 times higher for R3.

Although, dose rate decreased after 28 days (the end of floor surface decontamination) the value it is at about 5 times greater than the limit of 10 μSv/hour.

By comparing the total initial dose rates for the mechanical worker performing all decontamination steps, at the inception of the decontamination process, it can be noticed that the RESRAD dose (5.42 mSv/h) is 1.5 lower than the dose evaluated by standard method (7.97 mSv/h). The difference can be explained by RESRAD model complexity. Consequently, the working time of the mechanical worker must be less than 3.7 hours/year.

In the proposed scenario, according to the hot spot’s activities, the greatest risk is presented by the ⁶⁰Co and ¹³⁷Cs [2]. The risk is very high because the decontamination scenario considers that the task is performed manually.

The radiation level remains significant after three decontamination cycles since the stainless-steel lining is activated.

4. Recommendation and future perspective

Due to high dose, the risks for the workers are considerable. In such circumstances we also consider that the clean-up process should be performed using robots [13] instead of the workers, according with the ALARA principle.

In order to avoid the dosimetrist and mechanical worker over exposure, prior decontamination, the sources located inside on the Hot Cell are extracted using a remote-controlled car (see Figure 8). The process is monitored by the dosimetrist from distance using a portable digital survey meter [2], (see Figure 9).

Figure 8.

Robot for sorce removal.

Figure 9.

Sources removal monitoring.

The decontaminant gel should be also sprayed from distance by the worker located outside of the Hot Cell (see the Figure 10). Then the decontamination should be performed using a Schunk robotic arm mounted on a Neobotix Platform [13, 14].

Figure 10.

Outside decontaminat spraying.

5. Conclusions

Dose rates received during a nuclear research reactor Hot Cells decontamination were assessed by a standard as well as a numerical method.

For both assessments resulted that the dose for professional exposed is higher than the limit. The risk is very high due to the fact that decontamination process was performed manually.

After three decontamination cycles, the radiation level remains significant because the stainless-steel lining is activated. The clean-up and decontamination should be performed by the robots in order to prevent workers over exposure.

The dose rates calculated with RESRAD Build code are lower and more accurately than the those obtained by standard method due to model complexity.

Despite small differences, it can conclude that both methodologies for dose assessment are in agreement and useful for similar exposure situations.

Acknowledgments

The authors offer many thanks to Dr. A.O. Pavelescu for his suggestions and feedback when clarifications of issues were required and also to Mr. Adrian Zorliu for his support for samples sampling.