**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].

*Dose Rates Comparative Study for Workers Involved in the Hot-Cells Clean-Up Activities… DOI: http://dx.doi.org/10.5772/intechopen.99901*

**Figure 2.** *Floor scanning [2].*

**Figure 3.** *TLD-s position [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].

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 [5–8].

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].

#### *Radiopharmaceuticals - Current Research for Better Diagnosis and Therapy*

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].

*Dose Rates Comparative Study for Workers Involved in the Hot-Cells Clean-Up Activities… DOI: http://dx.doi.org/10.5772/intechopen.99901*

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** 4**c**) [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].

The total external dose at time t, over the exposure duration, ED, to a volume source containing radionuclide n in compartment i, ( ) *<sup>n</sup> D t iV* , was calculated using (Eq. (1)) [9]:

$$D\_{iV}^{u}\left(t\right) = \left(\frac{ED}{\textbf{365}}\right)F\_{iu} \cdot F\_i \cdot \overline{C\_{sV}^{u}} \cdot DCF\_V^{u} \cdot F\_G^{u} \tag{1}$$

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);

**Figure 5.**

*RESRAD Build code exposure pathway [9].*

#### **Figure 6.**

*RESRAD Build code input parameters on floor decontamination.*

Fin is the fraction of time spent indoors; Fi is the fraction of time spent in compartment i; *<sup>n</sup> DCFV* is the FGR-12 dose conversion factor for infinite volume source [(mrem/yr)/(pCi/g)]; *<sup>n</sup> FG* is the geometrical factor for finite area, source thickness, shielding, source material, and position of receptor relative to the source for radionuclide n; *<sup>n</sup> CsV* 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.


*Dose Rates Comparative Study for Workers Involved in the Hot-Cells Clean-Up Activities… DOI: http://dx.doi.org/10.5772/intechopen.99901*


#### **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].


#### **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].

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).


#### **Table 3.**

*Dose rate assessed by standard method [2].*

*Dose Rates Comparative Study for Workers Involved in the Hot-Cells Clean-Up Activities… DOI: http://dx.doi.org/10.5772/intechopen.99901*


**Table 4.**

*Dose rate assessed by numerical method.*

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**.

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.

**Figure 7.** *RESRAD Build dose rate.*

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