96 G. Yunwen: Radioprotection 2025, 60( 1), 91 – 98
Table 3. Actual measurement results.
Hospital No. |
1 |
2 |
3 |
4 |
Number of beds for thyroid cancer |
12 |
24 |
3 |
22 |
Total volume of decay tanks( m 3) |
350 |
135 |
99 |
198 |
Volume per tank( m 3) |
70 |
45 |
33 |
66 |
Sampled decay tank |
1 |
1 |
1 |
B |
Actual collection volume( m 3) |
70 |
43.2 |
33 |
66 |
Collection period |
2023.8.21-2023.10.19 |
2023.12.4-2024.1.5 |
2023.8.7-2023.10.07 |
2023.10.14-2023.11.24 |
Collection duration( days) |
59 |
32 |
61 |
41 |
liquid effluent Input |
Toilet þ Shower |
Toilet |
Toilet |
Toilet |
Specific activity of 131 I on the day of the |
4.28Eþ05 |
1.67Eþ06 |
1.35Eþ06 |
1.62Eþ06 |
tank is full( Bq / L) |
|
|
|
|
Theoretical total activity( kNA, Bq) |
2.10Eþ11 |
4.21Eþ11 |
5.26Eþ10 |
3.86Eþ11 |
Measured total activity( specific activity |
3.00Eþ10 |
7.21Eþ10 |
4.46Eþ10 |
1.07Eþ11 |
volume, Bq) |
|
|
|
|
Theoretical minimum storage time( days) |
116.43 |
124.46 |
100.39 |
123.45 |
Monthly discharge after minimum |
1.28Eþ06 |
1.54Eþ06 |
7.62Eþ06 |
2.49Eþ06 |
storage time( Bq) |
|
|
|
|
Below monthly discharge limit( 9Eþ6Bq) or not |
Yes |
Yes |
Yes |
Yes |
The theoretical activity is derived under the assumption of the hospital operating at full capacity, with consecutive batches of patients and no intervals, and with conservative assumptions in terms of discharge ratio. Therefore, the result obtained from the calculation formula is actually the upper limit of the total activity of 131 Ior 177 Lu in the tank.
For the 131 I isotope, the minimum storage time T min is proportional to ln( N), where N is the number of patients. The volume of liquid effluent discharged into the decay tank from the 131 I treatment ward per day is also proportional to N. According to equations( 12) and( 13), the volume of the decay tank required for 131 I is proportional to N ln( N).
For the 177 Lu isotope, the required minimum volume of the decay tank is directly proportional to the number of patients.
The daily water consumption per patient W 0 has a significant influence on the required volume of decay tanks. For medical institutions with limited expandable land space, limiting the daily water consumption per patient can reduce the required volume of the decay tanks, thereby lowering land requirements and construction costs.
7 Conclusions This paper presents the theoretical activity formulas for 131 I and 177 Lu in a full decay tank, demonstrating the limit on
maximum allowable discharged activity specified in Section 8.6.2 of GB 18871and the specific activity of total betaemitting radionuclides at the outlet of decay tanks specified in GB 18846 are the practical restrictions for the isotopes 131 I and 177 Lu, respectively. It also proposes a method for determining
the decay tank volume and the minimum storage time for liquid effluent.
For hospitals and regulatory authorities, if the intended number of patients, hospitalization cycle, average dosage, daily liquid effluent volume discharged into the decay tank are determined, the method provided in this paper can be used to determine the required volume of decay tanks. The minimum storage time can also be calculated. For regulatory authorities, this serves as a basis for assessing whether the planned decay tank volume meets the requirements when approving environmental impact assessment documents.
Given that the theoretical activity derived in this paper represents the upper limit of the actual total activity of 131 Ior 177 Lu in the decay tank, the actual total activity is often
significantly lower than the theoretical calculation provided by this method. Therefore, if the storage time of the liquid radioactive effluent exceeds the calculated minimum storage time, it can be theoretically proven that the discharge meets regulatory requirements without the need for monitoring data. Actual monitoring results from four hospitals also proved the conclusion.
Acknowlegments
The author would like to thank the Provincial Radiation Station for providing the monitoring data support.
Funding
This research did not receive any specific funding.
Conflicts of interest
The author declares that they have no conflict of interest. Data availability statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.