DISMANTLEMENT OF AN UNIT OF METABOLIC TREATMENTS WITH IODINE 131

Transcription

1 DISMANTLEMENT OF AN UNIT OF METABOLIC TREATMENTS WITH IODINE 131 Autores: J.M. Jiménez 1, A.R. Cortés 2, P.Alcorta 2 jmjimenez@hsan.osakidetza.net 1 Unidad de Radiofísica. Hospital Santiago Apóstol, Calle Olaguíbel Vitoria-Gasteiz 2 Servicio de Medicina Nuclear. Hospital Santiago Apóstol, Calle Olaguíbel Vitoria-Gasteiz Abstract. With the construction in our hospital of a new room for metabolic treatments, the closing of the current unit is planned, including the disassembly of the liquid discharging system. This item consists of a series of deposits whose emptying is carried out by a control system that includes sensors for the radiation detection, filling control sensors and some bombs and electrovalves. During the dismantlement stage, an Excel file was designed by the Radiofísica department as a support for the calculation of manual daily working times in the emptying bomb from the deposits, and a radiation monitor, a dose calibrator and a gamma counter from the Nuclear Medicine department; as indirect indicators of the process optimization. These measurements are taken in order to not exceeding the regulation limits authorized by the competent authority in the most unfavourable case. A comparative study with some recognized references was made. Once it was completed the full emptying, it was verified that the deposit walls were cold. That was verified with the help of detection instruments and the closing of the unit with the disassembly of the system as a conventional waste was done afterwards. Due to the high volume of liquid waste stored in the deposits, and to avoid the long time of storage in these ones that it could cause the deterioration and leakage or similar, the manual discharging controlled by indirect measurements is considered as the most suitable and safe solution for the dismantlement of one system of this kind. 1. Introduction During the year 2003 it was built in our hospital a new room for Nuclear Medicine treatments. In parallel it was projected the dismantlement of the old system of elimination of liquid waste being built a new one in a different location. In this paper the whole process is detailed with all phases. After 8 months of decay plus controlled evacuation since the last patient treated, the classification of the whole system was as conventional waste. In accordance with the Real Decreto 783/2001 [7], the public's protection supposes the reduction in a factor of five with respect to the previous dose limits [6]. Being based on these last ones, the Reglamento de Protección Sanitaria contra las Radiaciones Ionizantes of the year 1992 [6] tabulated the ALI (Annual Limit on Intake) values for workers and the public in general (adult). With the adoption of the new dose limits, the new Regulation [7] introduces the values of Committed Effective Dose per unit of incorporation for ingestion and inhalation (Factors of Dose) and it doesn't show values of ALI. In this paper a relationship between these two magnitudes has been made. 2. Material and methods The unit to be dismantled is a mixed system of treatment of radioactive waste manufactured by Aplicaciones Tecnológicas, model AT-131E [4] Basically it is composed of two basic elements: - Toilet Sanitrid provided with pumping system - Processing and control unit 1

2 This last one is the most tedious and it is composed of three deposits: D1, D2 and D3 of 20 litres, 400 litres and 50 litres respectively. In the Figure 1 it is shown the remote control unit. The patient's urines are directly driven to D1 from the toilet Sanitrid after the previous filtration with the aid of some resins of ionic interchange. From D1 using a bomb B1 (Figure 2), with a pumping capacity of 32 litres per minute at 3 meters head, the filtered urines go to D2 (Figure 4) that is divided in two containers in cascade: the front one with the coldest urines and the rear one with the most active liquid wastes. The working times of the bomb are programmable and it can be used in both modes, manual or automatic. Figure 1 During the regular operation of the installation with patients treated, the operation mode was automatic and during the dismantlement, that is presented in this paper, the whole process it has been carried out manually. There is a third deposit D3 for the cooling of the resins after the decrease of the capacity of the same ones. The bomb B2 (Figure 2), with a pumping capacity of 32,5 litres per minute at 3 meters head, will be used at first for the discharge of the front container. B1 B2 B3 D1 D2 D3 Figure 2 2

3 Using the bomb B3 (Figure 2), in this case it was necessary to calculate the pumping capacity 21,6 litres per minute, it was carried out the discharge of the rear container once completed the emptying of the front one. In the same way, with the aid of B3 the decayed resins were evacuated from the deposit D3 to the mains, once verified the no activity with a radiation detector G2 located in the deposit D3 (Figure 3) Selection of G2 Activity Concentration of D3 Figure 3 A spreadsheet was made to quantify along the time the working times of the bombs, the left activity and the remaining volume of both containers. With the help of a tape measure it was measured the free height after the dumping of each container and it was compared with the calculated one using the spreadsheet (Figure 4). The objective of this measurement was to confirm a good operation of the bombs and their pumping capacities after every dumping. D2 THIN WALL Rear Container Measurement of the free height Front Container Figure 4 To estimate the initial activity, an scintillation counter make Packard, model Auto-Gamma Cobra II was used. To start, it was necessary to calculate the counter efficiency by using some radioactive sources of known activities with a dose calibrator located in the Nuclear Medicine Department make Veenstra, model VDC 404 and that is submitted to the periodical quality control according to the regulations of the Spanish Legislation. 3

4 3. Results Estimation of the Counter Efficiency: Starting with the activity of four tubes of Iodine 131 (liquid) of precise activity and measured with the dose calibrator, it was also carried out a counting in the scintillation counter, using as parameters: central energy of 365 kev and window of energy of 20%. The minimum efficiency (1) is determined for the formula: C. P. S. ε 100 (1) Actividad where C.P.S. are the net counts per second obtained in the counter. With the four measurements made for the four tubes it was found a minimum efficiency of 0,327 % For each container of D2 they were taken three tubes with a volume of 1 ml that were counted afterwards in the counter. Due to the high variability detected in the measurements, it was selected the maximum value, in accordance to conservative criteria Counter Reading (C.P.M.) FRONT CONTAINER C.P.S. Activity (Bq/ml) Table I Counter Reading (C.P.M.) REAR CONTAINER C.P.S. Activity (Bq/ml) 79,4 1, ,9 9, The emptying of the front container began 3 days after the estimation of the activities shown in the Table I. Meanwhile, the rear container was allowed to decay. For both containers it was calculated with the twentieth part of the ALI (Annual Limit on Intake) for ingestion of ICRP-61 [2] and assuming a consumption of 2,5 litres of water per day for a man. In the following section, Discussion and conclusions, it is discussed this election. With a maximum concentration of activity of 2,2 Bq/litre and an exit volume of water of litres per day, the evacuation will be of 2,2 MBq per day. This daily activity was evacuated in several times every day. Dumping of the Front Container: Known the pumping capacity of the bomb B2, the time of its manual working was calculated until the full emptying. The starting activity corresponding to an initial volume of the container of 135 litres (30x80x56 cm 3 ) was 4, Bq (1,14 mci) In the Table II it is shown the spreadsheet of follow-up for the front container: 4

5 Table II Date 09/06/03 10/06/03 11/06/03 12/06/03 13/06/03 16/06/03 Bq (1 ml) 3,13,E+02 2,9,E+02 2,6,E+02 2,4,E+02 2,2,E+02 1,7,E+02 Bq/Litre 3,13,E+05 2,87,E+05 2,63,E+05 2,42,E+05 2,22,E+05 1,71,E+05 A Depos Bq 4,21,E+07 3,7,E+07 3,2,E+07 2,7,E+07 2,3,E+07 1,6,E+07 A Depos mci 1,14 0,99 0,86 0,73 0,62 0,43 Bq max/day 2,2,E+06 2,2,E+06 2,2,E+06 2,2,E+06 2,2,E+06 2,2,E+06 L per day 7,0 7,6 8,3 9,1 9,9 12,8 Tbomb(s) 12,0 14,0 15,0 16,0 18,0 23,0 Remaining litres Free Height (cm) Measur. Height (cm) 31 34, ,5 50 Date 17/06/03 18/06/03 19/06/03 20/06/03 23/06/03 Bq (1 ml) 1,6,E+02 1,4,E+02 1,3,E+02 1,2,E+02 9,3,E+01 Bq/Litre 1,57,E+05 1,44,E+05 1,32,E+05 1,21,E+05 9,33,E+04 A Depos Bq 1,3,E+07 9,5,E+06 6,7,E+06 4,1,E+06 1,5,E+06 A Depos mci 0,34 0,26 0,18 0,11 0,04 Bq max/day 2,2,E+06 2,2,E+06 2,2,E+06 2,2,E+06 2,2,E+06 L per day 14,0 15,2 16,6 18,1 23,5 Tbomb(s) 25,0 28,0 30,0 33,0 43,0 Remainig litres Free Height (cm) FULL Measur. Height (cm) 55 61,5 67,5 74,5 VOIDING Dumping of the FRONT Container Litres /06/03 13/06/03 12/06/03 11/06/03 10/06/03 09/06/03 08/06/03 23/06/03 22/06/03 21/06/03 20/06/03 19/06/03 18/06/03 17/06/03 16/06/03 15/06/03 Date Dumping of therear Container: It is started after having emptied completely the front one. Since the first samples were taken for both containers until the beginning of the emptying of the rear one have passed 34 days. The calculated decayed activity was 160 Bq/ml for the rear container. It has been decided to precise the activity to get a new set of samples and a new counting. Applying the minimum efficiency calculated (1) it has been estimated a starting activity of 115 Bq/ml. 5

6 09/07/03 10/07/03 11/07/03 12/07/03 13/07/03 14/07/03 15/07/03 16/07/03 17/07/03 The starting activity corresponding to an initial volume of the container of 130 litres (30x80x54 cm 3 ) was 1, Bq (0,40 mci) In the Table III it is shown the spreadsheet of follow-up for the rear container: Table III Date 10/07/03 11/07/03 14/07/03 15/07/03 16/07/03 17/07/03 Bq (1 ml) 1,15,E+02 1,1,E+02 8,1,E+01 7,5,E+01 6,8,E+01 6,3,E+01 Bq/Litre 1,15,E+05 1,05,E+05 8,14,E+04 7,46,E+04 6,85,E+04 6,28,E+04 A Depos Bq 1,49,E+07 1,2,E+07 7,3,E+06 4,7,E+06 2,3,E+06 1,1,E+05 A Depos mci 0,40 0,32 0,20 0,13 0,06 0,00 Bq max/day 2,2,E+06 2,2,E+06 2,2,E+06 2,2,E+06 2,2,E+06 2,2,E+06 L per day 19,1 20,8 26,9 29,4 32,0 34,9 Tbomb(s) 52,0 57,0 74,0 81,0 88,0 96,0 Remaining litres Free Height (cm) FULL Measur. Height (cm) ,5 60,5 69 VOIDING Dumping of the REAR Container Litres Date 4. Discussion and conclusions a) The method used for the dismantlement based on the decay added to the controlled evacuation, has been considered as valid, quick and safe. It fulfils the basic principles of the radiological protection as they are the optimization and the dose limitation for the members of public. b) With the limit of effective dose for the public of 1 msv per year, the Annual Limit on Intake is adopted (ALI) for ingestion in agreement with ICRP-61 [2], being for the members of public the value Bq (twentieth part of the limit for workers). Applying, as it has been written before, the principle of optimization, it is adopted the twentieth part of the ALI (2000 Bq) c) To start from the most unfavourable value Factor of Dose h(g) for ingestion for Iodine 131, in accordance with [5] and [7] for the range between 0 and 2 years that is 1, Sv/Bq, it 6

7 is calculated the Committed Effective Dose (D efc ) in one year that is 0,36 msv (< Dose Limit of Public) d) For the six groups of age according to the new regulations [7], the Committed Effective Dose during 1 year is shown in the Table IV. Table IV Range of Age h(g) (Sv/Bq) D efc (msv) 1 yeat 1, , years 1, , years 1, , years 5, , years 3, ,07 > 17 years 2, ,04 e) It is obvious that considering the twentieth part of the ALI, the effective dose is less than half the dose limit for the most unfavourable group of age ( 1 year). If the ALI were taken directly for the age group 1 year, the effective dose would be 7,2 msv and this would exceed the dose limit. f) From the Factor of Dose for adults it is possible to determine a new Annual Limit on Intake for ingestion of Bq, value that overcomes in more than 10% the one obtained from ICRP-61 [2] (more restrictive). g) If it was kept in mind the dumping as a function of the age groups, the ALI for the most unfavourable case would be decreased in one order of magnitude with regard to the ALI for adults: Bq. It becomes necessary to apply a factor of restriction on the ALI for adults. h) Therefore, for the ingestion of Iodine 131, it is appropriate to take the twentieth part of either the ALI from ICRP-61 [2] for the members of public (as it has been made in this paper) or the ALI calculated for the adults from the Factors of Dose h(g). REFERENCES 1. International Commission on Radiation Protection, Limits for Intakes of Radionuclides by workers. Publication 30. Annals of the ICRP, Vol. 2, No. 3/4. Pergamon Press, Elmsford, NY (1979) 2. International Commission on Radiation Protection, Annual Limits of Intake of Radionuclides by Workers Based on the 1990 Recommendations. Publication 61. Annals of the ICRP, Vol. 21, No. 4. Pergamon Press, Elmsford, NY (1991) 3. International Commission on Radiation Protection, Dose Coefficients for Intakes of Radionuclides by Workers. Publication 68. Annals of the ICRP, Vol. 24, No. 2. Pergamon Press, Elmsford, NY (1995) 4. AT-131E User s Manual. Aplicaciones Tecnológicas 7

8 5. Council of the European Union. Council Directive 96/29/EURATOM. Official Jounal of the European Communities. Vol. 39, NºL 159, p (1996) 6. Real Decreto 53/1992 de 24 de enero. Reglamento sobre Protección Sanitaria contra las Radiaciones Ionizantes. Boletín Oficial del Estado (Spanish Legislation). BOE núm. 37 (1992) 7. Real Decreto 783/2001 de 6 de julio. Reglamento sobre Protección Sanitaria contra las Radiaciones Ionizantes. Boletín Oficial del Estado (Spanish Legislation). BOE núm. 178 (2001) 8