1. Introduction
Teletherapy
60Co unit has a specific energy and is easy to maintain. It was a useful tool for treating head and neck cancers and superficial tumors during several decades. However, the effectiveness of linear accelerators has greatly improved over recent years. Many old
60Co units are decommissioned because of weak activity as well as long treatment times required. The Alcyon CGR MeV teletherapy unit has been used in different radiation oncology institutes in Egypt for over 15 years. Recently it was decided to decommission those units and transfer them to the centralized waste facility at the Egyptian Atomic Energy Authority (Hot Labs & Waste Management Center, HLWMC). This paper describes the planning and dismantling steps of such type of
60Co units, and personal doses measured during the procedure.
The safety development of nuclear energy and progressive increasing use of radioactive materials and sealed sources in industrial, medical and research applications increase the demand of establishing a procedure for removal and control of high spent radioactive sealed sources (SHARS) from its devices such as a head of teletherapy unit that contains more than 1000 Ci of 60Co. These activities have determined the necessity to formulate and apply an institutional strategy to assure harmless and ecologically rational management of spent sealed sources and radioactive waste conform national safety as well as security and in parallel with and international recommendations.
2. Material and Method
According to the law No.7/2010, Egyptian Atomic Energy Authority represented in the Hot Laboratories & Waste Management Center (centralized waste facility, HLWMC) has the responsibility of collecting, conditioning, storing and management of all types of radioactive waste from all Egyptian territory including spent radioactive sealed sources (SRSSs).
The removal process was performed including radiation dose measurement in contact and at 30 cm, and illustrated in Figure 1. The leak test was performed for possible contamination using a swap test and measured by Contamat (THERMO). The radiation dose also was performed for individuals who ranged from 50 to 60
Figure 1. Radiation dose measurements on contact and at 30 cm from the head.
µSv/hr.
The option, to reduce the risks for accidents or incident with spent radiation sources, is the conditioning of high risk sources including embedding in a matrix of cement mortar. This method is applicable for radiation sources with activities up to a few terabecqeurel (TBq), depending on the radionuclide and half life. The method has the advantage of using a simple and cheap technology, material and equipment that is available and it gives a waste package, the stability for a long time under interim storage for conditioning [1,2] and transfer to the storage facility area.
2.1. Planning
HLWMC received formal requests from different radiation oncology institutes to unload the 60Co teletherapy unit of type Alcyon CGR MeV (Figure 2) either to install a new fashion one or replaced with of linear accelerator unit. Figure 2 also describes the source head. The first step of the decommissioning was to calculate the current activity of the 60Co source according to the kinetic law of radioactivity. The initial radioactivity level was 222.64 TBq ≈ 6000 Ci. The current radioactivity on the removal procedure date was 21.1 TBq ≈ 600 Ci. The exposure rate was measured by a Thermo FH40G-L at 30 cm and in contact. The source was also identified using identifiFINDER Target insight, Germany. Figure 3 illustrateS the gamma ray spectrum results which revealed two gamma energy lines at 1332 & 1121 KeV.
2.2. 60Co Head Description and Installation
2.2.1. Radiation Head
1. lead shield;
2. wolfram shutter;
Figure 2. Head of 60Co source; metallic support and its accessories.
3. depleted uranium sheets inside the head;
4. “beam off” position of the source;
5. primary collimator;
6. light fiber optical system;
7. quartz lamp;
8. mirror;
9. cast shell;
10. source information:
a. source number: 3897
b. radioactivity level: 222,621 TBq
c. date: 12 June 95
d. Radionuclide: 60Co.
2.2.2. Installtion
The installation/system teletherapy machine, Alcyon-II with cobalt 60Co source with initial activity Ai = 222,621 TBq in Oncological Department of the “ASSUIT” University Hospital Center has been in operation since 1996. This system contains four important panels as follow: 1) revolving head with 60Co source; 2) metallic support with its accessories; 3) electronic, electrical & surveillance instruments; 4) the ventilation installation.
The solid metallic frame was putted over the head of tele-therapy machine welding with a metallic dray with sizes: 800 × 800 mm and thickening 45 mm. The source’s window (collimator) was fixed by down head at the metallic plinth-sheet.
The Alycon II is both a teletherapy head and source shipping container, the head is placed in a protective shell made of steel frame, poly wood panels, ceramic wool, wood and belt. The shipping case is a single use item and includes an installation hoist which remains with customer or CGR district manager. CGR Baltimore will stock an installation hoist for emergency use. Source exchanges are not made in the field; rather, a decayed source head is replaced with a new, full-strength source head. The source capsule is contained in a source-holder barrel by means of bayonet type pins with locking clips. As a further precaution, CGR Baltimore, will have a part plug machined to fit the collimator part to ensure source containment within the head. There is an optional beam stop, 50 cm in diameter and 14 cm thick, made of lead with 4% antimony encased in steel, maximum transmission through the beam stop is 0.1% of the primary beam, 99.9% efficient. The gantry will rotate the source head through 0 to 3600 at a variable rote from 0.05 to 1 rpm the source head cannot be angled in relation to the gantry. The iso center is 80 cm ± 2 mm from the source and 120 cm above the floor with a maximum field size of 32 × 32 cm.
3. Result and Discussion
3.1. Report of Routine Swipe Test for Contamination [3]
Equipment containing radioactive sources shall be wipe tested for removable radioactive contamination at intervals not to exceed six months. Transport packages for radioactive material also shall be wipe tested for removable radioactive contamination on receipt and prior to shipment. Records of test results shall be kept in microcuries or S.I units and maintained for inspection by the appropriate Regulatory Authority. Sensitivity Factors [4] for 60Co:40 c/min = 0.005 µCi (185 Bq) and 238U:140 c/min = 0.005 µCi. If any wipe tests exceed than the background count per minute the calculation should be in microcuries. The wipe showed no contamination and the results was less than 0.005 µCi for 60Co and 238U.
3.2. Removal Procedures
1) Remove the rigid plastic cap cover from the head as well as the patient bed and performing the dose rate measurement, in this step the background was 80 nSv/hr;
2) Follow the safeguard guidelines. The dismantling steps are performed using the following;
a) Remove the cover of the collimator as illustrated in Figure 2.
b) Rotate the machine manually or electrically clockwise with 180˚ angle, Figure 4.
c) Remove, and store the collimator in suitable package.
d) In each step the dose rate was recorded.
3) Fix the body of the machine to prevent unbalance change (when the head is removed) using stopper stick in the back which prevent the rotation of the machine due to the weight balance as shown in Figure 5.
Figure 5. Fixation of machine stoppestick.
4) Rotates the head again anticlockwise 90˚ to start the removal of the head, Figure 2. This procedure was achieved using the following steps.
a) Disconnect and shut down the power source, Figure 6.
b) Start to safe the source to be in safe position by three different locks to prevent the rotation of the shutter. Figure 7. This was done using red metal fixed sheet in semi vertical position. The second step is to put the screw with angle 70˚ in contact perpendicular to the rotating disc. The final step is to put the last screw perpendicular to the rotating wheel.
c) Remove the four screws that being in contact with the head as in Figure 7, after being carried on the crane and safely transfer the head to the container, Figure 8.
Figure 1 elucidates the skeleton of the head with the dose rate measurement from different positions, in each case before and after dismantling the collimator as well as dismantling the head, and the dose rate measurement on the final package Figure 9.