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A) Design & Built New Decay Tank Syatem For Radioiodine 131 (Ir-131) @ IPPT.
  • Work Order.  Click  HERE
  • Decay Tank Contract Documentation : -
           I. ) Low Level Radioactive Waste Management :
    • Preamble,
      • Radioiodine 131 Treatment In IPPT
In Radioiodine treatment, a patient is given Radioiodine 131 (Ir-131) in a drink or capsule form. The average activity is 150mCi per patient. The patient is expected to stays in one of the 3 isolation rooms for 3days. Urine and feces are highly radioactive and 70% of the 150mCi is expected to be excreted. A system of decay and delay is necessary to allow a safe discharge to the main sewer.
      • Legislative Infrastructure and Radioactive Waste Management In Malaysia.  
Atomic Energy Licensing Act requires that we manage radioactive waste. Atomic Energy Licensing Board has issued legislations on radiation protection, which also cover radioactive waste management. However, a new legislation, substituting the old one, covering the discharges of radioactive effluents from the licensed establishments is to be legislated in the near future. It is therefore prudent that the system in IPPT be in line with this new discharge limit for Ir-131.
      • Liquid Radioactive Wastes
The liquid radioactive wastes from IPPT can be discharged to the sewage system according to the concentration limits of  27micro curies per meter cube. Short-lived radionuclide such as Ir-131 with half-lives of about 8.4 days can be discharged to the sewage system at this discharge limit. The legislation requires monitoring of Ir-131 discharges for hospital.


           II.) Design Foundamentals For IPPT Decay Tank System. 

    • Waste Volume Calculation;
3 rooms will have a maximum of 6 patients/ week.
And each patient stays for 3 days with 8 flushes per day then : 
Based on 1.5 liters / flush.
1 day for 1 patient : 8 x 1.5 liters = 12 liters
For 3 days stay per patient : 3 x 12 liters = 36 liters
In a week, there will be a total of 6 patients using the 3 rooms.

For 6 (MAX) patients per week = 6 x 36 liters = 216 liters/week.
For 1 month : 216 liters x 4 weeks = 864 liters per month of radioactive waste will be discharged to the tank.
    • Tank Capacity;
For the capacity of a septic tank, the size of the tank should follow as below:
1 liters is equal to 0.001m3.
For decay tank to hold 864 liters of wastes, each of the 5 tanks shall have a minimum capacity of  : 864 x 0.001m3 = 0.864m3 per tank.
In practise an extra capacity of 20% is included ie : therefore the tank capacity shall not be less than 1.1m3 per tank capacity.
    • Ir-131 Decay Chart;
Based on the formula below formulated a new Ir-131 decay chart, suitable for the new discharge limits for decay process calculation. This is due to the fraction of remaining activity available in there decimal points for the old decay chart. Our decay process calculation for Ir-131 needs to be correct to at least six decimal points for more accuracy since the 'soon to be released' discharge limit is 27uCi per m3. The earlier discharge limits calculation was based on 10 half life. However, the tolerance included in our design using 5 tanks allows us to accommodate and satisfy the new discharge limits even in a worse case scenario. 

A(t)  = A(0) 2 - (t/hl)  ( Source from I-131 Decay Chart - DEHS, UMN )

A(0) : activity at time t = 0
t : elapsed time
hl : half time
Ir-131 half life : 8.04 days


Table 1 : Fraction of Original Radioactivity Remaining after  t  Days

Day0123456789
01.00000000.91739940.84162170.77210320.70832710.64981880.59614340.54690160.50172720.4602843
100.42226450.38738520.35538700.32603180.29910140.27439540.25173020.23093710.21186160.1943617
200.17830730.16357900.15006730.13767170.12629990.11586750.10629670.09751660.08946160.0820721
300.07529290.06907360.06336810.05813390.05333200.04892670.04488530.04117780.03777650.0346561
400.03179350.02916730.02675810.02454790.02252020.02066000.01895350.01738790.01595170.0146341
500.01342530.01231630.01129900.01036570.00950950.00872400.00800340.00734230.00673580.0061795
600.00566900.00520080.00477120.00437710.00401550.00368380.00337960.00310040.00284430.0026094
700.00239380.00219610.00201470.00184830.00169560.00155560.00142710.00130920.00120110.0011018
800.00101080.00092730.00085070.00078040.00071600.00065690.00060260.00055280.00050720.0004653
900.0004268









    • Design Criteria for 6 Patients - Radioactivity;

Average activity is about 150mCi ( as confirmed by end user ) for one patient. 6 patients are treated in a day. All calculation is based on the Table 1.

150mCi x 6 patients per day = 900mCi

30% of the activity is absorbed and 70% is excreted in 72 hrs ( 3days) = 23.33% per day Assuming that the Decay Tank is filled in one month with 864 liters and every patient stays for 3 days. 6 new patients start therapy every fourth day throughout the month unless it's a Sunday.

Total activity after one month in receiving tank = 640.354mCi ( refer decay chart 2 ).

Total activity in 1st Decay Tank

  • 30 days : 640.354 mCi x 0.0752929 = 48.2141mCi
  • 60 days : 48.2141  mCi x 0.0752929 = 3.6302 mCi
  • 90 days : 3.6302    mCi x 0.0752929 = 0.3722 mCi
  • 116 days : 0.2733  mCi x 0.1062967 = 0.0291 mCi

During the transfer from receiving tank to decay tank we assume 200 liters of water from rinsing being diluted with 864 liters of waste. Total waste volume is 1064 liters.

Total activity end of 116 days in decay tank per total waste volume in liters :

= 0.0291 mCi / 1064 l

= 0.0000273 mCi / l

Total activity end of 116 days in decay tank per total waste volumn in cubic meter :

= 0.0000273 mCi / l x 1000

= 0.273 mCi / cubic meter.

Total activity for 1m3 is 27uCi / m3

Discharge activities comply with the discharge requirement of  27 uCi / m3.


Radioiodine 131 Decay Chart ( 6 Patients ).

Day
Activity = 150mCi x 6 patients per day. 30% absorbed and 70% excreted in 72hrs i.e. 23.33%/day
Fraction of remaining activity as per Ir-131 decay Chart
Remaining Activity mCi








Total monthly activity in Tank
0
216
1.000
216.000









1
216
0.9173994
198.158









2
216
0.8416217
181.790









3
216
0.7721032
166.774
216.000








4
216
0.7083271
152.999
198.158








5
216
0.6498188
140.361
181.790








6
216
0.5961434
128.767
166.774








7
216
0.5469016
118.131
152.999
216







8
216
0.5017272
108.373
140.361
198.158







9
216
0.4602843
99.421
128.767
181.790







10
216
0.4222645
91.209
118.131
166.774
216.00






11
216
0.3873852
83.675
108.373
152.999
198.158






12
216
0.3553870
76.764
99.421
140.361
181.790






13
216
0.3260318
70.423
91.209
128.767
166.774






14
216
0.2991014
64.606
83.675
118.131
152.999
216.000





15
216
0.2743954
59.269
76.764
108.373
140.361
198.158





16
216
0.2517302
54.374
70.423
99.421
128.767
181.790





17
216
0.2309371
49.882
64.606
91.209
118.131
166.774
216.00




18
216
0.2118616
45.762
59.269
83.675
108.373
152.999
198.158




19
216
0.1943617
41.982
54.374
76.764
99.421
140.361
181.790




20
216
0.1783073
38.514
49.882
70.423
91.209
128.767
166.774




21
216
0.1635790
35.333
45.762
64.606
83.675
118.131
152.999
216.000



22
216
0.1500673
32.415
41.982
59.269
76.764
108.373
140.361
198.158



23
216
0.1376717
29.737
38.514
54.374
70.423
99.421
128.767
181.79



24
216
0.1262999
27.281
35.333
49.882
64.606
91.209
118.131
166.774
216.00


25
216
0.1158675
25.027
32.415
45.762
59.269
83.675
108.373
152.999
198.158


26
216
0.1062967
22.960
29.737
41.982
54.374
76.764
99.421
140.361
181.790


27
216
0.0975166
21.064
27.281
38.514
49.882
70.423
91.209
128.767
166.774


28
216
0.0894616
19.324
25.027
35.333
45.762
64.606
83.675
118.131
152.999
216.00

29
216
0.0820721
17.728
22.960
32.415
41.982
59.269
76.764
108.373
140.361
198.158

30
216
0.0752929
16.263
21.064
29.737
38.514
54.374
70.423
99.421
128.767
181.790
640.354

    • Functional Design Brief ;

Basic Diagram of Decay Tank System

As one of the best available technique, waste tanks are used to collect and decay radioactive waste before the discharge of effluents into the sewage system. The single waste tank system makes use of  the "decaying while filling" principle. Therefore, by the time that the tank is filled, the total activity in the waste tank is many times lower than the total input so that the overall capacity requirement can be greatly reduced. The use of multiple waste tanks resolves most of the problem of single waste tank system. However, it is important to design waste tank system with optimum tank number and capacity.

Designed a system using a five tank systems, tow tanks receiving the discharge from the toilets while other three tanks will be used as decay tanks. The design is based on three rooms with six patients a week. It will collect all the waste from the toilets of the rooms and store it in the receiving tank 1 for the first month or 30 days. Once the receiving tank 1 is filled up with waste for 30 days, it will be transferred to decay tank 1. Waste in decay tank 1 will be kept for 116 days  before it being discharged into the sewage system. This is to satisfy the discharge concentration limits of 27 micro curies per meter cube.

The receiving tank 2 will start to be filled up with waste for the second month. After 30 days, the waste from receiving tank 2 will be transferred to decay tank 2 and the waste from the toilet will continuously start to fill up the receiving tank 1 again for the third month. Once the receiving tank 1 is filled up with waste for the whole month, it will be transferred and kept in decay tank 3. All the waste in three decay will be stored for 116 days before it being discharged into the sewage system, Each tank will have at least 1 month time gap for washing and maintenance purpose.


    • Measurements ;

I.  Level Detectors to measure the level of all tanks to automatically stop overflow and   to inform operator the status of tank level at the remote computer display.

II. Radiation Detectors to measure the radiation levels of all decays tanks and to inform operator the status of tank radiation level at the remote computer display.

    • Special Processes ;
  • Macerators
  • Mixers in each tank to keep particles suspended.
  • Aerators in each tank to eliminate odour.
  • Venting of foul gas from tank and bunker area.
  • Sampling Points for decay tank before discharge.
  • Tank rinsing cycle for each tank.
  • Shunting of sewage to and from each of the five tanks shall be built into the system capability.
    • Control System ;
  • Graphical display to display the status of all measurement parameters and conditions.
  • Computerized control of all valves, pumps, motors and switches.
    • Specifications ;

System Capability

Radioactive Waste Management system shall be fully automatic system with manual override system and to minimise the Physicist to enter the Decay Tank housing.

Toilets

Type        :     Wall Mounted Vacuum Toilet

Quantity  :     3 nos

Tanks

Type        :      FRP or HDPE

Quantity  :      5 nos

Volume    :      Minimum of 1300 Litres

Measurement parameters

Level    :  Millimeter. To be monitor by level sensors suitable for sewerage purposes.

Radiation : mCi. To be monitor by radiation sensors probe at each of Decay Tank.

Control system

Computer system :  System is to provide supervision of radioactive waste management at Physicist room and also would provide operator wit information and database required for the ease of operation.

PLC System  :  PLC system to logically control for all the equipments, sensors and indicator.

Shunting Process

Receiving tank  : The radioactive sewerage shall transfer from the toilet to the receiving tank by using vacuum pumps.

Decay tanks      : The collected radioactive sewerage at receiving tank shall be transferred to Decay Tank by using grinder / cutter pump once it has reached 30 days or the highest level of sewerage in the tank or whichever earlier.
 
Discharge Control

Computer guided : The controls of discharge process are via the Local Control Panel ( LCP). PLCs are to be provided in the LCP for the automatic control of discharge process. Signals and status of Decay Tank are linked back to the Physicist room.

Discharge Limits : The discharge shall be manually opened by the Physicist

Emergency Control

Manual override : All motorised valve had equipped with manual override mechanism. In the event of power failure, UPS system ( 10 min ) shall be provided as back up power supply for the PLC in the control panels while the awaiting the essential supply from the hospital generator.

Conclusion

This document should be read on the whole and fully understood by the vendor who is bidding for the decay tank system.

The vendor must accept full and complete responsibility of the efficient functioning of the system as it is designed to do base on this document.

the vendor is expected to work closely with the Radiation Consultant to achieve the planned discharge limits.


 
Nuclear Solution (M) Sdn Bhd (855707-W)
No.3-2, Jalan SL 1/11, Bandar Sungai Long, 43200 Cheras, Selangor D.E. Malaysia
    Email: nuclearsolution123@gmail.com
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