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CONTRACT : Design, Built & Commissioning of New Decay Tank System for Iodine - 131 Room Toilets 

1. Overview
Radioactive Waste Decay Tank is also called as decontamination plants which are individually designed according to the requirements of the respectively hospital. In the era of new technology whereby the Radioactive Waste Decay Systems  will be equipped with modern central process control system including process visualization regulates the plant and guarantees a high safety standard.

When thyroid diseases are treated with radio iodine ( I-131) in the nuclear medical departments of hospitals, a large quantity of radioactive isotopes is deposited. The radioactive wastewater may not be discharged into public sewer, but it must be stored until radioactivity has decayed below the legally defined value.

Due to the quite long retention time of the wastewater in the decay tank, an anaerabe process must be prevented to avoid biogas generation. Therefore, an aerobic process takes places when the wastewater is periodically aerated. Consequently, it had replaced the neutralization process which had been used in former decay plants.

The scale of above plant is mainly depending on the daily water volume discharged from the water closet. However, with the new technology of vacuum toilets system, the daily sewage discharge into recipient tanks is reduced by 50% compare to the conventional type and in the other words, the quantity of storage versus the numbers of days for decaying process had been increased.

The Radioactive Waste Decay Systems consists of the following systems : -
  • Vacuum Toilet Systems 
  • Holding Tank Systems
  • Decay Tank Systems 
  • Sump Pit Discharge Systems
  • Aeration Systems
  • Rinsing Systems
  • SCADA Systems
A. Vacuum Toilets Systems

I ) Introduction 

Vacuum technology is used for collecting and conveying wastewater in domestic, commercial and industrial areas, for both outdoor and indoor systems. It also offers innovative solutions in both economic and ecologic respects for the discharge of wastewater from buildings.

In terms of flexibility and cost saving, the vacuum toilets system required a minimal usage of water i.e. approximately 1.2 litres per flush compare to conventional toilet system because air is used as transport medium instead of precious water. In addition to that, the pipe works laying does not have any restriction on slope or building structural. Wastewater can even lifted up to a height of 4.50m.

II ) Functionality

The pneumatic vacuum toilet is hooked into vacuum sewer line, which may consist of a single or multiple series of sewer lines. The vacuum sewer line are connected to the vacuum station where vacuum pumps create the required negative pressure ( approximately - 0.3 bar ). Therefore, once patient uses the toilet and flushes it, the flushing actions opens a valve in the toilet water closet, and the vacuum sucks the contents of the toilet out and transfers it to Holding Tank for next process.

III) Advantages
  • Closed systems with no leakage or smell.
  • Low water usage per flush compare to conventional water closet.
  • Reduce the scale of the plant
  • Small diameter pipelines.
  • Flexible pipeline construction.
B.  Holding Tank System
I ) Introduction

Holding Tank Systems is used to collect the continuous waste water and store the wastewater up to designed numbers of days or high level of the tanks before transfer to designated tank for decaying purpose. In additional to that, an aerobic process will takes place by periodically aeration to avoid any biogas generation to the tanks.

II ) Functionality

Holding Tank System consists of 2 nos FRP/HDPE tanks ( 1 duty and 1 standby ) to receive the continuous Radioactive waste from the vacuum system until for the extension of 37 days or the designated High-Level whichever come earlier. It also equipped with Motorized Ball Valve with manual override and Cutter pump which acts as macerator and transfer pumps.

Upon receiving Radioactive Waste from the vacuum Toilet System, the waste will be stored in the either one of FRP/HDPE tanks which will be act as 1 duty and 1 standby. All the waste will be store at FRP/HDPE tanks for 37 days limited or high permitted level whichever come earlier.

Simultaneously the filled Holding Tank will transfer to Decay Tank System thru a cutter pump which also acts as a macerator to the radioactive waste.

III ) Advantages
  •  To ease the sequencing process and also to acts as additional standby tank for any unforeseen circumstances.
  • The combination of macerator and transfer pump will eventually reducing the maintenance cost for operation of the plant.
C.  Decay Tank System
I ) Introduction

Decay Tank systems is used to receive the wastewater from the Holding Tank and store the wastewater up to designed numbers of days or radioactivity has decayed below the legally defined value before it can be discharge to public sewer.

In additional to that, an aerobic process will takes place by periodically aeration to avoid any biogas generation to the tanks.

II ) Functionality

Decay Tank System consists of 3 nos FRP / HDPE tanks to receive and store the wastewater accumulation for 37 days or High - Level from the Holding Tank system to the extension of 111 days or the radioactivity has decayed below the legally defined value.
This system will be equipped with Motorized Ball Valve with manual override and Radioactivity sensor for the continuous monitoring purposes.

Upon receiving Radioactivity Waster from the Holding Tank System, the waste will be stored in the one of FRP /HDPE tank for additional 111 days or radioactivity has decayed below the legally defined value whichever comes earlier.

Once the above criteria is meet, the Medical Physicist are required to take a sample from the Decay Tank to double confirmation of the radioactivity has decayed below the legally defined value before it permitted to proceed to discharging process.

III )  Advantages
  • Continuous monitoring and recorded trending for decaying process.
  • Additional safety feature designed for user to carry put sampling and double confirmation the radioactivity reading before discharging to public sewer.
D. Sump Pit Discharge System
I )  Introduction

Sum Pit Discharge System is used to collect and transfer back any wastewater collected during abnormal criteria and also as a pre-discharge tank before the discharge of permitted radioactivity waste water to public sewer.

II ) Functionality

Sump Pit Discharge System consists of 1 no of Return Pump to transfer back any radioactive wastewater which flowing to the sump bit back to Holding Tank system when in abnormal conditions. Besides that, there is also 1 no of manual gate valve at the outlet of sump pit for Medical Physicist to open upon confirmation of sampling is ready to discharge and also to control the flow rate of discharging.

Upon detection of high-level at Sump Pit and all of Decay Tank's discharge still in CLOSED mode ( abnormal condition ), the return pump will be activated and pump the radioactive waste back to Holding Tank System.

During Normal Condition where one of the Decay Tank discharge valve is in OPEN mode,
the return pump will be deactivated and the Medical Physicist shall open the manual gate valve upon confirmation to discharge.

III ) Advantages
  • To ensure all un-permitted radioactive waster not been discharge
  • To enable Medical Physicist to control the flow rate of discharge manually.
E.  Aeration System
I ) Introduction

Since the wastewater is a biodegradable waste, an aeration system is used to create aerobic process by periodically aeration to the wastewater in order to  prevent any anaerobe process and avoid any biogas generated. Further to that, the aeration system will also preventing any possibility of sludge creation and reduce to odor in the tank.

II ) Functionality

Aeration System consists of 1 no of Air Blower and 2 nos of submerged coarse bubble diffuser which will be install at each of Holding Tank and Decay Tank.

III ) Advantages
  • Odor removal.
  • Prevent of biogas generation.
  • Prevent of sludege sedimentation.
F.  Rinsing System
I ) Introduction

Rinsing System is used for tank cleaning before receiving the new batch of wastewater.

II ) Functionality

Rinsing System consists of 1 no of Water Pump and 5 nos of solenoid valve which will be installing at each of Holding Tank and Decay Tank. It had designed for the purpose of tank cleaning after radioactive waste had been transferred.

When the process of transferring / discharging of radioactive waste, the rinsing system will be activated and the rinsing process will start when the radioactive waste in the tank remained 20% of volume and stop after one ( 1 ) minutes of rinsing.

III ) Advantages
  • To remove or ensure all radioactive waste had been transferred / discharged to the next process.
G. Scada System
I ) Introduction

SCADA systems are typically used to perform data collection and control at the supervisory level. Some SCADA systems only monitor without doing control, these systems are still referred to as SCADA systems.

The supervisory control system is a system that is place on top of a real-time control system to control a process that is external to the SCADA system. This implies that the system is not critical to control the process in real-time, as there is a separate or integrated real-time automated control system that can respond quickly enough to compensate for process changes within the time-constants of the process. The process can be industrial, infrastructure or facility based as described below :
  • Industrial processes include : manufacturing / production / power generation / fabrication / refining - continuous, batch repetitive or discrete.
  • Infrastructure processes may be public or private and include : water treatment and distrubution, wastewater collection and wastewater treatment, oil & gas pipelines, electrical power transmission and distribution and large communication systems.
  • Facility processes in private or public facilities including : buildings, airports, ships or space stations in order to monitor and control : HVAC, access control , energy consumption management.

The SCADA systems for these applications all perform Supervisory Control And Data Acquisition, evne though the use of the systems are very different.

II ) Functionality

When the Physicist selected all equipments in the LCP to 'Remote' mode, the whole equipments in the Decay Tank System will be run as automatically based on written PLCs. Any alarms, trending and reading which required will be display in the workstation in the physicist room and it had been set up for three types of user as a security features as below :

  • Admin Level ( High ) - the user can control and print reports for all the Decay Tank system in their workstation.
  • Engineer Level ( Medium ) - the user can control only for any related to engineering works in the Decay Tank System thru their workstation.
  • Technician Level ( Low ) - the user can on view the Decay Tank systems and NOT allow to control any of the Decay Tank System.
III ) Advantages
  • To automate, retrieving and record all activities happen in the Decay Tank System during operation.
  • To facilitate the user to do reporting purposes.

2. Operational of the Radioactive Decay Tank System

The radioactive waste discharges from the ward goes into one of the holding tanks, HT1 or HT2, i.e. one is on duty and one on standby. (Say HT 1)

After receiving radioactive waste discharges for 37 days, the waste is pump through a cutter pump which acts also as a macerator to one of the decay tanks, DT 1 or DT 2 or DT 3.  ( Say DT 1 )

HT 1 now goes on standby. HT 2 starts receiving another 37 days discharge from the ward.

Once HT 2 has received discharges for 37 days, the waste is pump through a cutter pump which acts also as a macerator to one of the two other decay tanks, DT 2 or DT 3. ( Say DT 2 )

The waste that was already in DT 1 is now 74 days old.

HT 2 now goes on standby. HT 1 starts receiving another 37 days discharge from the ward.

After HT 1 has received discharges for 37 days, the waste is pumped through a cutter pump which acts also as a macerator to the remaining decay tank DT 3.

Waste in DT 1 is now 111 days months old and can now permit to be discharged into the public sewer.

Waste in DT 2 is now 74 days old and will be discharged to the public sewer in another 37 days time.

3. Leakage and Structural Failure of Tanks.
The possibility of leakages cannot be discounted and this has been taken into account. Structural failure of the tanks is very remote but has been designated to be replaced with ease.

During operations, one of the receiving tanks and one of the delay tanks are always on standby, i.e. they are empty.

Should a leak occur, say in HT 1, which can be detected, as all five tanks are equipped with level sensors, the contents are transferred to HT 2 via valves and pump. HT 1 can then be replaced or repaired.

Similarly with the DT 1, DT 2 or DT 3. The contents from the leaking DT Tank will be transferred to an empty HT/ DT tank.

4. Spillages

The floor of the reinforced concrete bunker containing the tanks are sloped to one corner, where a small sump collects spillages. Surrounding of bunker will be facialites with water hose for rinses the floor and water from this goes into the sump.

A small pump empties the sump into the incoming pipeline from the ward. This will go into either HT1 or HT2.

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