SEAN MOBBERLEY

/RadioactiveWasteSystem

 

Introduction

 

Every institution involved in radiopharmaceutical production using an atomic accelerator (or "cyclotron") must comply with some level of regulation governing effluent radioactive waste release. Though the acceptable release limits and enforcement may vary widely around the world and region-to-region, effective containment of radioactive waste gas is crucial, and a simple, reliable and centralized solution allows the radiochemistry team to concentrate their efforts on production and development.

The method for containment of radioactive gases resulting from radiopharmaceutical synthesis has historically been improvised, often with a singular purpose and tailored to the specific synthesis module. These methods have included 1) plastic capture bag or balloon, 2) long-tube delay line, 3) large volume flow-through diffusion tank and 4) storage or buffer tanks.

Because of the unique waste gas output requirements for each synthesis module (very high or low flow rates, tolerance to backpressure or vacuum, total volume of waste gas release and half-life of radionuclide), none of these systems can provide a combined solution for all synthesis applications.

The system developed here is an automated, interactive, reliable and centralized system for the containment of radioactive waste gas produced as a by-product of radiopharmaceutical synthesis and based on the compressed gas storage tank model.

 

Front panel of the system's LabVIEW interface.

 

 

System Components

 

The system is based on a compressed waste gas cylinder storage model. This model is easier to leak test and less likely to develop leaks over time.

For radiation containment reasons, all components of this system are located in the cyclotron vault, except for the controlling micro-PLC and the distributed waste gas input panels. The following describes the location and basic composition of each of the five basic sub-systems that make up the complete waste gas evacuation/storage system.

 

Rugged, Web-Access Micro-PLC Controller:

A Compact FieldPoint device (National Instruments) provides the base for the system: a simple-to-program platform with numerous plug-in modules for analog, digital and control signal i/o and a user-friendly, network-accessible web interface. This interface allows the user to control the parameters of automated operation as well as change to manual control of the pump and all valves.

 

The system's network-accessible microcontroller.

 

Multi-Input Distributed Manifold Panels:

With one placed behind each group of hot cells, these panels receive the waste gas from each synthesis module while adding scalability to the system. A check valve to vent on each panel prevents the system from drawing a vacuum.

 

Hot cell manifold.

 

Buffer Tanks with Proportional Flow Paths:

These tanks, pumped down to 1/3 atm, act to draw the waste gas from the manifolds. As the vacuum is depleted in one tank, the other takes over, then the first is rapidly pumped to the storage tanks. This provides continuous draw even when there is a relatively high-flow waste gas supply. Also the flow from the manifolds can be controlled by diverting though one of three flow-regulated paths.

 

Buffer and storage tanks, along with associated flow paths and pump for the system.

 

Results & Conclusions

The vacuum pump used for multiple purposes is capable of drawing down one of the buffer tanks in a very short period of time. The system has a storage tank capacity of 1,350 litres, and will operate continuously at flows approaching 1L/min; or higher flows for short periods of time.

The design is limited to simple components: solenoid valves, pressure transducers, needle valves, check valves, standard 50-litre gas cylinders, a vacuum pump and a dedicated PLC controller. Though numerous, all of the parts were selected for ruggedness. The total cost for all components was less than 12,000 USD.

This waste gas system, operational in daily use for 4 years has proven itself to be an effective mechanism for the containment of radioactive waste gases produced by our radio-pharmaceutical synthesis modules. The benefits of this system over existing commercial and ad hoc containment systems are: Reliability, Scalability, Continuous Function, Automated Leak Checking, Scheduled Automated Release Capability, Simplicity of Design & Implementation, and Adaptability to Synthesis Requirements.

If you would like to read more, please see our published work on this project.