WM 00 Conference, February 27 March 2, 2000, Tucson, AZ RELEASE SURVEYING OF SCRAP METALS WITH THE IONSENS CONVEYOR A CASE STUDY

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1 RELEASE SURVEYING OF SCRAP METALS WITH THE IONSENS CONVEYOR A CASE STUDY E Miller, J. Peters, and D. Nichols Manufacturing Sciences Corporation (MSC) (a BNFL Inc. company) Oak Ridge, Tennessee R.D. Gunn,, N.A. Troughton, R.J.Corless, and F.W. Gardner BNFL Instruments Sellafield, England Los Alamos, New Mexico ABSTRACT The decommissioning of the various buildings of the Oak Ridge Gaseous Diffusion Plant (ORGDP) on the East Tennessee Technology Park (ETTP) is a major undertaking with many challenges. One interesting challenge is the monitoring required to classify metal scrap from the site, where large quantities of steel and copper require disposal. The D&D project costs can be reduced by realising the equipment and metal value through recycle rather than paying for disposal as low level radioactive waste (LLRW). The requirements for instrumentation to survey the metal before it could be released off-site are stringent. The instrument must be capable of identifying contamination at the levels specified by DOE Order supplemented by National Regulatory Commission (NRC) Regulatory Guide At the same time, a high through-put of material is required without compromising the quality of the measurement. For this case study, the material to be surveyed were parts of the gaseous diffusion plant s tanks and vessels which had been size-reduced and then decontaminated using a shot blasting technique. The radioactive contaminant of most concern is technicium-99. This radioisotope has a half-life of years, and decays emitting beta particles with energies of up to 0.3MeV. No significant gamma rays are emitted during this decay. The characteristics of this decay and the high throughput requirement dictated the high sensitivity type of instrumentation to be deployed. The IonSens TM Conveyor comprises a 4 ft wide open mesh belt with seven gas flow proportional detectors positioned above and seven below the belt. The gas flow proportional detectors are positioned such that they can detect the weak beta particle from the technicium- 99, even at the high throughput required. The key design features and method of operation of the conveyor are discussed. INTRODUCTION The requirement for the rapid surveying of large masses of scrap metal, potentially contaminated with very low levels of residual surface activity, enabled the key design features of the IonSens Conveyor to be determined. The Minimum Detectable Activity (MDA) of the system had to be less than the regulatory release limits specified for removable

2 activity. The system had to be capable of surveying all surfaces of every item, with detection set points established and noted as part of the final survey record. Finally the system had to be capable of automated operation over extended periods in an operational decommissioning environment and be sufficiently robust for the handling of heavy pieces of decontaminated scrap metal. OAK RIDGE GASEOUS DIFFUSION PLANT The Oak Ridge Gaseous Diffusion Plant (GDP) was constructed in the 1940s as part of the U.S. Army's Manhattan Project. The plant's mission was production of highly enriched uranium (HEU) for nuclear weapons. Enrichment was initially carried out in two process buildings, K-25 and K-27. Later, the K- 29, K-31, and K-33 buildings were built to increase the production capacity of the original facilities by raising the enrichment of the feed material entering K-27. After military production of highly enriched uranium was concluded in 1964, the two original process buildings (K-25 and K-27) were shut down. For the next 20 years the plant's primary mission was production of only low enriched uranium (LEU) to be fabricated into fuel elements for nuclear reactors. The United States Department of Energy (DOE) has contracted BNFL Inc. to Decontaminate and Decommission (D&D) three buildings located in the East Tennessee Technology Park (ETTP). The purpose of the project is to dismantle, remove, decontaminate and economically maximise the recycle of process equipment and materials within the Gaseous Diffusion Plant (GDP) buildings K29, K-31 and K-33. The objective of the project is for the three LEU buildings of the GDP cascade (K-29, K-31, and K-33) to be decontaminated for reuse in a brown-field industrial state/standard by It is estimated (1) that the overall Decommissioning and Decontamination (D&D) project costs to DOE are reduced by nearly 20% due to the recapture of equipment and metal value through recycle revenues collected by the contractor(s). This strategy avoids condemning 5550 tonnes of material as waste. DECOMMISSIONING STRATEGY To ensure the optimal application of a surveying system, the system must form an integral part of the decommissioning strategy and be deployed at the most appropriate point in the decommissioning process. The decommissioning follows 5 key phases. The Initial Survey identifies the scope of the task and also areas needing particular attention. The process equipment is then removed from the buildings. Equipment is cut up. Equipment is decontaminated

3 Final release survey. If the metal fails, further decontamination may be necessary. The IonSens Conveyor forms a key decision point within this strategy, not only by measuring material as being clean and uncontaminated but also by providing an auditable Quality Assurance trail of documentary evidence that each batch of material was clean and uncontaminated and that the system functioning correctly, as when it was commissioned. RELEASE SURVEY REGULATORY REQUIREMENT The regulatory requirements which are employed for the determination of recycle release are DOE Order supplemented by NRC Regulatory Guide 1.86 Termination of Operating Licenses for Nuclear Reactors. These release criteria contain the Department of Energy s requirements for controlling and releasing property containing residual radioactive material. These authorised limits are established to provide that, at a minimum, the basic dose limit standards will not be exceeded under the worst case or plausible-use scenarios. To ensure that the IonSens Conveyor was capable of surveying material well within the required limits extensive Verification and Validation (V&V) trials were carried out. These trials determined that the most effective way of determining the presence of contamination was by measuring the soft beta emission from technitium-99. The V&V trials also allowed the physical envelope of pieces of potentially contaminated metal on which contamination could be detected. This was determined to be a contaminated surface, at an angle of 30 degrees located 10 cm away from the furthest detector with the conveyor travelling at a speed of 0.5 inches per second. SURVEY SYSTEM The IonSens Conveyor comprises three main components. The system control and data acquisition software / hardware. The detector / pulse processing system and the conveyor system. Figure 1. The Conveyorized Survey Monitor

4 MATERIALS TO BE SURVEYED The material surveyed is generally parts of the gaseous diffusion plants tanks and vessels which have been size reduced and then decontaminated using a shot blasting technique. In operation, the vessels and tanks were subjected to surface exposure by uranium isotopes and technetium. These have remained on the surface with no penetration. No significant neutron flux was present to cause activation of the steel. Thus, a surface scan is all that is required in order to demonstrate the presence or absence of contamination. The uranium isotopes can all be fingerprinted to 99 Tc, making this the key isotope of interest. This radioisotope has a halflife of years, and decays emitting beta particles with energies of up to 0.3MeV, no significant gamma rays are emitted during this decay. The characteristics of this decay dictated the type and geometric configuration of the detectors deployed in this system. GAS FLOW PROPORTIONAL COUNTERS To optimise the efficiency of the measurement, two banks of detectors are deployed, above and below a mesh conveyor belt. The large open proportion of the mesh (80% open) provides confidence that any activity on the lower side of a piece of metal will be detected by the lower detector bank. The detectors deployed are Gas Flow Proportional (GFP) counters. These each comprise a large flat metal box with an open side covered in a thin film of aluminised plastic. Inside the detector box an anode wire is wound backwards and forwards down its length. The space within the detector is continually purged by a slow flow of a mixture of argon and methane gas known as P10. A voltage applied to the anode central wire creates an electric field inside the detector. Alpha or beta particles pass through the plastic film and create ionisation in the gas inside. The negative ions are drawn onto the anode wire by the electric field and are collected by a charge amplifier. Alpha particles, having more energy and a shorter range in the detector gas, produce larger charge pulses than beta particles allowing both particles to be measured separately. Figure 2 The Detector Arrangement

5 The detectors are arranged in two off-set banks both above and below the conveyor belt. The off-set allows the sensitive areas of the detectors to overlap with respect to the direction of movement of the conveyor and so eliminates the possibility of contamination being missed. Should contamination be detected the system electronics immediately halts the conveyor to allow manual removal of the contaminated item. MECHANICAL ARRANGEMENT The mechanical arrangement comprises a 4 feet wide, open mesh conveyor belt, which moves scrap metal between two banks of detectors. The detectors are located inside a lead shield mounted on a removable maintenance trolley. This enables a detector which has failed to be quickly replaced, minimising system downtime. Scrap metal is manually fed onto the belt via a heavy-duty slide which prevents the belt being damaged by very heavy pieces of metal being loaded onto it. At the other end of the belt the scrap metal falls into a standard skip. DATA ACQUISITION SYSTEM The data acquisition system accepts signal pulses from the detector systems, stores the pulses in counting scalers and continuously comperes the count rate within these scalers with pre-set alarm values determined during commissioning. The GFPs incorporate integral amplifiers which are connected to a PC containing Multi- Channel Scaler (MCS) hardware. The MCS allows data to be collected in a series of short, discrete scaler channels known as time bins. The count time for each time bin is selected as a function of the speed of the conveyor. The detection system runs optimally when the time bins are set up to be half the length of time the measured item is within the detection field - known as the dwell time. This ensures that activity present on a piece of metal will be in full view of the detector for one complete time bin allowing the counting statistics to be optimised. This method of data collection allows an optimised measurement to be made both when the activity is concentrated (as in a hot spot ) and also if distributed activity levels are gradually increasing towards the regulatory limits. Gradually increasing activities can present problems if data averaging methods are applied. 120 Counts Background Source Ent er ing Det ector Source Below Detector for Ent i re Count Source Leaving Detector Time bins ( 2 second integrals) Figure 3. Count Rate Histogram

6 SYSTEM OPERATING PARAMETERS The basic operating parameters for the system are initially determined during commissioning. These parameters are stored in a data file, accessible only to the engineers carrying out the calibration operations. The operating parameters are being checked routinely to ensure the correct operation of the system. This is particularly appropriate if the system is subject to changing operating conditions or measurement requirements. The basic operating parameters which are determined and subsequently used during measurements include the sensitivity factors, at a range of conveyor speeds and also at a range of heights of the top array of detectors. The factors take into consideration the range and types of radionuclide species which are to be measured by the system (i.e. for the Oak Ridge application the relatively low sensitivity arising from the soft beta emitter 99 Tc ) The factors also include the statistical probability factors used in the limit of detection calculation allowing the number of false positives to be minimised. During commissioning the choice of whether Gaussian or Poisson statistics should be applied was determined, based upon the observed background count rates. STANDARDISATION CONTROL CHECK AND BACKGROUND CONTROL CHECK. Prior to any measurement the operator is obliged to go through a standardisation and background measurement routine. The standardisation routine serves as a control check to ensure that the detectors are all operating within their expected efficiency range when exposed to a source of known activity. The detector efficiencies are determined during onsite commissioning. Depending on the background radiation levels and the thickness of the materials being monitored a shadow screen may have to be placed in the detection area of the monitor during the background measurement. The shadow screen will allow the reduction in the observed background to be accounted for due to the shielding effect of waste metals as they pass between the detectors. This background measurement will allow the Limit of Detection to be determined and allow the alarm level to be selected in terms of surface activity. Operator Interface Control and Display Figure 4 The operator interface

7 IonSens CSM FRONT END DISPLAY The front end control display shows graphically the array of detectors. If any detector detects activity that particular detector is highlighted on the display, in addition to the conveyor stopping and an alarm sounding. This display allows operators to quickly identify the piece of metal which is contaminated. When in alarm the operator must manually reverse the conveyor to allow the contaminated piece of metal to be identified and removed. This ensures that no other pieces of contaminated metal can move through the detector array when the system is in alarm. The windows based display guides the operator through the background and measurement control checks and prevents material from being surveyed if the system is not operating within its design specification. PERFORMANCE During commissioning the system was subjected to 4 x 16 hour periods of continuous operation to ensure all subsystems were functioning correctly. Extensive trials involving the simulation of rejection conditions were carried out using certified radioisotope sources attached to typical scrap metal to ensure that the system successfully identified items which were not compliant with the regulatory release criteria. In routine operation, typically 4-5 tonnes of material are released in an 8-hour shift. Figure 5 Typical Material arising from D&D Operations at Oak Ridge GDP.

8 CONCLUSIONS The BNFL Instruments IonSens Conveyor Monitor has facilitated the efficient and reliable monitoring of scrap metal, permitting the categorisation of the material at the required regulatory release levels. This categorisation has enabled the operators to recycle the material, releasing its intrinsic value and avoiding the costs associated with Low Level Waste disposal. REFERENCES 1. Dr. Vincent Adams, and Dr. Michael Gresalfi, A survey of scrap metal released and reused in the USA, TUV Nord meeting, Hamburg, November Multi-Agency Radiation Survey and Site Investigation Manual, (MARSSIM), DoD, DoE, EPA and NRC, December Nuclear Regulatory Commission (NRC). 1997b. Minimum Detectable Concentrations with Typical Radiation Survey Instruments for Various Contaminants and Field Conditions., NUREG/CR-1507, Final, NRC, Washington, D.C. 4. Lloyd A.Currie, Limits for Qualitative Detection and Quantitative Determination, Analytical Chemistry Division, National Bureau of Standards, Washington, D.C.