Measurement Methods Training and Software Developed for Inspection of Small Gaseous Diffusion Plants

Transcription

1 Measurement Methods Training and Software Developed for Inspection of Small Gaseous Diffusion Plants Rollen, H.Y.; Smith, S.E.; Whitaker, J.M.; Mayer II, R.L.; McGinnis, B.R.; Bonino, A.D.; Righetti, M.A. and Gryntakis, E. Presentado en la 42 th Annual Meeting on the Institute of Nuclear Material Management, Indian Wells, EE. UU., julio 2001

2 Measurement Methods Training And Software Developed for Inspection of Small Gaseous Diffusion Plants H. Y. Rollen Jr., S. E. Smith, J. M. Whitaker Y-12 National Security Complex National Security Program Office 1099 Commerce Park Drive Oak Ridge, Tennessee, R. L. Mayer II, B. R. McGinnis United States Enrichment Corporation P.O. Box U.S. Rt. 23 S Piketon, Ohio, A. D. Bonino, M. A. Righetti Nuclear Regulatory Authority Av. Del Libertador 8250 (1429) Buenos Aires, Republic of Argentina E. Gryntakis International Atomic Energy Agency Wagramer Strasse 5, P.O. Box 100 A-1400 Vienna, Austria ABSTRACT A suite of measurement methods and software was developed to verify uranium inventory at small gaseous diffusion plants and to enable the International Atomic Energy Agency (IAEA) to perform inspections at the gaseous diffusion plant at Pilcaniyeu, Argentina. In addition, the Brazilian- Argentine Agency for Accounting and Control of Nuclear Materials (ABACC) will use the software to support its inspections at the facility. In November 2000, training was conducted on these measurement methods and software, and inspectors from the IAEA, ABACC, and other agencies participated. The training sessions were held in the Mock-Up Facility at the Pilcaniyeu plant. Inspectors received classroom training, used the measurement methods and software to calibrate instrumentation, performed quantitative measurements of process equipment and waste, located and quantified measurable deposits in process piping, performed enrichment measurements, and compared measurement results with operator declarations. Measurement results compared well with previous measurements on this equipment and with operator declarations. INTRODUCTION The gaseous diffusion plant at Pilcaniyeu located near San Carlos de Bariloche, Argentina, in the Province of Rio Negro is designed to enrich uranium to a nominal level of 5% 235 U at a nominal capacity of 20,000 SWU/year (SWU, separative work unit). The plant is operated by the Comisión Nacional de Energía Atómica of Argentina (CNEA). Since 1993, this facility has been subject to international safeguards applied by the International Atomic Energy Agency (IAEA) and the Brazilian-Argentine Agency for Accounting and Control of Nuclear Materials (ABACC). Both the IAEA and ABACC are responsible for periodically verifying the inventory of 235 U in the gaseous 1

3 diffusion cascade. This facility was the first enrichment plant using gaseous diffusion technology to be placed under IAEA safeguards. In April 1992, a joint program was initiated between the U.S. Department of Energy (DOE) and the Argentine Regulatory National Authority (ARN) to develop non-destructive assay (NDA) measurement methods for use by IAEA during inspections of the plant. During the development effort, it became apparent that classical NDA measurement methods employed by IAEA were not sufficient for conducting all aspects of the inventory verification, particularly for the process holdup and waste strata. It was also recognized that the calibrations and computation methods associated with several of the selected techniques would require the use of computer software to permit verification in a reasonable amount of time. As a result, it was determined to write a software package that would assist inspectors in performing the necessary measurements and analyzing the results of those measurements. The software development project was initiated under direction of the National Security Program Office (NSPO) of they-12 National Security Complex. Technical support personnel were provided jointly from NSPO, the Applied Nuclear Technology (ANT) Department of the Portsmouth Gaseous Diffusion Plant, and the Technical Support Department of the ARN. Personnel traveled to Pilcaniyeu in the mid-1990s to conduct tests with ARN personnel on the measurement techniques. Periodically, since then, DOE and ARN personnel have traveled to Pilcaniyeu to test and improve the methodology. The results of those efforts was a new software package for verifying uranium inventory at small gaseous diffusion plants: the Inventory Verification Analysis Software for Pilcaniyeu (IVASP). The IVASP software was demonstrated for IAEA and the ABACC at the Portsmouth Gaseous Diffusion Plant in October The purpose was to permit IAEA and ABACC to use and evaluate the software and to validate that the software provides correct analyses when provided appropriate measurement data. Subsequently, in December 1998, a validation exercise was carried out for IAEA and ABACC at the Ezeiza Atomic Center on the latest version of the IVASP software. The purpose of this demonstration was twofold: (1) to allow IAEA to verify calibration data and measurement techniques utilized by the ARN personnel during the validation exercise and (2) to enter the calibration data into the latest version of the IVASP software and validate the volumetric calculation methods employed by the IVASP software. ANT personnel presented the final software package to IAEA at the annual U.S. Support Program Review Meeting at IAEA Headquarters in Vienna, Austria. At this time, the software was formally presented to the meeting and in smaller groups to the IAEA inspectors who will use the package at Pilcaniyeu. A final demonstration and validation was conducted at the Pilcaniyeu gaseous diffusion plant during November PLANT CONFIGURATION The Pilcaniyeu gaseous diffusion plant contains 21 modules (cells) under safeguards, including one in the mockup facility. Each module contains 20 units (diffuser plus compressor) and associated piping. Loose components and process waste streams (typically crushed barrier) are monitored as well. Inspectors encounter diffusers of multiple size and design. Most of the diffusers have three visible sections, while some have only two. Height and diameter varies, and wall construction can be steel, aluminum, or a combination of both. 2

4 Because there are several designs and configurations, these must be accounted for during analysis by IVASP, as the measurements are sensitive to size, materials of construction, and attenuation characteristics. Therefore, design verification during inspections must confirm the number of sections, diameter of each section, total height, wall thickness, and wall material. Periodically, transmission is verified as well. SOFTWARE DESCRIPTION The IVASP software developed to meet the needs of the Pilcaniyeu plant consists of five major subroutines (i.e., branches). Each subroutine performs a specific task related to inventory verification. The five subroutines and their primary divisions are as follows: CALIBRATE Neutron Calibration NaI Quantification NaI Scanning SCAN Module Piping QUANTIFY Module Unit Piping Loose Diffuser Drum VERIFY Design Gamma Transmission Enrichment MEASUREMENT CONTROL Enter Measurement Data View Control Chart View Detector Ranges Edit Detector Edit Standard Edit Records Calibrate Calibration is performed on all detectors used for quantitative and scanning measurements. Although scan results do not include estimates of uranium quantity, calibration is performed on scanning detectors so that results from different inventory periods can be compared to one another. Calibration for scanning detectors is accomplished by placing a 235 U source with known effective mass at a predetermined distance in front of the detector face. Calibration is performed on gamma-ray quantitative detectors by placing a 235 U calibration source on a predefined grid. The data are fitted to a calibration equation that accounts for distance from the detector and angle of incidence of the gamma rays to the detector. The calibration equation is presented below: E i = 1 a5 [ 3 i 4 i ] 2 ( z + a ) 1+ a Θ + ( a Θ ) i 2 a where E i is the efficiency of the detector for position i; the constants a 1 through a 5 are fitted using the calibration data; and the values for z i and È i represent the source-to-detector distance and angle of incidence, respectively. Calibration of the neutron detector is accomplished using a similar process, except that a 252 Cf source is used as the calibration source. All calibration results are stored in a database. 3

5 Scan The Scan subroutine permits scanning of a module or piping. Scanning is accomplished by moving the detector slowly over the surface of the item being measured. The analyzer is placed in the rate meter mode for this measurement. The maximum value obtained for each item section is recorded and entered into the IVASP software. The data are then stored in a database. Any prior results for the scanning measurement of the item are retrieved for comparison. Quantify The Quantify subroutine permits quantification of entire modules of up to 20 units, individual units within a module, interconnecting piping, loose diffusers, and drums of waste. All measurements can be performed using gamma-ray quantitative measurement methods. In addition, measurement of waste drums can be performed using neutron quantitative measurement methods. The size, wall thickness, and matrix attenuation characteristics of each item are entered into the IVASP software prior to the measurement data. Transmission measurements are typically performed to estimate the gamma-ray linear attenuation factor for waste items. Transmission measurements on selected diffusers have previously been made to estimate the linear attenuation factor for the diffusion barrier. The software performs computations of 235 U quantity by dividing the item into nodes that are spaced evenly throughout the item, both radially and axially. The detection efficiency is then estimated for each node using the calibration model. For gamma-ray measurements, the path length through the matrix and item wall are also computed for each node, and attenuation correction factors are estimated. A numerical integration is performed to estimate the overall efficiency, and then the entered count rate data are used to estimate 235 U quantity. Prior measurement results are displayed along with the current estimates for comparison. Verify The Verify subroutine enables the inspector to verify that the physical configuration of the plant has not changed since the previous inspection. In addition, physical configuration data for all diffusers to be measured must be entered prior to making quantitative measurements. Transmission measurement data are entered into the gamma transmission subroutine to estimate linear attenuation coefficients for waste drums. Any enrichment data are entered into the enrichment subroutine. All entered data are stored in a database for later retrieval. In addition, any prior measurements on the entered items are displayed for comparison. Measurement Control The Measurement Control subroutine is used to provide assurance that the detector system operating characteristics have not changed since calibration. Measurements are taken of sources of known strength. The data are compared to previous measurement data using control charts and other statistical computational methods. The software supplies the inspector with a message that the system is in control or out of control. If the system is out of control, the failed tests are indicated. 4

6 COMPARATIVE MEASUREMENT DATA In-situ cascade measurements conducted at the Pilcaniyeu gaseous diffusion plant using IVASP have been relatively consistent, as one would hope considering its steady-state operational status. Four measurement exercises were held in 1994, 1995,and Prior to each campaign, design verification was conducted to ensure no changes had taken place between exercises. Unit (diffuser) parameters were verified as follows: Measurement parameters were set as follows: Unit diameter cm Height 375 cm Outer shell thickness cm Linear Attenuation Coefficient cm -1 Detector 7.62 cm x 7.62 cm (3 x3 ) Collimation depth 12 cm Collimator thickness 1 cm Standoff, collimator to unit 46.9cm In the Pilcaniyeu plant, the mock-up facility is known as Module 0. Although a module normally contains 20 units (diffuser plus compressor), only ten units (11 20) in Module 0 have ever received UF 6 feed and, thus, contain the majority of uranium holdup in the module. These ten units were measured by teams including members from DOE, ARN, and ABACC. The results were reasonably consistent as shown in Table 1. (Note that the operator-declared value is 231 grams 235 U.) Table 1. Comparative results of successive measurement campaigns [grams 235 U] Campaign ARN 1994 ABACC / ARN DOE / ARN CLASS Unit Total mass

7 VALIDATION BY DESTRUCTIVE ANALYSIS During 1997, ARN performed an experiment where the uranium contents of two diffusers of different size and without the outer casing were carefully measured by NDA techniques. After the measurements, the barrier was destroyed, and sampling was performed for chemical analysis of 235 U content. The results of this analysis determined grams 235 U for the smaller unit, and grams U 235 for the larger. Subsequent analysis by IVASP provided comparable results, grams 235 U and 55.8 grams 235 U, respectively (see Table 2). Table 2. Comparative analysis of 235 U Small diffuser (g 235 U) Larger diffuser ( 235 U) Chemical analysis IVASP analysis EXERCISES ON-SITE A final demonstration and validation was conducted at the Pilcaniyeu gaseous diffusion plant (Figure 1). The plant site is located approximately one hour outside of the nearest city of size, San Carlos de Bariloche, in a remote valley a few kilometers from the village of Pilcaniyeu (Figure 2). The site is operated by the CNEA Pilcaniyeu Atomic Center in Bariloche. During the second and third week of November 2000, sixteen instructors and students traveled to Pilcaniyeu to conduct insitu measurements, testing, and coursework. Participants included representatives from DOE, IAEA, ARN, ABACC, and CNEA. Figure 1. Plant entrance. Figure 2. Pilcaniyeu Station. The training sessions were held in a mock-up facility at the Pilcaniyeu plant. Inspectors received classroom training, used the measurement methods and IVASP software to calibrate instrumentation (Figures 3-4), performed quantitative measurements of process equipment and waste (Figure 5), located and quantified measurable deposits in process piping (Figure 6), performed enrichment measurements (Figure 7), and compared measurement results with operator declarations. 6

8 Figure 3. NaI qualitative calibration. Figure 5. NaI quantification. Figure 4. Neutron calibration exercise. Figure 6. NaI scanning of piping. Figure 7. Class on enrichment measurement. 7

9 SUMMARY The inspector training at Pilcaniyeu represents the culmination of several years of development effort by participants from DOE and ARN related to measurement methods and software for inspection of the Pilcaniyeu gaseous diffusion plant (the first and only gaseous diffusion enrichment plant under IAEA safeguards). IAEA provided great assistance in adapting the methodology for use in routine inspections. Inspectors from several different organizations were successfully trained on the measurement methods used in performing an inventory of this plant. In addition, inspectors were provided with software to aid in accomplishing this purpose, and were trained in its use. Participants successfully measured a module, which represents the smallest portion of the cascade for which a declaration is made. In addition, they performed all other measurements needed to support inspections at the facility. These measurements included quantification of piping and waste materials, and enrichment measurements. REFERENCES 1. R. L. Mayer II, "Software for Uranium Quantitative Estimates from Passive Neutron and Gamma Ray Measurement," Portsmouth Gaseous Diffusion Plant publication POEF-TS-04, May, R. L. Mayer II, B. R. McGinnis, J. N. Cooley, J. M. Whitaker, T. D. Reilly, "Nondestructive Assay Measurements in Support of the Cooperative Effort Between the United States and Argentina," Portsmouth Gaseous Diffusion Plant publication POEF-TS Anibal Bonino, et al., "DOE-ARN Proposed Method to Verify Uranium Inventory at the Pilcaniyeu Gaseous Diffusion Enrichment Plant," Portsmouth Gaseous Diffusion Plant publication POEF-LMUS-100, July 21, R. L. Mayer II, J. Bailey, R. C. Hagenauer, B. R. McGinnis, R. R. Royce, "A Comparative Study of Nondestructive Assay Estimates to Chemical Recovery and Operator-Declared Inventory for Large-Scale Gaseous Diffusion Process Equipment," INMM 35 th Annual Proceedings, July 1994, Vol. XXIII, pp (Photos courtesy of CNEA, November 2000) 8