DEVELOPMENT OF BUILDUP FACTORS FOR UPDATING THE ANSI/ANS-6.4.3STANDARD

摘要

The ANSI/ANS-6.4.3-1991 Standard, Gamma-Ray Attenuation Coefficients and Buildup Factors for Engineering Materials, is currently being updated by an American Nuclear Society (ANS) Working Group. The ANSI/ANS-6.4.3, 1991 standard, which is "withdrawn" due to the failure to meet the requirement of the ANS to have standards updated every ten years, contains buildup factor values that are derived from data that is over seventeen years old. In addition, computer technology has significantly improved since 1991, allowing for more complicated, computationally demanding codes to be utilized. Therefore, the ANSI/ANS-6.4.3-1991 standard is being re-visited to include updated data and to include modern codes.Gamma-ray buildup factors and attenuation coefficients are being generated for common shielding materials (e.g., concrete, steel, water, etc.) utilizing the ENDF/B-VI.8 cross-section data library, which is distributed by Brookhaven National Laboratory's National Nuclear Data Center (NNDC). One modern code, Monte Carlo N-Particle (MCNP5/MCNPX), is being compared with PALLAS-ID (VII) and Anisotropic Source-Flux Iteration Technique (ASFIT-VARI), which were used to develop values included in ANSI/ANS-6.4.3-1991. PALL AS-ID (VII) is a code for direct integration of transport equation in one-dimensional plane and spherical geometries while ASFIT-VARI is a gamma-ray Iransport code system for one-dimensional finite systems. MCNP5 is a general purpose Monte Carlo radiation transport code that tracks particles (e.g., neutron and photons) at numerous energies in a three dimensional configuration of materials.The MCNP5 radiation transport code require response function input to provide dose and exposure output. Mass energy-absorption coefficients and mass energy-transfer coefficients are required to develop absorbed dose and exposure responses. The National Institute of Standards andTechnology (NIST) provide and maintain the most up-to-date mass energy-absorption coefficient and mass energy-transfer coefficient database currently available. The NIST values are used in these initial buildup factor calculations to prove the validity of the methodology used and allow for preliminary comparisons.The energy absorption (dose in material) and exposure buildup factors are calculated at mfp values of interest, consistent with ANSI/ANS-6.4.3-1991 up to forty mfp.Comparisons between the new buildup factors and the previous results presented in the ANSI/ANS-6.4.3-1991 standard indicate that there is fairly good agreement. Differences in buildup factor values can be attributed to differences in cross-section data libraries, numerical methods, and physics treatments within the respective codes.