Abstract

The first edition of the current American National Standards Institute (ANSI)/American Nuclear Society (ANS) standard ANSI/ANS-8.1-2014, “Nuclear Criticality Safety in Operations with Fissionable Materials Outside Reactors,” was published in 1964 as ASA N6.1-1964, “Safety Standard for Operations with Fissionable Materials Outside Reactors.” In 1969, that standard was revised as ANSI N16.1-1969, “Nuclear Criticality Safety in Operations with Fissionable Materials Outside Reactors.” ANSI N16.1-1969 includes a variety of subcritical limits (SCLs) for uniform aqueous solutions and metals containing fissile nuclides of ²³³U, ²³⁵U, and ²³⁹Pu. Furthermore, SCLs are also included for uranium-water lattices. In the 1983 version of ANSI/ANS-8.1 (a revision of ANSI N16.1-1975), the suite of SCLs in the standard was expanded to include ²³⁵U enrichment limits for homogeneous uranium-water mixtures and dry/damp oxides, uniform aqueous solutions of low-enriched uranium, and uniform aqueous mixtures of Pu(NO₃)₄ containing ²⁴⁰Pu in addition to the SCLs included in ANSI N16.1-1969. The SCLs have changed little in subsequent revisions (ANSI/ANS-8.1-1998 and ANSI/ANS-8.1-2014). The ANSI/ANS-8.1-2014 standard is currently being revised to include new SCLs (uranium metal and compounds with enrichments up to 20 wt% ²³⁵U) and possible updates to the current SCLs already in the standard, although these new/updated SCLs will not be available to the nuclear criticality safety (NCS) community for a number of years. The original bases for these SCLs were documented in papers in journals such as Nuclear Science and Engineering and in ANS meeting transactions; however, these bases are ambiguous enough that sites and regulators in the United States have not widely endorsed them for safety purposes. The purpose of this paper is to present the results of a comparison study for the SCLs in the ANSI/ANS-8.1-2014 standard using modern codes (SCALE and MCNP) and cross sections (ENDF/B-VIII.0) to provide some assurance about their quality (bias and bias uncertainty) for use in NCS applications and for consideration by the ANS-8.1 Working Group as a reference for future revisions.

摘要

当前美国国家标准协会 (ANSI)/美国核学会 (ANS) 标准 ANSI/ANS-8.1-2014 的第一版“反应堆外可裂变材料操作中的核临界安全”于 1964 年作为 ASA N6 出版。 1-1964，“反应堆外可裂变材料操作的安全标准”。1969 年，该标准修订为 ANSI N16.1-1969，“反应堆外可裂变材料操作中的核临界安全”。ANSI N16.1-1969 包括各种亚临界限值 (SCL)，适用于含有²³³ U、²³⁵ U 和²³⁹裂变核素的均匀水溶液和金属浦。此外，铀-水晶格还包括 SCL。在 ANSI/ANS-8.1 的 1983 版（ANSI N16.1-1975 的修订版）中，标准中的 SCL 套件被扩展为包括均质铀水混合物和干/湿氧化物的²³⁵ U 浓缩限制，均匀除了 ANSI N16.1-1969 中包含的 SCL 之外，低浓缩铀的水溶液和含有²⁴⁰ Pu的 Pu(NO ₃ ) _{4 的}均匀水性混合物。SCL 在随后的修订版（ANSI/ANS-8.1-1998 和 ANSI/ANS-8.1-2014）中变化不大。ANSI/ANS-8.1-2014 标准目前正在修订，以包括新的 SCL（铀金属和浓缩物高达 20 wt% ²³⁵U) 以及标准中现有 SCL 的可能更新，尽管这些新的/更新的 SCL 将在数年内无法提供给核临界安全 (NCS) 社区。这些 SCL 的原始基础记录在《核科学与工程》等期刊和 ANS 会议交易中的论文中；然而，这些基础不够明确，以至于美国的站点和监管机构并未出于安全目的广泛认可它们。本文的目的是展示使用现代规范（SCALE 和 MCNP）和横截面（ENDF/B-VIII.0）对 ANSI/ANS-8.1-2014 标准中的 SCL 进行比较研究的结果，以提供一些保证它们的质量（偏差和偏差不确定性）用于 NCS 应用和供 ANS-8 考虑。