[en] The U.S. Nuclear Regulatory Commission (NRC) issued its Interim Staff Guidance (ISG) Fuel Cycle Safety and Safeguards (FCSS)-ISG-10, 'Justification for Minimum Margin of Subcriticality for Safety, ' in 2006. The NRC developed this ISG in response to several licensee amendment requests for reduced margins of subcriticality for operational efficiency. FCSS-1SG-10 considers the minimum subcritical margin as arising from uncertainty about the ability to accurately calculate k_eff, and discusses five criteria that a reviewer should consider in making a risk-informed judgment of the adequacy of the margin: (1) conservatism in the calculational method; (2) confidence in adequacy and applicability of the criticality code validation; (3) sensitivity of the system to possible nuclear data errors; (4) other independent knowledge of subcriticality (e.g., knowledge of system neutronics); and (5) risk of attaining the modeled configuration. Even after the promulgation of this guidance, there seems to be the widespread impression that NRC is imposing an arbitrary margin of 5% on the nuclear industry. This paper discusses how the principles in FCSS-ISG-10 have been applied in recent licensing reviews, including new facility licensing. It also discusses how these principles could be broadened to make risk-informed decisions about the adequacy of the subcritical margin under repository conditions. (authors)

[en] Nuclear fuel cycle facilities, like much of the nuclear industry, are increasingly relying on digital instrumentation and control (DI and C) systems such as programmable logic controllers (PLCs) to maintain system variables for both production and safety purposes. Fuel cycle manufacturing processes are increasingly automated and relying on active engineered controls. Compliance with the double contingency principle requires that DI and C trains credited for criticality safety are independent. The additional requirements imposed as part of performing an Integrated Safety Analysis (ISA) in accordance with Title 10 of the Code of Federal Regulations (10 CFR) Part 70, Subpart H, mean that such DI and C systems must meet more stringent requirements than would otherwise be required to meet industry standards. This paper discusses the current status of the U.S. Nuclear Regulatory Commission's (NRC's) Digital I and C Working Group, and the requirements that would be imposed on such systems are a result of having to comply with the double contingency principle and the performance requirements of 10 CFR 70.61. Existing applicable NRC Interim Staff Guidance is summarized, and several case studies from ISA technical reviews are presented. (authors)

[en] We consider the random sequence x_n = x_n-1 + γx_q, with γ > 0, where q = 0, 1, ..., n - 1 is chosen randomly from a probability distribution P_n(q). When all q are chosen with equal probability, i.e. P_n(q) = 1/n, we obtain an exact solution for the mean < x_n> and the divergence of the second moment < x²_n> as functions of n and γ. For γ = 1 we examine the divergence of the mean value of x_n, as a function of n, for the random sequences generated by power law and exponential P_n(q) and for the non-random sequence P_n(q) = δ_q,β(n-1)

[en] Nuclear Regulatory Commission (NRC) fuel cycle licensees handling, possessing, or storing fissionable materials are required to ensure that equipment and processes are subcritical under the most reactive credible upset conditions. NRC licensees make commitments in their licenses regarding how criticality safety analyses will be generated. Typical commitments include ensuring subcriticality under the most reactive reflection condition physically possible, typically 30.48 cm (12 inches) of tight-fitting water (full reflection). If this does not meet the upper subcritical limit, a bounding credible reflection condition may be employed. Most NRC licensees use a 2.54 cm (1 inch) tight-fitting water reflector as the minimum reflection condition (incidental reflection), intended to bound nearby materials that are not explicitly modeled or transient reflectors, such as nearby containers or operators. Typically, a licensee will commit to establish controls to ensure that more reactive reflection conditions do not inadvertently occur. When a licensee is unable to demonstrate subcriticality with incidental reflection, another bounding reflection condition may be used. Two conditions frequently seen are gapped water reflectors and large water blocks (representing human beings). This paper compares the effects of various gapped reflectors and water blocks on typical fuel systems with the effects of tight-fitting full and incidental reflectors. The systems used were cylinders containing high- and low-enriched uranyl nitrate solution and gloveboxes with several containers of high- and low-enriched uranium oxide. The paper also analyzes a variety of other reflection conditions to gain further insight. The paper will examine whether a single parameter can be developed to characterize different reflection conditions, will seek to explain why the observed effects occur, and will discuss controls appropriate to maintain the chosen reflection conditions. (authors)

[en] Considerable interest has been expressed for developing a stable U.S. production capacity for medical isotopes and particularly for molybdenum-99 (99Mo). This is motivated by recent reductions in production and supply worldwide. Consistent with U.S. nonproliferation objectives, any new production capability should not use highly enriched uranium fuel or targets. Consequently, Aqueous Homogeneous Reactors (AHRs) are under consideration for potential 99Mo production using low-enriched uranium. Although the Nuclear Regulatory Commission (NRC) has guidance to facilitate the licensing process for non-power reactors, that guidance is focused on reactors with fixed, solid fuel and hence, not applicable to an AHR. A panel was convened to study the technical issues associated with normal operation and potential transients and accidents of an AHR that might be designed for isotope production. The panel has produced the requisite AHR licensing guidance for three chapters that exist now for non-power reactor licensing: Reactor Description, Reactor Coolant Systems, and Accident Analysis. The guidance is in two parts for each chapter: (1) standard format and content a licensee would use and (2) the standard review plan the NRC staff would use. This guidance takes into account the unique features of an AHR such as the fuel being in solution; the fission product barriers being the vessel and attached systems; the production and release of radiolytic and fission product gases and their impact on operations and their control by a gas management system; and the movement of fuel into and out of the reactor vessel.

[en] The requirements of low-cost and high-temperature corrosion resistance for bipolar interconnect plates in solid oxide fuel cell stacks has directed attention to the use of metal plates with oxidation resistant coatings. The performance of steel plates with multilayer coatings, consisting of CrN for electrical conductivity and CrAlN for oxidation resistance, was investigated. The coatings were deposited using large area filtered arc deposition technology, and subsequently annealed in air for up to 25 hours at 800 °C. The composition, structure, and morphology of the coated plates were characterized using Rutherford backscattering, nuclear reaction analysis, atomic force microscopy, and transmission electron microscopy techniques. By altering the architecture of the layers within the coatings, the rate of oxidation was reduced by more than an order of magnitude. Electrical resistance was measured at room temperature.

[en] We report a measurement of the asymmetry in spin-dependent quasielastic scattering of longitudinally polarized electrons from a polarized ³He target. The neutron magnetic form factor Gⁿ_M has been extracted from the measured asymmetry based on recent PWIA calculations using spin-dependent spectral functions. Our determination of Gⁿ_M at Q²=0.19 (GeV/c)² agrees with the dipole parametrization. This experiment represents the first measurement of the neutron magnetic form factor using spin-dependent electron scattering

[en] We present the results of a measurement of the spin-dependent asymmetry in ³H rvec e(rvec e,e') inelastic scattering at kinematics on the low-energy transfer side of the quasielastic peak, including the region near the breakup threshold. Comparison with existing calculations based upon the plane wave impulse approximation shows significant deviation between the data and the model near the breakup threshold. Good agreement between data and theory is seen at higher energy transfer

[en] We report a measurement of the asymmetry in spin-dependent quasielastic scattering of longitudinally polarized electrons from a polarized ³He target. The neutron magnetic form factor Gⁿ_M has been extracted from the measured asymmetry based on recent PWIA calculations using spin-dependent spectral functions. This work represents the first measurement of Gⁿ_M using spin-dependent electron scattering. copyright 1995 American Institute of Physics

[en] We report a measurement of the asymmetry in spin-dependent quasielastic scattering of longitudinally polarized electrons from a polarized ³He target. The neutron magnetic form factor Gⁿ_M has been extracted from the measured asymmetry based on recent PWIA calculations using spin-dependent spectral functions. This experiment represents the first measurement of the neutron magnetic form factor using spin-dependent electron scattering. copyright 1995 American Institute of Physics