Denise B. Pelowitz

Initial MCNP6 Release Overview

MCNP6 is simply and accurately described as the merger of MCNP5 and MCNPX capabilities, but it is much more than the sum of those two computer codes. MCNP6 is the result of five years of effort by the MCNP5 and MCNPX code development teams. These groups of people, residing in Los Alamos National Laboratory’s (LANL) X Computational Physics Division, Monte Carlo Codes Group (XCP-3), and Decision Applications Division, Radiation Transport and Applications Team (D-5), respectively, have combined their code development efforts to produce the next evolution of MCNP. While maintenance and bug fixes will continue for MCNP5 1.60 and MCNPX 2.7.0 for upcoming years, new code development capabilities only will be developed and released in MCNP6. In fact, the initial release of MCNP6 contains 16 new features not previously found in either code. These new features include the abilities to import unstructured mesh geometries from the finite element code Abaqus, to transport photons down to 1.0 eV, to transport electrons down to 10.0 eV, to model complete atomic relaxation emissions, and to generate or read mesh geometries for use with the LANL discrete ordinates code Partisn. The first release of MCNP6, MCNP6 Beta 2, is now available through the Radiation Safety Information Computational Center, and the first production release is expected in calendar year 2012. High confidence in the MCNP6 code is based on its performance with the verification and validation test suites, comparisons to its predecessor codes, the regression test suite, its code development process, and the underlying high-quality nuclear and atomic databases.

The MCNPX Monte Carlo Radiation Transport Code

MCNPX (Monte Carlo N‐Particle eXtended) is a general‐purpose Monte Carlo radiation transport code with three‐dimensional geometry and continuous‐energy transport of 34 particles and light ions. It contains flexible source and tally options, interactive graphics, and support for both sequential and multi‐processing computer platforms. MCNPX is based on MCNP4c and has been upgraded to most MCNP5 capabilities. MCNP is a highly stable code tracking neutrons, photons and electrons, and using evaluated nuclear data libraries for low‐energy interaction probabilities. MCNPX has extended this base to a comprehensive set of particles and light ions, with heavy ion transport in development. Models have been included to calculate interaction probabilities when libraries are not available. Recent additions focus on the time evolution of residual nuclei decay, allowing calculation of transmutation and delayed particle emission. MCNPX is now a code of great dynamic range, and the excellent neutronics capabilities allow new opportunities to simulate devices of interest to experimental particle physics, particularly calorimetry. This paper describes the capabilities of the current MCNPX version 2.6.C, and also discusses ongoing code development.

MCNPX Improvements for Threat Reduction Applications

Enhancements contained in the current MCNPX 2.6.0 Radiation Safety Information Computational Center (RSICC) release will be presented, including stopped‐muon physics, delayed neutron and photon generation, and automatic generation of source photons. Preliminary benchmarking comparisons with data taken with a muon beam at the Paul Scherrer Institute Spallation Neutron Source accelerator will be discussed. We will also describe current improvements now underway, including Nuclear Resonance Fluorescence (NRF), pulsed sources, and others. We will also describe very new work begun on a threat‐reduction (TR) user interface, designed to simplify the setup of TR‐related calculations, and introduce standards into geometry, sources and backgrounds.

Comparison of Codes and Neutronics Data Used in the United States and Russia for the TOPAZ‐2 Nuclear Safety Assessment

The TOPAZ‐2 reactor system is a heterogeneous epithermal system fueled with highly‐enriched fuel based on uranium oxide, cooled by a sodium‐potassium liquid metal (NaK), using a zirconium hydride moderator, with 37 thermionic fuel elements (TFEs) built into the core. The core is surrounded by a radial beryllium reflector which contains rotating regulating drums with moderating segments. An important problem is the guaranteeing of nuclear safety upon the accidental falling of the TOPAZ‐2 reactor into water, which leads to the growth of the reactivity of the reactor. It has turned out that it is necessary to use the Monte‐Carlo method for the conduct of neutronics calculations of such a complex reactor. In the United States (U.S.) and Russia, different codes based on the Monte‐Carlo method are used for calculations — the MCNP code in the U.S., and the MCU‐2 code in Russia. The goal of this work is the comparison of the codes and neutronics data used in the U.S. and Russia for the basis of the TOPAZ‐2 nuclea...

Pre‐Orbital Criticality Safety for the Proposed NEPSTP Mission

In December 1991, the Strategic Defense Initiative Organization (SDIO) proposed investigating whether launching a Russian Topaz‐II space nuclear power system could be done safely and within budget constraints. Functional safety requirements developed for the potential US space application of the Topaz‐II mandated that the reactor remain subcritical when immersed in water. Topaz‐II is an epithermal, enriched‐uranium‐fueled, NaK‐(liquid metal alloy with 22% sodium and 78% potassium) cooled, and zirconium hydride‐moderated reactor. A radial beryllium reflector containing 12 rotatable control drums surrounds the core. We prepared a computer model of the Topaz reactor that explicitly represented all major reactor components. Initial analyses indicated that in several water‐immersion scenarios, the reactor would not remain subcritical. After additional calculations, modifications were proposed that would assure subcriticality under such conditions. This paper describes the analyses and the proposed modifications.

Dry critical experiments and analyses performed in support of the TOPAZ-2 safety program

In December 1991, the Strategic Defense Initiative Organization decided to investigate the possibility of launching a Russian Topaz‐2 space nuclear power system. Functional safety requirements developed for the Topaz mission mandated that the reactor remain subcritical when flooded and immersed in water. Initial experiments and analyses performed in Russia and the United States indicated that the reactor could potentially become supercritical in several water‐ or sand‐immersion scenarios. Consequently, a series of critical experiments was performed on the Narciss M‐II facility at the Kurchatov Institute to measure the reactivity effects of water and sand immersion, to quantify the effectiveness of reactor modifications proposed to preclude criticality, and to benchmark the calculational methods and nuclear data used in the Topaz‐2 safety analyses. In this paper we describe the Narciss M‐II experimental configurations along with the associated calculational models and methods. We also present and compare t...

Water/sand flooded and immersed critical experiment and analysis performed in support of the TOPAZ‐II safety program

Presented is a brief description of the Narciss‐M2 critical assemblies, which simulate accidental water/wet‐sand immersion of the TOPAZ‐II reactor as well as water‐flooding of core cavities. Experimental results obtained from these critical assemblies, including experiments with several fuel elements removed from the core, are shown. These configurations with several extracted fuel elements simulate a proposed fuel‐out anticriticality‐device modification to the TOPAZ‐II reactor. Preliminary computational analysis of these experiments using the Monte Carlo neutron‐transport method is outlined. Nuclear criticality safety of the TOPAZ‐II reactor with an incorporated anticriticality unit is demonstrated.