Criticality Calculations for Step-2 GPHS Modules

Conference · Sat Feb 09 23:00:00 EST 2008

OSTI ID:990029

Hensen, Danielle Lynn ^[1]; Lipinski, Ronald J. ^[1]

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1. Sandia National Laboratories (SNL-NM), Albuquerque, NM (United States)

The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) will use an improved version of the General-Purpose Heat Source (GPHS) module as its source of thermal power. This new version, referred to as the Step-2 GPHS Module, has additional and thicker layers of carbon fiber material (Fine Weaved Pierced Fabric) for increased strength over the original GPHS module. The GPHS uses alpha decay of ²³⁸Pu in the oxide form as the primary source of heat, and small amounts of other actinides are also present in the oxide fuel. Criticality calculations have been performed by previous researchers on the original version of the GPHS module (Step 0). This paper presents criticality calculations for the present Step-2 version. The Monte Carlo N-Particle eXtended code (MCNPX) was used for these calculations. Numerous configurations of GPHS module arrays surrounded by wet sand and other materials (to reflect the neutrons back into the stack with minimal absorption) were modeled. For geometries with eight GPHS modules (from a single MMRTG) surrounded by wet sand, the configuration is extremely sub-critical; k_eff is about 0.3. It requires about 1000 GPHS modules (from 125 MMRTGs) in a close-spaced stack to approach criticality (k_eff = 1.0) when surrounded by wet sand. The effect of beryllium in the MMRTG was found to be relatively small.

• Research Organization: Sandia National Laboratories (SNL-NM), Albuquerque, NM (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA), Nuclear Criticality Safety Program (NCSP)
• DOE Contract Number: AC04-94AL85000
• OSTI ID: 990029
• Report Number(s): SAND2007-5190C
• Country of Publication: United States
• Language: English

Related Subjects

30 DIRECT ENERGY CONVERSION
97 MATHEMATICS AND COMPUTING
ABSORPTION
ACTINIDES
ALPHA DECAY
BERYLLIUM
CARBON FIBERS
CONFIGURATION
CRITICALITY
General-Purpose Heat Source (GPHS) Module
HEAT SOURCES
Monte Carlo N-Particle eXtended Code (MCNPX)
Multi-Mission Radioisotope Thermoelectric Generator (MMRTG)
NESDPS Office of Nuclear Energy Space and Defense Power Systems
NEUTRONS
Nuclear Criticality Safety Program (NCSP)
OXIDES
RADIOISOTOPES
SAND
THERMOELECTRIC GENERATORS