Program - Brookhaven National Laboratory

kinetic model based on Avery-Cohn model is simple, calculable. The model introduces simple probability
relationships essential to calculating the coupling parameters between core and reflector [1] and derives
the reflected-core inhour equation which contains multiple decay modes. However, Spriggs model cannot
describe well the multiple time constants of the thermal reflected reactor. In this kind of reactor, thermal
neutrons with long lifetime contribute much to the time constant. Because of importance of thermal
neutrons in such fast-thermal reactor, we present a simplified two-group; two-region kinetic model (2G2R)
based on Spriggs model, and rewrites the reflected-core inhour equation. With the help of the numerical
method, we perform the analytical calculation on the coupling parameters, neutron lifetimes and first
and secondary time constant of a hypothetical reactor, and compared with the numerical methods, the
Monte-Carlo method and the stochastic neutron kinetics. The results of 2G2R model agree well with the
Monte-Carlo time fitting method which can be thought as an experiment in computer.
[1]. G. D. Spriggs et al., “Two-Region Kinetic Model for Reflected Reactor”, Ann. Nucl. Energy, 24, 205,
(1997).
LD 4 4:40 PM
ATR HEU and LEU Core Kinetics Parameters Calculation by MCNP5 Version 1.60
Adjoint-Weighted Method
G. S. Chang
Idaho National Laboratory
C. R. Glass
Idaho National Laboratory
The Advanced Test Reactor (ATR), currently operating in the United States, is a high power research
reactor used for nuclear material testing at very high neutron fluxes. Powered with highly enriched uranium
(HEU), the ATR has a maximum thermal power rating of 250 MWth. Because of the large core fuel loading
with 235 U ∼ 43 kg, the ATR is assessing the feasibility of converting HEU driven reactor cores to lowenriched
uranium (LEU) cores. The present work investigates the impact on the safety related kinetic
parameters of the LEU alternate Monolithic foil-type (U-10Mo) fuel with external 10 Be absorber (0.58
g) in fuel element side-plates, as compared to the HEU reference case. The calculated safety related
kinetic parameters, such as the effective neutron generation time, Λeff , and the effective delayed neutron
fraction, βeff , are of considerable importance in the fast transient safety analysis of the ATR. These
parameters are typically calculated using deterministic codes. For complex nuclear systems, such as the
ATR with its HEU/LEU fuel elements, water coolant, and beryllium (Be) reflector, the Monte Carlo
method, specifically MCNP5 version 1.60, is the preferred calculation tool because of its ability to handle
continuous energy cross-sections and detailed geometries. MCNP5 version 1.60 has a newly added option
(KOPTS KINETICS=YES) to calculate the effective (i.e. adjoint-weighted) neutron generation time
(Λeff ), and delayed neutron fraction (βeff ), which can be executed to accurately calculate the ATR HEU
and LEU core’s kinetics parameters. In this work, the neutron generation time and delayed neutron fraction
for ATR HEU/LEU core will be directly calculated using a single code, without having to generate energydependent
cross-sections or approximate geometries.
1. Monte Carlo Codes (XCP-3), X Computational Physics Division, Los Alamos National Laboratory
“MCNP5 - 1.60 - A General Monte Carlo N-Particle Transport Code”, Version 5/1.60, Forrest Brown,
Brian Kiedrowski, Jeffrey Bull , “MCNP5-1.60 Release Notes,” LA-10-06235. 2. B. C. Kiedrowski, F.
B. Brown, P.H. Wilson, “Adjoint-Weighted Tallies for k-Eigenvalue Calculations with Continuous-Energy
Monte Carlo,” LA-UR-10-01824, 2010. 3. “Upgraded Final Safety Analysis Report for the Advanced Test
Reactor,” Idaho National Laboratory, SAR-153, 2011.
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