Abstract
The default energy deposition model in the CASL neutronics code MPACT assumes all fission energy is deposited locally in fuel rods. Furthermore, equilibrium delayed energy release is assumed for both steady-state and transient calculations. These approximations limit the accurate representation of the heat generation distribution in space and its variations over time, which are essential for power distribution and thermal-hydraulic coupling calculations. In this paper, an improved energy deposition model is presented in both the spatial and time domains. Spatially, the energy deposition through fission, neutron capture, and slowing-down reactions are explicitly modeled to account for the heat generation from all regions of a reactor core, and a gamma smearing scheme is developed that utilizes the gamma sources from neutron fission and capture. In the time domain, the delayed energy release is modeled by solving an additional equation of delayed heat emitters, similar to the equation of delayed neutron precursors. To allow the explicit heat generation coupling, the interfaces between MPACT and CTF were updated to transfer separate heat sources for different material regions (fuel, clad, moderator and guide tube). The results show that the distributions of the energy deposition between MPACT and MCNP agree very well for various 2-D assembly and quarter-core problems without TH feedback. The MPACT/CTF coupled calculation for the hot full power quarter-core case exhibited a reduced peak pin power by 2.3% and a reduced peak fuel centerline temperature by 17 K when using the explicit energy deposition and heat transfer. The new model also shows a maximum 100 pcm keff effect on assembly depletion problems and an increased overall energy release by 7% in a PWR reactivity-initiated accident (RIA) problem.
Highlights
The energy released by a fission event consists of various energy modes
To consider the heat directly generated in the moderator via neutron and gamma reactions, a direct moderator heating fraction is presently used when coupled with CTF for TH calculation. Several limitations of this energy deposition model include: (1) time dependence of the delayed energy modes is not considered in the time-dependent problems, such as depletion and transient problems; (2) a typical value of the neutron capture and slowing-down heat is built into fission kappa without considering the fuel composition explicitly; (3) all heat generation is assumed to be deposited in fuel rods when computing the pin power; and (4) no gamma smearing effect is considered
Without solving the gamma transport equation, an approximate gamma smearing model was developed in this work based on the following rationale: (1) the spatial gamma energy sources can be estimated according to the fission and capture rates; (2) the gamma energy is assumed to be redistributed within a predefined block of n×n pin cells due to the longer mean free path of gamma rays; (3) “pseudo pins” with zero gamma source are placed in the radial reflector to consider the leakage effect; and (4) according to Knoll [6], the gamma interaction cross sections are essentially proportional to the power of isotope atomic number
Summary
The energy released by a fission event consists of various energy modes. In addition to the direct fission energy release, the capture of neutrons via (n,J ) or (n,D ) reactions, for example, and the slowing down of neutrons deposit energy. To consider the heat directly generated in the moderator via neutron and gamma reactions, a direct moderator heating fraction (user-defined) is presently used when coupled with CTF for TH calculation. Several limitations of this energy deposition model include: (1) time dependence of the delayed energy modes is not considered in the time-dependent problems, such as depletion and transient problems; (2) a typical value of the neutron capture and slowing-down heat is built into fission kappa without considering the fuel composition explicitly; (3) all heat generation is assumed to be deposited in fuel rods when computing the pin power; and (4) no gamma smearing effect is considered. An improved energy deposition model is developed in MPACT to eliminate these assumptions
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