Irradiation experiments for the pebble bed modular reactor PBMR fuel (coated fuel particles and pebble
fuel) are planned at the South African First Atomic Reactor Installation (SAFARI-1). The experiments
are conducted to investigate the behavior of the fuel under normal operating and accelerated/accident
simulating conditions because the safe operation of the reactor relies on the integrity of the fuel for
retention of radioactivity.
For fuel irradiation experiments, the accurate knowledge and analysis of the neutron spectrum of the
irradiation facility is required. In addition to knowledge of the neutron spectrum in the irradiation facility,
power distributions and knowledge of nuclear heating values has to be acquired. The SAFARI-1 reactor
boosts operating fluid temperatures of about 300 K. On the contrary, the PBMR can reach temperatures in
up to about 1370 K under normal operating conditions. This calls for design of high temperature
irradiation rigs for irradiation of the PBMR fuel in the SAFARI-1 reactor. The design of this instrument
(rig) should be such that to create an isolated high temperature environment in the SAFARI-1 reactor, to
achieve the requirements of the PBMR fuel irradiation program. The design of the irradiation rig is
planned such that the rig should fit in the existing irradiation channels of the SAFARI-1 reactor, a time
and cost saving from the licensing perspective.
This study aims to establish the know-how of coated particle and pebble modeling in using the Monte
Carlo N-Particle code (MCNP5). The study also aims to establish the know-how of rig design. In this
study, the Necsa in-house code Overall System for the Calculation of Reactors (OSCAR-3), a software
known as OScar 3-Mcnp INTerface (OSMINT) linking OSCAR-3 and MCNP5, also developed at Necsa,
as well as MCNP5 code developed and maintained by the Los Alamos team, are used to calculate
neutronic and power distribution parameters that are important for fuel irradiations and for rig design.
This study presents results and data that can be used to make improvements in the design of the rig or to
confirm if the required operational conditions can be met with the current preliminary rig design. Result
of the neutronic analysis are presented for the SAFARI-1 core, core irradiation channel B6 (where the
PBMR fuel irradiation rig is loaded for the purpose of this study), the rig structure and the pebble fuel are
presented. Furthermore results of the power distribution and nuclear heating values in the reactor core, the
irradiation channel B6, the rig structures and the pebble fuel is also presented.
The loading of the PBMR fuel irradiation rig in core position B6 reduces the core reactivity due to the
fact that the loading of the rig displaces the water moderator in channel B6 introducing vast amounts of
helium. This impacts on the keff value because there will be less neutron thermalization and reproduction
due to the decreased population of thermal neutrons. The rig is found to introduce a negative reactivity
insertion of 46 pcm. The loading of this rig in the core leads to no significant perturbations on the core
power distribution. The core hottest channel is still localized in core channel C6 both with RIG IN and RIG OUT cases. A power tilt is observed, with the south side of the core experiencing reduced assembly
averaged fission power, with correspondingly small compensations from the assemblies on the north side
of the core.
The perturbations on the core assembly averaged fluxes are more pronounced in the eight assemblies
surrounding B6. Core position B6 suffers an 18% neutron flux depression with the loading of the rig. The
fluxes in core positions A5, A6, A7, B5, B7 and C7 are increased when the rig is loading. The largest
increases are noted as 12% in A7, 9% in A6 and 6% in A5 and B7. All the eight core positions
surrounding B6 experience reduced photon fluxes with the loading of the rig. Core position B6 shows a
flux depression of up to 20%, with 10% reduction in core position A6. The remainder seven positions
surrounding B6 shows flux depressions of no more than 5%.
Further on, due to decreased moderation effects, the axial neutron flux in core position B6 is reduced by
20% when the rig is loaded. The energy dependent neutron flux in B6 decreases by 50% in the thermal
energy range with corresponding increases of up to 50% in the resonance and fast energy regions. The
axial and the energy dependent photon flux in core position B6 decreases by up to 20% when the rig is
loaded.
The magnitude of the neutron and photon fluxes is found to have a direct proportion on the neutron and
photon heating values. While the amount of neutron heating in core position B6 increases by one order of
magnitude, when the rig is loaded, the photon heating values increases by up to 60% in the region
spanning ±10cm about the core centerline. The amount of photon heating in the rig structural materials
dominates neutron heating, except in the helium regions of the rig, where neutron heating dominates
photon heating. In the fuel region of the pebble, fission heating (3803W) largely dominates photon heating (119W). / Thesis (M.Sc. (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2009
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/4030 |
| Date | January 2009 |
| Creators | Makgopa, Bessie Mmakgoto |
| Publisher | North-West University |
| Source Sets | North-West University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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