The build–up of 6Li and 3He, that is, the strong thermal neutron absorbers or the so called "neutron
poisons", in the beryllium reflector changes the physical characteristics of the reactor, such as
reactivity, neutron spectra, neutron flux level, power distribution, etc.; furthermore,gaseous isotopes
such as 3H and 4He induce swelling and embrittlement of the reflector.
The SAFARI–1 research reactor, operated by Necsa at Pelindaba in South Africa, uses a beryllium
reflector on three sides of the core, consisting of 19 beryllium reflector elements in total. This
MTR went critical in 1965, and the original beryllium reflectors are still used. The individual
neutron irradiation history of each beryllium reflector element, as well as the impact of beryllium
poisoning on reactor parameters, were never well known nor investigated before. Furthermore,
in the OSCAR{3 code system used in predictive neutronic calculations for SAFARI–1, beryllium
reflector burn–up is not accounted for; OSCAR models the beryllium reflector as a non–burnable,
100% pure material. As a result, the poisoning phenomenon is not accounted for. Furthermore,
the criteria and hence the optimum replacement time of the reflector has never been developed.
This study presents detailed calculations, using MCNP, FISPACT and the OSCAR{3 code system,
to quantify the influence of impurities that were originally present in the fresh beryllium reflector,
the beryllium reflector poisoning phenomenon, and further goes on to propose the reflector's
replacement criteria based on the calculated fluence and predicted swelling. Comparisons to
experimental low power flux measurements and effects of safety parameters are also established.
The study concludes that, to improve the accuracy and reliability of the predictive OSCAR code
calculations, beryllium re flector burn–up should undoubtedly be incorporated in the next releases
of OSCAR. Based on this study, the inclusion of the beryllium reflector burn–up chains is planned
for implementation in the currently tested OSCAR–4 code system. In addition to beryllium
reflector poisoning, the replacement criteria of the reflector is developed. It is however crucial
that experimental measurements on the contents of 3H and 4He be conducted and thus swelling
of the reflector be quantifed. In this way the calculated results could be verified and a sound
replacement criteria be developed.
In the absence of experimental measurements on the beryllium reflector, the analysis and
quantifcation of the calculated results is reserved for future studies. / Thesis (M.Sc. Engineering Sciences (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2011.
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/6930 |
| Date | January 2011 |
| Creators | Moloko, Lesego Ernest |
| Publisher | North-West University |
| Source Sets | North-West University |
| Detected Language | English |
| Type | Thesis |
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