Life-extension in a research reactor: calculation of the neutron-induced ageing of the core-box, grid-plate and beryllium reflector

Abstract

The SAFARl-1 research reactor has been operating at NECSA- the South African Nuclear Energy Corporation - since 1965. SAFARI-1 is a tank-in-pool type research reactor of similar design to the original Oak Ridge Materials Testing Reactor (MTR). It is a light-water-moderated and cooled, beryllium and water reflected research reactor designed and built as a general research tool, falling in the class of research reactors commonly known as MTR. The reactor currently uses low enrichment uranium (LEU) plate-type fuel. Until the early 1990s, SAFARI-1 was used as a research reactor, but since 1994, it is mainly used as a Dedicated Isotope Production Reactor(DIPR) for the production of medical radio-isotopes. The SAFARl-1 reactor was originally envisaged to have an operational life of 40 years, but this has been extended to circa 60 - 65 years, in line with ageing management programmes at similar reactors worldwide. An Al-2014-T6 core grid-plate carries the entire weight of the content of the reactor core, which is bounded on its 4 sides by an Al-5052 core-box. The core-box and core grid-plate are manufactured from Al alloys. Three sides of the core contain beryllium neutron reflectors. As a reactor ages, the above three metal structures will age as well, by several ageing mechanisms. Firstly, neutron absorption by nuclei in irradiated structures will cause a class of nuclear reactions that will "transmute" one chemical element to another element, e.g. the element Al will be depleted and transmuted to Si. An alloy that initially contained e.g. 0.23% Si can, after 60 years of neutron irradiation, contain more than 5% Si. In metal alloys, such transmutations will cause the alloys to "go out of specification" which will typically increase hardness and tensile strength but decrease ductility, i.e. the metal will become more brittle. Secondly, fast neutrons will cause atomic displacements, which will also gradually render a metal alloy harder but more brittle, i.e. less ductile. In the case of the Be reflector, He gas will be produced by nuclear reactions, and cause the Be reflector elements to swell, complicating maintenance work. Furthermore, three significant neutron poisons are formed when Be is irradiated − ⁶Li, ³He and ¹ºB. Poisoned Be-reflector elements will reflect less neutrons, necessitate "burning" more 235U to produce the same quantity of commercial radio-nuclides, i.e. it will have a negative impact on the economy of reactor operation. The aim of this study is to answer questions such as: (1) At which rate will the mass-percentage of elements in irradiated alloys change, and how will this impact mechanical properties? (2) How will the build-up of neutron poisons in the Be reflector impact neutron economy? (3) Will it be necessary to replace all of or some of the SAFARI-I core-box, the core grid plate and Be reflector elements? MCNP transport code and material activation and isotopic evolution code FISPACT II were used to ascertain the nature and magnitude of the radio-activation of the core box, the grid plate and beryllium reflectors. The results obtained using these tools indicates neutron irradiation changed both 20I4-T6 Al-alloy and 5052-0 to other forms of Al-alloys. Beryllium reflectors must be rotated in its position and repositioned to avoid uneven swelling and bowing of these elements caused by neutron flux on one side of the element that is more exposed than others. Although the change occurred, the study concludes that it is safe to continue operating the reactor beyond 2030 as prolonged neutron irradiation did not compromise the integrity and safe operation of the reactor. It is recommended that grid plate and core box be replaced with 6082 Al-alloy and 606I-T6 respectively towards 2040 or alternatively, the reactor can be decommissioned at circa 2040.