Abstract
Highly localized in-core measurements are necessary for the validation of neutron transport calculations with high spatial resolution. In the present work, a miniature neutron detector developed at EPFL in collaboration with PSI was used to carry out a set of thermal neutrons counting measurements in the zero-power CROCUS reactor core within a spatial range in order of mm. The miniature detector, positioned close to the core reflector, shows a gradient of +(4.29 ± 0.10)% in the count rate profile in the radial direction within 1.3 cm, with higher values pointing towards the core reflector because of the higher share of neutrons in the thermal range. On the contrary, in a control rod guide tube the count rate gradient is -(4.37 ± 0.10)% and it is directed towards the core center. The measured values are compared with the azimuthal trend of the normalized 6Li reaction rate calculated with an iterative three-steps method performed with the Monte Carlo code Serpent 2. These measurements proved the feasibility of resolving spatial effects in the mm-range and they represent a basis for further investigating highly spatially-resolved phenomena in the CROCUS core.
Highlights
The increasing complexity of nuclear reactor concepts has been driven by the continuous endeavor to improve fuel performances and utilization in view of cost optimization by nuclear power plant operators
Radial and azimuthal fission cross sections have been measured within a fuel pin of a Westinghouse SVEA-96 Optima2 boiling water reactors (BWRs) assembly installed in the zero-power reactor PROTEUS [3] at Paul Scherrer Institut (PSI)
In the control rod (CR) position, a gradient of -(4.37 ± 0.10)% is present between the NW and the SE detector positions, meaning that the higher neutron count rate is pointing towards the core center
Summary
The increasing complexity of nuclear reactor concepts has been driven by the continuous endeavor to improve fuel performances and utilization in view of cost optimization by nuclear power plant operators. Modern light water reactors (LWRs) core designs present heterogeneities at multiple levels: from mixed core arrangements [1] to heterogeneous boiling water reactors (BWRs) fuel assemblies with one or more water channels and partial length fuel rods (e.g. the latest Westinghouse TRITON11 fuel assemblies [2]) The presence of such heterogeneities requires the precise knowledge of the spatial distribution of the neutron population in the core for both safety reasons and performances optimization. Despite the success of this experiment, the employed neutron activation technique represents a limit whether the localized phenomena want to be observed in an online manner during dynamic neutronics events [4],[5] Detection techniques with both a spatial resolution in the order of the millimeter and a good efficiency are a paramount to investigate local static and dynamic effects in zero-power reactors. A good insensitivity to the gamma radiation field present in the core and a high flexibility are desirable characteristics for such measuring devices
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