The large negative reactivity is measured in Semi·Homogeneous Experimental facility (SHE). Experimental methods are Sjostrand's pulsed neutron, source multiplication and rod drop methods beside revised King-Simmons' pulsed neutron methods. Neutron detectors are placed at various points in the core region for multi-points measurement.Usual one-point reactor model analysis resulted in the reactivity values, strongly dependent on the detector position with the increase of subcriticality. In addition, disagreements between the used experimental methods are also pointed out.In order to overcome these difficulties due to the spatial higher harmonics and the kinetic distortion in the neutron flux distribution, an integral version analysis is applied, in which use is made of multi-points reactor model. In the analysis, space integration of the neutron counts obtained throughout the core region is made with weights of the adjoint function of fast neutrons, calculated using the two-or three-dimensional diffusion code. The negative reactivity values determined by the integral version analysis agreed well with each other within the uncertainty of -5% in the reactivity range down to -50 dollars.It is concluded that all the experimental methods are adequate for precise determination of the large negative reactivity of reactor provided that the integral version analysis is utilized or that correction is made for the change of the neutron generation time using precise calculation.
Measurements of the neutron slowing down times in light water, ice, paraffin and santowax have been made by pulsed neutron technique. Bursts of D·T neutrons of 0.1 psec width were generated in moderator cubes of 40 X 40 X 40 cm 3 • The slowing down neutrons were detected by bare as well as energy-selective filter-covered BF 3 counters, and analyzed with a 256-channel time analyzer. Slowing down times in the moderator were determined by interpreting the increment of the difference of events between the two counters as attributable to the fraction of the neutrons slowing down at the time of measurement below the cut-off energy of the filter.The measured slowing down times below 0.63 and 0.43 eV agreed well with theoretical values on the 0°K free gas model. On the other hand, the measured values below 0.20 e V were found to be appreciably greater than the theoretical, which would appear to indicate that he effect of thermal agitation and chemical binding come into play at this range of energy.
The overall temperature coefficient of reactivity of VHTRC-1 core was measured at the Very High Temperature Reactor Critical Assembly (VHTRC) to examine the calculation-accuracy of temperature dependent neutronic characteristics of the High Temperature Engineering Test Reactor (HTTR). The core of VHTRC consists of pin-in-block type fuel using coated particles of low-enriched U02. The whole assembly was heated by electric heaters and the reactivity change due to the temperature rise up to 2oo•c was measured by the pulsed neutron method to determine the overall temperature coefficient of reactivity in isothermal condition. The experimental results indicated that the temperature coefficient was-1.71 x 1Q-4 .dk/k/"C on the average in the measured range from 25 to 2oo•c and the absolute value was smaller by 20%' near room temperature than at the higher temperature. The calculation by the SRAC code system using the ENDF /B-IV nuclear data well predicted the experimental results.
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