Neutron flux distribution

The 211 control rods were moved in and out of the core by winches driven by electric motors. Power and neutron flux distribution were measured by in-core self-powered ion chambers, which were inaccurate at lower power. At low power, ion chambers in the graphite reflector were used. 

The geometrical buckling depends on the neutron flux distribution in the reactor. This distribution in turn dep ds on the g ieral geometry of the assembly, including boundary... 

The neutron flux distribution is the shape function for the reactor that is running on an asymptotic period ... 

Following Robkin and Clark (id), we define the neutron birth-rate density Q(t, E, Q,)dr dE dQ, as the rate at which neutrons in the energy range dE around , moving in the direction dft around ft, are emitted from collisions at dr around r. It is related to the neutron flux distribution as follows ... 

FIG 27. Core configuration andfast neutron flux distribution inMK-III core. 

A second desirable characteristic of the small-core light water—moderated reactor is that the thermal-neutron flux distribution is essentially flat in the core and for a short distance into the beryllium reflector is equal to or greater than the average core flux. The holdup of thermal neutrons in the... 

A4.5 7 ensnrement of Neutron Fluxes. The data presented below have been analyzed and compared with previous calculations. Neutron flux distributions were measured by determining the saturated activity (A ) induced in bare and cadmiUm-covered metal foils. The measurements can be divided into three series of experiments, based on the experimental conditions and the informstion sought in each series. 

Arnette, T., and Jones, H., Thermal Neutron Flux Distribution in a Standard Graphite Pile, CP-2804, April 30, 1945. 

A neutron zero power facility with only SOkg U-235 has been built up in 1970, then moved to the South-West center of Reactor Engineering in Sichuan P rovince. Basic zero power physics experiments have been done at diis facility including critical parameter measurements, fission rates, neutron flux distribution, neutron spectrum, material reactivity etc. in 1988, it was removed to ClAE again, and now it has been rebuilt and will be used for proving of the neutronics experiment medtods which will be served to CEFR first start-up and to primary test for die neutronic and other radiation detectors. It is considered also it will be valuable to the evaluation of some specimen nuclear cross section using its hard spectrum. 

Again, the reaction rate constant (Rf) may be described as the integral of the product of neutron flux distribution and the cross section as a function of neutron energy ... 

The material buckling of the lattice was determined from the thermal-neutron flux distribution measured parallel to the vertical axis of the assembly. A cell of the lattice is shown in cross section in the figure. From this distribution and the extrapolated width of the assembly, a value of the buckling was found to be 10.89 a 0.05/m. ... 

The lattices were assembled in a tank of 1.83-rn diam with fuel rods which were made of 94.9% TD UOj pellets of 12.5-mm diam and a 0.76-nun-tiiick A1 cladding of 14.2-mm o.d. The fuel rods were arrayed in square lattices n X n, where n = 17, IS,..., 3, with lattice pitch of 19.56 mm. The central fuel rod was replaced by an A1 tube of 18-mm o.d., in which a BFj counter was traversed to measure the vertical neutron flux distribution and a Cf neutron source of i.5 mCi was positioned at an elevatioti of 16.8 cm lower than the bottom of fiiel active zone. A pdyethylene rod, 18 cm long, was set between the source and the fUel active zone to shield the direct neutron beam.. The active height of the lattices was fixed at 122.5 cm, which was the critical water level of the n > 17 lattice. ... 

The above relation shows that keff depends on three important quantities neutron flux distribution, nuclear cross sections, and neutron leakage. Neutron flux diftribution... 

The determination of asymptotic concentrations of neutron-absorbing intermediate fission products whose presence have a strong effect on subsequent fc-effective and neutron flux distributions... 

The analysis of quickly varying time dependence of fc-effective and neutron flux distribution, which gives us the needed information to control reactor total power and power spatial distribution changes with time... 

Calculation of the fission power distribution in Step 1 involves the handling of a tremendous amount of geometric details of the reactor because the reactor might contain tens of thousands of fuel elements, control elements, instrument ports, coolant flow channels, structural components, etc. In addition to the spatial detail, the designer needs to know the neutron flux distributions in energy and direction as well. This is an overwhelming amount of detail. 

Typical fast and thermal neutron flux distribution in the FUJI-233Um core is shown in Fig.XXX-2... 

FIG. XXX-2. Neutron flux distribution in FUJI-233Um core. 

Since the calculation of f first requires a calculation of the neutron flux distribution across the lattice cell (dis vantage factor) and then a conqputation of the relative absorptions in each cell region (SeeEquation 2.5.4.1) and both these parts of the calculations depend on the cross sections the functional dependence of f on the water teoperature ultimately reduces to just one of cross section dependence which in turn depend on the coolant temperature and density ... 

It will be recalled that the diffusion approximation is based upon the assumption that the neutron-flux distribution is nearly isotropic (see Secs. 5.1b and 5.1d). This assumption is equivalent, analytically, to the requirement that the series expansion for the flux (7.33) be well repre-.sented by the first two terms. Consider, therefore, the set of Eqs. (7.46) for values of a = 0,1. This means that in the series expansion (7.33) we retain only the terms involving , and , all coefficients of... 

The TWINKLE code is used to predict the kinetic behaviour of a reactor for transients that cause a major perturbation in the spatial neutron flux distribution. TWINKLE was used in the analysis performed in support of the Sizewell B PCSR (Reference 5.7). There is therefore a high degree of confidence that an acceptable verification statement can be made in the context of the UK regulatory regime. 

Provision has been made in the Winfrlth reactor for the insertion of flux measuring wires along the axes of the fuel elements. Wires may be loaded or withdrawn with the reactor on power. Copper wires were used during the zero energy experiments to check the radial and axial neutron flux distribution. 

Precision in Neutron Flux Distribution Deviations in radial flxox distribution will affect equally all of the temperature differences as given by Eq. (l) through (5). Since the coolant temperature rise depends on the integrated axial flux distribution, the effect of deviations in axial distribution will be less for the coolant rise than for the temperature drops through the fuel element. 

Figure 1 The neutron flux distribution for three different moderators at the ILL reactor and for the liquid hydrogen moderator at the ISIS accelerator. The accelerator flux distribution is adjusted by a fector 10 to represent the increased efficiency for time-of-flight experiments due to the pulse structure. Modified with permission from Price DL and Skdid K (1986) Introduction to neutron scattering in Neutron Scattering, Part A, Skdid K and Price DL (eds). Orlando Academic Press.

By 1 (X) am, on April 26, the operators were able to stabilize the power back at 2(K) MWt, but this was as high as they could get it due to the xenon poison buildup that had started during the excursion to lower power and was still continuing. To drag the reactor up to 200 MWt, the operators had pulled far too many of the manual control rods out of the reactor, and the neutron flux distribution in the core was such that the reactivity worth of those rods that would be effective in the first few centimeters of travel back into the core was limited to the equivalent of six to eight fully inserted rods. 

At 1 22 30 a.m., the operators obtained a printout from the fast reactivity evaluation program, giving them the position of all the rods and showing that the operating reactivity margin had fallen to a level that required immediate shutdown of the reactor. But they delayed long enough to start the test. There was clearly a failure to appreciate the basic reactor physics of the system, which had rendered the control rods relatively worthless. The neutron flux distribution in the core had been pulled into such a distorted shape that the majority of the rods would have go to well into the core before they would encounter sufficient neutron flux for their absorption to be effective. 

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