Neutronic parameters

The reference (initial) core state needs to be established for the analysis of accidents, especially reactivity initiated accidents. 

The actual core characteristics of the reactor under consideration should be taken into account together with the maximum permissible deviations of the parameters, first of all characterizing neutronic field non-uniformity. 

The reference core state is established with regard to  

All the reactivity effects are calculated for the reference state of the core. 

The time lag that exists between the generation of appropriate signals and the start of actions must be specified. The analysis involves conservative time lag values, i.e. maximum time intervals either given in the system specifications or based on specific tests. 

---

In support of this conclusion, Hanson et al. (1973) found, for a large data set on sucrose (with sin 0/k < 0.81 A-1), a mean difference between the phases ipx from a spherical-atom refinement and spherical-atom calculation with neutron parameters of 1.8°. A better approximation to the true phases is... 

The X-N technique is sensitive to systematic errors in either data set. As discussed in chapter 4, thermal parameters from X-ray and neutron diffraction frequently differ by more than can be accounted for by inadequacies in the X-ray scattering model. In particular, in room-temperature studies of molecular crystals, differences in thermal diffuse scattering can lead to artificial discrepancies between the X-ray and neutron temperature parameters. Since the neutron parameters tend to be systematically lower, lack of correction for the effect leads to sharper atoms being subtracted, and therefore to larger holes at the atoms, but increases in peak height elsewhere in the X-N deformation maps (Scheringer et al. 1978). 

Table 21.1 Delayed neutron parameters for thermal fission...

Values of delayed neutron parameters and the problem of stiffness... 

Table 21.1 gives values of delayed neutron parameters, Pi, ki, for thermal fission, while Table 21.2 gives... 

These initial results show that there is not a relative error in the assumed value for Pett ot to cause the errors noted in reactivity. The worth of the drop in is almost independent of the choice of delayed-neutron parameters for results near the drop. For the ZPR-3 Assembly 56, the result is within 1% of 132 for any reasonable constants (dbS0% in assumed fission rate). This value can be used as a standard for other relative measurements so that only a small uncertainty is inherent in reactivity measurements stressed in cents. This would also be true if the isotcqiic values for S s and X s are all allowed to vary with reasonable limits. 

The one-velocity neutron parameters Za and D are constant over an infinite medium. Outside a spherical region of radius Rj the absorption cross section is made up solely of radiative-capture cross section inside the spherical region, the absorption cross section has the same value as outside but is made up of fission cross section as well as radiative capture. For every neutron which is absorbed inside the spherical region, vZf/ La neutrons are born from fission there are no other sources of neutrons. 

The reactor to be examined consists of a multiplying core surrounded by a single nonmultiplying reflector. It is assumed that this system is critical with the cross sections and neutron parameters listed below ... 

The RA-8 critical facility has been designed and constmcted to measure neutronic parameters typical of the CAREM core. It provides a reactor shielding block and a reactor tank that can be adapted to contain custom designed reactor cores. Experiments were performed using fuel rods of the same radial geometry and pitch as in the CAREM-25 fuel element. Components of the neutronic calculation lines were validated with the use of data for VVER type reactors obtained in the experiments at ZR-6 Research Reactor (Central Research Institute for Physics, Academy of Sciences, Hungary) and data for PWR critical experiments. 

Parish-TA Charlton-WS Shinohara-N Andoh-M Brady-MC Raman-S,"Status of Six-Group Delayed Neutron Data and Relationships Between Delayed Neutron Parameters From the Macroscopic and Microscopic Approaches", Nuclear Science and Engineering vl32 Number 2 2 Feb 1999, pp 208-221... 

The current situation on fast reactor calculations is that, while a combination of basic differential cross section data, adjusted by correlation methods, with two-dimensional transport or three-dimensional diffusion codes, can give reasonable agreement for quantities such as critical mass or relative reaction rates in central regions of an FBR core, more precise data are still required for accurate prediction of differential effects such as the Doppler and sodium void coefficients. These two effects, together with the influence of the delayed neutron parameters for a fast reactor, are most conveniently dealt with in the following section, which considers the dynamic behavior of the system. 

Core-averaged and peak temperatures and neutronic parameters for cooling at the... 

The final temperature fields were computed with a multiregion, multigroup coupled neutronic/ thermohydraulic simulator developed for this purpose. This simulator takes as input temperature-dependent multigroup neutronic parameters—collapsed from detailed computations by specialized computer codes— and temperature-dependent thermal conductivities for the core and reflector materials. The simulator implements in a 2-D geometry an iterative inner-outer difference-scheme to solve the multigroup multiregion diffusion equation coupled to the heat transfer equations. 

NEUTRONIC PARAMETERS AND THEIR TEMPERATURE DEPENDENCE FOR EACH OF THE FOUR MATERIAL REGIONS... 

The Argonaut is introduced to students by tour and lecture. Next, the students perform a complete reactor checkout and the approach-to-critical experiment using a control rod. This is followed by a brief study of some reactor kinetics and an experimental determination of some delayed-neutron parameters. 

D. Delayed Neutrons Make the reactor critical at approximately 100 W. When you are satisfied that all transients have died out, scram the reactor. Make a similog plot of the resulting flux as a function of time until the flux as indicated by the lowest range on Linear is within about 50% of chart zero. Then analyze the resultant curve to determine the delayed-neutron parameters, i.e., periods and approximate relative abundances. Only the three longest periods can be resolved. 

Knowledge of the thermal-neutron lifetime and the delayed-neutron parameters of the reactor in question will enable a plot of reactivity versus stable period to be made. This plot for the AGN-201 reactor is made in... 

The reactivity changes that occur in the reactor when an absorber is placed at its center will be measured in terms of asymptotic positive periods. The stable period of a supercritical reactor is related to the reactivity through the basic inhour equation. This expression involves the prompt neutron lifetime and the delayed neutron parameters (see Appendix B). 

Through use of a "rabbit" facility at the Argonaut reactor, the delayed-neutron parameters of U are measured, A small sample of is irradiated to equilibrium in a neutron flux and subsequently rapidly removed to a shielded counting pig. By means of a neutron counter and a time analyzer, the neutron emission versus time is determined. From these data, at least four of the five so-called groups can be resolved and characterized in terms of their half-lives and relative abundances. 

This experiment is a measurement of the thermal-fission delayed-neutron parameters of U . The parameters are determined from an analysis of the neutron counts versus time observed when a U sample is irradiated in the rabbit facility and ejected to a remote neutroncounting system. The pre-experiment briefing session will include a general discussion of delayed neutrons, the rabbit facility design, and the theory and procedure of the measurement. 

The measured delayed-neutron parameters are dependent on the neutron energy spectrum of the irradiation flux. 

D. A. Daavettila, H. Muller, and L. S. Zingoni, Design of the Argonaut Rabbit Facility and Its Use to Measure the Delayed Neutron Parameters of U s, Argonaut Internal Memo (Sept 1963). 

The delayed-neutron parameters for U thermal fissions were measured with a 1.2-g sample at 93 percent enriched uranium metal foil. The reactor loading was a 6-fuel box, north-quadrant core, containing 1,93 kg U . The reactor power level was 50 W, and the rabbit stringer was in the J-8 position. 

Since a nuclear reactor is a statistical system, it will show fluctuations in neutron intensity. These fluctuations, or pile noise, are not commonly considered of interest in themselves, but only as interference to other experiments. However, since the nature of the pile noise depends strongly on important reactor parameters, its study can enable the determination of quantities less easily accessible by other means. In particular, Moore (f) points out that the noise spectrum of such a system, that is, the mean square noise amplitude per unit band width, is proportional to the square modulus of the transfer function or to the Fourier cosine transform of the autocorrelation function. Thus, observation of the noise spectrum of a reactor could yield information about the shape of its transfer function. To test this technique, pile noise analyses were done on various low-power experimental reactors at Argonne National J aboratory. Since these reactors operate at such a low level that power effects on reactivity do not appear, the shape of the low-frequency portions of their transfer functions would depend only on fairly well-known delayed neutron parameters, and thus would be of little interest. However, the high-frequency rolloff portion of the transfer function is strongly dependent on the quotient of the effective delayed neutron fraction over the prompt neutron... 

---