Effective delayed neutron fraction

The goal of this programme was to reduce uncertaincies as to the effective delayed neutron fraction, peff, from 10 % ((2 o) to 5 % (2 a) in order to obtain better prediction of the reactivity scale. 

Table n gives a comparison of the measured and calculated temperature coefficients and eigenvalues between 74 and 464 F. The temperature defect between 74° and 464 F was estimated from integrated rod worths to be about 3%. Rod worths and temperature coefficients were b d on a calci ted effective delayed neutron fraction fi of 0.0074 (i.e., p/p = 1.16). In addition to having a small absolute bias (all calculated eigenvalues are within + 0.5 and - 0.9% of measured values), the model describes the temperature effects quite accurately. 

G. A.. Price,"Migration Areas and Effective Delayed Neutron Fractions by Critical Exq[>eriments, J. Nuclear Energy, fO. Ill (1959). 

Core Effective delayed neutron fraction, Plqrsical coresize (in.) Gd septum (w/o) Measured keff Variational treatment keff Normalized iterative treatment keff... 

The ratio of the effective delayed neutron fraction (3eff) to the effective prompt-neutron generation time (f ) was measured for several unpoisoned configurations, employing both pulsed neutron and nOise analysis methods. The two methods were in satisfactory agreement, giving a best value of 4.76 x 10 sec". The correspon ng value for the beryllium-reflected reactor has been measured as 1.38 X lO sec". The much greater f for the water-reflected assembly is attributable to reflector delayed neutrons. 

A si rles of experiments with unreflected and unmoderated cylinders of enriched-uranium metal (93.15% U-23S) has been performed at the ORNL Critical Experiments Facility to determine the dependence of the prompt-neutron lifcitime on the cylinder dimensions. Five cylinders ranging in diameter from 17.77 to 38.09 cm and in height-to-diameter ratio from about 0.2 to 0.7 were assembled and their prompt-neutron decay crnistants measured at delayed critical by the Rossi-a technique. Prompt-neu-tnm lifetimes were obtained from the measured decay constants and effective delayed-neutron fractions calculated by transport theory. The average uranium density for each assembly was greater tium 18.7 g/cm. . 

A computational study has been carried out on several plutonium-fueled and mixed-lueled critical assemblies of small to medium size to verify the plutonium cross sec-tlons of the twenty-six groiq> set recently produced at Argonne National Laboratory. Plutonium worths in some larger uranium-fueled reactors have been computed to ascertain the effect of softer spectra on the comparisons. Other reactor parameters included in the study were detector-response ratios and U-235,U-238 and B-10 central clanger coefficients, as well as effective delayed-neutron fraction and prompt-neutron lifetime. 

Central reactivity worths were calculated in spherical. geometry by void replacement. Effective delayed-neutron fractions were determined both by subtraction of delayed-group spectra from the fission-source distribution and by the standard diffusion-theory perturbation method. Prompt-neutron lifetimes were calculated by 1/v poison and by perturbation methods. 

T. MIHALCZO, The Effective Delayed Neutron Fraction from Firakm in an Unreflected Ur um Sphere from tirne Correlation Measurements with Californiuro-252, iVrici Set ng., 60.262 (1976). 

After the plant is brought to the nominal parameters, the reactor changes over to the power self-control mode when the compensation group absorber elements are all located in the upper part of the core they create an operating reactivity margin within the limits of one effective delayed neutron fraction (Peff), which does not threaten reactor safety under an erroneous or even malevolent personnel action. 

The delayed neutron fraction of U is lower than that of U and half of the delayed neutrons are generated outside the core therefore, the effective delayed neutron fraction in the FUJI-233Um is relatively small. However, safe control of the reactor is possible because of a large negative reactivity coefficient on fuel salt temperature and small overall reactivity margin ... 

In the design basis reactivity insertion accident (RIA) [XXX-25], the maximum reactivity insertion in the MSR corresponds to the drop of one graphite control rod into the core. Since the worth of a single graphite rod is only 0.06 %5K/K and less than one effective delayed neutron fraction, such initiating event does not result in any prompt criticality of the FUJI. 

When an amount of positive reactivity equal in magnitude to the average effective delayed neutron fraction is inserted into a critical reactor, the reactor is ... 

The delayed neutron fraction, p, is the fraction of delayed neutrons in the core at birth, that is, at high or fast energies. The effective delayed neutron fraction, is the... 

Define the effective delayed neutron fraction, Peff. 

The effective delayed neutron fraction, 0gff, is used in equation (3.10) because it is this relative number of delayed neutrons actually reaching thermal energies and causing fission that measures the impact on reactor control. Since the total fraction of core neutrons is one, the denominator of equation (3.10) is one. Also, since is small, the numerator... 

Measurements of subcriticality relative to the effective delayed neutron fraction can be made by calibrating a reference fine control rod by means of asymptotic period measurements following rod withdrawal, or by inverse kinetics analysis of the reactor power response following rod drop or rod withdrawal (fitting the response using the delayed neutron kinetics equations).There are imcertainties in total delayed neutron yields and in the time dependence of delayed neutron emission, the accuracy of this reactivity scale being estimated to be 5%. 

The effective delayed neutron fraction, p, is not only smaller for a plutonium-fueled fast reactor than for a uranium-fueled thermal system iP = 0.(X)21 for Pu, p = 0.0065 for but is also more difficult to predict accurately because of the relatively large number of isotopes (including fertile species such as and Pu " ) which may be making a significant contribution to the delayed neutron population. Taking into account the effect of the fast fission in which has a delayed neutron fraction of... 

The effective delayed neutron fraction ( Seff) represents an important reactor kinetics parameter. In circulating-fuel systems, because of the delayed neutron precursor drift, the Seff calculation requires special techniques. The coupled neutronics/CFD simulations represent a necessary step for the accurate calculation of the effective delayed neutron fraction in the MSFR (Aufiero et al., 2014). Fig. 7.3 shows the distributions of the prompt (right) and delayed (left) neutron sources obtained in OpenFOAM and adopted to calculate fieti in the nominal MSFR conditions. 

Aufiero, M., Brovchenko, M., Cammi, A., Clifford, I., Geoffroy, O., Heuer, D., Laureau, A., Losa, M., Luzzi, L., Merle-Lucotte, E., Ricotti, M.E., Rouch, H., 2014. Calculating the effective delayed neutron fraction in the Molten Salt Fast Reactor anal3dical, deterministic and Monte Carlo approaches. Annals of Nuclear Energy 65, 78—90. 

Solid lines are fitted for all data to the exact VTMR equation including all delayed neutron terms, and for all variables related with VTMRs -counting efficiency (counts/fission), reactivity, Pu-239 fission portion (assuming fissions are occured only by U-235 and Pu-239), neutron generation time and effective delayed neutron fraction. 

Operation of LAPRE-1. Final tests on the LAPRE-1 system were made with a O.ol M UO3 in 7.25 M H3PO4 fuel solution. Data were obtained at room temperature in terms of control-rod position at delayed critical versus volume of fuel injected into the system, and results were interpreted in terms of a simplified calculational model to obtain control rod worths. For the five control rods, four located on a 3iVin. radius and one central rod, measurements yielded a total worth of 6.3%. The latter re,suits were in good agreement with period measurements at cold critical. Also inferred from the data was an effective delayed neutron fraction of 0.0091. 

Here jS is the effective delayed neutron fraction. The delayed neutron data for the Super LWR are given in Table 5.10. 

This experiment is designed to determine the reactivity worth of the control rods by a pulsed neutron technique. A burst of neutrons is injected into the reactor, and the decay rate of the resultant neutron flux is measured. The decay rate measured is that of the prompt fission neutrons and is proportional to the prompt critical reactivity of the reactor. Measurements will be made with the reactor in subcritical conditions and at delayed critical. The decay rate at delayed critical yields the constant of proportionality between the decay rate of the neutron flux and the reactivity in dollars. This constant is equal to the ratio of the effective delayed-neutron fraction to the prompt-neutron lifetime. 

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... 

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