Rossi-α measurement of the effective delayed-neutron fraction

Main Scientific Center of the Russian Federation — Physics and Power Engineering Institute. Translated from Atomnaya énergiya, Vol. 78, No. 3, pp. 152–155, March, 1995.     

Method of experimental estimation of the effective delayed neutron fraction and of the neutron generation lifetime in the IBR-2 pulsed reactor

Experimental investigations of random fluctuations in the power of the IBR-2 periodic pulsed reactor carried out in the steady-state mode of its operation and in the pulsed mode made it possible to obtain estimations of the main parameters of the kinetics – the effective fraction of delayed neutrons and the average lifetime of prompt neutrons. Functionals were measured relating the main parameters of the kinetic behaviour: the prompt neutron decay constant (steady-state mode) and the relative dispersion of pulse energy fluctuations. It was shown that the experimental values of parameters of the kinetic behaviour are close to the calculated ones. In addition, the power of the spontaneous neutron source was estimated on the basis of the analysis of neutron noise.     

Measurement of the average neutron generation lifetime in IBR-2

The purpose of this work was to measure the lifetime of a neutron generation in an IBR-2 core as a function of its state and the core environment. The main problem was to study the possibility of decreasing the duration of a neutron pulse. Different measurement methods were used. The main one was the α-Rossi method. For comparing with experiment, the lifetime was estimated by a computational method. It was shown that the results obtained by all measuring methods used agree with one another. For the standard state of a reactor, the lifetime of neutrons in the IBR-2 core is 62 ± 2 nsec. The contribution of individual elements of the core and its surroundings to the total lifetime of a neutron generation is presented. It is noted that in experiments with part of the radiation shielding moved away there are discrepancies in the estimate of the effective fraction of the delayed neutrons of a factor of 1.5 as compared with the standard state of the reactor. No explanation has been found for such a discrepancy. __________ Translated from Atomnaya énergiya, Vol. 103, No. 3, pp. 166–172, September, 2007.     

Calculation of the effective delayed neutron fraction and the prompt neutron lifetime with the indigenously modified CEBIS code: MCEBIS

The CEBIS code has been modified to enable the calcination of both the effective delayed neutron fraction and prompt neutron generation lifetime in any nuclear thermal reactor, especially reactor types such as TRIGA, SLOWPOKE, and MNSR. The new version, called MCEBIS, includes sonie special subroutines which will be called up as part of the input to calculate the above two dynamic parameters. In addition, some control flags have been added to recognize any important reactor components such as beryllium as a reflector or heavy water as moderator and pence calculate their photo-neutron fractions.

The MCEBIS code has been tested using two reactor models: TRIGA and MNSR. These models were developed mainly to verify the modified code. Each model represents a 1-D neutronics model of the reactor. Calculated results for the effective delayed neutron fraction and prompt neutron generation lifetime in both reactors have been compared with published data. Good agreement with published results has been established.     

Measurement of the effective delayed-neutron dose by the α-Rossi method in water-containing media

A modified method of determining the effective delayed-neutron dose is proposed. The method is based on a combination of determining the pseudoreactivity of the ²⁵²Cf source and measuring the α-Rossi time distribution. The advantage of the method proposed here is that there is no need to measure the absolute rate of fissioning of the nuclei in the medium. It is proposed that the latter be expressed in terms of the intensity of a known source of spontaneous fissions of ²⁵²Cf nuclei. The measurements are performed on an assembly with a hydrogen-containing moderator. The influence of the positions of the detectors (³He counters) on the measured effective delayed-neutron fraction is studied. __________ Translated from Atomnaya énergiya, Vol. 100, No. 5, pp. 393–399, May, 2006.     

Calculation of effective delayed neutron fraction with Monte Carlo perturbation techniques

New methods are proposed to estimate the effective delayed neutron fraction β_eff in Monte Carlo calculations: the eigenvalue methods jointly used with the differential operator sampling and correlated sampling techniques. In particular, the eigenvalue method with the differential operator sampling technique has a distinct feature that it theoretically gives an exact β_eff value. To verify the proposed methods, Monte Carlo calculations are performed for several systems with simple geometry. It is found that the results obtained with the proposed methods agree with the reference deterministic results within sufficiently small statistical uncertainties. The indirect perturbed source effect must be taken into account to estimate an exact β_eff value.     

Pulse characteristic of the feedback in the IBR-2 reactor

Joint Institute of Nuclear Research. Translated from Atomnaya Énergiya, Vol. 70, No. 5, pp. 326–329, May, 1991.     

Safe external reactivity perturbation limits in IBR-2

For the IBR-2 pulsed reactor safe reactivity perturbation limits have been determined by mathematical simulation of reactor dynamics in the plane “input reactivity level–input speed”. From power transients calculated using a reactor model with experimental power feedback parameters safe reactivity perturbation limits corresponding to different years of reactor operation have been obtained.