Experience gathered in the use of evacuated fission chambers in the IBR-2 reactor

Translated from Atomnaya Énergiya, Vol. 69, No. 5, pp. 324–326, November, 1990.     

Model of IBR-2 power feedback dynamics taking account of slow components

A dynamical model of total feedback is constructed and a procedure for determining the parameters of the dynamics of the slow power-feedback components is investigated. Using the quasistationary equation governing the activity balance, the general effect of power feedback, consisting of the sum of fast and slow components of the reactivity change, is identified. The form of the curves of this effect indicates that the model of dynamic power feedback in IBR-2 for slow processes consists of three components: a proportional link and two slow inertial links (fast and slow links with positive and negative transfer coefficient and time constants of several hours and several tens of hours). The parameters of the dynamics of slow action are determined. The reactivity variation curves obtained according to the model are identical to the measured curves. __________ Translated from Atomnaya énergiya, Vol. 104, No. 4, pp. 198–203, April, 2008.     

Influence of automatic regulator parameters on power transition processes of the IBR-2 reactor

Using a model of the IBR-2 reactor based on the block structure with z-transformation of constants and experimentally determined feedback parameters, power transition processes have been evaluated at different values of the parameters of the automatic power regulator (APR). It has been shown that, at regular reactivity excitations, the best transition processes correspond to higher APR rates at the elimination of the APR smoothing unit. The recommendations are given for choosing the APR parameters when at normal operation of the IBR-2 reactor there are random reactivity excitations.     

Radiation experiments with hydrogen-containing materials on the URAM-2 cryogenic irradiation facility at the IBR-2 reactor

The URAM-2 irradiation system facility at the IBR-2 reactor is described. The results of experiments performed with this facility to study the behavior of water ice, solid methane, aromatic hydrocarbons, and other materials exposed to fast neutrons and rays are presented. The rate of energy accumulation in these materials under irradiation and the amount of accumulated hydrogen and the temperature at which the hydrogen leaves the material matrices are determined. Recommendations are made for the working temperature of these materials for use in cold neutron moderators.Translated from Atomnaya Énergiya, Vol. 97, No. 3, pp. 183–189, September, 2004.     

Influence of the nearest environment of the core on the power pulse dynamics in the IBR-2 reactor

An analysis of the IBR-2 reactor power pulse shape measured over the entire dynamic range of neutron flux variation (10⁴), i.e. from the maximum pulse power to the background power between pulses, has been carried out. Three variants of the model describing the reactor dynamics during the power pulse have been investigated. The best approximation to the experimental data has been obtained by adding to six equations describing the effect of delayed neutrons on the power pulse of two analogous ones describing the effect of the neutrons reflected from the structural elements of the reactor. It is shown that the most probable source of additional groups of neutrons may be the neutron moderators enveloping the core as well as the elements of the biological concrete shielding that are the closest to the core. These additional groups of neutrons influence essentially the formation of the power pulse.     

Estimate of the parameters of feedback on power and the stability of IBR-2 for different average power levels

Experimental and model studies of the parameters of fast feedback on power as a function of the average power of IBR-2 have been performed. Transient power processes caused by square fluctuations of reactivity have been investigated. The changes in the parameters are estimated for average power ranging from 0.5 to 1.5 MW. The results obtained are compared with data from previous experiments performed in 1984–1996. It is noted that the influence of feedback on power decreases as the reactor operating time increases. The model of a reactor with parameters of feedback on power which correspond to one series of experiments is investigated for stability by the frequency method. It is shown that at the regular average power level 1.5 MW a reactor in a self-regulating regime (i.e., without an automatic regulator) possesses an adequate margin of stability. __________ Translated from Atomnaya énergiya, Vol. 103, No. 2, pp. 89–93, August, 2007.     

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.     

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.     

IBR-2 dynamics with power shedding

Calculations have shown that in the presence of power shedding the reactivity depends strongly on the moment of shedding relative to the start of the reactor cycle, the stopping time of the reactor after power shedding, and the number of sheddings. It is shown that reactivity effects due to power shedding continue to act for 2 days after shedding, after which these effects can be neglected. The total change of the reactivity after power shedding can range from 0.12 to –0.05%Δk/k. A region of negative reactivity forms in most practical cases after power shedding. The depth of this region can reach –0.05%Δk/k, and the duration varies from several hours to several tens of hours. Knowledge of the reactivity after power shedding makes it possible to determine more accurately the consequences of different emergency situations and to predict the critical state of the reactor when the reactor is once again brought up to power.     

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.     

Solution of the point reactor kinetics equations with temperature feedback by the ITS2 method

The ITS2 method is used to solve the point-reactor kinetics equations in the integral formulation with arbitrary number of delayed neutron groups and Newtonian temperature feedback. The method is based on low-order Taylor series expansions of neutron density and reactivity functions and uses variable time steps to control the numerical instabilities resulting from the stiff nature of the governing equations. Time steps are determined through an analytic criterion relating their magnitudes to the maximum admissible truncation error in the neutron-density expansion series. Temperature feedback is included in the reactivity as a function of the neutron density for different input types, including step change with adiabatic temperature feedback and compensated ramp functions. An iterative procedure is applied to determine the time steps while simultaneously updating the reactivity function. Numerical results show the ITS2 method is highly accurate for solving point reactor dynamics problems with temperature feedback.     

Calculation of the effective delayed-neutron fraction and prompt-neutron lifetime in IBR-2M

The main parameters of IBR-2M are presented: the effective delayed-neutron fraction β_eff and the promptneutron lifetime τ, calculated using the DORT two-dimensional multigroup neutron-transport compute code and the SCALE4 code with a system of multigroup nuclear constants. For a regular IBR-2M regime β_eff = 0.00216 ± 0.00007, τ = (6.5 ± 0.5)·10^–8 sec, the delay-neutron value γ = 0.980, the prompt-neutron decay constant in the critical state α = 3.5·104 sec^–1. The calculations showed that the effective delayed-neutron fraction for IBR-2M is identical, within the error limits, to the measured value for IBR-2, the prompt-neutron lifetime is approximately 5% longer (β_eff = 0.00216, τ = (6.2 ± 0.2)·10^–8 sec). It is shown that β_eff and τ increase somewhat as the IBR-2 core size increases in the radial direction.