At present VNIIEF is developing a nuclear physics facility—a physical model of a stationary reactor-laser (RL) with transverse flowing of the laser medium [16—19]. The facility includes an IKAR-500 pulsed reactor and LM-16 laser module.

The IKAR-500 core is a graphite matrix (a cube with 2,400-mm sides) with nine reach-through holes that are 500 x 500 mm in cross section, which accommodate the reactor modules. In the graphite matrix (between the modules) at the top and on the sides, zirconium channels are mounted to accommodate the reactor control systems. An LM-16 laser module can be placed in one of the reach-through holes instead of a reactor module. The planned energy release in the reactor core for a startup with a duration from fractions of a second to tens of seconds is 500 MJ; the expected energy of laser radiation is >20 kJ.

Calculations of the nuclear physics characteristics of the IKAR-500 type facil­ities are a very difficult problem, because the reactor core has significant dimen­sions, high porosity due to the presence of a large number of laser channels, and a clearly pronounced anisotropy of the leakage neutron field, which is not typical for traditional reactor designs. The calculations of reactor core characteristics of such facilities must therefore be confirmed by direct physical experiments. In this regard,

VNIIEF has developed the IKAR-S critical stand (Figs. 6.8 and 6.9) for experi­mental investigation of the nuclear physics characteristics of the IKAR-500 reactor [18,19]. The basic difference between the IKAR-S critical stand and the IKAR-500 reactor is the absence of full-fledged control elements.

Absorbing rods made of boron carbide, B4C, are used as control elements for reactivity adjustment on the critical stand. Each of the control elements includes two absorbing rods. The reactivity control elements are functionally divided into four groups: two groups of reactivity control rods and two groups of emergency shutdown rods. The KNK-4 and KNK-15 chambers are used to register neutrons. Two plutonium-beryllium neutron sources are included as standard in the design of the critical stand.

Each reactor module consists of two fuel sections, which constitute a set of alternating layers of graphite and dispersed uranium-aluminum fuel elements (72 fuel elements in each section). A fuel element is a 5 x 60 x 900 mm uranium — aluminum plate containing 2.5 % uranium (90 % enriched in 235U), confined in a vacuum-sealed case made of Zr-Nb alloy with walls 0.5-mm thick.

Recently performed experimental and computational investigations [19—21] of the nuclear-physical characteristics of the IKAR-S stand made it possible to determine the effective neutron multiplication factor at various stages of core assembling, and showed the possibility of forecasting the critical mass parameters of the RL.

It was initially assumed that the IKAR-S would consist of sections in accordance with version 1 (Fig. 6.10a), which most fully models the IKAR-500 reactor core,

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Fig. 6.9 Structural diagram of IKAR-S critical stand [19]: (1) reactor module of two fuel sections or LM-16 laser module; (2) graphite assembly; (3) neutron detector channels; (4) neutron source mechanisms; (5) control rods; (6) emergency shield rods; (7) horizontal channels; (8, 9) shielding gates reproducing the structure of the laser modules. The results of preliminary estimates using Nuclear Data Library (BAS) neutron constants for version 1 yielded an effective neutron multiplication factor kef = 1.076. The actually obtained value of kef = 0.906.

To bring IKAR-S to a critical state, the decision was made to increase the content of graphite in the sections while keeping the same number of fuel elements, which was supposed to lead to a shift of the neutron spectrum to the thermal range and a reduction in the core porosity, and accordingly, to a growth in kef. The corresponding calculations of kef were made using ENDF-B5 neutron constants, which were normalized to the experimental results obtained when IKAR-S was assembled per version 1. A diagram of the reactor sections, with a 46 % increase in the graphite content (version 2), is shown in Fig. 6.10b. In this case kef = 0.988 was obtained.

A further increase in reactivity was obtained when sections of the core’s lower row were modified in accordance with Fig. 6.10c (version 3) by raising the fuel elements to the upper part of the section (closer to the center of the reactor core). According to calculations, the expected increase in kef was 0.012, while the value obtained experimentally was 0.004; that is, as a result of reconfiguration of sections in the module of the lower row, kef = 0.992 was obtained.

Some discrepancy of calculated and experimental results may be explained by inadequate allowance for the specific features of the reactor core in the computa­tional model. In particular, when different versions of the reactor core were calculated, the significant influence of the quantity of aluminum (alloys) in the sections on the reactivity was noted; an increase in the quantity of aluminum (all other conditions remaining equal) led to a reduction in reactivity.

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Fig. 6.10 Diagram of reactor section: (a) per version 1; (b) per version 2; (c) per version 3; (d) per version 4 of the studies and was examined in further experiments

In order to study this influence and achieve a critical state of the reactor core, the sections of version 2 of the central module were replaced with sections of version 4 (Fig. 6.10d), in which the aluminum details simulating radiators for cooling of the gas mixture were removed. The quantity of fuel elements and zirconium in the sections was not changed, while the quantity of graphite was increased by roughly 2 %. A value of kef = 0.998 was obtained.

After removal of the central source mechanism, a massive component of alumi­num alloy, from the reactor core, the system achieved a critical state; moreover, the reactivity margin of the reactor core was 0.62^eff. To reduce the reactivity margin of the reactor core, a mockup of the temperature sensor was additionally (in mirror- image symmetry) placed in the horizontal channel of the core where the sensor is placed. As a result, the margin reactivity amounted to 0.35^eff. The reactivity was determined from the reactor acceleration period, which was 14 s. Figure 6.11 shows the configuration of the reactor core that was assembled as a result of the studies and was examined in further experiments.

Fig. 6.11 Diagram of IKAR-S stand with nine modules in a critical state

In the course of the studies on physical startup of the critical stand, the charac­teristics (kef) were determined for roughly 40 different multiplying systems. The studies looked at how reactivity was affected by a reduction in reactor core porosity (by increasing the graphite content in the sections), an increase in the number of fuel elements in the central module, and a reduction in the quantity of aluminum alloy modeling the cooling radiators for the gas medium in the RL. Selection of the critical configuration of the critical stand core was accompanied by Monte Carlo calculations of kef. As new experimental data were obtained, the computational model was adjusted, which ultimately made it possible to quite accurately describe the multiplying properties of an IKAR type reactor core.

It was proposed that the IKAR-500 reactor be built on the basis of the IKAR-S critical stand. For this it was necessary to:

• select the reactor section configuration that would assure the required reactivity margin (~3^eff);

• ensure obtaining of a neutron pulse of the required (rectangular) shape;

• ensure safety of reactor operation.

The results of the physical startup showed that the proposed IKAR-500 reactor core configuration with sections per version 1 (Fig. 6.10a) did not assure the required reactivity margin when the existing fuel elements, with a content of 2.5 % uranium in a uranium-aluminum alloy, were used. One of the options to resolve the problem is to use “bare” fuel cores when the entire fuel section is sealed, which greatly reduces the mass of the zirconium alloy in the reactor core.

To assure the required pulse shape, it is proposed that a so-called reactivity modulator be used. This is a structure made of absorbent rods arranged in horizontal channels and controlled by linear stepper motors.

For fast transition of the reactor to a subcritical state, a system of fast-acting emergency shielding was developed based on the control elements existing on the IKAR-S stand. The absorbent rods are dropped to the lower position using compressed-air “guns” which reduces the reactor shutdown time to 0.2 s.

The basic problem in ensuring the safety of the IKAR-500 reactor is the lack of a temperature coefficient for reactivity damping when the characteristic pulse times are ~1 s. To ensure internal emergency shielding of the reactor, a system was developed that assures poisoning of the reactor core with a neutron-absorbing gas (3Не or BF3) [22]. Another option is to use uranium-graphite fuel similar to the fuel of the IGR reactor [7] in one or several reactor modules. This ensures the presence of an “instantaneous” temperature coefficient for reactivity damping.

It is proposed that all of the listed developments be tested on the critical stand of the IKAR-S. Thus the developed critical stand makes it possible to determine the parameters of the IKAR-500 reactor and to develop the real design of its primary assemblies and systems.