Seminars

Time-tested in a new project

On 18 January, a FLNP seminar was held, at which Yu.N.Pepelyshev presented preliminary calculations by a team of authors (Yu.N.Pepelyshev, A.V.Vinogradov, A.D.Rogov, D.Sumkhuu) for the IBR-4 pulsed reactor.

Discussion of options for the design of a new neutron source that is expected to be developed at FLNP before the operation of IBR-2 is completed, started several years ago. Employees of the FLNP Nuclear Safety Group proposed that after the decommissioning of the IBR-2 reactor, not to develop a completely new neutron source that would require large-scale development work, but to use, to a large extent, technical solutions tested at IBR-2 and other nuclear facilities (MBIR, BOR -60 and others).

"This is our conceptual vision of the project," Yuri Nikolaevich began his speech. It can be refined during subsequent development. In this case, special attention was paid to the issues of stable reactor dynamics. In our previous publications, this task was not highlighted. Now, when preparing this proposal, we took into account the relevant experimental data obtained at the IBR-2 and IBR-2M reactors. Particular attention is paid to the issue of stability in the operation of the reactor: reducing the influence of a number of factors causing fluctuations in pulse energy.

In the concept of the new reactor, the authors propose to use the MBIR reactor configuration with a reactivity modulator of the PO-3 type that is currently successfully operated at IBR-2. A significant design difference of the proposed concept from the IBR-2 and IBR-2M reactors is the movement of the control and protection system components inside the core, as is implemented in the BOR-60, MBIR research reactors and the BN-600 power reactor. It allows to bring the moderators closer to the reactor vessel and to increase the flux density of thermal and cold neutrons without increasing the average power of the reactor. IBR-4 is supposed to use already tested plutonium dioxide as fuel, to limit the power to 5 MW (the limitation is associated with an increase in pulse energy fluctuations with increasing power) and the control and protection system components are supposed to be made of boron carbide enriched with boron-10. The authors propose repeating the MBIR reactor vessel, changing the geometry of the core, cooling with liquid sodium and surrounding the core with water moderators.

Citing some characteristics of the core of the basic version of IBR-4, the author emphasized that the large calculated margin for fuel burnup (10%) is a significant operational advantage. Some neutron and physical parameters of the proposed reactor option are average power - 5 MW, pulse frequency 10 per second, fuel - plutonium dioxide, fast neutron pulse duration 200 ?s, pulse power 1900-2370 MW, background between pulses 8.5%. If you replace plutonium dioxide with plutonium nitride, the core will become smaller and the power can be increased; accordingly, the flux density of thermal neutrons on the surface of the moderator will increase. But this way of developing a neutron source with fuel in the form of plutonium nitride was not considered in detail by the authors, since it required lengthy additional research that went beyond the scope of the IBR-4 concept. To extend the service life of the core, the authors propose to consider a new radiation-resistant cladding steel EK164.

Speaking about the oscillatory instability of pulsed reactors in general, the author gave some examples from the practice of IBR-2 and IBR-2M. He went on to talk about modifying the basic layout of IBR-4 in order to reduce low-frequency fluctuations in pulse energy. To understand the frequency properties of the reactor, a dynamics model of "IBR-4" was built as a pulsed automatic control system with feedback. Feedback on power changes was obtained experimentally on the IBR-2 reactor and its modernized version IBR-2M and adapted to the IBR-4 reactor. The model has been tested experimentally and has been well describing the dynamics of pulsed reactors for more than 25 years. The authors determined a general criterion for the stability of IBR-4 that depends on power feedback that in turn, depends on the parameters of the fuel rods. Calculation and experiment using three variants of fuel rods MBIR, IBR-2 and BN-1200 showed that in order to reduce resonance phenomena in IBR-4 it is necessary to equalize the energy release in its core. For this purpose, changes have been made to the design of fuel elements: it is proposed to insert tungsten or uranium rods into the central fuel elements of some fuel cassettes and further strengthen their rigidity.

The authors' conclusions are the following: the IBR-4 composition, based on the maximum use of reference technical solutions, allows to implement the presented version of the neutron source at lower costs as compared to other options; the oscillatory dynamics of "IBR-4" compared to IBR-2M is significantly reduced; at a power of 5 MW, the thermal neutron flux density on the surface of a flat water moderator is 1.5-1.75 times higher than on IBR-2M and on the surface of comb moderators - 2.5-3.1 times higher. The duration of the fuel campaign for IBR-4 is 20 years.

The authors expressed great gratitude to all the personnel of the IBR-2 reactor and all the staff members of the FLNP Nuclear Safety Group for their help, support and fruitful discussions.

The speech raised numerous questions and comments from the staff of FLNP and NIKIET named after N.A.Dollezhal that gathered in the conference hall and online.

Olga TARANTINA