Neutronic Analysis of Absorbing Materials for the Control Rod System in Reactor Allegro

Cajko, Frantisek, VÚJE

25th Symposium of AER on VVER Reactor Physics and Reactor Safety (2015, Balatongyörök, Hungary)
Nuclear fuel Cycle Perspectives and Sustainability


Demonstrator Allegro is a gas cooled fast reactor in the design stage. Its reactivity control
system consists mainly of control rods filled with enriched boron carbide as an absorber. The
boron enrichment of 70% is needed to ensure enough worth of the control rods. This paper
deals with searching for other possible absorbing materials that could be used in the design in
order to lower the boron enrichment needed while increasing the control rod worth and
keeping the safety margin. The worth of the control rods filled with known absorbing
materials is compared using the transport Monte-Carlo code SERPENT in a computational
supercell. A possible benefit of a neutron trap concept is explored as well. It is shown that the
best absorbers and the usable moderating materials for the neutron trap are compounds
containing hydrogen (HfH2 and ZrH2, respectively). The worth of the control rods containing
the selected materials (B4C, EuB6, ReB2) has also been calculated by SERPENT using the full
model of the Allegro reactor including the radial reflector and the radial absorber. The
corresponding integral characteristics are presented.

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