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1. Introduction
In Korea, an accelerator driven system (ADS), which is called HYPER (Hybrid Power Extraction
Reactor) is currently under development for transmutation of TRUs (Transuranic Elements) [1].
Concerning the uranium-free fast reactors like HYPER, one of the big problems is a very large
reactivity swing, regardless of the sub-criticality of the core. In an ADS, a large burn-up reactivity
swing means a large reservation of the proton beam current. This large reserved proton current may
result in several unfavourable safety features as well as adverse impacts on the economics of the
system. Also, another concern associated with the large reactivity change is a one-way change of the
radial power distribution during depletion of the core. To resolve this problem, an on-power refuelling
concept, as in CANDU, was studied previously for HYPER [1]. However, the on-power refuelling
makes the system fairly complex and may cause serious engineering concerns.
Several types of burnable absorbers such as boron, gadolinium, and erbium are successfully used
to suppress the initial excess reactivity and to control the power distribution in thermal reactors like
PWRs [2]. Regarding the critical fast reactors such as LMR (Liquid Metal Reactor), poisoning the
core with a burnable absorber is not used to control the reactivity. This is mainly because the excess
reactivity in conventional LMRs is fairly small due to self-generation of fissile elements and thus the
burn-up reactivity swing can be easily controlled by control rods. Of course, it is well recognised that
there is no effective burnable absorber for fast neutron systems due to small neutron capture crosssections.
When it comes to the fast-neutron ADS loaded with TRUs, however, the situation is quite
different from those of the conventional critical thermal and fast reactors. Basically, any excess
reactivity, which should be suppressed by external control mechanisms, should not be allowed in ADS
in order to guarantee its surmised advantages. Consequently, control rods or absorber-containing
coolant cannot be used to control the reactivity of an ADS. Thus, fixed burnable absorbers, if any,
could only be utilised as the reactivity control mechanism for ADS.
Previously, Stone et al. [3] studied a dual spectrum core to reduce the reactivity swing of the
ATW core, where a thermal spectrum zone is placed in the periphery of the core and 237 Np and 241 Am
are loaded in the thermal region. They showed that the reactivity swing could be reduced by a factor of
2 in the dual spectrum ATW. However, the smaller reactivity change in the modified ATW is mainly
due to the reduced power density. In addition, the dual spectrum core may lead to a large power
peaking in the interface region between hard and soft spectrum regions.
The simplest way to reduce the reactivity swing is to adopt a low power density core. However, a
low power density needs a large core volume, thus it is not favourable from the economics point of
view. Recently, Hejzlar et al. [4] studied burnable absorbers for a critical Pb-Bi-cooled transmutation
reactor. They evaluated various candidate materials such as B, Re, Hf, Gd, Er, etc. They showed that
10 B has the largest neutron capture cross-sections and can reduce the reactivity swing a little. Finally,
they discarded the burnable poison option, in favour of excess reactivity compensation through control
rods.
In this paper, we have re-evaluated the potential of 10 B as a burnable absorber for the sub-critical
HYPER core to reduce the burn-up reactivity swing and also to control the radial power distribution.
All calculations are performed with a Monte Carlo code, MCNAP [5], which was developed at Seoul
National University, Korea. It is worthwhile to note that MCNAP has its own built-in depletion
routine
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