炉物理の研究第76号（2024年3⽉）

Feasibility of Rotational Fuel-shuffling Breed-and-Burn Fast Reactor with Nitride Fuel andSodium Coolant
Tsendsuren Amarjargal, Jun Nishiyama, and Toru Obara*
Laboratory for Zero-Carbon Energy, Institute of Innovative Research, Tokyo Institute ofTechnology, Ookayama 2-12-1-N1-19, Meguro-ku, Tokyo 152-8550, Japan*Corresponding email: tobara@zc.iir.titech.ac.jp
Introduction
In recent years, renewable energies like solar and wind have gained attention as a way to reducegreenhouse gas emissions. However, the intermittent nature of renewable energies makes itchallenging to meet the base load demand. Nuclear power is a potential solution(1), as it produces
large amounts of electrical energy per unit mass of fuel and emits no greenhouse gases. Despite
these advantages, concerns about nuclear waste disposal, safety, proliferation, and the short-term
availability of uranium reserves have led to scrutiny from governments and environmentalists.
To address these concerns, in 2001 scientists established the Generation IV International Forum(GIF) to develop advanced nuclear reactor designs, culminating the following year in the selectionof six designs based on sustainability, economics, safety and security, and proliferation resistancecriteria. Fast breeder reactors, such as the sodium-cooled fast reactor(2), are a promising option
for sustainable energy production because they can use nearly 100% of natural uranium's energy
value. To achieve this high fuel utilization, it must reprocess the fuel when the peak radiation
damage reaches a certain threshold. However, due to the separation of plutonium or trans-uranium
elements from used or spent fuel, reprocessing poses a significant proliferation risk. To avoid thisissue, breed-and-burn (B&B) reactors(3) have been proposed that can sustain a fission reaction
when fueled directly with natural uranium without the need for reprocessing. To initiate the chain
reaction, the B&B core must first be fed an adequate amount of fissile fuel such as enricheduranium. Thereafter, the B&B core is capable of continued operation while being fed solely withnatural or depleted uranium. However, B&B reactors require improved neutron economy(4,5) toestablish and maintain criticality in normal operating conditions. One proposed solution isrotational fuel shuffling(6), which involves loading fresh fuel from the edge of the core and moving
it stepwise towards the center while discharging spent fuel. This helps to maintain a high neutron
economy by ensuring that high burnup fuel is always in close proximity to the central region of
the reactor core. Detailed analyses of small rotational fuel-shuffling breed-and-burn fast reactors(RFBBs) have been performed(7,8).
A small RFBB reactor with nitride fuel and sodium coolant was analyzed, and the results
confirmed the feasibility of the concept with a simplified core design(9). Moreover, the sodium
void reactivity coefficient in RFBB-NS was investigated, revealing that it was the same as that inconventional SFR designs in the equilibrium condition. However, it is important to note that thecore design was a simplified one, lacking both a fuel assembly duct and control rod assemblies,
which are necessary to realize the reactor concept. Still, the presence of such devices may diminish
the neutron economy, which could make the reactor concept unfeasible. The purpose of this study
is to design the RFBB-NS, including assembly duct and control rod assemblies, and to show itsfeasibility.
Brief results of the study

炉物理の研究第76号（2024年3⽉）

Neutron transport and fuel burnup analyses were performed for the improved RFBB-NS design.
The reactor reaches a critical state and k-eff converges at the equilibrium state by the use of lead‒bismuth reflectors. The equilibrium state was obtained after 35 shuffling steps with an 1120-day
shuffling interval (cycle). Also, it can be seen that the reactivity swing between shuffling steps wassmall due to burnup performance.
The burnup characteristics of the equilibrium cycle were analyzed in detail. Burnup is expressedin megawatt-days per kilogram of initial heavy metal, and the results indicate that the burnup at
the top and bottom of the zones was ~166 MWd/kgHM, which is significantly higher than in thefeasibility study(9). This higher burnup resulted in a lower Pu vector in the discharged fuel, thus
solving one of the issues that arose in the previous study.
Moreover, the burnup at middle zone was ~395 MWd/kg/HM, resulting in an average discharge
burnup of ~299 MWd/kgHM or 31% FIMA. However, the high burnup values achieved in theimproved RFBB-NS design require a fuel cladding material that can withstand high radiation
damage. The estimated DPA of the cladding was about 743, while the maximum allowable
constraint for radiation damage in cladding material is around 650 DPA for this study. Therefore,a key challenge in the design of an RFBB will be the development of a fuel cladding material
capable of withstanding such high irradiation.
In rotational fuel shuffling, fresh fuel assemblies were loaded at the core periphery and discharged
at locations far from the core center. The power distribution peaked at the core center and gradually
decreased in the radial direction. Changes of power distribution at the BOEC and EOEC weresmall. This implies the safe operation of RFBB-NS. While the uncertainties associated with thepower distribution have been assessed, it is found that they exert minimal impact on the heatremoval calculation. The uncertainty from Monte Carlo calculation were around 0.00162% for
Beginning of Equilibrium Cycle (BOEC), suggests that the variations in power distribution havenegligible consequences for the heat removal assessment. Despite the small magnitude of these
uncertainties, rigorous evaluations continue to be essential to guarantee that the obtained resultsalign with acceptable limits for temperatures of fuel, cladding, and coolant in the system.
The temperatures of the fuel, cladding, and coolant were all found to be within acceptable limits.
This indicates that the design was successful in controlling the heat produced by the reactor andaverting any potential harm to the coolant, cladding, or fuel. The pressure drops in the hot channel
remained within acceptable limits. This suggests that the coolant was able to flow through thereactor without encountering significant resistance or blockages, and that the pump was not alimiting factor.
Brief conclusion
The practical design of an RFBB-NS was studied. Burnup analysis revealed that the reference
design of the RFBB cannot reach criticality due to the presence of duct material and control rod
channels. To address this, the core was modified by applying lead‒bismuth reflectors. The thusmodified design of RFBB-NS showed promising results in neutron transport, burnup, and heatremoval analyses. By decreasing the active height of the core and placing axial reflectors at its topand bottom, the pressure drop in the long coolant channel was reduced to acceptable levels. The
use of lead‒bismuth reflectors improved the neutron economy, allowing the reactor to reachcriticality and the k-eff to converge at equilibrium. The improved design showed superior burnup
performance compared to the previous study, with higher burnup values achieved in the top and炉物理の研究第76号（2024年3⽉）

bottom zones. However, the peak radiation damage to the cladding material slightly exceeded theconstraint set used in this study. During BOEC and EOEC, the power distribution in the radial
direction remained constant, and the maximum fuel centerline temperature, cladding outertemperature, coolant outlet temperature, and pressure drop in the coolant channel were all within
acceptable limits.

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