Research and Development of Advanced Nuclear System
1-10 Technology for Removal of Excess Oxygen from
MOX Fuels during Irradiation
=Research and Development for MOX Fuel by Using Oxygen Getter=
2000
＊
1400
1200
1000
800
Core Fuel Region
＊
TiOx (700 ͠, closed face 2)
＊
TiOx (580 ͠, open face 1)
700 ͠
600
400
Cladding inner temp.
200
Pellet surface temp.
Pellet center temp.
0
0
200
400
600
800
1000
1200
＊1
＊2
exposed to the atmosphere
interface between two discs
250
1400
Distance from the lower end of LABR (mm)
Thickness of oxidation layer (Ǵm)
Temperature (͠)
1600
Upper Axial Blanket Region
Lower Axial Blanket Region
TiOx (700 ͠, open face 1)
1800
200
150
100
50
0
Fig.1-25 Analyzed fuel-temperature distribution
From the temperature that is less than 570 ℃ at the pellet-cladding
gap, the temperature at the bottom end of the upper axial blanket
region is expected to be 728 ℃.
Cladding
MOX pellets
Insulator UO2 pellet
Blanket UO2 pellet
titanium roll pellet
dummy cladding
0
50
100
150
200
250
Time (h)
Fig.1-26 Dependence of titanium-oxidation-layer
growth on heating time
Quick oxidation was confirmed at 700 ℃, but oxidation
was very slow at 580 ℃.
Fig.1-27 Method for preparing titanium roll pellet and
cross sections of a produced roll pellet
Gaps between titanium layers and a gap between roll and
cladding can be observed.
titanium roll
Increasing the burnup is essential for attaining economical
competitiveness of fast reactor systems. Because excess
oxygen accumulates in the fuel and accelerates cladding
corrosion in proportion to the burnup, it should be removed.
The use of an oxygen getter is one of the solutions for
removing excess oxygen, but almost no papers have reported
the application of the option in removing excess oxygen from
pellet-type mixed oxide (MOX) fuels. Therefore, a feasibility
study on the oxygen getter option for pellet-type MOX fuels
is in progress.
First, a survey of oxygen getter materials was carried out
and titanium was selected after considering factors such as
the oxygen potential, melting point and neutron absorption.
Second, the configuration of the getter material and the
loading position were investigated. Contact of the getter
material with the end surfaces of MOX pellets is not
allowable because of the problem of compatibility. Therefore,
there is practically no choice with respect to the loading
position except the pellet-cladding gap or upper axial blanket
region. A computational-analysis result showed that the
temperature of the oxygen getter material at those locations
was 396~728 ℃ (Fig.1-25). Experimental results showed fast
oxidation of titanium at 700 ℃ but quite slow oxidation at
580 ℃ (Fig.1-26). This suggests that absorption of the excess
oxygen by titanium is expected only when it is located at the
lower end of the upper axial blanket region where the
temperature reaches 728 ℃. The volume of titanium
increases during oxidation to TiO2. To avoid mechanical
interaction with the cladding, the getter titanium must be
installed with low smear density that is less than 50%.
Although some options like metallic particle packing or
porous pellet installment were also discussed, installment of
titanium pellets rolled up in titanium foils with a thickness of
less than 100 μm was the most recommended option. The
results of a test production confirmed that a titanium pellet
roll with a controlled pellet diameter and with adequate gaps
between foils could be produced without welding (Fig.1-27).
These results show the feasibility of using titanium pellet
rolls.
Reference
Morihira, M., Feasibility Study of Oxygen Getter materials for FBR MOX Fuel (2)-Investigation of Loading Options, Evaluation of Oxidation Behavior and
Compatibilities of the Candidate Materials-, JAEA-Research 2011-018, 2011, 32p. (in Japanese).
20 JAEA R&D Review 2011