              


Intensity (cps) 

periphery

4000




Central voidU

periphery

3000

Pu

2000
1000

Am
0

0

1000

2000

3000

4000

5000

6000

7000

Distance (μm)

1mm


     
Central void

UPu

Am

500μm

               

MOX fuels containing several percent concentrations ofminor actinides (MA) and fission products (FP) arepromising candidates for a closed nuclear cycle system basedon a fast reactor. Cycle management of MA such as Am, Npand Cm is of crucial importance from the viewpoint of
reduction of environmental burden, effective use of naturalresources, and enhancement of nuclear nonproliferation. For
evaluating the irradiation performance of the fuel, it isindispensable to understand irradiation behavior, such as Am
redistribution and fuel cladding chemical interaction.
In order to confirm the effect of MA addition on irradiation
behavior of MOX fuel pellets, an irradiation program named“Am-1” is being conducted in JAEA. The Am-1 program
consists of two short-term irradiation tests of 10-minutes and24 hours, and a steady-state irradiation test. The two shortterm irradiation tests have been successfully completed. Afterthe reactor power was raised continuously to the targeted
peak linear heating rate of 430W/cm and kept there for 10min, the power was manually shut down. The second
irradiation test was performed with almost the same power
history up to the targeted peak heating rate of 430W/cm andheld for 24 hours.
Post irradiation examinations such as ceramography andelectron probe microanalysis (EPMA) have been conducted          in order to evaluate the irradiation behavior of the fuel.
No sign of fuel melting was found in any of the specimens
taken from the Am-MOX fuel pins irradiated for 10 min and24 hour. This implied that the thermal design had a sufficient
safety margin for these test conditions. The configuration of
the microstructure after 10 min irradiation was similar to thatof conventional oxide fuels irradiated for a short time at a highlinear heating rate (Fig.1-5(a)). After the 24 hour irradiation,a central void developed, and the change in structure seen at
the beginning of irradiation was completed (Fig.1-5(b)).
The results of EPMA revealed that Am migrated to theradial center of the fuel pellet up the temperature gradientduring the initial stage of irradiation. This tendency is similar
to that of Pu (Fig.1-6).
It has been reported that the melting point temperature isdecreased by the addition of a large amount of Am, whichleads to a smaller design margin for fuel melting. The present
EPMA results indicated that careful consideration must begiven to the redistribution behavior of Am as well as that ofPu for evaluating its impact on the thermal performance ofAm-MOX fuels.
The irradiation behavior at a high linear heat rate observedin the Am-1 program will be useful for the modeling anddesign study of Am-containing MOX fuel.



Tanaka, K. et al., Microstructure and Elemental Distribution of Americium-Containing Uranium Plutonium Mixed Oxide Fuel under a Short-Term
Irradiation Test in a Fast Reactor, Journal of Nuclear Materials, vol.385, issue 2, 2009, p.407-412.

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