show the assumed core conditions and RZ geometry for parametric survey, respectively. The calculation methods were as follows. Core burn-up characteristics were
analyzed with the burnup calculation code STANBRE [13]. Reactivity coefficients
were analyzed using the diffusion calculation code DIF3D [14]. The effective cross
sections used in these calculations were obtained by the cell calculation code
SLAROM-UF [15], based upon 70 group cross sections from JENDL-4.0 [16]
with a self-shielding factor table as a function of background cross section. This
method for the production of the effective cross sections is considered to be
adequate to take into account the influence of each diluting material upon the
self-shielding effect of heavy isotopes for the parametric study. Concerning material compositions, a homogeneous model of fuel, diluent, and spectrum moderator
was used.
To begin with, in the survey to improve Doppler feedback, 21 elements to
enhance resonance absorption were evaluated as a diluent material for the TRU
alloy: Cr, Mn, Fe, Ni, Nb, Mo, Tc, Ru, Rh, Pd, Nd, Sm, Gd, Tb, Dy, Er, Tm, Ta, W,
Os, and Au. Moreover, the effect by neutron moderators such as BeO,
7 Li 2 O,
11 B 4 C
(100 % enrichment of
11 B was assumed), and ZrH 2 were investigated to clarify the
impact against Doppler feedback by neutron spectrum softening. To compare the
Doppler effect enhancement of various diluent materials and neutron spectrum
moderators in a simple manner, each material was hypothetically added to
TRU-10wt%Zr alloy. The amount of each material added was adjusted case by
case to maintain 1.0 of k-effective at the end of cycle.
Next, in the evaluation to decrease the burn-up swing, the effects of the measures
taken to increase the fissile amount at the beginning of the cycle were studied. The
effects on burn-up reactivity swing were evaluated by reducing the core height,
installing B 4 C shield at core peripheral, and increasing the number of refueling
batches, which all lead to increase of the fissile amount at the beginning of the
cycle.
Last, reflecting the results obtained by the parameter surveys, an optimal
uranium-free TRU metallic fuel core was specified, and its feasibility in light of
Doppler feedback and burn-up swing was evaluated by core performance analysis.
15.3.2 Analysis Results for Doppler Feedback Enhancement
The effects of measures taken to enhance Doppler feedback, that is, diluent and
spectrum moderator, are evaluated in this section.
As shown in Fig. 15.3, 6 among 21 diluent materials are found to enhance
Doppler feedback more than Zr, the typical metallic fuel alloy. Although Nb,
Ni, W, Mo, Fe, and Cr have greater potential to enhance Doppler feedback than
Zr, there are some deficiencies that cannot be ignored. First, the melting points of
Pu-Ni alloy and Pu-Fe alloy are below 500
C, which is too low for nuclear fuel
[17]. Second, the melting point of Pu-W alloy is too high to fabricate fuel by
injection casting because the melting temperature of W itself is above 3,000
C.
160
K. Ishii et al.
analyzed with the burnup calculation code STANBRE [13]. Reactivity coefficients
were analyzed using the diffusion calculation code DIF3D [14]. The effective cross
sections used in these calculations were obtained by the cell calculation code
SLAROM-UF [15], based upon 70 group cross sections from JENDL-4.0 [16]
with a self-shielding factor table as a function of background cross section. This
method for the production of the effective cross sections is considered to be
adequate to take into account the influence of each diluting material upon the
self-shielding effect of heavy isotopes for the parametric study. Concerning material compositions, a homogeneous model of fuel, diluent, and spectrum moderator
was used.
To begin with, in the survey to improve Doppler feedback, 21 elements to
enhance resonance absorption were evaluated as a diluent material for the TRU
alloy: Cr, Mn, Fe, Ni, Nb, Mo, Tc, Ru, Rh, Pd, Nd, Sm, Gd, Tb, Dy, Er, Tm, Ta, W,
Os, and Au. Moreover, the effect by neutron moderators such as BeO,
7 Li 2 O,
11 B 4 C
(100 % enrichment of
11 B was assumed), and ZrH 2 were investigated to clarify the
impact against Doppler feedback by neutron spectrum softening. To compare the
Doppler effect enhancement of various diluent materials and neutron spectrum
moderators in a simple manner, each material was hypothetically added to
TRU-10wt%Zr alloy. The amount of each material added was adjusted case by
case to maintain 1.0 of k-effective at the end of cycle.
Next, in the evaluation to decrease the burn-up swing, the effects of the measures
taken to increase the fissile amount at the beginning of the cycle were studied. The
effects on burn-up reactivity swing were evaluated by reducing the core height,
installing B 4 C shield at core peripheral, and increasing the number of refueling
batches, which all lead to increase of the fissile amount at the beginning of the
cycle.
Last, reflecting the results obtained by the parameter surveys, an optimal
uranium-free TRU metallic fuel core was specified, and its feasibility in light of
Doppler feedback and burn-up swing was evaluated by core performance analysis.
15.3.2 Analysis Results for Doppler Feedback Enhancement
The effects of measures taken to enhance Doppler feedback, that is, diluent and
spectrum moderator, are evaluated in this section.
As shown in Fig. 15.3, 6 among 21 diluent materials are found to enhance
Doppler feedback more than Zr, the typical metallic fuel alloy. Although Nb,
Ni, W, Mo, Fe, and Cr have greater potential to enhance Doppler feedback than
Zr, there are some deficiencies that cannot be ignored. First, the melting points of
Pu-Ni alloy and Pu-Fe alloy are below 500
C, which is too low for nuclear fuel
[17]. Second, the melting point of Pu-W alloy is too high to fabricate fuel by
injection casting because the melting temperature of W itself is above 3,000
C.
160
K. Ishii et al.