Study on transmutation efficiency of the VVER-1000 fuel assembly with different minor actinide compositions

_ 
241
Am 580.05 223.76 38.58 877.75 313.70 35.74 1536.57 482.05 31.37 
243
Am 269.52 49.80 18.48 158.24 26.79 16.93 241.75 38.26 15.83 
244
Cm 81.54 -42.09 -51.62 29.94 -28.17 -94.09 50.18 -39.56 -78.84 
245
Cm 4.79 -4.94 -103.14 1.65 -3.10 -187.40 4.17 -3.90 -93.42 
Total 1832.67 366.73 20.02 1832.67 427.57 23.33 1832.67 473.97 25.86 
The results illustrated in Fig. 3 also 
imply that the MAs with the content of up to 
10 wt% can be loaded into the VVER-1000 
LEU fuel assembly without significantly 
affecting the fuel cycle length by means of 
reducing the gadolinium content and the 
boron concentration to offset the negative 
reactivity insertion by the MAs. For the MA 
loading up to 10 wt%, it was found that the 
lower excess reactivity and equivalent cycle 
length as compared to the reference case can 
be obtained with the gadolinium content 
reduced to around 2.5-3.0 wt% and the boron 
concentration reduced to around 350-400 
ppm. As a result, loading 10 wt% of MA is 
recommended for the sake of excess reactivity 
control and high loading amount of MAs 
while keeping almost the same cycle length 
with the reference case. 
It is found that the case of loading MA 
vectors from the VVER-440 shows the highest 
k-inf while that from the VVER-1000 exhibits 
the lowest k-inf. This also makes the excess 
reactivity at the beginning of the cycle when 
loading the MA vector from the VVER-440 
spent fuel higher than the two others. 
However, Fig. 3 indicates that the fuel cycle 
length is mostly unaffected when loading with 
different MAs vectors. 
The transmutation of MA isotopes is 
shown in Table II for the cases when loading 
10 wt% of MAs and adjusting the gadolinium 
content to 3 wt% and the boron concentration 
to 350 ppm. It can be seen that the 
concentrations of 
241
Am and 
243
Am decreased 
with fuel burnup while those of 
244
Cm and 
245
Cm accumulated with fuel burnup. The 
concentration of 
237
Np decreased with burnup 
when loading the VVER-440 and PWR-1000 
MA vectors. After 306 days, the 
237
Np 
concentration was reduced ~15.63 % when 
loading the VVER-440 MA vector and ~15.47 
% when using PWR-1000 MA vector. The 
241
Am concentration reduced ~38.58 %, ~35.74 
% and ~31.37 % while the 
243
Am concentration 
reduced ~18.48 %, ~16.93 % and ~15.83 % in 
correspondence with loading VVER-440, 
PWR-1000 and VVER-1000 MA vectors. 
Meanwhile, those of 
244
Cm and 
245
Cm 
increased ~51.62%, ~94.09 %, ~78.84 % and 
~103.14 %, ~187.40 %, ~93.42 % 
corresponding to VVER-440, PWR-1000 and 
VVER-1000 MA vectors. The results 
demonstrate that the transmutation of MAs 
STUDY ON TRANSMUTATION EFFICIENCY OF THE VVER-1000 FUEL ASSEMBLY 
22 
recycled from spent nuclear fuel in the VVER-
1000 fuel assembly is feasible from neutronic 
viewpoint and the total transmutation rate 
higher than ~20% can be achieved. Besides, it 
is noticed that in case of loading the VVER-
1000 MA vector without 
237
Np, the transmuted 
amount of 
241
Am was much larger compared 
with the two other cases since the initial 
loading amount of this isotope was more than 
two times larger. This explained why the case 
of loading the VVER-1000 MA vector 
exhibited the highest total MA transmutatiton 
mass and efficiency as can be found in Table 
II. It is also worth noting that more than 90% 
of the radiotoxidity of MAs from long time 
storage spent fuel (more than hundred years) 
come from 
241
Am (half-life of 432 years). 
Thus, with the significant amount of 
241
Am 
that was transmuted in the VVER-1000 fuel 
assembly, it could contribute to a significant 
reduction of radiotoxicity level of the 
radioactive waste. 
B. Coating a thin layer of MAs to the 
UGD pins 
In the case of heterogeneous loading of 
MAs in the UGD pins of the VVER-1000 LEU 
fuel assembly, MAs were coated as a thin layer 
at the outside of the UGD pellets as shown in 
Fig. 4. The thickness of the cladding was kept 
unchanged and the outer radius of the UGD 
region was reduced to accommodate the layer 
of MAs. For the purpose of MA burning and 
keeping the fuel cycle length, the MA content 
of 10 wt% was selected in this investigation. 
The MA coated layer (see Fig. 4) equivalent to 
homogeneous loading with 10 wt% of MA is 
0.01981 cm thick. Similar to the case of 
homogeneous mixing, the gadolinium content 
and boron concentration were also reduced to 
compensate the negative reactivity insertion by 
the MAs. 
Fig. 4. Coating a thin layer of MA to the UGD pellet 
The results of the k-inf of the VVER-
1000 LEU assembly versus burnup when 
coating MAs to the UGD pins and reducing 
the gadolinium content and boron 
concentration are shown in Fig. 5 in relation 
to the reference case. It was found that the 
cases of reducing only the gadolinium 
content led to a significantly lower excess 
reactivity at the beginning of the cycle and 
a considerably shorter cycle length. This 
behavior of the k-inf versus burnup is 
similar to the cases of homogeneous loading 
as above mentioned. For that reason, the 
boron concentration was reduced to 400 
ppm, 350 ppm, and 300 ppm with respect to 
the gadolinium content of 2 wt%, 2.5 wt%, 
and 3 wt%. It is worth noting that the 
amount of boron concentration reduction in 
these cases was about 50 ppm larger than 
the respective ones of homogeneous loading 
due to the self-shielding effect of MAs. The 
excess reactivity at the early burnup steps 
when reducing the gadolinium content to 2 
wt%, 2.5 wt%, and 3 wt% was generally 
lower than the reference case; except that it 
was slightly higher for the case of the VVER-
440 MA vector with the gadolinium content 
of 2 and 2.5 wt% (Fig. 5). Sooner or later the 
k-inf in the three cases became smaller than 
the reference case. However, the cycle length 
with gadolinium content of 2.5 and 3 wt% 
was almost the same with the reference case 
while that with gadolinium content of 2 wt% 
was somewhat shorter. Consequently, 
TRAN VINH THANH et al. 
23 
reducing the gadolinium content to 3 wt% and 
boron concentration to 300 ppm is 
recommended when coating with 10 wt% of 
MA to the UGD pellets. The transmutation of 
MA isotopes when coating with 10 wt% of 
MAs and reducing the gadolinium content to 
3 wt% and boron concentration to 300 ppm is 
given in Table III. Comparing Tables III and 
II, it is shown that the difference in the 
transmutation rate of MA isotopes between 
homogeneous and heterogeneous loadings was 
relatively small. However, the transmutation 
mass in the case of heterogeneous loading was 
slightly higher than that with homogeneous 
loading, in particular for the case of VVER-
440 MA vector. Table III also signify that the 
highest total MA transmutation mass and 
efficiency was again achieved for the case of 
loading the VVER-1000 MA vector as 
compared to the two other cases. 
Fig. 5. The k-inf of the LEU fuel assembly versus burnup when coating a layer of MAs to the UGD pins and 
reducing GD to 2 wt% (upper), 2.5 wt% (middle) and 3 wt% (lower) 
STUDY ON TRANSMUTATION EFFICIENCY OF THE VVER-1000 FUEL ASSEMBLY 
24 
Table III. Transmutation capability in case of heterogeneous loading of 10 wt% MA 
Isotope 
VVER-440 MA vector 
PWR-1000 MA vector 
VVER-1000 MA vector 
Initial 
amount 
(g) 
Mass reduced 
after 306 days 
Initial 
amount 
(g) 
Mass reduced 
after 306 days 
Initial 
amount 
(g) 
Mass reduced 
after 306 days 
(g) (%) (g) (%) (g) (%) 
237
Np 896.79 150.40 16.77 766.06 118.35 15.45 0.00 __ __ 
241
Am 580.05 238.34 41.09 877.12 317.87 36.24 1536.83 494.08 32.15 
243
Am 269.51 51.13 18.97 157.98 24.03 15.21 241.56 34.39 14.23 
244
Cm 81.53 -41.27 -50.62 29.87 -25.87 -86.61 50.12 -36.42 -72.67 
245
Cm 4.79 -7.81 -142.27 1.65 -3.44 -208.80 4.17 -4.85 -116.46 
Total 1832.67 391.79 21.38 1832.67 430.93 23.51 1832.67 484.39 26.43 
IV. CONCLUSIONS 
In this study, the efficiency of MA 
transmutation as burnable poison in the 
VVER-1000 LEU fuel assembly was 
examined using the SRAC code for the MA 
homogeneous and heterogeneous loading 
parterns with different vectors of MAs 
recycled from the spent fuel of VVER-440, 
PWR-1000, and VVER-1000 reactors. The 
gadolinium content and the boron 
concentration were reduced correspondingly 
to compensate the negative reactivity 
insertion by MA loading. The results show 
that, with 10 wt% of MAs loading, 2.5-3.0 
wt% of gadolinium content and 400-350 ppm 
of boron concentration were recommended 
for homogeneous mixing MAs in the UGD 
pins while 3 wt% of gadolinium content and 
300 ppm of boron concentration were 
recommended for heterogeneous loading of 
MAs in the UGD pins. It was also found that 
237
Np, 
241
Am, and 
243
Am could be 
significantly transmuted with a transmutation 
rate as high as ~40% for 
241
Am. With this 
transmutation capability, burning MAs in the 
VVER-1000 fuel assembly could contribute 
to a significant reduction of radiotoxicity 
level of the radioactive waste since more 
than 90% of the radiotoxidity of MAs from 
long time storage spent fuel (more than 
hundred years) come from 
241
Am (half-life 
of 432 years). Furthermore, the case of 
loading the VVER-1000 MA vector is 
highly recommended as it could lead to the 
highest 
241
Am transmutation mass as well as 
the highest total MA transmutation mass 
and efficiency. 
In addition, it was shown that the MAs 
loading in combination with the reduction in 
gadolinium and boron concentration could help 
facilitate the excess reactivity control at the 
beginning of the fuel cycle without significant 
effect on the cycle length. Moreover, the MA 
coating approach could increase slightly the 
MA burning efficiency when comparing with 
homogeneous MA mixing model because of 
the self shielding effect on MAs, especially for 
the VVER-440 MA vector. Besides, the results 
indicate that, although loading of different MA 
vectors slightly affected the fuel cycle length, 
loading the MA vectors with lower amount of 
237
Np and higher amount of 
241
Am could help 
significantly reduce the excess reactivity at the 
beginning of the cycle. 
TRAN VINH THANH et al. 
25 
Further investigation on transmutation 
of MAs and radiotoxicity reduction at a full 
core level and MOX core of the VVER-1000 
reactor is being planned. 
ACKNOWLEDGMENTS 
This research is funded by Vietnam 
National Foundation for Science and 
Technology Development (NAFOSTED) 
under grant number 103.99-2018.32. 
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