Conceptual nuclear design of a 20 MW multipurpose research reactor

d MTR standard and control fuel assemblies 
B. Core Arrangement 
The core has 23 lattices that consist of 
fourteen standard assemblies, four control 
assemblies and three in-core irradiation sites. 
The heavy water reflector tank of 200 cm in 
diameter and 120 cm in height surrounds the 
core. The reactor regulating system shares 
control rods with the reactor protection system. 
Fig. 2 shows the horizontal cross sectional 
view of the AHR and MTR cores. Some 
specifications of the cores are listed in Table II. 
Fig. 2. The horizontal cross sectional view of the AHR and MTR cores 
 a) AHR standard b) AHR Control c) MTR standard d) MTR control 
NGUYEN NHI DIEN, HUYNH TON NGHIEM, LE VINH VINH, VO DOAN HAI DANG 
29 
Table II. The specifications of the cores 
Reactor type AHR MTR 
Core volume (cm3) 1199.5 x 70 1527.7 x 70 
Fuel assembly Number 16 S + 4 C 16 S + 4 C 
Control rod Number 4 4 
Absorber material Hf Hf 
Total weight U-235 (kg) 9,87 10,12 
In-core irradiation sites 3 3 
IV. NUCLEAR ANALYSIS 
To confirm that the conceptual cores 
satisfy the functional and performance 
requirements, nuclear analyses are performed 
for fresh core and equilibrium core with 
several code systems such as MCNP [5], MVP 
[6], HELIOS [7], etc. 
A. Fresh Core 
The basic analysis of the core 
characteristics was performed for the fresh core 
with and without irradiation facilities. 
The core configuration should be 
designed to meet the functional and 
performance requirements. The neutron flux at 
the in-core irradiation sites and the reflector 
region of the cores without irradiation facilities 
was calculated by the MCNPX code [8] using a 
mesh tally. On the other hand, the power 
distribution, the reactivity of the core and the 
reactivity worth of control rods were also 
assessed to meet the requirements. Two core 
configurations with one and three in-core 
irradiation sites were proposed. Although the 
first configuration (with one irradiation site) is 
better in the fuel saving point of view, the 
configuration with three in-core irradiation 
sites was selected to meet predicted utilization 
of in-core irradiation in the future. 
As the ultimate goal of a research reactor 
is its utilization, the irradiation facilities should 
be designed in conformity with the user's 
requirements. The required irradiation facilities 
should be located at proper positions to 
maximize neutron utilization and minimize 
reactivity effect. Based on the neutron flux 
distribution of the reflector region, the 
arrangement by their purposes has been studied 
to achieve the objectives above. Their 
Fig. 3. The layout of the experimental sites of the AHR and MTR 
CONCEPTUAL NUCLEAR DESIGN OF A 20 MW MULTIPURPOSE RESEARCH REACTOR 
30 
reactivity worth is considered as a priority 
because of the influence to the reactor core. 
Various layouts of the irradiation facilities 
were proposed, and one of them was selected. 
To evaluate the stability of neutron flux at the 
irradiation sites, their neutron fluxes were 
calculated when the control rods are located at 
300 mm and fully withdrawn. 
The reactivity effect by the irradiation 
facilities was estimated to be 20.2 mk and 28.9 
mk and the total control rods worth 182.4 mk 
and 217.7 mk for AHR and MTR, respectively. 
Table III shows the neutron fluxes at the 
irradiation facilities. Figure 4 presents the 
thermal and fast neutron distribution of the 
AHR fresh core. 
Table III. Neutron fluxes at the experimental sites 
Neutron flux [n/cm2/sec]( Thermal1.0MeV) 
AHR MTR 
Maximum Average Maximum Average 
Thermal Fast Thermal Fast Thermal Fast Thermal Fast 
CT 4.46E+14 1.46E+14 3.04E+14 9.80E+13 4.01E+14 1.13E+14 2.87E+14 8.06E+13 
IR1 3.21E+14 1.18E+14 2.23E+14 8.29E+13 3.37E+14 9.31E+13 2.49E+14 6.76E+13 
IR2 3.16E+14 1.20E+14 2.23E+14 8.26E+13 3.33E+14 9.16E+13 2.46E+14 6.65E+13 
CNS 8.71E+13 1.15E+12 7.01E+13 8.76E+11 8.49E+13 1.69E+12 6.48E+13 1.24E+12 
ST1 1.37E+14 1.96E+12 - - 1.40E+14 3.23E+12 - - 
ST2 2.40E+14 3.47E+12 - - 1.79E+14 1.01E+13 - - 
NR 1.28E+14 3.20E+11 - - 1.28E+14 1.32E+12 - - 
NTD1 4.74E+13 1.13E+11 4.31E+13 8.12E+10 4.93E+13 4.19E+11 4.26E+13 3.19E+11 
NTD2 4.63E+13 9.91E+10 4.24E+13 7.60E+10 5.29E+13 4.84E+11 4.57E+13 3.56E+11 
NTD3 5.16E+13 2.43E+11 4.70E+13 2.04E+11 4.64E+13 5.21E+11 3.93E+13 3.78E+11 
HTS1 6.96E+13 3.42E+11 5.96E+13 2.71E+11 7.02E+13 6.30E+11 5.79E+13 5.03E+11 
HTS2 2.23E+13 2.07E+10 1.93E+13 1.39E+10 2.25E+13 2.81E+10 1.97E+13 2.29E+10 
NAA1 1.39E+14 4.96E+11 1.20E+14 3.88E+11 1.22E+14 8.15E+11 1.05E+14 6.27E+11 
NAA2 4.11E+13 - 3.59E+13 - 4.00E+13 - 3.55E+13 - 
NAA3 1.74E+13 - 1.52E+13 - 1.53E+13 - 1.35E+13 - 
RI1 3.53E+14 1.47E+13 2.60E+14 9.05E+12 2.31E+14 1.49E+13 1.69E+14 9.28E+12 
RI2 3.44E+14 1.42E+13 2.57E+14 8.91E+12 2.18E+14 1.47E+13 1.58E+14 9.13E+12 
RI3 2.46E+14 4.03E+12 1.85E+14 2.54E+12 2.10E+14 1.53E+13 1.58E+14 9.56E+12 
RI4 2.48E+14 4.23E+12 1.86E+14 2.82E+12 2.03E+14 1.45E+13 1.52E+14 8.92E+12 
RI5 2.24E+14 3.12E+12 1.67E+14 2.10E+12 2.15E+14 1.55E+13 1.58E+14 9.51E+12 
NGUYEN NHI DIEN, HUYNH TON NGHIEM, LE VINH VINH, VO DOAN HAI DANG 
31 
Fig. 4. Neutron flux profile at the AHR fresh core 
B. Equilibrium Core 
An equilibrium core is dependent on an 
operation strategy, so there may be various 
equilibrium cores according to a reactor 
operating strategy. In this report, an 
equilibrium core is proposed and analyzed to 
meet the established design requirements. 
Fuel Management 
A candidate model for an equilibrium 
core can be easily obtained by considering 
target discharge burnup, cycle length and 
excess reactivities at begin of cycle (BOC) and 
end of cycle (EOC). There are many candidate 
models according to the number of reloaded 
fuel assemblies and the loading pattern. The 
equilibrium cores with 2 or 3 fuel assemblies 
reloaded for an operation cycle (the 9-batch or 
6-batch core) are assessed. The 9-batch cores 
show a high discharge burnup and a good fuel 
economy, but the cycle lengths are less than 30 
days. They look proper for a low utilization 
condition of the reactor. The 6-batch cores with 
a cycle length greater than 30 days are suitable 
for the design requirements, so they are 
selected for evaluating in detail. In the 6-batch 
core, three of the standard fuel assemblies or 
two of the standard fuel assemblies and two of 
the control fuel assemblies are replaced for an 
operation cycle, so the whole core will be 
replaced for 6 cycles according to the loading 
strategy. There are many loading patterns that 
they depend on the fuel management strategy. 
The loading pattern showed in Table IV is 
evaluated in detail. 
Table IV. Loading location of the fuel assemblies for 6-batch cores 
Cycle Assembly Number 
(standard+control) 
Loading Location 
AHR MTR 
1 2+2 H14,H16,C1,C3 H9,H12,C1,C3 
2 3+0 H8,H10,H12 H14,H15,H7 (move H14,H15,H7 to 
H2,H4,H6) 
 a) Thermal Neutron Flux b) Fast Neutron Flux 
CONCEPTUAL NUCLEAR DESIGN OF A 20 MW MULTIPURPOSE RESEARCH REACTOR 
30 
3 3+0 H7,H9,H11 H13,H10,H16 (move H13,H10,H16 to 
H3,H5,H1) 
4 2+2 H13,H15,C2,C4 H8,H11,C2,C4 
5 3+0 H2,H4,H6 H14,H15,H7 (move H14,H15,H7 to 
H2,H4,H6) 
6 3+0 H1,H3,H5 H13,H10,H16 (move H13,H10,H16 to 
H3,H5,H1) 
Once a cycle length and a loading 
pattern are determined, an equilibrium core is 
obtained by numerical iterations. The initial 
core is loaded with the new FAs then the 
burnup calculations are iterated by the loading 
pattern until the parameters of burnup and 
reactivity are stable over 6 cycles. Table V 
presents the calculated results of the average 
burnup and reactivity of 6 cycles for different 
cycle lengths. From these results, it can be 
concluded that the 36 days cycle for AHR and 
34 days cycle for MTR meet the performance 
requirements. 
Table V. Burnup and reactivity of the equilibirum cores 
Reactor type AHR MTR 
Cycle Length (days) 35 36 37 33 34 35 
Average Burnup (%U-235) 
 - BOC 23.43 24.02 24.61 22.38 23.04 23.70 
 - EOC 31.82 32.65 33.47 29.08 29.94 30.81 
 - Discharge 50.35 51.77 53.18 48.65 49.91 51.17 
Reactivity (mk) 
 - BOC (no Xe) 111.9 109.9 107.8 87.8 85.8 83.6 
 - Fuel Depletion 37.5 38.7 39.9 15.1 16.7 18.3 
 - Xenon Buildup 38.1 38.1 38.0 36.2 36.3 36.3 
 - Power Defect 3.0 3.0 3.0 3.0 3.0 3.0 
 - EOC (eq. Xe) 33.4 30.1 26.9 33.5 29.8 26.0 
 - Shutdown Margin 15.0 17.1 19.6 22.2 24.2 26.4 
Power Distribution 
The power distribution is strongly 
dependent on the positions of the control rods 
and it was checked for all possible positions at 
5 cm intervals. The largest maximum linear 
power of the equilibrium cores was observed at 
a 300 mm position of the control rods. The 
power distribution for the equilibrium cores of 
6 cycles at a 300 mm position of the control 
rods was calculated. Table VI shows maximum 
total peaking factors for the 6 cycles equilibrium 
cores and Table VII shows the power 
distributions and peaking factors at the cycle 
that total peaking factor reaches the maximum 
value. The maximum local power peaking factor 
for AHR and MTR are 2.56 and 2.79 
respectively. 
32 
NGUYEN NHI DIEN, HUYNH TON NGHIEM, LE VINH VINH, VO DOAN HAI DANG 
33 
Table VI. Maximum total peaking factor for the equilibrium cycles 
Reactor 
type 
Parameter 
Cycle 
1 2 3 4 5 6 
AHR 
Position of FA H02 C2 H01 H03 C3 H04 
Fq(peaking factor) 2.47 2.56 2.5 2.46 2.56 2.49 
MTR 
Vị trí FA H09 H04 H01 H11 H04 H01 
Fq 2.69 2.77 2.74 2.76 2.76 2.79 
Table VII. Power distribution and peaking factor for equilibrium cores 
(cycle 5 for AHR, cycle 6 for MTR) 
Location 
AHR MTR 
Total Power (kW) Fq Total Power (kW) Fq 
H01 1117 1.93 1167 2.79 
H02 1061 1.72 1158 1.82 
H03 1314 2.21 1229 1.96 
H04 897 1.58 1092 2.51 
H05 1064 1.73 1223 2.02 
H06 1305 2.18 1143 1.82 
H07 811 1.38 1033 2.12 
H08 1071 1.64 1179 2.21 
H09 1329 2.37 985 1.67 
H10 1063 1.76 1066 2.31 
H11 1088 1.65 1213 2.26 
H12 1298 2.29 987 1.66 
H13 1224 1.78 1137 2.01 
H14 991 1.4 1093 1.91 
H15 1184 1.57 1170 2.05 
H16 1050 1.42 1174 1.96 
C1 577 2.44 445 1.28 
C2 491 2.04 527 1.55 
C3 589 2.56 449 1.29 
C4 476 1.95 531 1.56 
Reactivity Coefficients 
To affirm the inherent safety, the 
reactivity coefficients should be determined. 
They include temperature coefficients of fuel, 
light water and heavy water. Physical changes 
of water due to a temperature change could be 
considered in two ways: one is a density 
change, and the other is a cross section change 
for a nuclear reaction. There are the gaps of the 
flow tubes for AHR. The light water in the fuel 
region is to cool the fuel assemblies, and so 
called a ‘coolant’ and the light water in the 
gaps of the flow tubes is called a ‘moderator’. 
Nuclear characteristics of these two light water 
regions are somewhat different, and a heat 
transfer between them is small. Therefore, their 
temperature variations following a power 
change are also different, thus the respective 
temperature coefficients were computed 
separately. The effect of a spectrum hardening 
CONCEPTUAL NUCLEAR DESIGN OF A 20 MW MULTIPURPOSE RESEARCH REACTOR 
34 
of neutrons following a temperature increase 
for heavy water is so small that it can be 
negligible. Table VIII presents the result of 
temperature and void coefficients. From this 
result, they are negative (except temperature 
coefficient of moderator. where almost of 
arriving neutrons are slowed down) and meet 
the functional requirements. The temperature 
variation of moderator is so small, therefore its 
contribution to power coefficient is small. 
Table VIII. Reactivity coefficients of temperature and void 
Parameter AHR MTR 
Fuel temperature coefficient (mk/K) <-0.002 <-0.02 
Light water temperature coefficient (mk/K) 
 - Coolant -0.059 -0.11 
 - Moderator 0.06 
Light water void coefficient (mk/%) 
 0 - 5 % -1.23 -1.79 
 5 - 10 % -1.37 -1.97 
 10 - 20 % -1.48 -2.25 
Heavy water void coefficient (mk/%) 
 0 - 5 % -1.26 -0.79 
V. CONCLUDING REMARKS 
From the functional and performance 
requirements, two reactor models AHR and 
MTR were proposed and investigated. The 
reference reactors are the light water cooled 
and moderated, heavy water reflected and 
open-tank-in-pool type research reactors with a 
20 MW power. 
The maximum fast and thermal neutron 
flux in the core region are greater than 1.0×10
14
n/cm
2
s and 4.0×10
14
 n/cm
2
s, respectively. In 
the reflector region, the thermal neutron peak 
occurs about 28 cm far from the core center 
and the maximum flux is estimated to be 
4.0×10
14
 n/cm
2
s. 
For the equilibrium cores, the cycle 
length is greater than 30 days, the whole core 
will be replaced for 6 cycles, and the assembly 
average discharge burnup is greater than 50%. 
For the proposed fuel management scheme, the 
maximum peaking factor Fq is less than 3. The 
shutdown margins by the 1st and 2nd 
shutdown systems are greater than 10 mk and 
the temperature coefficients are negative 
showed the inherent safety feature. The 
parameters for utilization and for the safety 
aspects of the reactor respectively meet the 
performance and functional requirements. 
The comparison of cores loaded with 2 
different fuel types, AHR and MTR, shows that 
the AHR fuel type core has a little longer 
operation cycle and higher discharge burn up as a 
result. In the safety point of view, the MTR core 
has an advantage because of shutdown margin, 
temperature and coolant void coefficients are 
higher compared to those of AHR core. 
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Conceptual Nuclear Design of a 20 MW 
NGUYEN NHI DIEN, HUYNH TON NGHIEM, LE VINH VINH, VO DOAN HAI DANG 
35 
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