An application of deuteride moderator for fast reactor cores is proposed for power flattening that can mitigate thermal spikes and alleviate the decrease in breeding ratio, which sometimes occurs when hydrogen moderator is applied as a moderator. Zirconium deuteride is employed in a form of pin arrays at the inner most rows of radial blanket fuel assemblies, which works as a reflector in order to flatten the radial power distribution in the outer core region of MONJU. The power flattening can be utilized to increase core average burn-up by increasing operational time. The core characteristics have been evaluated with a continuous-energy model Monte Carlo code MVP and the JENDL-3.3 cross-section library. The result indicates that the discharged fuel burn-up can be increased by about 7% relative to that of no moderator in the blanket region due to the power flattening when the number of deuteride moderator pins is 61. The core characteristics and core safety such as void reactivity, Doppler coefficient, and reactivity insertion that occurred at dissolution of deuteron were evaluated. It was clear that the serious drawback did not appear from the viewpoints of the core characteristics and core safety.
In order to flatten radial power distribution in fast reactors, ordinary fast reactor cores employ two enrichment zones where outer zone has higher plutonium enrichment. Even in such design the power is dropped at the outer zone of the outer core due to the neutron leakage at the peripheral regions. Zirconium hydride has advantages of high moderation ratio as well as the stability to neutron irradiation as no gas emission occurs at neutron absorption. On the other side, it sometimes induces thermal spikes at the fuel pins adjacent to the moderator zones and reduces breeding ratios due to the large absorption cross section of hydrogen contained even in the fast reactor hard spectrum. However, such features of generating thermal spikes will be useful to increase the power at low power region such as core peripherals if the moderator is appropriately arranged, and it can provide the flattening in power distributions in fast reactors.
There are many studies [
The author proposes zirconium deuteride instead of zirconium hydride for flattening power distributions because it has a relatively high moderation ratio and very small neutron absorption cross sections. In this paper, an application of deuteride moderator for fast reactor cores is proposed for power flattening that can mitigate thermal spikes and alleviate the decrease in breeding ratio, which sometimes occurs when hydrogen moderator is applied as a moderator. The power flattening can be utilized to increase core average burn-up by increasing operational time or reducing fuel inventory which can be achieved by the reduction of core height, for example, under the same restriction of the maximum linear heat rate. In this study, the power flattening and the increase in the core average burn-up were evaluated under the above assumptions for MONJU core. The influences of core characteristics, such as sodium void reactivity, Doppler coefficient, and control rod worth by the introduction of deuteride moderator were evaluated. In the safety aspect, reactivity insertion occurred at dissolution of deuteron was also evaluated.
The evaluated moderators include three types of zirconium compound. One is zirconium deuteride, another is ordinary zirconium hydride and the other is zirconium hydride of 25% smear density, where the effective volume ratio of the zirconium hydride is diluted to 25% of the inside cross sectional area of the pin. Such low smear density pins are usually fabricated by employing hollow pellets or pore rich materials.
The reason to select this arrangement is the mitigation of thermal spike, which sometimes occurs even in the fast reactor core including hydride materials. If the moderator is located at core peripherals, thermal spike is not important because linear heat rates of fuel pins are relatively low. This thermal spike can be utilized to enhance the flattening of power distribution if it occurs at low-power regions such as core peripherals.
The moderator pin arrangements in the moderator assembly are selected as parameters. The pin arrangements are shown in Figure
Pin arrangements in moderator assembly.
The specifications of the MONJU core [
Specification of MONJU core.
Items | Unit | Spec. |
---|---|---|
Reactor thermal power | MWt | 714 |
Core configuration | — | Homogeneous 2 region core reactor |
Operation cycle length | days | 123 |
Core height | mm | 930 |
Number of fuel assemblies (IC/OC/total) | — | 108/90/198 |
Number of moderator assemblies | — | 54 |
Total number of pins in the moderator assembly | 61 | |
Number of Moderator pins | 9 or 24 or 61 | |
Number of radial blanket pins | 52 or 37 or 0 | |
Pin diameter | mm | 10.6 |
Diameter of moderator | mm | 9.5 |
Moderator material | Zr deuteride |
The nuclear analysis method is listed in Table
Analytical method.
Items | Methods | Notes |
---|---|---|
Computation method | Three-dimensional continuation energy Monte Carlo analysis code |
1,200,000 neutron histories with 120 Batches, |
Nuclear data | JENDL-3.3 library | |
Calculation model | Pin heterogeneous model |
Typical neutron history number employed is 1000000 divided to 100 batches and other 20 batches are used for generating the initial source distribution. The statistical error of pin power is about 2% in 1
MVP-BURN (burn-up routine for MVP) [
Figure
Core arrangement of MONJU.
For pin-wise power distribution in the second row of the outer core has been evaluated for each pin row as shown in Figure
Details of pin arrangement at outer core region.
Figure
Pin wise power peaking factor in the 2 row assembly of the outer core.
The zirconium hydride (ZrH1.7) case shows the largest spike at the outer pins which peak power is about 3 times of that of the inner most row pins. On the other hand, the peak of the ZrD1.7 case does not exceed the power of the inner pins, and the power distribution over the assembly is flattened relative to that of no moderator case. The 25% smeared zirconium hydride (25% ZrH1.7) case has small peak at the outer pins but it has a dip behind the peak.
Figure
Assembly averaged pin power distribution in radial direction.
Tables
(a) Dependence of core performances on the number of ZrD1.7 moderator pins. (b) Dependence of core performances on the number of ZrH1.7 moderator pins.
Number of moderator Pins | Power peaking factor in core | Power peaking factor of 2nd row assembly in outer core | Power peaking factor in blanket assembly | Increase of core average burn-up (%) |
---|---|---|---|---|
0 | 1.233 | 1.173 | 1.420 | — |
9 | 1.253 | 1.130 | 1.703 | 0.9 |
18 | 1.231 | 1.095 | 1.251 | 2.9 |
24 | 1.230 | 1.141 | 1.927 | 1.2 |
61 | 1.202 | 1.312 | — | 7.0 |
Number of moderator pins | Power peaking factor in core | Power peaking factor of 2nd row assembly in outer core | Power peaking factor in blanket assembly | Increase of core average burn-up (%) |
---|---|---|---|---|
0 | 1.233 | 1.173 | 1.420 | — |
9 | 1.236 | 1.373 | 2.223 | 4.7 |
18 | 1.204 | 1.890 | 1.649 | 7.4 |
24 | 1.223 | 1.379 | 2.035 | 4.9 |
61 | 1.264 | 2.748 | — | 8.2 |
Comparison of breeding ratio between cores with and without deuteride moderator pins in Table
Comparison of breeding ratio between cores with and without moderator.
Core | Cycle | Breeding ratio |
---|---|---|
Core with 61 deuteride moderator pins | Equilibrium cycle | 1.02 |
Core without moderator pins | Equilibrium cycle | 1.11 |
The influences of core characteristics, such as sodium void reactivity, Doppler coefficient, and control rod worth by the introduction of deuteride moderator pins were evaluated.
Comparison of Doppler coefficient between the cores with and without ZrD1.7 moderator pins is shown in Table
Comparison of Doppler coefficient between cores with and without moderator.
Core | Cycle | Doppler coefficient |
---|---|---|
Core with 61 deuteride moderator pins | Equilibrium cycle |
|
Core without moderator pins | Equilibrium cycle |
|
Comparison of sodium void reactivity between the cores with and without ZrD1.7 moderator pins is shown in Table
Comparison of sodium void reactivity between cores with and without moderator.
Core | Cycle | Sodium void reactivity |
---|---|---|
Core with 61 deuteride moderator pins | Equilibrium cycle | 1.05 |
Core without moderator pins | Equilibrium cycle | 1.14 |
Comparison of control rod worth between the cores with and without ZrD1.7 moderator pins is shown in Table
Comparison of control rod worth between cores with and without moderator.
Type of control rods | Core | Control rod worth |
---|---|---|
Main control rods |
Core with 61 deuteride moderator pins | 8.13 |
Core without moderator pins | 8.51 | |
Back-up control rods (6 rods) | Core with 61 deuteride moderator pins | 6.08 |
Core without moderator pins | 6.65 |
Figure
Neutron spectra of outer row pins of outer core 2nd row fuel assembly.
The linear power of the moderator was two orders below that of the fuel pins around them because the main energy source is gamma deposition generated at the core fuel. The neutrons by gamma-n reactions in ZrD1.7 are roughly estimated five orders below those of neutron generations of MONJU at the normal operation.
One problem of the hydride use in fast reactor is positive reactivity insertion that occurred at the dissolution of hydrogen. The dissolution of the deuterium will also occur in zirconium deuteride, though the penetration rate of the deuterium across the cladding is considered to be smaller than that of hydrogen due to the larger atomic mass. The reactivity insertion by the dissolution of deuterium was evaluated. The reactivity insertion by the dissolution of deuterium is about
Reactivity insertion occurred at dissolution of deuteron.
Ratio of deuteron in moderator pin | Reactivity ( |
---|---|
1.0 | — |
0.9 | 0.029 |
0.7 | 0.103 |
0 | 0.255 |
Deuterium will be transmuted to Tritium by neutron absorption where the generation rate of Tritium is estimated to be about 2 decades smaller than that generated from B-10 (
An application of deuteride moderator for fast reactor cores is proposed for power flattening that can mitigate thermal spikes and alleviate the decrease in breeding ratio, which sometimes occurs when hydrogen moderator is applied as a moderator. Zirconium deuteride is employed in a form of pin arrays at the inner most rows of radial blanket fuel assemblies, which works as a reflector in order to flatten the radial power distribution in the outer core region of MONJU. The power flattening can be utilized to increase core average burn-up by increasing operational time. The core neutronics has been evaluated with a continuous-energy model Monte Carlo code MVP and the JENDL-3.3 cross-section library. The result indicates that the power peaking factor in the core is the smallest when the number of deuteride moderator pins is 61, and the core average burn-up can be increased by about 7% relative to that of no moderator core due to the power flattening. Major core characteristics and core safety including sodium void reactivity, Doppler coefficient, control rod worth and reactivity insertion that occurred at dissolution of deuteron were also evaluated. The breeding ratio of the deuteride case is a little less than that of no moderator case. It is also clear that the influence of major core characteristics and safety aspect by the introduction of deuteride moderator is small.
The present study indicates that the use of deuteride moderators in the blanket region has a large potential to improve performances of FBR cores.
The author deeply appreciates Dr. T. Yokoyama of Toshiba Nuclear Engineering Services Corporation, Mr. Akira Nagata and Mr. Y. Tsuboi of Toshiba Corporation, and Mr. S. Aoyagi of ISA Corporation for their cooperation and advices. Present study includes the results of “Study on Optimization of Core Arrangement of Core Fuel and Blanket Fuel on FBR” entrusted to “Tohoku University” by the Ministry of Education, Culture, Science and Technology of Japan (MEXT).