炉物理の研究第65号（2013年3月）＜炉物理部会賞受賞記念寄稿＞

Design study on small and simple nuclear reactors intended for large-diameter
Si crystal doping using PWR fuel assemblies
Byambajav Munkhbat1,* and Toru Obara2
1

Department of Nuclear Engineering, Tokyo Institute of Technology
Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology
2-12-1-N1-19 Ookayama, Meguro-ku, Tokyo, Japan, 152-8550
Corresponding email: (1) munkhbat.b.aa@m.titech.ac.jp, (2) tobara@nr.titech.ac.jp
2Introduction
Nowadays, the use of electric vehicles such as hybrid cars and electric trains is increasing rapidly.
Each such vehicle requires a considerable amount of semiconductor devices, suggesting an ever-growing
demand for such devices in the near future. Thus, the mass production of the semiconductor material is
becoming a very important issue for many manufacturers. There are several methods for mass production,
but Neutron Transmutation Doping of Silicon (NTD-Si) is the most promising method for the productionof silicon based semiconductor. Natural silicon has three stable isotopes: Si-28 (92.3%), Si-29 (4.7%) andSi-30 (3.1%), and these stable isotopes are homogenously mixed in natural silicon crystal. The principleof NTD-Si is that conversion of a small amount of natural stable isotope Si-30 into the stable isotope P-30
by nuclear reaction; uniformly doped silicon crystal can therefore be expected after irradiation[1].
The production of doped Si is usually carried out in research reactors because the suitable
irradiation condition exists in these types of reactors. In other words, the research reactors can easily bemodified for this purpose; therefore, all the silicon irradiation facilities in the world have been built
around these research reactors. The current NTD market is dominated by 5 and 6 inch ingots, and theshare of 4 inch ingots has been decreased drastically. Recently the demand for irradiation of 8 inch ingots
has grown, and its market share could expand in the future. Since the typical diameter of silicon ingots
has become much larger, a large amount of silicon can be irradiated at an irradiation site. The CZ-Si
market has jumped from 8 to 12 inches. Therefore, the next wave of NTD ingots would be 12 inchesrather than in between 8 and 12 inches such as a 10 inch ingot. It seems that the industry is now
developing technology for 18 inch CZ-Si production but NTD for 18 inch ingots may be difficult tosatisfy not only in terms of irradiation uniformity but also accommodation at reactors[2].
The doped Si is widely used in various fields such as high quality semiconductor devices, sensors
and other standard devices, but mainly used for high power devices such thyristor and several types of
insulated gate bipolar transistors (IGBT). These high power devices are mostly adopted in hybrid electricvehicles, carrying both conventional and electric propulsion systems, for controlling the traction. The
performance of such vehicles such as motor power is important, and it depends on the performance of thekey component like IGBT. It is known that the doped Si is suitable base material for high performance
IGBTs because of its higher uniformity compared to others doped by conventional methods. Rapid
increase in demand of hybrid cars is expected due to the high cost of fuel and environmental concerns. A
recent survey made by Korean Atomic Energy Research Institute (KAERI) indicated that 50 million
hybrid electric vehicles would be produced, and almost 1000 tons of NTD-Si would be needed in2030[3-4].
At present, the worldwide capacity of the NTD-Si facilities is estimated to be 150Ң180 tons perannum. This capacity cannot be increased drastically because most research reactors with NTD facilities
were constructed many years ago and they have little potential for expansion; in addition, number of
research reactors to be constructed in the future is few. Another important issue is that the present
research reactors are not NTD-dedicated facilities, and thus a stable and adequate supply is not expected.
If the hybrid electric vehicles increase according to the current forecast, the existing research reactors
cannot supply future demand. As such, a new doping facility with a large irradiation capacity for NTD-Si
may need to be constructed to ensure an adequate supply of doped silicon.
Purpose of study
The purpose of the study was to design small and simple nuclear reactors for large-diameter(12 inches) NTD-Si using PWR fuel assemblies. By using conventional fuel assembly, which is1

炉物理の研究第65号（2013年3月）

commercially available fuel, the reactor concept can be easily realized, and the supply of the fuel can also
be assured. The proposed reactor concepts are for industry, not for research, and they need to beeconomically profitable; therefore, the stable operation and reliable fuel supply are major requirements.
There are several facilities fabricating PWR fuel assemblies in the world, and it can be said that PWR fuelsupply is more reliable than supply of plate type fuel which is mainly used in the research reactors.
Another important advantage of using conventional fuel assembly is the spent fuel from reactor, and thespent fuel can be sent and accepted at existing fuel reprocessing facilities.
Design concepts
Two design concepts were proposed. The neutronic and thermal hydraulic analyses were
performed to obtain the optimum core composition, operating period, necessary condition for uniform
doping and the reactor production rate. Criticality and burn-up analyses were performed using thecontinuous energy Monte Carlo code MVP/GMVP II[5] with the JENDL-4.0[6] data library. The coreburn-up calculation was carried out by auxiliary code MVP-BURN[7]. The single-channel steady-state
thermal hydraulic analysis was performed using COMSOL Multiphysics[8] which is a finite-element
analysis, solver, and simulation software. A neutron transport calculation was performed in the reflector
region by the continuous energy Monte Carlo code MVP/GMVP II with the JENDL-4.0 data library todetermine the necessary condition for uniform irradiation.
First one was a design concept of a small and simple nuclear reactor for large-diameter NTD-Si
using full-length conventional PWR fuel assemblies[9-10]. The idea was to use commercially available
conventional PWR fuel (fabricated for power reactors) directly without any modification. A conventional
17×17 PWR fuel assembly was used. The reactor power was 15 MWth, and operating pressure was 1 atm.
The excess reactivity was managed by a combination of the burnable poison Gd 2O3 and control rod. The
criticality and burn-up calculation results showed that the operating period of the reactor concept wasover 18 years without refueling. The thermal hydraulic analysis showed that the heat removal from coreunder 1 atm operating pressure was possible, and no need to pressurize the reactor systems. The necessary
condition for uniform doping was determined. Si ingots, 12 inches in diameter, up to 20 cm in height
could be doped with sufficient uniformity. The reactor semiconductor production rate was estimatedassuming the reactor would be in 24–hour operation for 8 months per year, and the estimated production
rate varied between 111 tons/year and 140 tons/year for 50 Ωcm target resistivity depending on thecontrol rod positions. The disadvantage was the height of the core.
Second one was a design concept of a small and simple nuclear reactor for large-diameter
NTD-Si using short PWR fuel assemblies[11]. The core height was reduced by using shorter assemblies.
But the irradiation capacity was reduced drastically. Thus, the reactor design was modified and optimized
for maximum capacity as possible. The criticality and burn-up calculation results showed that theoperating period of reactor concept was over 3 years without refueling. The thermal hydraulic analyses
showed that the heat removal from core under 1 atm operating pressure was possible. Large Si ingots with12 inches in diameter were able to be doped with sufficient uniformity. The estimation of reactor
production for 50 Ωcm target resistivity showed that the rate varied between 48 tons/year and 70
tons/year depending on the position of the control rod. The study showed that short PWR fuel assemblies
could be used for this concept, and the reactor production rate was a good rate, as compared to the present
worldwide capacity.
Conclusions
The reactor concept using full-length conventional PWR fuel assemblies had a very goodperformance, for instance, a long operating period and a large production rate, but the size of the core
could become big which could lead a large construction cost (initial investment). On the other hand, thereactor concept using short PWR fuel assemblies might require a less construction cost due to the smallercore, and the production rate was not that bad, but the operating period was much shorter than firstconcept. It should be mentioned here that only the fuel assemblies may need to be replaced every 3 years.
This concept with short assemblies may be attractive for industry if the shorter PWR assemblies can be
fabricated without major difficulties in existing fuel manufacturers, needless to say that they aretechnically capable to produce such assemblies.
These concepts were intended for industry, not for research. The PWR fuel assemblies were used
as a fuel in both concepts to ensure the stable and reliable supply of the fuel. The study showed that it2

炉物理の研究第65号（2013年3月）

could be possible to design small and simple reactors for large-diameter NTD-Si using such PWR fuelassemblies. If these concepts were realized for semiconductor industry, it could make a great contribution
in the worldwide supply of doped Si.
References
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M. Tanenbaum, A.D. Mills, Preparation of Uniform Resistivity n-Type Silicon by Nuclear
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M.S. Kim, S.J. Park, and I.C. Lim, Estimation of Future Demand for Neutron-Transmutation-Doped
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T. Singh, A. Bhatnagar, K. Singh, V.K. Raina, Neutron Transmutation Doping Technology of Siliconand Overview of Trail Irradiations at CIRUS Reactor, BARC/2007/E/023, Bhabha Atomic ResearchCentre, Mumbai, India, 2007.

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COMSOL-3.4, Multiphysics Modeling and Engineering Simulation Software, COMSOL AB, 1994–2007; software available at http://www.comsol.com/.

9.

B. Munkhbat, T. Obara, Design concept of a small nuclear reactor for large-diameter NTD-Si using a
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10. B. Munkhbat, T. Obara, Design simplification of a small nuclear reactor for large-diameter NTD-Si
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