炉物理の研究

第 64 号（2012 年 3 月）

＜炉物理部会賞受賞記念寄稿＞

Simplified Small Pebble Bed Reactor and the Use of Natural Uranium
Dept. of Nuclear Engineering, Tokyo Institute of Technology
Dwi Irwanto

Nowadays, natural uranium resources are decreasing rapidly because of its utilization as
nuclear fuel; therefore, innovative concepts are needed to use it more efficiently. A fast reactor isa good option for efficient natural uranium use, but the slow developments of this type of reactor
forces us to look at other available options immediately. However, almost all commercial
reactors operating today are the thermal reactor type, which is considered a proven technology
due to its operation history, and many studies and experiments have been conducted on thisreactor type. Moreover, high-temperature gas reactors have more advantages than theconventional water-moderated thermal reactors. The thermal efficiency of these reactors ishigher than that of light water reactors, and their high output temperature can be utilized forother industrial purposes such as hydrogen production as well as for district heating. Pebble bedreactors (PBR) offer even further advantages including the efficient use of natural uranium,
which can be seen from the facts that PBR has small excess reactivity during reactor operationand that the moderator material (graphite) has a small absorption cross section. In addition, asmall PBR is the most promising reactor system to adopt inherent and passive safety features:
namely, the coating of fuel particles, which limits the release of radioactive material into theenvironment, and the use large amount of graphite, which make high heat capacity of the core, asa moderator and reflector. Currently, these safety features have become a very important issue in
avoiding severe nuclear accidents due to active system failure such as failure of the coolingpump system.
However, the high cost and complexity of pebble bed reactors, especially the unloadingmachinery, forces us to think about innovations for this reactor design that would be simpler bothin term of design and operation and at the same time optimize the use of uranium resources by
increasing the burnup value. In our study, we introduce and analyze an innovative design for apebble bed reactor that could be constructed and operated at a lower cost, have easier1

炉物理の研究第 64 号（2012 年 3 月）

construction and maintenance processes, and increase efficiency of uranium use in the reactor
system. Namely, we simplify the pebble bed reactor by removing the unloading devices from thesystem. For such kind of system, a suitable fuel-loading scheme is the Peu a Peu fuel loading
scheme.

The purposes of our study were to design a simplified pebble bed reactor by removing theunloading devices from the system and then to optimize the peu à peu fuel loading scheme in thereactor system by performing optimization in term of reactor configuration and fuel composition,such as uranium enrichment and packing fraction of coated fuel particle inside the pebble ball; sothat the system could achieve good burnup characteristics.
Moreover, to improve the burnup characteristics of this reactor design while at the sametime using natural uranium more effectively, we also considered the use of thorium. Thoriumbased fuel is gaining interest for many researchers in the nuclear industry as an alternative to theexisting uranium-plutonium-based fuel. The abundance of thorium resources worldwide, which
is several times larger than the uranium resources, could be expected to significantly extend therole of nuclear power as one of the world's energy resources. Many studies have been performed
to examine the use of thorium as a nuclear fuel, but additional time and effort are needed to makeits use on the industrial scale possible. In the meantime, using natural thorium with existing
technology, for example, as a fertile material mixed with existing uranium dioxide fuel, is one
way to extend the nuclear horizon by reducing the level of natural uranium utilization while
waiting for the thorium technology to become more mature.
In our study, we trying to optimize the burnup characteristics of a small simplified PBR
by introducing thorium into the fuel configuration as a fertile material. We also performed
optimization of the thorium fraction in the UO2 fuel, of uranium enrichment and of the packing
fraction of coated fuel particles inside the pebble ball, so that the system could achieve better
burnup characteristics and use scarce uranium resources more efficiently.
As the results, calculation and optimization for several small simplified PBR designs
have been performed using both uranium fuel and mixed thorium-uranium fuel. These reactor
designs have 110 MWt power and could achieved high burnup value compared to that ofconventional PBRs. In terms of the use of natural uranium, from this study, it was confirmed2炉物理の研究第 64 号（2012 年 3 月）

that these design could decrease the use of natural uranium for every electric power generated
per year as around 33%.

Regarding the RPD (Reactor Physics Division) Award 2011, I am very happy and feelhonored to received such kind of acknowledgement from the nuclear society in Japan. Thus, Iwould like to express my special appreciation to the Reactor Physics Division of AESJ. I hopesomeday, I could share the experiences interacting with AESJ to the nuclear community in mycountry. In addition to that, I want to express my deep gratitude to Obara-sensei as my academic
advisor in Tokyo Institute of Technology for all his efforts all this time, and also for members ofthe Obara-lab. I hope that this kind of award could increases motivation of the students wholearns reactor physics, including me, to work harder and do many good researches in the future
and make many contributions to the peoples and society. Even though that right now nuclear
industry facing a great challenge due to a certain nuclear accident, however I hope and I believethat the condition will be recovered and nuclear industries and it related field will be raise again,
soon. Because I believed that the peacefully use of nuclear energy have very important roles forhuman life in the world, including Japan and also for a developing country like my country,
Indonesia, which is hoping to introduce nuclear energy as one of the energy source in the near
future. Hopefully.

Thank you,
Dwi Irwanto
Dept. of Nuclear Engineering, Tokyo Institute of Technology
irwanto.d.aa@m.titech.ac.jp

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