25 years of Fugen ATR

1 October 2003

On 29 March 2003, the Fugen heavy water moderated, boiling light water cooled, pressure tube type reactor shut down after 25 years of operation. By Toshiyuki Furubayashi, Takashi Iijima, Susumu Shibuya and Tsuyoshi Okawa

As a country with very limited energy resources, Japan's nuclear policy is based on the establishment of a nuclear fuel cycle that reuses plutonium which is considered to be a domestic energy resource in order to maximise the use of uranium. In accordance with this policy, the Advanced Thermal Reactor (ATR) was designed to burn plutonium, and construction of the Fugen prototype reactor was carried out as a national project.

After the 1995 decision to abandon the ATR project, Fugen continued operation for eight years and completed its role in improving operational management and plutonium technologies.

Progress of Fugen

According to the "General Policy for Development of Power Reactors", PNC took responsibility for development of FBR-Monju and ATR-Fugen. At that time, similar heavy water power reactors to Fugen were under construction in the UK (SGHWR, 100MWe) and in Canada (Gentilly, 266MWe). The former reactor used enriched uranium and the

latter used natural uranium for fuel. Meanwhile, Fugen was designed to use a mixture of recovered plutonium from LWR spent fuel and natural uranium with the aim of avoiding dependence on importing enriched uranium from overseas. Another aim was to decrease the total amount of natural uranium consumption by effective use of plutonium from LWR spent fuel.

Fugen took up two major missions from the beginning of its development. One was to demonstrate full-scale plutonium use to establish the independent nuclear fuel cycle. The other mission was, as described in the term 'self-development', to make progress in Japanese science and technology and to thus contribute towards the development of the country's industrial growth.

At first, Fugen planned to operate with natural uranium because the USA was the only source of enriched uranium supply and domestic enrichment technology was still under development. Later, Fugen shifted the role from a plutonium burning-reactor using LWR recovered plutonium to a complementary reactor in the transition from LWR to FBR technology. Although the development of Fugen was in the middle of that transition, the ATR's flexible core characteristics made it possible to adapt to these roles without changing the type or constitution of reactor.

However, with the advances in plutonium use in LWRs, it became clear that LWRs could almost substitute for the role of the ATR in burning plutonium. This resulted in the termination of the ATR project, and operation of Fugen ceased in March 2003.

Self-Development of Fugen

Self-development of Fugen aimed to:

• Design and establish Japanese major equipment technologies.

• Carry out research and development specifying what and when technical information was required.

• Use existing technologies with evaluation and verification as needed.

• Construct large-scale testing facilities and operate them as soon as possible in order to implement the above aims.

The full-scale testing facilities (deuterium critical assembly, heat transfer loop, component test loop and ATR safety test facilities) were constructed at PNC's Oarai Engineering Center (OEC) and many kinds of experiments were implemented there. Acquired data for designing, such as nuclear characteristics in the core, heat transfer mechanisms, coolant behaviour in accident conditions as well as in normal conditions was analysed, evaluated and verified. This data formed the basis of the approach and development of design calculation codes. A number of experimental, testing and improvement work was also carried out on the original design of the equipment that comprised ATR's unique core.

Due to these efforts, equipment performance, credibility and durability were well demonstrated. In addition, Fugen's operation provided data that fed back to prove the design and approach, and to enable further improvement. In this way, the self-development spirit was embodied in the interconnected activities of OEC and Fugen, resulting in the development of systemised ATR technologies.

Features of Fugen

Fugen is a 165MWe reactor described in the Table below (heavy water moderated, light water cooled, pressure tube type). It is the only plant to load MOX fuel in Japan. As the reactor uses heavy water to moderate neutrons, its core structure is different from that of a LWR. Otherwise, it has a similar system to BWR in that steam generated at the core directly turns the turbine for power generation.

The heavy water as moderator is kept in a calandria tank of approximately eight metres in diameter. In the tank, 224 pressure tubes, 12cm in diameter are installed vertically. One fuel assembly is set in each pressure tube, where light water flows as a coolant. The pressure tubes are surrounded by calandria tubes to make a double-tube structure so that heavy water as moderator and light water as coolant are completely separated. Carbon dioxide gas flows in the space between the pressure tube and the calandria tube to mitigate the heat transfer from the fuel side to heavy water side and to detect the leakage of the coolant. Heavy water is cooled by circulation to maintain its fixed temperature (49ºC at the reactor entrance) in the core. Furthermore, boric acid is added to the heavy water to control the reactivity in the core. Helium is used as a cover gas for maintaining the purity of heavy water. In total, there are 49 control rods, which are inserted from the reactor top into the heavy water via guide tubes installed in the calandria tank.

The reactor cooling system consists of two independent reactor-cooling loops with 112 fuel channels for each. Coolant is supplied to the core through an inlet feeder tube that distributes coolant to each channel from a lower header of each loop. In the core, the cooling water is heated up to two phase flow, which enters a steam drum, the steam goes directly to a turbine, and the heated water, along with feedwater, returns to the lower header through four downcomers pressured by two recirculation pumps.

Fuel assemblies

The fuel assembly consists of 28 fuel rods and has a cylindrical structure for the insertion into pressure tube. Fugen used MOX fuel along with enriched uranium fuel. Although Fugen used fuels with relatively small quantities of fissile material 1.4% to 1.5% (assembly average) in the initial operation it later used 1.9% to 2.0% fissile fuels. In addition, experimental MOX fuel assemblies, consisting of 36 fuel rods, have been irradiated as a part of the fuel development of the ATR demonstration plant. The burnup of these experimental fuel assemblies increased with higher plutonium content, from 2.4% to 3.5% fissile material (assembly average), compared to Fugen's standard fuel assemblies.

MOX fuel in ATR

In terms of safety and operation, use of MOX fuel in LWR has been verified (see Panel above). Here, the inherent advantages of plutonium use in Fugen are compared with the use in LWRs.

Neutron cross section and nuclear reaction

Neutrons produced by nuclear fission are moderated sufficiently to thermal neutron energy range in low-temperature (60ºC) heavy water region away from fuel. Hence they are relatively unsusceptible to the resonance absorption by plutonium. The number of the neutrons produced by plutonium fission can be used in the same energy range as U-235. Additionally, better nuclear performance of Pu-241 than Pu-239 in the thermal neutron range compensates neutron absorption followed by the increase of Pu-240 and Pu-242. As long as the sum of fissile plutonium and uranium are kept at a constant value, it is possible to gain equal burnup although the isotopic composition of plutonium used for MOX fuel differs.

Effect of Am-241

Most neutrons are moderated sufficiently in heavy water away from fuel. Fuel reactivity is not affected by resonance absorption of the neutrons with Am-241. On the other hand, in the case of LWR, inadequate moderated neutrons in the thermal energy range exist in the fuel region, since the neutrons are moderated mainly in light water close to fuel assemblies. Therefore, the LWR is susceptible to resonance absorption of Am-241 compared to Fugen.

Plutonium enrichment

In the case of Fugen, decrease of fissile plutonium and increase of Am-241 with higher order plutonium used for MOX fuel has little effect on fuel burnup. For the above reasons, fissile plutonium enrichment is not needed, even if isotopic compositions of plutonium become higher order. On the other hand, in LWR, there is a high possibility of nuclear fission reaction by neutrons, which is not adequately moderated; accordingly when higher order plutonium is used, plutonium enrichment needs to be increased.

Relation between MOX fuel loading ratio and control rod worth

ATR fuel assemblies loaded at intervals of large heavy water region make fuel assemblies independent from each other. For this reason, there is little effect of the shift of neutron spectrum (from thermal neutron energy to the higher neutron energy) on uranium fuel by thermal neutron absorption with plutonium in MOX fuel, when uranium fuel and MOX fuel coexist. Furthermore, MOX fuel loading has little influence on control rod worth because control rods are inserted into the heavy water rich in thermal neutrons. For this reason, full loading of MOX fuel is possible without design change of the reactor.

MOX benefits

For the past 25 years since Fugen attained the first criticality with 22 MOX fuel assemblies in March 1978 Fugen continued operation loading 772 MOX fuel assemblies (including 1850kg plutonium, metal weight) and 687 uranium fuel assemblies. Although Fugen experienced one failure from a uranium fuel assembly in April 2002, no failure was detected on MOX fuel.

Fugen has no restriction on loading of MOX fuel in the reactor core. The loading ratio, from 34% to 72% according to the supply and demand status of plutonium, shows flexibility of Fugen's core. As for the disposition of MOX fuel in the core, 96 MOX fuel assemblies were loaded in the centre of the core in the early stage of operation to conform the characteristics of plutonium core.

The maximum burnup of regular MOX fuel at Fugen is approximately 20GWd/t, which is half of uranium fuel burnup at recent LWRs. However the experimental MOX fuels irradiated at Fugen as part of the ATR demonstration plant's fuel development, reached 38GWd/t.

Plutonium used for MOX fuel in Fugen was imported until September 1981, when Fugen started using the plutonium recovered at Tokai reprocessing plant using spent fuel from domestic LWRs. As for uranium fuels loaded after December 1982, enriched uranium processed at Ningyo-Toge Works has been used. Additionally, in May 1984, four MOX fuels were loaded of which base material was made from uranium recovered from spent fuel from domestic LWRs at Tokai Works. Since then, recovered uranium has also been used for MOX fuel fabrication.

Spent MOX fuel of Fugen was reprocessed at Tokai Works and the recovered plutonium from these fuels was refabricated into four MOX fuel assemblies, which were newly loaded to Fugen in May 1988. In this way, the nuclear fuel cycle was completed. This means Fugen proceeded to establish and demonstrate the nuclear fuel cycle at reactor scale.

Fugen produced a total of 21.9TWh and recorded an average load factor of 62% after 25 years of operation. These records were achieved based on

six-month operation cycles with two refuellings in one year. These are comparable with commercial LWRs with one-year operation cycles, and are unparalleled achievements in the world as a single prototype reactor.

There was no successor to the ATR project, in terms of going beyond the prototype stage in Japan. However, its contribution to the industry and in plutonium use played a great role in the history of nuclear development in Japan.

Industry self-development

In the early stage of Japan's nuclear development, major equipment for Fugen (recirculation pump, lower header check valve and main steam isolation valve) were domestically designed and manufactured.

The development activities for these parts contributed to the build up of the confidence of researchers and engineers, and to the Japanese technological advances. It also fostered manufacturing corporations of related equipment. In addition, technologies used at large-scale testing facilities continued to improve. Development of Fugen contributed greatly to the improvement of Japan's nuclear field level and reinforced industry.

Plutonium use

The 772 MOX fuel assemblies loaded at Fugen in the 25 years after the first criticality represents one-fifth of loaded MOX fuel in LWRs in the world. In addition, this is the largest number of loaded MOX fuel in a single reactor in the world. This is also a great achievement in the nuclear fuel cycle based on the principle of Japan's nuclear energy policy.

The nuclear fuel cycle was completed reusing plutonium recovered from LWR fuel and Fugen's MOX fuel at the Tokai reprocessing plant. Plutonium use in Fugen served as the driving force to establish the basis of nuclear fuel cycle technology in Japan (since it required the accumulation of reprocessing technology and technology of MOX fuel fabrication). In this way, Fugen played a significant part in the development of nuclear power in Japan.
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