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The design of this supersafe, supercompact fast reactor (RAPID-L) has achieved
fully automated reactor operation. This is the first reactor concept ever established
in the world, while providing advanced safety, compact size and maintenance free. |
| 1) Fundamental Nuclear Research Promotion System (FNRPP) |
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In consideration of the importance of fundamental research in the field of nuclear
energy, JAERI started the "Fundamental Nuclear Research Promotion Program
based on public applications of research topic proposals in FY1998 in order to
stimulate the activation of nuclear research and to foster human resources in this
field. In FY1998, the FNRPP invited applications by proposing the theme of "re-
search into supersafe, supercompact reactors in pursuit of extreme size and weight
reduction" with an aim to introduce new ideas in nuclear technology. JAERI ex-
amined the applications from the viewpoint of their potential impact in the domain
of nuclear science and technology and originality of concept and thus, adopted the
topic on the lithium-cooled supercompact fast reactor using uranium nitride fuel
(RAPID-L), which was proposed by Senior Researcher Mitsuru KAMBE of CRIEPI. |
| 2) Type of the reactor |
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The reactor is a lithium-cooled fast reactor. Figure 1 shows the overall plant con-
figuration assuming its installation at a manned lunar base and Figure 2 shows its
internal construction. Since there is no water on the lunar surface, it is not possible
to adopt a system cooling the waste heat from the reactor using water as on the earth.
Instead, the waste heat is emitted by radiation from radiator panels. With such a
system, the temperature of the radiator panels reaches around 500°C and that of the
primary coolant becomes about 1,100°C. This means that a light water reactor
cannot be used in this situation and that, even when a fast reactor is used, a lithium
coolant featuring a high boiling temperature must be used because the sodium used
in "Monju" has lower a boiling point. |
| 3) Core |
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The uranium nitride fuel is selected because of its high thermal conductivity, high
fuel density and low swelling characteristics. The uranium enrichment is 52% for
the inner region and 60% for the outer region. The fuel rods have an outer diameter
of 8 mm and an effective height of 600 mm, and about 2,700 rods are mounted in the
core. Considering the use under a high-temperature environment, the Fuel cladding
is made of Mo-Re alloy (molybdenum-rhenium). |
| 4) Supersafety by fully automated operation |
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Past large-scale accidents in nuclear plants, such as those at Three Mile Island and
Chernobyl, were caused by human errors. In order to eliminate human errors, the
new reactor has been designed to enable fully automated operation.
While conventional reactors use control rods to control the reactor by adjusting the
absorption of neutrons, RAPID-L has no such control rods. Instead, it is equipped
with the systems developed by CRIEP based on new concepts, such as the LEM
(Lithium Expansion Module) for controlling the reactor, the LIM (Lithium Injection
Module) for shutting it down and the LRM (Lithium Release Module) for starting it
up. Figure 3 (1) to (3) show the principles. All of these modules feature liquid lithium 6, a neutron absorber, sealed inside the closed tubes installed inside the reactor. It is the expansion and contraction of lithium 6 according to changes in temperature that make the unmanned operation of the reactor possible. As the closed tubes are installed with a certain margin in number, there is no impact on safety even if some of them are failed. The absence of control rods has eliminated the probability of reactivity insertion accidents. In the progress of this research, full testing of LEM and LIM was conducted using JAERI's facilities. In October 2000, the criticality test to measure the reactivity worth of lithium 6 was performed using JAERI's Fast Criticality Assembly (FCA). In November 2000, the neutron radiography tests for visualizing the movement of lithium 6 was conducted in the JAERI's research reactor JRR-3M, enabling con- firmation of its function through clear images. |
| 5) RAPID refueling concept |
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One of the advantages of the fast reactor in comparison to the light water reactor is
the long life core. RAPID-L core enables continuous operation for up to ten years. In
addition, "RAPID refueling concept" is adopted. This is the method using a car-
tridge-type integrated fuel assembly (Figure 2) to enable quick and simplified re-
fueling. By the refueling, RAPID-L can be operated another 10 years. CRIEP has already studied the possibility of applying this refueling concept to the terrestrial fast reactor RAPID in which refueling every five years is planned. Such an integrated fuel assembly has definite advantage in view of proliferation resistance. |
| 6) Size and weight reduction |
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The reactor has a total weight of 7.6 tons, which is small and light enough to be
launched by an ordinary launch vehicle. The reactor structure (Figure 2) has a
diameter of 2 meters and a height of 6.5 meters. Other systems than the reactor, including the cooling system, thermoelectric energy conversion system, radiator panel and so on, have a total weight of 8.2 tons. |
| 7) Thermoelectric energy converter system |
| Based on the policy to eliminate movable equipments to improve the reliability, RAPID-L employs a thermoelectric energy converter system for power generation. Thermoelectric converters have already been employed to power the space satel- lites, but their energy conversion efficiency was very low at about a few percent and their output was limited to as low as about 100 watts. If such converters were adopted in RAPID-L, the system weight would be enormous and not realistic. Therefore, RAPID-L adopts the "high-performance thermoelectric conversion system with compliant pads", which is expected to offer twice as much as con- ventional converters. The compliant pads (thermal stress relief pads) are cushions to enhance heat flux to the thermoelectric cells and protecting them. For this research, compliant pads that can be used at 1,000°C were prototyped assuming the possible operating environments of RAPID-L. This system is also expected to be applicable in the utilization of waste heat on the terrestrial systems. |
