Yoshitomo Inaba

Establishment of floating support technology applied to high-temperature components and piping in HTTR

In the primary cooling system of the High Temperature Engineering Test Reactor (HTTR) with an outlet coolant temperature of 950°C, high-temperature components and piping such as an intermediate heat exchanger and coaxial double piping reach very high temperature, and large and complex thermal displacements arise in them. In order not only to absorb the thermal displacements but also to withstand earthquakes, the HTTR has adopted a new three-dimensional floating support system. In the limited space of the containment vessel, the support system can support the components’ and pipings own weights and follow the thermal displacements and have seismic capacity. On the other hand, the adoption of the support system was unprecedented in nuclear plants. Thus, the effectiveness of the support system was demonstrated through the HTTR operation. In this paper, by using the HTTR operation data, the thermal displacement behavior of the high-temperature components and piping is investigated, and the behavior and characteristics are simulated numerically. In addition, the aftermath of the Great East Japan Earthquake on the HTTR is confirmed. As a result, the effectiveness of the three-dimensional floating support system adopted by the HTTR is verified.

Evaluation of maximum fuel temperature in HTTR

In order to ensure the thermal integrity of fuel in the high temperature engineering test reactor (HTTR), it is necessary that the maximum fuel temperature in the normal operation is to be lower than a thermal design limit of 1495°C. In the core thermal and hydraulic design of the HTTR, the maximum fuel temperature was estimated to be 1492°C, which satisfied the thermal design limit. However, the estimated temperature was derived by using hot spot factors with a large safety margin for the consideration of uncertainties in the design stage without the HTTR practical operating data, and thus there is no doubt that the estimated temperature includes excessive conservativeness. In order to obtain the maximum fuel temperature with appropriate conservativeness, the maximum fuel temperature has been re-evaluated on the basis of the HTTR operating data. In this paper, the random factors of the hot spot factors are revised by using the HTTR first fuel fabrication data, and the new maximum fuel temperature is estimated. As a result, the estimated maximum fuel temperature can be reduced to 1424°C. The reduction of the maximum fuel temperature leads to a larger thermal margin in nuclear and fuel designs.

Investigation on dust captured by quintuple filters installed upstream of primary gas circulators in HTTR

A main objective to install filters upstream of primary gas circulators in the high temperature engineering test reactor (HTTR), besides having a primary helium purification system, is the reduction and removal of circulating dust in the primary circuit. A problem encountered with the filters during the initial operations of the HTTR was that the differential pressure across the filters had increased excessively over the duration of the operations so that the differential pressure would be expected to exceed the limit value regulated in the HTTR operation manual. It was speculated that either the carbon traced back chemical reactions, the debris from mechanical contacts or both of these sources might be captured by the filters. Then, the filters were replaced and inspected to identify the cause of the increase of the filter differential pressure. As a result, it was found that the increase is caused by clogging of the filters by the dust traced back to the physical contact of the piston rings of the gas circulators equipped in the primary helium purification system. Hence, prismatic block-type very high temperature reactors (VHTRs) do not continuously supply carbon dust from the core during its operation.

Status of development on 99 Mo production technologies in JMTR

The Japan Materials Testing Reactor (JMTR) is now under refurbishment, and the operation of the new JMTR will be started in FY 2011. The new JMTR has a plan to produce 99Mo, which is the parent nuclide of 99mTc, and two 99Mo production technologies have been developed; the one is a solid irradiation method, and the other is a solution irradiation method. In the solid irradiation method, it was found that JMTR can provide about 20% of the 99Mo imported into Japan. In the solution irradiation method, the fundamental characteristics of the aqueous molybdate solutions selected as the irradiation target were cleared by the gamma-ray irradiation test. Aiming at the domestic production of 99Mo in Japan, the development of 99Mo production technologies in JMTR has been continued.