[1] Basic principles of thermal-hydraulic design

Thermal-hydraulic design of fast reactors is made so that the allowable thermal design limit is not exceeded at normal operation and abnormal operational occurrences and that fuel integrity is kept until the designed burnup. The basic principles are explained below.

Normal operation indicates the conditions where startup, shutdown, operation in power and refueling are intentionally carried out and the operational condition is within the limit. Anticipated operational occurrences indicate the conditions where the operation is disturbed due to a single failure of equipment or its malfunction, which are anticipated during its lifetime, or a single operator failure. A condition having similar frequency and leading to an unplanned state is also an anticipated operational occurrence. The allowable thermal design limit indicates the limit of allowing fuel damage for safety design and continuous operation of the nuclear reactor facility.

The basic principles of fast reactor thermal-hydraulic design are, at normal operation and anticipated operational occurrences: (i) prevent sodium from boiling and (ii) prevent fuel from reaching the allowable design limit.

Item (i) is followed because there is a possibility of positive reactivity insertion due to boiling and an excess increase in the fuel cladding temperature due to change in heat transfer characteristics. For satisfying the allowable thermal design limit, the maximum fuel centerline temperature and the fuel cladding temperature are limited from the viewpoint of thermal-hydraulic design. As for the maximum fuel centerline temperature, fuel melting is not allowed in the mainstream design in order to avoid failure of fuel cladding by excessive stress caused by thermal expansion of the fuel pellets. The design limit of the maximum fuel centerline temperature is determined by the melting limit test data with consideration of the design margin. As for the cladding temperature, the cladding temperature itself, which dominates the material properties, is limited instead of limiting the heat flux on the cladding surface because the heat transfer characteristic of sodium is better than that of water. For avoiding cladding failure by an increase in the cladding temperature for a short period at anticipated operational occurrences, the limit of maximum cladding temperature at anticipated operational occurrences is established. Stainless steel is generally used as the fuel cladding material of fast reactors. The limit of fuel cladding temperature at anticipated operational occurrences is determined based on out-of-pile rapid heat-up tests of irradiated target mate­rials. Since the internal pressure creep dominates the mechanical strength of the fuel cladding for ensuring fuel integrity until designed burnup, the limit of fuel cladding temperature at normal operation, which dominates the creep strength, is determined based on the creep test data of the target materials. This limit for sodium cooled reactors is set as 650-700 °C in many designs.

In the thermal-hydraulic design of fast reactors, the flow allocation among the fuel assemblies is provided according to the power distribution in order to satisfy the allowable design limits above and to efficiently remove the heat generated in the reactor. That is the important feature of fast reactors compared to LWRs.