19.3.1 IMR

The IMR is an iPWR concepts and was proposed by MHI (Hibi et al., 2005) in cooperation with the JAPC (Japan Atomic Power Company) (Okazaki et al., 2011). The targets for the SMR design are summarized in Table 19.1. The schematic view of the reactor concept is shown in Figure 19.1. Its major specifications are summarized in Table 19.2. Although it is based on the PWR concept, boiling of the primary coolant, i. e. water, is allowed around the top area of the core (average void fraction at the core outlet is about 20%) and the core is cooled by the natural circulation of the coolant. Boiling of the core coolant is favourable to increase the driving force for the natural circulation core cooling. In this concept, the primary

Table 19.1 Targets for the SMR design

Power output: 300-600 MWe

Construction cost: The same as large reactors or less Capacity factor : More than 90%

Safety: At least as safe as existing plant Construction period: Less than 24 months License suitability: Suitable for the present license

Control rod drive (CRD) integrated into reactor

Containment

vessel

Reactor

vessel

Steam

generator

(helical)

Control

rods

Reactor core J (short length)

cooling system and the steam generators (helical once-through type) are enclosed in the reactor pressure vessel and hence any large break LOCA is eliminated. In this reactor concept, several major components are eliminated, such as reactor coolant pumps, pressurizer and primary piping.

The power output is 350 MWe. The average discharge burnup is 45 GWd/t and the refueling interval is 26 months. Considering this longer lifetime of fuel rods as well as the increased coolant temperature up to 345 °C, a Zr-Nb alloy is applied to the cladding tube. The system pressure is 15.5 MPa and is the same pressure as in the normal PWR. For the safety system design, the IMR is significantly simplified. That is, it does not require the ECCS and only requires the steam generator cooling system for decay heat removal. The steam generator cooling system uses pumps, driven by diesel and gas turbine, as the active equipment.

As the IMR concept is different from the PWR in the flow characteristics, two experiments were conducted to check the flow characteristics inside the reactor vessel of the IMR. One was conducted under the actual conditions of the temperature, the pressure and the length of axial direction of IMR reactor system. Based on the results, it is confirmed that the natural circulation core cooling is available under the operation and design conditions of IMR. The other was conducted using a simulator of the main structure to confirm the three-dimensional flow characteristics in the reactor vessel. By the test, flow characteristics of the main structure inside reactor vessel could be understood and the methods for evaluating two-phase flow behaviours were developed.

For further developments of the IMR, optimization studies will be performed. In addition, possibilities to develop the concepts with various output ranges will be considered, utilizing the technology gained in the previous development. For example, a plant with 100 MWe output has been developed based on the IMR.