Korea Atomic Energy Research Institute, Republic of Korea

XVIII — 1. Introduction

The SMART (System-Integrated Modular Advanced ReacTor) is an advanced pressurized light water reactor that is being continuously studied at KAERI (Korea Atomic Energy Research Institute) with a rated thermal power of 330 MW. The reactor is proposed to be utilized as an energy source for sea water desalination as well as for small scale power generation. Advanced technologies such as inherent and passive safety features are incorporated in establishing the design concepts to achieve inherent safety, enhanced operational flexibility, and good economy. The SMART is designed to supply 40,000 tons of fresh water per day and 90MW of electricity to an area with an approximate population of 100,000 or an industrialized complex. In order to demonstrate the relevant technologies incorporated in the SMART design, the SMART-P (i. e. a Pilot plant of the SMART) project is currently underway at KAERI.

The prominent design feature of SMART is the adoption of integral arrangement. The major components of the NSSS such as the core, steam generators, main coolant pumps, and pressurizer are integrated into a reactor vessel without any pipe connections between those components. The schematic diagram of the SMART NSSS is shown in Figure XX-1.

The SMART core is currently being designed with the fuel design based on existing Korea Optimized Fuel Assembly (KOFA) which is in 17 x 17 rectangular rod arrays. The SMART core design is characterized by an ultra long operation cycle with a single or modified single batch reload scheme, low core power density, soluble boron-free operation, enhanced safety with a large negative Moderator Temperature Coefficient (MTC) at any time during the fuel cycle, a large thermal margin, inherently free from xenon oscillation instability, and minimum rod motion for the load follow with coolant temperature control. Due to soluble boron-free operation, an important design requirement for the SMART CEDM is a fine maneuvering capability to control the excess core reactivity. A linear step motor type CEDM is employed for easy maintenance. The minimum step length is 4mm that is short enough for the fine reactivity control. Forty-nine CEDMs are installed in the fifty-seven fuel assemblies of the SMART core.

Twelve identical SG cassettes are located in the annulus formed by the RPV and the core support barrel. Each SG cassette is a once-through type with helically coiled tubes wound around the inner shell. The primary coolant flows downward in the shell side of the SG tubes, while the secondary feedwater flows upward in the tube side. Therefore, the tubes are under compressive loads from the greater primary pressure, reducing the stress corrosion cracking and thus reducing the probability of tube rupture. The 40°C superheated steam at the exit of the helically coiled tubes eliminates the necessity of a steam separator during normal operations. The twelve SGs are divided into four sections. Each section consists of the neighboring three steam generator cassettes which are connected together with the steam and feedwater pipes. If there is a leakage in one or more of the tubes, the relevant section is isolated and SMART can be operated with reduced power until the scheduled shutdown.

The SMART adopted an in-vessel self-controlled pressurizer (PZR) located in the upper space of the RPV. The volume of the PZR is filled with water, steam, and nitrogen gas. The self-pressurizing design eliminates the active mechanisms such as spray and heater. The system pressure is determined by a sum of the steam and nitrogen partial pressures. In order to minimize the contribution of the steam partial pressure, a PZR cooler is installed for maintaining the low PZR temperature, and wet thermal insulator is installed to reduce the heat transfer from the primary coolant. The coolant temperature of the core outlet is controlled during a power maneuvering so as to minimize the system pressure variation by counterbalancing the increase of the coolant volume of the hot part with the decrease of the coolant volume of the cold part.

The SMART MCP is a canned motor type pump that eliminates the problems connected with conventional seals and associated systems. Four MCPs are installed vertically on the RPV annular cover. MCP is an integral unit consisting of a canned asynchronous 3-phase motor and an axial flow single-stage pump. The motor and pump are connected through a common shaft rotating on three radial and one axial thrust bearings. The impeller draws the coolant from above and discharges downward directly to the SG. This design minimizes the pressure loss of the flow.

There are many inherent safety features in the SMART design. Those include a large negative moderator temperature coefficient due to the boron-free operation, a low core power density, and the reduced xenon oscillations. Furthermore, enhanced safety of the SMART is accomplished with highly reliable engineered safety systems. The engineered safety systems consist of a reactor shutdown system, passive residual heat removal system, emergency core cooling system, safety vessel, reactor overpressure protection system and containment overpressure protection system. As the result of the probabilistic safety assessments for 10 internal events, the core meltdown frequency is predicted as 8.56 x 10’7.