CNEA,

Argentina

III — 1. DESCRIPTION OF THE CAREM DESIGN

CAREM is an Argentine project for design and technology development and construction of an innovative, simple and small nuclear power plant (NPP). This nuclear power plant is based on an indirect cycle nuclear reactor with some distinctive and characteristic features which simplify design, and contribute to enhanced safety. A detailed description of the CAREM design and features is presented in [III-1, III-2].

The first step of this project is the construction of a prototype of about 27 MW(e) (CAREM-25) [III-2]. Main features of the CAREM approach and, specifically, the CAREM-25 design, are the following; see Fig. III-1:

• Integrated primary coolant system;

• Primary cooling by natural circulation (for CAREM-25 and CAREM designs below 150 MW(e));

• Self-pressurization (active pressurizer is eliminated);

• Safety systems relying on passive features.

Main characteristics of the CAREM nuclear power plant are given in Table III-1.

In order to simplify design, the whole high energy primary system, including the core, the steam generators, primary coolant and the steam dome, is contained inside a single reactor pressure vessel. This considerably reduces the number of pressure vessels and simplifies the layout.

The absence of large diameter piping associated with the primary system, removes the possibility of large break loss of coolant accidents (LOCA). The elimination of large break LOCA substantially reduces the necessity for emergency core cooling system (ECCSA) components, alternate current (AC) supply systems, etc.

Large coolant inventory in the primary circuit results in large thermal inertia and long response time in the case of transients or accidents.

The reactor primary coolant system operates on natural convection. Water enters the core from the lower plenum. After being heated, the coolant exits the core and flows up through the riser to the upper dome. In the upper part, water leaves the riser through lateral windows, going to the periphery region of the in-vessel space. Then it flows down through the modular steam generators, with decreased enthalpy. Finally, the coolant exits the steam generators and flows down through the down-comer to the lower plenum, closing the circuit.

The CAREM primary coolant system is self-pressurized.

Due to the innovative design of the reactor core cooling system (RCCS), an extensive experimental plan has been developed and is being implemented [III-2, III-3].

RCCS modelling and qualification are supported by tests performed in a high pressure natural circulation rig (CAPCN), covering thermal hydraulics and techniques of reactor control and operation. The CAPCN rig reproduces all dynamic phenomena of the RCCS, except for 3D effects.

The fuel is enriched UO2. Core reactivity is controlled by the use of Gd2O3 as a burnable poison in special fuel rods and moveable absorbing elements belonging to the reactor control and adjustment system. Liquid chemical compositions (like boric acid solution) are not used for reactivity control during normal operation.

Each absorbing element (AE) consists of a cluster of rods linked by a structural element (namely, ‘spider’), so that the cluster moves as a single unit. Absorber rods fit into guide tubes. The absorber material is the commonly used Ag-In-Cd alloy. Absorbing elements (AE) are used for reactivity control during normal operation (control and adjustment system) and to interrupt nuclear chain reaction promptly when required (fast shutdown system).

The shutdown system is diversified to fulfil the requirements of the Argentine regulatory authority.

The first shutdown system (FSS) consists of gravity driven neutron-absorbing elements. In CAREM-25, this system provides a total negative reactivity of 6880 pcm in a cold shutdown state, with all rods inserted.

During normal operation, elements of the FSS are kept in the upper position. They are designed to provide a minimal dropping time, so it takes only a few seconds to completely insert the absorbing rods into the core. In CAREM-25, this system has a minimum worth of 3500 pcm, with one rod unavailable.

The second shutdown system (SSS) is a gravity driven injection device based on high pressure borated water. In CAREM-25, this system provides a total negative reactivity of 5980 pcm in a cold shutdown state, assuming a single rod failure.

Twelve identical ‘mini-helical’ vertical steam generators (see Fig. III-2) of the once-through type are placed equidistant from each other along the inner surface of the reactor pressure vessel (RPV) [III-1, III-2]. They are used to transfer heat from the primary to the secondary circuit, producing superheated dry steam at 47 bar. The secondary system circulates upwards within the tubes, while the primary is in counter current flow. An external shell surrounding the outer coil layer and adequate seal form the flow separation system. It guarantees that the entire stream of the primary system flows through the steam generators.

To achieve rather uniform pressure-loss and superheating on the secondary side, the length of all tubes is equalized. For safety reasons, steam generators are designed to withstand the primary pressure without pressure in the secondary side and the live steam system is designed to withstand primary pressure up to the isolation valves (including the steam outlet/water inlet headers) in case of SG tube breakage.