The FSS is designed to shut down the core when an abnormality or a deviation from normal operation occurs and to maintain the core in a subcritical condition during all shutdown states. This function is achieved by dropping neutron absorbing elements into the core, driven by gravity. Each neutron absorbing element is a cluster composed of a maximum of 18 individual rods coupled together in a single unit. Each unit fits into the guide tubes of a fuel assembly.

The internal hydraulic control rod drive (CRD) eliminates the mechanical shafts passing through the reactor pressure vessel (RPV) or through the extension of the primary pressure boundary and, as the whole device is located inside the RPV, contributes to the elimination of large break LOCAs. This design is an important element in the CAREM concept. Many of the control rods belong to a fast shutdown system. A simplified diagram of the fast shutdown system hydraulic CRD is shown in Fig. III — 3. During normal operation, fast shutdown system control rods are kept in the upper position, where the piston partially closes the outlet orifice and reduces water flow leaking into the RPV dome.

The CRD of the control and adjustment system is a hinged device controlled in steps and fixed in position by pulses over a base flow, designed so that each pulse produces only one step.

Both types of devices perform the reactor scram function by using the same principle: ‘rods are dropped driven by gravity when the flow is interrupted’, so that the malfunction of any powered part of the hydraulic circuit (i. e., a valve or a pump failure) causes immediate shutdown of the reactor. CRD of the fast shutdown system is designed with a large gap between piston and cylinder to obtain a minimum dropping time (of a few seconds) to insert absorbing rods completely into the core. CRD manufacturing and assembling allowances are stricter, and clearances are narrower for rods of the control and adjustment system, but there is no stringent requirement on dropping time.

1: First shutdown system 2: Second shutdown system

3: Residual heat removal system 4: Emergency injection system 5: Pressure suppression pool 6: Containment

7: Safety valves

A: Core B: Steam generators C: Reactor building

FIG. III-2. Containment and safety systems of CAREM.

Second shutdown system (SSS)

The SSS is a gravity driven injection device using borated water at high pressure. It acts automatically when the reactor protection system detects a failure of the FSS or in the case of a LOCA. This system consists of two tanks located in the upper part of the containment. Each of them is connected to the reactor vessel by two pipelines; one is from the steam dome to the upper part of the tank, and the other is from a position below the reactor water level to the lower part of the tank. When the system is triggered, the valves open automatically and the borated water drains into the primary system, driven by gravity. The discharge of a single tank produces the complete shutdown of the reactor.