FBNR is an innovative nuclear reactor being small and simple in design, with the characteristics of inherent safety, passive cooling, and environmental friendly.

The FBNR core is in suspended state; namely the flow of water coolant keeps the spherical fuel elements inside the reactor core in a fixed and appropriate condition that allows the reactor becomes critical and in operating condition. In the case of any malfunction of the reactor, any signal from any of the numerous detectors that do the surveillance of the reactor will cut electricity from the coolant pump. The stop in flow will result in falling out of fuel elements from the reactor core by the force of gravity into the fuel chamber below where they become stored in a subcritical condition and cooled by natural convection. The normal condition of the pump switch is “off”, and it will turn “on” when all the signals from all the safety detectors show safe operational conditions. This results in total safety for the FBNR concept.

The Fixed Bed Nuclear Reactor (FBNR) is modular in design, and each module is assumed to be fuelled in the factory. The fuelled modules in sealed form are then transported to and from the site. The FBNR has a long fuel cycle time and, therefore, there is no need for on­site refuelling. The reactor makes an extensive use of PWR technology.

It is an integrated primary system design. The basic modules have in its upper part the reactor core and a steam generator and in its lower part the fuel chamber. The core consists of two concentric perforated zircaloy tubes of 20 cm and 160 cm in diameters, inside which, during the reactor operation, the spherical fuel elements are held together by the coolant flow in a fixed bed configuration, forming a suspended core. The coolant flows vertically up into the inner perforated tube and then, passing horizontally through the fuel elements and the outer perforated tube, enters the outer shell where it flows up vertically to the steam generator. The reserve fuel chamber is a 40­cm diameter tube made of high neutron absorbing alloy, which is directly connected underneath the core tube. The fuel chamber consists of a helical 25 cm diameter tube flanged to the reserve fuel chamber that is sealed by the international authorities. A grid is provided at the lower part of the tube to hold the fuel elements within it. A steam generator of the shell-and-tube type is integrated in the upper part of the module. A control rod slides inside the centre of the core for fine reactivity adjustments. The reactor is provided with a pressurizer system to keep the coolant at a constant pressure. The pump circulates the coolant inside the reactor moving it up through the fuel chamber, the core, and the steam generator. Thereafter, the coolant flows back down to the pump through the concentric annular passage. At a certain pump velocity, the water coolant carries up the 15 mm diameter spherical fuel elements from the fuel chamber into the core. A fixed suspended core is formed in the module. In a shut down condition, the suspended core breaks down and the fuel elements leave the core and fall back into the fuel chamber. The 15 mm diameter fuel elements are made of TRISO type microspheres used in HTGR.

Any signal from any detector due to any initiating event is assumed to cut-off power from the pump, causing the fuel elements to leave the core and fall back into the fuel chamber, where they remain in a highly subcritical and passively cooled condition. The fuel chamber is cooled by natural convection transferring heat to the water in the tank housing the fuel chamber.

The pump circulates the water coolant in the loop and at the mass flow rate of about 141 kg/sec, corresponding to the terminal velocity of 1.64 m/sec in the reserve fuel chamber, carries the fuel elements into the core and forms a fixed bed. At the operating mass flow rate of 668 kg/sec, the fuel elements are firmly held together by a pressure of 9.5 bar forming a stable fixed bed. The coolant flows radially in the core and after absorbing heat from the fuel elements enters the integrated heat exchanger of tube and shell type. Thereafter, it circulates back into the pump and the fuel chamber.

The long-term reactivity is supplied by fresh fuel addition and a fine control rod that moves in the center of the core controls the short-term reactivity. A piston type core limiter adjusts the core height and controls the amount of fuel elements that are permitted to enter the core from the reserve chamber. The control system is conceived to have the pump in the “not operating” condition and only operates when all the signals coming from the control detectors simultaneously indicate safe operation. Under any possible inadequate functioning of the reactor, the power does not reach the pump and the coolant flow stops causing the fuel elements to fall out of the core by the force of gravity and become stored in the passively cooled fuel chamber.
The water flowing from an accumulator that is controlled by a multi redundancy valve system cools the fuel chamber as a measure of emergency core cooling system. The other components of the reactor are essentially the same as in a conventional pressurized water reactor.

This reactor concept has a core life of at least 10 years and refueling is done simply by changing the fuel chamber which is connected to the reactor by two flanges. The core life is flexible and can be designed according to the customer’s requirement. The variables involved to design for particular core life are the fuel enrichment and the amount of fuel in the reserve fuel chamber.

The coolant pump is controlled by a frequency control system, thus its flow can be controlled very smoothly. The coolant maintains the fuel elements in the fixed bed tightly together at the pressure of about 10 bar, thus any common fluctuation in the pump pressure will not affect the configuration of the core .
The reactor concept has the flexibility to be a multipurpose reactor. It may generate electricity alone or as a double purpose plant generate electricity and produce desalinated water. It may be designed as a district heating reactor.

Summary of the parameters of the Fixed Bed Nuclear Reactor:

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