GENERAL CONCEPT
Fluidized Bed Nuclear Reactor

 

The nuclear energy is suffering from the lack of public acceptance everywhere mainly due to the issues relating to reactor safety, economy and nuclear waste.The Fluidized Bed Nuclear Reactor (FBNR) concept has addressed these issues and tried to resolve such problems.

 

The FBNR is small, modular and simple in design contributing to the economy of the reactor.It has inherent safety and passive cooling characteristics.It's spent fuel being small spherical elements may not be considered nuclear waste, and can be directly used as a source of radiation for applications in in1dustry and agriculture resulting in reduced environmental impact.

 

The 8-mm diameter spherical fuel elements being fluidized by water at desired porosity form a suspended reactor core.The reactor is critical when everything operates under designed conditions.Any anomaly will cause the reactor becomes subcritical by it's nature and eventually the suspended core is automatically removed to outside to the fuel chamber which is passively cooled.The reactor for the reasons of it's smallness and inherent safety characteristics can be built in urban areas with no need for containment, but for esthetic reasons, it is provided with a simple underground containment.
 

This reactor concept utilizes the well-dominated Pressurized Water Reactor (PWR) technology; thus there are no need for developing a new technology.Only one experiment in relation to the stability of the core needs to be performed to prove the feasibility of the concept.This means that with a budget of less than one million dollar we can have the possibility of having a new nuclear reactor so simple that even the unsophisticated public can understand its principles and believe in its safety and other advantages.

 

A new century is emerging and we are faced with new paradigms. Please contribute to the well being of a humanity that can no longer ignore the utilization of nuclear energy by making a critical review of the proposed nuclear reactor concept pointing out all the conceivable problems and merits of the concept.Meanwhile your suggestions on how to improve the concept and the hints about how to bring it into reality are most welcomed.

 

A SIMPLE AND SAFE NUCLEAR REACTOR AT THE SERVICE OF MANKIND

 

Reactor Description

The nuclear reactor based on the fluidized bed concept proposed by Dr. Farhang Sefidvash, professor of Nuclear Engineering at the Federal University of Rio Grande do Sul, is modular in design such that any size of the reactor can be constructed from the basic module.

The upper part of the reactor module which include the core and the steam generator consists of a 25 cm diameter fluidizing tube being circumscribed by a hexagonal channel. The lower part of the module consist of a 10 cm diameter fuel chamber surrounded by a circular channel which in turn is covered by a graphite jacket.

 

An ring is formed between the fluidizing tube and hexagonal channel and at its extension between the fuel chamber and the circular channel where the coolant flows down the module. In the upper part of the core, a movable sieve acting as a fluidized bed level limiter separates the core from the steam generator. A cylindrical neutron absorber shell connected to the sieve moves along with it. A steam generator of the shell and tube type is integrated into the upper part of the module.

 

Inside the fuel chamber exist the spherical fuel elements of about 0,8 cm in diameter made of slightly enriched uranium dioxide cladded by zircaloy. The fresh fuel elements are fed into the reactor core through the hollow shaft of the level limiter. The bottom of the fuel chamber is provided with a fuel discharge valve. The valve is operated by a hydraulic system allowing the fuel to be discharged from the fuel chamber into a permanently cooled storage tank. The module is provide with a pressurized system which is to keep the pressure a constant, and a depressurizer valve which leads the steam to the condenser intended to be used for reducing the pressure to allow the opening of the valve for refueling purposes.

 

The cooled coolant gaining pressure in the pump enters the combustion chamber after crossing the perforations in the distributor. It rises in the module and after exceeding a certain velocity limit, carries with it the fuel pellets into the reactor core, and thereafter fluidizing them. The coolant after gaining heat from the core, transfers it to the steam generator, returning to the pump through the ring space.

 

The reactor is surrounded by a graphite reflector and a biological shield.
 
 

Reactor Operation

 

The reactor physics calculations show that the reactivity of the reactor increases with the fluidized bed height or porosity to a maximum value and thereafter decreases.

 

The decrease in reactivity due to fuel depletion and fission product accumulation is compensated by increase in porosity. The bed porosity is controlled by varying the coolant velocity governed by the pump.

 

For additional safety, the level limiter is set at a certain distance away from the critical height where it defines the maximum reserve reactivity. The level limiter also prevents the escape of fuel elements from the module in the case of a flow excursion due to a loss of coolant accident.

 

In the case for a probable accident, the motor of the pump is made to stop or rotate at a low velocity resulting in the precipitation of fuel elements from the reactor core into the fuel chamber where due to the geometric configuration stay in a highly subcritical condition. The fuel elements can be discharged from the reactor through the discharge valve and be stored in a permanently cooled spent fuel storage tank or in the pool of water which is provided under the reactor. Also the water can be injected into the pool to raise its level to cover the base of the module in order to absorb the produced decay heat.
 
 

Reactor Control



The four major areas of reactor control are startup, steady state operation, shutdown, and transients.

 

When initially starting up the reactor, the core is at a much lower temperature than the operating temperature. Therefore due to negative temperature coefficient, the reactor must be brought to full power by simultaneous adjusting of flow velocity and raising of the level limiter in various steps. During the normal operating condition, the small reactivity control is done through coolant flow velocity control and larger ones including the burnup effect is performed by raising the level limiter. Shutdown of the reactor, as rapidly as desired, is easily obtained through the decrease of pump velocity causing decrease in core porosity. The collapsed bed due to stopped flow is highly subcritical.

Schematic design of a 19 module reactor:

 

 

01- STRUCTURAL SUPPORT
02- HYDRAULIC VALVE OPENER
03- FUEL DISCHARGE VALVE
04- GRAPHITE JACKET
05- REACTOR CORE
06- LEVEL LIMITER SHAFT
07- DESPRESSURIZER
08- STEAM EXIT
09- LEVEL LIMITER DRIVE
10- FUEL FEED
11- PRESSURIZER
12- WATER ENTRANCE
13- SREAM GENERATOR
14- LEVEL LIMITER
15- ABSORBER SHELL
16- HEXAGONAL CHANNEL
17- FLUIDIZATION TUBE
18- CIRCULAR CHANNEL
19- FUEL CHAMBER
20- DISTRIBUITOR
21- ENTRANCE PERFORATIONS
22- COOLANT ENTRANCE
23- COOLANT EXIT
24- PRIMARY PUMP
25- GRAPHITE REFLECTOR
26- BIOLOGICAL SHIELD

 

The Merits of the Reactor

 The perceived advantagens of the proposed reactor concept over conventional reactors are as follows:
 The design is simple and therefore the reactor is cheaper and more reliable and can be constructed by the developing countries.
 The reactor is modular in system: therefore, any reactor size can be constructed from the basic standard module. Only a standard module need to be developed, designed, and licensed. It will result in a lower capital cost, Also pant size is flexible and small increments can be added to match demand growth.
 No heavy and expensive pressure vessel fabrication is needed. Factory fabrication of components is maximized and site construction is minimized. This will result in higher fabrication quality of the components and reduced cost due to shorter construction time.
 Relatively simple and cheap fuel fabrication since there is no fuel assembly to be fabricated and all the pellets have the same enrichment.
 Continuous and on power refueling system without the use of a complicated refueling machine. It will greatly increase the economy of the power production since 43% of shortfall in capacity factor is due ro refueling.
 This reactor concept eases plant decommissioning tasks.
 Simple and safe control system. There is no need for design of additional control systems resulting in higher safety and economy.
 Inherent safety which results in reduced capital cost and increased public acceptance of the nuclear power. It makes it conducive to siting close to industrial and urban areas.
 Due to inherent safety, there exist the possibility of having completely automatic control, minimizing and even eliminating the reactor operators.
 Integrated primary circuit reduces the cost and the probability of a loss of coolant accident.
 There is no problem of seismic effect on the fuel.
 The reactor does not suffer from the consequences of a probable loss of coolant accident. This results in a better public acceptance and reduced cost due to elimination of sophisticated emergency core cooling system.
 There is no need for big and strong containment building, since the modular nature of the reactor eliminates the possibility of large energy release.
 The high heat transfer rate due to large surface area and high turbulence existing in the fluidized bed results in low critical temperatures for the fuel element.
 There is no need for solid or soluable burnable poison, thus a better neutron economy is obtained.
 Due to the random motion of the fuel pellets, uniform fuel burnup is obtained.
 Any desired neutron spectrum condition may be created in the reactor which allows the possibility of manipulation in fuel cycle.
 The possibility of the reactor operating with thorium cycle or with natural uranium and heavy water, or with organic coolant.
 The spent fuel elements may not be considered nuclear waste. They can be used as a source of radiation for radiotherapy, food irradiation and industrial applications.

 

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