Material and Equipments used in Reactors:

Various materials and equipment used in reactors are :

• nuclear fuels

• moderators

• coolants

• controls

• safety devices

• instrumentation

• pumps

• radiation detectors

• heat exchangers

Fuel Materials :

The fuels or fissionable materials used in reactors are the isotopes of uranium, thorium and plutonium.

Only U-235 is the naturally occurring isotope. Rest of the isotopes can be formed from Th-232 and Th-238 by the absorption of one neutron.

Uranium is the basic fuel for the reactor. Natural and enriched uranium is needed for starting the reactor. It is used either in the form of pure metal or as a solution of uranium salt.

Uranium in the form of powder ignites even at room temperature. It melts at 1100°C. Fuel used in reactor should have good mechanical strength. Upto 660°C uranium exists in alpha phase with a mechanical strength of 2.7 N/mm2 which can be increased to 13.8 N/mm2 by cold rolled process. Uranium is stable in the beta phase i.e., from 660°C to 770°C. The brittleness increases and it becomes much harder. Above 770°C the uranium exists in gamma phase with cubic lattice crystal structure. The metal becomes soft and fragile. The operating temperature is thus much below the first phase temperature limit. With pure uranium it is impossible to eliminate all dimensional changes. Hence it is alloyed with certain materials. Alloyed uranium is less subjected to radiation damage. 0.1 to 0.2% chromium is added to uranium to improve its properties. Other alloying elements are molybdenum, niobium and zirconium.

U-233, U-235 and Pu-239 are fissible species and Th-239 and U-238 are fertile materials. Thorium is stable to radiation. At low temperature the crystal orientation of plutonium is similar to uranium but as temperature increases it is severely effected by the radiation damage.

In order to prevent the contamination of the coolant by fission products, a protective coating or cladding is given to Fuel.

Ceramic fuels have been developed to increase the operating temperature of reactors. The graphite uranium is mu Siable IC) radiation. Ceramic fuel, beryllium oxide and uranium oxide mixture is better suited because they provide less radiation damage

Moderators :

Moderator, in a reactor, is used to reduce neutron energy by collisions. It may be in the form of solid, liquid or gaseous. Materials of moderator are graphite, beryllium, beryllium oxide, carbon, wilier, heavy -water or hydrogen and its isotopes such as deuterium.

Graphite for reactor is obtained from petroleum coke mixed with coaltar pitch and graphitised at 2500 to 2800°C.

Beryllium and Beryllium oxide offer excellent chemical and corrosion resistance. They are not affected by radiation damage. But because of high cost, poor ductility and high toxicity these are rarely used, Beryllium oxide is preferred because its melting point is 2520°C and is heat resistant.

Light water is the cheapest moderator but it should be used is pure forms. It should not specially contain sodium salts. Water corrodes the metal at high temperatures and pressure. Radiation damage is also more severe with water moderator.

Heavy water is the best suited moderator. It is deuterium oxide. It serves as a coolant as well. It has high moderating ratio. Only drawback with heavy water is that it is very costly. Helium gas (He) is also used as a moderator in some of the reactors.

Coolant :

Coolant is the heat transfer medium. It takes away the heat from reactor to the heat exchanger. It should

• absorb only a little number of neutrons.

• have high heat transfer coefficient.

• have low melting point.

• have high density and specific heat.

• have less radioactivity from neutron capture.

• have low moderation ratio. But it should not corrode the metal and should he free from radiation damage.

Water and liquid metals are common coolants. Sodium, aluminium, bismuth, lead and magnesium are common liquid metals. Alloys of sodium and potasium and lithium are also used as coolants.

Potassium is dangerous because it undergoes explosive reaction and forms potasium oxide. Sodium is a cheap metal with better coolant properties. It does not react with stainless steel upto 580°C.

Gas coolants such as Carbon Dioxide and helium are also as coolants. But Helium has poor heat transfer capacity.

Organic fluids such as polyphenyls, di phenyl, terphenyl etc., are also used as coolants but their heat transfer capacity is less and higher volume of liquids are needed to take away heat from reactors.

Control Rods :

Control rods in a reactor are used to start the nuclear chain reaction, maintain the reaction and to shut down the reactor.

Materials used for 'control' purpose should have absorption cross -section for thermal neutrons. Boron and cadmium are widely used because of their cheapness. The boron forms alloy either with steel or aluminium and commonly used in the form of rods. Cadmium is either plated on steel, or aluminium rod, or it is employed as sandwich material with steel or aluminium.

Reactor Control :

Each reactor is equipped with three types of control rods

• shim rods

• fine control rods

• safety rods

Shim rods absorb the excess reactivity during normal operation of the reactor. At the time of starting, the rods are taken out of the reactor. Afterwards they are positioned in the reactor to maintain the constant power level. These rods are adjusted after certain interval of time to compensate for the poisoning and other effects of fission products in the core. The fine control rods are designed to absorb small percentage of reactivity caused due to changes in pressure, temperature and density of fuel material. These rods can be operated manually or automatically.

The safety rods provide protection against hazards like earth quake and excessive power. Because of poisoning effect, it is impossible to start up the reactor with safety rods inside. In case of energy i.e., in the event of potential hazards or dangerous pressure surges the rods enter the reactor core and instantaneously reduce the multiplication factor well below unity.

The effect of control rods is the function of neutron flux at the point of insertion of rods and it varies as the square of neutron flux. When a single rod is provided it must be centrally placed in the fuel core.

Other methods of reactor control are

• adjustment of fuel quantity

• adjustment of moderator quantity

• adjustment of coolant quantity

Heat Exchanger :

Heat exchanger is a device in which transfer of heat takes place from one medium to another or from one system to another. It is a heat transferring component.

Heat exchanger is provided with two circulatory systems. in one system heated liquid circulates while in another system heat receiving medium circulates in opposite direction. A thin metallic wall separates the two systems. For different types of reactors different heat exchangers are developed.

Figure shows one of such types of heat exchangers used in nuclear reactors using sodium - potassium alloy (liquid metal) as coolant.

As this coolant is highly radioactive and will penetrate into the secondary circulatory circuit, two systems are separated by a gap filled up with mercury. Mercury also circulates in thin narrow gap. Any leakage of radioactive particles can be detected using measuring instruments. The main drawback with this system is that leakage of mercury into the primary coolant will poison the reactor because of high corrosion effect of mercury.

Secondary coolant absorbs heat from primary one (reactor side) and leaves the heat exchanger. The heated secondary fluid transfers heat to water (not shown) which evaporates into steam and passes to steam turbine.

In natural re-circulation heat exchanger, the heated fuel is pumped through the pipe as shown which contains stainless tubes. The top drum also contains number of stainless steel tubes. Water circulating in the staggered cross piping system get evaporated into steam after absorbing heat in fuel.