COOLANTS FOR NUCLEAR REACTORS

The coolant which passes through the nuclear reactors is used to transport the reactor heat either to a boiler where steam is raised to run a conventional turbine or it is used as a thermodynamic heat engine fluid and passes directly into the turbine and back to the reactor. Pressurized water, organic liquids, sodium, and most gas cooled nuclear power plants employ an intermediate steam boiler. Boiling water and some gas cool reactors use the coolant directly in the turbine.

Regardless of the method used, coolants should ideally have the following properties:

Low melting point.
High boiling point.
Non-corrosive properties.
Low neutron absorption cross section.
High moderating ratio.(for thermal reactors)
Radiation stability.
Thermal stability.
Low induced radioactivity.
No reaction with turbine working fluid.
High heat transport and transfer coefficient.
Low pumping power.

No single coolant has all of these properties, and as a result a number of different coolants have been used in nuclear reactors. Each coolant with its own particular advantages for certain type of reactors. Among these coolants are light and heavy water (both pressurized and boiling), organic liquids, sodium, sodium potassium mixtures, fused salts, and a number of gases - air, carbon dioxide, helium, nitrogen, hydrogen and steam.

PRESSURIZED WATER REACTOR

The first pressurized water reactor, the Mark I prototype for the Nautilus submarine, began operation in May 1953 at the National Reactor Testing Center in Idaho. Since that time the development of pressurized reactors for military and civilian purposes has been intensively pursued specially in the United States.

By definition, in a pressurized water reactor the fission heat is removed from the fluid elements by the water coolant without bulk boiling occurring. This implies a two circuit heat transfer system - a primary loop containing the reactor and one side of steam generator containing a steam side of the steam generator and the turbine generator.

The extensive use of water as a reactor coolant is related to the relatively low pressure drops accompanying flow at significant rates, and the relatively high heat transfer coefficients. (See figure below.) Figure shows the relationship of heat flux to the temperature difference between the fuel element surface and the water. The nucleate boiling stage is attractive for reactor operation, because of the favorable heat fluxes that can be obtained. However, as the temperature difference increases to the point where bulk boiling and film boiling occurs, there is a drop in heat flux and the danger that the fuel element surface temperature will rise above their melting point (burn out).

Some properties of water:

Coolant   Temp(F)  Density(lb/cuft)  Specific Heat(BTU/lb.F)  Thermal Conductivity  Viscosity(lb/(hr)(ft))
m.p. 32F  212	   60	             1.006	              0.395	            0.70
b.p. 212F	   482	             50	1.21	              0.35	            0.45

Some nuclear properties of Light Water:

absorption cross section (barn)    scattering cross section (barn)    fractional energy loss/collision    moderating ratio
0.66                               44.4                               0.925                               62

ADVANTAGES OF PRESSURIZED WATER REACTOR:

Water technology well known.
Water is cheap.
Water is very effective moderator of neutron energy
core is compact.
Water has high heat capacity.
Negative temperature coefficient.
Ordinary leakage can be tolerated.
Fission products are contained, not circulated.
Radioactivity of coolant is short-lived if kept pure.
Conversion ratio may be high.
Superheating steam in separately fired superheater is possible.
Appreciable fast fission effect attainable.

DISADVANTAGES OF PRESSURIZED WATER REACTOR:

Water must be highly pressurized to achieve even reasonably high temperature without boiling.
Fuel element fabrication expensive.
The temperature is limited in metallic fuel elements.
Fission product activity in the core builds up to high a level.
Pure hot water is highly corrosive, requires special materials for the primary loop.
Fuel must be at least slightly enriched.
Heat exchanger and control rods required.
Large excess reactivity at operating temperature.
Heat transfer only moderately efficient.
Fuel reprocessing a difficult task.
Rector must be shut down to unload and reload core.
Water would flash to steam in case of rupture of primary loop.
Water reacts with uranium, thorium, and structural metals under certain conditions.
Low thermal heads make heat exchanger, pumps and pipins large.
Hot-channel factors are significant.

BOILING WATER REACTOR

Although reactors using boiling heat transfer had been considered during the World War II Manhattan project in the United States, it was generally believed at that time such reactors would be unstable in operation. During the planning of the water cooled submarine reactors in 1947, the stability question was still unresolved and high pressure non-boiling reactors were chosen for the submarine program. The first test of basic heat transfer phenomenon was made at Argonne National Lab in 1952.

Some pictures of flow pattern in the boiling water reactor are shown in below:

slugs:

film boiling:

slugs:

the above pictures are taken from A Heat Transfer Textbook by Lienhard

Some properties of steam:

Coolant   Temp(F)  Density(lb/cuft)  Specific Heat(BTU/lb.F)  Thermal Conductivity  Viscosity(lb/(hr)(ft))
Steam	  440	   0.027	     0.48                     0.018	            0.044
          1160     0.017             0.52                     0.038                 0.072

ADVANTAGES OF BOILING WATER REACTOR:

Some intermediate heat exchange equipment eliminated.
Pressure is lower for given steam output conditions than in pressurized water reactor.
Metal surface temperatures are lower for given steam output conditions than in pressurized water reactor.
Power excursion quickly damped by formation of steam.
Overall thermal efficiency quite high.
Water is cheap.
Core is compact if void coefficient is low.
Negative temperature coefficient.
Ordinary leakage can be tolerated.
Fission products are contained, not circulated.
Radioactivity of coolant is short-lived if kept pure.
Conversion ratio may be high.
Heat may be taken from circulating water increasing power output.

DISADVANTAGES OF BOILING WATER REACTOR:

Boiling limits power density.
Radioactivity builds up in turbine.
Changes in load on turbine reflected back to reactor as pressure changes.
Separately fired superheater cannot conveniently be employed.
System must be designed to overcome tendency to react negatively to load increases.
Fuel must be at least slightly enriched.
Fuel handling necessitates complex equipments.
Reactor must be shut down to unload and reload core.
Water flashes to steam in case of rupture of primary system.
Condenser leak may cause serious trouble.

HEAVY WATER REACTOR

Although the reactors mentioned above are classified generally by the type of coolants used, the title heavy water refers to the use of heavy water as a moderator. The reactors discussed here are cooled by heavy water.

Some nuclear properties of Heavy water:

absorption cross section (barn)    scattering cross section (barn)    fractional energy loss/collision    moderating ratio
0.0011                             10.5                               0.504                               5000

ADVANTAGES OF HEAVY WATER REACTOR:

Any fuel including natural uranium can be used.
Heavy water is excellent moderator.
Heavy water has high heat capacity.
Specific power high.
Negative temperature coefficients.
Fission product are contained, not circulated.
Radioactivity of coolant is short lived if kept pure.

DISADVANTAGES OF HEAVY WATER REACTOR:

Heavy water expensive.
Special precautions must be taken to make primary loop leak proof and to prevent loss of heavy water during refueling operations.
Primary loop must be highly pressurized to achieve even high temperatures without boiling.
Pure hot heavy water is highly corrosive.
Fuel suffers radiation damage.
Heat exchanges and control rod required.
Heat transfer only moderately efficient.
Fuel reprocessing a difficult task.
Reactors must be shut down to unload and reload core.
Heavy water would flush to steam in case of rupture of primary loop.
Heavy water reacts with uranium, thorium and structural metals.
Low thermal heads make heat exchangers, pumps, and piping large.
Hot channel factors are significant.

ORGANIC COOLED REACTORS

Organic cooled reactors use certain types of organic liquids, particularly mixtures of diphenyl and diphenyl oxide have been used as high-temperature heat transfer media. Three major reasons for which it never became popular are: a general unfamiliarity with organic heat transfer agents; the poorer heat transfer properties of organics compared to water; and, particularly, the sensitivity of organics to radiation.

Some properties of Terphenyl:

Coolant   Temp(F)  Density(lb/cuft)  Specific Heat(BTU/lb.F)  Thermal Conductivity  Viscosity(lb/(hr)(ft))
mp 250F   600      53                0.60                     0.066                 0.77
bp 750F

Some nuclear properties of Polyphenyl:

absorption cross section (barn)    scattering cross section (barn)    fractional energy loss/collision    moderating ratio
0.33                               25.0                               0.84                                 62

ADVANTAGES OF DIPHENYL REACTOR:

Boiling point of diphenyl is very high.
Extremely good moderator.
Fission products are contained, not circulated.
Negative temperature coefficients.
Coolant would not vaporize to any great extent in case of rupture of primary loop.
Coolant doesn't become radioactive.
Ordinary leakage can be tolerated.
Corrosion of metal surface is negligible.

DISADVANTAGES OF DIPHENYL REACTOR:

High temperatures tend to polymerized diphenyl.
Decomposition products of diphenyl may be deposited on fuel elements and other heat transfer surfaces.
Fuel suffers radiation damage.
Fuel reprocessing a difficult task.
Heat transfer not so efficient as in water system.
Hydrogen gas is produced.

LIQUID METAL COOLED REACTORS

The extraordinary heat transfer properties of liquid metals make them attractive reactor coolants for both thermal and fast reactors. In addition, liquid metal systems operate at low pressure require the minimum of pumping power and are capable of operating at the high temperatures required to generate steam for modern turbine generators. Two liquid metals, sodium and eutectic mixtures of sodium and potassium have been used extensively in nuclear reactors.

Some properties of Sodium:

Coolant   Temp(F)  Density(lb/cuft)  Specific Heat(BTU/lb.F)  Thermal Conductivity  Viscosity(lb/(hr)(ft))
mp 208F   752      53                0.306                    41.1                  0.65
bp 1621F  1022     51                0.301                    37.4                  0.54

Some properties of Sodium Potassium:

Coolant   Temp(F)  Density(lb/cuft)  Specific Heat(BTU/lb.F)  Thermal Conductivity  Viscosity(lb/(hr)(ft))
mp 66F    752      51                0.252                    16.0                  0.56
bp 1518F  1020     49                0.248                    16.4                  0.47

Some nuclear properties of Sodium:

absorption cross section (barn)    scattering cross section (barn)    fractional energy loss/collision    moderating ratio
0.45                               4.0                                0.83                                0.89

Some nuclear properties for sodium potassium:

absorption cross section (barn)    scattering cross section (barn)    fractional energy loss/collision    moderating ratio
1.1                                3.2                                0.0774                              0.225

ADVANTAGES OF SODIUM FAST REACTOR:

No moderator required.
Full advantage is taken of excellent heat removal characteristics of sodium.
Sodium doesn't react with uranium and thorium.
Fuel can be bonded to container with liquid metal.
Electromagnetic pumps can be used with fair efficiency.

DISADVANTAGES OF SODIUM FAST REACTOR:

Sodium reacts violently with water and actively with air.
Radiation damage a serious problem, except with molten fluid.
Sodium must be kept free of oxygen.
High thermal stress complicates reactor vessel and steam generator design.
Fuel handling very difficult.
Sodium is strongly activated by nuclear bombardment.
Special precautions must be taken to contain sodium which may leak out of the primary or secondary loop and to prevent its contracting atmosphere or water.
Provisions must be made to heat coolant if it freezes.

GAS COOLED REACTOR

The gas cooled natural uranium fuel reactors were not first built for the primary purpose of power production but rather for the production of plutonium. At the present state of the reactor art, it is difficult to argue that these gas cooled reactors are any more or less economic or reliable than the water, organic and sodium cooled reactors.

Some properties of Carbon Dioxide:

Coolant   Temp(F)  Density(lb/cuft)  Specific Heat(BTU/lb.F)  Thermal Conductivity  Viscosity(lb/(hr)(ft))
          440      0.0670             0.241                    0.0194                0.0463
          1160     0.0372             0.287                    0.0359                0.0875

Some properties of Helium:

Coolant   Temp(F)  Density(lb/cuft)  Specific Heat(BTU/lb.F)  Thermal Conductivity  Viscosity(lb/(hr)(ft))
He        1160     0.00339           1.248                    0.1570                0.1003

ADVANTAGES OF CARBON DIOXIDE GRAPHITE REACTOR:

Corrosion by coolant is negligible.
Coolant doesn't react with fuel or with other core materials.
Coolant will not flash into vapor if primary system is ruptured.
Coolant has very low capture cross section.
System can be designated with negative temperature coefficient.
Any fuel can be used, including natural uranium.
Coolant is cheap.
Ordinary leakage can be tolerated.
Gas turbine may be employed.

DISADVANTAGES OF CARBON DIOXIDE GRAPHITE REACTOR:

Reactor vessel and heat exchangers large and expensive.
Heat transfer efficiency is low.
Coolant must be pressurized.
Power density is low.
Carbon dioxide dissociates above 300 Centigrade.

BIBLIOGRAPHY
Nuclear Energy Technology, Knief.
The Science and Engineering of Nuclear Power, Goodman.
Introduction to Nuclear Engineering, Lamarsh.
Thermal Hydraulics of Advanced Nuclear Reactor, Y. A. Hassan.
Heat transfer and Fluid Flow in Nuclear Systems, Henri Fenech.
Nuclear Engineering, Bonilla.
Nuclear Power Plants, Loftness.
A Guide to Nuclear Power Technology, Rahn.

RIZWAN AHMED

NE 161

NOVEMBER28, 1994