Nuclear Reactors: Coolant Materials The coolant, as the name implies, is used to remove the heat produced in the core and from other parts of nuclear reactors where heat is produced. The choice of coolant for a given type of a reactor is governed by various factors such as nuclear properties, heat transfer and transport properties, physical properties, chemical properties, corrosion behavior, stability, induced radioactivity, and cost. In practice, the coolant is chosen as a compromise between the various requirements. The main nuclear properties are neutron absorption and scattering cross-sections. The coolant should have a low neutron absorption cross-section; the value of the scattering cross-section depends on the reactor type: thermal or fast reactor. In the case of thermal reactors the scattering cross-section should be large, and in the case of fast reactors it should be small. The coolant should have a high heat transfer coefficient and require low pumping power. The boiling point should be high, except in reactors where boiling is desired, and the melting point should be low. The coolant should be compatible with the fuel cladding and the structural materials with which it makes contact. In the case of coolants other than water, the compatibility of coolant with water is an important aspect, in view of possible leaks in the steam generator. The coolant must not give rise to a fire hazard in case of a leak to the atmosphere. The coolant should be stable under operating temperature and radiation environment. The induced activity in the coolant should be a minimum to reduce the need for shielding. Finally, the coolant must be cheap and easily available.
sorption cross-section of heavy water is much smaller, about 1\500 of that of light water. Except for the very significant advantage of the neutron-moderating property of heavy water in comparison to light water, the difference between them in terms of the thermophysical and transport properties of interest as coolants is small. However, heavy water is very expensive. The pressurized heavy water reactor (PHWR) (Fig. 3) uses the pressure tube concept. Heavy water flows at high pressure through these tubes which contain the fuel. Hot heavy water in turn transfers heat to the feed water in the steam generators and the steam produced is sent to the turbine. There are a few reactors which employ heavy water as the moderator in combination with another material, such as light water, as the coolant. The fuel choice in this particular case is different from natural uranium. There is no serious neutron activation problem in water-cooled reactors. "'N formed by the (η,p) reaction with "'O has a 7.4 s half-life and therefore the activity is negligible after a few minutes of reactor shutdown. A serious drawback of water as a coolant is its high vapor pressure. The coolant system pressure in BWRs, PWRs, and PHWRs is approximately 7 MPa, 15 MPa, and 10 MPa, respectively. The steam temperature at the turbine inlet in these types of reactors
1. Reactor Coolants 1.1 Water Water has many desirable characteristics and most reactors for power production use ordinary (light) water as the coolant. In a boiling water reactor (BWR) (Fig. 1), the light water is allowed to boil and the separated steam is fed to the turbine. The condensed steam returns to the reactor through feed water heaters and pumps as in a conventional fossil fuel power station. In a pressurized water reactor (PWR) (Fig. 2), the light water is maintained at a sufficiently high pressure so that the temperature of coolant leaving the reactor is below the saturation temperature. The highpressure hot water from the reactor transfers heat to the feed water in the steam generators and the steam produced is sent to the turbine. In both BWRs and PWRs, light water is both the moderator as well as the coolant and the fuel is based on either enriched uranium or a mixture of natural uranium and plutonium. The principal advantage of heavy-water moderated and cooled reactors is that natural uranium can be used as the fuel. The thermal neutron ab-
Figure 1 Boiling water reactor.
Figure 2 Pressurized water reactor.
Nuclear Reactors: Coolant Materials peratures. Helium is capable of working at extremely high temperatures, and helium-cooled reactors are being employed and considered in some countries for power generation based on both steam turbines (Fig. 4) and direct-cycle gas turbines and to provide process heat in chemical and other industries as well as for domestic heating purposes.
1.4 Liquid Metals
Figure 3 Pressurized heavy water reactor.
is rather low, approximately 532–573 K, compared to 811 K in modern fossil fuel power stations. The lower steam conditions in these reactors result in lower overall efficiency, of the order of 33% for BWRs and PWRs and 29% for PHWRs, compared to about 42% for fossil fuel power plants.
1.2 Organic Coolants Coolants based on diphenyl and terphenyl have been used in reactors. Their good moderating properties, high boiling point, noncorrosive nature, and relative cheapness make them attractive as coolants. Their major drawback is decomposition by nuclear radiation. There is virtually no interest in organic-cooled reactors.
1.3 Gases The heat transfer characteristics of gases are poor in comparison to liquids. To improve heat transfer and reduce pumping power, it is necessary that the gas system be pressurized. Heat removal can be further improved by having fins on the fuel elements. Gases as coolants have the advantages of good radiation and thermal stability, ease of handling, and compatibility with structural materials. Carbon dioxide has been extensively used in graphite-moderated gas-cooled reactors, as in the Magnox and advanced gas-cooled reactors (AGRs) in the UK. AGRs operate with an overall efficiency of about 40% and are comparable in this respect to modern fossil fuel power stations. The heat transfer coefficient of helium is higher than that of carbon dioxide and it also has advantages with respect to lower pumping power and neutron absorption. Carbon dioxide has the additional drawback of interacting with the graphite moderator at high tem2
Sodium, sodium–potassium alloy, mercury, lead, and lead–bismuth alloy are the liquid metals that have been used as coolants in nuclear reactors. Sodium is the coolant for all fast reactors in operation. Alkali metals in particular have excellent heat transfer properties and high boiling points. Among the various liquid metals, sodium has found the widest use in fast reactors because of its high heat transfer coefficient, poor moderation, low pumping power, very high boiling point (1155 K), low melting point (371 K), compatibility with structural materials provided oxygen impurity is controlled, and commercial availability at a relatively low price. In a liquid metal-cooled fast breeder reactor, the sodium coolant in the primary circuit picks up the heat produced in the core and transfers it to a secondary sodium circuit, which in turn produces the steam in the steam generators (Fig. 5). Sodium has drawbacks associated with its intense chemical activity (catching fire) in the case of a leak to the atmosphere. This leads to the incorporation of a double envelope for the primary circuit with an inert interspace. The violent reaction of sodium with water makes the design of sodium-heated steam generators a challenging task. Another drawback of sodium is its activation. #%Na as a result of the (n,γ) reaction and ##Na as a result of the (n,2n) reaction are the main activation products. The half-lives of #%Na and ##Na are 15 h and 2.6 yr, respectively. As a result, sodiumcooled reactors need biological shielding. Also, there is a delay of a few days after reactor shutdown before access can be safely gained to the primary circuit for any maintenance activity. The excellent heat transfer of sodium coupled with its low specific heat leading to higher coolant temperature rise makes the thermal stresses of great significance in the mechanical design of sodium components. The other alkali metals have less favorable properties in comparison to sodium. The eutectic alloy of sodium with potassium (NaK), however, has the advantage that it is in the liquid state at room temperature. Among the heavy metals there is renewed interest in lead–bismuth and lead as coolants for fast reactors to overcome the serious drawback of the chemical activity of sodium. Using these coolants, the secondary circuit associated with the sodium-cooled reactor is unnecessary. Lead–bismuth and lead have relatively poor heat transfer and transport properties compared to sodium, control of oxygen is needed to minimize
Nuclear Reactors: Coolant Materials formation of polonium and the higher cost, lead as a coolant instead of lead–bismuth is under study, particularly in Russia. Figure 6 shows a schematic diagram of a lead-cooled fast reactor. The high melting point (600 K) and the structural material compatibility are the two challenging tasks in the design and operation of a lead-cooled reactor. Mercury was used in Clementine, the first fast reactor. However, it is not an attractive coolant owing to its relatively poor nuclear and heat transfer properties, toxicity, and high cost. Liquid metal-cooled reactors, in general, generate steam conditions to provide an overall efficiency of around 40%.
1.5 Molten Salts Figure 4 High-temperature gas-cooled reactor.
Figure 5 Sodium-cooled fast reactor.
Molten salts, especially fluorides, have been used as reactor coolants because they provide advantages associated with a fluid-fuel type of reactor and have favorable high-temperature properties. In one reactor of this type (molten salt reactor experiment), a molten mixture of the fluorides of lithium, beryllium, zirconium, enriched uranium, and thorium is pumped through channels provided in graphite moderator elements. As the molten mixture passes through the core, the system becomes critical and the fission energy is absorbed directly in the fluid. The hot fluid then transfers the heat to a secondary coolant, consisting of a mixture of lithium fluoride and beryllium fluoride, in an intermediate heat exchanger and returns to the reactor. Molten salt reactors have the potential to provide an overall efficiency of about 44% and breeding. However, molten salt reactors have some serious drawbacks, like the highly corrosive nature of the molten salts and a requirement for remotely operated equipment for maintenance and repairs, which do not permit continued interest in their development for power generation.
1.6 Chemistry Control of Nuclear Reactor Coolants
Figure 6 Lead-cooled fast reactor.
corrosion of structural materials, and the melting point of bismuth (398 K) is also higher than sodium. A serious drawback to the use of lead–bismuth alloy is the formation of toxic polonium through neutron capture by bismuth. To overcome the problem of the
Chemistry control of coolant is of importance in reducing the corrosion of structural materials and in minimizing the man-rem consumption during operation and maintenance of the reactor. The control of pH and oxygen are the two important factors in PWRs and PHWRs. Lithium hydroxide is normally used for pH control of primary water, the pH being between 6.9 and 7.4 at operating temperature. Hydrogen is normally used for oxygen control. BWRs, unlike PWRs, operate at close to neutral pH, as the system is not closed. Oxygen impurity in sodium is the key factor in the corrosion of structural materials in sodium-cooled systems. Oxygen impurity in sodium is kept normally below 5 ppm by purification of sodium in a cold trap. Lead- or lead–bismuth-cooled reactors 3
Nuclear Reactors: Coolant Materials need a controlled amount of oxygen in the coolant to reduce the corrosion of structural materials.
2. Future Developments Water will continue its dominant position as a coolant for thermal reactors. Helium will receive more attention as a coolant in high-temperature reactors for process heat and power generation purposes. As alternatives to sodium, lead and lead-bismuth are likely to be seriously evaluated as coolants for fast reactors. See also: Nuclear Reactors : Moderator and Reflector Materials; Nuclear Reactor Materials
Bibliography Bukousky J, Haack K 1994 Heay Water Handbook. Risø National Laboratory, Roskilde, Denmark Glasstone S, Sesonske A 1967 Nuclear Reactor Engineering. Van Nostrand Reinhold, New York IAEA 1999 Status of Liquid Metal Cooled Fast Reactor Technology, TECDOC-1083. International Atomic Energy Agency, Vienna Lamarsh J R 1983 Introduction to Nuclear Engineering. AddisonWesley, Reading, MA Rahn F J, Adamantiades A G, Kenton J E, Braun C 1984 A Guide to Nuclear Power Technology. Wiley Interscience, New York Yevick J G, Amorosi A 1966 Fast Reactor Technology: Plant Design. MIT Press, Cambridge, MA World Nuclear Industry Handbook 1999 Nuclear Engineering International, Wilmington, NJ
S. C. Chetal
Copyright ' 2001 Elsevier Science Ltd. All rights reserved. No part of this publication may be reproduced, stored in any retrieval system or transmitted in any form or by any means : electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the publishers. Encyclopedia of Materials : Science and Technology ISBN: 0-08-0431526 pp. 6361–6365 4