1.1 INTRODUCTION

1.1.1 Choice of Coolant

This chapter describes the engineering of the remainder of the plant in a fast reactor electricity-generating station, apart from the reactor core that is the subject of Chapter 3. The nature of the plant depends primarily on the coolant, which is the heat-transfer medium. The main considerations determining the choice of the coolant were explained in sections 3.2.3 and 3.2.4. The most important is that the high power density of a fast reactor core demands a high-density coolant and high coolant velocities. The relative advantages and disadvantages of the various possible coolants can be summarised in terms of the choices available to a reactor designer, as follows.

Liquid or Gas. Helium has the advantage that it is chemically inert and is therefore appropriate for use in a high-temperature reactor. CO2 has the advantage that there is extensive experience of its use in thermal reactors. Neither presents significant problems of corrosion or erosion. However any gas coolant has to be pressurised to make it dense enough to transport heat out of the core without unreas­onably high velocities. The major consequent disadvantage is that it is then very hard to guarantee that decay heat could be removed safely in the event of an accidental loss of pressure. It would be

necessary either to accept relatively low power density in the core (compared with what is possible if a liquid coolant is used) or to provide elaborate emergency cooling equipment for use in the event of a major breach of the primary coolant system. For this reason no gas-cooled power-producing fast reactor has, at the time of writing, been built and thus there is no operating experience, but that does not mean that gas coolant may not at some time in the future become attractive.

Water or Liquid Metal. Water is almost inevitably ruled out as a coolant for a fast reactor because the moderating effect of the hydro­gen would degrade the neutron energy spectrum to the extent that its advantages — either as a breeder of fissile material or as a consumer of radioactive waste — would be lost or at least drastically reduced. A fast reactor cooled with supercritical water (“supercritical” in the thermo­dynamic sense, at a pressure above the critical pressure of 22.12 MPa) has been suggested but never taken beyond the stage of an outline design.

All liquid metals have the major advantage that they do not have to be pressurised so the reactor structure can be relatively light. In the case of an accident the decay heat can be removed by ensuring that the fuel stays immersed in the coolant, and it may be possible to arrange the primary coolant circuit so that even if the pumps fail natural convection cooling is adequate.

An important disadvantage of liquid metals is that they are opaque, which makes inspection of the core and coolant circuit structures and components difficult.

Light or Heavy Liquid Metal. The alkali metals lithium, sodium and potassium all suffer from the major disadvantage that they react chemically with air and water. They have the advantage that they are light, and they are not corrosive. They have low melting temperatures so that it is relatively easy to avoid freezing, but also low boiling temperatures so that there is a possibility that the cooling may be impaired under extreme accident conditions. They are all moderators.

Sodium and potassium are cheap. Lithium is too expensive and too much of a moderator to be considered.

Heavy liquid metals such as lead or bismuth have the major dis­advantages that they corrode steel and that at more than moderate velocities they cause erosion and cavitation damage, particularly in pump rotors. In addition they are heavy and expensive and they have high melting temperatures. Their advantages are that they do not react chemically with water, they have high boiling temperatures, and they are poor moderators (so they do not degrade the neutron energy) while having high scattering cross-sections (so they reduce neutron leakage from the core).

Mercury was the coolant for very early experimental fast react­ors in the United States and the Former Soviet Union but has never been used since. It would not be contemplated now because it is too expensive and too toxic.

Sodium or Potassium. Although both are chemically reactive potassium is rather more hazardous than sodium. Compared with potassium, sodium has the disadvantage that it becomes radioactive by the 23Na(n, y)24Na reaction. The resulting 24Na decays with a 15- hour half-life. While the reactor is operating the specific activity of the sodium primary coolant may exceed 30 GBq/kg.

It has never proved possible to eliminate the possibility of a leak in a sodium-heated steam generator, and in fact such leaks have occurred quite frequently. A large steam-generator leak generates large volumes of steam and hydrogen accompanied by tens or hun­dreds of kilograms of NaOH, and the only way to protect the reactor is to vent these reaction products to the atmosphere. This cannot be contemplated if they are radioactive. Therefore the steam gener­ators cannot be heated directly by the radioactive primary sodium, and intermediate nonradioactive secondary sodium circuits have to be interposed.

Sodium, being lighter, has a greater moderating effect than potassium. This is a disadvantage not only in that it degrades the neutron energies but also because removal of sodium from the centre of the core causes a positive reactivity change. This gives an undesir­able positive contribution to the temperature coefficient of reactivity and, in a severe accident that causes the sodium to boil, a substantial reactivity increase.

However sodium has the overwhelming advantage that it is cheap and readily available, and for this reason, in spite of its disadvantages, it has been used almost universally as the coolant for fast power reactors.

Pure potassium has never been used as a reactor coolant but in a few cases sodium-potassium alloy, usually referred to as “NaK”, has proved attractive. Pure sodium melts at 97.8 °C so care has to be taken to avoid freezing, but the admixture of potassium reduces the melting point. Eutectic NaK (77%K, 23%Na) freezes at -12.6 °C.

Lead or Lead-Bismuth Alloy. The melting point of pure lead is 327 oC so extensive reliable trace heating of the primary coolant circuit has to be employed to avoid freezing. However, as in the case of NaK, the addition of bismuth can reduce the requirement substantially. Eutectic Pb-Bi (55%Bi, 45%Pb) freezes at 123.5 °C.

The main disadvantage of lead-bismuth alloy is the reaction 209Bi(n, y)210Po. The resulting 210Po is а-active with a half-life of 3.5 x 106 years and constitutes a major hazard in reactor maintenance and refuelling. In addition bismuth is expensive. In spite of these disad­vantages lead-bismuth was chosen as the coolant for the fast reactor power plants of the innovative — but secret — USSR “Alpha”-class submarines, and their deployment has provided many reactor-years of operating experience. When in the 1990s the Russian authorities began to release this information lead-bismuth came to be regarded as a serious alternative to sodium as a coolant for fast reactors.