Helium is one of a family of gases (which also includes argon, neon, and xenon) commonly referred to as the inert gases or noblegases. Apart from some exceptions of purely academic interest, atoms of these gases do not form com­pounds with other elements (hence their description as inert). Helium, which has a molecular weight of 4, is present in small quantities in the atmosphere but is more commonly derived from oil and natural gas wells.

The inert gas argon (atomic mass 40) is much more available; air contains

0. 94% by volume of this gas. Unfortunately, argon is not suitable as a reactor coolant, since irradiation by neutrons causes it to form a radioactive isotope (argon-41) that decays with a half-life of 1.8 h, emitting both p and y rays. This neutron absorption and the resultant activation of the coolant circuit are unac­ceptable. Helium, though more expensive than argon, is not activated in a neu­tron flux and is, therefore, much more suitable.

Helium has been employed in the so-called high-temperature gas-cooled re­actor (HTR). Here, the fuel is in the form of uranium carbide clad in graphite, which acts as both the cladding material and the moderator. With helium it is possible, in principle, to operate such reactors at very high temperatures (typi­cally in excess of 800°C) without any chemical attack on the moderator-dad. However, it is usually impossible to maintain the helium coolant in a pure state, because in an actual circuit there will be a small leakage of water vapor from the boilers, ingress of air and other materials through leaks of the circulators, and release of gases originally adsorbed on the graphite. Although the helium itself does not react with the graphite or the steel structures even at high oper­ating temperatures, the impurities do, and this limits the temperatures that can

3.1.1 Steam

Steam has better thermodynamic properties as a coolant than carbon dioxide. Its high specific heat allows good heat transport with lower mass flow rates and smaller, more compact piping systems than in other gas-cooled units. This has led to a number of studies of the possibility of using steam as a reactor coolant. However, at high temperatures and pressures, steam is a highly corrosive oxi­dizing fluid, and stainless steels may be the only suitable construction materials for use with steam at temperatures above about 600°C.

In a conventional oil-fired or coal-fired boiler, it is normal to superheat the steam (i. e., increase its temperatures above the saturation temperature) before feeding it to the turbine. This increases the overall thermodynamic efficiency of the power generation of the cycle. In the normal nuclear boiler (e. g., the B^WR the steam is not superheated. However, there have been a number of attempts to introduce superheating in nuclear boilers and to make the nuclear reactor closer to a conventional system. In this case, the steam can be regarded as a supplementary coolant to the boiling water in the other parts of the reactor. In general, it is not economically attractive to introduce superheating in this way, mainly because it requires the use of stainless steel cans and hence an increase in the enrichment of the fuel. However, a number of plants in the former Soviet Union do employ superheating on a regular basis.