T. D. Burchell

4.10.1 Introduction

There are many graphite-moderated, power — producing, fission reactors operating worldwide today.1 The majority are in the United Kingdom (gas-cooled) and the countries of the former Soviet Union (water — cooled). In a nuclear fission reactor, the energy is derived when the fuel (a heavy element such as 92U235) fissions or ‘splits’ apart according to the following reaction:

235 1 236

92U +0 n ^92 U! Fi + F2 + n

+g — energy

An impinging neutron usually initiates the fission reac­tion, and the reaction yields an average of 2.5 neutrons per fission. The fission fragments (F1 and F2 in eqn [I]) and the neutron possess kinetic energy, which can be degraded to heat and harnessed to drive a turbine — generator to produce electricity. The role of graphite in the fission reactor (in addition to providing mechani­cal support to the fuel) is to facilitate the nuclear chain reaction by moderation of the high-energy fission neu­tron. The fission fragments (eqn [I]) lose their kinetic energy as thermal energy to the uranium fuel mass in which fission occurred by successive collisions with the fuel atoms. The fission neutrons (n in eqn [I]) give up
their energy within the moderator via the process of elastic collision. The g-energy given up in the fission reaction (eqn [I]) is absorbed in the bulk of the reactor outside the fuel, that is, moderator, pressure vessel, and shielding. The longer a fission neutron dwells in the vicinity of a fuel atom during the fission process, the greater is its probability of being captured and thereby causing that fuel nucleus to undergo fission. Hence, it is desirable to slow the energetic fission neu­tron (E ~ 2 MeV), referred to as a fast neutron, to lower thermal energies (^0.025 eV at room temperature), which corresponds to a velocity of 2.2 x 1015 cms-1.

The process of thermalization or slowing down of the fission fast neutron is called ‘moderation,’ and the material in a thermal reactor (i. e., a reactor in which fission is caused by neutrons with thermal energies) that is responsible for slowing down the fast fission neutrons is referred to as the moderator. Good nuclear moderators should possess the following attributes:

• do not react with neutrons (because if they are captured in the moderator the fission reaction cannot be sustained);

• should efficiently thermalize (slowdown) neutrons with few (elastic) collisions in the moderator;

• should be inexpensive;

• compatible with other materials in the reactor core;

• meet the core structural requirements; and ideally

• do not undergo any damaging chemical or physi­cal changes when bombarded with neutrons.

In the fast neutron thermalization process, the maxi­mum energy lost per collision occurs when the target nucleus has unit mass, and tends to zero for heavy
target elements. Low atomic number (Z) is thus a prime requirement of a good moderator. The density (number of atoms per unit volume) of the moderator and the likelihood of a scattering collision taking place must also be accounted for. Frequently used ‘Figures of merit’ for assessing moderators are the ‘slowing down power’ and the ‘moderating ratio.’ Figure 1 shows these Figures of merit for several candidate moderator materials. The slowing down power accounts for the mean energy loss per collision, the number of atoms per unit volume, and the scatter­ing cross-section of the moderator. The tendency for a material to capture neutrons (the neutron capture cross-section) must also be considered. Thus, the sec­ond figure of merit, the moderation ratio, is the ratio of the slowing down power to the neutron absorption (capture) cross-section. Ideally the slowing down power is large, the neutron capture cross-section is small, and hence the moderating ratio is also large.

Practically, the choices of moderating materials are limited to the few elements with atomic number <16. Gasses are of little use as moderators because of their low density, but can be combined in chemical compounds such as water (H2O) and heavy water (D2O). The available materials/compounds reduce to the four shown in Figure 1 (beryllium, carbon (graphite), water, and heavy water). Water is rela­tively unaffected by neutron irradiation, is easily contained, and inexpensive. However, the moderating ratio is reduced by the neutron absorption of hydro­gen, requiring the use of enriched (in 235U) fuels to maintain the neutron economy. Heavy water is a good moderator because 1H2 and 8O16 do not absorb neu­trons, the slowing down power is large, and the mod­erating ratio is therefore very large. Unfortunately,

the cost of separating the heavy hydrogen isotope is large. Beryllium and beryllium oxide are good mod­erators but are expensive, difficult to machine, and suffer toxicity problems. Finally, graphite (carbon) is an acceptable moderator. It offers a compromise between nuclear properties, utility as a core structural material, and cost. It also has the advantage of being able to operate at very high temperatures (in the absence of oxygen). Unfortunately, the properties of graphite are markedly altered by neutron irradiation and this has to be considered in the design of graphite reactor cores.