3.19.
Limitation on the isotopic composition of the elements in the fissile material, or restriction to a certain type and chemical compound of the fissile material, or a combination of both, is essential for ensuring criticality safety in many cases. Effective safety measures should be applied to ensure that:
The limits on the isotopic composition of the elements in the fissile material are complied with;
The compound to be used cannot change to become a more reactive compound;
A mixture of different types or different compounds resulting in a higher effective neutron multiplication factor cannot occur.
3.20.
As the last two events listed above could, in specific situations, occur — for example, the precipitation of a U/Pu nitrate solution — they should be taken into account in the criticality safety assessment and proven to be subcritical.
3.21.
The presence of neutron moderating materials should be considered, as these can significantly reduce the critical mass of the fissile material. Hydrogen and carbon contained in materials such as water, oil and graphite are common moderators. Low atomic mass, low neutron absorption materials (such as deuterium, beryllium and beryllium oxide) are less common but can be very effective moderators. Consideration should be given to replacement of a moderator with an alternative substance having lower or no moderating properties; in the case of oils, for example, there is the possibility that long chain CH2 type oils (i.e. aliphatic hydrocarbons) could be replaced with oils containing (for instance) fluorine or chlorine.
3.22.
The presence of neutron reflecting material should be considered. Material present outside the system of fissile material will act as a neutron reflector and can increase the neutron multiplication factor of the system. Criticality safety assessments usually consider a light water reflector of a thickness sufficient to achieve the maximum neutron multiplication factor, known as ‘total reflection’ or ‘full light water reflection’. However, the possible presence of other reflector materials (such as polyethylene, concrete, steel, lead, beryllium and aluminium), or several reflector materials used in combination, should be considered, if this could result in a greater increase of the neutron multiplication factor than by full light water reflection.
3.23.
The presence of neutron absorbers should be considered. Neutron absorbers are mainly effective for thermal neutron systems. Therefore, any neutron spectrum hardening, i.e. an increase in the distribution of neutron energy, caused by operating conditions or accident conditions, should be considered, as this may result in a decrease in the effectiveness of the neutron absorption. Therefore, when the safety function of a neutron absorber is necessary, safety measures should be applied to ensure that the effectiveness of the neutron absorber is not reduced. Consideration should be given to monitoring the credible long term degeneration and/or degradation of neutron absorbers.
3.24.
The geometrical distribution of neutron absorbers and credible changes in their distribution should be considered. Changes in the geometrical distribution of neutron absorbers could include slumping, evaporation or compression.
3.25.
Neutron absorbers that are homogeneously distributed in a thermal neutron system are usually more effective than if they were heterogeneously distributed (however, heterogeneously distributed absorbers may be easier to control by administrative means). In a thermal neutron system consisting of a heterogeneous arrangement of fissile material and a fixed neutron absorber (e.g. the storage of fuel assemblies), the neutron absorber may be more effective the closer it is located to the fissile material. Any material (e.g. water, steel) located between the absorber and the fissile material can change the effectiveness of the absorber. Solid, fixed neutron absorbers should be tested and/or validated prior to first use in order to demonstrate the presence and uniformity of the distribution of the absorber isotope (e.g. 10B). Demonstration of the continued presence and effectiveness of neutron absorbers throughout their operational lifetime should be considered.
3.26.
Material (e.g. steam, water mist, polyethylene, concrete) located between or around fissile material may act not only as a reflector but also as a moderator and/or a neutron absorber and can therefore increase or decrease the neutron multiplication factor of the system. Any change in the neutron multiplication factor will be dependent on the type and density of the material positioned between or around the fissile material. Materials with low density (such as steam or foam) can cause a significant change in the neutron multiplication factor. The inclusion or omission of any materials from the criticality safety assessment should be justified by evaluating the effect of their treatment on the neutron multiplication factor.
3.27.
Interaction between units of fissile material should be considered, as this interaction can affect the neutron multiplication factor of the system. This control parameter can be used to ensure criticality safety, for example by specifying minimum separation distances (or in some cases maximum distances, e.g. to limit interstitial moderation between fissile material units) or by introducing screens of neutron absorbers. Wherever practicable, separation should be ensured by engineered means, for example fixed storage racks for storage of arrays of drums containing fissile material.
3.28.
Heterogeneity of materials such as swarf (turnings, chips or metal filings) or fuel pellets can result in neutron multiplication factors greater than those calculated by assuming a homogeneous mixture, particularly for low enriched uranium systems or for mixed uranium and plutonium. Therefore, the degree of heterogeneity or homogeneity used or assumed in the criticality safety assessment should be justified. Safety measures should be applied that ensure that heterogeneity of the fissile material could not result in a higher neutron multiplication factor than considered.
3.29.
The temperature of materials may cause changes in density and in neutron cross-section, which may affect reactivity. This should be considered in the criticality safety assessment.