5.1 The molten salt reactor is a nuclear reactor which uses graphite as reflector and structural material and fluoride molten salt as coolant. The graphite components will be submerged in the molten salt during the lifetime of the reactor. The porous structure of graphite may lead to molten salt permeation, which can affect the thermal and mechanical properties of graphite. Consequently, it is important to assess the effect of impregnation of molten salt on the properties of the as-manufactured graphite material.

5.2 The purpose of this guide is to report considerations that should be included in the preparation of graphite samples representative of that after exposure to a molten salt environment. The degree to which the molten salt will infiltrate the graphite will depend upon a number of factors, including the type of graphite and the type and extent of porosity, the properties of the molten salt, the impregnation pressure and temperature, and the duration of the exposure of the graphite to the molten salt.

5.3 The user of this guide will need to select impregnation parameters sufficiently representative of those in a molten salt reactor based on parameters provided by the designer. Alternatively, the user may select a standard set of impregnation conditions to allow comparisons across a range of graphites.

5.4 This guide is not intended to be prescriptive. A typical apparatus and associated procedure is described. Some indication of the sensitivity of the procedure to graphite type and impregnation conditions is given in He, et al.3

5.5 There are four major practical issues that must be addressed during the impregnation process:

5.5.1 The density of molten salt is greater than that of graphite. A specially designed tool is required to submerge graphite samples in the molten salt during the impregnation process.

5.5.2 Some molten salts (for example, FLiBe) are poisonous and it is therefore necessary to provide containment by performing procedures within a glove box.

5.5.3 The graphite must be kept away from air to avoid oxidation at high temperature. This can be achieved by performing the impregnation process within a glove box with a controlled atmosphere.

5.5.4 Pressure control of the molten salt can be difficult to achieve. A specially designed autoclave is needed to hold the sample and molten salt.

5.6 In order to assess the quantity of molten salt in the graphite, parameter D is used as a variable in measuring the mechanical and thermal material properties. For a low impregnation pressure, the parameter D_o is the ratio of salt volume to open pore volume. For a high impregnation pressure, the parameter D_t is the ratio of salt volume to total pore volume. The saturated value of D_o can be greater than 1 when the molten salt impregnation takes place at high pressure. It is postulated that the internal microstructure of graphite has been damaged by the impregnation pressure and some closed pores have been opened. In this case, the parameter D_t is more appropriate to represent the impregnation process.