Significance and Use
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 specimens 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 are described. Some indication of the sensitivity of the procedure to graphite type and impregnation conditions is given in He, et al.5
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 specimen 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. Parameter Do is the ratio of salt volume to open pore volume. Parameter Dt is the ratio of salt volume to total pore volume. The saturated value of Do 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 high impregnation pressure and some closed pores have been opened. In this case, the parameter Dt is more appropriate to represent the impregnation process.
Scope
1.1 This guide covers procedures for the impregnation of graphite with molten salt under a consistent pressure and temperature. Such procedures are necessary if the user wishes to prepare graphite specimens for testing that represent material that has been exposed to a molten salt environment in a molten salt nuclear reactor. The user will need to ensure that impregnation temperature and pressure conditions reflect those pertaining to the molten salt environment, noting that the properties of the material will change once it becomes irradiated.
NOTE 1: The term impregnation is used throughout this guide as this is the correct term for the described process. Other terms such as infiltration and intrusion may be encountered by the user in other texts and the term intrusion is commonly used to describe penetration of open porosity in graphite in a molten salt reactor environment.
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this guide.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
