## 1 Introduction  
The increased energy demand of our age may require answers to environmental issues such as the management and disposal of radioactive waste [1]. Today, the most effective solution to this problem is the disposal in deep geological repositories [2]. The primary task of such a facility is to be inert and permanently isolated from the outside world with a high level of technical safety over the long term (in some cases more than 100,000 years [3]. To achieve such safety, natural and engineered barriers work together to ensure the content of the waste. The most critical natural barrier is a geological barrier: the host rock, which has several primary functions of long isolation. The geological, hydrogeological, geophysical, geochemical, and rock-mechanical parameters of the host formations must meet very strict rock mechanical requirements [1, 4].  
Granite--or its metamorphosed type geniss--is used to dispose of radioactive waste in Finland, Sweden, and Hungary. In Hungary, low- and intermediate-level radioactive waste is stored at the Batapati National Radioactive Waste Repository, located at the foot of the Eastern-Mecsek Mountains. The research project first examining the site started in 1993, after which the construction and the repository started in 2012. Now, it is in full operation, hosting low- and medium-level radioactive waste [5]. The disposal chambers are located below the surface at 200-250 m, with more than 1.7 km-long access tunnels leading to the disposal chambers. The host rock forms a part of the Carboniferous Moragy Granite Formation [6].  
Granite has already proven their worth as a host rock type many times before and, therefore, is considered one of the best target rocks for radioactive waste disposal. Among the many investigated mechanical features of this rock type, one of the most crucial is the mechanical strength loss due to elevated temperatures. The study of thermal effects on rock properties is an emerging area of research with publications on the thermal behavior of several lithologies, including limestone [7; 8; 9], sandstone [10; 11], and granite [12; 13; 14]. Besides the mechanical changes, mineralogical changes and other physical transformations are linked to increasing temperature in rocks such as granite [15; 16] or especially carbonate rocks such as limestone [17; 18; 19].  
One of the main safety factors of a deep geological radioactive waste repository is to ensure that high temperatures do not occur and that the facility operates within normal temperature conditions. However, in certain failure events (such as a sudden fire or heat caused by the decay of radioactive waste), the temperature of the facility can rise to several hundred degrees Celsius. In previous works, it has been demonstrated that micro-cracks are already formed at lower temperatures [20] due to thermal expansion [21], cooling [22], or being linked to thermal cycles of 20 \({}^{\circ}\)C to 105 \({}^{\circ}\)C [23; 24]. Temperature increase generates stresses in granitic rocks [25] which is also associated with the anisotropy of granite micro-texture [26; 27]. Understanding such behavior and cracking-linked damage of the granitic rocks need further studies. Suppose the waste disposal site is subjected to elevated temperatures for short or long periods. In this case, the facility's structure and the host rock are exposed to the negative consequences of the thermal effects, which is one of the critical risks in safety [28]. For these reasons, the changes resulting from thermal treatment must be analyzed and interpreted in the design.  
This study focuses on the thermal-induced physical alteration and color change of the primary rock type of the Moragy Granite Formation.