# Optimized Water Distillation Layout for Detritiation Purpose
## 1 Introduction  
The Breeding Blanket (BB) represents the step forward from ITER to a Fusion Power Plant (FPP), being the most important and novel components. Its central role in the development of a FPP is demonstrated by the great attention posed by the European fusion community (e.g., the EUROfusion consortium) on the testing and qualification strategy for the EU-DEMO BB [1]. The essential functionalities of a FPP BB can be summarized as (i) the need to absorb the largest part of the fusion energy transported by neutrons and delivers it to the Primary Heat Transport System (PHTS), (ii) the ability to breed enough tritium for self-sufficiency of the machine, and (iii) the contribution, along with the vacuum vessel, to the shielding of various components from nuclear radiation. As such, the BB must ensure an efficient heat transfer toward the PHTS through large and thin metallic heat exchange surfaces and high temperatures, along with a proper tritium breed and removal, achieved with high-tritium-specific activities. These pose the basis for the tritium permeation issue from the BB to the primary coolant [2] and, once permeated into PHTS, its migration toward rooms and environment and the possible contamination of the Power Conversion System (PCS) through leaks and further permeation [3]. Within the EU-DEMO framework, the comprehensive nature of this process promoted a holistic investigation of the integration issue between the BB and its ancillary systems [4]. The mitigation approach for the tritium migration was conceived with preventing and protective measures consisting in anti-permeation barriers and in the Coolant PurificationSystem (CPS), respectively. The first works at preventing the tritium permeation toward PHTS by applying specific coatings to the metallic surfaces [5, 6], whereas the latter removes tritium permeated into primary coolant, keeping its content inside the PHTS below a target value [2]. The EU-DEMO Coolant Purification System is the topic of this activity. The choice of the technologies involved in the CPS primarily depends on the coolant medium. Currently, two BB concepts are under investigation for the EU-DEMO, namely the Water-Cooled Lithium Lead (WCLL) and the Helium-Cooled Pebble Bed (HCPB) [7, 8]. The present paper focuses on the WCLL concept in which water works as coolant medium and liquid lithium-lead alloy works as a breeder and neutron-multiplier [9]. Tritium generated inside the breeder can diffuse toward metallic surfaces and permeate into the PHTS. Due to the isotopic exchange reaction between HT and H\({}_{2}\)O, most of the tritium permeated inside the PHTS is trapped in water as HTO. Therefore, the function of the CPS is to remove tritium with a rate equal to the permeation rate and to keep HTO concentration below a target value derived from safety considerations. The reference technology for the EU-DEMO water CPS is currently the water distillation, benefiting on its intrinsic simplicity and safety [10]. Distillation is the most mature and widely used thermal separation process in the chemical industries [11], and the interest of fusion community is growing as demonstrated by experimental [12, 13, 14] and modelling [15] activities. The main drawback of this technology is the high energy consumption due to its operative principle. According with the preliminary design of the EU-DEMO, the power consumption of water CPS in the reboiler and condenser ranges between 0.6 and 5 MW depending on the target tritium-specific activity fixed in the PHTS [3, 10]. The current EU-DEMO baseline considers a fusion power of 2 GW with a target net power production of 500 MW [16]. Assuming a PCS net electric efficiency of 33.6% [17], the available power for ancillary systems (e.g., the fuel cycle, the heating and current drive systems, the toroidal field coils, and the cryoplant) is less than 200 MW. Therefore, optimization in the power consumption of these systems is desirable, also for the CPS that represents one of the most power consuming system of the fuel cycle.