This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE TRANSACTIONS ON PLASMA SCIENCE 1 A New Concept for a Higher Burn-Up Fraction Improvement in DEMO Reactor Yuri Igitkhanov , Christian Day, and Stylianos Varoutis Abstract—The possibility of improving the fuel burn-up frac- tion by recirculating a part of the helium and fuel particles and separating helium atoms from fuel atoms is investigated for the DEMOnstration Power Plant conventional divertor configuration. It is shown that the burn-up fraction can be considerably improved by a continuous exhaust gas reinjection. Index Terms—Burn-up fraction, DEMOnstration Power Plant (DEMO), DEMO divertor, helium enrichment, helium recycling. I. I NTRODUCTION O NE of the key challenges for a fusion power plant is the need to significantly increase the fuel burn- up fraction at least above 5% in order to make fusion energy sufficiently attractive [1]. For 2-GW DEMOnstra- tion Power Plant (DEMO) fusion power, the fueling rate necessary to replenish the burnt fuel is rather small (∼2.7 Pa m 3 /s) compared with the estimated fuel throughput (∼200–350 Pa m 3 /s) needed mainly for the replenishment of collateral fuel lost due to helium ash removal [2]. Therefore, the fuel burn-up fraction is rather small (∼1%), indicating the need to maintain the lowest possible fuel throughput for reducing the required tritium inventory. Different methods have been discussed to improve the burn-up fraction and helium ash exhaust. The use of helium retention in the first wall to keep the helium accumulation below an admissible level is limited because of the wall saturation, erosion, blistering, etc. For nonmetallic plasma-wall interaction, helium recycling with the wall is large enough so that under such high-recycling conditions, the exhaust of several percent of the recycling helium particles is sufficient to keep the helium concentration in the main plasma below a permissible level. However, this situation is unrealistic in DEMO, for which we expect an all-tungsten wall under high wall temperature operation. The sufficient pellet fueling in the hot high density active reactor zone in order to increase the effective particle confinement time is still an open issue. As for the helium ash exhaust, it is shown by Monte-Carlo simulation that helium particles could be enriched in the divertor region and the ash exhaust can be achieved by a relatively moderate pumping speed [3]. The degree of enrichment is, however, not Manuscript received June 30, 2017; accepted March 2, 2018. This work was supported by EURATOM Research and Training Program 2014–2018 through the EUROfusion Consortium under Grant 633053. The review of this paper was arranged by Senior Editor E. Surrey. (Corresponding author: Stylianos Varoutis.) The authors are with the Institute of Technical Physics, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany (e-mail: stylianos.varoutis@kit.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2018.2814785 so high to allow for a considerable amount of fuel particles to be pumped out simultaneously. Furthermore, experiments in the detachment regime show that the plasma density in the divertor region is fairly high. In this case, the enrichment is further reduced and the amount of fuel pumped out may become much larger. Another suggestion of helium separation is to integrate a palladium alloy membrane in the pumping port for selective pumping [4]. However, the erosion of the membrane during the long-term operation could be a problem. In this paper, we propose a new concept, which is based on the organization of a recirculation of helium and fuel particles between the divertor plenum and the scrape-off layer (SOL) . This approach can be considered as a variant of the direct recycling concept presented in [5]. This recirculation will reduce the amount of fuel eventually to be pumped out. The reduction can be achieved by employing the fact that the helium atoms after neutralization on the divertor plate readily penetrate into private flux region (PFR), thus, further increasing the helium enrichment in the divertor. This will mitigate the load on the pumping system and reduce the circulating fuel amount (especially reducing the tritium inventory). A divertor configuration with the vertical plates and particle bypassing loop connecting the plenum with the SOL area at the baffle region is suggested for facilitating the helium removal with moderate pumping flow and, consequently, the tritium inventory. II. FUEL BURN-UP FRACTION The fuel burn-up fraction is defined as the ratio of burn-up fuel rate 8 burn to the particle throughput 8 in , f b ≡ 8 burn /8 in , where the particle throughput equals the sum of the burn rate and the pumping rate: 8 in = 8 burn + 8 pump . The burn rate can be expressed as 8 burn = nV a /τ F and the pumping rate as 8 pump = nV/τ ∗ p , where τ F is the burning time, τ F = 1/nhσ DT v i, n is the plasma density and the fusion rate hσ DT v i= 3.68 × 10 -18 T -2/3 exp(-19.94 · T -1/3 ) m 3 /s [6], which strongly depends on the temperature profile, and consequently, τ ∗ p = τ p /(1- R) is the effective par- ticle confinement time. Here, τ p is the particle loss time related to the particle transport and R is the recycling coefficient. The values are averaged over the plasma volume V and the volume of active zone V a , where a stable fuel burn occurs. Using the temperature and density radial profiles from [7] and [8], the active zone for DEMO can be defined from the inequal- ity: n p (r )τ E ≤ 12T (r )/ E α hσv i F , where E α = 3.5 MeV, τ E ∼ 4 s, which for the DEMO reactor is fulfilled in the 0093-3813 © 2018 EU