A fusion chamber design with a liquid first wall and divertor
Summary (2 min read)
Introduction
- In their designs, modeling of the plasma edge defines the heat loads, and thermal-hydraulic analyses show that the heat removal is adequate.
- In FY2002 the authors began their design with the molten salt, Flinabe, described later.
II. CHAMBER CONFIGURATION
- In their design, strong radiation from the core plus strong radiation from the divertor region is used to reduce the peak heat load in the divertor and balance the power loads of the divertor and first wall.
- The recent plasma transport modeling by Rognlien and Rensink, finds a stable operating window for a highly radiating edge plasma including steady state modeling solutions in which about 95% of the power coming into the scrape-off layer is radiated near the X-point for alpha powers in the range of 300-360 MW.
- This work is described briefly in their companion paper[1] and elsewhere[2,31].
- The plots are maps of flourine radiation in the lower portion of the chamber cross section with strong impurity line-radiation concentrated near the X-point and below.
III. FLOW PATHS AND HEAT TRANSFER
- Fig. 3 shows the flow streams and bulk temperatures.
- The approach is based on the standard “K-_” model used widely in engineering applications to characterize turbulent flow, and was modified by Smolentsev and others to include the effects of MHD on the turbulence, particularly in the region of fluid near the free surface.
- Near the bottom of the chamber, these first wall streams become the inboard and outboard divertor flow2.
- The rise in surface temperature for the divertor stream is about 135ºC; this, added to the bulk temperature of 420ºC leaving the first wall gives a peak of about 555ºC.
- The Flinabe flow enters each side channel of the blanket at 422°C and 0.18 m/s, flows to the front and then enters the multiplier region at 483°C and flows at 0.013 m/s into the interior space most of which is an open volume.
IV. ENERGY CONVERSION AND MATERIALS
- For good power conversion efficiency, the authors need a large operating temperature window.
- The power balance in their design is based on (a) the distribution of the alpha and auxiliary power plus (b) the management of coolant flow that includes a recirculating stream for the first wall, shown in Figure 3.
- The authors have little data on the physical properties of Flinabe; they believe these are similar to Flibe, but with a lower melting temperature and the same BeF2 concentration at the same temperature.
- This lower melting point extends the window of operating temperature enough that a workable design appears possible.
- The primary structural material for the blanket, and auxiliary structures, is an advanced ferritic steel.
V. MECHANICAL DESIGN
- The authors believe the designs can be made robust in terms of the mechanical and EM forces that such a structure must withstand.
- The authors have had neither time nor resources to develop engineering details that would show mechanical response of the structure to various types of off normal events and transient loading that are associated with a detailed engineering design and safety analysis.
VI. NEUTRONICS
- The lower Li concentration requires a comparatively more of the neutron multiplier beryllium to improve the tritium breeding ratio (TBR).
- Ref. [2] summarizes an initial assessment of tritium breeding and a final assessment, the latter corresponded to the design described here with the blanket having an advanced ferritic steel structure with a 60-mm-thick Be bed of 57% packing density.
- In the space here the authors can present only a brief summary of the work on tritium breeding and neutronics.
- The minimum shield thicknesses, based on endof-life neutron fluences, are ~56 cm in the inboard side and ~26 cm in the outboard side3.
- The total 24Na activity is much lower than the total structure activity and is expected not to be a major concern when Flinabe is used in fusion systems.
VII. TRITIUM PROCESSING
- Using this solubility along with the rates of tritium production and flow and temperature of Flinabe, Author Sze calculates that the tritium partial pressure over the Flinabe to be about 40 Pa at the exit of the reactor.
- Gas purging is the easiest method for recovering tritium from Flinabe, and a vacuum disengager process is proposed for this purpose.
- A key step in the process is using a vacuum system to pump tritium from the molten salt coolant.
- Whether secondary systems for tritium recovery are affordable has not yet been evaluated.
VIII. SAFETY
- Safety assessments[50,53] were performed for their blanket design, but here the authors have space here only for the conclusions.
- Given the rate of releases from the APEX liquid wall blanket design, the facility must be isolated within an additional two weeks to remain below the 10 mSv limit.
- The authors are strongly supported by the APEX and ALPS Teams and through significant commitment by the Dept. of Energy’s US Fusion Energy Science Program they have utilized expertise in plasma edge modeling, advanced mechanical and systems design, and heat transfer.
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References
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