From a neutronic point of view, the blanket must be designed to provide an adequate tritium breeding ratio to sustain a substantial volumetric power density, allowing for continuous power extraction via heat exchange with the coolant, and to serve as shielding for equipment, especially for the superconducting magnet coils in MCF, and personnel.

Tritium is produced by the neutron-induced reactions

jNdN, <ov>d[d3r

(13.22)

associated with isotropically distributed neutrons, Nn(vn), colliding with lithium nuclei at rest. Here N6 and N7 are the 6Li and 7Li atom densities in the blanket volume Vb, and On6 and <3n7 are their corresponding microscopic neutron absorption cross sections; Nn is the speed dependent density of neutrons in the blanket, vn is the neutron speed and Vc is the fusion core volume. We point out that the n(7Li, n, a)t reaction, Eq.(13.21b), is a threshold reaction and requires an incident neutron energy in excess of 2.47 MeV. For that, the neutron flux (Nnvn) has to be considered in the fast energy range, that is the second integral in the numerator of Eq.(13.22) yields zero in the thermal and epithermal neutron energy range.

In the absence of tritium from other sources, it is necessary to have С, > 1 in order to compensate for tritium transport losses during extraction and transfer as well as for its decay before injection into the fusion core. Indications for breeding capabilities can be obtained from upper limit estimates given in Table 13.1 for various blanket materials; as shown, adequate tritium breeding may be obtained if a neutron multiplier such as 9Be or Pb is added.

Early blanket concepts employed liquid lithium as a coolant, thereby providing adequate tritium breeding. However, other considerations like pumping power requirements, the effect of magnetic fields on a flowing metal, and materials compatibility forced the development of alternative designs with lithium added in other forms. This includes various solid lithium compounds, molten salt fluids (e. g. 2LiF + BeF, called FLIBE), and a lithium-lead eutectic,

Material	Estimated Upper Limit Breeding Ratio, Ct
*Li	1.1
Natural Li	0.9
9Be + 6Li (5%)	2.7
Pb + li (5%)	1.7
Table 13.1: Tritium breeding ratios for various materials, encompassing the entire fusion core.

17-Li 83-Pb. Table 13.2 summarizes calculated breeding ratios obtainable for a variety of materials in a "typical" blanket 1 cm thick with 10% volume fraction of 316 stainless steel, preceded by a 1 cm steel front-wall and backed by a 100 cm thick shield. Only the metallic lithium, Li02, and two of the Li-Pb eutectics appear to offer adequate tritium breeding. Consequently, use of the various solid breeders generally requires an added neutron multiplier.
	Calculated Tritium
Material	Breeding Ratio, Ct
17-Li 83-Pb	1.6
LiPb	1.4
FLIBE	1.1
LiA102	0.9
Li02	1.3
Li2Si03	0.9
Li2Zr03	1.0

Table 13.2: Tritium breeding attainable with typical lithium bearing materials.

Since the blanket is exposed to high energy neutrons entering from the fusing plasma, the neutron density is a maximum in the first wall domain and then attenuates rapidly, even if a reflector zone completes the blanket composition. A consequence of this is that energy deposition will similarly vary with the depth of blanket penetration, Fig.13.8. The general trend of an exponential fall-off from the plasma side to the blanket interior must be considered in designing the coolant flow pattern and also in calculations of breeding, radiation damage, and activation. A maximum power density of ~ 80 Wcm’3 occurs in the multiplier zone, while the average is ~ 15 Wcm’3. For comparison, power densities of < 100 Wcm’3 apply to light and heavy water fission reactors. The fusion blanket region should operate at a high average temperature (> 1000 K) in order to facilitate a reasonable thermodynamic conversion efficiency.

Fig. 13.8: Power density for a typical blanket using a beryllium neutron multiplier zone followed by a high concentration of LiA102 and some H20 with an outer graphite reflector.

Safety aspects suggest considering helium as an appropriate coolant since it is an inert gas which reacts with neither the lithium, the beryllium neutron multiplier nor other structural material. Further, it offers the advantage that the bred tritium is conveniently transported out of the blanket with the helium coolant flow.

In assessing the energetic performance of a fusion reactor blanket, we refer to the internal energy flows illustrated in Fig. 13.9, from where it is evident that the energy removable from the blanket is

El = b(fn Efu + Erad) + 2 Ent (13‘23)

i

where a blanket coverage factor b depending on the specific blanket geometry is introduced, since a fusion reactor blanket will feature several channels through its structure, e. g. for injection tubes, diagnostic equipment, etc., and hence cannot completely envelope the fusion plasma. Further, in Eq.(13.23), Ent accounts for the total energy released by an t — type neutron-induced reaction. If exothermic, these reactions can then provide for multiplication of the energy of fusion neutrons having initially entered the blanket. To generalize such energy enhancement, it is convenient to account for it by the explicit blanket multiplication factor

Mb=——————- і—————————————— (13.24)

fnE*fu

allowing us to now rewrite Eq.(13.23) in the following form:

El = MbfnE*ju + bE*rad ■ (13.25)

Fig. 13.9: Energy flows into and from a fusion reactor blanket.

Introducing specific power expressions based on reaction rate densities and the corresponding reaction Q-values, we find for Eq.(13.24)

2 RneQnedir

Mh=b + — f—£—————— (13.26a)

fn}RfuQfud3r

vc

or, respectively, for d-t fusion and assuming lithium as the only neutron reactive substance in the blanket, we obtain

Qn6 f j <7n6^6^n(vn)vndvnd3r + Qn7 j °7Nn(vn)vndvп(1Ъr

Mb =b +—- ^^—————————————————————————————— .

fn4,Qd,)NdN, <ov>dtd3r

Vc

(13.26b)

With the blanket composition and dimension known, and for a specified fusion plasma, Mb can be readily calculated and is seen to range from 1.3 to 1.8 for pure fusion blankets-those which do not contain fissionable material.