Page 312 - Fundamentals of Magnetic Thermonuclear Reactor Design

292 Fundamentals of Magnetic Thermonuclear Reactor Design

To perform these functions, the blanket must meet a number of engineering
requirements, the most important being the following:

1. A tritium breeding ratio (TBR) of no less than 1.05–1.1. The TBR is defined
as the average number of tritium atoms bred per one incident neutron, or, in
a wider sense, a ratio between tritium extracted from the blanket and tritium
fed to the reactor and burnt. The TBR > 1 requirement stems from the need
to make up for the inevitable tritium losses in the blanket itself and the ex-
traction/purification/transportation systems, as well as for the natural decay
in the storage system.
2. A higher than 40% efficiency for converting neutron power to electrical en-
ergy. This is only attainable with at least 350°C coolant temperature, mean-
ing that respective structural materials must have very special thermal me-
chanical properties and high radiation endurance.
3. High durability of the blanket structure and materials to allow for the short-
est possible duration of maintenance and repair and enable an at least 0.6
capacity utilisation ratio for a demonstration fusion reactor. For commer-
cial reactors, the capacity utilisation ratio must be much higher (it is up to
0.85–0.9 for present-day nuclear plants).
4. The ability of the blanket (together with the vacuum vessel) to ensure desired
durability of the magnetic field coils. For example, the Russian DEMO-S
10
−2
project is expected to allow for a 1 × 10 cm fast neutron fluence and
a 50 kW energy release in toroidal field coils over 30 years of the reactor’s
lifetime.
5. Low residual activity of the blanket’s structural, shielding and tritium-breed-
ing materials. The rationale behind this requirement is the need to maintain
the blanket, minimise the gravity of potential accidents and allow for ma-
terials recycling and disposal after the blanket decommissioning. In some
projects, radioactive waste is required to decay to an acceptable safety level
of <2.5 µSv/h within 500 years.
Let us try to see if the existing and prospective materials and design solu-
tions comply with the mentioned requirements.
Tritium can be produced within the reactor under irradiation of lithium,
contained in the blanket, with neutrons, supplied by the D–T fusion reaction,
through the following reactions:

4
+
n
Li6+n→T+He4+4.8 MeV 6 Li +→ + He 4.8MeV
T
4
n
n
T
Li7 + n→T + He4+n′−2.8 MeV. 7 Li +→ + He + ′ − 2.8MeV.
The first reaction has a larger cross-section, but natural lithium only contains
7.5% lithium-6. Therefore, an isotope mixture enriched with lithium-6 is used in
the blanket to increase the TBR.