A breeding blanket is a device used in nuclear engineering to transmute quantities of an element, using the neutron flux from a fission reactor or fusion reactor. In the fission context, breeding blankets have been used since the 1950s in breeder reactors, to manufacture fission fuel from fertile material. In the fusion context, they have been conceptualized for the manufacture of tritium from lithium-6. In both scenarios, neutron radiation is converted into thermal energy in the blanket, leading it to require its own cooling system.
Contents
Fission blanket
Breeder reactors come in two types: thermal and fast. The former use thermal neutrons to activate thorium-232, ultimately producing uranium-233:
Th
90
232
+
0
1
n
→
(n,
γ
)
Th
90
233
→
21.8 min
β
−
Pa
91
233
→
27 days
β
−
U
92
233
{\displaystyle {\ce {{^{232}_{90}Th}+_{0}^{1}n->[{\text{(n,}}\gamma {\text{)}}]{^{233}_{90}Th}->[\beta ^{-}][{\text{21.8 min}}]{^{233}_{91}Pa}->[\beta ^{-}][{\text{27 days}}]{^{233}_{92}U}}}}
The latter use fast neutrons to activate uranium-238, ultimately producing plutonium-239:
U
92
238
+
n
0
1
⟶
U
92
239
→
23.5
min
β
−
Np
93
239
→
2.356
d
β
−
Pu
94
239
{\displaystyle {\ce {^{238}_{92}U + {}^{1}_{0}n -> {}^{239}_{92}U ->[\beta^-][23.5\ {\ce {min}}] {}^{239}_{93}Np ->[\beta^-][2.356\ {\ce {d}}] {}^{239}_{94}Pu}}}
Historically the production of both was more common in rod assemblies, such as in the Hanford Site and Mayak nuclear weapons production facilities. However, blankets are used to minimize the neutron and energy loss rate. Examples include the Experimental Breeder Reactor I and Shippingport Atomic Power Station initial core in the 1950s.
Fusion blanket
In conceptual fusion power plants, including both magnetic and inertial confinement schemes, a breeding blanket can serve multiple purposes:
Absorbing fusion neutrons to breed tritium from lithium
Multiplying the neutron flux
Absorbing fusion neutrons to produce thermal energy from the reactor
Cooling the interior reactor components such as the first wall
Shielding the exterior reactor components from neutron radiation and limited X-ray radiation
It is only the breeding portion that cannot be replaced by other means. For instance, a large quantity of water makes an excellent cooling system and neutron shield, as in the case of a conventional nuclear reactor. However, tritium is not a naturally occurring resource, and thus is difficult to obtain in sufficient quantity to run a reactor through other means, so if commercial fusion using the D-T cycle is to be achieved, successful breeding of the tritium in commercial quantities is a requirement.
Tritium breeding
The primary purpose is to breed further tritium fuel for the nuclear fusion reaction through the reaction of neutrons with lithium in the blanket:
3
6
Li
+
0
1
n
⟶
1
3
H
+
2
4
He
+
4.8 MeV
{\displaystyle _{3}^{6}{\text{Li}}+{}_{0}^{1}{\text{n}}\longrightarrow {}_{1}^{3}{\text{H}}+_{2}^{4}{\text{He}}+{\text{4.8 MeV}}}
3
7
Li
+
0
1
n
⟶
1
3
H
+
2
4
Coolant system
The blanket may also act as a cooling mechanism, absorbing the energy from the neutrons produced by the reaction between deuterium and tritium ("D-T"), and further serves as shielding, preventing the high-energy neutrons from escaping to the area outside the reactor and protecting the more radiation-susceptible portions, such as ohmic or superconducting magnets, from damage.
ITER runs a major effort in blanket design and will test a number of potential solutions. The four main concepts are the
Dual-cooled lithium lead (DCLL)
Helium-cooled lithium lead (HCLL)
Helium-cooled pebble bed (HCPB)
Water-cooled lithium lead (WCLL)
Light water, helium, and lead coolant systems, and understanding of their neutronic behaviors, have already been developed for various fission reactors. Six different tritium breeding systems, known as Test Blanket Modules (TBM) will be tested in ITER.
To date no large-scale breeding system has been attempted, and it is an open question whether such a system is possible to create.