A fusion neutron entering a lithium blanket first meets a beryllium multiplier

PHYSICS · Multi-part · Fermi University Physics · Nuclear Physics · Plasma Physics · Nuclear reactions

Energy from Physics, Ch Fusion as a power plant: gain, beta, wall loading and breeding

Problem

A fusion neutron entering a lithium blanket first meets a beryllium multiplier. With probability

f_m

it undergoes an

(n,2n)

reaction that turns it into two neutrons, so

(1+f_m)

neutrons per source neutron go on into the lithium. Each of these is captured by lithium-6, making one triton, with probability

f_6

, or is lost to structure and leakage. Separately, fast neutrons drive

^7\mathrm{Li}(n,n't)\alpha

, which makes a triton and passes its neutron on, so it takes no neutron away from the lithium-6 budget; count its contribution as

y

tritons per source neutron. ((a)) Show that the tritium breeding ratio is

\mathrm{TBR} = (1+f_m)f_6 + y

. ((b)) A blanket design captures

f_6 = 0.85

of the available neutrons on lithium-6 and gets

y = 0.08

from lithium-7. Find the beryllium

(n,2n)

fraction

f_m

needed to reach a breeding ratio of

1.10

, the margin a plant needs above unity to sustain its own fuel cycle over time.