474
8 Nuclear Physics – II
The counting rate,
R = N σ νn(r )
(10)
As the absorption obeys the 1/v law, the product σ.v = constant. We then have
250
60
= 10
20
× 3,000 × 10
−24
× 2.2 × 10
5 n(r )
Or n(r ) = 6.313 × 10
−5
/cm
3
Substituting the values: D = 5 × 10
5
, r = 300, L = 50 and R = 250/60 in
(9), we find
Q = 4.8 × 10
7
/s.
8.79 The absorption rate in the fuel is
Σ au φ u V u
where V is the volume, and the absorption rate in the moderator is
Σ am φ m V m
The fraction of thermal neutrons absorbed by the Uranium fuel as compared
to the total number of thermal neutron absorptions in the assembly is known
as the thermal utilization factor f and is given by
f =
Σ au φ u V u
Σ au φ u V u + Σ am φ m V m
=
1
1 +
Σ am φ m V m
Σ au φ u V u
Fig. 8.12 shows a unit cell of a heterogeneous assembly in which the uranium rods of radius r , are placed at regular intervals (pitch). The equivalent
cell radius r 1 is also indicated.
Fig. 8.12 Unit cell of a
heterogeneous assembly
8 Nuclear Physics – II
The counting rate,
R = N σ νn(r )
(10)
As the absorption obeys the 1/v law, the product σ.v = constant. We then have
250
60
= 10
20
× 3,000 × 10
−24
× 2.2 × 10
5 n(r )
Or n(r ) = 6.313 × 10
−5
/cm
3
Substituting the values: D = 5 × 10
5
, r = 300, L = 50 and R = 250/60 in
(9), we find
Q = 4.8 × 10
7
/s.
8.79 The absorption rate in the fuel is
Σ au φ u V u
where V is the volume, and the absorption rate in the moderator is
Σ am φ m V m
The fraction of thermal neutrons absorbed by the Uranium fuel as compared
to the total number of thermal neutron absorptions in the assembly is known
as the thermal utilization factor f and is given by
f =
Σ au φ u V u
Σ au φ u V u + Σ am φ m V m
=
1
1 +
Σ am φ m V m
Σ au φ u V u
Fig. 8.12 shows a unit cell of a heterogeneous assembly in which the uranium rods of radius r , are placed at regular intervals (pitch). The equivalent
cell radius r 1 is also indicated.
Fig. 8.12 Unit cell of a
heterogeneous assembly