158
C. H. Pyeon
4
5
6
7
8
9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 2 0 2 1 2 2
2 3 2 4 2 5 2 6
A Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr
Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr
F
r
G
r
G
r
G
r
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B
f
r
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r
G
r
G
r
G
D
r
G
p
p
p
p
p
p
p
p
p
p
p
r
G
E
p
r
G
p
p
p
p
p
p
p
p
p
p
p
r
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G
r
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p
p
p
p
p
p
p
p
p
p
p
r
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H
p
p
p
p
r
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I
F F F
r
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p
p
p
p
p
p
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J
F
F F F
F
r
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p
p
p
p
p
r
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K
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F F F
F
r
G
r
G
p
p
p
p
p
p
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L
F
F F F
F
r
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p
p
p
p
p
r
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F
f
f
f
F
p
p
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O Gr
p
p
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f
f
f
F
p
p
p
G r
Q Gr
p
p
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f
f
f
F
p
p
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p
p
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F F F
F
p
p
G r
T G r
p
p
p
p
p p
p
p
p
p
p
G r
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p
p
p
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p p
p
p
p
p
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V G r
p
p
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p p
p
p
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Z' Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr Gr
Normal fuel (1/8"p60EUEU)
Special fuel (1/8"Pb40p20EUEU)
Polyethylene moderator
Fission chamber
Polyethylene reflector
Graphite
Aluminum sheath
Uncompensated ionization chamber
Control rod
Safety rod
Neutron source (Am-Be)
N
N
FC
UIC
S
C
C2
C3
S5
S6
FC#3
FC#2
FC#1
UIC#5
S4
C1
UIC#4
UIC#6
Fig. 6.1 Top view of the KUCA core (reference core) for critical irradiation experiments on MA
(Ref. [1])
A unit cell is composed of two highly enriched uranium (HEU) fuel plates 2 × 2
square and 1/8
thick (1/16
× 2), polyethylene plates 2 × 2
square and 1/8
thick,
for the normal fuel plate “F” (HEU-PE). The special fuel assembly “f” (HEU-Pb) is
composed of 40 unit cells with two HEU fuel plates 1/8
(1/16
× 2) thick, lead (Pb)
plate 2 × 2
square and 1/8
thick, 20 unit cells of HEU and polyethylene plates as
well as in the normal fuel assembly.
6.1.1.2 Back-to-Back Fission Chamber
Critical irradiation experiments for measuring the fission reaction rate ratio were
carried out with the back-to-back (BTB) fission chamber (Fig. 6.3) set inside a special
void element, containing two foils: a test foil neptunium-237 (
237 Np) or americium241 (
241 Am) and a reference one uranium-235 (
235 U). The reference foil
235 U was
a normalization factor that has the fission reactions
235 U(n, f ), to obtain two fission
reaction rate ratios:
237 Np/
235 U and
241 Am/
235 U. The main characteristic of the BTB
chamber is to obtain uniquely an original signal of fission event of one foil attached
at the position of sample deposit, and a different signal of fission event of the other
foil attached at the opposite side without any disturbance of the foil on the front
side. The chamber has a structure of separating it from two electro-deposited foils
that receive independently pulsed signals of fission events accumulated as electric
pulses generated by fission fragments through the ionization process of filling gas