128
C. H. Pyeon
the assumption that, in the thermal neutron region, the cross sections of
235 U(n, f )
are proportional to those of
115 In(n, γ)
116m In shown in Fig. A2.7. The In wire was
set along the vertical direction (14, 13–P, A
) at the axial center position shown in
Fig. A2.1. An Al (10 × 10 × 1 mm, (15, H) in Fig. A2.1) and an In (10 × 10 ×
1 mm, (15, H) in Fig. A2.1) foils were attached at the location of the target to monitor
information on the generation of protons and spallation neutrons through
27 Al(p, n +
3p)
24 Na and
115 In(n, n
)
115m In reactions (threshold energy of 0.3 MeV), respectively.
For an easy understanding of experimental analyses, the methodology of normalized
reaction rates was introduced with the comparison between the experimental and
calculated results at the location of the target and in the core region: the
115 In (n,
n
)
115m In and
115 In(n, γ)
116m In reaction rates normalized by
27 Al(p, n + 3p)
24 Na
and
115 In(n, n
)
115m In ones, respectively, were estimated in the experiments, and
interpreted as actual values of proton yield and neutron yield, respectively, in terms
of the influence of the external neutron source. The main characteristics of proton
beams were as follows: 100 MeV energy, 0.7 nA intensity, 20 Hz beam repetition,
100 ns beam width and 1.0 × 10
7 s
−1 neutron generation. The irradiation time of
all the foils and wire was about 3 h. The measured subcriticality, 2,900 pcm, of
the core was obtained by the full insertion of control (C1, C2 and C3) and safety
(S4, S5 and S6) rods, as shown in Fig. A2.1. The reactivity worth of all control and
safety rods was evaluated by the rod drop method and the positive period method
beforehand. The measured reaction rates varied according to the kind of solid target
used, as shown in Fig. A2.8, under the subcritical level 2,900 pcm. The reaction
rates were high with the combined use of W and Be targets, and the moderation
(thermal) peak caused by the high-energy neutrons was observed in the polyethylene
region, mostly in the two-layer target (W-Be target). Neutron multiplication has
been obtained successfully by the
115 In(n, γ)
116m In reaction rates in the core region
because the relation between
115 In(n, γ)
116m In and
235 U(n, f ) reaction rates in the core
region is apparently applicable to the subcritical multiplication analyses through the
proportionality of
115 In(n, γ)
116m In and
235 U(n, f ) cross sections in the thermal region
[2].
5.1.2.3 Numerical Simulations
The numerical calculations were performed with the combined use of MCNPX and
JENDL/HE-2007 for high-energy protons and high-energy neutrons, JENDL-4.0 [7]
for transport, and JENDL/D-99 [8] for reaction rates. With MCNPX, the calculated
reaction rates were obtained from the evaluation of volume tallies of activation foils,
and since the effects of their reactivity are not negligible, they were included in
the simulated geometry and transport calculations. The eigenvalue calculations were
conducted for 1,000 active cycles of 100,000 histories. The subcriticalities in the
eigenvalue calculations had statistical errors within 0.01 %k/k (10 pcm), and the
reaction rates in the fixed source calculations were within 3% as determined with
the use of the total 1 × 10
8 histories. The precision of numerical subcriticality in the
C. H. Pyeon
the assumption that, in the thermal neutron region, the cross sections of
235 U(n, f )
are proportional to those of
115 In(n, γ)
116m In shown in Fig. A2.7. The In wire was
set along the vertical direction (14, 13–P, A
) at the axial center position shown in
Fig. A2.1. An Al (10 × 10 × 1 mm, (15, H) in Fig. A2.1) and an In (10 × 10 ×
1 mm, (15, H) in Fig. A2.1) foils were attached at the location of the target to monitor
information on the generation of protons and spallation neutrons through
27 Al(p, n +
3p)
24 Na and
115 In(n, n
)
115m In reactions (threshold energy of 0.3 MeV), respectively.
For an easy understanding of experimental analyses, the methodology of normalized
reaction rates was introduced with the comparison between the experimental and
calculated results at the location of the target and in the core region: the
115 In (n,
n
)
115m In and
115 In(n, γ)
116m In reaction rates normalized by
27 Al(p, n + 3p)
24 Na
and
115 In(n, n
)
115m In ones, respectively, were estimated in the experiments, and
interpreted as actual values of proton yield and neutron yield, respectively, in terms
of the influence of the external neutron source. The main characteristics of proton
beams were as follows: 100 MeV energy, 0.7 nA intensity, 20 Hz beam repetition,
100 ns beam width and 1.0 × 10
7 s
−1 neutron generation. The irradiation time of
all the foils and wire was about 3 h. The measured subcriticality, 2,900 pcm, of
the core was obtained by the full insertion of control (C1, C2 and C3) and safety
(S4, S5 and S6) rods, as shown in Fig. A2.1. The reactivity worth of all control and
safety rods was evaluated by the rod drop method and the positive period method
beforehand. The measured reaction rates varied according to the kind of solid target
used, as shown in Fig. A2.8, under the subcritical level 2,900 pcm. The reaction
rates were high with the combined use of W and Be targets, and the moderation
(thermal) peak caused by the high-energy neutrons was observed in the polyethylene
region, mostly in the two-layer target (W-Be target). Neutron multiplication has
been obtained successfully by the
115 In(n, γ)
116m In reaction rates in the core region
because the relation between
115 In(n, γ)
116m In and
235 U(n, f ) reaction rates in the core
region is apparently applicable to the subcritical multiplication analyses through the
proportionality of
115 In(n, γ)
116m In and
235 U(n, f ) cross sections in the thermal region
[2].
5.1.2.3 Numerical Simulations
The numerical calculations were performed with the combined use of MCNPX and
JENDL/HE-2007 for high-energy protons and high-energy neutrons, JENDL-4.0 [7]
for transport, and JENDL/D-99 [8] for reaction rates. With MCNPX, the calculated
reaction rates were obtained from the evaluation of volume tallies of activation foils,
and since the effects of their reactivity are not negligible, they were included in
the simulated geometry and transport calculations. The eigenvalue calculations were
conducted for 1,000 active cycles of 100,000 histories. The subcriticalities in the
eigenvalue calculations had statistical errors within 0.01 %k/k (10 pcm), and the
reaction rates in the fixed source calculations were within 3% as determined with
the use of the total 1 × 10
8 histories. The precision of numerical subcriticality in the