4 Effective Delayed Neutron Fraction
103
4.3 Evaluation of β eff/
4.3.1 Experimental Settings
4.3.1.1 Core Configurations
ADS experiments were carried out in a uranium-lead (U-Pb) core with 14 MeV
neutrons (Fig. A3.1) and spallation neutrons (Fig. A3.2) [15]. The core comprised
normal fuel rods (1/8
p60EUEU) of highly enriched uranium (HEU: 2
× 2
×
1/16
) and a polyethylene moderator (p: 2
× 2
× 1/8
) in an aluminum sheath 2.1
× 2.1
× 60
, as shown in Fig. A3.3a, and U-Pb fuel rods composed of an HEU plate
and a Pb plate (Pb: 2
× 2
× 1/8
) in the center, as shown in Fig. A3.3b. The core
spectrum was approximately hard in the central region for the spectrum of the actual
ADS, and the driver (normal fuel) region has an H/U (hydrogen/uranium) ratio of
approximately 50 in the thermal reactor.
14 MeV neutrons were generated by the injection of a deuteron beam (intensity
0.3 mA, pulsed width 10 μs, and pulsed frequency 20 or 50 Hz) onto a tritium
target located at (14–15, Y; Fig. A3.1). A proton accelerator (FFAG accelerator) was
operated to inject 100 MeV protons (beam spot 50 mm, intensity 10 pA, and pulsed
frequency 20 Hz) onto a lead-bismuth (Pb–Bi) target (50 mm diam. and 18 mm thick)
located at (15, D; Fig. A3.2) to generate spallation neutrons.
Time evolution according to the injection of external neutrons was obtained from
the signals of four BF 3 detectors set around the core. Furthermore, in the case of
spallation neutrons, an optical fiber type detector containing a Eu:LiCaAlF6 scintillator [16] was additionally installed at location (10, U; Fig. A3.2) symmetrical to
BF 3 #2 so as to examine the influence of the detector on measured subcriticality.
Subcriticality was obtained by full insertion of control and safety rods, and by the
substitution of the fuel assembly for polyethylene rods, as shown in Table 4.11a. In
Cases I-1 to I-5, the subcriticality was experimentally deduced with the combined use
of control rod worth and its calibration curve obtained by the positive period method.
Moreover, in Cases II to VII (Table 4.11b), some of the fuel rods “F” (Figs. A3.1 and
A3.2) were substituted for polyethylene reflectors and configured as shown in Figs.
A3.6 and A3.7. The subcriticality in dollar units was acquired experimentally by the
extrapolated area ratio method [1]. Also, α was obtained by the α-fitting method in
PNS experiments. The subcriticality level ranged between 500 and 7500 pcm.
4.3.1.2 Numerical Analyses
Numerical analyses were conducted by using MCNP6.1 together with ENDF/BVII.1 (total histories 5E + 08: 5E + 05 histories per cycle and 1E + 03 active cycles)
and by PARTISN [17] (with mesh size less than 10 × 10 × 10 mm; transport cross
section instead of P L scattering treatment; EO 16 quadrature for S N [18]; Fig. 4.5). In
PARTISN analyses, seven effective cross Sects. (7-energy group) were generated as
103
4.3 Evaluation of β eff/
4.3.1 Experimental Settings
4.3.1.1 Core Configurations
ADS experiments were carried out in a uranium-lead (U-Pb) core with 14 MeV
neutrons (Fig. A3.1) and spallation neutrons (Fig. A3.2) [15]. The core comprised
normal fuel rods (1/8
p60EUEU) of highly enriched uranium (HEU: 2
× 2
×
1/16
) and a polyethylene moderator (p: 2
× 2
× 1/8
) in an aluminum sheath 2.1
× 2.1
× 60
, as shown in Fig. A3.3a, and U-Pb fuel rods composed of an HEU plate
and a Pb plate (Pb: 2
× 2
× 1/8
) in the center, as shown in Fig. A3.3b. The core
spectrum was approximately hard in the central region for the spectrum of the actual
ADS, and the driver (normal fuel) region has an H/U (hydrogen/uranium) ratio of
approximately 50 in the thermal reactor.
14 MeV neutrons were generated by the injection of a deuteron beam (intensity
0.3 mA, pulsed width 10 μs, and pulsed frequency 20 or 50 Hz) onto a tritium
target located at (14–15, Y; Fig. A3.1). A proton accelerator (FFAG accelerator) was
operated to inject 100 MeV protons (beam spot 50 mm, intensity 10 pA, and pulsed
frequency 20 Hz) onto a lead-bismuth (Pb–Bi) target (50 mm diam. and 18 mm thick)
located at (15, D; Fig. A3.2) to generate spallation neutrons.
Time evolution according to the injection of external neutrons was obtained from
the signals of four BF 3 detectors set around the core. Furthermore, in the case of
spallation neutrons, an optical fiber type detector containing a Eu:LiCaAlF6 scintillator [16] was additionally installed at location (10, U; Fig. A3.2) symmetrical to
BF 3 #2 so as to examine the influence of the detector on measured subcriticality.
Subcriticality was obtained by full insertion of control and safety rods, and by the
substitution of the fuel assembly for polyethylene rods, as shown in Table 4.11a. In
Cases I-1 to I-5, the subcriticality was experimentally deduced with the combined use
of control rod worth and its calibration curve obtained by the positive period method.
Moreover, in Cases II to VII (Table 4.11b), some of the fuel rods “F” (Figs. A3.1 and
A3.2) were substituted for polyethylene reflectors and configured as shown in Figs.
A3.6 and A3.7. The subcriticality in dollar units was acquired experimentally by the
extrapolated area ratio method [1]. Also, α was obtained by the α-fitting method in
PNS experiments. The subcriticality level ranged between 500 and 7500 pcm.
4.3.1.2 Numerical Analyses
Numerical analyses were conducted by using MCNP6.1 together with ENDF/BVII.1 (total histories 5E + 08: 5E + 05 histories per cycle and 1E + 03 active cycles)
and by PARTISN [17] (with mesh size less than 10 × 10 × 10 mm; transport cross
section instead of P L scattering treatment; EO 16 quadrature for S N [18]; Fig. 4.5). In
PARTISN analyses, seven effective cross Sects. (7-energy group) were generated as