112
M. Yamanaka
In qualitative calculation property, the ω-mode calculation involves −α/v value
in absorption term compared to the λ-mode calculation, indicating softer neutron
spectrum by decreasing the total absorption cross section of −α/v (since lower the
energy, the more increase −α/v value). Furthermore, the adjoint neutron spectrum is
estimated softer by the λ-mode calculation to induce fission reactions, resulting in
the overestimation of β eff value in the λ-mode calculation since the importance of
delayed neutrons is emphasized by the adjoint neutron flux. Furthermore, the different
estimation of adjoint neutron spectra is also considered to influence the value of
since observation showed an overestimation of β eff and underestimation of in
the λ-mode calculation. Accordingly, calculated k eff by the λ-mode calculation is
implied to be different from actual neutron multiplication factor (in the subcritical
system) by the combined use of α, β eff , and since the spectrum calculation was
inadequate in the λ-mode calculations for super-critical and subcritical cores.
Investigation of the cause in the variation of is considered limited since the
number of energy groups is insufficient for characterizing the neutron spectra of φ
and φ
+ . Thus, further investigation with a higher number of energy groups is needed.
4.4 Neutron Generation Time
4.4.1 Experimental Settings
4.4.1.1 Core Configuration
Critical cores set in the A-core (Figs. A4.1 and A4.4) have polyethylene moderator
and reflector rods, and three different fuel assemblies: normal “F” (Fig. A4.2a),
partial “8”, and “4” (Figs. A4.2b and A4.5) corresponding to Figs. A4.1 and A4.4,
respectively. The normal fuel assembly “F” is composed of 60 unit cells, and upper
and lower polyethylene blocks about 23
and 21
long, respectively, in an Al sheath
(2.1 × 2.1 × 60
). A unit cell is composed of two HEU fuel plates 2 × 2
square
and 1/8
thick (1/16
× 2), polyethylene (p) plate 2 × 2
square and 1/8
thick, for
normal fuel plate “F.” Numeral “8” represents a partial fuel assembly composed of
eight unit cells, with two HEU fuel plates and a polyethylene plate as in the normal
fuel assembly, providing 52 unit cells of two Al plates 2 × 2
square and 1/8
thick
(1/16
× 2), and 1/8
polyethylene plates. Also, the numeral “4” corresponds to four
unit cells of fuel assembly with 56 unit cells composed of Al and polyethylene plates.
4.4.1.2 Experiments
In the two critical cores [23] shown in Figs. A4.1 and A4.4, criticality was reached
at positions of control (C1, C2, and C3) and safety (S4, S5, and S6) rods for the total
number of HEU fuel plates: 3016 and 3008, as shown in Tables 4.14 and 4.15, respec-
M. Yamanaka
In qualitative calculation property, the ω-mode calculation involves −α/v value
in absorption term compared to the λ-mode calculation, indicating softer neutron
spectrum by decreasing the total absorption cross section of −α/v (since lower the
energy, the more increase −α/v value). Furthermore, the adjoint neutron spectrum is
estimated softer by the λ-mode calculation to induce fission reactions, resulting in
the overestimation of β eff value in the λ-mode calculation since the importance of
delayed neutrons is emphasized by the adjoint neutron flux. Furthermore, the different
estimation of adjoint neutron spectra is also considered to influence the value of
since observation showed an overestimation of β eff and underestimation of in
the λ-mode calculation. Accordingly, calculated k eff by the λ-mode calculation is
implied to be different from actual neutron multiplication factor (in the subcritical
system) by the combined use of α, β eff , and since the spectrum calculation was
inadequate in the λ-mode calculations for super-critical and subcritical cores.
Investigation of the cause in the variation of is considered limited since the
number of energy groups is insufficient for characterizing the neutron spectra of φ
and φ
+ . Thus, further investigation with a higher number of energy groups is needed.
4.4 Neutron Generation Time
4.4.1 Experimental Settings
4.4.1.1 Core Configuration
Critical cores set in the A-core (Figs. A4.1 and A4.4) have polyethylene moderator
and reflector rods, and three different fuel assemblies: normal “F” (Fig. A4.2a),
partial “8”, and “4” (Figs. A4.2b and A4.5) corresponding to Figs. A4.1 and A4.4,
respectively. The normal fuel assembly “F” is composed of 60 unit cells, and upper
and lower polyethylene blocks about 23
and 21
long, respectively, in an Al sheath
(2.1 × 2.1 × 60
). A unit cell is composed of two HEU fuel plates 2 × 2
square
and 1/8
thick (1/16
× 2), polyethylene (p) plate 2 × 2
square and 1/8
thick, for
normal fuel plate “F.” Numeral “8” represents a partial fuel assembly composed of
eight unit cells, with two HEU fuel plates and a polyethylene plate as in the normal
fuel assembly, providing 52 unit cells of two Al plates 2 × 2
square and 1/8
thick
(1/16
× 2), and 1/8
polyethylene plates. Also, the numeral “4” corresponds to four
unit cells of fuel assembly with 56 unit cells composed of Al and polyethylene plates.
4.4.1.2 Experiments
In the two critical cores [23] shown in Figs. A4.1 and A4.4, criticality was reached
at positions of control (C1, C2, and C3) and safety (S4, S5, and S6) rods for the total
number of HEU fuel plates: 3016 and 3008, as shown in Tables 4.14 and 4.15, respec-