Chapter 13
Study on Neutron Spectrum of Pulsed
Neutron Reactor
Takanori Kitada, Thanh Mai Vu, and Noboru Dobuchi
Abstract The neutron spectrum of a pulsed neutron reactor at subcritical state is
different from that evaluated by k-eigenvalue mode, because of the time needed in
the neutron slowing-down process from fast to thermal energy range. The time
needed in slowing down does not depend on the degree of subcriticality, but the
decreasing speed of neutron flux becomes fast as the subcriticality becomes deep.
Therefore, the neutron spectrum becomes soft as the subcriticality becomes deep.
This fact suggests to us that group constants to be used in the design study should
change with the degree of subcriticality of the target system, even in the case of the
same composition.
Keywords ADSR • Alpha-eigenvalue • k-Eigenvalue mode • Neutron spectrum
• Subcriticality • Time-dependent mode
13.1 Introduction
The accelerator-driven subcritical reactor (ADSR) is considered as one of the best
candidates to annihilate the radioactivity of nuclear waste and has been investigated
in many institutes for many years. The ADSR is operated by the pulsed proton beam
as an ignition of spallation reaction to produce many neutrons. Kyoto University
Critical Assembly (KUCA) is one of the facilities to demonstrate the ADSR by
using accelerated protons for the spallation reaction or deuterons for the deuteriumtritium (DT) reaction.
This study focused on the transient behavior of the neutron spectrum in a
subcritical system after the injection of DT neutrons to know and discuss the
physical behavior of the neutron spectrum in a subcritical system through the
analysis of the experiments performed at KUCA. The subcritical system with
pulsed neutrons has been widely analyzed in the steady state, although transient
behavior of the neutron spectrum after the injection of DT neutrons can be analyzed
in the transient state. This chapter focuses on the neutron spectrum evaluated in
T. Kitada (*) • T.M. Vu • N. Dobuchi
Osaka University, Graduate School of Engineering, 2-1 Yamada-oka, Suita,
Osaka 565-0871, Japan
e-mail: kitada@see.eng.osaka-u.ac.jp
© The Author(s) 2015
K. Nakajima (ed.), Nuclear Back-end and Transmutation Technology for Waste
Disposal, DOI 10.1007/978-4-431-55111-9_13
129
Study on Neutron Spectrum of Pulsed
Neutron Reactor
Takanori Kitada, Thanh Mai Vu, and Noboru Dobuchi
Abstract The neutron spectrum of a pulsed neutron reactor at subcritical state is
different from that evaluated by k-eigenvalue mode, because of the time needed in
the neutron slowing-down process from fast to thermal energy range. The time
needed in slowing down does not depend on the degree of subcriticality, but the
decreasing speed of neutron flux becomes fast as the subcriticality becomes deep.
Therefore, the neutron spectrum becomes soft as the subcriticality becomes deep.
This fact suggests to us that group constants to be used in the design study should
change with the degree of subcriticality of the target system, even in the case of the
same composition.
Keywords ADSR • Alpha-eigenvalue • k-Eigenvalue mode • Neutron spectrum
• Subcriticality • Time-dependent mode
13.1 Introduction
The accelerator-driven subcritical reactor (ADSR) is considered as one of the best
candidates to annihilate the radioactivity of nuclear waste and has been investigated
in many institutes for many years. The ADSR is operated by the pulsed proton beam
as an ignition of spallation reaction to produce many neutrons. Kyoto University
Critical Assembly (KUCA) is one of the facilities to demonstrate the ADSR by
using accelerated protons for the spallation reaction or deuterons for the deuteriumtritium (DT) reaction.
This study focused on the transient behavior of the neutron spectrum in a
subcritical system after the injection of DT neutrons to know and discuss the
physical behavior of the neutron spectrum in a subcritical system through the
analysis of the experiments performed at KUCA. The subcritical system with
pulsed neutrons has been widely analyzed in the steady state, although transient
behavior of the neutron spectrum after the injection of DT neutrons can be analyzed
in the transient state. This chapter focuses on the neutron spectrum evaluated in
T. Kitada (*) • T.M. Vu • N. Dobuchi
Osaka University, Graduate School of Engineering, 2-1 Yamada-oka, Suita,
Osaka 565-0871, Japan
e-mail: kitada@see.eng.osaka-u.ac.jp
© The Author(s) 2015
K. Nakajima (ed.), Nuclear Back-end and Transmutation Technology for Waste
Disposal, DOI 10.1007/978-4-431-55111-9_13
129