Chapter 21
Options of Principles of Fuel Debris
Criticality Control in Fukushima Daiichi
Reactors
Kotaro Tonoike, Hiroki Sono, Miki Umeda, Yuichi Yamane,
Teruhiko Kugo, and Kenya Suyama
Abstract In the Three Mile Island Unit 2 reactor accident, a large amount of fuel
debris was formed whose criticality condition is unknown, except the possible
highest
235
U/U enrichment. The fuel debris had to be cooled and shielded by
water in which the minimum critical mass is much smaller than the total mass of
fuel debris. To overcome this uncertain situation, the coolant water was borated
with sufficient concentration to secure the subcritical condition. The situation is
more severe in the damaged reactors of Fukushima Daiichi Nuclear Power Station,
where the coolant water flow is practically “once through.” Boron must be endlessly added to the water to secure the subcritical condition of the fuel debris, which
is not feasible. The water is not borated relying on the circumstantial evidence that
the xenon gas monitoring in the containment vessels does not show a sign of
criticality. The criticality condition of fuel debris may worsen with the gradual
drop of its temperature, or the change of its geometry by aftershocks or the retrieval
work, that may lead to criticality. To avoid criticality and its severe consequences, a
certain principle of criticality control must be established. There may be options,
such as prevention of criticality by coolant water boration or neutronic monitoring,
prevention of the severe consequences by intervention measures against criticality,
etc. Every option has merits and demerits that must be adequately evaluated toward
selection of the best principle.
Keywords Criticality control • Fuel debris • Fukushima Daiichi
21.1 Introduction
In normal nuclear facilities, the goal of criticality control is to secure subcritical
conditions of fissile materials, which is achieved by regulating the composition,
geometry, or mass of the fissile materials [1]. In the accident of Three Mile Island
K. Tonoike (*) • H. Sono • M. Umeda • Y. Yamane • T. Kugo • K. Suyama
Japan Atomic Energy Agency, 2-4 Shirakata Shirane, Tokai, Ibaraki, 319-1195, Japan
e-mail: tonoike.kotaro@jaea.go.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_21
251
Options of Principles of Fuel Debris
Criticality Control in Fukushima Daiichi
Reactors
Kotaro Tonoike, Hiroki Sono, Miki Umeda, Yuichi Yamane,
Teruhiko Kugo, and Kenya Suyama
Abstract In the Three Mile Island Unit 2 reactor accident, a large amount of fuel
debris was formed whose criticality condition is unknown, except the possible
highest
235
U/U enrichment. The fuel debris had to be cooled and shielded by
water in which the minimum critical mass is much smaller than the total mass of
fuel debris. To overcome this uncertain situation, the coolant water was borated
with sufficient concentration to secure the subcritical condition. The situation is
more severe in the damaged reactors of Fukushima Daiichi Nuclear Power Station,
where the coolant water flow is practically “once through.” Boron must be endlessly added to the water to secure the subcritical condition of the fuel debris, which
is not feasible. The water is not borated relying on the circumstantial evidence that
the xenon gas monitoring in the containment vessels does not show a sign of
criticality. The criticality condition of fuel debris may worsen with the gradual
drop of its temperature, or the change of its geometry by aftershocks or the retrieval
work, that may lead to criticality. To avoid criticality and its severe consequences, a
certain principle of criticality control must be established. There may be options,
such as prevention of criticality by coolant water boration or neutronic monitoring,
prevention of the severe consequences by intervention measures against criticality,
etc. Every option has merits and demerits that must be adequately evaluated toward
selection of the best principle.
Keywords Criticality control • Fuel debris • Fukushima Daiichi
21.1 Introduction
In normal nuclear facilities, the goal of criticality control is to secure subcritical
conditions of fissile materials, which is achieved by regulating the composition,
geometry, or mass of the fissile materials [1]. In the accident of Three Mile Island
K. Tonoike (*) • H. Sono • M. Umeda • Y. Yamane • T. Kugo • K. Suyama
Japan Atomic Energy Agency, 2-4 Shirakata Shirane, Tokai, Ibaraki, 319-1195, Japan
e-mail: tonoike.kotaro@jaea.go.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_21
251