geological formations remains sub-critical for tens of thousands to millions of
years. For various repository concepts, CSA is considered to include three major
stages in a chronological order: (1) the stage before package failure, (2) the stage
after package failure, while fissile nuclides remain within the engineered barriers,
and (3) the stage in which fissile nuclides originated from multiple packages deposit
in far-field host rocks.
This paper summarizes our previous works [2, 3] on neutronics analysis for the
disposal of damaged fuels from Fukushima Daiichi reactors, during the three stages
in CSA. Current understanding about the conditions of the damaged fuel is very
limited, and the location and design of the repository have not been determined.
Therefore, the primary objective of our study is to establish a consistent methodology to evaluate the criticality safety for certain repository conditions and engineered barriers designs. The methodology could be further improved and utilized to
assist the repository system design and criticality safety assessment in the future.
For stages (1) and (2), neutronics analysis for the engineered barrier region consisting of a single waste package containing damaged fuel debris, failed overpack
and the buffer materials [2] will be reported in Sect. 2. For stage (3), our study on
the criticality conditions for uranium depositions in geological formations resulting
from geological disposal of damaged fuel [3] will be reported in Sect. 3.
2 Neutronics Analysis on Engineered Barrier System
Containing Damaged Fuel Debris
2.1 Model and Assumptions
The repository is assumed to be in a water-saturated reducing environment. The
neutronics model consists of a canister containing fuel debris from Fukushima
Daiichi Unit 1 reactor and the buffer surrounding the canister. Because there is no
current design for the disposal system for the damaged fuels, the composition and
dimension of the canister and buffer are assumed based on the design for spent fuel
disposal [4]. The damaged fuel is assumed to be disposed of after 50 years of
cooling. The fuel composition after the accident was calculated by burnup code
ORIGEN, which was reported in [1]. Gaseous, soluble, and volatile neutron
absorbing nuclides in the fission products (such as Xe and Cs) might have been
separated from the fuel and released during and after the accident [5]. Therefore, in
this study, only physically and chemically stable, and strongly neutron absorbing
nuclides in fission products are considered, which include Gd, Nd, Sm, Rh, and Eu
isotopes.
The present work considers six nominal time steps for neutronics analysis: the
emplacement time (t = 0), the canister failure time (t = T f ), and four steps during
the dissolution of debris particles (t = T f + 0.2 T l , t = T f + 0.4 T l , t = T f + 0.6 T l ,
and t = T f + 0.8 T l ). At t = 0, the canister only contains fuel debris. The failure time
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X. Liu
years. For various repository concepts, CSA is considered to include three major
stages in a chronological order: (1) the stage before package failure, (2) the stage
after package failure, while fissile nuclides remain within the engineered barriers,
and (3) the stage in which fissile nuclides originated from multiple packages deposit
in far-field host rocks.
This paper summarizes our previous works [2, 3] on neutronics analysis for the
disposal of damaged fuels from Fukushima Daiichi reactors, during the three stages
in CSA. Current understanding about the conditions of the damaged fuel is very
limited, and the location and design of the repository have not been determined.
Therefore, the primary objective of our study is to establish a consistent methodology to evaluate the criticality safety for certain repository conditions and engineered barriers designs. The methodology could be further improved and utilized to
assist the repository system design and criticality safety assessment in the future.
For stages (1) and (2), neutronics analysis for the engineered barrier region consisting of a single waste package containing damaged fuel debris, failed overpack
and the buffer materials [2] will be reported in Sect. 2. For stage (3), our study on
the criticality conditions for uranium depositions in geological formations resulting
from geological disposal of damaged fuel [3] will be reported in Sect. 3.
2 Neutronics Analysis on Engineered Barrier System
Containing Damaged Fuel Debris
2.1 Model and Assumptions
The repository is assumed to be in a water-saturated reducing environment. The
neutronics model consists of a canister containing fuel debris from Fukushima
Daiichi Unit 1 reactor and the buffer surrounding the canister. Because there is no
current design for the disposal system for the damaged fuels, the composition and
dimension of the canister and buffer are assumed based on the design for spent fuel
disposal [4]. The damaged fuel is assumed to be disposed of after 50 years of
cooling. The fuel composition after the accident was calculated by burnup code
ORIGEN, which was reported in [1]. Gaseous, soluble, and volatile neutron
absorbing nuclides in the fission products (such as Xe and Cs) might have been
separated from the fuel and released during and after the accident [5]. Therefore, in
this study, only physically and chemically stable, and strongly neutron absorbing
nuclides in fission products are considered, which include Gd, Nd, Sm, Rh, and Eu
isotopes.
The present work considers six nominal time steps for neutronics analysis: the
emplacement time (t = 0), the canister failure time (t = T f ), and four steps during
the dissolution of debris particles (t = T f + 0.2 T l , t = T f + 0.4 T l , t = T f + 0.6 T l ,
and t = T f + 0.8 T l ). At t = 0, the canister only contains fuel debris. The failure time
270
X. Liu