SIV-6
安全研究と日本の原型炉概念への展望	 
Prospect of Safety Research and DEMO Reactor Conceptual Study	 
飛田	  健次
Kenji TOBITA
原子力機構
JAEA
1. Introduction
As a result of the increasing public interest for
stable energy supply by means of safe sources after
the Fukushima nuclear accident, there is a need toreassess and reconfirm the safety advantages offusion energy. Various safety studies in the past
have identified the advantages in safety andenvironmental aspects of fusion power plants.
However, to increase the level of confidence of thepredictions, there is a need to revisit and expand on
work done in the past and use more relevant source
terms, system configuration, materials selection andplant parameters which have not necessarily been
considered in some of the previous work. Given thissituation, safety research has been started in theDEMO Design Activities of the Broader Approach.
2. Scope of safety research
The goal of the safety research is to analyze
accidental sequences potentially anticipated in
DEMO as radiological risks, to develop possible
projections of the results to safety design forpreventing and mitigating serious accidents, andeventually to compile the safety design guidelines.
In general, the process of licensing a nuclear
plant requires a comprehensive safety assessment
covering the safety analysis of the plant in depth indifferent
operational
phasesandincidental/accidental situations as well as the doses
to public and to workers in normal and abnormalconditions. The overall assessment comes to beimplementable when the detailed design of the plant
has been completed and the annual meteorological
data in the planned construction site are given.
In contrast to this, the safety research starts as
early as in the pre-conceptual design phase whenneither key design parameters nor plantconfiguration is determined. It is a rather unusual
situation compared with most of previous safetyassessments, but, looking at this from a different
angle, such a situation can provide a definite

advantage of applying effective countermeasures to
DEMO conceptual design toward improved safety.
3. Main research issues
3.1 Analysis of reference event sequences
By paying attention to radioactive source terms
and energies showing a difference between DEMO
and ITER, we can identify potentially importantreference events for DEMO among a number ofaccidental sequences considered in the ITER safetyassessment. In the same way, a comparison of thesource terms with other power plants will help us tonarrow down reference events for safety analysis,
as well.
The reference event sequences will be analyzed
using reliable and dedicated computer codes. The
analysis will cover the transient analysis of thetemperature of in-vessel components during a lossof coolant accident (LOCA), the temporal evolutionof pressure in the confinement chambers during anover-pressure event caused by an in-vessel LOCA,
and the effects of prevention or mitigation systems
against these accidents. In the case of the Japanese
DEMO concept based on water-cooling solidbreeders, hydrogen explosion caused by thesteam-beryllide reaction could be a safety concern.
In addition, the possibility of explosion resulting
from other reactions such as dust explosion andsteam-metal reactions needs to be carefullyexamined.
3.2 Projection to DEMO design
What is required today is exploring every
possible design option for ensuring passive safety toenhance the value of fusion energy. For the purpose,
the analysis results should be fed back to thereference DEMO design so that the impacts ofintroducing various prevention and mitigation
systems against abnormal events would beconfirmed. In particular, in the wake of theFukushima accident, secure safety systems forresidual heat removal in a loss of coolant accident(LOCA) and a loss of flow accident (LOFA) need
to be investigated. Admittedly, the heat removal
capability largely depends on the assemblyarchitecture of the in-vessel components and thearrangement of cooling systems. This means thatthe LOCA and LOFA analysis can provide notable
implications for narrowing down the in-vessel
component architecture and the cooling systems.
In order to reduce the burden for future
generations, the impact of the design on radioactive
waste needs to be identified. The evaluation ofradioactive waste would be performed to outline themanagement and disposal strategy of the waste to a
relatively limited extent.
3.3 Beyond design basis events (BDBEs)
In addition to the analysis of reference event
sequences based on design basis events, theanalysis of the sequences following low-probability
events is sometimes required. The events are called“beyond design basis events” (BDBEs). Despitewith extremely low probability, it is required toassure that no consequences resulting from theBDBEs lead to a cliff-edge, i.e., a cataclysmic
change of event causing a catastrophe. The
Fukushima accident revealed a fact that a boundary
between “design basis” and “ beyond design basis”
events in safety assessment is uncertain.

In the safety research, after the examples ofprevious safety assessments, the following eventswould be considered as BDBEs and the envelope of
their consequences would be analyzed.
l Total loss of on-site power for a long periodl Total loss of coolant from all loops in the plant
for a long periodl Multiple failures sequentially or simultaneously
The analysis results will be projected to the DEMO
safety design as possible, albeit BDBEs.
3.4 Safety design guidelines
The final target of the DDA Safety Research isto compile the safety design guidelines, which will
be fundamental principles for the successive
DEMO conceptual design, together with otherdesign guidelines on physics, engineering, etc.
The safety design guidelines are rationales tosatisfy the “generic safety requirements” which will
be set in the early time of the DDA Safety Research
as a target to assure inherent safety of DEMO. For
instance, the guidelines will include rationales on
confinement barriers for radioactive materials, thesecure power control of burning plasma, theintegrity of the confinement barriers, the residual
heat removal system, countermeasures against lossof coolant and power accidents, and the prevention
and mitigation systems against abnormal events.
The guidelines will include the management anddisposal strategy of radioactive waste.