5 Nuclear Safety Research
To Support Safety Regulations and to Ensure Nuclear
Safety and Confidence
Research programs conducted inaccordance with the policy determinedby the Japanese Nuclear Safety Commission
・Probabilistic safety assessment (including
study on emergency preparedness andanalysis of operating experience)・Safety of high burnup fuel・Thermohydraulic safety of advanced
utilizations of LWRs
・Safety assessment of plant aging
・Safety of nuclear fuel cycle facilities
・Safety of waste disposal and decommissioning

Provide technical datafor regulatory judgment

・Ensure safety
・Increase public
Confidence

Fig.5-1 Major Subjects and Tasks of Safety Research

The latest scientific and technical knowledge is essential forthe development and improvement of the safety guidelines andcriteria with which the regulatory authorities perform safety
regulation to assure the safety of nuclear installations.
The Nuclear Safety Commission (NSC) proposed a“Prioritized Plan for Nuclear Safety Research” in July 2004, tobe carried out in order to meet the future regulatory needs. The
main tasks expected of us are shown in Fig.5-1.
The results of nuclear safety research contribute to themaintenance and improvement of safety of the nuclear facilities
and also to fostering public confidence in nuclear safety.
Furthermore, we are supporting the regulatory body Nuclear
and Industrial Safety Agency (NISA) and the Japan Nuclear
Safety Organization (JNES) by conducting research on various
technical issues in safety regulation which they have contracted
us to do.
The programs specified in the NSC’s prioritized researchplan and these contracts with the regulatory bodies are steadily
providing a wide spectrum of useful results. The following
paragraphs briefly describe the current status of research andnew results from the programs shown in Fig.5-1.
The program on probabilistic safety assessment (PSA)
is developing PSA methodologies for nuclear fuel cycle
facilities and studying effective uses of risk information forsafety regulation and management. The research program on
emergency preparedness is applying the methodologies of PSA
and environmental consequence analysis to the development ofguides for decision making in various emergencies, including
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JAEA R&D Review 2008

action levels, area, and timing of protective actions. Analysisof operating experience aims to obtain lessons from incidents
that have occurred in nuclear facilities. An analysis of reported
events of primary water stress corrosion cracking (PWSCC)
provided information useful for early detection of PWSCC(Topic 5-1).
For the study of high burnup fuel safety, the FEMAXI-6
code for simulation of fuel behavior under normal operation
was improved for application to MOX fuel and was tested bycomparison with an experiment (Topic 5-2). A test at the nuclear
safety research reactor (NSRR) simulating a reactivity initiatedaccident (RIA) clarified the fission gas release behavior in fuelpellets of high-burnup fuel (Topic 5-3)
In the study of safety assessment on advanced uses of lightwater reactors, tests at the Large Scale Transient test Facility(LSTF) under the international collaboration with the OECD/
NEA are in progress and have produced test data useful for
diagnosis of accident conditions and validation of analysis
codes (Topic 5-4). A new measurement system for rapid boilingbehavior in coolant channels in fuel bundle was developedand obtained test data for validation of RIA analysis codes(Topic 5-5).
For research on structural integrity assessment of reactor
components, data on the neutron irradiation embrittlement
mechanism such as intergranular embrittlement were obtained.
The data suggested that the effect of phosphorus (P) impurities
is not significant even with high values of neutron fluence.
Moreover, we proposed a modified “Master Curve” method
for fracture toughness evaluation of reactor pressure vessel
steel, that enables a conservative fracture toughness estimate
considering intergranular embrittlement (Topic 5-6).
For research on safety evaluation of nuclear fuel cycle
facilities, we performed tests of the degree to which fission
products in spent fuel reduce reactivity (burn up credit), andusing this data examined the accuracy of the computation
analysis used in criticality safety assessment of spent fuelstorage facilities (Topic 5-7).
In the field of radioactive waste disposal and facility
decommissioning safety research, we developed procedures
to derive the upper bound of radioactivity concentration
for three disposal methods of transuranium (TRU) wastes:
trench disposal, concrete vault disposal and intermediate
depth disposal. Calculation results of these procedures wereincorporated in a report by the nuclear safety commission andin the ordinance of Ministry of Economy, Trade and Industry in2008 that prescribed requirements on disposal of TRU wastes(Topic 5-8).
For the safety assessment of geological disposal of high-level
radioactive waste (HLW), we developed a computer code forprediction of long-term performance alteration of compacted
bentonite that covers the metal container encapsulating theHLW (Topic 5-9).