PRELIMINARY STUDY OF PRE-CLOSURE SAFETY ASSESSMENT IN THE
NUMO SAFETY CASE
K. Yamashina, S. Suzuki, S. Kubota
Nuclear Waste Management Organization of Japan
Mita NN Bldg. 1-23, Shiba 4-Chome, Minato-ku, Tokyo 108-0014 Japan
Key words; repository, safety assessments, pre-closure, hazard
Abstract
The methodology of assessment for pre-closure safety has been developed as a part of NUMO safety case.
The ongoing update of the safety case for co-disposal of HLW and TRU waste in Japan will include a more
extensive assessment of pre-closure safety than has been carried out in the past. The pre-closure safety case aims
to assure both radiological and conventional safety. Radiological safety for public and workers requires radiation
shielding and radionuclide containment within the disposal facilities in case of operational perturbations.
Radiation control and facility design are based on guidelines applied for other nuclear facilities. Within theradiation-controlled zones, most operations will be remote-handled or will involve appropriate shielding,
avoiding any significant dose to workers. For conventional safety for workers, the working environment will bemaintained to ensure worker comfort and safety during normal operation. After identifying the evaluation
scenarios that take into account the progress of abnormal situation, an impact analysis is conducted to evaluate
the damage to waste packages and/or workers. In the evaluation of the abnormal state, the relationship between
the abnormal state that may occur at the geological disposal facility and the potential countermeasures areschematized by taking the same approach as the event tree analysis method.

1.

Introduction

NUMO has developed a generic safety case forgeological disposal in Japan. This safety casepresents technical evidence to support the feasibility
and safety of geological disposal, which willencourage stakeholder confidence in the safe
implementation of geological disposal and will
provide the basic structure for a safety case which
will be applicable to any potential site. The target
range of the safety case is not only after closure of
the repository but also the pre-closure period.
In the previous generic safety studies1, 2, thefeasibility of geological disposal in Japan of vitrified
high-level waste (HLW) and various types wastefrom reprocessing of spent fuel and frommixed-oxide (MOX) fuel fabrication (termed “TRU
waste” in Japan) was demonstrated, focusing on
long-term post-closure safety but not discussing
operational safety in detail.
NUMO has been developing the key technologies
required for the safe implementation of the geologicaldisposal project and published a comprehensive
technical report3 in 2011. The target range of thesafety case is not only after closure of the repository
but also the pre-closure period.
Based on the design of the disposal facility andthe procedures for construction and operation, etc., a
Hazard Database is established to allow transparent
scenario development for the pre-closure safetyassessment. It is important to discuss how operationalsafety can be secured in the disposal facility. The
present study was, therefore, undertaken in order todemonstrate the prospect of ensuring safety.
During the period for construction and operation,
the state of the disposal facility is classified into thenormal state and the abnormal state. The normal stateis defined as the state of the facility assumed as
normal operation in the design. A state that deviatesfrom planned operation due to abnormal events suchas internal fire, equipment failure or loss of power isdefined as an abnormal state.
In the evaluation of the abnormal state, therelationship between the abnormal state which canoccur in the geological disposal facility and potential
countermeasures is schematized using the event tree
analysis method. These results are compiled into theHazard Database for operational safety. After
identifying the evaluation scenarios based on the
Hazard Database, an impact analysis is conducted toevaluate the damage to waste packages and/or
workers.
The pre-closure safety assessment aims to assure
both radiological and non-radiological protection of
the public and workers. This paper describes the key
aspects of safety case concerned with the assessmentof pre-closure safety.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan2. Operational procedures and design of thegeological disposal facility
Specified radioactive waste final disposal plan4
requires disposal of 40,000 vitrified waste packages
and 19,000 m3 of TRU waste. NUMO plans to design
a geological repository in accordance with thisdisposal plan. The repository design consists of thevarious surface facilities and an undergroundrepository.
As shown in Fig. 1, the operational processes forgeological disposal can be consisted of sevenprocesses. These images illustrate the sevenprocesses for HLW. The processes for TRU waste arealmost the same as for HLW.
1. Transport to therepository site

7. Backfilling and sealing thedisposal tunnel
2. 3. 4.

1.
5.

2. Removal of HLW containers
from transport casks

6.
7.

transportation

れ3. Inspection andencapsulation
収納上蓋溶接

6. Emplacement in a disposal
tunnel検査

4. Transfer of overpacks onto atransporter

emplacement

5. Transport to the repository
disposal level

access ramp

Fig. 1. Operational processes for HLW waste
The underground facilities consist of an accessramp and connecting tunnels for transporting thewaste from the surface facilities to the undergroundfacilities, disposal tunnels for emplacing the waste
and shafts for ventilation, drainage, transportation of
excavated rocks and workers.

Based on the Defence in Depth Concept (reference)
Accident

Normal operation Abnormal operation（Design basis）
Level 1

Level 2

Level 3

Level 4
Accident management

Success

Limited within thedesign basis

Control of accident conditions

Success

Beyond design basis
accidents

Control of accidents within thedesign basis

Success

Recovery process

Fire
Loss of power
Failure of systems, etc.

Design basis incidents

Natural phenomenon
Other events

Control of abnormal operationand detection of failures

Success

Normal operation

Initiating events

Prevention of abnormal
operation and failures

External hazard

3. Concept of ensuring operational safety
Radiological protection of the public andconventional safety for workers during the period forconstruction and operation of the repository are
discussed. During the period for construction andoperation of the disposal facility, the public andworkers are protected from radiation by maintaining
the functions "containment" and “radiation shielding”
for the radioactive waste in the transport container
and the surface facility. In addition, conventionalsafety for workers will also be implemented.
Radiological protection is planned according to
the Reactor Regulation Act and guidelines for othernuclear facilities6, 7.
On the other hand, conventional safety forworkers is based on the Industrial Safety and HealthAct. The measures for ensuring operational safety arethus implemented based on the measures for existingatomic energy facilities and infrastructure such as
road tunnels, mines or underground power plants.

Fig. 2 shows schematically the conceptual flow of
event sequences and safety measures based on the
defense in depth concept8.
Firstly, "Measures to prevent the occurrence of anabnormal state" are applied to prevent the occurrenceof an abnormal state of the facility or wastes due toexternal hazards such as natural events,
anthropogenic events and equipment failure. This is
the first protection step.
The second protection step includes measures forpreventing the escalation of the abnormal state. In theevent of failure or incorrect operation during
construction and operation, it is necessary to detect
the abnormal state at an early stage, remedy it, or takemeasures to prevent progression to an accident state.
"Measures to prevent progression of the abnormal
state", such as the use of incombustible materials andflame-retardant materials, duplication of lines, etc.,
are prepared for abnormal states such as fire,
overpack dropping or loss of power.
Furthermore, if the abnormal state exceeds theassumed magnitude, it may be that "measures toprevent the progression of the abnormal state" will
become invalid. In this case, in order to prevent theabnormal state from progressing to an accident
situation involving release of radiation by keeping theabnormal state within the design basis and therobustness of the facility and the waste, it isnecessary to take measures such as emergency fire
extinguishing facilities and limitation of lifting heightof the overpack. This is the third protection.

Limited accidental
radioactive releases

Mechanical impact and/or
thermal impact on the waste
packageFig. 2. Conceptual flow of event sequences andsafety measures5
The Hazard Database for operational safety wasconstructed to protect the public and workers fromradiation and to ensure conventional safety for theperiod for construction and operation of thegeological disposal facility5. The process ofdevelopment and justification of scenarios areprovided in chapter 4.
4. Scenario development for pre-closure safety
This chapter illustrates how to construct theHazard Database for the period of construction andoperation. In the evaluation of abnormal situations,

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

the Hazard Database for operational safety wasconstructedtoallow
transparent
scenariodevelopment for pre-closure safety and to promote
protection of the public and workers from radiationand ensure conventional safety for workers.
In the evaluation of the abnormal state, therelationship between an abnormal state which canoccur in the geological disposal facility and thepotential countermeasures is schematized using theevent tree analysis method. These results arecompiled into the Hazard Database for operationalsafety.
In constructing the Hazard Database for
operational safety, we proceeded with considerationof international safety standards9, 10.
The development processes for event identificationand event evaluation are shown in Fig. 3.
Five steps are set within this scope of work.
Hazard Database development is included in thescope of STEP 4. The contents of each step arepresented in section 4.1 to 4.5.
Finally, based on the results of the Hazard
Database development, evaluation scenarios relating
to mechanical and thermal impacts are constructed
and the effects of these impacts are evaluated byanalysis.
1. Identification of hazards and initiating events

2. Screening of hazards

3. Evaluate the event sequence

4. Examine the countermeasures

5. Developing the evaluation scenarios
Evaluate the robustness of disposal concept

Fig. 3. The process of development and justificationof scenarios5
The relationship between abnormal conditions
that may occur in the geological disposal facility andthe safety measures taken against them isschematized using the event tree analysis method.
The details are described in section 4.1 to 4.4.
The summary of these results represents theHazard Database for operational safety. It isdisplayed in the database in the form of an event tree
and is linked with hazard datasheets showing thedetails of the progress of each event.

4.1 Identification of hazards and initiating events
The guideline for similar facility, “Regulations
and guides for waste management facilities“, is used
as a reference, and the external hazards that may
cause the abnormal state of facility or abnormal
situation of the wastes are identified. And the hazards
are identified from relevant laws and regulations inJapan shown in Table 1.
Table 1. Relevant laws and regulations in Japan
Relevant laws and regulations in Japan
Regulations on the location, structure andequipment standards for waste management
facilities
Disaster Countermeasures Basic Act
Industrial Health and Safety Act（second and third editions）
Regulations for Enforcement of the Mine Safety
Act
First of all, natural events and other events thatare the cause of the hazard are extracted based on
relevant laws and regulations as shown in Table 1.
According to the guidelines for waste
management facilities, all events are classified into
natural events and other events.
The events listed in the regulations for similar
facilities have different causes and effects, affecting
the facilities and/or wastes. This makes it difficult toorganize the hazards.
For this reason, the extracted hazards were firstclassified into causes and effects. With thisarrangement, it is possible to avoid complicating thework of organizing the events causing the abnormal
state in the next step. Also, all the events arereviewed and the relationship between cause andeffect may be revised in the next step, as may someof the classifications.
For natural events 11 events such as earthquake,tsunami, floods, wind, tornado, rain, snow, lightning,
volcanic influence, biological event and forest fire
were identified.
For other events 13 events such as ballistic
fragment, dam collapse, explosion, fire inneighboring installations, toxic gas, ship collision,
electromagnetic interference, illegal intruders, carryon of the explosive and harmful material,cyberterrorism, mine inundation, ground pressure,and terrestrial heat were identified.
In this way, 11 natural events and 13 other eventswere extracted, totaling 24 events. Also, because
pre-closure safety is required to ensure both radiation
protection and conventional safety, the effects ofevents on radiological safety and conventional safety
for workers were arranged separately.
After this identification, ballistic fragments (forexample aircraft crash, etc.), dam collapse, fire in

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanneighboring installations and ship collision were
excluded at present. It is difficult to consider these
events at the current stage because whether thefacility will be affected by these events depends onthelocation,
topography
andsurrounding
environment at a specific site to be selected in the
future. Other external events (e.g. criticality events,
decay heat, direct release of radiation, etc.) areexcluded because they are evaluated to be lessfrequent or are considered in other event sequence
analyses.
4.2 Screening of hazards
The natural events and other events are taken as
the starting-point and the event sequences that might
affect the facilities, waste and workers are identified.
The initiating events of an abnormal state in thedisposal facility are identified. As a typical example,
Fig. 4 shows the result of an event sequence analysis
due to an earthquake.
Effect

Cause

Progress of the event

Level 1

Level 2

Level 1Naturalevent

Earthquake

Occurrenceof Crack

Level 2

Level 3

Initiating
Eventspring waterin a short time

Flooding

Flooding

The damage of theelectric equipment

Loss of power

Loss of power

Turnover, Slant,
Distortion

Mechanicalmalfunction

Failure or
malfunction ofsystems

Distortion of theground foundation

Damage tostructures
Landslide,earth pressure

Damage tostructures ortunnels

Slope collapse, earthpressure
Distortion of thenatural ground

Damage to tunnels

Fig. 4. Event sequence analysis due to earthquake5
As shown in Fig. 4, when an earthquake is theinitiating event, progression will eventually lead toflooding, loss of power, failure or malfunctioning ofsystems and damage to structures or tunnels. Event
sequence analyses were organized for all events inthe same way.
In arranging these event sequences, if all
combinations were to be considered the combinations
would be too numerous to allow clear arrangement.
Events which cannot be reasonably explained are also
included in the initiating events to be considered. For
this reason, we organized these event developmentsin the following way.
We investigated the types of natural disasters
worldwide in the past, the occurrence situation andthe number of occurrences of each.
By comparing with the accident in the past, it ispossible to screen out the events which are notplausible or low frequency of occurrence.
Also, in judging whether these could be an
initiating event or not, we organized them based onwhether or not they should be considered for thegeological disposal facility. By proceeding in thisway, it was possible to extract event sequences to beconsidered from among the many combinations ofevents.
Natural events and other events listed in Tables 2
and 3 were arranged in the same way for each.
As a result, for radiological protection all hazards
can be organized into six initiating events. Taking theconventional safety for workers into account, a poor
working environment should be added as a seventh
initiating event as shown in Table 2.
Table 2. Initiating events affecting to the disposal
facility
Initiating events
1. Fire2. Explosion
3. Flooding
4. Loss of power
5. Failure or malfunctioning of systems
6. Damage to structures or tunnels
7. Poor working environment
4.3 Event tree analysis for event sequences
In the evaluation of the abnormal state, therelationship between the abnormal state and potential
countermeasures are schematized using the event tree
analysis method, with the initiating event specified in
the previous section as the starting-point.
As a typical example, Fig. 5 shows the result ofan event tree analysis for failure or malfunctioning ofsystems leading to dropping of the overpack. This
figure shows the analysis results for progression of
the initiating event caused by the machine failures,
malfunctioning and human error, etc. assumed in thesurface facilities. As a result, events that impact on
the overpack are arranged in four cases and thepreventive measures are indicated in the next chapter.
Occurrence ofabnormal event
Failure or
malfunction ofsystems

Abnormal state

Wire cutting

Control of abnormal operation anddetection of failures

Incidents within thedesign basis

Control of incidents
within the design basis

Mechanical impact
Thermal impact

Limiting the height
to lift up

The impact on
overpack

Limiting the height
to lift up

The impact on
overpack

Limiting the height
to lift up

The impact on
overpack

Limiting the height
to lift up

The impact on
overpack

within expectation

・Making the wire doubled
・ Monitoring by operators
・Fail-safe function・periodical maintenance

Recovery process
An overpack drop

unexpected

within expectation

Derails andoverruns ofoverhead crane・Lags to prevent derailment
・Installing end stoppers of girder

Incomplete
grasping・ visual check of grasping
condition・ operational manual, instruction
・Interlock to block the crane fromreleasing an overpack at lifting itup

Recovery process
An overpack drop

unexpected

within expectation

Recovery process
An overpack drop

unexpected

Release duringlifting it

・Interlock to block the crane fromreleasing an overpack at lifting itup
・ operational manual, instruction

within expectation

unexpected

Recovery process
An overpack drop

Fig. 5. Result of event tree analysis for dropping of
the overpack5
Through the analysis of event sequences, "hazard
datasheets" that show the details of the event
sequence are developed. "Hazard datasheets" show
not only the details of the progression but also themeasures to be taken against the events.
The hazard datasheet also describes incident cases,
relevant laws and regulations for related facilities andother facilities as reference information.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

Every initiating event was analyzed for each
operational process. Identified initiating events weretreated as the starting-point, every situation which is
generated by considering all combination of initiating
events and operational processes were analyzed with
Event Tree Analysis method. In this case, theoperational processes was grouped into three
processes in the surface facilities, access ramp andunderground facilities and analyzed in the same way.
As a result, 54 events that have mechanical orthermal effects on the overpack and waste packages
were identified; 48 events that affect workers andothers during the operational period were also
identified. In the construction period of theunderground facility, there were 52 events affecting
people such as workers.
4.4 Measures against abnormal situations
Event sequences are analyzed and the measures tobe taken in the case of these abnormal events are also
examined. Measures are classified into three stages:"Measures to prevent occurrence of the abnormal
state", "Measures to prevent progression of theabnormal state" and "Mitigation measures" based on
the defense in depth concept8.
The purposes of these measures are as follows:
・To prevent abnormal operation and failures
・To control abnormal operation and detection of
failures・To control incidents within the design basis by
mitigating the thermal or mechanical impact.
These safety measures are also included in hazard
datasheets.
4.5 Numerical analysis for dropping of theoverpack
After identifying the evaluation scenarios basedon the Hazard Database development, an impact
analysis is conducted to evaluate the damage to waste
packages and/or workers.
In the surface facility, overpacks are shipped tothe transport vehicle which transfers them throughthe access tunnel. In this process, it was assumed that
overpack would fall due to the failure of lifting
equipment.
For example, to evaluate the consequences of the
dropping impact on the overpack, Fig. 6 indicates theevaluation model of a finite element analysis whichassumes a maximum drop height of 9 m. The
evaluation focused on the possibility of cracks
penetrating the overpack.
For the purpose of conducting more precise
evaluation, analysis is performed with an analysis
model reflecting the following items.
 Modeling the parts of lid of overpack andwelds of it in order to consider the shearing
force and stress concentration at the weldedpart in the case of collision.

Modeling the vitrified waste and waste
canister inside the overpack in order toconsider the behavior such as a bounce
behavior inside the overpack in the case ofcollision.
Simulating the collision against concrete
floors in order to consider the effect of impact
mitigation.

Lid of overpack
Welding part
Overpack
Waste canister
A

Vitrified waste

A

Concrete floor

Fig. 6. Analysis model for evaluating the dropping
impact on the overpack
LS-DYNA Ver.8.0.0., which is general-purpose
structural analysis software, was used for thisanalysis.
The possibility of cracks penetrating the overpack
is judged by comparing the equivalent plastic strain
and breaking strain of the overpack material. In the
judgment, the penetrating cracks were judged tooccur when the region where the equivalent plasticstrain exceeded the fracture strain of the material wasconnected from surface to the inside of overpack.
The analysis results indicated that the area where
the equivalent strain exceeds the strain limit of the
carbon steel is generated only at the region
surrounding the impact point, but did not reach theinside of the overpack as shown in Fig. 7.

Fig. 7. Result of the analysis in case of dropping of
overpacks
In addition, the waste canister resulted inmaintaining structural integrity. There was noinfluence on welded part of an overpack due to the

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanjumping up of the vitrified waste and no influence on
the upper part of the canister.
This scenario is assuming the case of droppingfrom a lifted height of 9 m, the height expected forthe time of actual handling. Therefore, it is unlikely
to result in through crack in case of dropping of
overpacks.
5. CONCLUSIONS
A methodology for assessment that demonstrates
the prospect of ensuring pre-closure safety has beendeveloped in the NUMO safety case. The safety casefor the period of pre-closure includes both
radiological and conventional safety.
NUMO has also constructed a Hazard Databasefor operational safety during the period forconstruction and operation of the geologicalrepository. The Hazard Database is established toallow transparent scenario development for the
pre-closure safety assessment.
In the evaluation of the abnormal state, therelationship between the abnormal state which canoccur in the geological disposal facility and thepotential countermeasures is schematized using theevent tree analysis method. These results arecompiled into the Hazard Database for operationalsafety.
The compiled Hazard Database for operationalsafety will be continuously updated in the future andthe comprehensiveness for assessing the pre-closure
safety will be improved.

NUMO: “Pre-closure safety assessment insafety case development”, IHLRWM 2017,Charlotte, NC, April 9-13, 2017.
6. Nuclear
Regulatory
Authority(2013):
Regulation on location, structure and equipment
for waste management facilities，entered into
force on December 18, 2013 (In Japanese).
7. Nuclear
Regulatory
Authority(2013):
Interpretation of regulation on location, structureand equipment for waste management facilities，entered into force on November 27, 2013 (InJapanese).
8. IAEA (2005): Assessment of Defence in Depth
for Nuclear Power Plants, IAEA Safety Report
Series No. 46.
9. DOE (2014): Waste Isolation Pilot Plant
Recovery Plan, U.S. Department of Energy.
10. IAEA (2011): Disposal of Radioactive Waste,
IAEA Safety Standards Series No. SSR-5.
5.

ACKNOWLEDGMENTS
The authors gratefully acknowledge colleagues of
SHIMIZU CORPORATION for their useful input.
REFERENCES
1.

2.

3.

4.

JNC: “H12: Project to establish the scientificand technical basis for HLW disposal in Japan”,
JNC TN-1410-2000-01
JAEA & FEPC: “Second progress report on
research and development for TRU waste
disposal in Japan; Repository design, safetyassessment and means of implementation in thegeneric phase”, JAEA-Review 2007-010
NUMO: “Safety of the Geological Disposal
Project 2010”, NUMO-TR-11-01 (2011). (InJapanese; English summary available as
NUMO-TR-13-05).
Ministry of Economy, Trade and Industry:
Revision of the basic policy on specified
radioactive waste final disposal and thespecified radioactive waste final disposal plan,press release, 14 Mar, 2008 (In Japanese).

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan