Subsidy Project of Decommissioning and Contaminated Water Managementin the FY 2015 Supplementary Budgets

Development of Technology for Criticality
Control of Fuel Debris
Final Report

March 2018
International Research Institute for Nuclear Decommissioning (IRID)
All Rights Reserved. International Research Institute for Nuclear Decommissioning©International Research Institute for Nuclear Decommissioning

1
CONTENTS
1.1 Research background and purposes••••••••••••••••••••••••••• 2
1.2 Project Goals•••••••••••••••••••••••••••••••••••••••••••• 4
1.3 Implementation Items, Their Correlations, and Relations with
Other Research•••••••••••••••••••••••••••••••••••••• 6
1.4 Project Organization ••••••••••••••••••••••••••••••••••••••••••• 8
2. Implementation Items
2.1 Establishing Criticality Evaluation Technologies ••••••••••••• 9(1) Criticality Scenarios Creation•••••••••••••••••••••••••9(2) Evaluation of Behaviors During Criticality ••••••••••••• 12(3) Drawing up Criticality Control Technologies••••••••••••• 14
2.2 Development of Criticality Control Technologies•••••••••••••15(1) Critical Approach Monitoring Technologies ••••••••••••• 15(2) Development of Re-Criticality Detection Technologies• 21(3) Criticality Prevention Technologies(i) Insoluble Neutron Absorber ••••••••••••••••••••••••••••26(ii) Soluble Neutron Absorber•••••••••••••••••••••••••32
3. Overall Summary•••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 34
1. Plan Overview©International Research Institute for Nuclear Decommissioning1. Plan Overview

2

1.1 Research Background and Purposes[Purpose]
It is estimated that, currently, the fuel debris is not in a critical state. However, the criticality control method will beestablished to prevent criticality during fuel debris retrieval work in the future, triggered by changes in the shape of fueldebris and/or the amount of water. These technologies will also protect the general public and workers from excessive
exposure should criticality occur.

[Goals]• To determine criticality control methods for each fueldebris retrieval method, develop applicable element
technologies, and confirm their feasibility, based on thedecision on the fuel debris retrieval policy in the summerof 2017, as scheduled on the roadmap.

Reactor Core
Lower Part of RPV
Bottom of PCV (Inside
Pedestal)
Bottom of PCV(Outside Pedestal)

Liquid Waste
Treatment Facility
Expected fuel debris locations

The following technologies have been developed to establish criticality control methods during retrieving fuel debris:
1. Criticality Evaluation Technologies: (1) Evaluate criticality scenarios, (2) Evaluate behaviors during criticality, and (3)Develop criticality control technologies.
2. Criticality Control Technologies:(1) Critical approach monitoring technologies.
(Note: The development of a critical approach monitor for small circulation loops
was completed in FY 2013.)(2) Re-criticality detection technologies (for gas sampling system and neutron system).
(3) Criticality prevention technologies (development of insoluble neutron absorber andsoluble neutron absorber)©International Research Institute for Nuclear Decommissioning

3[Project Achievements Application]
Item/FY
Main Roadmap Schedule

2015

2016

2017

Decide on fuel debris retrieval policy [Development of Technology for
Criticality Control Methods] Project
1. Establishing Criticality Evaluation
Technologies(Evaluation of Criticality & Behaviors
During Criticality)

Evaluation and criticality control methods for each debris
processing technique

2. Developing Criticality Control
Technologies(1) Critical Approach Monitoring
Technologies

Advanced system review: Feasibility test(2) Development of Re-Criticality
Detection Technologies(3) Criticality Prevention Technologies
Insoluble Neutron Absorber

2018

2019

2020

2021

Finalize retrieval methods ∆ Retrieve fuel debris ∆Critical scenario review: Update with latest information

Criticality evaluation for internal
investigation & sampling
Technologies review: Feasibility test (verification ofprinciples)

Candidate material selection, application method
review, and feasibility test
Impact assessment on use & facility review:
Feasibility test

Reflect the development
results in method andfacility reviews

Soluble Neutron Absorber[Upgrading of Approach and Systems
for Retrieval of Fuel Debris andInternal Structures] Project©International Research Institute for Nuclear Decommissioning1.2 Project Goals (Goals and Basic Concepts of Criticality Control) No.4
4
The objectives of criticality control were set reflecting opinions of external experts (from Evaluation
Committee, CRIEPI in FY 2016).
[Criticality Control Objectives]
To prevent excessive exposure (radiation sickness) for the general public and workers by preventing criticality and by
detecting and suppressing criticality even if it occurs.
Criteria: General public on the site boundary: 5 mSv, workers: 100 mSv[Criticality Control Methods Based on Defense in Depth]
Level 1:Prevention of Abnormality

Criticality
Control

Monitoring

Prevention• Monitor critical• Limit the amount ofapproach using
debris that can be
critical approach
retrieved at a time
Specific Methods monitoring• Apply boric acid
(Major Ones)
techniquesolution/insoluble• Monitor levels ofneutron absorberwater and boron
concentration, etc.

Level 2:
Understanding the State of andTerminating Abnormality

Defection

Mitigation• Detectcriticality
based onneutronfluxes/FP gas
concentration• Terminate
criticality byinjecting boricacid
solution/insoluble
neutron absorber• To prevent criticality by monitoring critical • To promptly detect and suppress
approachcriticality
Objectives(Preventing the emission ofradioactive materials at a higher level
than under normal operating
conditions (*))

Portion which criticality control focuses on

Level 3:
Level 4:
Protection of the Addressing Events
General Public
BeyondExpectation
Outside the
Facility Perimeter• Measures are taken against
situations, such as leakage ofradioactive materials and fire, aspart of overall risk management.

• To prevent excessive exposure(radiation sickness) for the generalpublic and workers in the event ofan accident

Portion that should be accomplished bycomprehensive efforts* Going forward, this level should be reviewed fromall aspects of safety control
includingcriticality.
©International
Research
Institute for Nuclear Decommissioning

5[Criticality Control Methods Used During Debris Retrieval (Positioning of Element
For more comprehensive information, see 2.1
Technologies)](3) Drawing up Criticality Control Methods.
Normal Conditions
Abnormality Prevention (PS)
Abnormality Prevention Parameter Monitoring
Restrict Retrieval
Tasks
Prevent Criticality
Using Neutron
Absorber(*)

Abnormal Conditions

Accident

Detect & Terminate
Abnormality (MS)Protection of the General
Public(Mitigate Impact of Accident)

Objectives

Monitor & Maintain

2.1(1)
Parameters
Criticality Scenarios
Creation* When neutron absorbers (soluble/insoluble) are used

Failed(Critical Approach Occurs)

2.2 (3) Criticality Prevention
Technologies(i) Insoluble Neutron Absorber(ii) Soluble Neutron Absorber

Monitor Critical
ApproachPrevent
AbnormalConditions(Criticality) from
Occurring

2.2 (1) Critical Approach
Monitoring Technologies
Failed (Criticality Occurs)

Success (Critical Approach
Terminated Before Criticality andReturns to Normal)

Stop Retrieval Tasks

Criticality Detection
Neutron Detector/
Gas Sampling System

Success (Criticality
Terminates)

Inject Emergency Boric
Acid Solution

Success (Criticality Terminates and Emission of Radioactive
Materials is Suppressed to Outside the Facility)

2.2 (2) Re-criticality
Detection Technologies

Failed (Criticality
Continues)

Prevent
Escalation into
Accident

2.1 (2) Evaluation of
Behaviors During Criticality
Inject Additional Boric Acid
Solution
Reduce Water Level
Suppress Radioactive Gas
Emission

Prevent
Excessive
Exposure for the
General Public &Workers

Reviewed Comprehensively
by the Facility©International Research Institute for Nuclear Decommissioning1.3 Implementation Items, Their Correlations, and Relations with
Other Research6

1. Establishing Criticality Evaluation Technologies(1) Criticality Scenarios Creation(i) Revisiting Critical Risks with the Latest
Know-Hows(ii) Statistical Criticality Evaluation(iii) Criticality Evaluation

Update9
Statistical Criticality Evaluation
1011(2) Evaluation of Behaviors During Criticality(i) Evaluation during Debris Retrieval(3) Drawing up Criticality Control
Technologies

Objectives
To Establish Methods
for Each Process

12

14

Criticality Evaluation During Retrieval (Review Restrictions
Placed During Processing and Align with Methods)

When the PCV is Filledwith Water

Evaluation of Behaviors During
Debris Retrieval

Point of View Concerning
SafetyControl Method

2. Developing Criticality Control Technologies(1) Critical Approach Monitoring
Technologies

Feasibility Test (Sub-Criticality Level Measurement)
15

21

Verification of Feasibility & Application Method of
Advanced Gas System(i) Insoluble Neutron Absorber

26

Feasibility Test (Nuclear Properties, Long-Term Irradiation,and Workability)(ii) Soluble Neutron Absorber

32(2) Developing Re-Criticality Detection
Technologies(3) Criticality Prevention Technologies

Feasibility Test (Confirm Nuclear Properties)
Control Method

15

During Fuel Debris
Retrieval

How to Set Boron
Concentrationand other numbers in the same format indicates page numbers.
©International Research Institute for Nuclear Decommissioning

[Correlations with Related Projects]

7
Development of Sampling
Technology for Retrieval of Fuel
Debris and Internal Structures

Upgrading theComprehensive
Identification of Conditions
inside Reactor

Characterization of Fuel
DebrisDevelopment ofTechnology for
Investigation inside RPV

Properties and Distributionsof Fuel Debris

Development ofTechnology for
Criticality Control
Methods* This project is outside of the scope ofthe subsidized Government-led R&D
Program on Decommissioning andContaminated Water Management.

MethodInformation

Criticality Control
Technologies

Upgrading of Fundamental
Technologies for Retrieval of
Fuel Debris and Internal
Structures

Development of Repair
Technology for Leakage
Points inside PCVDevelopment ofTechnology for
Investigation inside PCV

Estimating the Fuel Debris
Locations Using Cosmic
Ray Muons*

Upgrading of Approach and
Systems for Retrieval of Fuel
Debris and Internal Structures

Information on
the Use of
Neutron Absorber
Impact on
Structural
Materials
Developing Corrosion
Control Technologies for
RPV/PCV

Impact on Storage
CanisterDevelopment of Technology
for Collection, Transfer andStorage of Fuel Debris©International Research Institute for Nuclear Decommissioning1.4 Project Organization

8

International Research Institute for Nuclear Decommissioning(Head Office) Planning Overall of Project and Supervising TechnicalManagement
 Supervising Technical Management Including Progress ofTechnical Development

Mitsubishi Heavy
Industries, Ltd.
1) Establishing Criticality Evaluation
Technologies
Drawing up criticality scenarios
Evaluation of Behaviors During
Criticality
Drawing up Criticality Control
Methods2) Developing Criticality Control
Technology
Criticality monitoring technology
Criticality prevention technologies(Soluble neutron absorber)

Toshiba Energy Systems &Solutions Corporation

Hitachi-GE Nuclear Energy,Ltd.

1) Establishing Criticality Evaluation
Technologies
Drawing up criticality scenarios
Evaluation of Behaviors During
Criticality
Drawing up Criticality Control
Methods2) Developing Criticality Control
Technology
Critical approach monitoring
technologies
Developing re-criticality detection
technologies(Neutron system)
Criticality prevention technologies(Insoluble neutron absorber)

1) Establishing Criticality Evaluation
Technologies
Drawing up criticality scenarios
Evaluation of Behaviors During
Criticality
Drawing up Criticality Control
Methods2) Developing Criticality Control
Technology
Critical approach monitoring
technologies
Developing re-criticality detection
technologies (Gas samplingsystem)Criticality prevention
technologies(Insoluble neutron absorber)©International Research Institute for Nuclear Decommissioning2. Implementation Details

9

2.1 Establishing Criticality Evaluation Technologies (2) Criticality Scenarios Creation(i) Revising Criticality Risks (Priority in Criticality Control) with Latest Know-Hows• To clarify critical scenarios for each process based on multiple methods and present the criticality control priority.
• To provide critical scenarios and risk assessment reflecting the latest information[Achievements]• Latest know-hows were reflected and side access methods were reviewed based on results of
Identification of Conditions Inside the Reactor Vessel Project in FY 2016, investigation inside the PCV of Unit 1(B2), investigation inside the PCV of Unit 2 (A2, A2'), investigation inside the PCV of Unit 3, and muons
measurements.
No new information was discovered that largely exceeded our original expectations, but the priority of controlling thesubmergence of the bottom of the PCV in Unit 2 was revised to small (See Table 1).
• Reviewed the operation and effectiveness of the prevention measures for reducing re-criticality risks[Issues for Practical Application]• To reflect the results of the Identification of Conditions Inside the Reactor Vessel Project, investigation inside the
PCV/PRV, and debris sampling Table
etc. 1 - Relative Priority of Criticality Control for Each Unit
[Purpose]

Components

Criticality Scenarios• Residual fuel submergence

Reactor Core

Lower Part of
RPV

CRD Housing

Bottom of PCV• Debris submergence• State changes during retrieval• Adhered debris submergence• Exposed debris submergence• State changes during retrieval(Including stirring up)

Unit 1

Unit 2

Unit 3Extremely low

MediumLow(Almost no fuel remaining)(Fuel may be remaining in thereactor core and peripheral area)(The possibility cannot be denied
that fuel remains in the peripheral
area)

Submergence: Low
Retrieval: Extremely low

Submergence: Medium
Retrieval: Low

Submergence: Medium
Retrieval: Low(Small residual amount)(A lot of exposed residual
amount)(A lot of exposed residual amount)Between extremely low
and low

Between extremely low
and low

Between extremely low andlow(Small risk based on adherence profile
and amount)(Small risk based on adherence profile andamount)(Small risk based on adherence profile andamount)Submergence: Low
Retrieval: Low

Submergence: Changed
from medium to low
Retrieval: Low

Submergence: Low
Retrieval: Low(Large amount and small
exposure)(Large amount and small exposure)(Slightly small amount andextensive exposure)Note: Relative priority of criticality control during debris retrieval
In the aspect of criticality control, a “high” priority case is where the original fuel profile is retained and the presence of a corresponding amount offuel debris is expected. The priority of each case is shown using this as the baseline.
©International Research Institute for Nuclear Decommissioning2.1 Establishing Criticality Evaluation Technologies (2) Criticality Scenarios Creation(ii) Criticality Evaluation by Introducing Statistical Approach

10[Purpose]- To evaluate criticality based on the estimated distribution instead of selecting
areas with more severe evaluation conditions[Implementation Items]- Use parameters such as fuel debris properties as random variables and estimate
the degree of sub-criticality (neutron multiplication factor).
- Develop the concepts and analysis methods for statistical criticality evaluation• Parameters treated as random variables
Debris particle size, porosity, debris volume occupancy, debris
composition (mixture ratio of structural materials, etc.), and Gd carrying
ratio• Case Settings
Figure 1 - Analysis System (Left: Entire System,
The PCV system of Unit 1 (Achievements of the identification of
Right: Pedestal Floor)
conditions inside theFrequency (Water level at 280 cm, general discrete distribution)
reactor vessel were reflected.)
Frequency (Water level at 190 cm, general discrete distribution){[Achievements][Issues for Practical Application]
To review the method to improve the reliability by introducing new
knowledge.

Normal distribution with mean and variance of histogram (Water level at 280 cm, general discrete
distribution)
Normal distribution with mean and variance of histogram (Water level at 190 cm, general discrete
distribution)

Frequency- Considered methods for estimating the current conditions using
statistical criticality evaluation• According to the neutron multiplication factor based on the radiation
concentration ratio of the Unit 1 gas management system, themixture ratio of metallic component was estimated using theBayesian estimation.
- Established evaluation methods that take into account fuel debris
properties and observed values, and conducted a statistical evaluationconsistent with the current estimates of fuel debris properties and theneutron multiplication factor. The results suggested that the system
was sufficiently subcritical.

NeutronMultiplication
Factor: 0.95

Neutron Multiplication Factor (Interval Median)Figure 2 - Comparing Neutron Multiplication Factor
Distributions Before and After Water Level Changes©International Research Institute for Nuclear Decommissioning2.1 Establishing Criticality Evaluation Technologies (2) Criticality Scenarios Creation 11(iii) Criticality Evaluation Reflecting the Review of Fuel Debris Retrieval Methods

Additional reactivity (%∆k)[Purpose]
To reflect the criticality evaluation conducted on each debris retrieval method in the safety requirements.
[Achievements] Boring processing: Reviewed the limit on the amount of debris
Enrichment: 5.0 wt%, Height: 80 cm, Porosity: 0.13
Enrichment: 5.0 wt%, Height: 30 cm, Porosity: 0.25
that can be retrieved at a time in order not to add excessive
Enrichment: 4.0 wt%, Height: 80 cm, Porosity: 0.18
reactivity.
Enrichment: 4.0 wt%, Height: 30 cm, Porosity: 0.29
Reviewed the possibility to ease the restriction on the amountof debris that can be retrieved at a time by revising theevaluation conditions.
• Enrichment: 5 wt% (highest of all pellets)  4 wt% (highest of mass
averages)(A realistic condition was adopted.)

Enrichment: 5.0 wt%
Enrichment: 4.0 wt%• Additional reactivity: 0.1% Δk/k or less  0.5% Δk/k or less
(The condition was eased to a level that does notcause the system to go prompt critical.)Concluded that the limit on the amount of debris that can be retrieved at a time can be reduced
from 12 cm cube or smaller to 16 cm cube or smaller
Laser Processing:
Surveyed the latest information and found out that the processing speed
was slow and the critical mass was not easily reached.
Plasma Processing: Conducted a minimum critical weight evaluation for a case where the debris
is being stirred up and determined that the criticality risk was small.

Length of a Side of a Retrieved Mass (Cube)

Figure 1 - Correlation Between Retrieved Amountat a Time and Additional Reactivity

Table 1 - Shortest Time to Reach Criticality by Stirred up Fine Debris
Processing
Minimum Burnup Composition
Average Core Burnup
Method(*)
per Mass
Composition(Processing Speed)(Minimum Critical Mass: 46 kg)(Minimum Critical Mass: 109 kg)
Laser Processing
2 hours 19 minutes
5 hours 30 minutes(330 g/min)
Plasma Processing(850 g/min)

54 minChange of Profile due to Processing
and Stirring up of Fine Particle DebrisLaser

2 hours 8 minutes(The above table only shows the laser and plasma processing methods, although there are other processing methods
including water jet and ultrasonic drilling. This is because these latter methods have a slower processing speed.)[Issues for Practical Application] • Align the requirements for each retrieval method with therequirements of the project for studying the fueldebris retrieval method.
©International Research Institute for Nuclear Decommissioning(*) Based on the results from the “Upgrading of Fundamental Technology for Retrieval of Fuel Debris and Internal Structures” conducted on FY 2013
Supplementary Budget for Government-Led R&D Program on Decommissioning and Contaminated Water Management in FY 2014.

2.1 Establishing Criticality Evaluation Technologies (2) Evaluation of Behaviors
During Criticality(i) Selecting Events to Evaluate[Subjects of Evaluation]
Impacts of criticality are evaluated to review measures taken for such events
assuming conservative conditions.
[Assumptions]• As a typical event during debris retrieval, crack generation and occurrence of
criticality due to processing fuel debris.
It is assumed the fuel debris is solidified in bulk (13 t) on the bottom of the RPV. The fueldebris is submerged. The fuel debris is processed by cutting. Cracks are generated
instantaneously across the fuel debris mainly around the position where it has been cut.
The volume of the cracks constitutes about 1% of the fuel debris. The water penetrates into
the cracks to create an optimal moderation condition causing the entire debris to reachcriticality. The critical excess reactivity is about 0.1 %dk. (Figure 1 - Criticality Generation
Model)• Criticality Detection: Neutron detector near the retrieval position
A neutron detector (critical approach monitoring technique) installed within a few dozen cm(preliminary) of the debris instantaneously detects neutrons generated by criticality. The
debris reaches criticality when the neutron count rate becomes 1000 times the initial value.
(Figure 2 - Criticality Detection Model)• Criticality Termination: Injection of emergency boric acid solution
Sodium pentaborate solution was started to be injected once criticality is detected. The
sodium pentaborate solution reaches the debris in ten minutes, starts to develop a negative
reactivity effect, and finally terminates the critical event. (Figure 3 - Criticality Termination
Model)• Exposure Assessment: Assessment of exposure dose assuming FP gas emission
Of the fission products generated before criticality is terminated, FP gas is emitted outsideof the building in the exhaust from the PCV gas control system. Internal and external
exposure for workers outside of the building and the general public on the site boundary
caused by the emitted FP gas cloud. Some of the FP gas is leaked from the cell to theoperation floor and cause exposure for operation floor workers. Worker evacuation iscompleted in one hour after criticality is detected and the exposure event is terminated.
(Figure 4 - Exposure Model Caused by Criticality)

12

Processing devices to cut the debris(candidates)• Disk cutter • Wire saw • Laser• Hydraulic cutter • Gas• Plasma ark • Plasma jet• Abrasive water jet (AWJ)

DebrisComposition

Fuel Debris: 13 t
(Composition)
U+Pu: 50 wt%
Zr : 25
SUS : 25
FP: None
B/Gd: None

Figure 2.1-1 - Criticality Generation Model

Neutron

Neutron
Detector

Figure 2.1-2 - Re-Criticality Detection
Model©International Research Institute for Nuclear Decommissioning2.1 Establishing Criticality Evaluation Technologies (2) Evaluation of Behaviors
During Criticality(ii) Evaluation of Behaviors During Debris Retrieval[Analysis Conditions]• Critical fuel debris: 13 t (including U+Pu/Zr/SUS, without FP/B/Gd)• Instantaneous reactivity after injection: 0.1 %dk (initial condition is criticality)• No delay in neutron detection• Criticality criteria for neutron detector: 1000 times the initial value• Delay in sodium pentaborate injection: 10 min• Part of FP gas generated in the PCV goes through the filters and is discharged outside
the building in the form of noble gas.

13
SodiumPentaborate
Solution Tank

• Exhaust from the PCV gas control system: 3000 m3/h
• Leak rate from the cell to the operation floor: 1%[Achievements]• Total number of nuclear fissions is up to about 1018.

Figure 3 - Criticality Termination Model• Public dose on the site boundary is less than the self-management goal (0.1 mSv).
Primary Boundary• Exposure dose for workers working outside the building is less than thestandard for normal operating conditions (20 mSv).
• Exposure dose for workers on the operation floor is even smaller by two orders ofmagnitude.
[Issues for Practical Application]• Present the facility requirements for minimizing the impact of exposure
even in the case of criticality.

Secondary Boundary

Leakage from Cell
1%
FP

Operation
Floor
Workers

Exhaust from the PCV
Gas Control System
3000 m3/h

FP

FP

FilterCriticality• Expand the event evaluation to other respective processes including investigation
inside the PCV/RPV in addition to debris retrieval.
• Expand data to statistically evaluate the uncertainties of the debris.

Site
Workers
Working Boundary
The Public
OutsideFigure 4 - Exposure Model Caused by Criticality©International Research Institute for Nuclear Decommissioning2.1 Establishing Criticality Evaluation Technologies (3) Drawing up Criticality Control
Technologies

14[Purpose]• To develop criticality control methods to meet the target in criticality control in preparation for the decision on the fuel debris
retrieval method.
• To confirm their consistency with debris retrieval methods and create procedures for the control methods.
[Achievements]• Drew up control methods used during debris retrieval.
Presented and aligned requirements for the retrieval facility & method from a standpoint of criticality control.
• Facility requirements: Installation of a detector for the critical approach monitoring system and the neutron absorber injection
facility (soluble and insoluble)• Consistency with daily retrieval schedule: Critical approach monitoring method (reduced time)• Control method for each processing method (See Table 1)- Control is unnecessary when the impact of criticality is negligible.
- Critical approach monitoring is used when the change is gradual and slow.
- Otherwise, measures such as the use of neutron absorbers are considered.
• Considered sub-scenarios (for other measures than retrieval position).
• Identified abnormal events and reviewed measures.
Table 1 - Control Method for Each Debris Processing Method[Issues for Practical
Application]• To clarify issues and pros and cons
concerning the optional methods inusing neutron absorbers.

Type

Processing Method

Pick-up

Pick-up

SuctionSuctionSurface
Machining

Laser gouging

BoringCore boring

Severing

Disk cutter, AWJ, andhydraulic cutter etc.

Crushing

ChiselCriticality Control

IssuesSpecial control not Measures against unexpected events, such as fallen
requiredequipment, are considered separately (Applicable toall the methods below).

Critical approachmonitoring

A measure against the outflow and accumulation ofmachined powder (Applicable to all the methods
below).
A measure against fallen pellets when severing stumpfuel.

IssuesAs the change occurs instantaneously, measures suchas the used of neutron absorbers are considered.

©International Research Institute for Nuclear Decommissioning

15

2.2 Development of Criticality Control Technologies (1) Critical Approach
Monitoring Technologies
PCV[Purpose]
Current Conditions
Nuclide analysis equipment of the PCV gas control system
 Monitoring maintained subcritical state (overall monitoring)

During Fuel Debris Retrieval
The fuel debris position and the state of moderation in thesurrounding area change
Local Monitoring near Sub-criticality Level may change (critical approach)
Retrieval Location

RPV
Overall
Monitoring
Gas Control
SystemCritical Approach
Monitoring System
Critical approach monitoring is used additionally at
the debris retrieval location

Up to 1000 Gy/h (*)

Developing Critical Approach Monitoring System• Applicable to up to 1000 Gy/h (*)• High sensitivity design that allows for realistic
measurement time
• Weight saving design capable of moving and holding

NeutronRetrieval
Equipment

Technologies

Reactor Noise
Methods

ASC Method (provided by
AMETEK)Y  Correlation Equation Method

Neutron Source Multiplication Method
Virtual Neutron Capture Method

A few dozencm

Fuel Debris (Cooling Period > 10 Years)

Monitoring Technologies Reviewed

Feynman α Method

Detector

Application Method
Main method

Advancement of the Feynman α method

Continuous monitoring during retrieval
Used as a reference for deep subcritical states, etc.

The Feynman α method was selected as
our main method because it provides a
sub-criticality measurement capability
through simple instrumentation
configurations and signal processing.
◼ Combinations of respective methods wereconsidered based on their characteristics.
(These methods differ in their signal©International
Research but
Instituteforbe
NuclearDecommissioning
processing
cancombined,as theirmeasurement system is the same.)◼

* Based on the results of investigation inside the PCV this may be able to be revised down to a lower

16

2.2 Development of Criticality Control Technologies (1) Critical Approach Monitoring
Technologies(iii) Sub-criticality Measurement Performance Test (KUCA)
g

Change keff according to the number ofcore fuel assemblies. It can be changed
with the polyethylene-to-uranium ratio.

h• The error of estimate was affected by the types and layouts of neutron
sources.
• The distance between fuel debris and neutron detector at whichmeasurements can be made with the above mentioned accuracy:

j
k
l
m
n

X (Distance between radiation
source and reactor core)

Cf Neutron Source (Test in thecase of no source)

o
p
q
r
s
t
u
v
w

Y (Distance between detector
and reactor core surface)

B10 Detector (used in a group of
1, 2, and 3)

xTime List Output

y
z
aa
ab
ac

Additional Neutron Detector Location
Fuel Cell
Control Rod Cell
Polyethylene Block Cell
Void Cell
Neutron Detector (B10)
Neutron Source
Location for Installing B-10 Detector for Reactor Noise (“w-15”
and “s-20”)

Figure 1 - KUCA Experiment Image

Effective Multiplication
Factor (Estimate)[Purpose]• To verify the operation of a prototype system.
• To confirm the feasibility of critical approach monitoring technologies.
[Test Method]• Establish a subcritical system at KUCA, analyze the neutron signals (forabout 30 min) using the Feynman α method, estimate the sub-criticality level,
and compare the result with the reference value (nucleus calculation result).
[Achievements]• The sub-criticality level with an error of estimate at about 1% near criticality(neutron multiplication factor keff ≈ 0.95) and about 10% at deep subcriticality (keff  0.7) was able to be measured.
 This provided an outlook that the method could be realized within the
period of time expected on the field.
• Confirmed that the impact on the error of estimate due to the uncertainty of thewater-to-fuel ratio (neutron spectrum) was small.

i

The Larger
the Distance,
the Larger
the Error

Wet condition : about 20 cm, dry condition: about 35 to 60 cm,
partially dry condition: about 25 cm
 Used to review specific application methods during debris retrieval.
[Issues for Practical Application]• To review the measurement time required on-site at 1F, requirements for
Effective Multiplication Factor (Calculated
retrieval equipment, and specific application methods, based on the data
Reference Value)
obtain from this test.
Figure 2 - Comparison of Sub-criticality Measurement Results andCalculated Reference Values©International Research Institute for Nuclear Decommissioning2.2 Development of Criticality Control Technologies (1) Critical Approach
Monitoring Technologies(iv) From maneuverability Test under High Radiation Fuel Assembly
Overhead
天井クレーン(Cooled for 14
Crane
to Detector Specification Review
Years)
Shielding
遮へい体

17
Measuring
計測装置[Purpose]• To confirm the maneuverability of the neutron detector under a highdose rate gamma ray environment derived from fuel.

[Test Method]• Spent fuel storage facility (Nippon Nuclear Fuel Development Co., Ltd.
(NFD))• Change the distance between B-10 proportional counter and spent
fuel assembly and measure the neutron count rate (test conducted in
FY 2016).

燃料集合体

吊り具

Instrument

Cable - 30 m
Long
Fuel燃料検査プール
Inspection
Pool
Neutron Detector
センサユニット及びGamma Ray
電離箱
Detector[Gy/h]Dose
率 [Sv/h]
線 量Rate[Achievements]• The test provided an outlook that the target neutron detection
Shielding (2 cm of Lead)
Figure 1 - Testing System Overview
sensitivity could be achieved even under a gamma ray environment at
certain dose rates derived from fuel by building an appropriate shield.
About 70 Gy/hVerification of102
1.E+02High-sensitivity B-10: 2 cps/nv, Small-sized B-10: 0.2 cps/nv
Outside theShield• The requirements for a shield (lead) under the 1000 Gy/h (*)
Shielding
Performanceenvironment are as follows:
0.14
High-sensitivity B-10: 2 cm, Small-sized B-10: 1 cm101
1.E+01• Reviewed the detector specifications based on the test results.
0.09
Size & Weight: 350 mm * 310 mm * 130 mm, 150 kg[Issues for Practical Application]
0
0.07
10
1.E+00• Test results revealed that the size and weight of a neutron detector
Inside theShieldinginstalled the shield, a high-sensitivity detector, were larger than
expected and should be installed closer to debris. Therefore, the-110
1.E-01
criticality control project cooperated with the project of fuel debris
00
10
20
30
40
50
10
20
30
40
50
retrieval method. * Based on the results of investigation inside the PCV,Distance from the Surface of Fuel Assembly
estimation can be revised down to a lower value.

燃料集合体からの距離 [cm]Figure 2 - Dose Rate at Test Position©International Research Institute for Nuclear Decommissioning

18

2.2 Development of Criticality Control Technologies (1) Critical Approach
Monitoring Technologies(v) Application Method during Debris Retrieval[Purpose]• To develop procedures for critical approach monitoring consistent withdebris retrieval tasks.

[Issues]• Sub-criticality level measurements based on the reactor noise method(Feynman α method) require 30 min or more of time.
The frequency of sub-criticality measurements affects the daily retrieval
workload.
 Review a technique that enables monitoring in a short period of time bycombining the reactor noise and neutron source multiplication
methods (Figures 1 and 2)[Achievements]• The review provided an outlook that, by reducing the measurement time
for a single debris processing or retrieval task to a few minutes, theimpact on the retrieval scheme can be removed.

At
Inspection
BeforeRetrieval

Measurement Using
the Reactor Noise
Method (Up to 1 hr)
Neutron Flux Before Processing,keff (φref, kref)

Measurement
Before Retrieval(1–2 min)
Neutron Flux Before Retrieval(φ)

Evaluation of keff Before
Retrieval Using the Neutron
Source Multiplication Method
𝐶𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒 𝒌 𝑏𝑎𝑠𝑒𝑑 𝑜𝑛 𝑡ℎ𝑒 𝑓𝑜𝑙𝑙𝑜𝑤𝑖𝑛𝑔 𝑟𝑒𝑙𝑎𝑡𝑖𝑜𝑛𝑠ℎ𝑖𝑝:

𝝓𝑟𝑒𝑓
𝟏−𝒌
=
𝝓
𝟏 − 𝒌𝒓𝒆𝒇Criticality Criteria
Maximum Additional
Reactivity Allowed for aSingle Retrieval Task

NeutronMultiplication
Factork-meas (Measured
Value)

k-alm
Alarm Triggering Level

Determination of Subcriticality Based on
Retrieval keff

OK

Task Suspended

Retrieve

Processing & Retrieving
DebrisMeasure

Not
OK
Task
Suspended

Proceed to the next retrieval task, if themeasured value k-mess < alarm level k-alm.
Time Elapsed

Figure 1 - Image of Criticality Prevention Through Critical Approach Monitoring

Figure 2 - (Proposed) Critical Approach
Monitoring Flow
©International Research Institute for Nuclear Decommissioning2.2 Development of Criticality Control Technologies (1) Critical Approach
Monitoring Technologies(vi) Outlook of Feasibility and Issues

19[Achievements]• Confirmed the neutron measurement performance of the B-10 detector under a gamma ray environment at
certain dose rates derived from fuel.
• Shielding of the neutron detector is not required under a gamma ray environment at a dose rate up to 100
Gy/h.
Under a gamma ray environment at a dose rate up to 1000 Gy/h (*), lead shielding is required with athickness of 2 cm (for high-sensitivity B-10) to 1 cm (for small-sized B-10).
• In a functional test of the prototype system using a high-sensitivity B-10 detector, an offline sub-criticality
measurement was able to be completed in a realistic period of time (i.e., within one hour).
• For factors that affect the sub-criticality measurement error (sub-criticality level, neutron spectrum, types andlayout of the neutron source, layout of the neutron detector, and underwater/atmospheric condition), sensitivity
data serves as a basis for design, and measurable conditions were identifies for a range of sub-criticality levels(0.7 < k < 0.95).
• It is necessary to position the detector within 20 cm from the debris in the water.
 Gained a general outlook for the critical approach monitoring technique currently being developed based on
the Feynman α method.

[Issues for Practical Application]• To review the superimposition of factors affecting the sub-criticality measurement error and consider additional
test conditions to confirm their effects.
• To put together system specifications and detector layout proposals based on the obtained data, and establish
the concepts of equipment by consulting with the debris retrieval system project.
• To consider technologies to improve a method (AMETEK method) to reduce the measurement error.
• Preferably to conduct a verification test in a complicated and large-scale system simulating the actual debris, as
the system that has been tested this time was relatively small and simple.
• To review small-sized detectors being considered by other projects as possible candidates.
©International Research Institute for Nuclear Decommissioning* Based on the results of investigation inside the PCV this may
be able to be revised down to a lower value.

2.2 Development of Criticality Control Technologies (1) Critical Approach Monitoring Technologies(v) Feasibility Evaluation of Critical Approach Detection System Using Virtual Neutron Capture Method

System Design

Feasibility Evaluation[Method]

Review a shielding structure for the 1F environment(1000 Sv/h) based on the operation confirm test under ahigh-dose rate condition (conducted later year).
[Achievements] The detection unit including the shieling is expected
to be 5 kg or less per unit.
HolderGamma Ray
Shielding(Thickness: 1 cm) Detector

3–5 cmSmall-Sized B-10
Proportional
Counter

Estimated
Minimum

Shielding Weight [kg]30 to 70 cm

Estimated
Maximum

Number of Detectors [Unit]

Figure 1 - Shielding Structure and Weight for the Actual Installation
Measure Neutrons Emitted from the Debris
Sub-criticality: Low

Subcriticality: Deep

Neutron Source Is Applicableon SitePassive Method(Reactor Noise)

Active Method(Neutron Source
Multiplication)

Not Applicable

Virtual Neutron
Capture Method

Estimate the Debris Multiplication Factor Before Starting a Task

Figure 2 - Flow Chart of Critical Approach Monitoring Technologies[Purpose] • To verify the operation of a prototype system.
• To evaluate the feasibility of a system that adopts the critical approachmonitoring technique.
[Method] • Make measurements using a prototype system simulating a reactor
core with various levels of sub-criticality.
• Evaluate the sub-criticality level using the virtual neutron capture andneutron source multiplication methods.
• Evaluate the correlation between measurement time and statistical error.
[Achievements]• For the virtual neutron capture method, the maximum
difference, 4.3 dk%, was observed under a deep sub-criticality
condition.
• The maximum measurement time is 15 seconds to make the statistical error
equivalent to 1 %dk or lower.
 This provided an outlook that measurements can be made within a practical
period of time.
[Issues for Practical Application]• To confirm the degree of dependency of the neutron source multiplication
method on the location of neutron source.

統計誤差(%dk)
Error (%dk)
Statistical[Purpose] To design equipment applicable to the neutron source
multiplication and virtual neutron capture methods.

Opening

20

4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1.0E-01

keff=0.95
keff=0.8
keff=0.7

1.0E+01

1.0E+03

Measurement
Time (s)計測時間(s)Figure 3 - Correlation between Measurement Time and Statistical Error©International Research Institute for Nuclear Decommissioning2.2 Development of Criticality Control Technologies (2) Developing Re-criticality
Detection Technologies(i) Overview & Purpose

21[Criticality Control Using a Gas Sampling System (PCV Exhaust Nuclides Analysis Monitoring) ][Purpose] To detect re-criticality at an early stage in the process from water filling the PCV to fuel debris
retrieval.
To apply it to the second layer (abnormality detection and impact mitigation).
[Current Condition] Continuous monitoring by measuring Xe-135 (250 keV) (criteria: 1 Bq/cm3).
[Proposal] The monitoring system is improved for measuring Kr-87 and Kr-88 and monitor criticality
using the following in combination with the Xe-135 measurement:(1) sub-criticality level evaluation for the entire system and (2) early critical approach.
monitoring.
• Estimate the neutron source multiplication factor byusing the yield difference between spontaneous andinduced fissions. Currently at 0.5–0.7.

• Detect critical approach quickly by detecting Kr88
that responds faster than Xe135.

(Analysis results of Unit 1 data)

Fission Yield

Cm244 SpontaneousFission
U235 Induced Fission

6.3%[Current]

I
135

0.08%

6.6 hrs

1.4%[Proposed]
A = 88The yields aredifferent for
induced andspontaneous
fissions.

A = 135
The yields arecomparable forinduced andspontaneous
fissions.

Br
88

Xe
135

9.1 hrs

1.7%
16.5 sec

Short
Lived
Precursor

Kr
88
A Half is
Directly
Generated

2.8 hrs

Short
Lived

Blue: Fission Yield, Orange: Half-Life
Mass Number (A)Figure 1 - Yields of Spontaneous and Induced
Fissions©International Research Institute for Nuclear Decommissioning

Figure 2 - Behaviors of Monitored Nuclides

22

2.2 Development of Criticality Control Technologies (2) Re-criticality Detection
Technology(ii) Achievements and Issues
No.

IssuesActions Taken

Status1

Select the measurement energy peak for target nuclides

On-site test at Unit 1

Completed2

Determine system specifications that allow for early detection

Review detector and chamber configurations

Completed3

Estimate a realistic detection time and set criteria for the actual
installation

Gas behavior analysis, detectability when the target ispartially in the airCompleted

4

Develop a gas activity concentration calibration technology (initial
Utilize overseas technologies& maintenance)

5

Create optimal system design consistent with the retrieval method
Existing Gas Sampler

Configure appropriate flow rate, channel, and detector
for the method

In the future

Server Computer Board* A desk study and test planning werecompleted in FY 2017.

Gas ChamberR

Ge Solid-State Detector

Existing Nuclide
Analyzer Board
PCV GasOutput Signal Cable
with an Accessory
Connector of the
Detector

Unused Output Pin

Under
consideration*

Detector Signal

Outlet Box No.4
NuclideAnalysis Data

BNC
Connector

R/2

Power Supply

Scope of Additional
Temporary Facilities

Pulse-Height
AnalyzerH• 40 L cylindrical chamber
H: 32.5 cm
R: 21.0 cm• The surroundings of thechamber are shielded withlead.
• Two Ge detectors are
positioned inside the chamber.

To Relevant
Equipment
Measurement Terminal(Laptop)

Temporary Monitoring Device

Figure 1 - On-Site Test System for Unit 1 Gas Sampling System

Figure 2 - (Proposed) Chamber Profile of the Enhanced
System©International Research Institute for Nuclear Decommissioning2.2 Development of Criticality Control Technologies (2) Re-criticality Detection
Technology(iii) Select the Measurement Energy Peak for Target Nuclides

23[Achievements]• For Kr-88, it was determined that measurements on the higher
energy side would be more appropriate.
• There were differences in the calculated concentration for
Table 1 - Calculated Concentrationdifferent energy levels at which measurements were made.
of Target Nuclides• There were differences in the activity concentration of
Calculated Concentration
Energy (keV)
systems A and B.
(Bq/cm3)
 Need to consider a calibration technology
249.8(1.16  0.01)10-3
402.6(7.72  0.70) 10-5[Issues for Practical Application]-5• To establish a calibration technology (leveraging overseas
2554.8(8.39  1.54) 10-4
technology).
196.3(2.06  0.11) 10• To determine the system specifications.
2392.1(1.69  0.06) 10-4[Method]• Conduct a test at the actual installation of
Unit 1

Xe-135
Kr-87
Kr-88

Confirmed that there were nointerfering nuclides near theenergy levels at whichmeasurements are made.
Number of Counts

NuclidesMeasurement Range of the Existing Radiation Monitor

Energy [keV]

Figure - Unit 1 Gamma Ray Spectrum Measurement©International Research Institute for Nuclear Decommissioning
Results (Total)

2.2 (2) Developing Re-Criticality Detection
Technologies[Estimate a realistic detection time and set criteria for theactual installation][Method](1) Estimate the detecting time by analyzing behaviors during criticality by capturing
diluted gas behavior.
• Evaluate the concentration rise of Kr-88 at the radiation monitoring position.
• Compare to actual measured radiation concentration.
• Incorporate it as a transfer function model into the analysis of behaviors during
criticality.
(2) Review the system’s capability to know the sub-criticality level under quasi-steady
operating conditions during retrieval in atmospheric conditions.

24

About 30 m3/h

NitrogenSupplyFacility

Regulator
Valve

Out-leak:
About 10 m3/h

About 20m3/h[Achievements](1) It is expected that it is possible to detect in a shorter period of time (about 0.5 hrs) than originallyestimated (3 hrs).
(2) It is expected that it is possible to estimate the sub-criticality of the entire system under quasisteady operating conditions.
 Capabilities to detect critical approach at an early stage and know the sub-criticality level of theentire system during a transition period.

Exhaust Fan

Figure 1 - Current System Configuration
Current Kr88: Detection Limit (1 hr)
Improved Kr88: Detection Limit (1 hr)

Transfer
Function*Attenuation

PipingEntry
Port
Multiplied byAttenuation

Radiation
MonitorFigure 2 - Capturing Noble Gas
Behaviors for Analysis of Behaviors
During Criticality

Current Xe135 Concentration Ratio

Improved Kr88 Concentration Ratio

Improved Xe135 Concentration Ratio

Concentration Criterion for
Criticality

Time [s]Figure 3 - Change of Activity Concentration During Criticality(Assuming a case where debris adhered to CRD is dropped)

Kr88 Current Flow Rate
Activity Concentration [Bq/cc]

Gas Phase Y(t) Gas Generation

Activity Concentration (Initial Ratio)

μ: Gas Generation Rate

Current Kr88 Concentration Ratio

ExhaustPort

Negative Pressure
Control: Regulator
Valve[Issues for Practical Application]• To review concrete details of the monitoring technology, including otheraspects besides the retrieval position.
Debris N(t) FP Generation

About 20m3/h

Neutron Source Multiplication Factor

Figure 4 - Applicability to Sub-Criticality
Estimate for Retrieval in a Partially Dry
Condition©International Research Institute for Nuclear Decommissioning2.2 Development of Criticality Control Technologies (2) Re-criticality Detection
Technology(iv) Calibration Technology of PCV Gas Radiation Monitor

25[Technical Issues for Calibration](1) How to generate and purify radioactive noble gas(2) How to determine the composition of radioactive noble gas(3) How to handle radioactive noble gas(4) Design a gas circuit for calibration procedures(5) Clearly identifying traceability and uncertainties(6) Conformance to the regulations[Research Partner for This Year: National Physical Laboratory (NPL) of UK]• A core research institution highly regarded internationally in metrology andmaterial science.
• Responsible for the development and maintenance of measurement
standards in the UK.
• Provides scientific support services to measurements in industry and healthcare.
[Achievements of This Year (Desk Study)]
Methods to generate radioactive noble gas by irradiating enriched uranium owned
by NPL inside a thermal neutron pile.
(1) Purification methods for Kr-87 and 88, including the removal of fine particles
and volatile substances.
(2) Methods to quantitatively determine the activity concentration of Kr-87 and 88
using γ ray spectrometry based on gas proportional counter.
(3) Review the gas circuit* and calibration procedures.
(4) Evaluation of expected levels of precision.
(5) Confirmationissues related to the regulations.

Figure 1 - Research Wing of NPL in U.K.

Figure 2 - Accelerator
Experiment Facility[Issues for Practical Application]
To quantify uncertainties by performing trial calibrations.
(* Gas circuit: A circuit that measures the activity of radioactive noble gas by
Figure 3 - Thermal Neutron
circulating it in the scavenging gas.)©International Research Institute for Nuclear DecommissioningPile

2.2 Development of Criticality Control Technologies (3) Criticality Prevention
Technologies (i) Insoluble Neutron Absorber(i) Overview

26[Concepts of Candidate Materials]• Developed as an alternative to the soluble neutron absorber. Used by directly spraying these materials
during debris retrieval.
• Candidate materials containing highly concentrated B or Gb in three forms to support various types offuel debris.
Expected Properties

Form

Ability to Fit
Invasiveness
Adhesivenesswith Changes
into Gaps
to Tilted
in Debrisand Cracks
DebrisShape Causedof Debris
Surfaceby Retrieval

Candidate Materials

5 mm

B4C sintered metal material
Solid





-

Liquid 
Solid(Solidificationmaterial)-

ViscousMaterialGlass material containing B or Gd
Gd2O3 particle
Cement/Gd2O3 granulated powdermaterial
Liquid glass/Gd2O3 granulated powdermaterial
Water curable resin/Gd2O3 powdermaterial
B4C gel material
Slurry/Gd2O3 particle

B4C
sinteredmetalmaterial

Glass
Gd2O3 particle
material
containing
B or Gd

Liquid glass/Gd2O3 granulated powdermaterial(After hardened)

Candidate materials were selected as one of the accomplishments in
FY 2017, based on the results of confirmation of neutronics, a longterm irradiation test, and a workability test (shaded in yellow).
©International Research Institute for Nuclear Decommissioning2.2 Development of Criticality Control Technologies (3) Criticality Prevention
Flow of Selecting Candidate
Technologies (i) Insoluble Neutron Absorber
Material(iii) Confirmation of Nuclear Properties
Radiation
Basic Physical[Sample Reactivity Worth Measurement in KUCA]•••

Property
Confirmation

Resistance
Performance
Test

27
Nuclear
Properties
Confirmation
Test

This is the final phase of selecting candidate materials.
Five types of materials, selected based on their performance in the radiation resistance performance test, etc., areconsidered.
The aim is to confirm their neutronics (neutron absorbing ability) , contribute in verification of analysis technologies, andacquire data required to obtain permissions and authorizations.
Sample reactivity worth measurement for neutron absorber.

[Achievements]•
Except for Gd2O3 particles, the measurement andanalysis values matched within a measurement
error of 3σ.
 Confirmed that the neutronics and analysis
precision were generally favorable.
For Gd2O3 particles, the reason for this result isestimated that, as their particle size is large (afew hundred μm), the layout inside the vessel cannot
be exactly simulated.

Table 1 - Comparison between Reactivity Worth
Measurements and Analysis Values
Neutron Absorber

Measured Value 1σ(%Δk/k)

C/E

B4C sintered metalmaterial

0.598  0.052

1.19

Glass material
containing B or Gd

0.656  0.048

1.20

Gd2O3 particle

0.536  0.036

1.28

Liquid glass

0.463  0.028

1.15• The tendency was comparable even when the
Water curable resin
neutron spectrum was changed (to over-moderation at
0.479  0.031
1.05
H/U235=322).
Optimal Neutron Spectrum Moderation (H/U235=107)[Issues for Practical Application]• To study performing measurements by changing the loading amount, location, etc. of the neutron absorber, andenhance the reliability by way of reproduction tests to reduce uncertainties.
©International Research Institute for Nuclear Decommissioning2.2 Development of Criticality Control Technologies (3) Criticality Prevention
Technologies (i) Insoluble Neutron Absorber(iv) Long-Term Irradiation Test

28

[Long-Term Irradiation Test]• Acquire data to evaluate secondary effects (hydrogen generation and corrosion) of post-retrieval process.
• Select candidate materials to be used in a long-term irradiation test and conduct the long-term irradiation test at
Takasaki Advanced Radiation Research Institute, QST.
QST: Quantum and Radiological
Science and Technology

Target in FY 2017:36 MGy B4C sintered metal material
Liquid glass/Gd2O3 granulated
powder material Glass material containing B or Gd
Solid B4C reagent (for comparison)* Gd2O3 particle

Water curable resin/Gd2O3 powdermaterial(Excluded from review)

Solidified
Material

 Liquid glass/Gd2O3 granulated
powder material
 Water curable resin/Gd2O3 powdermaterial(*) A pH range with less corrosion effect: 5–9

Irradiation dose (kGy)

Figure 1- Review Results and Plans for Corrosion Risk Arising from Leached Components
Caused by Long-Term Irradiation (Example)(Test solution pH after a leaching test of gamma radiation materials at 80C)[Achievements] • Selected candidate materials assuming they would remain in the reactor and be stored in storage
canisters.
©International Research Institute for Nuclear Decommissioning[Issues for Practical Application] • To improve reliability by adding data.

2.2 Development of Criticality Control Technologies (3) Criticality Prevention
Technologies (i) Insoluble Neutron Absorber(v) Work MethodDuring Debris Retrieval[Implementation Items]• After dropping a neutron absorber onto a specimen (lava) simulating thecorrugated surface of the debris, the expanse, coating thickness, and weight of
the adhered absorber was measured and the adhesiveness evaluated.
(Liquid glass based and water curable resin based neutron absorbers)• Conducted tests on flat plate, granular lava (about a few cm in size), and platelava specimens.
* Water curable resin was tested only for flat plate and plate lava specimens.
[Fundamental Adhesiveness Test][Achievements]• Evaluated the required input amount for the plate and granular lava specimens
simulating the debris.

29
Neutronabsorber flowing
down while
expanding in thewater

Plate Lava

Evaluate the area
where the thickness of
the adhered neutron
absorber meets theprescribed coating
thickness (tentatively
set to 1 mm)[Where to Apply These Achievements]• Review of the installation methods of insoluble
neutron absorbers.
• Reflect them on the final selection of candidate
materials, facility requirements, andprocedures for debris retrieval.

[Issues for Practical Application]• Combined test for transportability andworkability.
• Study on the applicable methods based on
the test data.
• Study on the impacts on the retrieval process.
(Processability, visibility,
impact on the water treatment
system, and equipment
configuration)

Measured Sectional Profile of Neutron Absorber Adhered to Plate Lava
(Materials: Liquid glass TX-10, Viscosity: 2000 [mPa･s], Distance between nozzle and substrate: 56[mm], Underwater environment, and Temperature: 20 [C])
Neutron Absorber Flowing Down
Measured Weight of Neutron Absorber Adhered to Granular Lava
in the Water (Input Amount:
Viscosity
Input
Number
WeightWeight Increase1000 [g])[mPa•s]Amount [g]of Tests

Increase
[g]

1

109.2

2

105.4

1

19.5

2

23.4

1000

Granular
Lava

107.3

2000
100

2 cm

(Average) [g]21.5©International Research Institute for Nuclear Decommissioning2.2 Development of Criticality Control Technologies (3) Criticality Prevention Technologies (i)Insoluble Neutron Absorber(v) Work Methods During Debris Retrieval - Evaluation of Compatibility with Anti-Corrosive
Agents[Implementation Items]• Conducted tests on flat plate, granular lava, and plate lava specimens and evaluatedthe impact of an anti-corrosive agent environment on the solidification characteristics
and adhesiveness of the insoluble neutron absorber (liquid glass based neutron
absorber).
• Evaluated the changes in anti-corrosive agent solution before andSampling for Water
after the test.
Quality Analysis[Test to Study the Impact of Compatibility with
Anti-Corrosive Agents][Achievements]• Evaluated the impact of an anti-corrosive agent environment on the solidification
characteristics and adhesiveness of the insoluble neutron absorber.
 Confirmed the solidification characteristics and adhesiveness to the lava
simulating the debris.
 Reactants were generated for anti-corrosive agent 1 (dependent on the amount ofboron).
• Analyzed and evaluated whether there were changes in an anti-corrosive additive
component contained in the solution.
 No significant changes were observed.

30

Image of Neutron Absorber Adhered to Plate Lava (Temperature: 20[C])(Left: Sodium pentaborate (anti-corrosive agent 1),
Right: Zinc/sodium molybdate mixed phosphate (anti-corrosive agent 3))
Before the Test
After the Test

Purifiedwater

Anti-corrosive Anti-corrosive Anti-corrosive Anti-corrosive
agent 4
agent 1
agent 2
agent 3

Water Analysis Results for Samples Before and After the Test
(Impact Assessment of Anti‐Corrosive Agent)[Where to Apply These Achievements]• Review of the work methods of insoluble neutron
absorbers.
• Reflect them on the final selection of candidate materials,
facility requirements, and procedures for debris retrieval.

Measured Weight of Neutron Absorber Adhered to Granular
Lava
Viscosity[mPa•s][Issues for Practical Application]• Combined test for transportability and workability.
• Study on the applicable methods based on the test data.
• Study on the impacts on the retrieval process.
(Processability, visibility, impact on the water treatment system,and equipment configuration)

Input
Amount
[g]

1000

2000

Measured Weight of Neutron
Absorber Adhered to Granular Lava

1000

1000

Test
Number Weight Weight Increase
Atmosphe of Tests Increase(Average) [g]re[g]

Anticorrosive
agent 1

1

141.2

2

179.1

Anticorrosive
agent 3

1

100.5

2

97.0

1

109.2

2

105.4

Underwate
r

160.2

98.8

107.3

Anti-corrosive agent 1: Sodium pentaborate©International Research Institute for Nuclear Decommissioning
Anti-corrosive agent 2: Sodium tungstate + Sodium pentaborate
Anti-corrosive agent 3: Phosphate-based anti-corrosive agent (Zinc/sodium molybdate mixed phosphate: ZSMMP)
Anti-corrosive agent 4: Phosphate-based anti-corrosive agent (Zinc/sodium carbonate mixed phosphate: ZSCMP)

31

2.2 Development of Criticality Control Technologies (3) Criticality Prevention
Technologies (i) Insoluble Neutron Absorber(v) Work Method During Debris Retrieval[Workability Evaluation]• Sorted out workability evaluation items for the candidate materials, and confirmed the adhesiveness and transportability of thesolidified materials in a laboratory-scale test.
• Confirmed that, for solid materials, no significant problems were found in a basic study of these materials’ ability to be manufactured
into a specified shape using a mass-producible method to demonstrate their gap permeability.
Mixability with
DebrisForm

Candidate Materials
Gap Permeation

Solid

Liquid 
Solid(Solidified
Material)

ViscousMaterial

Adhesiveness to Debris
Coating of Tilted
Coating of Horizontal
Surface - Flat Plate Surface - Flat Plate

Transportability

Plate Lava

Granular Lava

B4C sintered metal materialGlass material containing B or Gd

Gd2O3 particle
Cement/Gd2O3 granulated powdermaterial








*3

*3

*3

Liquid Glass/Gd2O3 Granulated
Powder Material



∆







Can be transported
50 m with a squeeze
pump by controllingthe viscosity.

Water curable resin/Gd2O3 powdermaterial

B4C gel material
Slurry/Gd2O3 particle

*1


*1


*1
*2

*1: B4C gel material was not tested since it did not meet the requirements for leaching
characteristics.
*2: Slurry/Gd2O3 particles were excluded from the review because they solidified as a result ofirradiation.
*3: Cement/Gd2O3 granulated powder was excluded from the review because its alkaline
components were leached as a result of irradiation.

The material was contracted and floated up during underwater work
(issue of underwater workability).
*1*2
Permeation depth of 2 mm widegap 100 mm or more 50-100 mm∆ Less than 50 mm

*1
*2

*1
*2
Coating of Tilted Surface: Good adhesiveness Relatively good adhesiveness∆ Need to improve adhesiveness[Achievements]• Confirmed the workability of expected debris shapes. It is expected that the insoluble neutron absorbers can be thrown in in an amountrequired for criticality prevention. Four types of candidate materials in two forms were selected (shaded in yellow).

[Issues for Practical Application]• To confirm the insoluble neutron absorbers’ ability to fit with the shape of the debris after it has been processes (crushed, for example).
• To confirm the transportability in a full scale test
©International Research Institute for Nuclear Decommissioning• To consider alternatives for cases where the location cannot be identified for a target such as accumulated debris powder caused by cutting.

32

2.2 Development of Criticality Control Technologies (3) Criticality Prevention
Technologies (ii) Soluble Neutron Absorber(i) Confirmation of Nuclear Properties[Purpose]• To confirm the neutronics (neutron absorption capability) of highly concentrated
sodium pentaborate solution and verify analysis technologies for calculating criticality.

g
h
i
j
k
l
m
n
o
p
q
r
s

[Test Details]• Sample reactivity worth measurement in KUCA
Tested for 6,000 and 12,000 ppm

t
u
v
w
x
y
z
aa
ab
ac

Compared with the nuclear calculation code analysis results[Test Results]• Confirmed the neutron absorber’s reactivity effect and that the predictability was
generally good for nuclear calculation using the criticality calculation code.

Sample Value

H/U235 of
the System

Boron
Concentration
ppm

322

6,000

SampleLoading
Positionr16 of Mount A

Measured
Value (C)%Δk/kDifference in
Measured andCalculated
Values
%Δk/k

0.017

0.005

Neutron Absorber Storage
Canister Cell
Fuel Cell
Control Rod Cell
Polyethylene Block Cell
Void Cell

追加していく
燃料

Sample Value

107

6,000

q14 of Mount B

0.224-0.001Polyethylene
ポリエチレン

グラファイトSample Value107

12,000q14 of Mount B

0.356

Container
容器-0.001Polyethylene
ポリエチレン

Sample Value(Reproduced)

燃料

107

6,000

q14 of Mount B

0.226-0.001[Achievements]• Test results proved that highly concentrated sodium pentaborate solution can behandled with the same precision in a conventional manner in evaluating criticality.

Figure 1 - Test System(Mount A)©International Research Institute for Nuclear Decommissioning2.2 Development of Criticality Control Technologies (3) Criticality Prevention
Technologies (ii) Soluble Neutron Absorber(ii) Facility Feasibility Confirmation for Maintaining Concentration

33

PCV Circulation Loops[Purpose]

Water Treatment
Facility

To study the system that collects sodium pentaborate solution
leaked from the PCV to the torus room and mixed withgroundwater, adjusts its concentration, and then returns it to thecirculation loop.

Water Intake from S/CWater Intake from Torus Room[Achievements]• Confirmed the basic feasibility of a proposed facility
configuration for maintaining the concentration of sodium
pentaborate solution (service conditions: 7,000 ppm) by
requesting to the project for upgrading fuel debris retrieval
methods (system review).
• Reviewed operation methods (under normal/abnormal
conditions).

Cooling Water Leakage Volume from
PCV & S/C to Torus Room: 4.4 m3/h

Facility for
Maintaining
Boron
Concentration

Groundwater Inflow
Amount: 2.1 m3/h

Figure 1 - Overview of the Facility for Maintaining Boron
Concentration
Gas System
Delivered to otherfacilities in the premisesetc.

Receiving
Tank

Condensation
Canister• Confirmed (on a calculation basis) that boron deposition does
Water Intake fromnot decrease even when the sodium pentaborate solution comes (i)S/C and Torus Room
into contact with the concrete.

Condensate
ReceivingTank(ii) To Circulation
Loop Water
Treatment Facility

Cooler[Issues for Practical Application]

Figure 2 - Boron Condensation System with Condensed Canister• The project for the study on fuel debris retrieval methods (system
Table 1 - Approximate Dimensions of Each Equipment
review) will continue to review the feasibility of the system using
Equipment Name
Volume, Throughput
Quantity
Approx. Dimensions per Unita boric acid solution.
• To select and review measures against operation
methods for maintaining the level of concentration andunexpected events in collaboration with the systemreview project.

Receiving Tank

About 40 m3/unit

2 units

φ4 m  3 m

Condensation
Canister

About 10 m3/h (Evaporation)

2 units

8 mW  7 mL  7 mH

Cooler

About 0.25 MW

1 unit

φ1 m  3 m

Condensate
Receiving Tank

About 10 m3/unit

2 units

φ2 m  3 m©International Research Institute for Nuclear Decommissioning

34

3. Overall Summary
Achievements[Establishing Criticality Evaluation Technologies]• Elaborate criticality scenarios and risk evaluation by reflecting
latest know-hows.
• Establish evaluation methods for behaviors during criticality andexposure, and confirm that the exposure dose is lower than thenormal level for representative events.
• Set the objective of criticality control and establish the concepts
based on the defense in depth[Developing Criticality Control Technologies]• Monitor critical approach:
Establish a critical approach monitoring method based on thereactor noise method and confirm its basic feasibility.
• Detect re-criticality:
Establish an enhanced criticality detection method using Kr
monitoring.
• Neutron absorber:
Select insoluble absorber candidates and confirm theirapplicability to the debris system.
Establish a soluble absorber facility and its application method.

Issues to be Solved Before Practical
Application• Continue to update based on the results ofinvestigation inside the PCV.
• Evaluation based on the conditions in eachprocess up to the full-scale retrieval.
• Determine concrete details of control method(e.g., policy for the application of neutron
absorbers, etc.) in light of the progress inretrieval method reviews.
• Validation assuming the on-site conditions of
1F (i.e. sub-criticality level measurement incomplexity systems and on-site measurement
system feasibility).
Determine concrete details of methods tomount the neutron detector to the retrieval
facility.
• Determine concrete details of the on-site
application method for, for example, Ge
detector calibration method, and confirm
detectability in light of system conditions up to
full-scale retrieval.
• Determine concrete details of the on-sitefacilities & application methods, and theircompliance assurance with the debris
processing method.
• Determine concrete details of the boric acidsolution system operation method.
©International Research Institute for Nuclear Decommissioning

End of Document©International Research Institute for Nuclear Decommissioning