Reactor Physics Summer Seminar (AESJ-RPSS), organized by AESJ Reactor Physics Division
Shikinoyu Gora Seiunso, Kanagawa, Japan
26-28 August 2015

Criticality Safety Control of Fuel Debris
Kenya SUYAMA
Research Group for Reactor Physics andStandard Nuclear Code System
Japan Atomic Energy Agency

Contents¾Criticality Safety – general issue.
¾Fuel Debris – what are difficult points?
¾Technical Development for Criticality
Safety for Fuel Debris in JapanIRID
 JAEA

¾Burnup Credit – is it feasible for
Fukushima Fuel
Debris?
2015 AESJ-RPSS in Gora, Hakone
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Criticality Safety – general issue

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Criticality Safety¾ Generally speaking, to prevent

criticality, outside of nuclear reactors.

¾ Basic principle of criticality safety
䠄Criticality safety handbook, JAERI1340 )

To prevent criticality for all situations
that are technically convinced.



To have enough safety margin in eachstage of design, fabrication,
construction, and operation of facilities.



Need to consider kind(sort), amount,physical and chemical status of nuclear
materials

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How to keep enough safety margin¾ Kind of nuclear
materials
Controlled andConfirmed

Which isotopes?

¾ Amount of nuclear
materials


Nuclide density?

¾ Consideration of physical and chemical status
Homogeneous/Heterogeneous(solution/array ofrods䠅
 uniform/un-uniform(concentration distribution)2015 AESJ-RPSS in Gora, Hakone

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Re-Criticality in Fukushima? –reported in 2011(1)¾ From IAEA web site(https://www.iaea.org/newscenter/focus/fukushima/status-update):
On 1 November, TEPCO detected the possible presence of xenon-133 andxenon-135 gases sampled from inside the Primary Containment Vessel(PCV) of Fukushima Daiichi Unit 2. The presence of these short-lived
radionuclides indicates that some nuclear fission may have recently
occurred. TEPCO reported that no increases in radiation levels have been
observed. According to TEPCO "even if a fission reaction is assumed to beon-going, its scale is extremely small and the reactor is in a stable condition
as a whole."

TEPCO responded to this development by injecting 10 tonnes of boric acidsolution (water containing 480 kg of boric acid) into the reactor from 02:48
to 03:47 local time on 2 November. Boric acid solution is used as acountermeasure to nuclear fission for its ability to absorb neutrons.



Further radionuclide analysis of the gas samples collected from Unit 2 is ongoing and will be conducted in collaboration with the Japan Atomic Energy
Agency (JAEA).
2015 AESJ-RPSS in Gora, Hakone
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Re-Criticality in Fukushima? –reported in 2011(2)¾ From IAEA web site(https://www.iaea.org/newscenter/focus/fukushima/status-update):
Based on further analysis, Japanese authorities have concluded that thexenon concentrations are not due to a criticality event but rather from thespontaneous fission of curium-242 and 244. (Spontaneous fission is a formof radioactive decay that does not involve chain reactions and ischaracteristic of very heavy isotopes. Spontaneous fission occurs in low
levels in all nuclear reactors.)This conclusion is based on three key factors outlined and discussed in thereport:

The inventory of Cm-242 and Cm-244 was calculated as was theconcentration of Xe-135, resulting from the spontaneous fission of Cm-242
and Cm-244. If nuclear fission of the reactor's uranium fuel were occurring,at the lowest possible level, the levels of xenon detected would be several
orders of magnitude higher than those measured. Current levels of xenon
are consistent with those that would be generated by spontaneous fissionof Cm-242 and Cm-244;
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Re-Criticality in Fukushima? –reported in 2011(3)¾ From IAEA web site(https://www.iaea.org/newscenter/focus/fukushima/status-update):

If the core had been experiencing a criticality event, the injection of boron
water should have stopped the criticality and terminated the generation of
xenon. However, the xenon levels were not influenced by injection of boron
water into the core; andIf the core was undergoing a criticality event the temperature and pressure
readings would be expected to rise as the event would increase heatproduction within the core. However, the temperature and pressure levels
have not undergone any significant increases either before or after xenon
were detected, indicating that no criticality event is occurring.

¾ No re-criticality event in Fukushima but very important issue.
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Fuel Debris – What are
difficult points?

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Difficulty of Fuel Debris¾ Kind of nuclear
materials


Not confirmed
nor controlled
after SA

which isotopes?

¾ Amount of nuclear
materials


Nuclide density?

¾ Consideration of physical and chemical status
Homogeneous/Heterogeneous(solution/array ofrods䠅
 uniform/un-uniform(concentration distribution)2015 AESJ-RPSS in Gora, Hakone

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Reality¾ We can say that the maximum 235U enrichment is lessthan 5wt% (fabrication condition).

¾ When we use the condition “235U enrichment is lessthan 5wt%”;
The estimated lower-limit critical mass is almost 30kg foruniform U-H2O system(JAEA-Data/Code 2009-010).
 In case total amount of U is about 100t/reactor, we need
~3,333 units/reactor to contain fuel debris. (This does notinclude safety margin.) Fuel debris is more than 100t because includes constructing
materials.
¾ Is it possible to have additional conditions to control
the criticality of debris to relax the situation?
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We evaluate…(1)¾ Fuels are not fresh (average burnup ~ 20GWd/t).
“Averaged residue 235U enrichment” is less than 5wt%
because of burnup.
 Accumulation of fission products.
 By using burnup calculation codes, it is an easy task toevaluate the averaged residue 235U enrichment.
¾ Burnup credit is an attractive option.
¾ However burnup credit is not adopted even in thecase of the intact fuel transportation in Japan.

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Example of change of kinf during
burnupPWR 17u17 Assembly
Aixal Direction – Infinite
Radial Direction – Reflective
235
Initial U Enrichment 4.7ORIGEN2.1 + ORLIBJ32
MVP + JENDL–3.2

1.4

kinf

1.3

Cooling
time

1.2

1.1

1.0
0

FP

Cooling = 0 Year , FP = No
Cooling = 10 Years, FP = No
Cooling = 30 Years , FP = No
Cooling = 10 Years , FP = Yes

10

20

30

40

50

Considering decrease in reactivity

C:/MyFiles/NGDATA/Effect–of–BUC/PWR–Kinf_r2.ngp

1.5

Burnup [GWd/t]
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Evaluation of benefit of BUC (1)Water¾ Condition
PWR 17u17
 235U;4.7wt%
 ORIGEN2.1+ORLIBJ32
 MVP+JENDL-3.2
 Axially infinite䚸Radial
Reflective Water rods are replaced
by UO2rods.


To obtain L value of Kinf = 0.98

L2015 AESJ-RPSS in Gora, Hakone

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Evaluation of benefit of BUC (2)
It is possible to store approximately doubled number of assemblies in unit surface
under specific condition.
C:/MyFiles/NGDATA/Effect- of- BUC/effect- of- buc.ngp

7.0

Cooling 0 Y : with FP
Cooling 10 Y : without FP
Cooling 30 Y : without FP
Cooling 10 Y : with FP

distance

3.0
Ratio of number of assemblies to be stored(to fresh fule case)

L: (distance between assemblies)/2 (cm)

8.0

2.5

6.0
5.0

2.0
4.0
3.0
2.0

1.5
ratio of assemblies
keff = 0.98

1.0
0

10

20

30

40

Burnup (GWd/t)
2015 AESJ-RPSS in Gora, Hakone

1.0
50
15

We evaluate…(2)¾ Adding structure materials (Fe, Si, Zr etc) into
the fuel debris increases the minimum
criticality mass/volume/size of the fuel debris.


Fuel debris should contain them after SA.

¾ We can estimate the material property of thefuel debris by computer simulations orexperiments.

¾ But no confirmation in Fukushima case.
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Effect of Steel, Concrete, Zry-2㻝㻚㻠¾ Uniform UO2-H2O

㻝㻚㻞
㻝㻚㻜

¾ Uniform UO2-H2O+

㼗㼕㼚㼒

㻜㻚㻤
㻜㻚㻢

Constructing Material㻿㼠㼑㼑㼘㻯㼛㼚㼏㼞㼑㼠㼑
㼆㼞㼥㻙㻞
Without FP㻿㼠㼑㼑㼘㻔㻲㻼䛺䛧㻕Without FP㻯㼛㼚㼏㼞㼑㼠㼑㻔㻲㻼䛺䛧㻕WithoutFP㼆㼞㼥㻙㻞㻔㻲㼜䛺䛧㻕

㻜㻚㻠
㻜㻚㻞
㻜㻚㻜
㻜

㻞㻡
㻡㻜
㻣㻡
㻾㼍㼠㼕㼛㻌㼛㼒㻌㻾㼑㼜㼘㼍㼏㼑㼙㼑㼚㼠㻌㼇㻑㼉¾ Negative reactivity


Fe>Zry-2>Concrete㻝㻜㻜2013AESJAnnualMeeting2015 AESJ-RPSS in Gora, Hakone

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We evaluate…(3)¾ If the constituent elements of the control rods
(Boron) exist in the fuel debris, the minimum
criticality mass/volume/size should be increased.
¾ If residual gadolinium in the UO2-Gd2O3 rods exists inthe fuel debris, the minimum criticality
mass/volume/size should be increased.
¾ During the process to compose the fuel debris, suchmaterials may be well mixed in the fuel debris.
 Born and gadolinium may have the same
melting/mixing behavior with U.

¾ But no confirmation in Fukushima case.
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Reality(1)¾ The criticality safety control requires theconfirmation/control/monitor of relevant parameters.

¾ Maximum/minimum of such parameters should bewithin a certain “limit”, which could becontrolled/confirmed.

¾ If we cannot confirm/control/monitor suchparameters, these should be set at the upper/lower
limit from the technical view point.

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Reality(2)¾ After the SA in Fukushima, people in this field quickly realized
the necessity of the criticality safety control of the fuel debris
especially when taking out it.

¾ Difficulty - this is because we have no procedures toconfirm/control/monitor the relevant parameters.

¾ People would like to find possibility to relax tough parametersand conditions.

¾ But it is just the problem of the possibility. There is no definitive
evidence nor confirmation procedure which could be adopted.

¾ We need further technical development.
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Reality(3)¾ Of course, criticality safety control is one of the key
issues for decommissioning of Fukushima site
considering the impact of the re-criticality on thesociety.

¾ We need clear strategy and consensus on this issue.
Everything should be under controlled.

¾ Which parameters could be used to control thecriticality of the fuel debris?

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Technical Development for
Criticality Safety of Fuel
Debris in Japan – IRID

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Fukushima - decommissioning¾ Mid-long term road map towards thedecommissioning (updated in June 2015).
The Council for the Decommissioning of TEPCO's
Fukushima Daiichi Nuclear Power Plant
 http://www.tepco.co.jp/en/decommision/planaction/in
dex-e.html
¾Removal of fuel assembly from SFP of
1F-1,2,3 – starting from FY2017¾Removal of fuel debris – starting fromFY20212015 AESJ-RPSS in Gora, Hakone

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IRID - General information¾ From August 2013, the technical development of
decommissioning of Fukushima site has been
carried out under the coordination of IRID
(International Research Institute for Nuclear
Decommissioning).
Composed of 18 institutes.(utilities, reactor vendors,
JAEA, CRIEPI etc.)
 To manage the funds and the technical program of thetechnology development for Fukushima
decommissioning.
¾ http://www.meti.go.jp/earthquake/nuclear/pdf/140731/140731_01_041.pdf
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IRID – Organization Framework

http://irid.or.jp/en/organization/2015 AESJ-RPSS in Gora, Hakone

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IRID for criticality safety(1)¾Managing the funds from the Agency
for Natural Resource and Energy.
¾Three main vendors of NPP of Japan(Hitachi, Toshiba and Mitsubishi) havebeen carrying out the technicaldevelopment for the criticality safetycontrol of the Fukushima debris.
¾JAEA also participates in the same
program.
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IRID for criticality safety(2)¾ R&D for Criticality Control Technique
Evaluation of the Fuel Debris Criticality Analysis Method for Power and FP Generation
Behavior at the Criticality Accident
 Sub-criticality Monitoring Technology for the Fuel
Debris Removal
 In-core Criticality Detection Technology
 Criticality prevention technology¾ See slides presented in Japan-Korea Summer
School in August 2015
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Technical Development for
Criticality Safety of Fuel
Debris in Japan – JAEA

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Technical development in JAEA(1)¾ Evaluation of basic
parameters of fuel debris


Minimum criticality mass,
radius¾ OECD/NEA/WPNCS/EGBUC/
Phase-3B benchmark (BWR
STEP-2 Fuel Burnup
Calculation)¾ JENDL-3.2¾ BUC isotopes of EGBUC
benchmarks.

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Example of Minimum Criticality
Size
10

Burnup=12GWd/t, V.F.=0%, with FP

3

10

keff=1.0

Sphere Radius
Infinite Cylinder Diameter

Burnup=12GWd/t, V.F.=0%, with FP

Sphere Radius [cm]Infinite Slab Thickness

[cm]

5%

10

Water Content [%]

2

50%
10%
20% 30%3

40%

Criticality Safety HandBook Ver.2 UO 2-H2O
235
3wt%
4wt%
InitialU Enrichment:

5%

10

2

Water Content [%]

50%

10%

40%
20%

30cm Water Reflector
1
10 0
1
10
10
H/(U+Pu) [-]10

keff=1.030%

1 30cm Water Reflector
10 0
1
10
10
H/(U+Pu) [-]2

10SizeofgeometrytoComparisonwithFreshFuelCase
ReachCriticality
2012AESJAnnualMeeting2015 AESJ-RPSS in Gora, Hakone

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2

Technical development in JAEA(2)¾International burnup calculation
benchmark for BWR STEP-III fuelassembly under
OECD/NEA/NSC/WPNCS/EGBUC
framework.
¾35 sets of results from 16 institutes
from 9 countries.
¾To be published in 2015 from
OECD/NEA
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OECD/NEA/NSC/WPNCS/EGBUC
Phase-IIIC Benchmark
U234
U235
U236
U238
NP237
PU238
PU239
PU240
PU241
PU242
AM241
AM243
CM244
SR90
MO95
TC99
RU101
RH103
AG109
I129
XE131
CS133
CS134
CS137
CE144
ND143
ND145
ND148
SM147
SM149
SM150
SM151
SM152
EU153
EU154
EU155
GD155
GD156
GD157
GD158

Case1bCase2aCase2bCase2cCase3bCase5bCase5cCase6bCase8bCase8cCase9bCase11bCase11c
Case12b

0.01

0.1

1

10

100

twostandarddeviations(2Ͳsigma(r))[%]2015 AESJ-RPSS in Gora, Hakone

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Technical development in JAEA(3)¾ Post Irradiation Examination(PIE) to measureassay data of used nuclear fuels of BWR andPWR
BWR – 1 sample
 PWR – 9 samples¾ TIMS, ICP-MS, New Chemical Separation,alpha-spectrometry
U, Np, Pu, Am, Cm
 Tc, Rh, Ag, Cs, Nd, Sm, Eu, Gd2015 AESJ-RPSS in Gora, Hakone

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Post Irradiation Examination tomeasure isotopic composition
234 235 236 238
238 239 240 241 242
Nd 142 143 144 145 146 148 150
Np 237Am 241 243
Radiation and Mass
Spectrometry
Cm 242 243 244 245
䖩 Pm 147
Sr
90
Chemical(Zr) 93
Separation
Ag 109Mo
95
Tc
99
Relatively
Ru 101easy toRh 103measureCs 133
䖩 Sm 147 148 149 150 151 152 154
Eu 153 154 155
Gd 154 155 156 157 158 160
U

TIMS

Pu

(Thermal ionization mass spectrometry)

Enriched Spike
Isotope Dilution Method

HR-ICP-MS(High Resolution Inductively Coupled Plasma Mass Spectrometry)

Important Isotopes for Criticality
Safety - chemical separation

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Post Irradiation Examination to measureisotopic composition‶⁛⁥⁥⁡⁞⁨⁗⁖‒⁓⁠⁖‒‶⁛⁞⁧⁦⁗⁖‒⁅⁢⁗⁠⁦‒‸⁧⁗⁞‒⁅⁓ ⁢⁞⁗134,137Cs

UTEVA Resin(remove U and Pu)

AmCm(ǂ-Spec)

154,155Eu(Ǆ-Spec)

Mo,Tc,Ru,Rh, Cs
Anion Exchange1

Anion Exchange 2Ag

U,Pu,Nd(TIMS)

Anion Exchange 3

In 2015
Except for 147Pm

Gd,Eu,Sm,Pm,Nd

HR-ICP-MS
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Technical development in JAEA(4)¾Modification of STACY in
NUCEF(Homogeneous (solution)system to heterogeneous(fuel pin array)) Funded by Nuclear
Regulation Agency of
Japan



¾Re-criticality of STACY is
expected to be FY2018
2015 AESJ-RPSS in Gora, Hakone-018-

The STACY modification has been
funded by the Nuclear Regulation
Authority (NRA) / the Secretariat of
NRA of Japan.

36Burnup Credit – is it feasible
for Fukushima Fuel Debris?

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To keep enough safety margin again¾ Kind of nuclear

Conservative
Isotopic
Compositionmaterials
What isotopes?

¾ Amount of nuclear
materials


Nuclide density?

¾ Consideration of physical and chemical status
Homogeneous/Heterogeneous(solution/array ofrods䠅
 uniform/un-uniform(concentration distribution)2015 AESJ-RPSS in Gora, Hakone

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Treatment of BUC in Japanesecriticality safety handbook¾ From initial publish in 1988, Japanese Criticality Safety Handbook(compiled by Nuclear Materials Regulation Division of Science andTechnology Agency ) includes the idea of BUC
.. if the degree of burnup can be determined with high accuracy, then acriticality safety assessment may take the degree of burnup into account(1st version).

… however, when the evaluation of burnup is valid from the criticality safety
evaluation viewpoint, the criticality safety evaluation may be conductedtaking account of changes in nuclide composition resulting from burnup(2nd version).

¾ Isotopic composition of uranium and plutonium (1st and 2nd versions)

Compilation of measured isotopic composition䠊

¾ Effect of burnup distribution should be considered separately ( 2ndversion).

¾ FP nuclides that can be considered in criticality safety evaluation ( 2ndversion).

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Current Status of BUC in Japan¾ BUC has already been introduced into thecriticality safety evaluation at RRP of Japan;

SNF receiving pools (operated from 1999)
 Non-destructive measurement to determine;
the burnup value of SNF and remaining enrichment of uranium-235.
 selection of a SNF storage rack of high capacity (BU<2%) or not (2.5<BU<3.5%).
Dissolver Measured burnup and initial enrichment
Addition of gadolinium nitrate solution in to dissolver forneutron absorber.
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Status of BUC in Japan
Wet
InterimTransport
Storage at Storage$
Reactor

RRP
SNF receiving
pool

Dissolver

PWR

N.A.

-

N.A.

(U,Pu)(U,Pu)

BWR

G.C.

N.A.

N.A.

(U,Pu)(U,Pu)

N.A.: Not Applicable, G.C.: Gd Credit is adopted instead of BUC, (U,Pu): U and
Pu isotopes are considered in the credit, $Dry Storage system using metalcasks. BWR fuel assemblies are considered.
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BUC for Fuel Debris¾ BUC or BUC-like method is considered to be
introduced to relax the tough condition to control
the criticality of Fukushima debris.

¾ It is a difficult task to take BUC for the fuel debris.
No method to evaluate the isotopic composition because of too
many variety of composition (U, Pu, MA, FP)
Wide range of burnup for fuel debris (can we measure theburnup?)Do all Fission Products exist in Fuel Debris (volatility)?

¾ Is it possible for us to introduce BUC in Fukushima Debris
criticality safety control even though we have not yet adopted
BUC in the intact fuel transport and SFP storage?
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Treatment of FP should be different
from intact fuel / reprocessing case.
Selection of FP to be included into BUC in Japan¾ Non volatile¾ Large reactivity effect- JAERI-Tech 2001-055 – more than
65% reactivity of total FP’s
contribution
149Sm, 103Rh*, 143Nd, 133Cs, 99Tc*,
152Sm, 155Gd, 145Nd, 147Sm, 95Mo*,
153Eu, 150Sm

Reactivity effect of each FP [%](to total FP effect)¾ Long half life or stable
㻝㻜㻜
㻥㻜
㻤㻜
㻣㻜
㻢㻜
㻡㻜
㻠㻜
㻟㻜
㻞㻜
㻝㻜
㻜

㻻㼠㼔㼑㼞㼟㻹㼛㻙㻥㻡㼀㼏㻙㻥㻥㻾㼔㻙㻝㻜㻟
㻿㼙㻙㻝㻡㻜
㻱㼡㻙㻝㻡㻟
㻿㼙㻝㻠㻣㻺㼐㻙㻝㻠㻡
㻳㼐㻙㻝㻡㻡
㻿㼙㻝㻡㻞㻯㼟㻙㻝㻟㻟
㻺㼐㻙㻝㻠㻟
㻿㼙㻙㻝㻠㻥
㻜㻚㻡㻠㻟㻜

Cooling Time (year)*exclude after dissolution process

After SA, can we confirm the existence of FP with fissile?
Seminar on BUC in KAIST

43

Conclusion

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Conclusion¾ Criticality safety control is one of the key issues fordecommissioning of the Fukushima site.

¾ We need clear strategy and consensus on this issue.
¾ IRID is the framework organizing R&D on Fukushima
decommissioning.

¾ JAEA has been carrying out relevant research
activities.

¾ Burnup Credit is one of the option for the criticality
safety control of the Fuel Debris. But not easy way.
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