INL/EXT-13-30078

Review of Destructive
Assay Methods for
Nuclear Materials
Characterization from theThree Mile Island (TMI)
Fuel Debris
Carla J. Miller
September 2013

The INL is a U.S. Department of Energy National Laboratory
operated by Battelle Energy AllianceINL/EXT-13-30078

Review of Destructive Assay Methods for Nuclear
Materials Characterization from the Three Mile Island(TMI) Fuel Debris
Carla J. Miller

September 2013

Idaho National Laboratory
Idaho Falls, Idaho 83415
http://www.inl.gov

Prepared for the
U.S. Department of Energy
Office of National Nuclear Security Administration
Under DOE Idaho Operations OfficeContract DE-AC07-05ID14517

DISCLAIMER
This information was prepared as an account of work sponsored by an agencyof the U.S. Government. Neither the U.S. Government nor any agency
thereof, nor any of their employees, makes any warranty, expressed or
implied or assumes any legal liability or responsibility for the accuracy,completeness, or usefulness, of any information, apparatus, product orprocess disclosed, or represents that its use would not infringe privately
owned rights. References herein to any specific commercial product, process,or service by trade name, trade mark, manufacturer, or otherwise, does notnecessarily constitute or imply its endorsement, recommendation or favoringby the U.S. Government or any agency thereof. The views and opinions ofauthors expressed herein do not necessarily state or reflect those of the U.S.
Government or any agency thereof.

ACKNOWLEDGEMENT
The author wishes to acknowledge the research performed by prior research teams at
Idaho National Laboratory that focused on studying the Three Mile Island 2 nuclear power plant
reactor accident including accident scenarios, reactor and fuel cleanup, fuel storage, anddecontamination and cleanup principles. I would specifically like to acknowledge Dr. Doug
Akers for the work that he performed in characterizing the molten fuel and containment debris
and providing destructive analysis measurements for post-accident nuclear material accountancy.

EXECUTIVE SUMMARY
This report provides a summary of literature reviewed discussing previous work performed at the
Idaho National Laboratory studying the Three Mile Island 2 (TMI-2) nuclear reactor accident andfocusses specifically on the melted fuel debris containing the majority of the fuel materials. The purposeof the literature review was to document prior published work that supports the feasibility of theanalytical techniques that were developed to provide quantitative results of the make-up of the fuel andreactor component debris located inside and outside the containment. The quantitative analysis provides
a technique to perform nuclear fuel accountancy measurements.
Debris for dissolution eas expected to have formed at high temperatures and contain silicates andother hard-to-dissolve materials. The debris was typically in a glassy matrix containing plutonium anduranium with radioactive fission products. The report 1) summarizes the TMI debris characteristics atvarious locations in the reactor system, 2) assesses the preprocessing and dissolution methods for
destructive assay of the various types of debris, 3) describes the radiochemical debris destructive
measurement methods, and 4) assesses the calibration and error estimation for measurements using
destructive assay.
TMI-2 samples were dissolved using a pyrosulfate fusion technique in a closed system. Iodine129 tracer was added to the intact sample before dissolution, and Sr-90 was added after dissolution. This
technique was used to allow measurement of the I-129 content of the sample. Corium samples that werelarger in size were processed through several steps including: grinding to get samples 5-10 mg up to agram maximum, a sequential dissolution beginning with nitric acid, then followed by hydrofluoric acid.
Some samples were leached using a potassium permanganate solution and sometimes a sodium or
ammonium (alkaline) permanganate solution was used. The most important step in all the analyticalprocesses was the pyrosulfate fusion to obtain the uranium amounts. Nitric acid was used to oxidize
metals and alloys to soluble nitrates and the hydrofluoric acid was used to get rid of any silicates anddissolve the oxides of Nb, Ta, Ti and Zr. Standard pyrosulfate fusions were used for nuclear
accountability.
This report is intended to provide knowledge obtained throughout the sampling, fuel storage, andcleanup processes at the TMI-2 nuclear reactor accident site as a path forward for nuclear material
accountancy measurements necessary for the Fukushima Daiichi nuclear power plant in Japan. Lessons
learned from the TMI-2 nuclear reactor accident may be applicable to the cleanup and nuclear material
accountancy of the destroyed Fukushima Daiichi nuclear reactor.

v

TABLE OF CONTENTS
1.

INTRODUCTION .............................................................................................................................. 9

2.

BACKGROUND ................................................................................................................................ 9

3.

2.1

CHARACTERIZATION OF FUEL DEBRIS FROM THE LOWER HEAD OF THE
REACTOR VESSEL ............................................................................................................. 9

2.2

CORE BORE SAMPLES BELOW THE DEBRIS BED IN THE LOWER REACTORREGION .............................................................................................................................. 11

DESTRUCTIVE ASSAY TECHNIQUES ....................................................................................... 15
3.1

Gravimetric Analysis ........................................................................................................... 15

3.2

Spectrophotometric Methods............................................................................................... 15

3.3

Electrometric Titration Methods ......................................................................................... 15

3.4

Fluorometry ......................................................................................................................... 16

3.5

X-Ray Fluorescence ............................................................................................................ 16

3.6

X-ray Absorption Edge Densitometry (K-edge) ................................................................. 17

3.7

Alpha Spectrometric Methods ............................................................................................. 17

3.8

Mass Spectrometric Methods .............................................................................................. 17

4.

TMI SAMPLE PREPARATION...................................................................................................... 18

5.

CONCLUSIONS .............................................................................................................................. 18

vi

TABLES
Table 1: Elemental analysis results of Debris Adjacent to the Lower Head of the TMI-2 Reactor Vessel 10
Table 2: Radionuclide analysis results of Debris Adjacent to the Lower Head of the TMI-2 Reactor
Vessel ............................................................................................................................................... 11
Table 3: Summary of post-accident core materials distribution (Estimated) ............................................. 13
Table 4: Fuel material and control rod materials distribution in the reactor vessel ................................... 13
Table 5: Fission product distribution in the reactor system ....................................................................... 14

vii

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viii

1. INTRODUCTION
This report provides a summary of the literature review that was performed and based on
previous work performed at the Idaho National Laboratory studying the Three Mile Island 2(TMI-2) nuclear reactor accident, specifically the melted fuel debris. The purpose of theliterature review was to document prior published work that supports the feasibility of theanalytical techniques that were developed to provide quantitative results of the make-up of thefuel and reactor component debris located inside and outside the containment. The quantitative
analysis provides a technique to perform nuclear fuel accountancy measurements.
The author has directly quoted many parts of this document; however, to make it more readable
the direct quotations have been omitted.

2. BACKGROUND
Unit 2 of the Three Mile Island pressurized water reactor (PWR) underwent a loss of coolant
accident on March 28, 1979 which resulted in severe damage to the reactor core. Samples of thelower core were taken to spatially characterize the chemical and physical state of the degradedcore. Nondestructive (visual examination, photography, sample weight, bulk sample density, andindividual particle density) and destructive examinations (optical metallography, scanningelectron microscopy (SEM), and radiochemical analysis) provided data of fission product release,interaction between core components, hydrogen generation, and core melt progression. The TMI
debris characteristics at various locations in the reactor system may be useful to describe similar
events. In addition, following the detonation of a nuclear device, solid debris samples are
expected to contain trace-level quantities of nuclear materials combined with material from theimmediate environment around the detonation site, which may have been activated and isassumed to have been vaporized and re-condensed. As such, debris for dissolution is expected tohave formed at high temperatures and contain silicates and other hard-to-dissolve materials. Solidfallout debris is typically in a glassy matrix containing parts per million (ppm) quantities of
plutonium or uranium with radioactive fission products. The following report 1) summarizes the
TMI debris characteristics at various locations in the reactor system, 2) assesses the preprocessing
and dissolution methods for destructive assay of the various types of debris, 3) describes theradiochemical debris destructive measurement methods, and 4) assesses the calibration and error
estimation for measurements using destructive assay.

2.1 CHARACTERIZATION OF FUEL DEBRIS FROM THE
LOWER HEAD OF THE REACTOR VESSEL
Fuel debris samples were taken from near the lower head of the reactor vessel [1]. Two types ofsamples were obtained: (1) nozzle and guide tube samples from the vessel and the flow
distributor and (2) fuel solidified debris samples removed from the surface of the lower head [1,2]. The debris samples ranged from <5 cm to 45 cm in the central part of the lower head.
Outside of this region the solidified debris was ~26 cm. The debris samples were taken fromwithin 30 cm of the lower head generally but the ones near the periphery of the molten pool weretaken within 15 cm [1]. Metallography was performed on the debris samples and results showedthat the debris samples consisted primarily of previously molten (U,Zr)O2 with pores formed instratified layers surrounded by microporosity and two phase structures (U,Zr)O2 and (Zr,U)O2 [1].
The microstructure is indicative of a solidified (U,Zr)O2 ceramic melt, rich in uranium. The
presence of two phases indicates that the samples were not rapidly quenched but underwent a
9gradual cool down and that the single phase regions solidified first [1]. Debris samples were fully
oxidized which suggests the presence of sufficient steam to oxidize all available zirconium [1].
The debris bed appears to be homogeneous based on the bulk elemental composition and iscomposed primarily of fuel element components with relatively small amounts of structuralcomponents. Elemental analysis results indicates the composition of the debris bed to be 70 wt%
U, 13.75 wt% Zr, and 13 wt% O, and 3 wt% elemental constituents of the stainless steel andinconel core components [1]. Striation or interconnected porosity seen in many of the samples
may be due to bubbling of steam or structural material vapors through the molten pool. Thesesamples show that the debris was liquid while on the lower head and remained liquid forsufficient time to allow bubble formation [1].
Scanning electron microscopy (SEM) analysis was also performed on debris bed samples. The
SEM analysis was used to determine the core constituents, primarily U, O, Zr, Ag, Al, Cd, Cr, Fe,
In, Mg, Mo, Mn, Nb, Ni, Sn, and some fission products. The samples were examined around theedge of large pores, metallic inclusions, secondary phases, and pores without secondary phases.
The homogeneous (U,Zr)O2 matrix had relatively low concentrations of Al, Mg, Sb, and Sn.
There was a zirconium-rich secondary phase around the pores and at grain boundaries. Therealso was the presence of oxidized Fe and Cr inclusions which suggests the remains of nozzle andother vessel components that were melted during the relocation of the fuel. [1] Metallic inclusion
samples are composed primarily of metallic silver with trace amounts of Zr and other metals.
The other control rod constituents (In and Cd) wer not present which suggests that the control rod
material had been heated sufficiently to volatilize the In and Cd from the Ag [1]. Secondary
phases around pores and in the debris matrix indicates the secondary phases are primarily(Zr,U)O2 with greater amounts of Fe and Cr present. The presence of the localized Fe and Cr
suggests that there was not a great deal of mixing after the material was deposited on the lower
head [1].
Radiochemical analysis was performed to assess bulk composition and radionuclide content.
Prior to destructive radiochemical analysis, intact samples were analyzed by gamma spectroscopy
to provide an estimate of the gamma-emitting radionuclide content. The samples were thendissolved and elemental analysis was performed using inductively coupled plasma (ICP)
spectroscopy techniques. The results appear in Table 1 [1]. From the table, it appears that thefuel melt is composed almost entirely of the constituents of the fuel rod and there are little
structural constituents.
Table 1: Elemental analysis results of Debris Adjacent to the Lower Head of the TMI-2 Reactor
VesselAverage Debris Composition
Element
Average Core Composition TMI-2(wt. %)(if core was homogeneously mixed
including end fittings)(wt. %)
U
Zr
O
Fe
Ag

65.8
18.0
8.5
3.0
1.8

Cr
Ni
In
Sn
Al

1.0
0.9
0.3
0.3
0.2

70.4
13.8
Cannot be measured with this technique
0.73
Not detected (some may have been lost
during analysis due to method used)
0.37
0.09
0.27
Not detected
Not detected

10

B
0.1
Not reported
Cd
0.1
Not reported
Mn
0.8
0.028
Nb
0.04
Not detected
Total
100.84--------Total minus O
92.34*
85.69**Difference in core composition and debris composition can be accounted for by the oxidation of the uranium andzirconium in the samples

Radiochemical analysis of the debris samples indicate that the volatile radionuclides (noble gases,
cesium and iodine) had volatilized from the bed with only the medium and low-volatile
radionuclide remaining [1]. High volatility fission products are the noble gases, halogens, alkali
metals, and heavy chalcogens. Only measurements of I-129 and Cs-137 were made. The
medium volatility fission products are the alkaline earth metals, some of the rare earth metals, andactinides. Radionuclides from this group are Sb-125, Sr-90, Eu-154, and Ru-106. The lowvolatility fission products are the noble metals, the remaining rare earth metals, tetravalents, andearly transition elements. The only radionuclide from this group that was measurable wascerium/praseodymium. Table 2 contains information from the radionuclide analysis of the debris
bed.
Table 2: Radionuclide analysis results of Debris Adjacent to the Lower Head of the TMI-2
Reactor VesselRadionuclide

ORIGEN2 CodeAdjusted
Radionuclide
Concentration*
Microcuries/g U

Debris Bed% composition
based on Uranium
analysis

Debris Bed
Radionuclide
Concentration
Microcuries/g U

Sr-90
Ru-106
Sb-125
I-129
Cs-137Ce-144Eu-154

8330
413
308
0.003
9680
617

64
2.9
7.6
91

5331
8.9
735.7
561.5

80
82
65.6
*Correction to the core average value to account for the fact that the peripheral 2.98% enriched fuel assemblies did notparticipate in the accident.

2.2 CORE BORE SAMPLES BELOW THE DEBRIS BED IN
THE LOWER REACTOR REGION
Core bores were taken from the region below the debris bed in the lower reactor core region.
Physical, metallurgical, and radiochemical measurements were performed. The upper reactor
core contained a void region. Below the void region was a layer of debris resting on a hard crustwith approximately 50% of the core volume located below the debris bed [3]. The core below thedebris bed had a region of previously molten material surrounded by a hard crust and a second
region of intact standing fuel rods extending from the bottom of the previously molten region to
the bottom of the core.
11

The core bores contained solid plugs of the upper and lower crusts, previously molten material
from between the crusts, and fuel rod stubs. Solid plugs from the upper and lower crusts werecomposed of agglomerated fuel and structural material components. The upper crust and thelower crust had different compositions from each other. The upper crust was a mixture of debris
agglomerated with metallic material. The lower crust consisted of fuel rods surrounded bysolidified molten material. The region between the crusts was relatively homogenous with longer
fuel rod stubs located near the periphery and shorter fuel rod stubs located near the core [3].
Gamma spectroscopy was performed on the core bores followed by isotopic analysis at 2.5cm
intervals over the length of the bore and at areas of high activity identified during gamma
radiation measurements [3]. Upper and lower crust regions had significant concentrations of Co60, Ru-106, and Sb-125. The molten material between the crust layers has low concentrations ofvolatile radionuclides such as Co-60, Ru-106, and Sb-125. The molten material between thecrusts has low concentrations of volatile radionuclides but has significant amounts of Ce-144 andEu-154. The fuel rod sections in the lower core have high concentrations of high volatile fission
products [3].
Metallurgical examinations of the molten material found in the coolant channels of the lower
crust area was a mixture of metallic structural and control rod components. Two metallic phases
were present with the main constituents of: 1) Zr, Fe, Ni, Cr and 2) Ag, In alloy. Some cadmiumfrom the control rods was present in the Ag phase [3].
Samples from the lower vessel plug suggest an interaction between the fuel rods and thestructural components which resulted in the dissolution of the zircaloy cladding and fuel by themolten structural materials. Fission products in the lower crust were retained in the fuel material[3].
The upper crust plugs had two phases: 1) ceramic phase containing mostly fuel material
components and 2) metallic phase of mostly structural components. The ceramic phase is aninteraction of fuel rods and structural/control rod components and is more concentrated in thelower crust region. There are no intact fuel rods in the ceramic phase and the structural materials
are present as oxides of nickel, silver, and indium. The ceramic phase also contains uranium andzirconium oxides with small amounts of iron, chromium and nickel [3]. Intact fuel pellet
remnants were encased intact in the ceramic matrix with the mixed oxides of uranium andzirconium. The ceramic phase is the phase where it is important to be able to do nuclear
material accountancy.
The peripheral crust is the region of the upper crust near the mid-radius of the core. This crust
has substantial amounts of metallic structural components: iron, nickel, silver, and indium [3].
Samples from the core interior particles are a mixture of both metallic and ceramic phases. The
metallic samples are dendritic and composed of iron, nickel, and chromium with circularinclusions of silver, indium and tin. Some contained spherical particles of chromium oxide. The
most common fission product in the metallic inclusions was ruthenium and technetium along withmeasurable concentrations of palladium and tellurium [3]. Fuel rods and guide tubes indicatedhydriding. No previously molten debris was present between the intact fuel and control rods inthe core bore [3].
Tables 3-5 summarize the core materials and fission product inventory from the TMI-2 reactor[4]. Table 3 is a summary of the post-accident core materials distribution, Table 4 is the fuelmaterial and control rod distribution on the reactor vessel, and Table 5 is a summary of the fission
12

product results on the reactor vessel [4]. The low volatiles were not released from the fuel andonly a small amount was transported to the reactor coolant system is intact or melted. These three
low volatility fission product numbers can be used to calculate the plutonium and uranium
concentrations for nuclear material inventories based on the ORIGEN2 code because they were
not lost during the TMI-2 accident. The Ce-144 concentration is the most accurate because Ce144 is produced by direct beta decay from fission of the UO2 while the Eu-154 and Eu-155 areproduced by neutron activation of fission products and are dependent on the neutron flux andspectrum and on the core location of the fuel material. Therefore the Ce-144 concentration wasused to calculate the nuclear material inventory for all samples sent to Idaho National Laboratory[4].

Table 3: Summary of post-accident core materials distribution (Estimated) [4]Core Region

Estimated
quantity (kg)

Intact fuel assemblies (partially or fully intact)
Central core region resolidified mass
Upper core debris bed
Prior molten material on the lower reactor vessel
head
Lower core support assembly (b)Upper core support assembly(b)Outside the reactor vessel

Uncertainty(a)(%)

Percent of
total core
(%)

44500
32700
26600
19100

5
5
5
20

33.4
24.5
19.9
14.3

5800
4200
100

40
40
(c)

4.3
3.2
0.3

(a) The uncertainty estimates are based on defueling. Those areas that have been defueled have lower
uncertainties.
(b) The lower core support assembly is the portion of the reactor vessel below the core that includes the lower
grid assembly and five flow distributor plates. The upper core support assembly is a coolant flow region
outside the vertical baffle plates that is the peripheral boundary of the core.
(c) Estimates of the amount of fuel material outside the reactor vessel are based on non-destructive evaluationsof reactor components in the reactor and auxiliary buildings. They range from 60-100 kg.

Table 4: Fuel material and control rod materials distribution in the reactor vessel [4]Core material repositories
Upper reactor plenum
Upper core debris
Upper crust region
x Ceramic
x Metallic
Consolidated region
x Ceramic
x Metallic
Lower crust region
x Ceramic
x Metallic
Intact fuel rods (d)Lower reactor vessel head
Lower core support assemblyCore material distribution of fuelmaterial (a)Uranium
Zirconium
Tin
(b)
(b)
(b)
24
13
(c)

Core material distribution of control rod
materials (a)SilverIndiumCadmium
1.0
(b)
(b)
1.8
(c)
(c)

1.3
--

1.2
0.3

2.3
6.1

1.2
2.4

3.6
3.3

0.65
0.39

12
--

18
0.2

-5.8

10
1.6

27
2.1

6.1
1.1

3.6
-33
15
4.6

2.8
5.6
33
11
3.3

9.3
26
33
(c)
(c)

7.3
11
11
(c)
(c)

7.2
16
11
(c)
(c)

1.4
2.9
11
(c)
(c)

13

Upper core support assembly

3.3

2.4

(c)

(c)

(c)

(c)

TOTAL

97

91

82

47

70

23

(a)
(b)
(c)
(d)

Percentage of the total amount of the element originally present in the core
Insignificant amount (<0.1 wt%) based on the upper plenum measurements
Elemental constituent not detected based on detection limits of approximately 0.1 wt%
Only 70% of the partially intact fuel assemblies contain control material as the balance (22.7%) areperipheral assemblies which do not contain control materials

Table 5: Fission product distribution in the reactor system [4]Fission Product Repositories

Ex-vessl
Containment atmosphere, basement,
and tanks
Reactor coolant system
Auxiliary building
In-vessel
Upper reactor plenum
Upper core debris-A
Upper core debris-B (c)Upper crust regionxCeramicx
metallic
Consolidated regionxceramicx
metallic
Lowe crustxceramicx
metallic
Intact fuel rods
Upper core support assembly
Lower core support assembly
Lower head-reactor vessel
TOTAL

Fission productdistribution
High volatility fission
products
Percent of inventory (a)Cs-137
I-129
Kr-85

Fission product distribution
Low volatility fission products
Percent of inventory (a)Fission product distribution
Medium volatility fission
products
Percent of inventory (a)Ce-144Eu-154Eu-155

Sr-90

Ru-106

Sb-125

0.01

(b)

(b)

2.1

0.5

0.7

(b)

(b)

54

47
3
5

(47) (c)
1
7

(b)
(b)
(b)

(b)
5.3
4.3
0.41

(b)
5.9
5.3
0.27

(b)
6
(b)
(b)

0.77

2.1

(b)

(b)
(b)

(b)
(b)

(b)
(b)

1
0.1

(b)
(b)

0.2
0.7

(b)
26
20
1.4

(b)
30
19
2.0

(b)
24
19
1.6

(b)
23
19

(b)
14
16

(b)
13
24

0.73
(b)

0.8
3.8

0.5
7.8

8.3
(b)

2.2
9.0

3.1
6.9
1.4

3.5

(b)

5.7
24
30
0.23
0.32
1.1

7.4
36
30
0.22
0.30
1.0

30
0.46
0.63
2.1

30
0.12
0.16
0.54

30
(b)
(b)
(b)

94

119

95

97

91

24

5.9

32

22

7.9

5.1

30
3.4
4.7
16

30
4.5
6.3
21

30
(d)
(d)
(d)

4.5
(b)
30
3.9
5.3
18

105

122

110 (d)93(a) Percentage of total amount of the fission product inventory calculated from comparisons with ORIGEN2(b) Insignificant amount (<0.1 wt%) based on the upper plenum measurements(c) Two sets of bulk sample measurements were performed on the upper debris bed. The A series was
performed on 16 cm3 sample from near the center of the core at a variety of depths whereas the B series werebulk samples from near the bottom of the debris bed. The data provide a range. For the totals, the B series
data were used.
(d) Measurements not performed for this radionuclide at this core location. The total shown value in parenthesis
is a total which assumes the same distribution as Eu-154 for the repositories where measurements were not
performed

14

3. DESTRUCTIVE ASSAY TECHNIQUESDA measurements provide a more accurate quantifiable measurement than non-destructive assay
techniques and can be used to determine if a small fraction of a material is present. Destructive
assay techniques are similar for all sample types. Common radiochemical techniques are:gravimetric methods spectrophotometric methods, electrometric titration methods(potentiometric, amperometric, and coulometric), fluorometry, x-ray fluorescence, x-ray
absorption edge densitometry (K-edge), alpha spectrometric methods, and mass-spectrometricmethods. The key to DA being quantitative and useful lies in the sample preparations. There arethree main types of sample preparations for non-liquid samples: fusion, wet-ashing including acidleaching and acid dissolution, and microwave digestion. Brief descriptions of the techniques areprovided in the following subsections [6].
Fusion and wet ashing are used to decompose most samples analyzed in radioanalyticallaboratories. Fusion techniques are used for total dissolution of a difficult sample matrix.
Leaching techniques are used to determine the soluble fraction of the radionuclide of interest. [6]3.1 Gravimetric Analysis
Gravimetric methods involve separating a compound of an element and igniting it to a constantweight stoichiometric compound. The technique is applicable to relatively pure materials such as
U3O8, UO2, or UF6 in product streams. Gravimetry may not be applicable for plutonium analysis
because only PuO2 is present in an acceptable for and it is hygroscopic. The amount and rate ofwater adsorption is dependent upon the ignition temperature [6].

3.2 Spectrophotometric Methods
Compounds or complexes in solution will absorb light of a specific wavelength in quantities
proportional to the concentration of the measured species. Specificity is a function of the
sharpness of the absorption bands, specificity of reagents, other elements or compounds present,
and the quality of the monochromators. Specificity can be improved using preliminary
separations, masking agents, and pH controls [6].
Spectrophotometric methods are of limited use for measuring uranium and plutonium in fuelcycle materials due to the non-specificity of reagents for uranium in the presence of plutonium
and because of the nuclear reaction products such as americium, neptunium, and the fission
products. Separation of these radioisotopes requires more in-depth time consuming separations[6].

3.3 Electrometric Titration Methods
Oxidation-reduction reactions can be used to determine both uranium and plutonium productswith high precision. There are three types of electrometric methods classified by how end-points
are detected: amperometric, coulometric, and potentiometric [6].

15Potentiometric titrations are based on measuring the change in potential of the system as acomponent is removed by oxidation or reduction. This technique is used to measure final
products in the nuclear fuel cycle of U3O8, PuO2, or Pu(NO3)4 [6].
Amperometric titrations measure the change in current between two electrodes maintained at aconstant potential as titrant is added. The technique is limited by the accuracy of the inflection
point measurement [6].
Coulometry can be used to determine uranium and plutonium in solution and is based on theprinciple that the weight of a substance oxidized or reduced at an electrode is proportional to thequantity of electrical charge passed through the electrode. Both uranium and plutonium aretitrated in the same sample without separation by performing successive titrations at different
potentials. The sample size of coulometric titrations is generally smaller than that needed for
either potentiometric or amperometric titrations. The determination of plutonium requires many
considerations and is generally not determined using this technique [6].

3.4 Fluorometry
Fluorometric determinations are applicable to low concentrations of uranium. It is not applicableto Pu. Fluorometry is based on the principle that uranium fluoresces when excited by ultraviolet
light. Samples are generally evaporated and fused in a flux. Samples are generally fused in a
carbonate flux or a NaF flux. Carbonate fluxes tends to provide better analytical precision
although fluoride fluxes provide better uranium sensitivity. However fluoride fluxes are sensitive
to flux temperature and cooling conditions. Most of the transition elements will interfere throughquenching or enhancement [6].
Fluorometry evolved as the standard method for determining small amounts (1-100 ng) of
uranium. High-throughput, off-line measurements of uranium in waste streams are made byprocessing samples in the analytical laboratory [6].

3.5 X-Ray FluorescenceThis technique is used primarily to measure solutions from the reprocessing of spent fuels but isapplicable to actinide analysis at all stages of the fuel cycle. The techniques are sensitive,
accurate, and capable of measuring microgram quantities of material to relative accuracies ofabout 1%. The typical analysis times are short ( 0.5 h). The predominant source is the x-ray
generator; however using radioisotopes is more common because the solid-state detector tomeasure the lower-intensity x-rays are readily available. Both uranium and plutonium in solution
can be measured by wave-length and energy-dispersive detection systems. Wavelength
dispersion has high resolution but low efficiency while energy dispersion is more efficient but haspoorer resolution. Energy dispersion using Ge(Li) or intrinsic germanium detectors is required formeasuring actinide K-series x-rays. To reduce matrix sensitivity, samples are often evaporated asthin film which reduces scattered background compared to the sample allowing for increased
sensitivity and decreased counting times [6].

16

3.6 X-ray Absorption Edge Densitometry (K-edge)
Transmitted intensity through the sample is measured for two x-rays or gamma rays (selected
above and below an absorption edge for the element being determined). This is an element
specific method that can be used in time, at-line or off-line to measure uranium and plutonium inintermediate process and final product solutions [6].
K-edge densitometry has a limited dynamic range and cannot be used for a precise plutonium
concentration in the presence of a large excess of uranium [6].

3.7 Alpha Spectrometric Methods
The measurement is based on the measurement of the alpha-radiation intensity of the sample.
The alpha particles are ejected with discrete energies for uranium, neptunium, plutonium andamericium isotopes. The energies are 4-5.5 MeV. This method is not quantitative unless prepared
as a thin, dry film because absorption of water and self-absorption of the sample occurs.
Detectors used for alpha spectrometry include standard radiation instruments such as proportionalcounters, scintillation detectors, and solid state devices. Special techniques were developed for
the PUREX process streams where the detector is in direct contact with the alpha radiation
sources [6].

3.8 Mass Spectrometric Methods
Mass spectrometry currently provides the most accurate isotopic analysis even in the presence offission products. The isotope dilution technique provides an overall accuracy of 0.3-1% for themeasurement of total uranium and plutonium in dissolver solutions [6].
Thermal-ionization mass spectrometry (TIMS) is used to determine the amount of each isotope ofuranium and plutonium and is used to measure the total uranium and plutonium in accountabilitytank samples. Isotope-dilution mass spectrometry is used for high-precision/high accuracy
measurements of plutonium and uranium in tanks. Sample preparation involves: 1) dissolving
and diluting with HNO3, 2) for isotope-dilution mass spectrometry, spiking part of the diluted
sample with accurately known amounts of U-233 and Pu-242 or Pu-244, 3) oxidizing Pu tohexavalent state, 4) sequentially separating fission products, plutonium and uranium by anionexchange or solvent extraction, 5) transferring aliquots of the separated uranium and plutonium
fractions to separate mass spectrometer filaments [6].
In TIMS, the light isotopes are evaporated and ionized preferentially, relative to the heavier
isotopes of an element. The temperature-dependent effect is a function of the mass, sample size,
and time. The abundances of major isotopes are determined with RDSs of 0.01 to 0.02%. Highgamma-radiation levels limit the mass-spectrometric analysis of dissolver solutions to extremely
small samples or to samples purified in shielded facilities [6].

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4. TMI SAMPLE PREPARATION
TMI-2 samples were dissolved using a pyrosulfate fusion technique in a closed system. Iodine129 tracer was added to the intact sample before dissolution, and Sr-90 was added after
dissolution. This technique was used to allow measurement of the I-129 content of the sample.
Based on communications with D.W. Akers [5], corium samples that were larger in size were
processed through several steps including: grinding to get samples 5-10 mg up to a grammaximum, a sequential dissolution beginning with nitric acid, then followed by hydrofluoric acid.
Some samples were leached using a potassium permanganate solution and sometimes a sodium or
ammonium (alkaline) permanganate solution was used. The main step in all the analyticalprocesses was the pyrosulfate fusion. Dr. Akers said that he would provide the exact steps on
his return to work in late September 2013.
Nitric acid was used to oxidize metals and alloys to soluble nitrates and the hydrofluoric acid wasused to get rid of any silicates and dissolve the oxides of Nb, Ta, Ti and Zr. Standard pyrosulfate
fusions were used for nuclear accountability is described below.
For pyrosulfate fusions to be successful, the sample must be oxidized to ensure that the sulfides,
metals, and organics contain chemically bound oxygen or has been removed. Fusion samples
only work if the sample has chemically bound oxygen such as oxides, carbonates, and silicates.
A crucible (generally platinum, quartz, or porcelain for uranium and plutonium analysis) is filled
half way with the sample mixed with a flux (potassium sulfate or sodium sulfate). The crucible isheated slowly and evenly to prevent ignition of the sample before the reaction with the molten
salt will occur. The crucible and the ingredients inside are taken up to a fusion temperature of up
to red heat. Once the salt is melted, the melt is swirled gently to monitor the reaction. The fusion
continues until visible signs of the reaction are completed (e.g. formation of gases, foaming,
fumes, etc). Usually a clear melt indicates the completeness of the sample decomposition. The
melt is swirled during cooling to spread it over the inside of the crucible. Thin layers of salt on
the sides of the crucible will crack and flake into small pieces during cooling making them easier
to dissolve. Once the fusion has reached room temperature, it is dissolved with a dilute sulfuricor hydrochloric acid to avoid hydrolysis and precipitation of titanium and zirconium. Niobium
and tantalum may precipitate but concentrated sulfuric acid, tartaric acid, ammonium oxalate,hydrogen peroxide or hydrofluoric acid can be added to prevent the precipitation. Mercury andthe anions of volatile acids are generally volatilized during the fusion.

5. CONCLUSIONS
The destructive assay of samples obtained from the high temperature ceramic debris
containing much of the destroyed and intact fuel rods and assemblies required innovativetechniques. These samples were used to obtain quantifiable data that was used to describe
the nuclear material accountancy of the destroyed TMI-2 nuclear reactor. The estimatedtemperatures at various regions of the core, in addition to recreations of the nuclear reactor
accident allowed for characterization of many of the volatile, medium volatile and highvolatile fission products. Sampling of the core itself and the debris that remained around thedestroyed core provided an understanding of what happened during the accident.
Unpublished work provided through communications with Dr. D.W. Akers, provided thedestructive assay techniques used for material accountancy of the debris and core samples
with the key component, being the pyrosulfate fusions that were performed after step by step18

sizing and preconditioning techniques. These or similar techniques, can be utilized fornuclear material accountancy at the destroyed Fukushima Daiichi nuclear reactor in Japan.

REFERENCES[1] Akers, D.W., Jensen, S.M., and Schuetz, B.K., “Examination of Relocated Fuel Debris
Adjacent to the Lower Head of the TMI-2 Reactor Vessel”, NUREG/CR-6195, 1994[2] Olsen, C.S., Akers, D.W., and McCardell, R.K., “Examination of Debris from the Lower
Head of the TMI-2 Reactor”, GEND-INF-084, EG&G Idaho, Inc., January 1988
[3] Olsen, C.S., Akers, D.W., and McCardell, R.K., “TMI-2 Core Bore Examination Results,”
Proceedings of the U.S. Nuclear Regulatory Commission 15th Water Reactor Safety Information
Meeting, Gaithersburg, MD, October 26-30, 1987, NUREG/CP-0091, Vol. 6 (February 1988), pp.
175-198.
[4] Akers, D.W., McCardell, R.K., Russell, M.L., and Worku, G., “TMI-2 Core Materials andFission Product Inventory”, Nucl. Eng. Design 118 (1990) 451-461[5] Phone conversation with D.W. Akers on August 24, 2013 discussing sample preparation
techniques for TMI-2 samples (specifically corium samples)[6] Gutmacher, R.G. and Jackson, D.D., “Survey of Chemical and Bulk Measurement Methods”,
Chapter XI in Fundamentals of Materials Accountancy for Nuclear Safeguards compiled by
K.K.S. Pillay, LA11569-M, 1989, pages 265-301

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