Ac In Nf Development

ROLE OF ANALYTICAL CHEMISTRY IN THE DEVELOPMENT OF NUCLEAR FUELS Power point presentation prepared by KL Ramakumar India

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Analytical chemistry in nuclear fuel cycle - indispensable - pivotal role in the entire nuclear fuel cycle - ore refining - conversion - nuclear fuel fabrication - reactor operation - nuclear fuel reprocessing - waste management 2

Chemical Quality Control

Chemical quality control provides a means to ensure that the quality of the fabricated fuel conforms to the chemical specifications for the fuel laid down by the fuel designer The specifications are worked out for the major and minor constituents which affect the fuel properties and hence its performance under conditions prevailing in an operating reactor. Each fuel batch has to be subjected to comprehensive chemical quality control for trace constituents, stoichiometry and isotopic composition.

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Why is CQC needed? Control of uranium and plutonium contents and their isotopic compositions is essential in order to ensure the required fissile content O/M can affect thermal conductivity, melting point, number of phases, chemical reactivity and mechanical strength. O/M has to be maintained in the specified limits of hypostoichiometry since the oxygen potential significantly increases with irradiation which may lead to clad corrosion Control of trace constituents in the fuel is necessary to obtain the designed burn-up

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Why is CQC needed? Fluorine, chlorine and moisture cause corrosion of the clad Moisture can modify the O/M of the fuel and also release hydrogen which can cause pressure buildup Carbon can react with oxygen forming gaseous carbon monoxide which facilitates transfer of carbon from fuel to the clad causing damage to it. Boron, cadmium and rare earths affect the neutron economy 5

Analytical methodology for chemical quality control measurements (1) preparation of starting materials (2) Sampling methodologies (3) Dissolution of samples (4) Thorium, uranium and plutonium content (5) Isotopic composition (for fissile and fertile content) (6) Americium content (7) Oxygen to metal ratio (8) Trace metals determination (9) Trace non-metals determination (10)Total gas content (11)Moisture content in the case of oxide fuels. 6

A number of analytical parameters need to be determined as a part of chemical quality control of nuclear materials. Classical to sophisticated instrumentation techniques New superior methodologies - reduction in the time of analysis - improvement in the measurement precision and accuracy - simplicity of the technique itself - ease of automation

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STARTING MATERIALS (UO2 or PuO2) UO2: Direct denitration, ammonium diuranate (ADU), ammonium uranyl carbonate (AUC), or peroxide precipitation followed by subsequent conversion to UO2. ADU precipitation agglomerates

conditions

decide

particle

size

and

ADU is converted to UO3 powder by calcination at about 3000C, reduced to UO2 with cracked ammonia at about 6500C. It is stabilized by partial oxidation to UO2.15 using CO2 or controlled amount of air. The particle size of powder is controlled within 2-10µm by ball milling and a surface area of 5m2/g is considered optimum 8

PuO2: Precipitation as plutonium(IV) oxalate is the most widely used commercial method of converting plutonium nitrate solution to dioxide as it offers both purification as well as better handling properties of the final product. Pu(III) oxalate precipitation and subsequent calcination can also be used. Plutonium is adjusted to trivalent state, by the addition of 1M ascorbic acid, in the presence of an oxidation inhibitor such as hydrazine. Finally the precipitate is calcined at 700ºC to obtain PuO2 Addition of excess of oxalic acid reduces the solubility of the product (minimises loses). Oxalate is decomposed to oxide 9

MOX fuel: 3 main routes 1. AU/PuC process (Ammonium Uranyl-Plutonyl Carbonate):

AU/PuC complex is precipitated by adding ammonia and carbon dioxide to a solution of uranium nitrate and plutonium nitrate followed by a calcination step 2. Micronizing method:

PuO2 powder is micronized with UO2 powder to form a “master blend”. This is mechanically mixed with free flowing AUC or ADU (Ammonium DiUranate) UO2 to obtain the specific plutonium content required. The very close contact between UO2 and PuO2 aggregates allows for sufficient inter-diffusion during sintering. 3. SBR (short binderless route): Homogenisation is attained by means of a high- energy attritor mill, which blends the oxide powders and a spherodizer in order to condition the powder to granules prior to pressing and sintering. At the milling stage, suitable lubricant and pore former are added in order to control the pellet density and obtain desired characteristics. 10

MOX Fuel: Microwave heating method can also be used as a preparation route for MOX fuel production. U/Pu nitrate solution is heated by microwaves and co-converted to (U,Pu)O2 powder. - Heating of the U/Pu nitrate solution to its boiling point - Concentrate by vaporization - De-nitration by decomposition of the molten nitrate mixture and formation of de-nitrated cake - Stabilization of the de-nitrated cake. (U,Pu)C: Monocarbide phase with a small admixture of M2C3. This fuel is fabricated by the carbothermic reduction of a mixture of the oxides of uranium and plutonium. 11

Analytical Chemistry in Dissolution of refractory Oxides and Carbides One of the most widely used methods of dissolution is to treat the sample with concentrated HNO3 and small quantities of HF. The dissolution rate generally decreases with increase in Pu/U ratio. Fusion with ammonium bisuiphate and later treatment with H2SO4 Sealed tube method in which a mixture of sample and concentrated acids is subjected to high pressure (4000 psi) at a ternperature of 350°C. (safety precautions with plutonium samples!)

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(U,Pu)C: Conventionally carbide is converted to oxide which can be dissolved using the oxide dissolution procedure. A direct dissolution method by refluxing the sample in 1:1 HNO3-H2SO4 mixture eliminated the oxide conversion step reducing the overall time required for dissolution. Alloy fuels like U-Zr are dissolved using 3M HNO3-1% 1M HF by heating at 800C for 4 hrs.

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Analytical Chemistry in Thorium, Uranium, and Plutonium Determination

Thorium: method.

Classical

EDTA

complexometric

titration

Pu interference could be eliminated by preferentially oxidizing it Pu (VI) by AgO prior to titration.

EDTA titration with potentiometric end point detection using Fe(III)/Fe(II) couple. Elimination of personal bias in the volumetric titration using xylenol orange as indicator for end point detection.

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Electroanalytical Chemistry (Multiple oxidation states) Titration of Plutonium 1) Pu + Ag(II) --------------- Pu(VI) + Ag(II) excess 2) Ag(II) excess --------------- Ag(I) using sulfamic acid 3)Pu(VI) + 2Fe(II) --------------- Pu(IV) + 2Fe(III) + Fe(II) excess 4) 2Fe(II) + Cr(VI) --------------- 2Fe(III) + Cr(IV) Titration of Uranium UO2++ + 2Fe++ + 4H+ -------- U4+ + 2Fe3+ + H2O Fe++ + NO3- + 2H+ -------- Fe3+ + NO2 + 2H2O U4+ + 2VO++ ------------ UO2++ + 2V3+ Cr2O7-- + 6V3+ + 2H+ ------------ 2Cr3+ + 6VO++ + 2H2O Cr2O7-- + 3U4+ + 2H+ ------------ 2Cr3+ + 3UO2++ + 2H2O 15

U determination in presence of Pu and Fe Ti(III) is used as the reductant in a mixture of H2SO4 and HNO3. Fe(II) and Pu(III) are selectively oxidized by the nitrous acid generated in the reaction between Ti(III) and HNO3. The U(IV) is determined by titration with K2Cr2O7 using biamperometry to detect the end point. The method is applicable to a variety of nuclear materials encountered at different stages of the nuclear fuel cycle and has no bias. The precision of the method is evaluated at different levels from 100 microgram to 100 milligram. The method is simple, rapid and convenient. 16

Controlled potential coulometric method for U

Reduction of uranium in 8M H2SO4 by Ti(III) followed by destruction of excess Ti(III) and selective oxidation of Fe(II) or Pu(III) to Fe(III) or Pu(IV), respectively, by sodium nitrite. U(IV) is subsequently determined by electrolytic oxidation at Pt electrode using Fe(III) as an intermediate. The precision obtained for uranium results was

±0.25%.

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The major technique applied is the Davies and Gray potentiometric titration, which is applied in its original NBL-modified form or in a robotized automate set-up using a scaled-down version. Both variants deliver routinely results with a precision and accuracy below 0.05% relative (standard deviation). Propagating all uncertainty components, including systematic uncertainties of calibration and certified values of primary reference materials, the total uncertainty for routine sample results (mean of two dissolutions and titrations) is estimated as ±0.07% relative (95% confidence interval).

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Pu determination Reductimetric titration of Pu(VI) to Pu(IV) using the reductant Fe(II) as titrant with amperometric detection of the end point after preliminary oxidation of the Pu to Pu(VI) using excess Ag(II) oxide as an oxidant. Random errors of 0.1 -0.2% are generally attained. Systematic errors are usually better than 0.1%. Random errors under the best conditions are better than 0.05%.

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Controlled Potential Coulometry of Pu Quantitative electrolytic oxidation of Pu(III) to Pu(IV) at an electrode maintained at a controlled potential with determination of the quantity of Pu from the quantity of electricity required for the complete oxidation. Random and systematic errors of 0.1% SAMPLE SIZE: 1 - 10 mg Pu SAMPLE TYPES: Pu metal, oxides, Mixed U, Pu oxides, Pu nitrate solutions, Dissolver solutions Applicable to most Pu materials when ion-exchange separation is used. ADVANTAGES: Absolute method. High precision and accuracy on relatively small quantities of Pu. Relatively free of interferences. Readily automated. Adaptable to remote operations and analysis of irradiated materials. 20

Other Specifications Specification

Methodology

Isotopic composition Pu in finished fuel pins

TIMS,Alpha spectrometry NDA techniques NDA technique

Pu-238 and Am-241

Alpha spectrometry

Total gas content

Vacuum extraction

Moisture content

Carbides: should not be present Manometry or coulometry Thermogravimetry, EMF and gas equilibration

O/M

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Scope for further improvements Necessitated by New and advanced nuclear fuels Nitrides/carbides: (Pu0.07Zr0.93)N, (U,Pu,MA)C, U,Pu,MA)N CerCer / CerMet: Mo - (Pu0.8Am0.2)O2 Particle Fuels for VHTR: (Zr,Y,Pu)O2

Precision and accuracy requirements (from CQC and NUMAC) Automated instrumentation New simplified multi-task capabilities

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Determination of microamounts of plutonium isotopes by luminescent radiation in the infra-red region of PbMoO4 crystallophosphors activated by plutonium is proposed. The determination is possible against the background of subtantial amounts of uranium, iron, rare earths and other elements. The detection limit of plutonium is 10–9 g in the samples tested. The accuracy is 0.1 for a confidence level of 0.95. Yu. P. Novikov, V. B. Gliva, S. A. Ivanova and B. F. Myasoedov, J. Radioanal. Nucl. Chem., 103, 337(1986)

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Techniques that use atomic emission for element detection and quantification encounter a large number of spectral interferences when analyzing plutonium samples due to the numerous emission lines of plutonium. Mass spectrometric techniques do not have that problem. In ICP-MS the main effect plutonium has on the elemental determinations is a signal suppression. With matrix dilution and signal normalization that problem is mitigated. The typical routine sample requires an acid digestion with a combination of nitric, hydrochloric, and hydrofluoric acids. The digested sample is split into a portion for ICPMS analysis, and a portion for ICP-AES analysis. Prior to the ICP-AES analysis, ion chromatography is used to remove the plutonium from the samples. 24

GDMS in nuclear technology Direct current GDMS applied to conducting and nonconducting nuclear samples. For the non-conducting oxide-based nuclear samples the relative sensitivity factors (RSFs), applied for quantitative analysis, are affected by the oxygen content in the matrix. For these samples the effect of getter metals, such as tantalum and titanium as binder material are used. Metallic alloys were analysed using several analytical techniques. The GDMS results obtained applying RSFs from the major metallic element uranium were in agreement with those from independent techniques, such as titration, thermal ionization MS and ICP-MS Maria Betti, J. Anal. At. Spectrom., 11, 855(1996) 25

A Rapid method for O/M in MOX The sample and the nickel metal were fused in a graphite crucible in a resistance heating furnace under helium flowing through the system. The oxygen in the MOX fuel reacted quantitatively with the crucible graphite producing carbon monoxide in proportion to oxygen content. CO intensity measured using non-dispersive infrared spectrophotometry (NDIR). Good agreement with the thermogravimetric values. The relative standard deviation was < 0.20% Only 30 mg of sample required Analysis time takes about 10 min Applicable over a wide range of plutonium contents T. Hiyama, Anal. chim. Acta, 402, 297(1999) 26

Specification

Methodology in 1970s (*)

Methodology currently in use

Carbon

Manometry, conductometry

IGF-TCD

Chlorine and fluorine

Pyrohydrolysis and Pyrohydrolysis and ion spectrophotometry/ISE chromatography

Nitrogen

Kjeldahl distillation and Nessler’s method

Kjeldahl distillation and indophenol blue method IGF-TCD Ion chromatography

Phosphorus

Spectrophotometry

Ion chromatography and spectrophotometry

Sulphur

Spectrophotometry

Combustion extraction-QMS

(*)M . V. Ramaniah in Pure & Appl.Chem., 54, 889 (1982) 27

Specification

Methodology in 1970s (*)

Methodology currently in use

Trace elements

Carrier distillation DCArc AEC AAS

ICP-OES, ICPMS HPLC, ion chromatography

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Periodic performance assessment of the analytical methodologies

Necessary for CBMs, Safeguards evaluation Proficiency testing Intercomparisoon experiments Sample exchange programmes

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Uranium analysis by Davis Gray Method 30

Pu analysis assessment 31

Method Gamma Spectrometry

HRGS

Alpha Spectrometry

Control material NBS020 NBS030

NBS947, SAL9984

NBS947, AEA90099

Limits Measurand 235U

(Wt%)

Control

Warning

0.50

0.33

241Am

1.97

1.47

238/239

3.56

2.66

240/23

1.93

1.46

241/239

1.50

1.13

Activity ratio

1.17

0.85

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Method

Control material

McD&S

UK-Pu-1 (as control material) D&G PuEC-110 Lab D&G ULab

EC-110

Limits Measurand

Control

Warning

Pu conc.

0.23

0.17

U conc.

0.26

0.19

U conc.

0.12

0.09

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limits

Method

control material

measurand

control

warning

Pu conc.

0.41

0.31

IDMS LSD

LSD control

U conc

0.22

0.17

Pu conc.

0.48

0.36

U conc

0.43

0.30

IDMS product

UK82522 or mixture NBL112A (NBS960) or mixture

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Method

Control material NBS947 UK-Pu-3

MassSpec Pu

UK-Pu-4 UK-Pu-5 UK-Pu-6

NBL-128

Measurand 240/239 242/239 240/239 242/239 240/239 240/239 242/239 244/239 240/239 242/239

CBNM-047a 244/239

Limits Control

Warning

0.19

0.14

0.51

0.38

0.11

0.085

0.21

0.16

0.18

0.13

0.18

0.13

0.32

0.23

0.36

0.27

0.15

0.11

0.20

0.15

0.30

0.22 35

Method

MassSpec U

Control material

Measurand

NBS010

235/238

NBS020a, NBS030a

235/238

0.20

235/238

0.22

0.16

233/238

0.26

0.20

236/238

0.15

0.11

NBS100 to NBS930 NBL116, GUS3568, CBNM072/2 CBNM072/2, GUS3568 GUS3568

Limits Control

Warning

0.36

0.27 0.15

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MATERIAL TYPE and METHOD

U Assay U Products (D&G titration)

UNCERTAINTY OF ROUTINE ANALYSIS RESULT (95% CONFIDENCE INTERVAL, relative) ±0.07 %

U Assay U/Pu Products (D&G)

±0.15 %

U Assay U/Pu Products (IDMS)

±0.21 %

Pu Assay U/Pu Products (McD&S)

±0.14 %

Pu Assay U/Pu Products (IDMS)

±0.26 %

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MATERIAL TYPE and METHOD

U Isotopic [wt% 235U], natural U U Isotopic [wt% 235U], LEU

U Isotopic [wt% 235U], HEU

U Isotopic [wt% 235U], LEU, gammaSpec Pu Isotopic wt% 239Pu Pu Isotopic wt% 240Pu Pu Isotopic wt% 241Pu Pu Isotopic wt% 242Pu

Pu Isotopic wt% 238Pu, alpha-spec

241Am

by HRGS/MGA

UNCERTAINTY OF ROUTINE ANALYSIS RESULT (95% CONFIDENCE INTERVAL, relative) ±0.27 % ±0.12 % ±0.09 % ±0.26 % ±0.03 % ±0.13 % ±0.75 % ±0.37 % ±0.84 % ±1.47 %

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Standard reference materials Required to (1) calibrate analytical methods (2) evaluate the capabilities of analysts and (3) assure the reliability of routine measurements. Commercially available only for uranium and plutonium contents (chemical) and their isotopic composition (isotopic). Not accessible to India Indigenous efforts to prepare and characterise reference materials for thorium, uranium and plutonium. BARC has prepared rubidium uranium Sulphate and potassium plutonium Sulphate reference materials for U and Pu contents respectively. BARC-NFC inter comparison experiments: for characterising U3O8 and ThO2 samples as reference materials for trace elements. 39

Conclusions There are many areas in nuclear reactor fuel technology where analytical chemistry plays a crucial role like fuel manufacture, irradiation behaviour and reactor safety. CQC is an integral and indispensable component in nuclear technology. A wide gamut of analytical methodologies is required for the chemical quality control measurements on nuclear materials. Analytical chemists constantly strive to develop new analytical methodologies to address many issues in nuclear technology.

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