ISOTC85/SC5

Date:2010-06-08

ISO/NWIP – PART2

ISOTC85/SC5/WG1

Secretariat:BSI

NUCLEAR FUEL TECHNOLOGY - Dissolution of Plutonium Dioxide-Containing Materials

Dissolution d’échantillons contenant du plutonium dioxide

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This document is not an ISO International Standard. It is distributed for review and comment. It is subject to change without notice and may not be referred to as an International Standard.

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ISO/NWIP – PART2

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ContentsPage

1SCOPE......

2NORMATIVES REFERENCES......

3SUMMARY OF THE METHOD......

4APPARATUS AND REAGENTS......

4.1Apparatus......

4.2Reagents......

5SAMPLE DISSOLUTION......

5.1Procedure for common plutonium containing materials......

5.2Solution Adjustment (Optional)......

6QUALITY CONTROL......

Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

International Standards are drafted in accordance with the rules given in the ISO/IECDirectives, Part3.

The main task of technical committees is to prepare International Standards. Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75% of the member bodies casting a vote.

Attention is drawn to the possibility that some of the elements of this part of ISOxxxx may be the subject of patent rights. ISO shall not be held responsible for identifying any or all such patent rights.

ISOxxxxwas prepared by Technical Committee ISO/TC85, NUCLEAR ENERGY, Subcommittee SC5, Nuclear Fuel Technology.

ISOxxxxconsists of 2 parts, under the general title NUCLEAR FUEL TECHNOLOGY— Dissolution of Plutonium dioxide-Containing Materials.

Part 1 - Dissolution of Plutonium DioxidePowders

Part 2 – Dissolution of MOXMaterials

Introduction

©ISO2003— All rights reserved / 1

ISO/NWIP – PART2

Dissolution of Plutonium Dioxide-Containing Materials

Part 2: Dissolution of MOX Materials

1SCOPE

This document specifies how samples consisting of MOX pelletsor powders have to be dissolved to provide suitable aliquots for subsequent analysis of elemental concentration and isotopic composition.

2NORMATIVES REFERENCES

ISO 3696, Water for analytical laboratory use – Specification and test methods

3SUMMARY OF THE METHOD

Among the factors affecting the formation of solid solution and hence, the ease of dissolution is:

- the method of fuel fabrication (i.e. whether mechanically blended oxides, co-precipitated oxides, or sol-gel oxides.

- The degree of sintering

The radiological hazard of plutonium and the need to minimize the waste have to be taken into account when choosing the mass of the sample to be dissolved. A MOX mass of0,1gto 10g should be sufficient for most of the analyses. Some analysis may however require more material.

For the highest possible assay accuracy only gravimetric dissolution methods are recommended. However for a less critical assay, volumetric dissolution may be appropriate.

For g-size powders, the sample is transferred in a pre-weighed flask or vial. glass, high density polyethylene (HDPE), TeflonTM, NalgeneTM or other container types, but the choice of the container has consequences:

- when using a glass dissolution vessel the added fluoride will attack the glass (resulting in a falsification of the pre-weighed tare) but with the benefit that the fluoride is ultimately removed from the solution via the escape of volatile SiF4. The resulting reduction in fluoride content of the final solution guards against the formation of insoluble PuF3 (especially below 8M), which wouldnegatively bias the plutonium element content up to 0.1 % (or more when large quantities of HF are used). The tare weight of the etched glass vessel may be re-established by careful cleaning, drying and re-weighing of the vessel.

- HDPE, TeflonTM or NalgeneTM dissolution vessels are not reactive to HF, therefore tare weight remains unaffected. However,these materials have the disadvantage that fluoride ions in the dissolution liquor promote the precipitation of insoluble PuF3 when the oxidation state of plutonium is reduced from IV to III (e.g. by later volume adjustments using a low molarity acid).

An aqueous dissolving mixture comprising nitric acid (about 8 to 14 mol.l-1) and an effective catalytic amount of fluoride ions by either hydrofluoric acid (about 0,05 mol.l-1 to 0,1 mol.l-1) or ammonium bifluoride(about 2 g.l-1)is added and the pre-weighted dissolution vesselclosed with a reflux cap (or watch glass). The vessel is placed on a hotplate and heat is applied until the dissolution is completed. The solution is now prepared to get a concentration of plutonium and uranium per gram of solution suitable for subsequent analytical methods.

4APPARATUS AND REAGENTS

Because of the radiological hazard of Plutonium, it must be handled in glove boxes or hot cells and extraordinary care must be taken to prevent direct contact, inhalation or ingestion.

4.1Apparatus

  • Ananalytical balance, capacity up to 1200 g, with an accuracy of 1 mg
  • Dissolution vessels: dissolution flask or vial with a Teflon-lined screw cap. The volume of the flask should be in relation to the volume of solution for the appropriate concentration required for subsequent analysis. Teflon lined screw cap is recommended for the samples which will be stored for long term.
  • Glass or TEFLON reflux capor watch glass
  • Reflux condenser
  • Shielded hotplate equipped with a temperature control unit and a fume hood.
  • Teflon coated tweezers.
  • Normal laboratory equipment for a plutonium laboratory

4.2Reagents

Use only reagents of recognized analytical grade.

Prepare the reagents in compliance with the local laboratory safety instructions (Nitric acid and hydrofluoric acid are very corrosive and can cause painful burns; HF is particularly dangerous and proper safe working instructions must be available and understood).

4.2.1Water, complying with grade 3 of ISO 3696 is recommended

4.2.2Concentrated Nitric Acid, w(HNO3) = 65%, c(HNO3) =14,4 mol L-1(density C~1,39)

4.2.3Nitric Acid, c(HNO3) = C2mol L-1prepared by dilution of concentrated nitric acid (4.2.2) with water (4.2.1).

4.2.4Hydrofluoric acid,w(HF) = 40%, c(HF) = 22 mol L-1stored in a polyethylene dropper bottle.

4.2.5Ammonium bifluoride, NH4HF2

4.2.6Mixed nitric-hydrofluoric acid solution, c(HNO3) =14 molL-1or 7 to 8 mol L-1 , c(HF) = 0.05 to 0.1mol L-1

This reagent can be prepared in various ways. To prepare the solution a procedure can be:

In a 200 ml polyethyleneflask,

- Addabout 1 ml of hydrofluoric acid (4.2.4),

- Adjust the volume of the flask with concentrated nitric acid (4.2.2)or 7 to 8 mol L-1 nitric acid (4.2.3)

- Homogenize.

Note 1: Never add concentrated HF to a plutonium nitrate solution since this may cause precipitation of PuF4 which may not be re-dissolved.

4.2.7Mixed nitric-ammonium bifluoride solution, c(HNO3) =14 mol L-1, c(NH4HF2) = 2 g.l-1 c(HF-) = 0.07 mol L-1

4.2.8This reagent is prepared by dissolution of ammonium bifluoride (4.2.5) in concentrated nitric acid (4.2.2).

5SAMPLE DISSOLUTION

5.1Procedure for common plutonium containing materials

  1. Calibrate the balance and check the calibration with appropriate standard weights.
  1. Weigh a labeled dissolution vesselwith a labeled cap(M1).
  1. Transfer the sample into the vessel. Be very careful when transferring powder or crushed pellet to avoid the presence of sample on the outside or in the neck of the vessel since material on the outside will induce a weighing error and powder in the neck of the vessel is difficult to recover.
  1. Powders are also hygroscopic and therefore prone to moisture uptake. The weighing of powders should be carried out confidently and quickly with minimum exposure to the analytical cell atmosphere. It is preferable that such of manipulations are done in a dry inert atmosphere
  1. Close the vessel and weigh it again to obtain the gross weight(M2).

Sample mass is the difference M2-M1

  1. Calculate the quantity of the mixed nitric-hydrofluoric acid solution (4.2.6) or mixed nitric-ammonium bifluoride solution (4.2.7) needed,V1,which is proportional to the sample mass and add it to the weighed sample(i.e.about at minimum 5 ± 1 ml of solution for 1 g of solid sample are needed). It can be more according to the requirement for instance Pu concentration and acidity of the analysis performed on the solution. However larger amounts acid will result in an increased attack of glass dissolution vessels or may cause increased PuF3 formation when no-glass vessels are used.

Although the solution volume reduces decrease with heating, the heavy metal concentration for MOX product is estimated by the following formula:

(Eq. 1A)

Where

(M2-M1)is the sample mass in g,

V1 is the volume of the mixed nitric-hydrofluoric acid solution or mixed nitric-ammonium bifluoride solution in ml

0.88 is the Pu/Pu02mass fraction.

  1. In the case of MOX powder or crushed pellet the acid should be added slowly due to the high reaction rate between the acid and the UO2 powder. For MOX pellet the acid can be added in one step.
  1. Swirl the vessel to bring the whole sample in contact with the acid.
  1. Place the cap on the vessel.
  1. Weigh the labeled vessel with the cap (M3)
  1. Replace the cap with a reflux cap (or watch glass) and store the original cap in a safe place.
  1. Put the vessel on the hotplate. For pellets the temperature controller should be set in a way to bring the solution to a boil. In this case the dissolution has to be performed under reflux conditions. For powder or crushed pellet the temperature controller should be set at 70°C to 110°C to avoid boiling.
  1. Make a visual assessment of whether dissolution is complete, (a green, brown or green/brown colored solution, no more reaction, no undissolved residues.). Otherwise, continue the heating procedure.

For pellets a normal time of about 18 hours or more may be necessary. Occasional gentle swirling will speed up the dissolution.

  1. After cooling half an hour to two hours weigh the labeled vessel with the cap (M’3) and check that mass lost during dissolution is less than that defined in accordance with the intended accuracy of the analysis performed later.

Note 2: This is important when a volumetric analytical method is used. For gravimetric methods this requirement is less important.

For instance criteria can be < 1%

In the solution of dissolved MOX the heavy metal concentration can then still be estimated by Eq. 1A

5.2Solution Adjustment (Optional)

Depending on the choice of the analytical method, the obtained dissolved MOX solution may need further dilution with pure nitric acid solution to arrive at an appropriate cocentration. For example the target value for assay by K-edge densitometry would be 150g HM/l, whereas potentiometric titration or spectrophotometricassay requires far lower concentration.

Note 3: Never add pure water to a plutonium nitrate solution as this results in the formation of Pu colloids which may remain invisible to some analytical techniques. When volume adjustments are needed diluted nitric acid solution rather than water should be used.

Some techniques may also have specific requirement regarding the molarity of the final solution matrix. Using spectrophotometry as a specific example, optimum assay conditions may stipulate 20 g Pu/l in an 8mol L-1matrix of nitric acid. In this instance a PuO2mass of 0.5g and an amount of V1 of 10ml of fluorated nitric acid is appropriate and is later adjusted with 6 ml of a 0.2mol L-1nitric acid solution (V2).

  1. Calculate the volume V2 of HNO3C2 mol.l-1l (4.2.3) to add to the obtained HM solution in order to adjust the HM concentration and acidity :

HM concentration in g.l-1in the adjusted solution can be estimated by the following formula :

To obtain element concentrations for U and Pu requires prior knowledge of the approximate U/Pu ratio of the MOX material.

Acidity in mol.l-1in the adjusted solution can be estimated by the following formula:

Where

(M2-M1)is the sample mass in g,

V1 is the volume of nitric-hydrofluoric acid solution or mixed nitric-ammonium bifluoride solution in ml,

V2 is the volume nitric acid solution for adjustment in ml,

0.88 is the conversion Pu/Pu02and U/UO2 coefficient,

C1 is the acidity of the concentrated nitric acid (about 14 mol L-1),

C2 is the acidity of the nitric acid used for adjustment.

  1. After introducing V2 of nitric acid solution (4.2.3) with a density ρ2vessel is weighted (M4):

So:

And

6QUALITY CONTROL

Details given in point 5 regarding the dissolution temperature and the heating duration are dependant of the material to be dissolved (ie. pellet, powders or crushed pellets).

If changes in material types for example due to changes in the production process, occurs then the dissolution procedure should be critically reviewed and if necessary adjusted to optimize dissolution efficiency. Such a critical review involves a reproducibility study of chosen dissolution conditions.

Tools available for monitoring the effectiveness of the dissolution range from carefully visual inspection to filtration of the final solution eventually followed by characterization of the filter residues.

In accordance with the intended precision of the analysis the number of dissolutions on the same sample or on different samples is defined. A Good analytical practice would be to make at least two parallel dissolutions.

©ISO2003— All rights reserved / 1