FORMATION AND PROPERTIES OF SOLIDS FORMED DURINGOF SOLIDS FORMED DURING EVAPORATION AND STORAGE OF HIGHLY ACTIVE LIQUOROF HIGHLY ACTIVE LIQUOR.

Barbara Dunnett21st July 2015

Outline

What is highly active liquor (HAL)• What is highly active liquor (HAL).

• Concentration of HAL in the HA Evaporator.

• Highly active storage tanks.

l d f d d• Solids formed during evaporation.

• Solids formed during storage.g g

• Settling properties of solids.

• Rheology properties of solids.

• Summary.Summary.

Highly Active Liquor (HAL)

Nitric acid liquors arising from reprocessing of• Nitric acid liquors arising from reprocessing of irradiated nuclear fuel.

• Fission product (waste) from Pu/U chemical separation process.

• Heat producing up to ~ 2 Watts / litre.

• Activity up to ~ 20 TBq/litre• Activity up to ~ 20 TBq/litre.

• 99% of the radioactivity in the original spent fuel.

• 3 % of the spent fuel is high level waste.

Highly Active Evaporator

• HAL is concentrated by evaporation prior toHAL is concentrated by evaporation prior to interim storage.

• Reduced pressure (~0 1 atmosphere)• Reduced pressure (~0.1 atmosphere).

• Constant volume feed.

• Jacket and coils for heating or cooling.

• Evaporation factors of 20 to 100.

• Batch process.

• Bulk temperature 50-60°C• Bulk temperature 50 60 C.

• Water feed at end to reduce acidity.

Highly Active Evaporator

Highly Active Liquor Storage TanksTanks

Interim storage prior to Vitrification• Interim storage prior to Vitrification.

• In-tank evaporationIn tank evaporation.

• Agitation by jet ballasts and air lifts.

• Internal cooling coils and jacket.

• Bulk temperature of up to ~55°C.

• Liquor additions / transfers.

Highly Active Storage Tanks

HA Evaporator profile

Example of HA evaporator batch profile : 25 GWd/teU PWR.12

) Evaporation phase acid profile

8

10

conc

n. (

g/l p p p

4

6

M)

& M

o c

WAR phase

2

4

Aci

dity

(M acid

profile

00 10 20 30 40 50 60 70

Batch size (teU)

How is HAL simulated?

• Use non active isotopes• Use non-active isotopes.

• Can do full evaporation or ‘bucket mix’.

• A full simulant would be ~90 elements.

• Experience has shown that elements at• Experience has shown that elements at insignificant molar concentrations can be omitted. This gets to ~25 elements which is >90% of the

t id t twaste oxide content.

• Analogues may also be required (e.g. Re for Tc).

• Order of addition is vital.

Getting nit te/ id b l n e i l o i l• Getting nitrate/acid balance is also crucial.

Small Experimental Evaporator

Solid phases formed during evaporationevaporation

Crystallisation of metal nitrates (e g barium• Crystallisation of metal nitrates (e.g. barium-strontium nitrate and magnesium lanthanide nitrate).nitrate).

• Precipitation of micron-sized particles:• Zirconium hydrogen phosphate (ZHP).y g p p ( )• Caesium phosphomolybdate (CPM).

• Solids can: • Settle to base of vessel.P i l bl k i k• Potential to block pipework.

• Increased localised heat output (corrosion).• Be difficult to re-suspend (rheology)Be difficult to re suspend (rheology).

HAL (1 hr settled)

Zirconium phosphates

• Organic and inorganic phosphates• Organic and inorganic phosphates (e.g. Zr(OH)2(HPO4).xH2O).

• Present in evaporator feeds, or formed early in evaporator.y p

• White gel like materials (fine white particles).

• Can be viscous if compacted.

D it 1 3 k /l• Density ~1.3 kg/l.

Zirconium phosphates

• Usually seen to be flocculent which is• Usually seen to be flocculent, which is slowly settling and mobile.

• Increases the viscosity of the liquor.

• Can adsorb other elements (ion-exchange).

Barium strontium nitrate

Crystallises at high acidity/ and or high nitrate• Crystallises at high acidity/ and or high nitrate concentration.

• Readily soluble in weak acid or water.

• Density ~3.2 kg/l.e s ty 3 g/

• Rapidly settling.

• White cubic crystals.

• 5-10% Sr included in Ba nitrate matrix5 10% Sr included in Ba nitrate matrix(e.g. Ba0.9Sr0.1(NO3)2).

• Sr-90 decay yields heat.

Barium strontium nitrate

The crystals display a cubic morphology with isometric• The crystals display a cubic morphology with isometric {100} and {111} faces, as shown below, when grown from any concentration of nitric acid (0 -12M).any concentration of nitric acid (0 12M).

Barium nitrate Strontium nitrate Barium nitrate& 4 mole% strontium

Magnesium Lanthanide Nitrate

a k a Mg rare earth element (MgREE) nitrate• a.k.a. Mg rare earth element (MgREE) nitrate.• Mg3Ln2(NO3)12.24H2O where Ln = La, Pr, Eu, Ce, Nd Sm & YNd, Sm & Y.

• Crystallise at high concentrations. • Crystals dissolve with dilution lower acidity or• Crystals dissolve with dilution, lower acidity or heating.

Magnesium LanthanideLanthanide Nitrate

CPM

Caesium phosphomolybdate [Cs PMo O 14H O]• Caesium phosphomolybdate [Cs3PMo12O40.14H2O].

• Precipitates during evaporation, above 1.8 g/l Mo.

• Mass precipitated limited by availability of molybdenum or phosphorus, caesium being in excess.

• Nucleate very rapidly as very small molecular cluster size (

CPM

Largely insoluble in acid solution• Largely insoluble in acid solution.• Density ~3.8 kg/l, rapidly settling.Adh t f• Adheres to surfaces.

• Cs-137 decay yields heat.

Molybdate chemistry

Complex anions of molybdenum and oxygen• Complex anions of molybdenum and oxygen.

• Complexing with other elements, e.g. phosphorous.

• Range of complexity depending on acidity• Range of complexity depending on acidity.

• Type A Keggin complex most stable in high acidity

[PMo O ]3-[PMo12O40]3-.• Lacunary complexes - loss of molybdate unit.

radiation heat and acidity known causes– radiation, heat and acidity known causes.

– stabilise with suitably sized cation.

• High temperatures may disrupt Lacunary complexes• High temperatures may disrupt Lacunary complexes.

• Zr reacts readily with molybdates -> ZM.

ZM

Zirconium molybdate [ZrMo O (OH) 2H O]• Zirconium molybdate [ZrMo2O7(OH)2.2H2O].

• Form during storage of evaporated liquor.

• Forms by conversion of CPM (via solution).

• Conversion aided by low acidity, high temperature Co e s o a ded by o ac d ty, g te pe atu eand agitation.

• Insoluble in acid solution.Insoluble in acid solution.

• Density ~ 3.4 kg/l, rapidly settling.

Whit / ti l• White/grey particles.

CPM to ZM conversion

ZM Morphology

Morphology variable depending on the solution• Morphology variable depending on the solution from which they crystallise.

• Cubic morphology predominates in nitric acid• Cubic morphology predominates in nitric acid alone.

ZM Morphology

Crystal growth susceptible to interference by a• Crystal growth susceptible to interference by a range of elements in HAL, giving defects in the crystal structurecrystal structure

Rod (e.g. citric acid) Wheat-sheaf (e g tellurium)( g ) Wheat sheaf (e.g. tellurium)

Solids in stored HAL simulant samplessamples

Gravity settling

Viscosity of CPM and CPM:ZM at 50 °C50 C

50:50 CPM:ZM viscosity measurements at 50 °C- Effect of solid loading in 2 M Blend supernate

100

1000Visc at 0.05 s-1

Visc at 0.1 s-1Visc at 0.5 s-1

Visc at 1 s-1 100

1000

CPM viscosity measurements at 50 °C- Effect of solid loading in 2 M Blend supernate

Visc at 0.05 s-1

Visc at 0.1 s-1

Visc at 0.5 s-1

1

10

osi

ty (

Pa.s

)

Visc at 5 s-1

Visc at 10 s-1

1

10

sity

(P

a.s

)

Visc at 1 s-1

Visc at 5 s-1

Visc at 10 s-1

0.01

0.1

Vis

co

0.01

0.1Vis

cos

0.0010 10 20 30 40 50

Solid volume (vol %)

0.0010 10 20 30 40 50

Solid volume (vol %)

Yield stress of CPM, ZM and CPM:ZM beds at 50 °CCPM:ZM beds at 50 C

Summary

• During evaporation of nitric acid liquors arising from reprocessing of nuclear fuel:

• metal nitrates such as barium/ strontium nitrate and magnesium lanthanide nitrate can crystallise at high aciditymagnesium lanthanide nitrate can crystallise at high acidity and/or nitrate/metal concentration.

• Zirconium phosphates andZirconium phosphates and

caesium phosphomolybdate are precipitated.

• During storage of the evaporated liquor caesium• During storage of the evaporated liquor, caesium phosphomolybdate can convert to zirconium molybdate.

• Several morphologies of zirconium molybdate can form.p g y

• NNL have a considerable amount of data on properties of HAL solids for Sellafield Ltd who has kindly provided funding f h kfor the work.

Thank you very much for listening!Thank you very much for listening!

Any Questions?

...and finallyDisposal of standard waste has its own challenges, but there are further materials to be considered which will b kbe even trickier...

...an extreme case is corium/RPVs from Fukushima Daiichi

Even here we are making progress, although disposal probably will not occur within the next 30 yearsprobably will not occur within the next 30 years