# Presentation: 259910

## Nuclear Waste Forms for Actinides:Design and Selection  

**Rod Ewing**
**Geological Sciences**
**Materials Science & Engineering**
**Nuclear Engineering & Radiological Sciences**
**University of Michigan**
**Office of Science Workshop**
**Basic Research Needs for Advanced Nuclear Energy Systems**
**July 31****st****, 2006**

**Notes:**
AFTER THE PU06 MEETING, ONE WONDERS WHY WE WORK ON WASTE FORMS AS THERE SEEM TO BE MANY WAYS TO GET RID OF PU AND THE MINOR ACTINIDES WITH FAST REACTORS OR ACCELERATORS.  BUT DESPITE THE BEST LAID PLANS - A PRUDENT PERSON MUST EXPECT THAT THERE WILL BE WASTE STREAMS IN EVEN THE MOSTS ADVANCED NUCLEAR FUEL CYCLES - AND WE HAVE LEGACY WASTE THAT WILL REQUIRE ATTENTION.

HERE GIVE SOME OF THE HISTORY OF WASTE FORM DEVELOPMENT 

GOES BACK TO HATCH (1953) PROPOSED MINERALS AS NUCLEAR WASTE FORMS - MAINLY CLAYS

1978 First meeting of the MRS symposium.  Rustum Roy’s plenary address, “ Science Underlying Radioactive Waste Management” emphasized the “Neglected Role of the Waste Form”   In subsequent years, the controversey was mainly over glass vs. crystalline ceramics, such as SYNROC.  This fall we will have the 30th meeting of this symposium - the the argument is still going on.

1980’S CRYSTALLINE CERAMICS VS. GLASS AS A WASTE FORM PRIOR TO CONSTRUCTION OF THE DWPF AT SAVANNAH RIVER. 

 Although vitrification was selected for SRS, much excellent work was completed on alternative waste forms. MOST OF THIS WORK IS SUMMARIZED IN NUCLEAR WASTE FORMS FOR THE FUTURE (1988) THIS WAS MAINLY GLASS, SYNROC, SPENT FUEL - FOR HLW (COMPLEX COMPOSITION)

RESURGENCE IN EARLY 1990S FOR DISPOSITION OF EXCESS WEAPONS PU - GO TO NEXT SLIDE

RESURGENCE NOW BECAUSE OF APPLICATIONS IN ADVANCED NUCLEAR FUEL CYCLES.

I want to thank organizers for this opportunity to describe some of the recent work on nuclear waste forms for actinides.  As I will show you in a few moments the motivation for such work comes from two sources:  1.) concern for the fate of the so-called excess weapon Pu from dismantled nuclear weapons and 2.) renewed interest in nuclear power and the development of advanced fuel cycles and Generation IV reactors.

I want  immediately acknowledge collaboration with some of the people in this room:  Bill Weber, Jie Lian, Lumin Wang, Lynn Boatner, Alex Navrotsky and Bryan Chakoumakos who first called pyrochlore to my attention 20 years ago - with a presentation at  MRS in 1984 on actinide pyrochlores.

## Slide 2

**Notes:**
Since 1994 thousands of nuclear weapons have been dismantled, as a result of the first and second Strategic Arms Reduction Treaties, as well as unilaterial agreements between the United States and Russia.  This process of dismantling nuclear weapons as created approximately 37.5 metric tonnes of “excess” weapons plutonium, as well as hundreds of tonnes of highly enriched uranium, that require disposition.  This was the impetus in the mid-1990s for investigating solids that migh be used to immobilize, store and otherwise dispose of this Pu.

## Plutonium Inventories in 2000

**Plutonium Inventories in 2000**

**in spent nuclear fuel (70-100 tons/year)****	****1,000 MT**
**in operating reactors****	**** 80 MT**
**separated by reprocessing of ‘civil” SNF****	****210 MT**
**(France, 73 MT; UK, 60 MT; Russia, 30 MT; Japan 24 MT)**
**military inventories:**
**	****Russia****	****140 MT**
**	****USA****	****100 MT**
**	****others****	****15 MT**
**Estimated World Total****	****1,545 MT**
*Bare critical mass of **239**Pu is  ~ 6 kg*

**Notes:**
3.5 MT/year of 237Np

## Growth of “Civil” Pu-inventory

**Albright and Kramer (2004) Bulletin***** of the Atomic Scientists***

**Notes:**
A graph like this should figure prominently in our thinking as we consider advanced nuclear fuel cycles, particularly those that include breeder reactors.  

Even now roughly 15-20 metric tonnes of plutonium are separated from irradiaed power reactor fuel each year, whil only 10-15 metric tonnes are fabricated into MOX fuel for LWR.

A ten-fold increase in global installed nuyclear capacity over the 1986 level to 3,000 GW would create approximately 500,000 kg (500 MT) of separated Pu per year (Williams and Feiveson (1990).

However, if uranium resource limitations leads to nuclear power systems based on Pu-breeders with reprocessing and recycling as fresh fuel, a 3,000 GW nuclear system will place some five million kilograms (5,000 mT) into global commerce.

## Uranium/Plutonium Fuel Cycle

**U**
**237**

**U**
**239**

**Np**
**238**

**Pu**
**237**

**Np**
**239**

**Np**
**240**

**Pu**
**241**

**Pu**
**243**

**Pu**
**238**

**U**
**238**

**Pu**
**239**

**Pu**
**240**

**Pu**
**242**

**Am**
**243**

**Am**
**241**

**(n,2n)**

**(n,2n)**

**(n,2n)**

**(n,2n)**

**(n,g)**

**(n,g)**

**(n,g)**

**(n,g)**

**(n,g)**

**(n,g)**

**(n,g)**

**EC**

**6.75 **
**days**

**23.47**
**min**

**45.2 **
**days**

**2.117 **
**days**

**2.355**
**days**

**1.032**
**hr**

**99.99****+**

**14.4**
**yr**

**4.956**
**hr**

**=**

**=**

- Short-lived
- Beta Decay

- Long-lived
- Alpha Decay

******-**

******-**

******-**

******-**

******-**

******-**

******-**

**Notes:**
So, setting aside the issue of the fission product yield, particularly the long-term fission products (99Tc and 129I, have half-lives of 200,000 years, and 16 millions years, respectively), you see the problem immediately, neutron capture reactions are creating long-lived actinides, 237-Np, Pu, and 241-Am grows in by decay. From a nuclear engineering or nuclear weapons prospective this is GREAT BECAUSE 239-Pu is fissile.  However, Np, Am and Pu can all be used to make nuclear weapons. The quantity of 237-Np is not small (3.4 mT/yr).   All can be separated by chemical processing.

Again, “It is the Pu, stupid”

## “Minor” Actinides

**“****Minor” Actinides**

**237****Np ****	****3.4 mT/yr**
**241****Am + ****243****Am****	****2.7 mT/yr**
**244****Cm****	****0.35 mT/yr**

**Notes:**
237 Np half-life = 2.1 million years
241 Am = 433 years; 243 Am = 7,370 years
244 Cm = 18 yr

233 U = 160,000 years

Two strategies:  burn in MOX of IMF or immobilization or geologic disposal.

## Strategies for Disposition of Pu

**Use nuclear reactors to “burn” or reduce inventories of plutonium and “minor” actinides.**
**Geologic disposal of spent nuclear fuel and/or immobilized actinides.**

**Notes:**
Embrace the full nuclear fuel cycle with reprocessing and reuse of Pu.

Once through or one-cycle of reprocessing followed by geologic disposal.

I am not choosing between these two alternatives or recommending one over the other  - I am choosing to talk about the latter.  
	- no matter the fuel cycle we will need waste forms for actinides - these waste forms may initially be IMF
	- a prudent person would develop waste forms because the excess Pu develops because the global fuel cycle gets out of balance.  E.G. our ability to reprocess fuel and separate Pu is greater than our ability to get it back into a mixed oxide fuel and into a reactor.

## Near-field containment

**Near-field containment**
**“****Robust” understanding/models of **
**long-term behavior**
**Long-term source term**

## Principal Considerations

**Chemical Complexity of the Waste**
**Large Volumes of Waste**
**Ease of Processing**
**Radiation Safety**
**Reliance on Engineered/Geologic Barriers**
**vs.**
**Advanced Waste Forms with Long-term Durability**

## “New” Strategies

##  single nuclide composition

** ****single nuclide composition**
**  ****long half-life (24,100 years)**
**  ****fissile (decays to ****235****U)**
**  ****multiple oxidation states (3****+****, 4****+****, 5****+**** and 6****+****)**
**  ****transport by colloids**
**  ****toxicity**

## Actinide Waste Forms

**simple oxides:  ****	*****zirconia******	*****ZrO****2**
**complex oxides: ****	*****pyrochlore******	*****(Na,Ca,U)****2****(Nb,Ti,Ta)****2****O****6**
***	******murataite******	*****(Na,Y)****4****(Zn,Fe)****3****(Ti,Nb)****6****O****18****(F,OH)****4**
***	******zirconolite****** ******	*****CaZrTi2O7**
**silicates: ****	*****zircon*******	*****ZrSiO****4**
***	******thorite*******	*****ThSiO****4**
***	******garnet*******	*****(Ca,Mg,Fe****2+****)****3****(Al,Fe****3+****,Cr****3+****)****2****(SiO****4****)**
***	******britholite******	*****(Ca,Ce)****5****(SiO****4****)****3****(OH,F)**
**	*****titanite******	*****CaTiO****3**
**phosphates: ****	*****monazite*******	*****LnPO****4**
***	******apatite*******	*****Ca****4-x****Ln****6+x****(PO****4****)****y****(O,F)****2**
***	******xenotime*******	*****YPO****4**
***durable heavy minerals**

**Notes:**
Properties of waste form phases:  1.) incorporate actinides; 2.) chemically durable; 3.) sustain radiation damage from alpha-decay without deliterious changes in properties, e.g., the leach rate.

Actually, a substantial amount of research on these phases has been completed during the past decade.  There are active programs in the U.S., U.K., France, and Russia.

Note, that many occur as durable mineral phases.

Note, that zircon is the most studied phase, and this is important as it lays the groundwork for the connection between natural minerals, actinide-doped phases and ion beam irradiations.

## Slide 13

## Slide 14

**Notes:**
One to two orders of magnitude increase in dissolution rate.

Determined three ways:

Change in dissolution rate as a function of dose in natural minerals with variable concentrations of U/Th
Actinide doping experiments
-  Ion beam irradiation experiments.

## Weber, Ewing, Catlow, Diaz de la Rubia, Hobbs, . . . (1998) J. Mater. Res.

**Weber, Ewing, Catlow, Diaz de la Rubia, Hobbs, . . . (1998) ***J. Mater. Res.*

## IVEM-Tandem Facility

**IVEM-Tandem Facility**
**Argonne National Laboratory**

## Critical Amorphization Dose 

**Critical Amorphization Dose **
*vs***. Temperature**

**Wang & Ewing (2000) ***Phys. Rev. B*

## Pyrochlore

**A****1-2****B****2****O****6****(O,OH,F)****0-1**** pH****2****O**
**(Ca,Na,U, REE)****2****(Nb,Ta,Ti)****2****O****6****(OH,F)**
**(Ca,Gd,Pu,U,Hf)****2****Ti****2****O****7**
**CaZrTi****2****O****7**** ****(monoclinic)**
**CaPuTi****2****O****7**** ****(monoclinic to cubic)**

**Notes:**
Point out the X- and Y-anions.

Over 500 synthetic compositions.

## Pyrochlore to Fluorite

**Pyrochlore to Fluorite**

**Lian, Wang, Wang, Chen, Boatner and Ewing (2001) ***Phys. Rev. Lett.*

**Notes:**
This is a wonderfully elegant structure.  Apparently simple, but deceptively complex.

Ordering the A-site cations, doubles the unit cell edge.

With the origin at the B-site, there are two types of oxygens: 48f (X) and 8b (Y) (surrounded by A-site cations).  There is an oxygen vacancy at the 8a site. And the vacanices are ordered.

The oxygen atoms shift slightly toward the 8a vacancy site.

The cations and anion vacanices can disorder independently, but clearly compositional changes in the cation can induce anion disorder.

## Gd2Ti2O7 Irradiated with 0.6 MeVAr+

**(A)  No dose; (B) 3.5 x 10****14****; (C) 5.0 x 10****14****; (D) 6.5 x 10****14****; 7.6 x 10****14****  ions/cm****2**

**Wang, Wang, Ewing and Kutty (1999) *****Mater. Res. Soc. Proceedings***

**Notes:**
In this paper we also did GdZr2O7 and first showed its radiation resistance.

## Slide 21

**Notes:**
Note Cm doped samle

Explain the shape of the curve, dose and Tc

We take a higher Tc to mean reduced stability in a radiation stability - higher temperatures are required to caused recrystallization

## Radiation-Induced Amorphization 

**(1 MeV Kr****+****):  Gd****2****(Zr****x****Ti****1-x****)****2****O****7**

**Wang, Begg, Wang, Ewing, Weber and Kutty (1999) *****J. Mater. Res.***

**Notes:**
Other doses for Gd2Zr2O7
1.5 MeV Xe to 36 dpa at 25K

Same relation between the Ti- and Zr-end member compositions seen for Er-titanate and Er-zirconate at what dose in their Science paper in 2000 by Sickafus et al. 

200 keV Ti to 100 dpa at room temperature

## Radiation-Induced Structural Disordering

**Radiation-Induced Structural Disordering**
**Gd****2****Zr****2****O****7**

**(a)**

**(b)**

- O

- Zr

- O

- Gd

- Gd2Zr2O7, fluorite

- Zr/Gd

- 2 nm

**Wang, Begg, Wang, Ewing, Weber and Kutty (1999) *****J. Mater. Res.***

**Notes:**
Radius ratio for Gd:Ti = 1.46; Radius ratio for Gd:Zr = 1.14

## Energetics of Disordering

**Sickafus et al. (2000) *****Science***

**Notes:**
In the same paper where they noted the radiation stability of the Er zirconate pyrochlore, Scickafus et al. did some very nice calculations on the energetics of the disordering process.

This is a calculation of the energetics of disordering the A and B- site cations.

This outlines for us a good set of experiments: Ti, Sn, Zr, and also the Hf (ionic radius = 0.71 S-P)

Note, we would expect the REE titanate pyrochlores to be very similar.

As the Ra:Rb ratio approaches 1 then the fluorite structure is more stable.  Either Ra decreases or Rb increases 

## Periodic Table of the Elements(electronegativity)

_**Ti**_
**1.54**

_**Zr**_
**1.33**

_**Hf**_
**1.3**

_**Sn**_
**1.96**

- _Mg_
- 1.31

- _Ac_
- 1.1

- _Pr_
- 1.13

- _Am_
- 1.3

- _Bk_
- 1.3

- _Cf_
- 1.3

- _Es_
- 1.3

- _Fm_
- 1.3

- _Md_
- 1.3

- _No_
- 1.3

- _U_
- 1.38

- _Np_
- 1.36

_**Nb**_
**1.6**

- _Cd_
- 1.69

**Notes:**
Busy chart, the numbers are the electronegativities - always a good way to characterize bond type.

We can look at:

Look at Group 4A elements (same valence electron structure (4+) with 2 3d, 4d, 5d electrons, but with increasing size: Ti (0.61A), Zr(0.84) and Hf(0.84))

These can be combined with the lanthanides (3+) [important because of the fission product lanthanides and they give insight into behavior of 3+ and 4+ actinides]

With increasing atomic number we are adding electronis to the 4f shell (14 electrons), ionic radius contracts from Ce (1.14) to Lu (0.97) and electronegativity increases slightly making for slightly more covalent bonds.

In some cases we have multiple valence states: Ce3+ (1.14) and Ce4+(0.97)

The actinides are there because of doping experiments for radiation damage (Pu and Cm), but the Th and U in natural and doped samples provide valuable information on radiation response as actinide concentration increases.  Th(1.06),U (1.00), Pu. (0.96), Cm (0.95).

We can also look at II-V pyrochlores (Cd-Nb), where the electronegativity difference is less (more covalent)

Finally, we can have a look at much more covalent bonding by studying the <Sn-O> bonding, where the Sn (column IVB) has the highest electronegativity value (1.96).  Sn4+ = 0.690 A (rather much smaller than the other B-site cations 

## REE2Ti2O7 Irradiated with 1 MeV Kr+

**REE****2****Ti****2****O****7**** Irradiated with 1 MeV Kr****+**

**Lian, Chen, Wang, Ewing, Farmer, Boatner and Helean (2003) *****Phys. Rev. B***

**1****E+13**

**1****E+15**

**2****E+15**

**3****E+15**

**4****E+15**

**5****E+15**

**0**

**200**

**400**

**600**

**800**

**1000**

**1200**

**6E+15**

**T****c**** (K)**

**Amorphization fluence (Ions/cm****2****)**

**Notes:**
Note, there are really two parameters that one can use to speak of the radiation response:  1.) critical amorphization dose at low temperatures (e.g., room temperature) -  vary by factors of 2 to 5but the uncertainty of the measurement is high; 2.) critical temperature, which really is a measure of the tendency recovery to dominate over damage accumulation.  For waste disposal applications the critical temperature is an important parameter as we want to characterize the damage as a function of temperature in a repository where the temperature varies (decays) as a function of time.

Note, Gd-titanate has the highest critical temperature, 1120 K.

Also, think of this as a phase diagram with amorphous and crystalline domains.

600 K range

La = 1.04 A
Lu = 0.8   A

## Critical Temperature, Tc, vs. 

**A-site:B-site Radius Ratio**

**Ewing, Weber and Lian (2004)***** J. Applied Physics***

## Tc vs. Gd2(ZrxTi1-x)2O7

***Ewing, Weber & Lian (2004) J. Applied Physics***

**Notes:**
We can develop models of the change in critical temperature and the zirconium content when the thermal recovery process is dominant.

This is essentially the balance between the the activation energy of he dominatn thermal process and the local cross-section for amorphization and damage accumulation.  Ideally, we do this for specific actinide-bearing hosts.

Although the pure Zr concentration will not amorphize, as long as the temperature of the repository is above 300 to 550 K then there will be no significant accumulation of damage.

## Temperature Dependence of 

**Amorphization for Pu-pyrochlore**

***Ewing, Weber & Lian (2004) J. Applied Physics***

**Notes:**
Clearly, the critical temperature of Gd2Ti2O7 is above the anticipated range of repository temperatures, hence damage will accumulate and it will become amorphous.  For the Pu-bearning phases the critical temperature is lower and thermal annealing can prevent amorphization 

In the case of Ca239PuTi2O7 amorphization will not occur for several decades until the temperature falls below 520K.  Amorphization of 239Pu-containing Gd2ZrTiO7 will be delayed for 1000 years or more, until the temperature falls below about 330 K.

## Storage Time vs. Amorphization

**Ewing, Weber & Lian (2004)***** J. Applied Physics***

**Notes:**
Based on the fundamental understanding of the damage accumulation and annealing processes, it is possible to predict the dose and thus the time dependence of amorphization in actinide-host phases under repository conditions.

Gd2Ti2O7 will amorphize in about 1,000 years.

Gd2ZrTiO7 will be delayed by 1,000 years while the temperature cools below the critical temperature and then time to amorphization will proceed and full amorphization will occur in 3,000 years.

Gd2Zr2O7 will not amorphize but will disorder to the defect-fluorite structure type.

MAIN POINT:    We can design waste  forms for the waste stream composition and the repository conditions.

## Conclusion

**We can now design actinide waste forms with specific performance characteristics as a function of:**
**  ****dose**
**  ****temperature**
**  ****geochemical durability**
**  ****time**

## Research Priorities

**  ****Systematic* studies (experimental and computational) of actinide-(radionuclide) incorporation into ceramics (use the 5f electrons as a structural probe).**
**  ****Systematic* studies of chemical durability (rates and mechanisms) over a range geochemical, hydrologic, technological conditions.**
**  ****Development of new synthesis technologies.**

## Colleagues & Collaborators

**William Weber  (PNNL)**
**Lumin Wang (University of Michigan)**
**Al Meldrum (University of Alberta)**
**Shixin Wang (University of Michigan)**
**Jie Lian (Univeristy of Michigan)**
**Wendy Panero (University of Ohio)**
**Lynn Boatner (ORNL)**
**Kate Helean (UC - Davis)**
**Alex Navrotsky (UC - Davis)**
**Sergey Yudintsev (IGEM, Moscow)**
**Sergey Stefanovsky (SIA Radon, Moscow)**

## EXTRA SLIDES

## Zirconia to Pyrochlore to Murataite

**Murataite, 3M**

**Cubic zirconia, 1M**

**Pyrochlore, 2M**

**Zirconia ****to**** Pyrochlore ****to**** Murataite**

**Notes:**
3x3, 5x5, 8x8

## Gd2Ti2O7 Irradiated with 1.0 MeV Kr+

**(A)**

**(B)**

**(C)**

**(D)**

**5 nm**

**Wang, Wang, Ewing and Kutty (1999) *****Mater. Res. Soc. Proceedings****.*

**Notes:**
Note, halving of lattice fringes

##  Must incorporate actinides, be chemically 		durable and radiation “resistant”:

**Nuclear Waste Forms**

** ****Must incorporate actinides, be chemically ****	****	****durable and radiation “resistant”:**
** ****multi-element, often of complex composition**
**  ****multiple valence states**
**  ****variety of bond types**
**  ****often, lower symmetry structures**
**  ****best described by size, shape and connectivity ****	****   ****	****of coordination polyhedra**

## Gd2Ti2O7: Nano-Scale Ionic Conductors

**Gd****2****Ti****2****O****7****: Nano-Scale Ionic Conductors**

**Lian, Wang, Wang, Chen, Boatner and Ewing (2001) ***Physical Rev. Letters*

## Enthalpies of Formation

**Lian, Helean, Kennedy, Wang, Navrotsky and Ewing**
**(2006)***** J. Physical Chem. B***

**Notes:**
With increasing cation ionic radius ratio (I.e., a relative increase in the size of the A-site cation), the formation enthalpies from the oxides become more negative, consistent with the increased stability of the pyrochlore structure as the ionic radius ratio increases.  The stannates deviate, steeper slope - but the covalency of the <Sn-O> bond is increasing.

## Pourbaix Diagram for Pu

***W. Runde (2000) Los Alamos Science***

**Notes:**
Eh vs. pH diagram calculated for Pu in water containing hydroxide, carbonate and fluoride ions - comparable  to those found in J-13 well water at Yucca Mountain.  Pu concentration is fixed at 10 -5 M

The upper and lower parallel lines define the stability field for water.

The point is that all the solubility of Pu oxide and hydroxides is generally very low, from a geochemical perspective the multiple oxidation states provide a mechanism for dissolution, transport and precipitation.  This is why we have uranium deposits, formed by the sequential oxidation and transport of U(VI)  followed by reduction and precipitation of  U(IV).  We do not see the same types of concentration of Th in the earth’s crust because it has a single (IV) oxidation state - even though it is more abundant than U (10 ppm vs. 3-4 ppm in the crust).