WSRC-TR-2003-00127

Aqueous PUREX Solidification in Saltstone

A.D. Cozzi and C.A. Langton
Westinghouse Savannah River Company
Aiken, SC 29808

Key Words: Saltstone, Aqueous PUREX, Grout Waste Form

1.0 Summary

Saltstone samples prepared with simulated aqueous PUREX blended with salt solution hold tank solution meet the processing (gel time, set time and bleed water) and compressive strength requirements for Z-Area treatment and disposal.

New Solvent Storage Tanks (NSST) 33 and 35 contain approximately 900 gallons (3,400 L) and 10,800 gallons (40,870 L) of the aqueous PUREX waste, respectively. The Solid Waste Division (SWD) identified stabilization in Saltstone as the preferred treatment for the aqueous fraction of the PUREX waste stored in Tank 33 and Tank 35. A surrogate aqueous PUREX solution representing a 1:10 (by volume) Tank 33:Tank 35 mixture was produced from prior analysis of the tanks1.

Three mixes were prepared using the surrogate and Tank 50 solution from the Salt Solution Hold Tank (SSHT). The blends used were (by weight) 2:1, 1:1, and 0.5:1 Tank 50 salt solution:aqueous PUREX. All of the mixes gelled between 50 and 130 minutes. The 1:1 and 2:1 mixes set within five days. The 0.5:1 mix did not set after seven days. Subsequent to the processing evaluation, compressive strength samples were prepared from the 1:1 and 2:1 blends. The 1:1 Tank 50 salt solution:aqueous PUREX mix had an average compressive strength of 1.78 MPa (258 psi) and the 2:1 Tank 50 salt solution:aqueous PUREX mix had an average compressive strength of 2.24 MPa (325 psi).

When the aqueous fraction of Tank 33 and Tank 35 are composited, mixes will be prepared using the composited PUREX and Tank 50 solution to produce samples for processing evaluations and testing using the Toxic Characteristic Leaching Procedure (TCLP).

2.0 Introduction

2.1 Objective

The objective of this report was to confirm Saltstone as a treatment and disposal option of the aqueous PUREX by blending it with the Tank 50 salt solution purposely left in the SSHT2.

2.2 Approach

2.2.1 Surrogate Preparation

The analytical results in Reference were used to calculate a compositional blend of the Tank 33 and Tank 35 aqueous PUREX. Table 0-1 is the calculated blend used to develop a surrogate of the aqueous PUREX. Table 0-2 is the composition of one liter of the surrogate used for blending with material in the SSHT. The surrogate was 1.2% salt (98.8% water).

Table 2-1. Calculated Blend of Tank 33 and Tank 35 Aqueous Fractions.

Parameter

Tank 33

Tank 35

Blend

Volume (Gal)

900

12800

13700

PH

9.83

7.66

--

Density (g/mL)

1.0478

1.0008

--

       

Composition

μg/mL

μg/mL

μg/mL

NO3

2,786

<100

253

NO2

1,686

<100

153

C2O4

764

87

149

COOH

1,397

<100

127

Na

23,103

1,621

3,574

Tributyl Phosphate

1,600

130

264

Dibutyl Phosphate

85,465

3,769

11,196


Table 2-2. Composition Used for
One Liter of Aqueous PUREX Surrogate.

Chemical

Grams

Water

988.00

NaNO3

0.35

NaNO2

0.23

Na2C2O4

0.30*

NaCOOH

0.19

Tributyl Phosphate

0.26

Dibutyl Phosphate

11.20

Total

1000.53


*Targeted 0.23 g

2.2.2 Saltstone Mix Preparation

The maximum amount of Tank 50 salt solution (Tank 50 salt solution 24% salt; 76% water) that could remain in the SSHT and process the entire aqueous PUREX fraction (~14k gallons, Table 1) in one SSHT batch is ~36k gallons. The 50,000 gallon SSHT currently contains approximately 11,000 gallons of Tank 50 solution. In this study, these blending scenarios are represented by three blends of Tank 50 salt solution from the SSHT to aqueous PUREX. The blends used were 2:1, 1:1, and 0.5:1 by mass of Tank 50 salt solution:aqueous PUREX. These ratios tested would represent volumes of Tank 50 salt solution in the SSHT from ~7k to ~28k gallons.

The Saltstone mixes were calculated to have water to premix ratios of 0.6. The mass of premix (400 grams) and proportions of the components of the premix were held constant. For a fixed (400 grams) amount of premix, 240 grams of water is required. An algorithm was used to achieve the blends of the Tank 50 salt solution and aqueous PUREX while maintaining the proper amount of water. Samples were prepared by blending simulated aqueous PUREX with SSHT material and premix cement, slag, and fly ash from Z-Area. The ingredients and mix proportions in these Saltstone samples are summarized in Table 0-3. The Saltstone mixes were prepared in the 704-Z laboratory by Z-Area operators3. Gel time, set time and bleed water was determined for each of the mixes.

Table 2-3. Ingredients and Proportions of the
SSHT/Aqueous PUREX Saltstone Samples.

Ingredient
(Grams in sample/Wt. % total mix)

Mix 61

Mix 62

Mix 63

Cement (8% of Premix)

32
4.8 %

32
4.8%

32
4.7%

Slag (46% of Premix)

184
27.8%

184
27.3%

184
26.8%

Fly Ash (46% of Premix)

184
27.8 %

184
27.3%

184
26.8%

SSHT:Aqueous PUREX

2:1

1:1

0.5:1

Salt Solution Hold Tank (g)
Water from SSHT (g)

88
67

137
104

191
145

Aqueous PUREX (g)
Water from AP (g)

175
173

137
135

96
95

PreMix (wt%)
Cement-slag-fly ash = 8, 46, 46 wt %

60.3

59.4

58.2

Water to premix ratio

0.6

0.6

0.6


3.0 Results

Slurry processing results and compressive strength results for samples cured for 28 days are summarized in Table 0-1.

Table 3-1. Properties of the SSHT/Aqueous PUREX Saltstone Samples.

Property

Mix 61

Mix 62

Mix 63

SSHT:Aqueous PUREX

2:1

1:1

0.5:1

Gel Time (minutes)

130

100

50

Set Time (days)

*

2

2

Standing Water

0 @1 day

0 @1 day

0 @1 day

Compressive Strength† (psi)

--

258

325

*Did not set.
†After 28 day set.

Acceptable gel times are in the range of 30 to 120 minutes. Setting should occur within the first three days after mixing and acceptable formulations have zero standing water after three days curing in a closed container. The compressive strength after curing for 28 days must be at least 200 psi. All of the mixes gelled the day of preparation. The mixes were evaluated after 24 hours for set time and bleed water. There was no bleed water evident on any of the mixes. Mix #61 did not set in the required 72 hours and, did not set after seven days. Mixes #62 and #63 were reproduced to produce samples for compressive strength testing. Mix #62 had an average compressive strength of 1.78 MPa (258 psi) and Mix #63 had an average compressive strength of 2.24 MPa (325 psi).

4.0 Conclusions

Saltstone laboratory samples made with SSHT solution and an Aqueous PUREX surrogate met the Z-Area processing and compressive strength requirements. Acceptable formulations were prepared with a premix blend of 8 wt. % cement, 46 wt. % slag, and 46 wt. % fly ash. The premix was mixed with the SSHT solution – Aqueous PUREX at a water to premix ratio of about 0.60.

5.0 Path Forward

After the aqueous fraction of Tank 33 and Tank 35 have been combined, a sample of the combined aqueous PUREX will be tested in the Z-Area lab. The blends tested will be determined by knowledge of the volume of both the aqueous PUREX and the SSHT. The mix (or mixes) will be evaluated for processing properties, compressive strength and a minimum of one mix will be tested using the toxic characteristic leaching procedure (TCLP) at a laboratory certified by the state of South Carolina.

6.0 References

  1. WSRC-TR-2001-00546, C.A. Langton and G.M. Iversen, "Stabilization of Tank 33 and 35 Aqueous PUREX Waste (U)" 12/2001.
  2. M.G. Looper, Personal Communication, 10/29/2002.
  3. Procedure 704-Z-4401, "Preparation of Simulated Grout Sample (U)."