VITRIFICATION OF HIGH LEVEL WASTE AT HANFORD
Patrice Roux, E. Nicaise, S. Goetghebeur
SGN 1 rue des Hérons - 78182 Saint Quentin en Yvelines
ABSTRACT
In the framework of the Hanford Tank Waste Remediation System (TWRS), the DOE has chosen a multi-step privatization approach with a phase 1A proof of principle effort followed by a Phase 1B including the design, construction and operating of an active vitrification plant for Low Active Waste (LAW) and High Level Wastes (HLW).
In September 1996 a team led by Lockheed has been awarded by DOE for the Phase 1A proof of principle effort. Within this team SGN/COGEMA.Technologies, Inc. (formerly NUMATEC, Inc.) was the technical leader for the design of the high level waste vitrification facility.
The Phase 1A study was completed in January 1998 and this paper presents the main features of the proposed HLW Vitrification Facility with in particular the benefit of the implementation of the cold crucible technology developed by the French Commissariat à l’Energie Atomique (CEA) and main test results.
The cold crucible technology with direct induction heating in the molten glass bath allows high temperature, high glass throughput while keeping the maintainability related to a small metallic melter and its outstanding resistance to corrosion. This melter technology is therefore quite responsive to the Phase 2 issues and offers the best opportunity to address the Hanford challenge.
INTRODUCTION
Radioactive wastes have been stored in large underground storage tanks at the U.S. Department of Energy’s (DOE) Hanford Site since 1944. About fifty four million gallons of waste are currently being stored in 177 tanks. Some 40 different waste types have been generated from several nuclear spent fuel reprocessing and radionuclides recovery activities. These wastes are highly caustic and present in the form of liquids, slurries, saltcakes and sludges. The volume, complexity and variability of the waste make the Hanford tank’s clean up a unique challenge in the world.
The remediation pathway goes through the separation of these waste in two streams, a large Low Active Waste (LAW) stream and a minimized High Level Waste (HLW) stream. Both streams are to be vitrified.
In the framework of the Tank Waste Remediation System (TWRS), the DOE has chosen a multi-step privatization approach with a Phase 1A Proof of Principle effort followed by a Phase 1B including the design, construction and operating of an active plant (5 or 9 years of operation). A Phase 2 larger plant should complete the vitrification of the remaining waste from around 2010 to 2028 but is not yet requested.
In September 1996 a team led by Lockheed has been awarded by DOE for the Phase 1A proof of principle effort. Within this team SGN was the technical leader for the design of the high level waste vitrification facility.
MAIN OBJECTIVES OF THE HLW FACILITY
The HLW facility has to treat and vitrify the HLW stream defined in Envelope D and the Intermediate Waste Products issued from LAW pretreatment.
The considered HLW glass is a borosilicate glass, it must comply with the Waste Acceptance Product Specification for Vitrified High Level Waste Forms (WAPS).
The waste loading shall be a minimum of 25% by weight in the HLW glass on an equivalent oxide basis with no credit for the Na2O and SiO2 which leads to an actual waste loading of 40% minimum.
The facility must vitrify 245 metric tons (Na and Si excluded) in 5 years maximum, which means about 50 kg/h of glass.
The glass canister could be either a system used at Savannah River DWPF or West Valley Demonstration Project (WVDP) or a new one with the same diameter but 50% taller.
MAIN CHALLENGES OF THE HLW FACILITY
The factors of success of the HLW Facility rely on :
Workability of the glass
The workability range of the glass must comply with waste variability and allow :
and should be extendable to the more complex waste to be treated in Phase 2
This means that the temperature range for glass working must be 1200 to 1300°C or higher.
Melter Reliability
The melter must be reliable (low frequency of breakdowns and long life duration) despite :
These conditions are very stringent, therefore it is necessary to design for tolerant conditions near the melter walls which leads to a cold wall crucible.
Maintainability
Equipment maintainability is nearly as critical than the vitrification process for a HLW vitrification facility and relies on :
The CEA two step process with small metallic melter heated by induction has demonstrated its maintainability.
Minimization of Secondary Waste
The Minimization of Secondary Waste relies in priority on HLW waste minimization :
but also capabilities of:
French HLW vitrification process based on industrial experience
The Hanford HLW vitrification process proposed for the phase 1 active pilot is derived from the one successfully applied at vitrification facilities in Marcoule, La Hague, and Sellafield. The SGN vitrification process developed by CEA and operated by Cogema is a continuous two-step process in which liquid waste is calcined in a rotary calciner and melted with glass frit in a metallic crucible melter heated by induction.
By now more than 10,000 glass canisters (more than 4000 metric tons of glass) have been generated by the vitrification of HLW waste with the CEA process. Vitrified residues specifications have been accepted by Cogema Customers Safety Authorities and transportation to Japan and Germany has been already initiated.
HLW vitrification process customized for Hanford
In Hanford, to adress the complexity of the mixture of radioactive and non-radioactive chemicals with corrosive species, the high targeted waste loading (more than 40% with Na and Si included) and the ability to be scaled up for next phase 2, SGN has selected an advanced cold crucible melter developed by French CEA for 15 years and which can replace the hot metallic melter heated by induction in the double step CEA process. This melter can also be liquid fed (HLW Vitrification Facility, Sallugia Project, design and construction, 1997-Italy) with a smaller glass throughput.
In the cold crucible melter the bath of molten glass is heated by direct induction while the walls are cooled to freeze a protective glass layer .
The cold crucible has been ranked first with High Temperature Joule Heated Melter in 1994 by an international expert committee to address the HLW vitrification of Hanford Waste.
Overview of the Process
The HLW feed consists of insoluble suspended solids, containing most of the materials to be vitrified and most of the activity, in an aqueous slurry. After reception in the facility stirred reception tank the HLW feed is sampled and analyzed. Sludges are (i) separated from the supernatant in a continuous centrifuge; (ii) acidified by concentrated nitric acid; (iii) mixed with secondary products separated from the LAW feed and with dust scrubbing solution recycled from the calciner off-gas treatment; (iv) further concentrated if necessary by evaporation to reach the nominal oxide concentration for calciner feeding.
The calciner is fed alternately from two tanks, in which the concentrated feed is sampled, analyzed and adjusted. The calciner is fed at a constant flowrate by metering pumps.
The electrically heated calciner evaporates liquid in the waste and partially converts solid nitrates into oxides. The oxides are mixed with specially-formulated glass frit at the calciner outlet and the mixture drops by gravity into the induction-heated cold crucible melter. Calcine is incorporated into the molten glass at a temperature of 1,200°C or higher.
The melter has a design capacity of 50 kg of glass per hour.
The glass is poured in a 3 or 4.5 m high glass canister 61 cm in diameter. Pouring is initiated and stopped by opening/closing the pouring valve.
The glass canister is then cooled and remotely welded. After decontamination and non contamination control by a wipe test, it is transferred to interim storage. After acceptance by DOE, the immobilized high level waste product is transferred by cask to the Hanford Canister Storage Building.
Tests Results
A comprehensive test program was conducted in 1997 to demonstrate the process and glass formulation feasibility. It was carried out jointly at Pacific Northwest National Laboratory (PNNL) and CEA.
A preliminary analysis was conducted to select working HLW feed composition ranges. Two baseline compositions were selected : AZ Blend, representative of NCAW sludges from Hanford tanks 241-AZ-101 and 241-AZ-102, and C106, representative of the waste generated by sluicing tank 241-C-106 content into tank 241-AY-102. Variability ranges over the two baseline compositions were defined for the principal elements based on available data and studies.
A single frit was selected by the CEA from a theoretical study using models developed at PNNL for both studied waste compositions. The frit was selected on the basis of 25 wt% oxide loading (excluding sodium and silicon), processing criteria (viscosity, electrical conductivity and liquidus temperature), and glass quality as measured by PCT (Product Consistency Test) and comparison of results to those of the EA (Environmental Assessment) glass. The results of the theoretical study were experimentally confirmed.
Further modeling studies investigated the effects of major constituents variability on glass properties.
For the C-106 and AZ-Blend wastes, the minimum waste loading can be achieved with margins to allow for compositional and process uncertainties.
Higher waste loadings are achievable with some additional composition constraints to adjust for viscosity and chemical durability.
The major constituents likely to limit higher waste loadings were identified.
The effects of some minor constituents were studied by experiment.
Although calcium and lanthanide phosphates may precipitate at high concentrations, there was no impact on the chemical durability of the glass.
The potential for separation of water soluble phases rich in sulphates and the sensitivity of the phenomenon to operating conditions was highlighted.
The HLW process feasibility was demonstrated in laboratory and pilot tests.
Laboratory tests with an active waste sample taken from the tank 241-C-106 and surrogate wastes demonstrated the HLW pretreatment operations feasibility of centrifugation, acidification, and concentration, and enabled initial determination of process parameters.
Pilot calcination and vitrification tests were conducted for the AZ Blend and C-106 surrogate wastes that produced glass at a rate of 20 kg/h which is nearly 40% of that of the full scale facility phase 1B.
The 241-C-106 active waste sample provided by the DOE was successfully vitrified and tested.
Surrogate 241-C-106 sample wastes were vitrified in preparation of the actual active sample vitrification, with waste loadings up to 28 wt%. The glass proved very satisfactory for the PCT and RCRA TCLP.
The actual active waste sample was vitrified with a waste loading between 27 and 29 wt%. The PCT results were similar to or lower than those measured for the surrogate waste glasses.
Layout of the Facility
The HLW Process Building will be a reinforced concrete, six-level, above-grade facility with some levels located below grade. The HLW Process Building will provide for process operations and personnel support functions. This building will also include below-grade vaults for storage tanks, and an interim storage area for filled HLW canisters. The HVAC equipment will be located on the upper floor.
The HLW Process Building is connected to the LAW Process Building with a three-level reinforced concrete connecting corridor structure. The lower level of the connecting corridor will be used for the movement of personnel between the two facilities. The upper level will be used for the maintenance cask transfer. Utilities will be routed through the middle level of the structure.
Overall dimensions of the building are about :
|
length |
: |
64.500 m, |
|
width |
: |
32.000 m, |
|
height below ground, without raft |
: |
6.800 m, |
|
height above ground |
: |
22.100 m |
|
overall volume, without raft |
: |
50,600 m3 |
|
height to width ratio |
: |
0,85 |
Operating experience from three generations of vitrification facilities is reflected in the process cells layout, which provides for a single vitrification cell containing the principal process equipment (calciner, melter, first stage of the off-gas system) and part of the support equipment (dismantling, turntable, elevating plate) : other cells are dedicated to canister handdling, decontamination, smear testing, and storage.
It was possible to integrate easily the cold crucible melter into facility design concept built around the hot crucible melter at the Marcoule, La Hague, and Sellafield vitrification facility because cold crucible melter and hot crucible melters are sensibly of the same size.
The small size of the key process equipment contributes to the small size of the vitrification cell and allows the building to be very compact and easily maintainable.
Maintenance Principles
The main process equipment, such as the cold cucible melter, the calciner, its feeding pumps, the first calciner off gas treatment equipment, are all located in the vitrification cell and are remotely maintained using master-slave manipulators and the cell crane. Maintenance is facilitated by the use of quick release jumpers that enable equipment exchanges in a matter of days. In the case of the melter, replacement takes only 3 days. After removal equipment are size-reduced in a dedicated area of the vitrification cell, sorted, decontaminated as necessary. Residual glass particles can be separated from the melter and either recycled in a glass batch or conditionned in HLW canisters, becoming non-routine HLW.
The melter itself and the other equipment become technological waste conditioning cell of the facility.
It must be noted that the small size of the equipment and the expected lifetime of the cold crucible melter will help keeping waste volume at a minimum.
For some other components, such as the main transfer pumps or the HEPA filters, replacement of which is foreseen during the lifetime of the facility, the maintenance principles used in the French La Hague reprocessing facility will be used. These components are designed in removable modular form to avoid the need to disconnect piping. Removal is performed under shielding and without break of containment through the use of a transfer cask that is connected to the cell containing the equipment to be removed. This transfer cask is called MERC (Mobile Equipment Replacement Cask).
Containment is preserved by an air-tight system consisting of coupled doors, with a special conecting gasket. This system can remove and replace failed equipment without spreading contamination to work areas.
CONCLUSION
The results of the feasability study during Phase 1A for the vitrification of the Hanford HLW tanks are quite positive. The Hanford HLW Vitrification represents a unique challenge that requires proven Operability Principles based on Industrial Experience and implementation of an Advanced Extended Performance Melter extrapolable to Phase 2.
Within the Lockheed Martin team, SGN/COGEMA/CEA can offer both of them with besides the unique knowledge of PNNL teams in the glass formulation of the complex waste of Hanford.
REFERENCES