DRAFT
CLIENT
DRAFT SUMMARY REPORT
Prepared by
Mr. Sebestian Roberts (Team Leader)
Mr. Tim Pegg
Mr. Sam Billing
Mr. Nick Petousis
Mr. Iain Hughes
Mr. Danny Thomas
Mr. Carl Young
Mr. Kevin Yong
Directed by
Dr. Phil Purnell
TABLE OF CONTENT
1.0 INTRODUCTION
1.1 Nirex
1.2 Radioactive Waste
1.3 Nirex’s Phased Disposal Concept
1.4 Multi-barrier Containment
1.5 Introduction of PGRC
1.5.1 Receipt Facility
1.5.2 Inlet Cell
1.5.3 Unshielded ILW Storage Vault
2.0 PROJECT OBJECTIVES
2.1 Inspection
2.2 Reworking
2.3 Unworkable Packages
2.4 General Considerations
3.0 CORROSION
3.1 ENVIROMENTS OF EXPOSURE
3.2 STAINLESS STEEL
3.3 FACTORS WHICH CAUSE CORROSION
3.4 CORROSION RESISTANCE
3.5 VUNERABLE AREAS
3.6 TREATMENT OF MINOR CORROSION
4.0 WASTE PACKAGE MONITORING
4.1 Potential Damage
4.1.1 Corrosion
4.1.2 Swelling
4.1.3Dropping
4.1.4 Cracking
4.2 Monitoring in Vault
4.2.1 ‘Parking Bay’
4.2.2 Position of ‘Dummy Packages’ & ‘Sensor Packages’
4.2.3Electronic Sensors
4.2.4Active RFID tags
4.2.5 Dummy Packages’ & ‘Sensor Packages’
4.2.6 Visual Inspection
5.0 INSPECTION CELL
5.1 INSPECTION PROCESSES AND METHODS
5.2 PARAMETERS TO BE INSPECTED
5.3 METHODS AND EQUIPMENT
5.3.1 Visual Inspection
5.3.2 Direct Viewing
5.3.3 Indirect Viewing
5.3.4 Radiation
5.3.5 Weight
5.3.6 Dimensions
5.3.7 Heat Generation
5.3.8 Chloride Levels
5.3.9 Advanced Methods of Testing
5.4 COMPARISON BETWEEN THE VARIOUS METHODS
5.5 LEVELOF INSPECTION
5.5.1 SCHEME 1
5.5.2 SCHEME 2
5.6 COMPARISON BETWEEN THE INSPECTION SCHEMES
6.0 INSPECTION CELL LAYOUT
7.0 OVERPACKING CELL………………………………………………………………………….26
7.1 Functional requirements of overpacking cell
7.2 Overpacking cell location
7.3 Overpacking, Repackaging and Reworking
7.4 Overpacking damaged packages
7.5 Overpacking damaged waste package
7.5.1 Stage 1 of overpacking damaged waste package
7.5.2 Stage 2 of overpacking damaged waste package
7.5.3 Stage 3 of overpacking damaged waste package
7.6 Waste package flow diagram
7.7 Grouting stage
7.8 Lid manipulation
7.9 Inspection of overpack waste package
7.10 Overpacking cell design layout
8.0 LOGISTICS
8.1 Emplacement
8.2 Monitoring
8.3 Package Transport
8.4 Maintenance
8.5 Transfer Tunnel
8.6 Ventilation
8.7 Safety features of cell designs
9.0 RELIABILITY AND RISK ASSESSMENT
9.1 General Package in vault
9.2 Corrosion
9.2.1 Modelling of pit initiation
9.2.2 Modelling of package failure with no inspection
9.2.3 Probability of monitoring system failing
9.3 Mishandling
9.3.1 Emplacement Period
9.3.2 Monitoring Period
9.4 Inspection Cell
9.4.1 Emplacement Period
9.4.2 Monitoring Period
9.5 Operational Risks
9.5.1 Contaminants Release
9.5.2 Shielding Malfunction
9.5.3 Fire
9.6 Overpacking Cell
10.0 CONCLUSION
11.0 GLOSSARY
DRAFT
SUMMARY REPORT
357976 / 1DRAFT
SUMMARY REPORT
1INTRODUCTION
1.1Nirex
UK Nirex Ltd is a company jointly owned by Defra and the DTI that advises nuclear site operators on the preparation of safety case submissions to the regulators for the conditioning and packaging of radioactive waste.[[1]]
1.2Radioactive Waste
Nirex had classified wastes into 3 main categories, which are Low Level Wastes (LLW), Intermediate Level Wastes (ILW) and High Level Wastes (HLW).
LLW / Radioactive wastes which releases radiation of not more than 4 GBq/ tonnesof alpha, or 12 GBq/ tonnes of beta/gamma activity.ILW / Radioactive wastes with radioactivity levels exceeding the upper boundaries of LLW, but do not require heat generation to be taken into account in the design of storage or disposal facility.
HLW / Radioactive waste which generates heat as a result of radioactivity. This factor needs to be taken into account in designing storage or disposal facilities.
Table 1: Types of radioactive waste[1]
According to Nirex’s standard specification, ILW is mainly contained in 500L drums, 3m3 boxes, 3m3 drums and 4m3boxes. The latter designed package incorporates radiation shielding with thick reinforced concrete walls. This project deals with the three unshielded ILW packages. Figures 1, 2, and 3 illustrate these waste packages.
Figure 1: 4 500L drums in a stillage [1]
Figure 2: 3m3 box[1]
Figure 3: 3m3 drum[1]
1.3Nirex’s Phased Disposal Concept
Nirex’s Phased Disposal Concept envisages the following phases.
The latter step would constitute deep disposal of radioactive waste, which is intended to provide long-term isolation. Each phase will be reversible, and sufficient time will be available to build confidence at each stage before moving to the next. [1]
1.4Multi-barrier Containment
Figure 4: Multi-barrier containment system[1]
The multi-barrier containment system is designed so that, after closure, it does not rely on the actions of future generations to ensure safety. The multi-barrier containment system is comprises engineered barriers and a natural barrier. [1]
1.5Introduction of PGRC
Figure 5: Concept drawings of the Phased Geological Repository Concept (PGRC)[1]
Phased Geological Repository Concept (PGRC) envisages emplacement of radioactive wastes in a facility constructed at depth within suitable host geology. The concept is assumed to be generic, rather than site specific. It is envisaged that the PGRC facility will to be operated as a store initially, where waste would be monitored and readily retrievable. The concept will undergo a 300 years monitoring period, where waste packages will be monitored, reworked and overpacked if necessary. The decision of backfilling, sealing and closure will be left to future generations. [1]
1.5.1Receipt Facility
Repository receipt facilities would accept packages transported by road and rail. On arrival, waste packages will be inspected, monitored and decontaminated if necessary. [1]
1.5.2Inlet Cell
Figure 6: Concept drawings of the inlet cell[1]
The inlet cell has the function of removing packages from the Reusable Shielded Transport Container (RSTC) and checking waste packages prior being transferred to the storage vault. Empty transport containers will be monitored and decontaminated before returning to the surface facility for reuse.
1.5.3Unshielded ILW Storage Vault
Figure 7: Concept drawings of an unshielded ILW storage vault[1]
The storage vault will be shielded from radiation to provide safety for both employees and the general public. The conditions within the storage vault will be monitored and controlled, in order to prolong the life expectancy of the packages. Every activity within the vault will be conducted remotely.
2PROJECT OBJECTIVES
The design brief received from Nirex was to create a concept for the Inspection and Waste Repackaging Cells for the PGRC. Considering the aforementioned information concerning the PGRC, and the details in the design brief, the following objectives were drawn up.
2.1Inspection
To formulate methods of inspection for the waste packages. The waste packages themselves would be examined for structural integrity, corrosion, degradation of the packaged wasteform and any change in the dimensions of the waste package.
2.2Reworking
If a package were deemed faulty following inspection, it could be ‘reworked’. This would be affecting slight repairs to the waste package, primarily polishing out pit corrosion and covering with a protective coating.
2.3Unworkable Packages
Solutions for damaged packages which are not workable were being devised. This is to ensure packages are deemed safe to the public and could be managed by the PGRC.
2.4General Considerations
Whilst working to provide solutions in these three areas, proven technology and simplicity were to be used where possible. Health and safety of both the public and workers should also be considered.
The design of the inspection cell and Overpacking cell should also be able to allow packages to be inspected and overpacked in a satisfactory rate.
3CORROSION
Waste packages which will be used in the PGRC will be made out of stainless steel. Stainless steels are iron alloys that have a minimum chromium content of 10.5% which leads to the formation of the chromium-rich oxide on the surface. This provides the high corrosion resistance and that is why it is named ‘stainless’ [[2]].
3.1ENVIROMENTS OF EXPOSURE
The waste packages will be subjected to various environments from their manufacture to their placement in the vaults. They will be subjected to atmospheric conditions during their storage and they will experience alkaline environment during their manufacture and over-packing stage. In addition, during storage, packages are expected to come in contact with water coming from ground water flows. Therefore, the packages must be highly resistant to atmospheric, aqueous and alkaline environments [2].
3.2STAINLESS STEEL
The reason why stainless steel has such a high resistance to corrosion is because the high chromium content creates a thin oxide film on exposure to air or water. This film acts as a shield to the underlying material against further reaction with the surrounding environment. When this film is damaged it reforms itself and therefore provides long term protection. However, the stainless steel is vulnerable to localised corrosion when the film breaks down in small areas. Localised corrosion usually takes the form of small pits (pit corrosion) which penetrate perpendicularly into the metal or crevices between the mated surfaces (crevice corrosion) [2].
3.3FACTORS WHICH CAUSE CORROSION
The main factor which causes corrosion is the presence of aqueous chloride ions. These ions tend to break the thin protection film and prevent it from reforming. The more chloride ions are present the higher the risk of corrosion will be. The relationship between the number of chloride ions and corrosion level follows an exponential relationship [[3]].
Another factor that causes corrosion is the, surface roughness. The more homogenous the surface is the lower the active sites for pit initiation are. Therefore, the lower the risk for corrosion will be [[4]].
Finally, sulphur tend to cause pitting corrosion. This is due to the fact that the chromium concentration in the vicinity of the sulphate inclusions is reduced [[5]].
3.4CORROSION RESISTANCE
The type of stainless steel which will be mainly used for the container packages will be 304L and 316L. For these types of alloys there is a large amount of previously collected data[[6]].According to this data the lifetime of the packages will be as follow:
Table 2: Estimated lifetime of a package from previous data. [6]
3.5VUNERABLE AREAS
3.6TREATMENT OF MINOR CORROSION
4WASTE PACKAGE MONITORING
The purpose of monitoring waste packages in the vault is to provide a periodic observations and measurements to determine changes in the physical condition of the packages over time [[7]]. By monitoring packages in the vault, the condition of the vault could also be predicted.
Monitoring of waste packages also had the advantage of increasing the efficiency of waste package inspection, by identifying damaged packages before being transferred into the inspection cell.
4.1Potential Damage
Although the condition of the vault is being monitored and controlled, packages are still prone to be damaged. The following are types of damages that were predicted to occur to the packages stored in the vault.
4.1.1Corrosion
Figure 8: A typical corrosion sensor used in gas pipes[[8]]
There are mainly two types of corrosion which are pit corrosion and stress corrosion. Corrosion is difficult to be detected because the sensors only detect corrosion locally.
Pit corrosion is mainly caused when corrosive agents are being deposited in a pit surface. Pit corrosion happens locally on the package’s surface, and corrodes the package progressively.
Stress corrosion occurs when a material experienced both tension and corrosion attacks. Stress corrosion could be detected by measuring strain instead.
4.1.2Swelling
Figure 9: A typical strain gauge mounted on the wall [[9]]
Swelling are mainly caused when the venting filter of the package are clogged. Building up of gasses within the package could cause packages to swell. Swelling of packages will be detected by a metallic strap around the package together with a strain gauge.
4.1.3Dropping
Dropping of packages are assumed mainly due to mishandling of packages. Dropping of packages could be detected by placing a load cell on the crane. An unpredictable decrease in the load on the crane could indicate a drop from happening.
4.1.4Cracking
Cracking could occur either by corrosive attack or mechanical damage. Cracking is difficult to be detected as it occurs locally. Cracking is only feasible to be detected by 3D mapping within the Inspection Cell.
4.2Monitoring in Vault
Monitoring of packages is being conducted by remote electronic sensors mounted on ‘Dummy Packages’ and ‘Sensor Packages’. These packages will take measurements periodically and signals will be transferred back to the inspection cell by active RFID tags.
4.2.1‘ParkingBay’
‘ParkingBay’ is empty space in a storage vault. Its purpose is to allow extraction of bottom packages without having to move packages on top of a stack to the end of thevault.
Figure 10: A drawing of a ‘ParkingBay’ in a vault
4.2.2Position of ‘Dummy Packages’ & ‘Sensor Packages’
Figure 11: A box in a box layout[[10]]
The arrangement for unshielded ILW packages within the vault will be 7 stillages across and 7 stillages in a stack. Assume 7 stillages across by 7 stillages high by 7 stillages along the vault to be named as a block.
‘Dummy Packages’ will be placed in all 8 corners of each cube, while ‘Sensor Packages’ will be placed at the centre of every face of each cube. Hence, a total of 16 ‘Dummy Packages’ and 12 ‘Sensor Packages’ in each block.
The following table summarises the statistics of packages in a storage vault.
Type of Package / ‘Real Packages’ / ‘Sensor Packages’ / ‘Dummy Packages’ / ‘ParkingBay’ (1 every 2 blocks) / Total Space500L Drums / 29464 / 276 / 304 / 336 / 30380
3m3 drums/boxes / 6520 / 265 / 292 / 77 / 7154
By introducing ‘Dummy Packages’ and ‘Parking Bays’, the effective storage capacity of the vault is only reduced by 2% for 500L drums and 5% for 3m3 packages. It is also found that 2% of the 500L drums will be monitored directly and 9% of the 3m3 packages will be monitored directly. It is arguably that the quality of the sampling methods is the same, because the distances between ‘Dummy Packages’ and ‘Sensor Packages’ are the same.
4.2.3Electronic Sensors
Electronic sensors and active RFID tags require batteries to be replaced. It is assumed that ‘Dummy Packages’ will have a battery life of 10 years, while ‘Sensor Packages’ will have a battery life of 5 years.
Figure 12: A typical ‘Sensor Package’
Electronic sensors which are attached in the ‘Sensor Packages’ include corrosion sensors, strain gauge, humidity and thermometer. It should be noted that strain gauge will not be equipped in ‘Dummy Packages’ because swelling is not likely to occur.
4.2.4Active RFID tags
Figure 13: A typical active RFID tag[[11]]
An active RFID tag will be used to transmit signals to the receiver placed on the crane. By performing and ‘RFID Sweep’, one could collect readings from ‘Dummy Packages’ and ‘Sensor Packages’ easily and quickly. [[12]]
Active RFID tag has the advantage of longer range and faster data transfer, compared to passive RFID tag which only has a range of 3 to 4 feet.
4.2.5 ‘Dummy Packages’ & ‘Sensor Packages’
‘Dummy Packages’ are packages which contain inert materials. They are being introduced for easy handling purposes and been used to obtain information of the condition of a typical ‘Real Package’.
‘Sensor Packages’ are however ‘Real Packages’ which are equipped with electronic sensors. These packages contain active wastes and hence require strain gauges to measure swelling of packages.
4.2.6Visual Inspection
Figure 14: A typical radiation resistant robotic crawler[[13]]
Visual inspection is only feasible to be carried out by remote CCTVs. Visual inspection will be the main monitoring method for packages within the vault. It has the advantage of being reliable, cheap, fast and provide and independent judgement from electronic sensors.
Figure 15: Corrosion in welded regions[[14]]
In order to aid visual inspection of corrosion, welded metal coupons will be attached in every stillages or 3m3 packages. Metal coupons are two metals of the same material welded together and being attached to the packages. Since welded regions are most susceptible to be corroded, by just viewing these coupons, one could justify if other welded regions are corroded.
The following illustrates the sequence of visual inspection to be carried out.
4.3Waste Package Monitoring Sequence
5INSPECTION CELL
5.1INSPECTION PROCESSES AND METHODS
5.2PARAMETERS TO BE INSPECTED
The inspection of a container will consist of two parts.
- Part 1: inspection for contamination and structural integrity of the package.
- Part 2: inspection of degradation and condition of waste stream.
For both parts of inspection individual data should be saved for each package. This will give information about the behaviour and degradation of the package. In addition, it will help to identify packages which show unexpected and dangerous behaviour for more frequent future inspection
The 1st part of inspection will primarily not include measurement of any parameters since direct and indirect viewing of the package will be available. Contamination on the package will be in form of corrosion which will probably occur in the corrosion susceptible area. [See corrosion technical report]
Inspection for structural integrity will consist of check for swelling, cracks or deformation of the package which could be caused by dropping or chemical reactions within the package’s waste stream.
The 2nd part of inspection requires more indirect inspection methods and measurement of various parameters will be necessary. The parameters that can be measured to indicate degradation and condition of the waste stream are:
- Radiation of the package
- Weight
- Dimensions
- Heat generation
- Chloride levels
For more specific information on the density change and degradation of the package more advanced methods of testing should be provided. These methods of testing should provide:
- Image of the waste stream within the package.
- Identification and quantification of the radionuclide within the waste stream.
- Check on the fissile content of the waste packages.
5.3METHODS AND EQUIPMENT
This section presents and discusses various methods and equipment which can be used to measure each of the parameters introduced in section 1.2.
5.3.1Visual Inspection
Visual Inspection may be direct, that is,looking at the package through an oil filled shielded window, or indirect, using cameras. It has been decided that this direct method, and a further indirect technique will be used. The latter will use athrough-wall Endoscope with built in camera.
Please refer to the Technical Report ‘Visual Inspection and 3D Mapping’ for more details. Furthermore, ‘Inspection Processes and Methods ’ considers all the methods of inspection and discusses the preferred processes for an overall inspection routine.
Visual inspection is one of the inspection methods proposed for use in the Inspection Cell. The pros and cons of the different methods are discussed below.