INSPECTION CELL

PART 1 INSPECTION METHODS

VISUAL INSPECTION

The visual inspection of a package can give information both on the structural integrity and on the contamination level of the package. It can be carried out either directly or indirectly. Direct visual inspection is simply viewing the package through a shielded window filled with oil. The thickness of such window is approximately 1.2m [[1]]. The indirect visual inspection consists of use of different types of cameras and robotics. The most common methods are the following:

  1. ‘Crawler-camera’
  2. ‘Fixed CCTV cameras’
  3. ‘Endoscope with built in camera’
  4. ‘Master/Slave Manipulator (MSM) held CCTV’

‘Crawler camera’ is a camera mounted on radio controlled robotic crawler that moves around the ‘inspection’ cell, sending images to a remote monitor [[2]]. ‘Fixed CCTV cameras’ are cameras fixed inside the inspection cell at the base, mid-height, and above a package inspection area. The ‘Endoscope with built in camera’ is based on the same principles as the ‘Fixed CCTV cameras’ but the camera is routed from the ‘cold’ to ‘hot’ side through the Inspection Cell wall [[3]].

‘Master/Slave Manipulators held CCTV camera’ are mechanical arms that reach into the ‘hot’ area holding a CCTV camera. Workers on the ‘cold’ side will operate these MSMs and they will be able to observe the package. They are already used with great success at the current waste retrieval facility at Nirex UK[[4]].

Figure 3: Rendering of Endoscope in the Inspection Cell [3]

Figure 4: Schematic of Endoscope [3]

CORROSION SENSORS

Measuring the corrosion levels is a direct way of checking each package for possible corrosion, uniform or localised. Corrosion sensors are already used in industry to measure corrosion levels in pipes or other structures made out of metals. The sensors are mounted n straps which are strapped in the suspicious areas of the package [Waste Package Monitoring Technical Report(Section 1.1)].

Figure 5: Typical corrosion sensor used in a decaying pipe [Waste Package Monitoring Technical Report(Section 1.1)].

CHLORIDE SENSORS

Chloride is a form of chlorine and is found in most natural water. It is the primary contaminant of concern which results in different types of localised corrosion [[5]]

Although chloride levels are expected to be controlled within the vaults, measurements on individual packages will indicate the likelihood of corrosion for the specific package. If high chloride levels are observed this may indicate that the corresponding area of the vault may also contain high levels and require further investigation.

Chloride levels can be measured in many ways such as wet-chemistry (titration), correlation with electrical conductivity measurement, colorimeters, [[6]] and chloride Ion-Selective Electrodes (ISE).

Figure 6: Measure of chloride levels using ISE [[7]].

PART 2 INSPECTION METHODS

RADIATION

The radiation of the package is an important issue since Nirex stipulates a maximum of 2mSvh-1 on its external surface [[8]]. The expected radioactive behaviour of different radioactive materials is shown in figure 7.

Figure 8: Theexpected total activity against time for various radioactive materials [5]

GET BETTER VERSION OF THIS FIGURE & CHANGE REF

As it can be seen from figure 7, most types of radioactive wastes are expected to gradually decay within the first hundred years but some wastes will carry on producing the same amount of radioactivity for up to hundred thousand years. Since the type of waste in each package will be known, the measurement of radiation will indicate whether the package decays the expected way or further inspection is required.

Measuring radiation is a relatively simple process. There is a wide variety of radiation sensors available which detect different radiation according to package’s content [[9]].

WEIGHT

The weight of the package indicates whether the waste stream degrades normally and loses water due to internal process. This loss of free water will reduce the risk of corrosion and the total activity of the package. [5].

The small levels of water will necessitate a high accuracy balance, which could be integrated with a lifting crane or on adedicated platform.

Which method is used will have a bearing on maintenance and access to the balance. 2.4.4 PART 2: DISCUSSION AND COMPARSION OF METHODS

DIMENSIONS

Change in dimensions of the package will indicate internal gas production due to reactions or phase changes of the waste. If large deformations occur, plastic deformation of the package may result in cracking of the package container. Therefore, change in the dimensions will be an indicator to the condition of the package in terms of the sorption, permeability and diffusivity [5].

Dimensions of the packages can either be measured directly with high accuracy types or more precisely withthree dimensional mapping (3D-Mapping). High accuracy tapes measure the height and diameter of the package.

3D-Mapping consists uses either‘laser scanning’ or ‘structured light ‘to produce a high resolution image of the package within seconds [[10]].

HEAT GENERATION

Heat measurements will provide further information as to change of the wasteform since exothermic reactions will produce heat [1]. Expected behaviour of the packages in terms of heat generation is shown in figure 9.

Figure 9: Expected heat generation of various radioactive materials [5].

As it can be seen from the figure 9 the general trend of the packages is to produce less heat over time. However, some materials such uranium tend to produce constant heat with some increase at the time of 10 000 years after their storage.

Heat generation can be measured using computer imaging, calorimeters and temperature gauges [5] and only takes a few seconds. Accuracy to the nearest Celsius degree will be satisfactory since the maximum heat generation of a package will be 50 Celsius [[11]].

ADVANCED METHODS OF INSPECTION

If specific information on the density and degradation of the package is required, more advanced methods would provide the following:

  • Image of the waste stream within the package.
  • Identification and quantification of the radionuclides within the waste stream.
  • Fissile content of the waste packages.

Real Time Radiography (RTR)

Real Time Radiography (RTR) is an advanced method of X-Ray which can produce images of the waste stream. It identifies the position of the waste within the package and therefore checks whether the waste is evenly distributed. If the nuclear waste is highly concentrated at one part of the package this may lead to corrosion. In addition, RTR detects wasteforms of unidentified type and gases and liquidsproduced by the degradation of the waste. These substances may also lead to corrosion of the package [[12]].

Figure 13: Real Time Radiography method.

Figure 14: The facility for Real Time Radiography [[13]].

High Resolution Segment Gamma (HRSG)

High Resolution Segment Gamma (HRSG) measures the amount and position of radionuclides within a package’s waste stream. The concept is similar to RTR since it can detect ‘hot’ spots on a package by measuring density variations and also identify the current level of the package (LLW or ILW, see Glossary)[5]. If for example, a certain package was originally classified as Intermediate Level Waste it would produce a high level of heat and therefore be placed in a relatively cold position in the vault. However, if after inspection it is found that its radionuclide content is no more active, it could be relocated a hotter area.

Figure 15: High Resolution Segment Gamma method.

Figure 16: The apparatus of HRSG measuring a package in the ‘hot’ area [[14]].

Passive Neutron Multiplicity Counting (PNMC)

The safe fissile material (SFM) for each package, according to Nirex standards, is up to 50g [[15]].

PNMCcan measure the fissile content within the waste stream for both encapsulated and non-encapsulated waste. Accuracy is ±50% but by using neutron physics computer modelling,this can be improved [5].

Figure 17: Passive Neutron Multiplicity Counting method.

Figure 18: Passive Neutron Multiplicity Counting apparatus in ‘hot’ area [[16]].

SCHEME 1

This scheme includes direct and indirect Visual Inspection. Radiation and heat generation sensors are also included. These processes would be carried out simultaneously and are expected to last up to an hour. If reworking takes place, visual inspection would be repeated. 3D mapping and HRSG will be the other two inspection methods that will be included in this scheme.

The levels of inspection for the scheme are shown in table 5

Table 5: Levels of inspection for scheme 1.

Scheme 1
Level 1
All packages /
  • Documentation validation
  • Direct visual inspection
  • Master/Slave Manipulator held CCTV’
  • Radiation
  • Heat generation

Level 2
40% of the packages or suspect packages /
  • 3D Mapping

Level 3
10% of the packages or suspect packages /
  • High Resolution Segment Gamma

The process of inspection for Scheme 1 is shown in the flow diagram in figure 19.

As it can be seen from the flow diagram most of the packages will take up to an hour to be inspected but if reworking is required inspection will take up to three hours. This is because packages which need to be reworked will go through Visual Inspection twice and will be considered as suspect therefore they will go through all the inspection levels.

SCHEME 2

This scheme includes only four methods of inspection and is divided into two levels. The levels of inspection and flow chart are shown in table 6 and figure 20 respectively.

Table 6: Inspection levels for scheme 2.

Scheme 2
Level 1
All packages /
  • Documentation validation
  • Direct visual inspection
  • Master/Slave Manipulator held CCTV’
  • Radiation

Level 2
40% of the packages or suspect packages /
  • 3D Mapping

As it can be seen from figure 10, the duration of this scheme will be up to an hour under normal circumstances, but with reworking, up to two hours. This is a more basic inspection scheme that favours the inspection of large quantities of packages within as short a time period as possible, rather investigating the waste stream in depth.

SCHEME 3

Scheme 3 uses direct visual inspection, ‘Master/Slave Manipulator held CCTV’ , radiation and heat generation sensors. From the Advanced Methods of Inspection it uses both Real Time Radiography and High Resolution Segment Gamma. For a very small percentage of packages it includes a destructive essay from which very specific information can be drawn. It must be pointed out that the destructive essay is a very expensive and time consuming process and is only used for scientific purposes. The levels of inspection and process are shown in Table 7 and figure 21 respectively.

.

Table 7: Levels of inspection for scheme 3.

Scheme 1
Level 1
All packages /
  • Documentation validation
  • Direct visual inspection
  • Master/Slave Manipulator held CCTV’
  • Radiation
  • Heat generation
  • 3D mapping

Level 2
40% of the packages or suspect packages /
  • High Resolution Segment Gamma
  • Real Time Radiography

Level 3
0.5% of the packages or destroyed packages /
  • Destructive essay

As it can be seen from figure 21, this scheme has a long duration and in depth

inspection, and with reworking will take up to three hours and twenty minutes. However, the first two levels provide high level of accuracy and a large amount of information for the waste steam condition.

How long is destructive essay ???

[1] Observations from the visit to Nirex laboratories.

[2] Inuktun Suppliers

Access date: 09/02/07

[3] IST Corp. (unknown) ONLINE “Radiation Tolerant Thru-Wall/Roof Viewing System”. [Online]. Last viewed 2007 March 11. Available:

[4]“Remote Handling Systems Development for the Spallation Neutron Source Target System”,P.T. Spampinato, E. C. Bradley, T. W. Burgess, J. B. Chesser, V. B. Graves, M. J. Rennich,

[5] “Options for Monitoring During Phased Development of a Repository for Radioactive Waste”, Contractor Report to United Kingdom Nirex Limited, June 2002.

Access date: 09/02/07

[6] “Chloride”, Hydrolab

Access date: 09/02/07

[7] “Chloride Electrodes and Systems”, Shelfsientific, 2004

Access date: 09/02/07

[8]Nirex Report no. N/104, Generic Repository Studies, “Generic Waste Package Specification”

Volume 1-Specification, June 2005: pages 25

[9]“Radiation Detectors”, Electronics Manufactures Directory

Access date: 15/03/07

[10]“3D Laser Mapping Technology”, Fujikura Europe Ltd

Access date: 15/03/07

[11] Nirex Report 484085, “ Summary Note for CoRWM on Repackaging of Waste”, September 2005: page 31

[12]“Real Time Radiography”, BNFL

Access date: 12/02/07

[13]Best Manufacturing Practises, Letterkenny Army Depot - Chambersburg, PA:Real Time Radiography Applied to Paladin Production

Access date: 13/03/07

[14] “ANTECH Model 3800 Combined Tomographic Gamma Scanner (TGS) for 400 Litre Drums”, Ortec

Access date: 13/05/07

[15]“Options for Monitoring During Phased Development of a Repository for Radioactive Waste”, Contractor Report to United Kingdom Nirex Limited, June 2002: p.114.

Access date: 09/02/07

[16] “ANTECH Model 2300 Combined Passive Neutron/Gamma Drum Monitor” Ortec online

Access date:13/05/07