Waste Package Monitoring

DRAFT

TECHNICAL NOTE

WASTE PACKAGE MONITORING

Prepared by: / Name / Kevin Yong
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STATUS : DRAFT FOR DISCUSSION / PROVISIONAL / INTERIM / FINAL
PROVISONAL or INTERIM status means this Technical Note has been prepared to facilitate Nirex’s work programme and does not necessarily reflect the company’s final position.

DRAFT

TECHNICAL NOTE

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DOCUMENT HISTORY
STATUS / REVISION / DATE / COMMENTS

SUMMARY

Packages stored in the storage vault will be subjected to damages such as pit corrosion, stress corrosion, swelling, dropping and cracking. The purpose of monitoring the packages in the vault is to provide a periodic observations and measurements to determine changes in the physical condition of the packages over time.

Monitoring of waste packages will be conducted by visual inspections and electronic sensors attached on ‘Dummy Packages’ and ‘Sensor Packages’. Electronic sensors include strain gauge, humidity sensors, and thermometer and conductivity sensors for corrosion indication.

Signals and readings will be transmitted by active RFID tags to a receiver mounted on the lifting crane. Electronic sensors and the active RFID tags will be powered by batteries which last for 5 or 10 years, for ‘Sensor Packages’ and ‘Dummy Packages’ respectively.

‘Dummy Packages’ and ‘Sensor Packages’ will be distributed evenly within the vault. The number of ‘Dummy Packages’ will not affect the effective storage capacity of the vault, while still providing a good representation of other ‘Real Packages’.

Visual inspection will be the primary method of inspection, due to its reliability, instant response and cost. Visual inspection will be conducted by remote cameras mounted on robotic crawlers. Metal coupons welded on stillages or attached on metal coupons will be inspected visually to check presence of corrosion in welded regions.

Waste packages monitoring sequence will begin with a RFID sweep followed by visual inspection. Packages identified or suspected to be damaged will be retrieved to the Inspection Cell for further inspection. If no package is identified damaged, random checks will be performed on ‘Real Packages’.

TABLE OF CONTENT

SUMMARY iii

AIMS 1

INTRODUCTION 1

1.0POTENTIAL DAMAGE 1

1.1PIT CORROSION 1

1.2STRESS CORROSION 1

1.3SWELLING OF PACKAGES 1

1.4DROPPING OF PACKAGES 2

1.5CRACKING 2

2.0MONITORING IN VAULT 2

2.1DUMMY PACKAGES 2

2.2SENSOR PACKAGES 3

3.0ELECTRONIC SENSORS 4

3.1CORROSION SENSORS 5

3.2HUMIDITY SENSORS 5

3.3RADIATION SENSORS 5

3.4STRAIN GAUGE 6

3.5THERMOMETER 6

4.0‘PARKING BAY’ 8

5.0SAMPLING LAYOUT 8

5.1BOX IN A BOX LAYOUT 9

5.2OTHER SAMPLING LAYOUT 10

6.0DIRECT VIEWING & CCTV 11

6.1WELDED COUPONS 11

7.0MONITORING OF STILLAGES 12

8.0WASTE PACKAGE MONITORING SEQUENCE 12

CONCLUSION 12

REFERENCES 13

357976 / iv

DRAFT

TECHNICAL NOTE

WASTE PACKAGE MONITORING

AIMS

The purpose of monitoring the packages in the vault is to provide a periodic observations and measurements to determine changes in the physical condition of the packages over time.[1] By monitoring packages in the vault, the condition of the vault could also be predicted.

INTRODUCTION

According to Nirex’s standard and specification (WPS/640), waste monitoring is defined as “continuous or periodic observations and measurements to determine changes in the physical condition of a waste package over time.”[1]

This report proposes designs in monitoring ILW waste packages in the vault. Packages include 500L drums, 3m3 boxes and 3m3 drums. Monitoring of the vault will be mainly by mobile cameras and electronic sensors. Monitoring of the waste packages are being conducted remotely.

1. POTENTIAL DAMAGE

Packages stored in the vault are prone to be damaged mechanically or chemically. It is thought that the following damages are likely to happen. These include pit corrosion, stress corrosion, swelling of packages, dropping of packages and cracking.

A detailed report about types of corrosion is discussed in the ‘Corrosion Technical Report’.

1.1. Pit Corrosion

Pit corrosion occurs due to the presence of a pit on the surface of packages and free water. When water evaporates, it deposits salt or chlorides on the surface of the pit. Chlorides then react with the stainless steel and hence initiate corrosion. The corrosion then progresses its way towards the pit and form a progressive corrosion mechanism.

Pit corrosion can be classified as a localised corrosion. Localised corrosions are often difficult to be detected by any sensors. This is because electronic sensor only detects the presence of corrosion in its surrounding, and it would be uneconomic to position many sensors around the package, as it could occur on any surface.

1.2. Stress Corrosion

Stress corrosion occurs when a material is subjected to both tension and corrosion. Mentioned above, it is often difficult to detect corrosions of a package. Thus, it would be easier and more feasible to measure the strain of the packages, rather than detecting the presence of corrosions.

1.3. Swelling of Packages

The wasteforms in every package differs from one another. Certain radioactive wasteforms releases gases throughout its decaying time. Swelling of packages is likely to happen when the venting filter of the package are clogged. Building up of gases within the package causes the package to swell.

1.4. Dropping of Packages

Dropping of packages would only occur due to mishandling of packages, assuming that it would only occur during lifting of packages. Hence, neglecting other mechanisms such as seismic movements in the PGRC, which are either site specific or has negligible probability of occurring. This ranges from human error to control systems of remote handling.

It is thought that such mechanism would be easier to be detected from the lifting crane or the grabber. By placing a load cell on each crane or grabber, one could measure the weight of the package. An unforeseen decrease in the load carried by the crane or grabber indicates a packages being dropped.

Such detection method however has its drawbacks. For example, when a package was accidentally dropped on to a stack of packages and it caused the whole stack of packages to fall over. In this scenario, the sensors only assume the dropping of one package only. Hence, it is recommended that a mobile CCTV to be dispatched to the spot.

1.5. Cracking

Cracking could occur either due to corrosion or mechanical damage. Such mode of failure is difficult to be detected. Cracking occurs locally and are difficult to be predicted. Cracking could only be detected by 3D mappings.

It was also thought that it is not feasible to conduct 3D mapping within the vault. This is due to the expensive cost of the equipment and the equipment might not be able to resist radiation. Hence, 3D mapping can only be conducted through shielded window.

3D mapping can only be conducted effectively when there’s no ‘blind spots’, such as packages being stacked together. Extracting packages and mapping them individually would somehow consume too much time.

2. MONITORING IN VAULT

It was thought that monitoring in the vaults play an essential role during the 300 years of emplacement period. Through monitoring of packages, not only one could determine the condition of packages but also predict any failure of the vault control system. ‘Dummy packages’ and ‘sensor packages’ will be use in order to monitor the conditions of the packages.

2.1. Dummy Packages

‘Dummy packages’ are packages of the same dimensions and material as other packages, but they will contain inert materials, such as grout or concrete. These ‘Dummy Packages’ will be equipped with a set of electronic sensors and a battery, which has an assumed battery life of 10 years. The batteries will be placed within the package and be shielded from radiation.

The main functions of these packages are to monitor corrosion, humidity and temperature of its surrounding area. The main advantage of these ‘Dummy Packages’ is being able to handle it without remote handling.

The following figure shows a drawing a typical ‘Dummy Package’ for 500L drums.

Figure 1: A typical ‘Sensor Package’ for 500L drum

2.2. Sensor Packages

‘Sensor Packages’ are real packages which are equipped with electronic sensors. Electronic equipments within the vault will include corrosion sensor, humidity sensor, thermocouple, strain gauge and a battery, with an assumed battery life of 5 years. Batteries will be strapped on the outside of the package and will be shielded from radiation.

The purpose of these sensor packages is to allow monitoring of the package and its surrounding area, without reducing the effective storage capacity of the vault. The diagram below illustrates the position of sensors in a ‘Sensor Package’.

Figure 2: A typical ‘Sensor Package’

3. ELECTRONIC SENSORS

Due to the radioactive environment of the vault, it is difficult and not feasible to monitor packages directly. Hence, electronic sensors will have to be used. Electronic sensors somehow have its limitations such as it require powering up and its reliability.

Powering up these electronic systems is the main drawback. Replacing batteries too often would increase the work load of the inspection cell, whereas having long lived batteries would reduce its reliability.

Several methods of powering up and charging batteries have been considered not feasible. Charging or replacing batteries with robotic crawlers somehow would prove difficult and time consuming. Hence, it is suggested that a battery life indicator should be included into the system, and batteries will be replaced.

From current proven technology, it is possible to design batteries which can last for at least 2 years. It is also possible to design such batteries to be small enough to be attached to the package, without affecting much of its properties. It is also assumed that these sensors will take measurements 3 times every 24 hours, rather than taking continuous readings.

3.1. Corrosion Sensors

Figure 3 A typical corrosion sensor used in gas pipes [2]

It was assumed that corrosion is most likely to happen in welded areas of the package. Hence, these corrosion sensors will be positioned at these potential areas. These regions include top side of the lid and underside of the lifting flange. Corrosion sensors will be mounted on a strap and be strapped around these areas. This is to prevent alterations of the packages itself.

Corrosion sensors worked by measuring the conductivity of the material. Stainless steel has a relatively high conductivity. The presence of chloride would result in a decrease in its conductivity and hence the presence of corrosion can be detected.

3.2. Humidity Sensors

Figure 4: A typical humidity sensor [3]

From experience of similar projects, it was found that corrosion usually occurs due to the presence of free-water. When water evaporates, it deposits chlorides and is the main corrosive agent. Hence, a humidity sensor is essential in predicting corrosion happening.

It was thought that free-water or moisture is likely to be present at the underside of the package, which also include stillages. Therefore, humidity sensors will be placed at the bottom of the package.

Humidity sensors also act as a fail-safe sensor, if the corrosion sensors malfunction. One could determine the condition of the packages by relying on two different sensors, without having to send robotic crawlers.

3.3. Radiation Sensors

The radioactive level of each package will be measured in the inlet cell before being transferred to the vault. The radiation level of the waste packages will only decrease through time, but will never increase.

The scope of the project only deals with unshielded ILW, hence measuring radiation level is thought to be redundant. Sensors such as a Geiger-Muller tube also proved to be expensive.

3.4. Strain Gauge

Figure 5: A typical strain gauge mounted on the wall [4]

It is thought that expansions of packages are more significant or more likely to happen in the planar direction, rather than axially. The expansion of the packages will be measured by a strain gauge on a metallic strap. The metallic strap will then be strapped at mid height around the package.

This method allows the strain gauge to obtain any expansion or within the package to be detected. But the metal strap will need to be replaced, depending on its material. This is due to creeping of the material. It is also noted that strain gauges will not be mounted on ‘Dummy-Packages’ as they don’t contain any waste materials.