kawalan kualiti bagi bekas pelupusan sisa radioaktif bagi projek

SeminarRnD14‐16Okt2014,NuklearMalaysia

Kawalan kualiti bagi bekas pelupusan sisa radioaktif bagi projek lubang gerek

Quality control of radioactive waste disposal container for borehole project

Mohamad Pauzi Ismail1, Suhairy Sani1, Azhar Azmi1, Ilham Mukhriz Zainal Abidin1, Noor

Azreen Masenwat1, Mohd Zaidi Ibrahim2, Norasalwa Zakaria2, Nik Marzukee Nik Ibrahim2,

Mohd Abdul Wahab Yusof2 and Anuar Abdul Rahman3

1Kumpulan NDT, 2Pusat Pembangunan Teknologi Sisa, 3Pusat Pembangunan Loji dan

Prototaip, Nuklear Malaysia

Abstrak

Kertas kerja ini menerangkan tentang kawalan kualiti bagi bekas pelupusan sisa radioaktif

yang akan ditanam dalam lubang gerek. Antara kaedah kawalan kualiti ialah menggunakan

kaedah Ujian Tidak Merosakkan (NDT). Bekas pelupusan dibuat dari keluli nikarat 316L dan

ujian‐ujian NDT yang boleh dilakukan termasuklah radiografi, ultrasonik, cecair penembus

dan arus pusar. Bekas ini akan diisi dengan kapsul radioaktif dan simen mortar akan diturap

ke dalam bekas pelupusan bagi mengisi ruang antara kapsul dan dinding bekas pelupusan.

Keputusan ujian NDT yang digunakan dalam kawalan kualiti bekas peluusan ini diterang dan

dibincangkan.

Abstract

This paper explained quality control of radioactive disposal container for the borehole

project. Non‐destructive Testing (NDT) is one of the quality tool used for evaluating the

product. The disposal container is made of 316L stainless steel. The suitable NDT method for

this object is radiography, ultrasonic, penetrant and eddy current testing. This container will

be filled with radioactive capsules and cement mortar is grouted to fill the gap. The results

of NDT measurements are explained and discussed.

Keywords: NDT, ultrasonic, radiography, penetrant, cement mortar, stainless steel, disposal

container, borehole


INTRODUCTION

Non-destructive testing (NDT) is a powerful tool for reducing costs, improving product quality and maintaining quality level. NDT is the name for a variety of subtle but effective test methods which measures some property of material or product without in any way interfering with its subsequent use. By definition, non-destructive tests differ from all tests and measurement which can be achieved only through damage or destruction to the test objects. The latter are known as destructive testing. Of many NDT methods, five of them are considered major and found widely applied in various sector of industry. They are radiography, ultrasonic, eddy current, dye penetrant and magnetic particles. Each method is especially suited for a particular task. Hence they do not compete but are complimentary to one another. One of the most important factors which adversely affect the mechanical strength of certain product and cause its premature failure is the presence of defects in the structure of the product. In the older design this may be taken care of by including a certain safety factor in the design of the product. At the same time designers of product are under pressure to save raw materials as well as to reduce the weight of their product. The new approach towards solving this problem is to minimize the safety factor and at the same time to ensure that the product is absolutely free from dangerous defects. This is where NDT plays an important role. Proper application of various NDT methods has helped industry to improve their product quality, reduced costs and maintain quality levels. BOrehole disposal of Sealed radioactive Sources (BOSS) is an IAEA project for developing a simple, economic yet safe means of disposal for disused sealed sources (DSS). For the BOSS system (Fig. 1), the required components are standard and may be briefly listed as follows:

1. stainless steel container with concrete insert that the stainless steel capsule (holding the sources) fits into;

2. borehole with containers equally spaced along it (spacing about 1 m) and with a no container placed at a depth of less than 30 m;

3. borehole casing; 4. concrete backfill completely filling void between casing and borehole wall; 5. concrete backfill completely filling void between containers and casing; 6. concrete plug above the disposal zone; 7. deflection plate (to deflect any drill that may be inadvertently or deliberately

attempting to drill a vertical borehole along the original trajectory); 8. surrounding geosphere; 9. lightly shielded (100 mm lead brick) conditioning unit in which low activity (<1

Ci Co-60 equivalent) DSS are welded into capsules and containers; 10. mobile hot cell in which high activity (<1000 Ci Co-60 equivalent) DSS are

welded into capsules and containers; and 11. transfer flask for moving high activity DSS to the borehole.

NDT group involved in the quality control of the item #1 above. NECSA [5] suggested ultrasonic testing as an NDT method to be used for quality control (QC) of the disposal container for the borehole project. However we suggest another two NDT methods i.e.


penetrant and eddy current as an additional QC tools. We were also asked to research on the cement mix for the grouting of the casing. It is a standard grout mix of river sand and Portland fly-ash cement. The fly-ash cement improves the impermeability and the sulphate resistance, and reduces the rate of chloride diffusion, thus improving the protection of embedded steel against corrosion.

Fig. 1: BOSS system

METHODOLOGY Disposal Container The disposal container is made of 316L stainless steel from full annealed solid bar with a minimum diameter of 120 mm. Figure 2and 3 show the drawing and picture of the container, respectively.


Fig. 2: Drawing of the container


Fig. 3: Photograph of the container

Ultrasonic Testing (UT) UT is widely used as QC of production and maintenance - various standards and other data are provided by technical societies. UT uses high frequency sound wave to penetrate materials. The pulse reflection method is the most widely used. The ultrasonic pulses are transmitted into specimen and ultrasound coming back to receiver after being reflected at any flaws or back surface is received and displayed on Cathode Ray Tube (CRT). Indication displayed on CRT is called echoes (Fig. 4). Flaws, back surface which reflect ultrasound in specimen are called reflectors. On the CRT, horizontal axis called time base indicates sound path distance and vertical axis indicates echo height which is proportional to sound pressure received. Flaw location is estimated from sound path and size from echo height. UT can be used to inspect metal and non metal, detect surface and internal defects, measure thickness and study material properties (elastic modulus).

w

w

flaw

probe

beam path distance

Fig. 4: Principle of ultrasonic pulse echo method and typical UT equipment


For this purpose, UT is used to measure the thickness and lamination checking of the container wall. The accuracy of the thickness measurement depends on various parameters such surface roughness, equipment, manpower and environment condition. For laboratory condition and smooth surface the accuracy is 0.3mm [1]. For marine and oil and gas industries the total uncertainty is 0.5mm [3] and 0.7mm [4] respectively. Penetrant Testing (PT) PT uses a liquid to penetrate into surface flaws on the test object based on capillary action. Image of flaws become visible after development. The principle of the method, typical penetrant materials and flaw indication are shown in Fig. 5.

1

2

3

4

5

Clean surface

Apply penetrant

Remove penetrant fromthe surface

Apply developer

Inspection

Fig. 5: PT principle, materials and indication

PT can be used to inspect non porous materials (metals, glass, ceramic, etc.) and only detect surface opening flaws. In the BOSS project, PT is used to lamination at the weld groove and any leakage on the container wall. Eddy current Testing (ET) ET is an electromagnetic technique and can only be used on conductive materials. It's applications range from defect detection, to the rapid sorting of small components for either flaws, size variations, or material variation. Commonly it is used in the aerospace, automotive, power generation, petrochemical, marine and manufacturing industries.


For the disposal container it is used to confirm the materials type/variation based on the calibration block provided.

Fig. 6: ET equipment and probe and calibration block

RESULTS AND DISCUSSION

Figure 7, 8 and 9 show UT, PT and ET reports for the disposal container. The results of NDT found that the disposal container is of acceptable quality. No defect detected by UT and PT ET confirmed that the material is as Stainless Steel 316L. Table I shows the results of ultrasonic thickness measurement. There are slight variations in wall thickness but no significant deviation of the container thickness from the specified drawing. The thinnest wall detected is 5.93mm (specification 6.0±0.1 mm).


Fig. 7: UT report


Fig. 8: PT report


Fig. 9: ET report


Table I: UT data

CONCLUSION

The quality of disposal container is acceptable by NDT. NDT is a reliable method of checking product quality if the correct procedure used with calibrated equipment and competent personnel. This condition can be achieved through accreditation program such as MS ISO/IEC 17020 or 17025 or participate in proficiency test program such as MS ISO/IEC 17043.

ACKNOWLEDGEMENT

The first author would like to express his appreciation to Mr. Khairul Azhar bin Saderi and Mr. Suaib Ibrahim of NDT support staff for their assistance in carrying out the inspection.

REFERENCES

1. Guidance Document for Estimation of Measurement Uncertainty in Non-Destructive Testing, 2004, Guidance Note: NDT001, Singapore Accreditation Council Singapore Laboratory Accreditation Scheme (SAC-SINGLAS).

2. IAEA Safety Standards for protecting people and the environment- Borehole Disposal Facilities for Radioactive Waste Specific Safety Guide No. SSG-1, IAEA, 2009


3. J.C. Drury, Corrosion monitoring and thickness measurement – what are we doing wrong?” INSIGHT Vol. 39, No. 1 January 1997

4. M. Lorenz, Procedure for LRUST45 for the manual ultrasonic inspection of pipes at support position for the detection and sizing of wall loss (>30%) caused by general corrosion.

5. NECSA - Road map and Procedures to implement the Borehole Disposal Concept, GEA- 1626, August 2005.

kawalan kualiti bagi bekas pelupusan sisa radioaktif bagi projek

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