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Optimisation of screens used with linear accelerator N.Rigaud EDF - Service Qualité des Réalisations - EVRY - France J.M. Tchilian Framatome - Division Qualité - PARIS LA DEFENSE - France J.P.Relet Framatome - Usine de Chalon - Saint Marcel- France Contact

ABSTRACT

This paper presents the results obtained in a study of AFCEN (French Society for Design and Construction Rules for Nuclear Island Components) in order to define the screens to use with linear accelerator in order to optimize the sensitivity of détection.
The testing conditions used are described and the results obtained with stainless steel, tantalum, lead screens are presented in terms of detectability on real cracks.
The consecutive modification of the RRCM, in order to define the minimum requirements, is presented.

Fig 1:

1. SCOPE

This study has been launched in order to determine the optimum assiociation of screens to use with linear accelarator to permit a good detection sensitivity on defects in order to introduce requirements in RCC-M code which up to now, had no precise stipulations on that point.

2. STUDY PROGRAMME

This study is carried out with a specific specimen containing five cracks of increasing importance(AFCEN block, see figure in annex 1). The images of these defects are well-known, and it is possible to «quantify» with a good accuracy the evolution of the detectability .This block is combined with steel pieces to obtain the thickness required.
The principle of the method is to make radiography of the Afcen block in RCCM conditions and to compare the results obtained with reference radiograms in terms of image quality, and detectability of defects.
The screens tested have been selected from literature (standardisation, codes) in terms of materials and thickness adapted to nuclear applications(energy and wall thickness).

The table 1 gives a synthetic abstract of the usable screens depending of the energy used. 1

Considered energy	Front metal screen (mm)	Back screen (mm)
E £ 4MeV	Stainless screen : 0,4 to 0,7	Stainless : 0,4 to 0,7 } or Lead Cu : 0,4 to 0,7 } 0,1 to 0,5 Lead : 0,1 to 0,6
4 < E £ 8 MeV	Stainless < 1,6 Tantalum e < 1,6 Lead: 1 to 1,6 Nickel 1 mm(+ front filter Lead 2 mm)	Stainless : < 1,0 Tantalum: < 1,0 Lead : 1 to 1,6 Lead : 0,25
8 < E £ 12 MeV	Stainless : 0,1 to 1 Tantalum: 0,1 to 1	Stainless : 0,1 to 1 Tantalum : 0,1 to 0,5
E > 12 MeV	Tantalum : 0,1 to 1 Tungsten: 0,1 to 1 Tantalum : 0,1 to 1 Tungsten: 0,1 to 1	No screen   } if Ep < 300 mm No screen   } Tantalum: 0,1 à 0,5} if Ep. ³ 300 mm Tungsten : 0,5 to 0,5}
Table 1

The table 2 gives the radiographed thickness in association with the energy source in the RCCM.

source	welds	casting
		To be welded areas	Other areas
X rays to 400 kV	to 70 mm	to 70 mm	to à 70 mm
Iridium 192	to 100 mm	to 100 mm	to 100 mm
Cobalt 60	³ 70 mm	³ 70 mm	³ 70 mm
Linear accelarator 1 - 1,5 MeV	³ 70 mm	³ 60 mm	³40 mm
Linear accelarator 2 - 6 MeV	³ 80 mm	³ 70 mm	³ 60 mm
Linear accelarator 8 - 12 MeV	³ 90 mm	³ 80 mm	³ 70 mm
Table 2: Energy source function of penetrated thickness

The tables 3 and 4 give the general retained conditions of exposure respectively to respect for welds and castings

thickness(mm)	Energy	Class system film	Front filter	Front screen(mm)	Back screen (mm)
70	1 Mev	C2		Inox : 0,4 Cu : 0,4 Inox : 0,7 Cu : 0,7 Pb : 0,2 Pb : 0,8 Pb : 1,6	Inox : 0,4 Cu : 0,4 Inox : 0,7 Cu : 0,7 Pb : 0,2 Pb : 0,8 Pb : 1,6
75	3 Mev	C2		Inox : 0,4 Cu : 0,4 Inox : 0,7 Cu : 0,7 Pb : 0,2 Pb : 0,8 Pb : 1,6	Inox : 0,4 Cu : 0,4 Inox : 0,7 Cu : 0,7 Pb : 0,2 Pb : 0,8 Pb : 1,6
80	6Mev	C2		Inox : 0,1 Inox : 0,5 Inox : 1,0 Tantalum : 0,1 Tantalum : 0,5 Pb : 1,0 Pb : 1,3 Pb : 1,6	Inox : 0,1 Inox : 0,5 Inox : 1,0 Tantalum : 0,1 Tantalum : 0,5 Pb : 1,0 Pb : 1,3 Pb : 1,6
Table 3: exposure conditions for welds

Note : « inox » or « In »mean, in text or tables, « stainless steel »

thickness (mm)	Energy	Class system film	Front filter	Front screen(mm)	Back screen (mm)
70	2 Mev	C3		Inox : 0,4 Cu : 0,4 Inox : 0,7 Cu : 0,7 Lead : 0,2 Lead : 0,8 Lead : 1,6	Inox : 0,4 Cu : 0,4 Inox : 0,7 Cu : 0,7 Lead : 0,2 Lead : 0,8 Lead : 1,6
80	6 Mev	C3	Lead :2mm	Inox : 0,1 Inox : 0,5 Inox : 1,0 Tantalum : 0,1 Tantalum : 0,5 Lead : 1,0 Lead : 1,3 Lead : 1,6 Ni : 1 mm	Inox : 0,1 Inox : 0,5 Inox : 1,0 Tantalum : 0,1 Tantalum : 0,5 Lead : 1,0 Lead : 1,3 Lead : 1,6 Lead : 0,25
90	9Mev	C3		Inox : 0,1 Inox : 0,5 Inox : 1,0 Tantalum : 0,1 Tantalum : 0,5 Lead : 1,0 Lead : 1,3 Lead : 1,6	Inox : 0,1 Inox : 0,5 Inox : 1,0 Tantalum : 0,1 Tantalum : 0,5 Lead : 1,0 Lead : 1,3 Lead : 1,6
Table 4: exposure conditions for castings

3. RESULTS

The table 5 gives an exemple of the results obtained for one of the level 3 who made the interpretation. The interpretation has been carried out by 3 interpretators named (T,N,R) in the all text and who are RT level 3 y from FRAMATOME and E.D.F.,
For each interpretation the IQI value is reported and the caracterisation of detectability of the 5 cracks based on sharpness and dimension of crack's images, is given.

Table 5:

The following tables 6 to 11 give the results in terms of detectability, table 6 to 9 are for welds and 10 to 12 are for castings.

	T	N	R
Pb 0,25	10	10	11
Pb 0,75	10	10	11
Cu 0,4	10	11	11
In 0,7	8	8	14
In 0,5	11	11	14
Pb 1,6	11	11	11
Cu 0,6	7,5	7,5	8
Table 6: MeV C2

	T	N	R
In 0,5	7,5	10	10
In 0,7	7	8	10
Cu 0,4	6	8	10,5
Cu 0,6	7	8,5	11
Pb 1	6	5	8,5
Pb 0,8	7	4,5	7
Pb 0,2	9,5	3,5	8
Table 7: 2 MeV C2

	T	N	R
In 0,1	5	4,5	5
In 0,5	4	4,5	5,5
In 0,1	3,5	5	6
Ta 0,1	2	4,5	4,5
Ta 0,6	6	6,5	7
Pb 1	5	4,5	5,5
Pb 1,5	5	3,5	5
Pb 2	4,5	5	5,5
Table 8: 6 MeV C2

	T	N	R
Pb 0,2	7,5	3	5
Pb 0,8	4	3	5,5
Pb 1	4,5	5	7,5
Cu 0,6	7,5	10	11,5
Cu 0,4	5,5	4	8
In 0,7	5	5	9
In 0,5	4,5	4,5	8
Cu 0,6	8,5	7,5	10,5
Table 9: 3 MeV C3

	T	N	R
In 0,1	4,5	6	7
Ta 0,1	4	4,5	5
In 0,5	2,5	3	5
Ta 0,6	4	4,5	6
In 1	5	3,5	4
Pb 1	3	3,5	5
Pb 1,5	4	5	6
Pb2	2	2,5	3
Ni 1	6,5	4,5	6
Table 10: 6 MeV C3

	T	N	R
In 0,1	5,5	7	7
In 0,5	3,5	3,5	3,5
In 1	5	3,5	3,5
Ta 0,1	5	5	5
Ta 0,6	4,	5,5	5,5
Pb 1	5	6	6
Pb 1,5	6,5	6	6
Pb 2	6,5	7,5	7,5
Table 11: 9 MeV C3

	T	N	R
Pb 0,25	65	65	68
Pb 0,75	62	62	71
Cu 0,4	80	80	57
In 0,7	61	61	84
In 0,5	89	89	82
Pb 1,6	88	88	68
Cu 0,6	59	59	52
Table 12: 1 MeV C2

	T	N	R
In 0,5	65	71	76
In 0,7	58	72	63
Cu 0,4	47	58	70
Cu 0,6	62	76	88
Pb 1	46	49	69
Pb 0,8	48	41	51
Pb 0,2	60	36	73
Table 13: 2 MeV C2

	T	N	R
In 0,1	53	54	50
In 0,5	45	54	56
In 1	53	55	57
Ta 0,1	52	49	50
Ta 0,6	64	71	63
Pb 1	47	51	51
Pb 1,5	57	52	48
Pb 2	55	57	55
Table 14: 6 MeV C2

	T	N	R
Pb 0,2	34	32	32
Pb 0,8	39	28	51
Pb 1	41	39	40
Cu 0,6	81	77	79
Cu 0,4	55	26	52
In 0,7	42	36	70
In 0,5	49	50	54
Table 15: 3 MeV C3

	T	N	R
In 0,1	49	44	57
Ta 0,1	51	46	53
In 0,5	47	28	42
Ta 0,6	42	53	56
In 1	57	32	47
Pb 1	50	43	46
Pb 1,5	52	64	75
Pb 2	27	32	37
Ni 1	55	64	56
Table 16: 6 MeV C3

	T	N	R
In 0,1	57	57	68
In 0,5	48	37	50
In 1	73	85	48
Ta 0,1	51	50	38
Ta 0,6	52	48	62
Pb 1	42	49	51
Pb 1,5	58	58	62
Pb 2	53	57	70
Table 17: 9 MeV C3

INTERPRETATION OF TESTS

The results of interprétation can be summarised in the table 18 where the best combinations of screens are given :

thickness	energy	class system film	front filter	front screen	back screen
70	1MeV	C2		In 0,5 to 0,7	In 0,5 to 0,7
				or Pb 1,6	or Pb 1,6
75	3MeV	C2		Cu 0,6	Cu 0,6
80	6MeV	C2		Ta 0,6 mm or In 1mm	Ta 0,6 mmor In 1mm
70	2MeV	C3		Cu 0,6	Cu 0,6
80	6MeV	C3	2 mm Pb	Ni 1 mm	Pb 1,5 or Ta 0,6
90	9MeV	C3		Pb 2 mm or In 0,1	Pb 2 mm or In 0,1
Table 18:

The same tests have been realised with double film technique.The results seem a little lower in terms of detectability and length, but the classification of the screen in terms of "performance" are in the same order.

5. CONCLUSIONS

This study has permitted to get a precise idea of the type of screens to used in conjunction with energy of source and system class film. It is now obvious that lead screens are not always the best solution and that stainless steel, tantalum and copper can give good results.
The conclusion of this study could be the following proposal to introduce in RCCM Code as requirements for radiography with linear accelerator;

Application	energy	front filter	front screen	back screen
Soudure Zone à souder des pièces moulées (C2 class system film)	1MeV		In 0,5 to 0,7	In 0,5 to 0,7
			or Pb 1,6	or Pb 1,6
	3 MeV		Cu 0,6	Cu 0,6
	6 MeV		Ta 0,6 mm or In 1mm	Ta 0,6 mm or In 1mm
Pièces moulées (C3 class system film)	2MeV		Cu 0,6	Cu 0,6
	6 MeV	2 mm Pb	Ni 1 mm	Pb 1,5 or Ta 0,6
	9 MeV		Pb 2 mm or In 0,1	Pb 2 mmor In 0,1


No intermediate screen is required when double film technique is used.

This study should be continued by the optimisation of the intermediate screens.

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