A SHIELDED STORAGE VAULT FOR RADIOACTIVE REFERENCE SOURCES. Une enceinte de stockage biindee pour les sources de reference radioactives

Transcription

1 AECL-9264 ATOMIC ENERGY I <dh\*\ L'ENERGIEATOMIQUE OF CANADA LIMITED V # s \ / DU CANADA UMITEE A SHIELDED STORAGE VAULT FOR RADIOACTIVE REFERENCE SOURCES Une enceinte de stockage biindee pour les sources de reference radioactives H.C. SPEIMCELEY Chalk River Nuclear Laboratories Laboratoires nucleates de Chalk River Chalk River, Ontario August 1986 aout

2 ATOMIC ENERGY OF CANADA LIMITED A Shielded Storage Vault for Radioactive Reference Sources by H.C. Spenceley Neutron and Solid State Physics Branch Chalk River Nuclear Laboratories Chalk River, Ontario KOJ 1J August AECL-9264

3 L'ENERGIE ATOMIQUE DU CANADA, LIMITEE Une enceinte de stockage blindée pour les sources de référence radioactives par H.C. Spenceley Résumé On a construit une nouvelle enceinte de stockage pour les sources de référence radioactives préparées et utilisées par la section des détecteurs et de la métrologie nucléaires de Chalk River. On donne les détails de la conception et construction. On stocke une partie des sources plus fortes dans des blindages ou tiroirs blindés particuliers pour réduire l'exposition au rayonnement lors du déplacement des sources plus faibles. La nouvelle enceinte permet une meilleure organisation et un meilleur accès ainsi que la réduction de l'exposition au rayonnement par rapport à l'ancienne enceinte. Physique des neutrons et de l'état solide Laboratoires nucléaires de Chalk River Chalk River, Ontario KOJ 1J août AECL-9264

4 ATOMIC ENERGY OF CANADA LIMITED A Shielded Storage Vault for Radioactive Reference Sources by H.C. Spenceley Abstract A new storage vault has been built for radioactive reference sources prepared by and used in the Nuclear Detectors and Metrology Section at Chalk. River. Design and construction details are described. Some of the stronger sources are stored in individual shields or shielded drawers to reduce radiation exposure when moving weaker sources. The new vault provides better organization and access as well as reduced radiation exposure compared to the one it replaced. Neutron and Solid State Physics Branch Chalk River Nuclear Laboratories Chalk River, Ontario KOJ 1J August AECL-9264

5 A new storage vault for radioactive reference and test sources has been built in Bldg. 115 which is occupied by the Nuclear Detectors and Metrology Section of Neutron and Solid State Physics Eranch. It replaces a vault in Bldg. 114 where the Radionuclide Metrology Laboratory was located before moving to Bldg. 115 in The new vault (Fig. 1) is situated in the northeast corner of Room 1, about 25 m from the counting room and 6 ra from any work station occupied on the average for more than about one hour per day. Since this location is beside the north door of the building, sources being moved to and from the Active Area need not be taken past the counting room. To support the vault the floor was reinforced from the crawl-space below; the bracing effectively prevents access to the space directly under the vault making shielding in the downward direction unnecessary. This allows a few stronger (typically HO MBq each) individually shielded sources to be stored in the vault on dollies at floor level (Fig. 2). Smaller sources are stored in a Lista B-313 steel workshop cabinet (576 mm wide x 700 mm deep x 1000 mm high) fitted with 13 G50 drawers (36 mm useful height) and two G125 drawers (111 mm useful height). The cabinet is supported 310 mm above the floor to make room for the dollies and put the drawers at a convenient height. The two deeper bottom drawers are used for storing active solutions sealed in ampoules for ion-chamber measurements (typically 1-10 MBq each). These drawers are shielded at the front with 50 mm of lead (Fig. 3). This shielding weighs about 25 kg per drawer, well below the 100 kg rating for G125 drawers. The shallow drawer immediately above the deep drawers is filled with lead to a depth of 25 mm and is kept closed except when being photographed (Fig. 4). The M 70 kg of lead in this drawer slightly exceeds the 60 kg rating for G50 drawers but this is of no concern because the drawer is never open in normal use. The remaining 12 shallow drawers are used to store beta- and gamma-ray point sources (typically kbq each) used for instrument calibration and testing (Fig. 4). When these weaker sources are being moved to or from the upper drawers the stronger sources in the lower drawers and on the dollies are still substantially shielded and contribute little radiation exposure. The total activity stored in the drawers is typically of the order of 200 MBq but fluctuates considerably as sources decay and are replaced. If the need arises, more lead shielding can be added at the front or partway back in any of the drawers.

6 - 2 - The cabinet and the space below It are shielded on all sides except the bottom with 50 mm of lead. If it were not for the shielded sources and drawers nearly twice as much peripheral shielding would have been required with a great increase in weight and cost. The supporting frame was welded from 50x50x6 mm (2"x2"xl/4" norainal) steel angle with 50x6 nun (2"xl/4" norainal) steel flat bar bracing. The cabinet and the shielding above it are each supported on a 6 ram thick steel plate. The door ("500 kg) is hung from a 50x50 mm I-beam constructed by welding 50x2.5x5 mm (2"xl"x3/16" nominal) steel channels back to back. The door rolls on four 32 mm 0D x 13 mm ID x 12.7 mm thick ball bearings (type ND-87013) located in pairs on 13 nun diameter shafts with 15 mm OD x 27 mm long spacers. Construction and assembly details can be inferred from Figs. 1 and 2. Structural drawings (E-HCS-001, -002, -003 and A-HCS-004) are available from the author. The shielding was assembled from 200x100x50 mm (8"x4"x2" nominal) lead bricks with enough half-sized bricks to allow staggered rows. A number of bricks were cut to size as required. Because the sources are widely dispersed within the cabinet it was judged unnecessary to use interlocking or overlapping bricks. Aluminum sheet 1.5 and 0.8 mm thick covers the two sides adjacent to the cabinet and the inside of the door respectively to prevent accidental displacement of the bricks. The completed vault weighs about 3 t. After all sources stored in the old vault were transferred to the new one the maximum gamma-radiation field at the front surface of the drawers with the vault door open was 8 mr«h~ and the average was 2 mr«h~. With the door closed no fields 2. *2 mr'h were detected. Accordingly the shielding in the drawers is adequate for the present stock of sources but could easily be increased if needed. The new vault provides better organization and access as well as reduced radiation exposure compared to the one it replaced. Acknowledgements R.H. Martin specified the drawer and shielding requirements. R.L. Nowell suggested the use of shielded drawers and sources to reduce both radiation exposure and peripheral shielding. D.G. Logan designed the floor bracing, the Sheet Metal and Welding Shop constructed the frame and supplied the lead shielding and the Photographic Laboratory provided the illustrations. G.A. Sims and G.A. Tapp assisted in assembling the shielding. J.G.V. Taylor initiated the project and helped with the manuscript.

7 V DANGER A * ^RAYONNEMENT Fig. 1 Vault with door closed.

8 Fig. 2 Vault with door open showing storage beneath the drawers.

9 - 5 - Fig. 3 View of partially opened ampoule drawer showing lead shielding behind the front panel.

10 - 6 - Fig. 4 View showing lead-filled drawer immediately above the arapoule drawers and a partially opened point-source drawer.

11 ISSN ISSN To identify individual documents in the series we have assigned an AECL- numbertoeach. Please referto the AECL- number when requesting additional copies of this document from Scientific Document Distribution Office Atomic Energy of Canada Limited Chalk River, Ontario, Canada K0J 1J0 Pour identifier les rapports individuels faisant partie decette serie nous avons assigne un numero AECL- achacun. VeuiIlezfaire mention du numero AECL- si vous demandez d'autres exemplaires de ce rapport au Service de Distribution des Documents Officiels L'Energie Atomiquedu Canada Limitee Chalk River, Ontario, Canada KOJ1J0 Price: $2.00 per copy Price: $2.00 per copy ATOMIC ENERGY OF CANADA LIMITED,