THE COBALT SPILL AT NRX, JULY 1958
On July 17, 1958 at 1504 hours the NRX reactor at the AECL, Chalk River project was shut down automatically by a loop fault. The fault was traced to a large leak that had developed in the WAPD CR-VI experimental loop. Radiation surveys of the lower header room showed fields of 1000 r/hr to be present. No accompanying airborne radioactivity was found. Melting of the loop specimens in the WAPD CR-VI loop due to low flow conditions was first suspected to be the cause of the fields. Sampling of the coolant in the loop indicated that the trouble lay elsewhere. No indications of activity in the effluent cooling water and exhaust air ruled out the possibility of a normal X-rod rupture. Inspection of the Hydraulic Rabbit rod and a uranium aluminum alloy fuel rod was prompted by several radiation surveys showing high activities at these positions. These rods were removed from the reactor before the real source of trouble was located in one of the cobalt isotope tray rods (T.6). After removal, inspection of the tray rod revealed that five of the twenty-five capsules therein had their bottoms partially or totally melted off. Each of the five melted capsules contained approximately thirty grams of nickel plated cobalt pellets. The cause of the overheating and subsequent melting of the five capsules was due to a serious reduction in the cooling air flow to the samples when a valve was accidentally left open in the lower end section of the rod during the July 7 shutdown. Evidence showed that the spill took place only a few hours after the startup on July 12. Cleanup of the estimated 20,000 pellets that had escaped from the five capsules into the lower header room was initiated on July 21st and continued on until the reactor start-up on August 8, 1958. Airborne radioactivity, usually associated with cohalt spills, was non-existent. This was due to the practice of nickel plating cobalt pellets before irradiation. An assortment of tools and methods were used to make the cobalt removal a success. Some of the more useful of these included the location of pellet sources in the headers with radiographic photographs, location of pellets on the floor utilizing beta radiation sensitive phosphor, displacement of pellets from the headers onto the floor by means of steam and water jetting, and removal of pellets to disposal flasks using vacuuming apparatus and magnets. Just prior to reactor start-up a general field of 1 r/hr persisted, arising from a few, difficult to remove sources of up to 300 r/hr in the headers. Since the field was considered to be sufficiently reduced, the reactor resumed normal operation on August 8. Cleanup of the remaining pellets in the headers was carried out during the monthly reactor shutdowns. Because of this incident all lower end sections of the tray and adjuster rods in the reactor were removed and replaced with modified end sections. (auth)
- Research Organization:
- Atomic Energy of Canada Ltd. Chalk River Project, Chalk River, Ont.
- NSA Number:
- NSA-17-017594
- OSTI ID:
- 4719209
- Report Number(s):
- IOI-158; AECL-1665
- Resource Relation:
- Other Information: Orig. Receipt Date: 31-DEC-63
- Country of Publication:
- Canada
- Language:
- English
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