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IMPACT ON PEOPLE AND THE ENVIRONMENT
Release

WORKED EXAMPLE: FIRE IN AN AIR COOLED REACTOR

 
Summary:
 
  • Large energy release during operations led to a fire
  • Air samplers at a distance of 800 metres showed activity levels 10 times higher than normal
  • A check of reactor temperatures indicated that about 150 channels were overheated
  • The reactor core was eventually cooled down
  • Activity released was estimated to be between 500 and 700 TBq of 131I and 20 to 40 TBq of 137Cs

Scenario

The graphite moderator of an air-cooled plutonium production reactor caught on fire, which resulted in a significant release of radioactive material. The fire started during a planned process of annealing the graphite structure. (During normal operation, neutrons striking the graphite result in a build-up of stored energy in the graphite, which is released by an annealing process). Unfortunately, in this case, excessive energy was released, resulting in fuel damage. The metallic uranium fuel and the graphite then reacted with the air and started burning. The first indication of an abnormal condition was provided by air samplers about 800 metres away. Radioactivity levels were 10 times above normal levels. Sampling closer to the reactor building confirmed that radioactive releases were occurring. Inspection of the core indicated the fuel elements in approximately 150 channels were overheated. After several hours of trying different methods, the fire was extinguished by a combination of water deluge and switching off the forced air cooling fans. The plant was cooled down. The amount of activity released was estimated to be between 500 and 700 TBq of 131I and 20 to 40 TBq of 137Cs. There were no deterministic effects and no one received a dose that would reach ten times the statutory annual whole body dose limit for workers.

INES procedure (click the highlighted flowchart items to follow the procedure)

Figure 4, page 145 of the Manual

Figure 5, page 146 of the Manual

Calculate radiological equivalence

Here's the calculation for the lower estimate, look at the upper estimate and see what rating you think the event would have.

(40 x 20(137Cs)) + (1 x 500(131I)) = 1 300 TBq 131

Here's the calculation for the upper estimate, look at the lower estimate and see what rating you think the event would have.

(40 x 40(137Cs)) + (1 x 700(131I)) = 2 300 TBq 131I

Check the answer

Answer

The total release was radiologically equivalent to between 1 300 and 2 300 TBq of 131I

Level 5 defines that the release should be:
“Equivalent to more than hundreds of TBq 131I”

Level 6 defines that the release should be:
“Equivalent to more than thousands of TBq 131I”

The boundary between the two ratings is about 5 000 TBq 131I.
(Footnote 2 on page 17)

Therefore:

The reactor fire leading to a radioactive release would be rated at Level 5.