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DEFENCE IN DEPTH
Defence in depth: facilities

WORKED EXAMPLE: NEAR CRITICALITY IN FUEL CYCLE PLANT

 

Summary:

 

  • Plutonium nitrate leaking from a pipe into a cell
  • Leak detection systems failed to detect the leak
  • Leak of 31 kg Plutonium nitrate found in daily inspection
  • Near critical crystalline structures formed

Scenario

At a plutonium recycling facility, a pipe carrying hot plutonium nitrate developed a leak. Over a period of about 24 hours, a total of 31 kg of the material leaked into the cell that housed the pipe. The leak was identified at the daily visual inspection. The hot plutonium nitrate ran over the outer surfaces of a hot plutonium evaporator and dripped onto the sloping stainless steel clad floor beneath. As the liquid ran over the various surfaces, it evaporated and deposited the plutonium in a crystalline form on the lowest part of the pipe and on the floor beneath, forming structures like a 'stalactite' and 'stalagmite'. The leak rate was such that the material failed to reach the detection sump as a liquid and was only identified through surveillance tours. The cell was subsequently decontaminated, the pipeline and evaporator replaced and the facility brought back into use.

The quantity of plutonium present on both the pipe and the floor was close to, but did not exceed, the minimum critical mass for the concentration of the material being handled at the time. Had the event taken place when more concentrated material was being handled, then the critical mass may have been exceeded.

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

Figure 10, page 151 of the Manual

Determine maximum potential consequences

The event needs to be considered in two parts:

  • First, with respect to releases from the facility, and
  • Second, with respect to doses to workers.

Dispersion of all the material accumulated in the cell could result in an environmental release equivalent to Level 5.

Return to flowchart

The event needs to be considered in two parts:

  • First, with respect to releases from the facility, and
  • Second, with respect to doses to workers.

The maximum potential consequence would be rated at Level 4 (fatal radiation exposure).

Return to flowchart

Determine number of safety layers and basic rating

There are two safety layers remaining to prevent the potential release:

  • The concrete structure of the cell containing the plutonium, and
  • The remaining building structure including primary and secondary ventilation systems.

There were no remaining safety layers to protect against a criticality.

The near criticality in the fuel cycle plant would be rated at Level 2.