Hydrogen Combustion in the Elevator Pit

Hydrogen Combustion in the Elevator Pit Frequency Bin IV, 10" > F 10- (Very Unlikely) No significant off-site radiological consequences YES... 

The frequency of each operational outcome or sequence for the event tree of Figure 3E-4 has been calculated as shown in Table 3E.4-1 below. The hazard analysis event corresponding to hydrogen combustion in the elevator pit is hazard event WE-4 of Appendix 3C. Event WE-4 frequency of occurrence was assessed as frequency bin IV. Sequence D also has a calculated frequency bin of IV so the event tree analysis is consistent with the hazard evaluation in assessed frequency of occurrence. 

Table 3E.4-1. Frequency of Occurrence for Each Sequence of the Event Tree Analysis Diagram for Hydrogen Combustion in the Elevator Pit...

The potential for hydrogen combustion in the elevator prt was examined by using event tree methodology. The sequence is initiated by the presence of an ignition source. Such a source exists in the elevator pit in the form of the hydraulic lift limit switch, which would close contacts if the lift were lowered to the bottom of the pit. it is assumed that if this switch is activated, and if oxygen levels are sufficient, ail free hydrogen at or above a concentration of 4% that exists in the pit would be ignited. 

The potential releases of energy and subsequent pressure and temperature excursions in the highly improbable event of deflagration of hydrogen in the elevator pit have been quantitatively assessed. If combustible levels of hydrogen and oxygen are ignited in the pit, the potential... 

Because the residual waste is in solidified form inside of steel containers in the elevator pit, and the combustion of hydrogen in the pit will not affect the solidified waste, the waste inventory is in a form which is not readily available for release (DOE 1994d). Thus, there is no credible source term of any significant magnitude. 

The Zone 2A elevator pit and Room 109 are the only two areas In the HCF where waste will be accumulated. By the time that the waste is moved into Room 109, the hydrogen generation rate is sufficiently low as to preclude accumulation to flammable levels. In the elevator pit, a flow of nitrogen of approximately 0.07 cubic feet per minute (cfm) wll be provided to the bottom of the pit to sweep out both hydrogen and oxygen, which will reduce the likelihood of hydrogen combustion due to depleted levels of both combustion constituents. The normal ventilation flow that will exist in Zone 2A will also preclude any buildup in that area. 

Sequence B The ignition source occurs due to inadvertent lowering of the elevator. However, nitrogen flow maintains hydrogen and oxygen levels below combustible levels, even in the event of a barrel failure that could release up to 65 liters of hydrogen into the pit in a short period of time (2.6% of the pit free volume). No combustion is possible. 

---