Explosion during testing of a LH2 pump

The event occurred during the testing of a LH2 pump. When the air turbine pump drive was initiated, the cool down of pump loop was underway, and the speed had increased to design condition and stabilized, an explosion took place.
The H2 alarms were actuated and the panel power in the control room turned off.
The explosion was felt from 700 to 1000 feet away, and a fireball centred on the test cell. A fire at the vents was extinguished after 50 minutes.
The vent piping between Dewar and cell was found distorted; severe damage to the cell and nearby building was incurred by the fire and explosion.
Fractures were found in the discharge section of the pump loop; the pump discharge nozzle was separated from the pump scroll and the female bayonet was broken within 1 inch of the valve disk. A leak was found at the vent connection of the top Dewar.
Based on damage, the explosion was equivalent to 2 to 3 lb TNT.
[Ordin, NASA (1974)]

The INITIATING CAUSE was an accidental release of cold hydrogen and its ignition, with consequent fireball.
The root cause analysis of this incident is complex, and the report is not conclusive, also regarding the initial explosion, which could be not related to hydrogen leak but to oil.
The fractures were due to the use of incorrect materials. The bayonet material was magnetic ferritic like the AISI 442 rather than the austenitic AISI 304 and the metal in the scroll, welds and nozzle were magnetic martensite 17-4P21 which are brittle at LH2 temperatures. The pump case, flux joint, and vent piping were unjacketed and increase the flow of liquid air falling on the top and under the turbine pump base.
Oil stains were found on the floor under the turbine-pump bedplate and with air could have been responsible for the initial explosion, which probably caused the fracture of vent line.
The explosion due to ignition of H2-air caused subsequent fracture at pump discharge nozzle and bayonet connection, adding H2 to fireball.
It was also considered possible that the Dewar bayonet fractured prior to the explosion due to the brittle material used. The fractured fitting could impart sufficient force to fracture the stressed brittle pump nozzle.

The ROOT CAUSE could be then identified in an inadequate design.

LH2 pump, vent stack

The test was just starting