The machine is used for cleaning commercial kitchen hoods, extractor filters, pots, pans and grills.
Mr Sellman-Leava was wearing gloves, but no protective goggles or full length apron and when the machine was opened and hot steam burned his face, he dropped some grill plates which splashed hot water and steam on to his body and legs.
The magistrates said Mr Sellman-Leava suffered a “significant physical injury” and there was no knowledge of what chemical substances were used in the decarboniser.
Although Wetherspoon had more than 100 risk assessments, it had no system to prioritise the risk of injury if “something dreadful happens”, magistrates said.
Wetherspoon said it spent £3.5m on staff training and had a good safety record, with only five health and safety convictions in a 30-year period.
“While the incident and injuries must have been frightening and unpleasant, they were not life-threatening or permanent,” the company’s lawyer said.