article: Father-of-six Peter O'Brien, 51, and 41-year-old Mark Sim died after a blast in the basement of the Celsa Steel UK plant in Splott on November 18, 2015. Five men were also injured in the incident. "What happened a year ago was the most tragic event in our history," said Celsa managing director Luis Sanz "And our thoughts remain with their loved ones. "We will be holding two minutes' silence to pay tribute to Mark Sim and Peter O'Brien," . The then Prime Minister David Cameron tweeted his support to "those affected" by the explosion at Celsa's rod and bar mill, while Wales' First Minister Carwyn Jones said he was "deeply saddened" by the deaths. More than 10 fire engines and ambulances attended the incident around 10:30 on November 18, 2015, while the air ambulance was also deployed. Mr O'Brien, who lived in the Llanishen area of Cardiff with his wife Marie, played rugby and baseball for St Peter's RFC and more than 1,000 people attended his funeral. Mr Sim, who lived in Caldicot in Monmouthshire with his wife Sam and their two children, was a fan of his home-town football club Newcastle United and played for his local darts team. "We have been very moved by the levels of support we have received throughout Cardiff, Wales and the UK," added Mr Sanz. "On behalf of everyone at the company, I want to say thank you for the kindness we have received. "We continue to work closely with the authorities to establish the causes and learn lessons. "The safety of employees is our absolute number one priority and we are deeply committed to the community here in Splott."
summary: Steelworkers will observe two minutes silence on Friday to remember two colleagues killed in an explosion at a Cardiff plant a year ago.