Hydrogen and oxygen only react at significant rate at high temperatures. In a hydrogen explosion, the rate initially starts off low, but as the reaction is high

Hydrogen and oxygen only react at significant rate at high temperatures. In a hydrogen explosion, the rate initially starts off low, but as the reaction is highly exothermic, heat from the reaction increases the temperature and causes the reaction to accelerate until reactant concentrations are sufficiently depleted. This means that hydrogen and oxygen can only react quickly if the concentration of hydrogen in a volume is high enough that the increase in temperature from the exothermic reaction is faster than the dissipation of heat, so the temperature will rapidly expand, causing an explosion. For this reason, hydrogen tends to build up and then explode, rather than burning continuously. It might be possible to trap hydrogen in a small volume and burn it in a series of small explosions; however, I don't think the reactors are designed that way.