Explosion in a nuclear reactor (Three Mile Island)

This event reports only the role of hydrogen in this nuclear accident. For a short description of the initial causes and the incident evolution, see "pre-event summary” and for a much more detailed and structured information see references.

After almost 2 hours from the start of the incident (a loss of pressurised water supply to the reactor core), the top of the reactor core was only in contact with steam and exposed to the intense heat. This condition enabled the exothermic reaction between the steam and the zircaloy nuclear fuel rod cladding, which produced zirconium dioxide, hydrogen, and additional heat. This reaction melted the nuclear fuel rod cladding and damaged the fuel pellets, which released radioactive isotopes to the reactor coolant. The hydrogen accumulated to a bubble in the reactor and is believed to have caused a small explosion in the containment building several hours later.

The INITIAL CAUSE was the failure of the pumps of the secondary water loop followed by a pressure relief valve in the primary system which failed in the pen position.

An escalation followed because of the plant operators failed to recognize that a loss-of-coolant accident was occurring.

ROOT CAUSES are related to
(1) to signals not well designed for this emergency or defective (design flaws, including poor control design), and
(2) to the fact that the operators were ill-prepared (lack training, inadequate operative procedures).

secondary water coolant pumps, steam generators, pressure relief valve, reactor core, etc.

This is the text is an adapted and shortened version of the NRC report (see reference for full text and pictures, video). It describes the evolution of the accident before hydrogen was formed:
The plant experienced a failure in the secondary, non-nuclear section of the plant (one of two reactors on the site), which prevented the main feed water pumps—component from sending water to the steam generators that remove heat from the reactor core.
This caused the plant's turbine-generator and then the reactor itself to automatically shut down. The pressure in the primary system began to increase. In order to control that pressure, the pilot-operated relief valve opened, located at the top of the pressurizer.
The valve should have been closed when the pressure fell to proper levels, but it became stuck open. Instruments in the control room, however, indicated to the plant staff that the valve was closed. As a result, the plant staff was unaware that cooling water in the form of steam was pouring out of the stuck-open valve. When the alarms want off flashing warning lights, operators did not realize that the plant was experiencing a loss-of-coolant accident. Because of other misleading or inadequate information, they concluded that there pressurizer water level was high enough and that the core was properly covered with water too. That wasn’t the case and took a series of actions that uncovered the core, such as turning off the reactor coolant pumps which had started to vibrate. They also reduced the flow of water from the emergency cooling water being pumped into the primary system. Without the reactor coolant pumps circulating water and with the primary system starved of emergency cooling water, the water level in the pressure vessel dropped and the core overheated, creating the condition for the Zircaloy –steam reaction.

The consequence of hydrogen explosion was negligible. For the overall consequence, see references

The explosion was followed by a jet fire. The Internal Operation Plan was launched and terminated 1h50 later.

The extensive are listed in the Report of the President's Commission (see references)