Reactor safety can mean different things to different people. At STL we believe that a safe reactor must have the following attributes:
- It must not be able to meltdown – ever! (even when subjected to the severity of a Fukushima type event).
- This means that fission products will always be retained within a safe containment structure.
- The containment structure will have multiple independent barriers to ensure fission product retention.
- During and after a worst case accident the evacuation of people from the surrounding area will not be necessary.
- Spent fuel will be passively safe – this means that no active cooling should ever be required for fuel storage to prevent release of fission products from spent fuel
The pebble bed reactor is designed to have a low power density of approximately 4-5MW/m³ as opposed to more than 100MW/m³ in most water cooled reactors. This low power density has the following advantages:
(1) Firstly it means that it is not critical to cool the core at all times as is the case in a water cooled reactor. In the event that the helium coolant flow is interrupted the reactor core temperature will take in the order of 20 hours to increase slowly to a maximum temperature of approximately 1550°C during which time it will automatically shut itself down (i.e. become subcritical) due to the effect of the negative temperature coefficient.
(2) Secondly the low power density means that the reactor geometry can be designed to make use of natural heat removal mechanisms to remove heat from the reactor and disperse the heat into the environment.
The safety features of the HTMR100 start with the fuel. During nuclear fission certain radioactive fission products are formed which are mainly in a gaseous form. The longer fuel remains in a reactor the more fission products are produced. As these fission products are transformed from a solid to a gas, the container in which these fission products are encapsulated will start to experience a pressure build-up. The coated particles are designed to withstand these pressures and have been tested and demonstrated to not release fission products. This capsule for the fission products is the first protective boundary in ensuring that the harmful fission products are not released into the atmosphere.
Another important characteristic of the fuel sphere containing TRISO coated particles is the fact that every single material is a ceramic with a very high melting point; therefor it is suitable for high temperature environments, even during postulated reactivity incidents, making it intrinsically safe.
Retention of Fission Products in fuel sphere
In the fuel sphere it has been proven that 99.99% of the fission products are retained within the 1mm small fuel particles, which are in effect small pressure vessels, coated with multiple silicon carbide and pyrolitic carbon layers. These layers provide excellent pressure and fission product retention to temperatures in excess of 1600°C
(Picture Retention Fission Products)