Physical Barriers to Environmental Release of radioactive releases in case of reactor accidents: Defense-in-Depth concept

The following five barriers are built into the reactor design to prevent radioactivity escaping from the reactor to the public:

1. Ceramic Fuel - The ceramic uranium dioxide fuel in pellet form entraps most of the fission products generated due to the fission reaction in the fuel. These fission products would be released if the fuel were to melt. The fuel has a high melting point, but continuous cooling is required so that the fuel is not over-heated resulting in melting of the fuel. Another safety feature of the ceramic fuel is that it is relatively chemically inert with the heavy water coolant.

2. Fuel Sheath (cladding) - The fuel pellets are enclosed in a high integrity, welded zircalloy sheath. This sheath contains the gaseous and volatile fission products which escape from the pellets. The sheath is designed to withstand the stresses resulting from pellet thermal expansion, gaseous fission product build-up and external hydraulic pressure.

3. Heat Transport System Boundary - The high integrity pressure tubes, piping, and vessels contain most fission products escaping via sheath defects until they are removed via the coolant purification system.

4. Containment Boundary - This is designed to withstand the pressure surge of a worst case LOCA, with a small ‘puff release’ during the overpressure transient. Post LOCA containment venting via a filtered, monitored pathway minimizes the environmental radioactive release.

5. Exclusion Zone - No permanent residence is allowed within a 1.6 km radius from nuclear reactor. This ensures significant dilution of an airborne radioactive release before it reaches any public habitation, thus reducing the resulting public dose.

Inadequate fuel cooling due to cooling system failure, the situation which is prevailing now at Fukushima nuclear power reactor in Japanese, results in overheating of the fuel, with potential for large scale fuel failures. In the event of large scale fuel failures, at least two of the five physical barriers would be breached, i.e., the fuel and the fuel sheath. In the case of a LOCA, the third barrier, the heat transport system is also breached, leaving only the containment and exclusion zone barriers.

In the case of a LOCA coincident with containment failure (dual failure), only the exclusion zone would remain as a physical barrier. Thus, the Containment boundary is a very strategic defense-in-depth barrier to fission product release in to the public domain. .