Some facilities have an advanced containment/protection system

Types

Containment building

A containment building, in its most common usage, is a steel or reinforced concrete structure enclosing a nuclear reactor. It is designed, in any emergency, to contain the escape of radiation to a maximum pressure in the range of 60 to 200 psi (410 to 1400 kPa). The containment is the final barrier to radioactive release (part of a nuclear reactor's defence in depth strategy), the first being the fuel ceramic itself, the second being the metal fuel cladding tubes, the third being the reactor vessel and coolant system.

The containment building itself is typically an airtight steel structure enclosing the reactor normally sealed off from the outside atmosphere. The steel is either free-standing or attached to the concrete missile shield. In the United States, the design and thickness of the containment and the missile shield are governed by federal regulations (10 CFR 50.55a).

While the containment plays a critical role in the most severe nuclear reactor accidents, it is only designed to contain or condense steam in the short term (for large break accidents) and long term heat removal still must be provided by other systems. In the Three Mile Island accident the containment pressure boundary was maintained, but due to insufficient cooling, some time after the accident, radioactive gas was intentionally let from containment by operators to prevent over pressurization. This, combined with further failures caused the release of radioactive gas to atmosphere during the accident.

Containment systems for nuclear power reactors are distinguished by size, shape, materials used, and suppression systems. The kind of containment used is determined by the type of reactor, generation of the reactor, and the specific plant needs.

Suppression systems are critical to safety analysis and greatly affect the size of containment. Suppression refers to condensing the steam after a major break has released it from the cooling system. Because decay heat doesn't go away quickly, there must be some long term method of suppression, but this may simply be heat exchange with the ambient air on the surface of containment. There are several common designs, but for safety-analysis purposes containments are categorized as either "large-dry," "sub-atmospheric," or "ice-condenser."

The ultimate safety system inside and outside of every BWR are the numerous levels of physical shielding that both protect the reactor from the outside world and protect the outside world from the reactor.

There are five levels of shielding:

1. The fuel rods inside the reactor pressure vessel are coated in thick Zircalloy shielding;

2. The reactor pressure vessel itself is manufactured out of 6 inch thick steel, with extremely temperature, vibration, and corrosion resistant surgical stainless steel grade 316L plate on both the inside and outside;

3. The primary containment structure is made of steel 1 inch thick;

4. The secondary containment structure is made of steel-reinforced, pre-stressed concrete 1.2–2.4 meters (4–8 ft) thick.

5. The reactor building (the shield wall/missile shield) is also made of steel-reinforced, pre-stressed concrete 0.3 m to 1 m (1–3 feet) thick.

If every possible measure standing between safe operation and core damage fails, the containment can be sealed indefinitely, and it will prevent any substantial release of radiation to the environment from occurring in nearly any circumstance.

Дата добавления: 2015-05-31