CHAPTER 1: General characteristics of Nuclear Power Plants and reactor types

General characteristics of Nuclear Power Plants and reactor types

Nuclear power plants consist of a nuclear reactor core, a reactor coolant system that extracts the heat from the reactor core and, for water cooled reactors, a steam and power conversion system that converts the heat into steam that drives a steam turbine, which is connected to a generator. Depending on the technology adopted for the conversion cycle, nuclear power plants can be distinguished in:

Direct cycle type, i.e. the steam which is generated in the reactor core by the boiling coolant is directly transported to the turbine;
Indirect cycle, where the heat of the core is transported to a steam generator, from where the generated steam goes to the turbine.

A Boiling Water Reactor (BWR) is a direct cycle reactor; a Pressurised Water Reactor (PWR, including VVER, which is a reactor type with horizontal steam generators) and the Pressurized Heavy Water Reactor (PHWR) are indirect cycle reactors. The reactor system (i.e. the pressure vessel or the pressure tubes) with the cooling circuit is housed in a leak tight building, the containment.

Hence, the scope of SAMG-D encompasses the following three types of water cooled nuclear power reactors:

1. Boiling Water Reactors (BWRs), which consist of a Reactor Pressure Vessel (RPV) and a coolant circuit. The steam is generated directly in the reactor core and drives the turbine. Fuel is slightly enriched uranium (up to about 5% enriched in 235U) and is contained in rods of about 1 cm in diameter, mostly bundled in closed canisters ('fuel elements') with 200-300 rods each. Shutdown rods are usually below the RPV and are driven in by hydraulic forces The BWR has a negative power coefficient, i.e. power decreases if the temperature increases. It also has a negative void coefficient, i.e. shutdown occurs automatically upon a large Loss Of Coolant Accident (LOCA). Refuelling is done by opening the RPV in an outage. The reactor is housed in a leak tight structure called the containment that is designed to keep any radioactive material inside that would otherwise be released in an accident involving the reactor core. This containment is usually equipped with a suppression pool to condense the steam that might be released in a LOCA.

2. Pressurised Water Reactors (PWR), which also consists of an RPV, but where the coolant water is not allowed to boil by keeping it under pressure. The coolant flows through steam generators. The steam produced in steam generators drives the turbine. The fuel is like the BWR fuel, but not in canisters (it should be noted that some VVER plants use fuel in canisters). Shutdown rods are above the RPV and fall in by gravity. Power and void coefficients are as with the BWR. Refuelling is as with the BWR. The reactor is housed in a containment, as with the BWR, but generally not equipped with a suppression pool (VVER-440/V213 reactors design includes pressure suppression pool). Most PWR containments use Fan Cooler System and Containment Spray System to reduce the pressure. Most reactors have vertical steam generators, but a Russian design uses horizontal steam generators, with the plants usually not denoted PWR but water-water energetic reactor (VVER).

3. Pressurised Heavy Water Reactors (PHWR), some of which have a pressure vessel like the PWR, but most have a fuel channel / pressure tube design and are referred to as CANada Deuterium Uranium (CANDU) reactors or IPHWR, Indian PHWR. The - mostly natural uranium - fuel is contained in fuel bundles, which are arranged horizontally inside pressure tubes, of which there are several hundred. The pressure tubes pass through an unpressurized vessel called the calandria which contains heavy water for neutron moderation. The heavy water coolant from the pressure tubes transfers heat to light water in steam generators like those in PWRs. Shutdown rods are placed above the fuel and fall in by gravity. Most PHWRs have a positive void coefficient of reactivity, i.e. power or reactivity increases as the void content inside the reactor increases, such as in a large LOCA. For this reason, many PHWR have a second fast-acting shutdown system. Many PHWR reactors are housed in a containment similar to the PWR. Some multi-unit CANDU stations have their local containments connected to a common vacuum building, kept at near zero absolute pressure.

Typical characteristic of a nuclear core is that it still produces heat when the nuclear chain reaction has been stopped, so-called decay heat. This amount of heat is about 6% of the nominal heat immediately after shutdown of the fission process and then decreases exponentially with time. For this reason, nuclear reactors are equipped with systems to evacuate the decay heat towards the ultimate heat sink, e.g. the Residual Heat Removal System (RHRS).

All reactors have means to cool the reactor core during loss of coolant accidents. These systems are called Emergency Core Cooling Systems (ECCS) and belong to a large family of the so-called Engineered Safety Features (ESF) of the nuclear power plant. The ECCS functions independently from the other reactor systems. In addition, emergency power systems (diesel generators) are provided in case of loss of normal power supply from the off-site grid or via the plants own transformer to provide electric power supply to the ESF.

The third barrier preventing the release of fission products to the environment is the reactor containment. Various designs exist for the containment:

• The BWR usually has a pressure suppression containment: the steam that escapes from the reactor circuit is condensed in a large pool, the pressure suppression pool.
• The PWR has often a full pressure containment, i.e. it just captures the escaping water and steam in its volume. In some PWRs the steam is condensed in an ice package, so-called ice-condenser containments. Some VVERs use pressure suppression pools.
• Candu PHWRs have a full pressure containment and, for multi-unit plants, a vacuum-building containment.