24 April 2012

PRESSURIZED WATER REACTORS (PWR)


The PWR was developed initially from the system used to power the USA's nuclear submarines. Ordinary water used both as the coolant and moderator in a common circuit. The fuel is 2-4% enriched uranium oxide clad in zirconium alloy. The system is highly pressurised to achieve a high coolant temperature. LWR's use a massive steel pressure vessel to hold the complete reactor core. In order to refuel, the reactor must be shutdown, cooled, and depressurised. The lid is then removed from the pressure vessel. Fuelling is carried out at intervals of 12-18 months with a required shutdown period of several weeks.

          The Pressurised Water Reactor (PWR) has 3 separate cooling systems. Only 1 is expected to have radioactivity - the Reactor Coolant System. The Reactor heats the water that passes upward past the fuel assemblies from a temperature of about 300 C to a temperature of about 334 C. Boiling, other than minor bubbles called nucleate boiling, is not allowed to occur. Pressure is maintained by a Pressuriser connected to the Reactor Coolant System. Pressure is maintained at approximately 15 MPa through a heater and spray system in the pressuriser. The water from the reactor is pumped to the steam
generator and passes through tubes. The Reactor Cooling System is expected to be the only one with radioactive materials in it. Typically PWRs have 2, 3, or 4 reactor cooling system loops inside the containment.

          In a Secondary Cooling System (which includes the Main Steam System and the Condensate-Feedwater Systems), cooler water is pumped from the Feedwater System and passes on the outside of those steam generator tubes, is heated and converted to steam. The steam then passes through the Main Steam Line to the Turbine, which is connected to and turns the Generator. The steam from the Turbine condenses in a Condenser. The condensed water is then pumped by Condensate Pumps through Low Pressure Feedwater Heaters, then to the Feedwater Pumps, then to High Pressure Feedwater Heaters, then to the Steam Generators.
          The diagram above simplifies the process by only showing the condenser, a pump, and the steam generator. The condenser is maintained at a vacuum using either vacuum pumps or air ejectors. Cooling of the steam is provided by Condenser Cooling Water pumped through the condenser by Circulating Water Pumps, which take a suction from water supplied from the ocean, sea, lake, river, or Cooling Tower. The first commercial PWR plant in the United States was Shipping Pennsylvania.

ADVANTAGES OF PWR:

      PWR reactors are very stable due to their tendency to produce less power as temperatures increase, this makes the reactor easier to operate from a stability standpoint.
      PWR reactors can be operated with a core containing less fissile material than is required for them to go prompt critical. This significantly reduces the chance that the reactor will run out of control and makes PWR designs relatively safe from criticality accidents.
      Because PWR reactors use enriched uranium as fuel they can use ordinary water as a moderator rather than the much more expensive heavy water as used in a pressurised heavy water reactor.
      PWR turbine cycle loop is separate from the primary loop, so the water in the secondary loop is not contaminated by radioactive materials.

Disadvantages of PWR:

      High strength piping and a heavy pressure vessel  are  required for highly pressurized coolant and hence increases construction costs.
      Most pressurized water reactors cannot be refueled while operating. This decreases the availability of the reactor- it has to go offline for comparably long periods of time (some weeks).
      Corrosion of carbon steel due to the presence of boric acid in coolant, limits the lifetime of the reactor and also increases the chances of radiation exposure.
      Water absorbs neutrons making it necessary to enrich the uranium fuel, which increases the costs of fuel production. If heavy water is used it is possible to operate the reactor with natural uranium, but the production of heavy water requires large amounts of energy and is hence expensive.
      Because water acts as a neutron moderator it is not possible to build a fast neutron reactor with a PWR design. For this reason it is not possible to build a fast breeder reactor with water coolant.
      Because the reactor produces energy more slowly at higher temperatures, a sudden cooling of the reactor coolant could increase power production until safety systems shut down the reactor.