Boiling Water Reactor Configuration boiling-water-reactor-configuration

Boiling Water Reactors

The boiler water reactor (BWR) is the second most widely used type of light-water reactor. The BWR has a single circuit, which is used to supply steam directly to turbines. The fuel used in the BWR is similar to that used for a pressurized water reactor (PWR), but the fuel rods have a larger diameter, and the bundles only contain about 50 sheaths. The moderator and coolant water temperature reach 286 °C at the core. The water is maintained at a pressure of only 70 bars (1000 psig), where it is transformed into steam. A BWR serves as a steam generator, which means the whole circuit is radioactive, and the water must be of very high purity.  

Boiling Water Reactor Treatment Circuits boiling water reactor

BWR Circuit Treatment Process

Makeup Water Treatment

Regenerable ion exchange resins are typically used to produce demineralized water that meets the required quality of the plant, while non-regenerable, highly-regenerated mixed beds are used for final polishing. These final polishing mixed bed resins are re-used in regenerable vessels after they are exhausted. Many nuclear units rely on service companies to supply makeup water to minimize the handling, storage and use of treatment chemicals and the production of chemical waste streams onsite.
 
Condensate Polishing
Condensate polishing is one of two methods to control the purity of the BWR coolant after makeup polishing. This type of polishing is primarily accomplished by using precoat filters on freshwater cooled units and deep bed ion exchange vessels for brackish and salt water cooled units. 

Units that rely on low-salinity cooling water sometimes use precoat filters with powder resins (Microlite® PrCH, PrAOH, MB and CG range). Although powdered resins have a disadvantage of rapid exhaustion in the event of a condenser leak, they have the following advantages:

  • Lower capital cost because regeneration facilities are not required.
  • Good filtration of corrosion products.
  • Powdered resins are used once and are at minimal risk of deteriorating chemically, even at relatively high temperatures.

In brackish and saltwater-cooled systems, whole bead resins are used in deep-bed polishers and are primarily high-capacity gel form resins. However, macroporous exchange resins offer exceptional selectivity for cobalt-60 and iron compared to gel-type resins. Mixed beds with an equivalent cation-to-anion ratio are preferred. Compared to the precoat polisher, deep-bed polishing systems ensure several hours of operation in the event of a cooling condenser leak. After the polishing step, condensate is sent directly to the reactor.

Reactor Coolant Purification
The reactor water or coolant purification system is the primary system for cleaning the coolant. This coolant becomes charged with radioactive corrosion products during the power cycle and requires removal before a refueling outage. This is achieved by diverting water through the suppression pool demineralizer. The suppression pool is an extensive reservoir of water located below the reactor that supplies flood-up water for fuel change outage activity and if the reactor water pumps fail. This demineralizer is loaded with a mixed bed ion exchange resin in equivalent ratios. Specialty macroporous anion resins used as an overlay in the reactor water cleanup vessel also have a greater affinity for particulate iron compared to gel resins. 
 
Mixed beds with macroporous cation bead resins (NRW3560) layered with a macroporous anion NRW5010 or NRW5070 have proven to be more effective for iron cleanup than gel resins. Cycling of reactor coolant is stopped when activity drops below 5.0E-2mRem.

Cleanup systems that use only powdered precoat resins have had success using a precoat mixed resin CG125H which has a blend of macroporous cation resins to remove metals such as copper. Microlite® precoat products used for RWC demineralizers include MB1:1H and CG19H.
 
Spent Fuel Pool Treatment
The exhausted or spent fuel is stored in large pools of cooling water to remove residual heat. This coolant becomes loaded with radioactive isotopes (primarily cobalt-60) released from the fuel bundles with low levels of iron. These pools must be treated to control clarity and remove activity. The suppression pool demineralization will often be used to treat the spent fuel pool.

Radwaste Treatment
Radioactivity originating from the plant is treated with mixed bed ion exchange resins to reduce activity to an appropriate level for release back to the reactor. The mixed bed resin that is most appropriate for this application is NRW3240. Makeup goes to the hot well and is further polished by the condensate polishing system before entering the boiler.