Boiling Water Reactors

Boiling Water Reactors (BWRs) are a fairly common design in electrical generation.  The advantage a BWR has over a Pressurized Water Reactor (PWR) is mainly the up-front cost.

The primary coolant loop in a PWR is kept at about 2200 psig (155 bar) to prevent boiling in the core.  This means the reactor vessel, pressurizer, and steam generators as well as all primary loop components, must be very robust.  In addition, they must be made from (or lined with) corrosion resistant materials, due to the addition of boric acid for reactivity control.

With a Boiling Water reactor, the primary coolant loop is also the steam loop.  There are no steam generators or pressurizer.  The operating pressure is about half that of a PWR, and no boric acid is used.  Thus a boiling water reactor can be built with less cost.  A BWR also has higher thermal efficiency than a PWR.

Below is a diagram of a BWR vessel.  While less expensive to build than a PWR, these are still pretty complex reactors. 

Below is a simplified overall diagram of an entire BWR steam/water circuit.

A few things you will notice that are different from a pressurized water reactor: 

• The top of the reactor vessel is used as a steam drum, to separate water and steam.
• Because the top of the reactor is a steam drum, control rods are moved to the bottom.
• Inside the reactor vessel are circulation pumps (#4).  More on these later.
• Steam directly from the reactor core flows through the entire steam system.
• Not shown in the above image are the feedwater and steam shutoff valves.  In the event of a steam line rupture, it is important to keep water in the core, and also to prevent the release of radioactive steam outside of the containment structure.
• Note the shielding required around the entire steam system. 

Some of the disadvantages of a BWR with respect to a pressurized and segregated reactor coolant loop become apparent. 

• The control rods and circulating pumps penetrate the bottom of the reactor vessel.  Should a leak develop there, it can drain all the water from the reactor vessel.  Since the fuel must be kept submerged in water (even for several years after use), this is a potential hazard.
• Water circulating in a power plant always contains minor levels of corrosion - flecks of iron from valves and pumps, and magnetite that falls from the inside of piping as temperatures expand and contract the pipe.  These corrosion particles can be carried through the reactor core and absorb a neutron.  They can become very radioactive, and then settle outside the reactor core.  This material is called CRUD, and is only detectable with a radiation detector.
• In a PWR, CRUD is all contained within the primary loop and inside the containment structure.  In a BWR, CRUD is everywhere within the steam system - the steam and drain lines, the turbine, the condenser, the condensate pumps, and feedwater pumps.
• There is an important nuclear reaction that occurs that involves the Oxygen in water molecules, H2O.  Oxygen-16, when hit with a high energy neutron, can absorb the neutron and eject\ a proton.  It then becomes radioactive Nitrogen-16.  This is a nasty isotope of nitrogen, mainly because it decays by emitting a gamma of 7.37 MeV.  That's a VERY hot gamma ray.  Nitrogen-16 has a half-life of 7.1 seconds, so that means it will be emitting these hot gamma rays while it is passing through the steam turbine, condenser, and settling in the hotwell.  Therefore massive shielding is required from all steam and feedwater systems during operation. 
• Due to the short half-life of Nitrogen-16, it quickly decays away.  Shortly after the reactor is shutdown, the only serious radiological hazard in the steam system is due to CRUD, which is composed of metals that have longer half-lives.

 Below is the generator for a BWR.  Note the tan-colored radiation shield blocking the view of the steam turbine.  This is to protect personnel on the operating floor from Nitrogen-16 gamma radiation.

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BWRs are a little more complex than PWRs from a reactor control standpoint.  A BWR, by its very description, has significant in-core boiling - primarily in the upper portion.  However a BWR, like the PWR, uses water as a moderator.  When water changes to steam, its neutron moderating properties change drastically - 1/1000 drop in density equates to about 1/1000 drop in moderating ability. 

With the phase change from water to steam there is an ongoing change in moderation and thermal neutron population, and therefore in the fission process.  It is therefore important to monitor neutron levels (flux) at many more points than would be necessary in a BWR, to ensure even power distribution.  Fuel burn-up (both axial and radial) is also difficult to calculate for the top end of a BWR, due to the dynamic two-phase activity in the core.

Interestingly, like the PWR, the BWR control rods are out of the core at full power, and reactivity is controlled by using the circulation pumps.  When the pumps are run at high speed, boiling occurs higher up in the core, and so more moderator is present, and more neutrons are available to create fissions.  Similarly, if reactor power needs to be reduced, the recirculation pumps can be slowed down, and boiling occurs lower in the core.  More steam in the core means less neutrons will be thermalized, and so fewer neutrons will be available to cause fission.

Lastly, BWR cores have to be somewhat larger than PWR cores for a given output, and that is mainly due to the fact that the upper part of a BWR core isn't generating much power, because steam is a poor moderator.