Light Water Reactors

Mina Bionta
December 30, 2009

Submitted as coursework for Physics 204, Stanford University, Fall 2009

Fig. 1: This figure shows a boiling water reactor (BWR). In this design there is only one contained loop of water, which both is heated by the reactor core, then turned into steam, which is used to drive the turbines and produce electricity.

Nuclear power can be integrated into civilian use through the creation of power plants. The successful integration of nuclear power is more widespread in Europe compared to the United States. In 2001, 80% of electricity in France came from nuclear origins, while only 17% of electricity in United States is from nuclear power plants. Overall 18% of the world's electricity is from nuclear power. "If electrical energy produced by nuclear fission is to play a major role in satisfying the needs of society, nuclear power must be competitive with other means of producing electricity" environmentally, economically and safely.

Safety is one of the main concerns when dealing with power plants. With many power plants comes the threat of terrorist attack, as there is a direct link between nuclear energy and nuclear weapons. Luckily, large civilian power reactors use only slightly enriched uranium, not capable of being used in weapons.

There are three steps to creating nuclear power. First are the neutronics. This step deals with the reactor core, fission events and the interactions of neutrons. Second deals with the thermohydraulics. This involves the transfer of heat from the reactor to the generating system via water. Finally is the turbogeneration step. This involves the mechanical process of the turbines to transform the heat into electricity. Only the first step involves nuclear physics, the second and third steps are the same for all power generation systems.

During the neutronics step, neutrons are emitted by each fission reaction. These neutrons must be slowed down or moderated in order to be useful to drive another fission reaction. They are moderated by graphite or heavy water (water with deuterium in place of hydrogen) before returning to fuel rods to cause fission. The fuel in power plants is a low-enriched fuel. This means that the fuel is only 4-5% U-235 (natural uranium is only 0.97% U-235, weapons grade uranium is ~95% U-235). For each U-235 atom destroyed by fission or neutron capture, the probably of another fission even occurring is 60%. This is because besides fissions of U-235, the U-238 can capture a neutron and transmute into Pu-238. Most of the Pu-238 that is created is destroyed by the end of the 4-year lifespan of the fuel rod.

Nuclear fuel rods are made of a ceramic of UO2. This provides the best possible alloy for heat conduction. The actually physical dimensions of the rods vary by reactor design and are described in more detail below.

There are two types of light water reactors in the United States: boiling water reactors (approximately 1/3 of the US water reactors) and pressurized water reactors (approximately 2/3 of the US water reactors). These reactors use ordinary light water as their moderator, at high pressure and temperature. The chain reactions are controlled by about 100 control rods made of boron-carbide. This boron-carbide alloy is a potent neutron absorber. These rods delay neutrons by about 10 seconds in order to make control possible.

Boiling Water Reactor

Fig. 2: This figure shows a pressurized water reactor (PWR). This design of reactor calls for two separate loops of water, one that is highly pressurized (and never boils) and heated by and core itself, and one that is heated by this heated pressurized water. The non-pressurized water then turns into steam and is used to drive the turbines and produce electricity.

In boiling water reactors (BWR), the core is located below and in the water, thus able to heat it to boiling in order to drive the generators. The reactor is used to directly heat the water that becomes the steam that drives the turbines. The heat generated from the reaction process flows from the interior of the ceramic fuel pellets to the surface. It crosses a gas filled gap to a metal tube and then into the flowing water. The water then turns into steam, which can then be used to turn a turbine to produce electricity. Fig. 1 shows a BWR reactor. In the figure you can see how the loop for the producing steam is the same loop of water that is heated by the reactor.

The fuel rods are arranged in pellets that are 8mm in diameter and 5m long that are each in their own individual tubes that have an outside diameter or 9.5mm. This provides a gas filled gap of 0.05mm that the heat flows through.

Pressurized Water Reactor

Pressurized water reactors (PWR) differ from BWR in that there are two separate loops of water in the system, thus facilitating containment of contaminated water. The water that touches the reactor core is pressurized to 153 atm so that it does not boil. Instead this water is used as a heat source to heat up a separate loop of water that boils into steam and then used to turn the turbines. Fig. 2 shows PWR reactor. In this figure you can see how there are two separate loops of water: one that is directly heated by the reactor and never boils and a second which then turns into steam to turn the turbine.

The fuel rods are arranged in tubes of 5m by 9mm in diameter. They differ from the fuel arrangement of a BWR in that they place 264 fuel rods in a 17x17 assembly, or fuel element. This leaves 25 slots for control elements.

© Mina Bionta. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] Garwin, Richard L., and Charpak, Georges, Megawatts and Megatons (Alfred A. Knopf: 2001) pp 31-57, 107-152.