Comment 61887

By Ryan (registered) - website | Posted April 05, 2011 at 17:19:45 in reply to Comment 61880

In a boiling light water reactor (BWR) like Fukushima Daiichi, pure demineralized water is circulated through the reactor core, which contains uranium fuel rods as well as partially inserted control rods. The water also acts as a neutron moderator, by absorbing some of the neutrons released by the uranium. (This is why spent fuel rods must remain submerged inside pools of water.)

The heat generated by uranium decay in the fuel rods boils the water, and the steam turns a turbine that generates electricity. The steam is then cooled in a condenser, and the water is cycled back into the reactor to be boiled again.

In the case of a shutdown, the control rods are inserted completely so that the uranium decay chain reaction stops altogether. However, some of the lighter elements that were produced when uranium atoms decayed are themselves unstable, and they will continue to decay into still lighter, more stable elements. The heat created by these decay elements is called decay heat, and it is this heat that caused all the problems at Fukushima Daiichi.

Because of the decay elements, a boiling water reactor must continue to have water flow through it to draw off decay heat even when it goes into "cold shutdown" with no more uranium chain reaction. Without cooling, the decay heat will keep increasing until the coating around the fuel rods melt, at which time uranium reaction will start again.

Eventually, the reaction will get so incredibly hot that the reactor casing itself melts. This is a meltdown. Reactors are designed so that, even in the case of a meltdown, the molten fuel rods should drop into a wide, shallow concrete foundation at the bottom of the reactor and spread out enough to stop criticality again.

Pressurized light water reactors (PWR) are a bit different from boiling water reactors. The water that runs through the reactor does not boil. Instead, it is maintained under high enough pressure that it cannot boil, and transfers its thermal energy via a heat exchanger to a secondary system in which the water does boil and turn a turbine.

Also, the control rods in a PWR are suspended above the reactor via an electromagnet (control rods in a BWR are held under the reactor via using high pressure hydraulic accumulators). If the power to the magnet fails, the control rods will automatically drop fully into the reactor and stop the uranium chain reaction.

PWRs tend to be more expensive than BWRs because they have to accommodate the high pressure system, but they also tend to be more stable. Maintaining two separate water systems further contains the radioactivity.

Another benefit to PWR is that the neutron absorption rate of the water increases as the pressure increases, so the chain reaction is inherently self-stabilizing.

However, both BWR and PWR reactors require enriched uranium - or uranium in which the concentration of the U-235 isotope has been increased beyond its naturally-occurring concentration of around 0.7% to between 2% and 5% - to maintain criticality, but CANDU reactors can operate with un-enriched uranium.

CANDU reactors are pressurized heavy water reactors. Heavy water is water in which the hydrogen atoms are deuterium, i.e. their nuclei contain both a proton and a neutron. The benefit to heavy water is that it has a much lower rate of neutron absorption than conventional water. That means un-enriched uranium can maintain a chain reaction, but it also means the uranium cannot maintain criticality in light water.

Even more than conventional PWR, CANDU reactors are designed with a number of such passive safety measures. The core is subdivided into a large number of individual pods so that damage, if it occurs, is localized. At the same time, the large number of narrow pipes feeding the individual pressure tubes provide more opportunity to radiate excess heat out instead of driving the fuel rods toward meltdown.

The cost savings from not having to enrich uranium in a CANDU system offset the higher cost of producing deuterium for the heavy water. CANDU reactors are the most efficient design in terms of megawatts produced per unit of uranium mined.

Other benefits to the CANDU design include the ability to replace fuel bundles without shutting down the reactor (on-power refueling) and the ability to operate using other low-fissile fuels, including spent fuel from light water reactors.

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