Preventing Fuel Damage in Nuclear Reactors

, director, Nuclear Safety Project | January 21, 2014, 8:23 am EDT
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Nuclear Energy Activist Toolkit #22

Lots of people have lots of reasons for preventing damage to nuclear fuel inside the core of nuclear power reactors. But what is done to help meet this goal?

The nuclear fuel for boiling water reactors (BWRs) consists of uranium pellets loaded inside hollow metal tubes called fuel rods. Metal caps are welded on the ends of fully loaded fuel rods to seal them. Dozens of fuel rods are put together to form a fuel assembly. A fuel assembly is about six inches wide, six inches deep, and over 12 feet long.

NEAT22 Figure 1 bwr-fuel-c

The reactor cores in U.S. BWRs consist of 500 to 800 fuel assemblies. Workers move individual fuel assemblies into and out of the reactor cores at the end of telescoping booms. The picture shows an irradiated fuel assembly glowing blue as it is transported above the reactor core of one of the BWRs at the Browns Ferry nuclear plant in Alabama. (Unlike a refrigerator, this light doesn’t go out when the reactor vessel’s lid is closed.)

NEAT22 Figure 2 bf-fuel-glow

When a BWR operates, three limits protect nuclear fuel from damage.

The fuel pellets as well as the metal fuel rods encasing them expand and contract as power changes cause local temperatures to change. The pellets and rods do not expand equally. The maximum power output from a fuel rod is limited to protect the fuel rod from being stretched to the breaking point by expanding pellets. This limit guards against fuel damage during normal operation. Two other limits guard against damage during postulated transients and accidents.

As implied by its name, water boils as it flows through the core of a boiling water reactor. Heat produced by atoms splitting within the fuel pellets is conducted through the pellets and the metal fuel rods to warm and boil water. In what is called nucleate boiling, the heat causes steam bubbles to form in the water wetting the outer surfaces of the metal fuel rods. These bubbles get swept away by the flow to join with lots of other bubbles in the steam carried off to the turbine. When too much heat is produced, the bubbles forming on the fuel rod surfaces can link up to become a solid blanket in what is called film boiling. Instead of allowing water to touch the fuel rod surfaces and carry away heat passing through the metal walls, the steam blanket essentially insulates the fuel rods keeping much of the heat inside and causing the fuel pellets to overheat. The maximum power output from a fuel assembly is limited to protect against fuel damage caused by film boiling.

During an accident, a ruptured pipe can quickly drain cooling water from the reactor vessel containing the reactor core. Instead of flowing past fuel rods to carry away heat, water can jet out the broken ends of the ruptured pipe. BWRs have an array of emergency core cooling pumps that will automatically start up and supply makeup water to the reactor vessel to restore the water level and sustain it despite inventory continuing to pour from the broken pipe ends. But these pumps are normally in standby mode and it takes them a few seconds to get off the bench and into the game. To guard against overheating damage during the seconds that fuel rods are left to fend for themselves during an accident, the maximum power output from a six-inch segment of a fuel assembly is limited during normal operation. (Recall that fuel assemblies are roughly six inches deep by six inches wide, so this six-inch segmentation essentially divides the core into six-inch cubes. The amount of decay heat generated by radioactive byproducts from fissioning atoms within fuel pellets is directly related to the initial power output. The third limit guards against the decay heat causing overheating damage to the nuclear fuel before the emergency core cooling systems can reflood the reactor vessel and resume carrying away the heat.

The maximum power produced by fuel rods, fuel assemblies, and six-inch segments of fuel assemblies are limited to protect against fuel damage during normal operation, transients, and accidents respectively.

Bottom Line

Fuel pellets and fuel rods are some of the barriers protecting workers and the public from dangerous radioactive materials. When intact, solid radioactive particles and gases stay in place rather than getting places where they can harm people. Three limits on nuclear fuel performance at BWRs attempt to keep the fuel rods intact during normal operation as well as during transients and accidents. (Pressurized water reactors employ comparable tactics for the same purpose.)

The three damaged BWR cores at Fukushima Daiichi demonstrate that these three limits provide protection rather than guarantees. All the limits were met on March 11, 2011, when the earthquake occurred. But the limit on maximum six-inch segment powers was predicated on the emergency core cooling systems restoring heat removal within certain timeframes. When those deadlines passed, so too did the chances of preventing core meltdowns.

The UCS Nuclear Energy Activist Toolkit (NEAT) is a series of post intended to help citizens understand nuclear technology and the Nuclear Regulatory Commission’s processes for overseeing nuclear plant safety.

 

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  • The point being that the new designs with passive cooling capable of removing heat after total power loss would reduce the number of potentially dangerous events from a total of three in the last half century(Chernobyl, TMI, and a mag nine quake ravaged reactor) to zero. All were the result of a loss of coolant.

    Not sure that is necessary considering that nuclear energy is the safest means of generating power we have, but public perception is everything.

    • pat b

      chernobyl was not a loss of cooling event, it was an accidental transient.

      The operators were attempting to hit a stable power level and the reactor went to positive void coefficient

  • Joyce Agresta

    There are many people who would like to prevent core melt down.One could imagine there many more who would like to achieve one or a hundred nuclear core melt downs. There’s probably not a single nuclear reactor in the world in a country free of enemies. Soon we will even begin fighting ourselves over where the waste will go. As there is no Nuclear Nirvana of a waste repository(that’s something of an industry illusionary promise but the impossible can‘t occur) or a safe place in the world to store it chances are the repository will end up in the most polluted areas. Westerners feel this is already on the east coast. Nobody wants it in their backyard. Where the next US Nuclear Core Melt Down occurs may well become a Nuclear Waste repository area.
    One may only conclude that there is a chance to prevent Nuclear Core Melt Down until a point in time when preventing Nuclear Core Melt Down has passed. In so much if a Nuclear reactor is operating that time may come. Such events have occurred time and time and time again. The potential for several Nuclear Core Melt Downs to happen at once however likely or unlikely that may be is ever present. Such a catastrophe could lead to ruination of the planet. The benefit simply does not out way the risk.
    Ever greedy grandiose pollution mongers put profits ahead of safety which will eventually help achieve the goal of one or a hundred Nuclear Reactor core melt downs.
    By the way how is it Calvert Cliff passed an NRC audit and less than two weeks later has a major glitch and is now under NRC review? It might be prudent to lower their operating license to a two week time period.