Nuclear Energy Activist Toolkit #34
In education, the three R’s are reading, ‘riting, and ‘rithmatic.
In nuclear plant safety, the three R’s are reactivity, residual heat, and radioactive material.
Considerable effort is expended in the design, operation, and maintenance of plant systems in order to properly manage these three factors.
Reactivity
Reactivity is to a nuclear plant like acceleration is to a vehicle. Reactivity does not reflect a nuclear plant’s power level any more than acceleration reflects a vehicle’s speed. Reactivity refers to the direction and rate of the change in a nuclear plant’s power level. Just as a vehicle can accelerate and de-accelerate, a nuclear power plant’s power level increases when positive reactivity is added and decreases when negative reactivity is added. Consequently, many systems at a nuclear plant are installed to control reactivity.
Failure to properly control reactivity caused the 1961 accident at the SL-1 reactor in Idaho and the 1986 accident at the Chernobyl nuclear station in the Ukraine.
Residual Heat
Residual heat refers to the thermal energy, or heat, released by the fission byproducts within a reactor core even after the nuclear chain reaction stops. Uranium and plutonium atoms fission into two smaller atoms during reactor operation—not always the same two atoms, but a large array of atom pairs. The atoms created when atoms fission are called fission byproducts. Some fission byproducts are stable, but many are unstable. The unstable fission byproducts seek stability through the emission of radioactivity such as alpha particles, beta particles, and gamma rays. These radioactive emissions release thermal energy, or residual heat, that must be removed to prevent the reactor core from overheating damage. Many plant systems are installed to remove residual heat.
Failure to properly remove residual heat caused the 1966 partial meltdown at Fermi 1 in Michigan, the 1979 meltdown at Three Mile Island in Pennsylvania, and the 2011 meltdowns at Fukushima in Japan.
Radioactive Material
Other byproducts of nuclear plant operation are radioactively contaminated solids, liquids, and gases. Examples of solids include worn-out gaskets replaced on a pump handling reactor water as well as the filter media used to remove particles and dissolved materials from reactor water. Examples of liquids include the water in the spent fuel pool as well as water drawn from the reactor coolant system for sampling. Examples of gases include fission byproducts (such as Krypton-87) escaping from minor cracks and holes in fuel rod cladding as well as the activation of particles in the water flowing through the reactor core. Many plant systems are installed to collect these radioactive materials and treat them for either re-use in the plant or discharge from the plant.
The table below illustrates how boiling water reactor (BWR) systems handle the three R’s of nuclear safety. (For background on how BWRs work, click here.)
Click to enlarge (Source: UCS)
Some systems, like the residual heat removal system, handle only one of the three R’s. (Pop quiz—which R does the residual heat removal system handle? Hint: it’s not reactivity.)
Other systems handle two of the R’s.
The main steam, reactor protection, and reactor vessel systems handle all three R’s.
The main steam system consists of the four large diameter pipes that carry steam produced inside the reactor vessel to the turbine. Isolation valves in these pipes automatically close when necessary to prevent the loss of inventory from the reactor vessel (the residual heat role) and to prevent potentially contaminated steam from leaving the containment structure (the radioactive material role). But the isolation valves do not close too quickly in order to minimize the pressure pulse racing back into the reactor vessel upon their closure (the reactivity role). A pressure increase inside the reactor vessel of a BWR collapses some of the steam bubbles and causing the neutron chain reaction to accelerate. Slowing down the main steam isolation valves’ closure provides time for the control rods to rapidly insert into the reactor core to more than offset the effects of the pressure pulse.
The secondary containment system functions to control radioactive material. During an accident, the secondary containment is “bottled up” (all unnecessary entrance and exit pathways tightly closed) to collect radioactive material leaking from primary containment and filtering it before discharging it to the environment. The primary containment system also has this function but has an additional residual heat function. The suppression pool inside the primary containment holds more than two million gallons of water—water used both to absorb residual heat as well as to provide a source of water for the emergency systems providing makeup supply to the reactor vessel.
Bottom Line
Which system is most important to nuclear safety?
The reactor protection system because it handles all three R’s?
The secondary containment system because it is the final barrier preventing radioactive material from reaching the environment?
These systems and the other systems listed in the table are all worthy candidates. But the most important system, by far, is the one that ensures all these systems are as reliable as possible.
The defense-in-depth approach to safety places many steps between a reactor problem and nuclear disaster. Pre-existing impairments reduce the number of steps remaining on that journey, making it more likely that awful destination someday gets reached.
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.