Overview of Nuclear Reactor Core Fundamentals

Updated 1/5/2019

There are many factors that play into the design of a modern reactor core. One of the first questions that come to mind is the enrichment percentage of the uranium fuel. A higher enrichment will allow for use of light water as a moderator which is more easily replenished; however, a lower enrichment will be less expensive while also requiring a heavy water moderator to compensate for the lower yield uranium.

Ultimately, it is a cost verses design analysis which will determine the core type to be used. Another question that will come up is fuel loading. How is the fuel to be loaded into the core? Higher and lower yield uranium can be mixed in a reactor core to produce the desired neutron flux distribution. Localized power peaking is never desired as it allows for a more likely chance to exceed core thermal limits and damage the fuel.

There are many factors that play into the design of a modern reactor core. One of the first questions that come to mind is the enrichment percentage of the uranium fuel. A higher enrichment will allow for use of light water as a moderator which is more easily replenished; however, a lower enrichment will be less expensive while also requiring a heavy water moderator to compensate for the lower yield uranium.

Another major concern with regards to reactor core design is poison loading. Fission poisons may be loaded into the core in order to exert a greater control over the flux profile typically early in core life. Burnable poisons are those that are expected to be used up earlier in core life to compensate for a higher fuel loading to allow a longer overall core life. Lumped poisons are those that exert a self-shielding effect. They will exhibit one absorption tendency early on and, due to the thickness and density of the poison, the inner portion of the poison will not be affected by the neutron flux.

As more transmutation of the lumped poison occurs, the inner segment is exposed and will result in a shift in the neutron flux profile at a designed time. Also of account are fission product poisons such as Samarium and Xenon. Xenon occurs primarily from the decay of Iodine. Samarium is produced from the decay of Neodymium. Both have high neutron absorption cross sections. Both are removed through burnout from fission. These poisons must be accounted for by the operator as they will build up while the reactor is at power.

Typical reactor control is exerted by control rods. Control rods, typically made of silver, hafnium, or cadmium, are materials with a high cross section of neutron absorption which act as movable poisons inside the nuclear reaction. By causing localized neutron flux absorption, the overall neutron flux profile of the reactor core can be shaped. Another major function of control rods is to provide a short term variation in overall reactor power levels. The action of changing control rod location will cause a change in neutron flux levels. As the steam demand on the reactor will remain constant there will be less heat produced than that being used. Temperatures will lower as a result.

Reactor power will change based on coolant density changing then return to its normal value. This allows a way to maintain power below a certain threshold for a brief period of time. The final use of control rods is as a method of conducting a reactor shutdown. Safety rods are control rods which act quickly enough and with enough negative reactivity to cause the reactor flux levels to lower to an amount that no longer adds heat to the system. The most common method of this is a reactor SCRAM. This is where many or all control rods are driven to the most negatively reactive position within the core in order to effect an immediate reactor shutdown. This could be due to a high reactor power density approaching thermal limits, a high reactor start up rate, or an improper flux profile being observed.

There are two types of control rod drive mechanisms. There are electric motor driven control rods and hydraulic operated control rods. The major benefit of electric control rods is a greater degree of control over rod speed. Rod speed is important as the operator must always be able to compensate for a continuous rod withdrawal casualty. Also, rod speed must also be high enough to prevent a Xenon-135 precluded startup due to operations at high power prior to a shutdown. Hydraulically driven control rods are more reliable in that an electrical fault will not cause a dropped rod or partial reactor SCRAM. Hydraulically operated control rods also allow for a more precise control over rod position. In a pressurized water reactor, rods are normally on the bottom of the core and are withdrawn. In a boiling water reactor, rods are held in the top of the core and pulled out from the bottom.

Other methods of establishing reactor control include the loading of steam through dump valves to raise reactor power or artificially cool down the reactor plant. Boiling water reactors control power level primarily through changing the circulation ratio inside the coolant stream by adjusting recirculation rates. This varies the nucleate boiling within the fuel channels which will change the reactivity inserted due to the voiding of the coolant.

As moderator is displaced by steam, fewer neutron thermalizations occur and reactor power will lower. The primary method that pressurized water reactors have of controlling overall reactor power levels is through what is called a chemical shim. Soluble boron is injected into the coolant causing it to have a higher cross section of neutron absorption. As a result, the number of neutrons absorbed in the fuel will be lower leading to fewer fission events. The soluble boron can be filtered out through a coolant purification system if necessary. Chemical shims are desirable because they will not lead to large variances in local neutron flux levels. The only major disadvantage is the actual use of boron inventory which is readily replenishable.

There are many factors in the physical construction of a reactor vessel. Not only must harsh pressure and temperature conditions over a long period of time be accounted for, but constant high neutron embrittlement must also be calculated for. Due to the cyclical stresses placed on a reactor vessel, there are many pressure and temperature limits that must be observed throughout operation of core life. Not only that, but there are hydraulic concerns as well. Crud buildup will occur on fuel surfaces which will degrade the thermal performance of the reactor and can lead to local temperature spikes. As a result, careful consideration must be made for the primary coolant loop’s equipment arrangement as well as vessel shape.

Steam generators are used in pressurized water reactors to convert the heat of the primary coolant loop into steam in the secondary loop. Typical construction is based on a u-tube bundle which has the discharge of the reactor’s hot coolant loop entering through a divider plate assembly. Coolant is circulated through the u-tubes back through recirculation pumps. The u-tubes are covered with feed water supplied through a feed ring assembly to prevent uneven cooling. The feed water is boiled and will rise through a series of moisture separators and out an outlet nozzle into the steam system. Moisture separators are necessary due to the extreme risk of moisture impingement damage on delicate turbine blades that rotate at a high rate of speed.

Steam generators are not used in the boiling water reactor design. This causes the BWRs to be more efficient as there is less heat waste in the system. However, the extra degree of separation between the reactor coolant and the steam system will allow pressurized water reactors to operate with lower ambient radiation levels. Also, there will be fewer concentration points for radioactive hot spots in a pressurized water reactor. Ideally, there should be zero radioactive contamination present in the secondary loop of a pressurized water reactor.

Nuclear safeguards are a vital part of safe plant operations. In the event of a loss of coolant accident, the displacement of the coolant, either through steam bubble formation due to saturation conditions existing or through the physical loss of the coolant through a rupture will cause local fuel temperatures to rise. As a result, blistering may occur due to the expansion of fission product gasses. These blisters may rupture and cause a release of radionuclides into the coolant stream. If a rupture is present, these radionuclides will be released outside of the reactor vessel.

Without a containment structure, radionuclides will be released into the environment. Containment structures take on many different types and designs. Also, due to cooling requirements, these structures must have some form of ventilation. This further complicates the engineers’ job of designing a structure capable of withstanding a reactor accident while maintaining ambient temperatures below a design basis. Through the use of hardened bulkhead penetrations, t-seals around valves, and pressure switch operated ventilation ducting, containment is enforced.

Emergency power is typically supplied by on site emergency diesel generators. These generators are typically started by air through fail-open solenoid valves and will automatically align power to the necessary busses to ensure adequate core cooling. It is also power for redundant power to be supplied through manual bus transfer devices located at off-site substations to provide commercial power from another provider.

There are two major forms of emergency cooling that are used: safety injection and coolant injection. While they sound similar, the functions are very different. Safety injection is used for a chemical shutdown of the reactor. This may be required for a number of reasons. It is possible that a loss of coolant accident has led to a partial core meltdown and poison pins have been melted. If normal coolant injection water were used, it could cause an inadvertent recriticality accident due to rewetting of the fuel with dense water.

Coolant injection is used to add makeup water to the reactor to ultimately prevent uncovering of the fuel plates. Also, it can be used as an emergency pressure control method in the event of a loss of pressure control. One possible method of emergency pressure control using an injection system would be to charge to the vessel until a pressure relief valve lifts. As long as the rate of injection doesn’t exceed the rate of coolant lost through the reliefs, the operator will be able to maintain pressure within design tolerances based on relief valve set points.

BWR

Seen here is a Boiling Water Reactor schematic. Note that there is no separation between the steam loop and the primary loop other than moisture separators at the top of the reactor vessel.

Seen here is a pressurized water reactor drawing. Note that there are three separate loops: a primary coolant loop, a steam loop, and a condensing loop.

2 Responses

  1. Richard Rhodes
    Richard Rhodes at |

    Tyler: I’m a science writer, with a book on energy in press. I’d like to include your helpful diagram of a PWR setup, one of about 100 illustrations in my book. Could you give me permission to reprint or let me know where you found the diagram on your website so I can track down permission from the owner? Thanks very much–Richard Rhodes

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