The Role of Control Rods and Neutron Poison in a Reactor

In the previuos post we have talked about control rods and burnable poisons is the reactor an their role in controlling the reactivity in the core, what are actually control rods and neutron poisons and what they do in the reactor to control the reactivity.

The control rod plays a major role in the control of the reactor. The physic behind this kind of control rod unique ability is that these control rods are able to absorb free neutrons. The free neutrons in the reactor will bombard a nucleus and promote the fission activity within the nucleus and this fission produces massive amount of power; so by absorbing the free neutron which bombard the nucleus. Thus, this will automatically reduce the fission occurrence within the reactor. By controlling the control rods position in the reactor we can subsequently control the reactivity of the reactor.

The materials used to manufacture the control rods are usually are silver, indium and cadnium graphite. This is because these materials have a very high capture cross section for neutrons. This will ensure that the neutron which are captured in the control rod will not be able to create another fission process in the reactor.

In most reactor designs, as a safety measure, control rods are attached to the lifting machinery by electromagnets, rather than direct mechanical linkage. This means that automatically in the event of power failure, or if manually invoked due to failure of the lifting machinery, the control rods will fall, under gravity, fully into the pile to stop the reaction. A notable exception to this fail-safe mode of operation is the BWR which requires the hydraulical insertion of control rods in the event of an emergency shut-down, using water from a special tank that is under high nitrogen pressure. Quickly shutting down a reactor in this way is called scramming the reactor. Urban legend has it that the control rods hung above the reactor, suspended by a rope. In an emergency a person assigned to the job would take a fire axe and cut the rope, allowing the rods to fall into the reactor and stop the fission. At some point the title of the person assigned this duty was given as SCRAM, or Safety Control Rod Axe Man (although this may be abackronym). This term continues to be in use today for shutting down a reactor by dropping the control rods.

The burnable poison in the other hand, has large neutron absorption cross-section. Usually this burnable poison or the neutron poison are added into the reactor  to lower the high reactivity of the reactors initial fresh fuel load. When a reactor is initially started with a fresh fuel, the reactivity in the reactor will be very high and moderators alone is not enough to reduce and maintain the reactivity rate in the most preferable and safe stage, so therefore to cope with the high reactivity usually burnable poisons are added into the reactor and mixed with moderator for better neutron absorption. There are 3 type of neutron poison they are burnable poison, non-burnable poison and soluble poison.

 The burnable poison is used to To control large amounts of excess fuel reactivity without control rods, burnable poisons are loaded into the core. Burnable poisons are materials that have a high neutron absorption cross section that are converted into materials of relatively low absorption cross section as the result of neutron absorption. Due to the burn-up of the poison material, the negative reactivity of the burnable poison decreases over core life. Ideally, these poisons should decrease their negative reactivity at the same rate that the fuel's excess positive reactivity is depleted. Fixed burnable poisons are generally used in the form of compounds of boron or gadolinium that are shaped into separate lattice pins or plates, or introduced as additives to the fuel. Since they can usually be distributed more uniformly than control rods, these poisons are less disruptive to the core's power distribution. Fixed burnable poisons may also be discretely loaded in specific locations in the core in order to shape or control flux profiles to prevent excessive flux and power peaking near certain regions of the reactor. Current practice however is to use fixed non-burnable poisons in this service.

 The non-burnable poison on the other hand,Soluble poisons, is one that maintains a constant negative reactivity worth over the life of the core. While no neutron poison is strictly non-burnable, certain materials can be treated as non-burnable poisons under certain conditions. One example is hafnium. The removal (by absorption of neutrons) of one isotop of hafnium leads to the production of another neutron absorber, and continues through a chain of five absorbers. This absorption chain results in a long-lived burnable poison which approximates non-burnable characteristics

Finally the soluble poison is also called chemical shim, produce a spatially uniform neutron absorption when dissolved in the water coolant . The most common soluble poison in commercial pressured water reactor (PWR) is boric acid, which is often referred to as soluble boron, or simply solbor. The boric acid in the coolant decreases the thermal utilization factor, causing a decrease in reactivity. By varying the concentration of boric acid in the coolant, a process referred to as boration and dilution, the reactivity of the core can be easily varied. If the boron concentration is increased, the coolant/moderator absorbs more neutrons, adding negative reactivity. If the boron concentration is reduced (dilution), positive reactivity is added. The changing of boron concentration in a PWR is a slow process and is used primarily to compensate for fuel burnout or poison buildup. The variation in boron concentration allows control rod use to be minimized, which results in a flatter flux profile over the core than can be produced by rod insertion. The flatter flux profile occurs because there are no regions of depressed flux like those that would be produced in the vicinity of inserted control rods. This system is not in widespread use because the chemicals make the moderator temperature reactivity coefficient less negative.