This is our group project for course MEHB513-Introduction Of Nuclear Technology. We are the final year students of Universiti Tenaga Nasional,Putrajaya Campus. This blog will delivered information about the nuclear energy to support our country Malaysia for having first nuclear plant to enrich the power plant system technology.
Hi.Today we would like to talk about control rod.As mention before,control rod is one of the main component of nuclear reactor.As we all know,the nuclear power plant can generate the tremendous power,heat and energy through the nuclear fission.However,we need to control it in predictable manner.The failure of controlling control rod will cause the incident like the Chernobyle disaster and Stationary Low-Power Reactor Number One explosion.Control rod can be made using chemical elements which can control the rate of fission of uranium.
Actually,why the nuclear reactor need to control? It is important to make sure the reaction is sustained rather than to start the chain reaction.In case of emergency situations such as a sudden mechanical or structural damage, the reactor needs to be shut down quickly in order to prevent any major disaster like Chernobyl which could be very costly in terms of loss to life and environment.Without having the nuclear reactor in control, the control rod might be melted down.
The control rod can absorb the amount of neutron to control the nuclear fission reaction.So the material being used in control rod must having high neutron absorbing capacity. Commonly material used are cadmium, boron, iridium and hafnium. For PWR nuclear reactor,the control rod is place on top of reactor but for BWR nuclear reactor,it is place at the bottom. This is because the stim generated inside the BWR reactor will push the control rod if control rod is place on top.
In the world today, many countries, especially big, heavily industrialized nations rely on nuclear power to satisfy their country’s energy needs. Such nations require huge amounts of energy to run their country. Various
consumers of energy and electricity include industries, residences, commercial establishments, military assets, governmental buildings, agriculture, transport and so on.
Massive usage of electricity on developed country.
Especially in extremely populated countries, there are in extremely high demand. The bitter truth is, it is just not possible to fill in such a high demand with just conventional sources of energy, countries look toward unconventional sources of energy such as solar, wind and nuclear energy. The yield of solar and wind energy is not that high. Nuclear energy is the answer for sustainable electric source in the future.
The reason why nuclear energy is important for future energy consumption is
1) Nuclear reaction release tremendous amounts of energy. Nuclear energy is so immense that one nuclear plant may provide electricity for a really large area. Thus it is important to have nuclear reactor to sustain future energy demand. This will be a stepping stone for a country such as Malaysia to be recognized as a developed country.
2) Nuclear energy doesn't emit anything into the atmosphere like polluted air, smoke or anything of that sort. France, which has embraced nuclear power has some of the most cleanest air in the world. this is mostly because of the heavy use of nuclear energy and minimum dependence of coal or oil based power plants. Future energy demand will be so demanding that it will be a major concern on the environment if we keep on using fossil fuel as our main source of energy and Malaysia need to take part in preserving the global environment.
Nuclear reactor only emits steam into the air (left) while conventional reactor emits greenhouse gas into the air (right)
What is nuclear power plant (NPP)? Actually, it can be defined as an electrical generating facility using a nuclear reactor as its heat source to provide steam to a turbine generator. In our country Malaysia, there are many type of electricity-generating power station in Malaysia. The make use of hydro-power, gas-fired, coal-fired (combined gas/coal), oil fired, biomass and hybrid power are remain as the main sources to generate electricity. Our country has enough skilled labor to operate power plant. It is simple because we have experience about how to operate hydro-power, gas-fired, coal-fired and etc.The whole system of power plant including steam turbine generator also we are capable of. How about if we replace the others heat sources with nuclear reactor? The concept stills the same, the steam turbine generator still being used. The different is only the heat source. It is sound easy, but the acceptation of Malaysian about nuclear reactor need to be considered. They need to be provided with simple and true information about nuclear power plant. Nuclear reactor is a device in which nuclear fission may be sustained and controlled in self-supporting nuclear reactor. It is heart of a nuclear power plant. A fissionable material of fuel, a moderating material, a reflector to conserve escaping neutrons, provisions of removal heat, measuring and controlling instruments and protective devices are include as parts of nuclear reactor. Figure above shows the general idea how to use nuclear reactor to produce the electricity.
What are the major components of nuclear reactors? There are 5 major components of nuclear reactor as listed as below.
ªNuclear fuel
ªModerator
ªCoolant
ªReactivity Control Material
ªStructural Material
The nuclear fuel is used to undergo nuclear fission by neutrons. It must have high thermal conductivity, irradiation, chemical stability and also excellent nuclear characteristics. The example fuel that can be used is uranium, etc. The moderator is used to reduce neutron energy by scattering without significant capture. It must have desirable characteristics to function efficiently such as large scattering cross section, small absorption cross section, large energy per collision and low atomic number. The example of moderator can be used are Deuterium, graphite and etc. For coolant, it is use to remove the fission heat from the primary heat source such as a reactor core or breeding blanket. The coolant must have excellent heat transfer, chemical stability, small pumping requirement, irradiation stability and abundance (low cost).The most natural coolant is water. The reactivity control material is needed to control the neutron flux in the core by absorbing neutrons. It can be done by using control rod. The structural material is used to maintain geometry. The structural must has high strength, ductility, corrosion resistance, small absorption cross section and high thermal and irradiation resistance. Zircaloy can be used as material.
Picture above shows the basic components of nuclear power plant. The most common of nuclear reactor type that been used nowadays are Pressurize Water Reactor (PWR) and Boiling Water Reactor (BWR). The picture above shows the example of PWR nuclear reactor type.PWR and BWR and the other type of nuclear reactor that commonly used will be discussed in other post.
Reference *Introduction to Nuclear Engineering MEHB513
In 1979 at Three Mile Island nuclear power plant in USA a cooling malfunction caused part of the core to melt in the second reactor. The TMI-2 reactor was destroyed. Some radioactive gas was released a couple of days after the accident, but not enough to cause any dose above background levels to local residents. There were no injuries or adverse health effects from the Three Mile Island accident.
The Three Mile Island power station is near Harrisburg, Pennsylvania in USA. It had two pressurized water reactors. One PWR was of 800 MWe (775 MWe net) and entered service in 1974. It remains one of the best-performing units in USA. Unit 2 was of 906 MWe (880 MWe net) and almost brand new.
The Chernobyl accident was occurred in 1986. Two Chernobyl plant workers died immediately and a further 28 people died within a few weeks due to acute radiation poisoning. The peoples around the case area were relocated. The April 1986 disaster at the Chernobylnuclear power plant in the Ukraine was the product of a not good design reactor and the also mistake made by reactor operator in terms of safety culture. The accident destroyed the Chernobyl 4 reactor, killing 30 operators within three months and several further deaths later. Two people were killed immediately at the site. The Chernobyl disaster was the only accident in the history of commercial nuclear power where radiation-related fatalities occurred .However, the design of the reactor is unique and the accident is thus of little relevance to the rest of the nuclear industry outside the then Eastern Bloc.
The Chernobyl incident had experienced positive (+ve) reactivity feedback effect. We need to know first what the reactivity is. Reactivity is a measure of the departure of a reactor from criticality. Actually the criticality is related to the effective multiplication factor, keff. Multiplication factor keff is the ratio of number of fission in one generation to the number of fission in preceeding generation as shown below.
In diffusion theory when k < 1, the condition of reactor is subcritical. Subcritical mean the neutron population is keep decreasing in each generation. If we have k =1, the condition of reactor is called critical. Critical means the neutron population in the reactor is neither decreasing nor increasing. It is also called self-sustaining for the neutron chain reaction. When we have k > 1, the population neutron is supercritical. We know that the neutron population is increasing to each generation. The k value is important to determine the criticality. Actually the reactivity, ρ is related with keff with the equation below:
Once we know the amount of reactivity, ρ in reactor core, the population of neutron can be determined. Hence we can predict the reactor power at any given time. We should noted that there are many factor that effect the power level in reactor such as fuel depletion, temperature of reactor, pressure of reactor and poisons. Because there are many parameters to consider, the reactivity coefficient (αx) is introduced. Actually, the reactivity coefficient (αx) is the amount of change in reactivity per unit change in parameter and is given by
Where Δρ = reactivity defects and Δx = change in variable parameter that effects reactivity. We can further observe the relationship of each variable in above equation. The Table 1 below shows the relationship of each variable
Δx
Δρ
αx
↑
+ve
+ve
↑
-ve
-ve
Table 1 : Relationship between each variable
Changes in the physical properties materials in reactor will results in change of reactivity. Hence the reactivity coefficient (αx) is very crucial in indentifying the reactivity change (Δρ) given the change in physical properties (Δx). In nuclear physics, there are four most important reacticity coefficient αx listed below
1)Moderator temperature coefficient of ρ, αTmod (moderator/coolant) 2)Fuel temperature coefficient of ρ, αTfuel (fuel) 3)Pressure coefficient of ρ, αP (moderator/coolant) 4)Void coefficient of ρ, αvoid (moderator/coolant)
Figure 1 : Variation of keff and its factors with the fuel-to-moderator ratio
Graph in Figure 1 is applicable to a large core fuelled with low-enriched fuel which is been used in nuclear industry today. We have been introduced with another ratio which is Nm/Nu ratio and also called moderator to fuel ratio. The amount of neutron will increase as we increase the amount of moderator Nm because the high amount moderator will make the leakage of neutron decreases. Then the absorption of neutron will be high and the thermal utilization factor will decrease and also resonance escape probability will increase. All of this trend can be seen in Figure 1. As we decrease the amount of moderator from the right to the left graph, the Nm/Nu ratio will decrease, then increase in slowing down time. This will increase the loss in neutrons by the resonance absorption thus increase the neutron in leakage. In practice, water-moderated reactor is designed with a Nm/Nu so that it can operated under moderated condition as shown in Figure 1. From Figure 1, when the temperature increases, the Nm/Nu decreases and because the water density becomes less, it’s creating the positive reactivity addition. The positive reactivity addition ρ will increase the effective multiplication factor and further increase the power and temperature in a dangerous cycle. This state called over moderated region. However when the same temperature increases would decreases the Nm/Nu and the density of water become decrease, the negative reactivity will be occurred. The effective multiplication factor keff will decrease and further decrease the power and the temperature in safe cycle. All of this can be explained in Figure 1. The Chernorbyl reactor power plan used the Boiling Water Reactor (BWR). It is uses water-moderated reactor. The disaster incident happened because there was no negative reactivity feedback effect in this reactor to lower down the reactor power as temperature. We hope that for the future, the engineer must take consideration of Nm/Nu ration in their design and put their design in under moderated region to make sure their reactor becomes more self-regulating.
In general, it seems the people increasingly supportive of nuclear when they feel better informed. Survey in Europe country shows that a quarter feel "completely uninformed" about them and a further half feel "not very well informed". As seen above, the feeling of being informed is linked to whether or not a country has nuclear power plants....
The mass media are peoples main source of information, with 87% getting their information from television, 44% from radio and 37% from local or regional newspaper. Only a few think that media provide sufficient information to form an opinion on nuclear issues. But then who is trusted to provide information on nuclear safety?, the table below provide some insight towards the issue (From Safety and Energy Technologies polls).....
Although the mass media are the most used sources to gather news, in the case of nuclear safety people tend not to trust journalist as much as more specialized sources of information such as scientist or environmental organisations. National Goverments are trusted by less than 1/3 of the people. lol! =P...
The public primarily trust scientist and NGOs on nuclear matters. Trust in national governments on these issues is low, presenting a clear problem for how governments can successfully communicate with their public. If Malaysia plan to build Nuclear Power Plant in the future, 60% nationwide must in favor of using nuclear energy.
This will give a rough idea on how to win people support for government plant on building nuclear power plant, that is by providing information on these matter by trusted source.
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