Topic B for 7th October 2010

Hi!! Guest what? This is actually my first official duty. Well more specifically after I created this blog. Hmm I found that my teammates’ posts are very lengthy so I want to diversify my post (not to say that I am good enough). My writing/post may not be as good as Mr. Story Teller or too technical as others’. This time I would like to post in a simplified point form format. Even though today’s post is relatively general topic but I prefer to keep my facts straight.hah..mind u that, if my post cause u perplexity or complexity and tend to drive u crazy..plz..plz..plz..

-Please refer to another source for further details-

Nuclear Power in the World Today

• The first commercial nuclear power stations started operation in the 1950s.
• There are now some 436 commercial nuclear power reactors operating in 30 countries, with 372,000 MWe of total capacity.
• They provide about 15% of the world's electricity as continuous, reliable base-load power, and their efficiency is increasing.
• 56 countries operate a total of about 250 research reactors and a further 220 nuclear reactors power ships and submarines.

The Economics of Nuclear Power

• Nuclear power is cost competitive with other forms of electricity generation, except where there is direct access to low-cost fossil fuels.
• Fuel costs for nuclear plants are a minor proportion of total generating costs, though capital costs are greater than those for coal-fired plants and much greater than those for gas-fired plants.
• In assessing the economics of nuclear power, decommissioning and waste disposal costs are taken into account.

Radiation and Nuclear Energy

• Natural sources account for most of the radiation we all receive each year. Up to a quarter of that received is due to human activity and originates mainly from medical procedures. 
• The nuclear fuel cycle does not give rise to significant radiation exposure for members of the public.
• Radiation protection standards assume that any dose of radiation, no matter how small, involves a possible risk to human health. This deliberately conservative assumption is increasingly being questioned.

World Energy Needs and Nuclear Power

• The world will need greatly increased energy supply in the next 20 years, especially cleanly-generated electricity.
• Electricity demand is increasing much more rapidly than overall energy use and is likely to almost double from 2004 to 2030.
• Nuclear power provides about 15% of the world's electricity, almost 24% of electricity in OECD countries, and 34% in the EU. Its usage is increasing.
• Nuclear power is the most environmentally benign way of producing electricity on a large scale. Without it most of the world would have to rely almost entirely on fossil fuels for continuous, reliable supply of electricity.
• Renewable energy sources other than hydro have high generating costs but are helpful at the margin in providing clean power.

AGAIN....

-Please refer to another source for further details-

Control Rods-(Monday summary)

hye...hye...everyone.Today post is about Control Rods in reactor...so, let's read together.=)

What is control rods (CR)?

Control rods is a rod made of chemical elements capable to absorbing many neutron without fissioning themselves.They are used in nuclear reactor to control the rate of fission of Uranium and Plutonium.

Why control rods is important?

Control rods like a 'heart' in nuclear reactor.Because too few fission events can slow down and automatically stop the chain reaction.Too much fission can overheat the core and lead to a meltdown.That's why control rods is much important here.

Materials of control rods 

• Silver,Ag
• Indium,In
• Cadmium,Cd
• Boron,B
• Hafnium,Hf

How CR work?

Nuclear engineers and technicians precisely control the amount of fission taking place by inserting control rods (upper left) into the fuel assembly(red box). The rods are made of a substance that readily absorbs neutrons, like graphite or cadmium. When things get too hot, technicians lower a few control rods into the core. The rods sop up some of the ricocheting neutrons, and the fission process slows down. The reverse is also true: control rods are removed to rev up the fissioning.

When control rods are lifted from the fuel assembly, neutrons (from the natural decay of uranium) bounce around and bombard other uranium atoms, causing them to split. This process gives off more neutrons and causes more splitting. This is a chain reaction. The heat generated from all this fissioning is converted into steam, which turns a turbine, which turns a generator that produces electricity.

REMEMBER, If the reaction gets too hot, the control rods are re-inserted to absorb neutrons. With fewer neutrons around, there is less bombardment and fissioning. The core cools; energy output slows down. 

CR effectiveness

CR effectiveness is depends on the how many ratio of the flux at the location of the rod to the average flux in the reactor. Figure 1 show, when a reactor has one CR,the CR is must be place in center part of reactor core.The CR has a maximum effect when  if it is placed in the reactor where the flux is maximum. At point A,if additional rods are added to this simple reactor, the most effective location is where the flux is maximum.

Figure 1:Effect of control rod on radial flux distribution

The exact value of reactivity that each control rods depends upon the reactor design. The reactivity caused by control motion  is referred to as control rod worth.

Type of CR

• Integral CR worth ( 'S' shape)

Figure below show the result of a value of rate of change of control worth as a function of control rod position.

Figure 2: Integral CR Worth

Function of integral CR worth curve is to define the ρ change due CR movement between two position. The integral CR worth is the total reactivity worth of the rod at the particular degree of withdrawal and is usually defined to be the greatest when the rod is fully withdrawn.

• Differential CR Worth (Bell shape) 

For figure tell us it has very low values at top and bottom of the core and a maximum at the center of the core. The curve has bell shape because of CR worth related to n flux and n flux max.Also,n flux max is highest in center of the core.

Figure 3: Differential CR Worth

 

At the bottom of the core, where there are few neutrons, rod movement has little effects,so change in rod worth is very little.The effect become greater, when the rod approach the center on the core. Basically, from center to the top inverse of the rod per inch will applied here.

Example of Control Rods 

Figure 4: PWR fuel with control rod CLUSTER

              figure 4: BWR fuel with CROSS ROAD design

Basic knowledge about Xenon

On Monday classes, Mr.Shamsul touch a little about Xenon. So,now...I would like to write something here as our knowledge. Let's check it out!!!

Xenon?

From my research, Xenon is also called ' STRANGER'. It name originates from the Greek word ' XENON'. Xenon is colorless, orderless, highly unreactive gaseous ( found in minute quantities in the atmosphere).

Characteristics of Xenon

Element  :Xenon (Xe)

Atomic no  : 54

Atomic mass   : 131.29 amu

Melting point : -111.9° C (-161.25°K)

Boiling point  : -108.1°C ( -165.05°K)

Neutron in Xenon  : 77

Crystal Structure   : Cubic

Color of  Xenon    : Colorless

Periodic Table: Group 18 ( Noble gases)

Did you realize?

If you realize, Xenon is commonly used as photographic strobe light ( lens camera,alarm light, arc lamp and etc). 

Xenon-135
Xenon is occurring consist of nine stable isotopes. For Xenon-135 is produces as a result of nuclear fission and acts as a nuclear absorber in nuclear reactor.

How it produce?
Xe¹³⁵ is produced directly from fission and from the BETA decay of Tellurium-135, as shown below.
Xe-135 subsequently beta decays to Cesium-135 then to Barium-135. The half-lives are shown in BLUE below the line.

Te¹³⁵  ======>   I¹³⁵  ======>  Xe¹³⁵=======>  Cs¹³⁵  =======>   Ba¹³⁵ 

<0.5 min               6.7 hr         9.2 hr        2 x 10⁶ yr

How it destroy?
It has two ways to destroy:

1. By its own radioactivity decay ( half-life 9.169 hours)
2. By neutron absorption to Xe136

From the observation, it take time to produced and also take a long time to destroy...ermmmmm???What it means that?you think???
Opsss...I got to go. I hope this simple info will give all some knowledge...I will update more later...adiosss! :)

Chernobyl continuity...

Owh ya ya I did said before that I will post the physics behind Chernobyl incident right? Unfortunately my lecturer doesn't have time to explain so here's how... I’ve posts the event sequence including the large picture related to effects of reactivity feedbacks before right. Try pinning it or relating it to this post. Sequence is the same so should be easy game to play. This is an article from

http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/xenon.html

A major contribution to the sequence of events leading to the Chernobyl nuclear disaster was the failure to anticipate the effect of "xenon poisoning" on the rate of the nuclear fission reaction in the Chernobyl nuclear reactor.

Neutron absorption is the main activity which controls the rate of nuclear fission in a reactor - the ²³⁵U absorbs thermal neutrons in order to fission, and produces other neutrons in the process to trigger other fissions in the chain reaction. To control the chain reaction, neutron absorbers in the control rods limit the rate of reaction, and the moderator (graphite in the case of Chernobyl) slows down the fast neutrons to enable the reaction to be sustained. It is a delicate balancing act requiring detailed knowledge and careful control.

One of the extraordinary sequences in the operation of a fission reaction is that of the production of iodine-135 as a fission product and its subsequent decay into xenon-135. Iodine-135 is a rather common fission product, reportedly amounting to up to 6% of the fission products. It has a rather small probability for absorbing a neutron, so it is not in itself a significant factor in the reaction rate control. But it has a half-life of about 6.7 hours and decays into xenon-135 (half-life 9.2 hours). The xenon-135 has a very large cross-section for neutron absorption, about 3 million barns under reactor conditions! This compares to 400-600 barns for the uranium fission event.

In the normal operation of a nuclear reactor, the presence of the xenon-135 is dealt with in the balancing of the reaction rate. Iodine-135 is produced, decays into xenon-135 which absorbs neutrons and is thereby "burned away" in the established balance of the operating conditions. There is an equilibrium concentration of both iodine-135 and xenon-135. But when the power level was drastically lowered at the Chernobyl reactor, the xenon-135 concentration began to increase because the parent iodine-135 was near full-power equilibrium concentration to produce it and the neutron flux necessary to "burn it away" was not present. It would eventually peak and decrease, but with a 9.2 hour half-life, that decrease would come too late!

When the persons conducting the tests on the Chernobyl reactor tried to increase the power at some point in their tests, it would not respond. They apparently did not have the understanding that the failure to increase was due to the absorption of neutrons by the xenon, so they completely removed the control rods to force the increase. The increased power then burned away the xenon and also caused voids in the cooling water, both of which rapidly increased the reaction rate, driving it out of control.

The "xenon poisoning" of the reaction rate had been known for many years, having been dealt with in the original plutonium production reactors at Hanford, Washington. In fact, it was dealt with in the original Manhattan Project where it presented itself as a dilemma - the researchers expected a given configuration to maintain a chain reaction and it failed to do so. They found that they had to increase the fuel concentration to overcome the xenon poisoning. So the phenomenon had been dealt with from the earliest days of our experience with nuclear fission, and should have been known by anyone who was in control of a nuclear reactor.

See this is a very good article. I picked it as it suits my writing style. It is easy to understand for those who prefer story telling format like me. If anyone got offended as a result of me doing some copy paste, I do apologize. The reason is that I don’t have sufficient time and understandings plus I don’t want to rob and ruin a very beautiful article. Sharing is caring…

By the way, maybe I’ll write on control rods within this week if possible. Or maybe one of my colleagues will do it.

Class Summary on 5th October 2010

This is my 3^rd official duty, sure you can distinguish my writing but did it bore you till death? If yes, then at least I know there are people out there reading my essay. I did mention my posts would be light and easy in accordance to my personal understandings but apparently my posts had been lengthy. Hope it could be enough to patch up our team’s weakness. Where to start...

As mentioned before, we would like to balance a nuclear reactor core at its critical state (k =1 , ρ = 0) all the time. Unfortunately it is impossible to achieve that due to fuel depletion, fission product build-up, and temperature changes.

Disturbance 1 – Temperature changes

In the previous posts we have understood the term reactivity (ρ) and 4 most important reactivity coefficients (α) needed to be considered in order to maintain a reactor at critical state.  Recall 1) Moderator temperature coefficient of reactivity which we can control, 2) Fuel temperature coefficient of reactivity which we can’t control, 3) Pressure coefficient of reactivity which is negligible in PWR, and 4) Void coefficient of reactivity which is negligible in PWR but crucial in BWR. As fuel temperature coefficient is always positive (thus providing positive reactivity feedback), moderator temperature coefficient is controlled to provide negative reactivity feedback most of the time. Hence we have dealt with temperature changes effect.

Disturbance 2 – Fuel depletion

Fuel depletes constantly during operation whereby after a long run, we don’t have enough fuel to run the core at its critical state. How to avoid this? We have to add on fuel but unfortunately we can’t add fuel in an operational reactor. It’s a onetime go for 60 years. So we have to place in excessive fuel in the reactor before it is started (when the reactor is built).

Still can’t get the idea? You work on Monday to Thursday, 4 days a week. Suppose you need ¼ tank of fuel to drive your car to work in a day but your boss allows you to claim for the fuel only in a single receipt per week.  So the best option is to fill up the whole tank on Monday before work. By doing this, there is some inevitable collateral damage - increasing the car’s weight and increase fuel consumption. Storing fuel for long would lower its combustible energy. Fuel is volatile and hence it evaporates and leaks out.  

Back to our main topic, similarly placing excessive fuel causes some inevitable collateral damage. It creates an excessive positive reactivity and need to be compensate with negative reactivity from neutron absorbing material such as Boron (remember that positive reactivity promotes to supercritical state [k>1, ρ>0] where the neutron population is increasing). Among the most popular neutron absorption device is the control rods but relying on it alone is undesirable or impractical for several reasons that would be discussed later. So what do we do if we have too many monkeys around our neighbourhood? Poisson them to death right? Similar here, we introduce neutron poison. Well it actually absorbs neutron, not killing it but as long as we manage to get rid of the excessive neutron means problem solved.

There are 2 classification of neutron poisons, 1) Burnable poisons 2) Non-Burnable Poisons. Difference? Burnable doesn’t actually mean caught fire and burn ya. Burnable poison absorbs neutron and converted into low neutron absorption cross section material. It is like hungry living things that eats only once then die. Non-burnable in the other hand has relatively constant neutron absorption characteristics over core life, example Hafnium (Hf), used to shape power and to prevent excessive flux power peaking near moderator regions.

Fixed burnable poisons are generally used in the form of compounds of Boron or Gd that are shaped into separate lattice pins or plates, or introduced as additives to the fuel whereby it is distributed more uniformly than control rods which results in less disruptive to the core power distribution. Advantages are that it can better shape or control core flux profile and does not affect the moderator temperature coefficient. Meanwhile soluble poison or chemical shim is a soluble neutron poison that is circulated in the coolant during normal operation, e.g. PWR: boric acid also known as solbor (solubleboron). Advantages are it has a spatially uniform effect and it is possible to regulate the amount of poison in the core during operation.

Disturbance 3 – fission product build-up

Fission fragments generated at the time of fission decay to produce a variety of fission products. Fission products are of concern in reactors primarily because they become parasitic absorbers of neutrons and result in long term sources of heat. So as the fission product that absorbs neutron is also called as neutron poison. The most substantial impact on reactor design and operation is the most powerful neutron poison: Xenon-135, a fission product (yield 6.3333%) produced 95% by the decay of Iodine-135. Recall that we actually introduce neutron poison to control reactivity, so fission products of neutron poisoning such as Xenon should be useful right?

Put it this way, you have 3 friends... All 3 of them are useful and you can’t live without either one of them. The problem is one of them had a pretty annoying and irritating attitude. What you normally do is get to understand his/her attitude, monitor closely, and adapt to it.

In the neutron poison clique, Xenon-135 is the one having an annoying and irritating attitude. It absorbs neutron and became Xenon-136 which is a non poisonous stable isotope as it won’t absorb neutron. Remember burnable poison? This is one of it. But only 10% to 50% of Xenon-135 produced in a reactor during operational undergo this neutron capture while the rest undergoes beta decay. Here is where the problem begins.

The rest of Xenon-135 is removed is by beta decay where it has a half life of about 9.1 hours. Iodine -135 has a half life of about 6.5 hours. So the parent live for 6.5 hours the while the daughter survive for 9.1 hours. This time differential is one of the factors that make Xenon such a problem for nuclear reactors. Since Xenon takes longer to decay than the Iodine takes to build in the Xenon, then there is a natural tendency for Xenon levels to increase in a reactor when not at equilibrium. Equilibrium refers to when the rate that Iodine decays into Xenon-135 (build in) is equal to the rate Xenon-135 decays plus the rate of Xenon burn out. The key is to keep it equilibrium.

Doesn’t sound much troublesome right? Fortunately my colleague found a comprehensive article on the web explaining in a pretty similar to my style about Xenon and therefore I could easily paste it here to share. I felt disgrace if I rob this article and claim it as mines so I’ll leave it in its original state. Written by Jack Gamble on 6/5/2010.

When Enrico Fermi fired up the first nuclear reactor at Hanford in 1944, he was in for quite a surprise. Shortly after the reactor went critical, power stalled and the reactor shut down. A few hours later, the reactor unexpectedly started up again all by itself. This was the result of poisoning brought on by Xenon-135 (Xe).

Xenon and Reactor Power Levels

The real problem with Xenon comes into play when power levels in the reactor change. When power rapidly decreases in the reactor, the rate of Xenon burn out drops. However, the existing Iodine-135 continues to decay and produce more Xe-135. This causes Xenon levels to increase, bringing the available neutrons down and lowering power. A few hours later, as Iodine-135 production slows, the Xe-135 levels off and power rises again. So you haven’t touched anything, but power is now higher than you left it.

The converse is also true. When power is rapidly increased, the rate of Xenon-135 burn out rises sharply but the Iodine-135 decay remains unchanged. This causes a lowering of Xe-135 concentration and increase in power. Eventually, the rate of Iodine-135 production and decay along with the rate of Xe-135 production and burnout reach equilibrium. Now power is lower than you left it because Xenon has built back in.

The end result of this Xenon-135 is a major nuisance to nuclear reactor operators and core engineers. The solution is placing limits on the rate at which a plant rises and lowers power. This enables operator to keep a close eye on Xenon and make sure the reactor is running in safe manner.

Xenon-135 contributes Xenon precluded startup (Inability of a reactor to be started due to the effects of Xe-135) and Xenon dead time (The period of time where the reactor is unable to override the effects of Xe-135). I think I’ll stop here... I’m exhausted and running out of time. Please visit link below to read on Xenon precluded startup and Xenon dead time.

http://www.chemie.de/lexikon/e/Xenon-135/

Topic B for 30th September 2010

This is the second official duty for me ya. For those who read my posts before can easily distinguish my writing and presentation concept. What to be written? Story tale again. Warm up.... Scientists discover, engineers deliver.  Actually I don’t have enough time and energy to write down but having deadline means that I must write down something even if it is nonsense. Let me apologize if this post isn’t fun to read.

Nuclear energy is the cleanest, greenest electricity... Obviously you just need to chop down a hectare of trees, flatten the ground, and build a nuclear power plant. That is all the amount of carbon footprint left for 60 years of useful life. Heat produced by the reactor boils up water and creates steams which then rotate the turbine to produce electricity. This process emits nothing to the air just like a steam engine in the 80’s but with different heat source.

Can Malaysians run nuclear power plants? Yes, we have the ability and knowledge to run nuclear power plants but are we expert in it? No. We are not an expert and this is among the reasons why we should have one.

Can’t get the idea? Suppose you went to a regular car workshop. Can the mechanics repair a BMW? Yes they can. In fact they can repair almost every type of car. Move on and visit a BMW specialist workshop. They are totally experts when handling with BMWs. Now compare these two workshops and you will notice that mechanics at BMW specialist workshop can repair BMWs better compared to the regular workshop. They had become BMW experts because they learnt about BMWs, drove BMWs, and some even possess a BMW. Apply this theory to our topic just now; Malaysians can be expert in nuclear power plant once we have one.

Nuclear versus RE. Nuclear power plant had been around since 1960’s. Developed countries had started to focus more on renewable energy (RE) and fusion technology. Why are we keen to learn nuclear? By time we became experts; nuclear is considered as old school. Don’t worry about RE, we are moving at the same pace as they are. In Malaysia, RE source we can consider is limited.
Wind... On 2008 maximum average wind speed recorded in Malaysia is at Mersing with a staggering 6m/s. Enough to blow several sheets of paper but insufficient to power a wind farm.
Tidal... Energy that we could harvest from tidal is too small.
Waves... Give me one spot in Malaysia where we can do surfing consistently for at least 180 days a year. If no, then forget about waves.
Geothermal... Do we have a volcanic eruption? Do we often have earthquakes over 3 on a richter scale? This is because we are not on top of an active tectonic activity and hence we have too little geothermal, sufficient for some hot spring bath. Unless we dig deep to the earth’s core.
Microhydro power plant... As the name clearly states micro which means small scale thus empowers only up to 20 houses in the rural area close to a fast flow river is close to its maximum potential.
Biomass, Biodiesel, and solar is the most potential RE in Malaysia but currently the cost to energy produced ratio is too small. The best photovoltaic solar panels using polycrystalline at its best have an efficiency of less than 35% without even considering the whole system’s power lost (inverter’s efficiency, tilt angle, dust, and irradiance)  whilst both biomass & biodiesel power plant runs at an efficiency lower than 50%. In fact biomass and biodiesel burn releases carbon to the atmosphere but it is consider as part of the natural carbon cycle and hence can be considered as RE.
Nevertheless RE might dominate the world in the future but by observing the trend; it might be another 150 years to come. Owh owh note that some people also considers nuclear power plant as RE because it emits nothing to the atmosphere but as it takes up large land area which interrupt the ecosystem (similar to Hydro power plant) plus nuclear waste plus limited recycling cycle of depleted fuels, its existence in the RE family had been denied. That’s it about RE.

Fusion... Our sun is the best example of fusion. Its potential even though the technology doesn’t exist yet is promising. Then why don’t we abandon nuclear power plant and focus on fusion? Because the basics of fusion is closely related to fission, which is the process used in a nuclear reactor.

Some say "then if it is hard enough then lets us remain on crude and coal burning". Boring topic. Dull and dry. Ask everyone, everywhere and it will be global warming and crude depletion. Maybe next time I’ll write down another story tale regarding this issue but this is the end for now.

Why we don’t want a moderator to boil in a PWR?

Let me try to answer this question in a form that is easy to understand in accordance with my understandings. I do apologize if there is any inaccurate information here as I am such a green horn in this field.

First, the function of moderator itself is to slow down neutrons. Take a small stone and assume it as a neutron. Throw it through a steam or smoke and observe whether it slows down significantly or insignificantly? Take another identical stone and through it through water and see the difference. This is why you don’t want to have steam in your reactor for PWR. Arrangements of atoms is the key factor here. Solids followed by liquids have stronger bondings where each atom are align closely to each other compared to gas. This also explains why solids have higher heat transfer rate where heat is transferred by conduction while gas such as water vapor have lower heat transfer rate where heat is transferred by convection.

Second, among the huge advantages of using PWR is that you can insert control rods from the top and hence utilizing gravitational force to create a passive system. Allowing steam in PWR reactor will diminish this advantage as steam will take up the volume (increase in pressure) as it expands upwards and pushes the control rods out from the reactor.

Third, steam is a form of gas which obeys the ideal gas law. Temperature increases as pressure increases. Having steam will increase pressure and thus cause the temperature to rise and hard to control. But if we keep the fluid close to its boiling point, temperature could be controlled. The boiling point can be controlled by regulating the pressure and thus explains why a pressurizer is introduced in PWR.

The reason is that a liquid that is about to vaporize is called a saturated liquid. Once boiling starts, the temperature stops rising until the liquid is completely vaporized. That is, the temperature will remain constant during the entire phase-change process if the pressure is held constant. Looking back at PWR, we have a pressurizer to hold the pressure constant, keep the fluid as saturated liquid means that temperature can be controlled or held constant.

In addition, if we keep it at saturated liquid state we will have some bubbles at the bottom, slug and churn in the middle, and annular at the top. The benefit is turbulence flow which means faster heat transfer.

Fourth, steam is a for of gas which means that heat transfer is by convection while in fluid or solids, heat is transferred by conduction. These two methods of heat transfer differ significantly whereby heat transfer by conduction is far greater than by convection.

-Please refer to another source for further details-

Basic of Nuclear Physics

History of nuclear physics

@ The history of nuclear physics
The discipline distinct from atomic physics starts with the discovery of radioactivity by Henri Becquerel in 1896,while investigating phosphorescence in uranium salts. The discovery of the electron by J. J. Thomson a year later was an indication that the atom had internal structure.
At the turn of the 20th century the accepted model of the atom was J. J. Thomson's "plum pudding" model in which the atom was a large positively charged ball with small negatively charged electrons embedded inside of it. By the turn of the century physicists had also discovered three types of radiation coming from atoms, which they named alpha, beta, and gamma radiation. Experiments in 1911 by Lise Meitner and Otto Hahn, and by James Chadwick in 1914 discovered that the beta decay spectrum was continuous rather than discrete.

@ There are the best things that we must know about the nuclear physics.
Nuclear physics is the field of  
-atomic nuclei
-nuclear power
-nuclear weapons
-nuclear medicine
-magnetic resonance imaging
-material engineering
-ion implatation
-archaeology
-radiocarbon dating 

For this reason, has been included under the same term in earlier times.

@ The atoms of which every element of matter is composed have a nucleus at the center and electrons whirling about this nucleus that can be visualized as planets circling around a sun, though it is impossible to locate them precisely within the atom. 
@ The nuclei of atoms are composed of protons, which have a positive electrical charge, and neutrons, which are electrically neutral. Electrons are electrically negative and have a charge equal in magnitude to that of a proton. 

What is the Nuclear (fission) energy:

• Commercially established since 1956 

  Calder Hall, gas-cooled Magnox NPP at Sellafield (UK), 50 MW (later 200 MW)

• Today: ~16% of world’s electricity generation (18% hydro, 66% fossil)

• Switzerland: ~40% (nearly all the rest: hydro)

This is about the structure of the atom (Rutherford’s model):

• Mass concentrated in the nucleus (mH/me ~ 1837)

• Nuclear charge: +Ze (Z: atomic number, e ~ 1.6.10-19 coulomb)

• Quantum mechanical basis for atomic, nuclear structure

• “Classical dimensions”: nucleus ~ 10-13 cm, atom ~ 10-8 cm

Energy units (1eV ~ 1.6.10-19 J)is the :

- Binding energy of outermost electrons ~ order of eV

– Energy involved in chemical reactions ~ same order

- Binding energy of nucleons (constituents of nucleus) ~ order of  MeV !

– Energy in nuclear reactions ~ x 106 times greater than in chemical

This is the Often encountered in nuclear engineering:

- Nuclear fuel, activation of materials, fission products, wastes

- Fundamental law: (λ : decay constant)

- Units of (radio)activity:

- Historical.. 1 curie (Ci) = 3.7 x 1010 dis/s (activity of 1 gm of Ra226)

- Actual.. 1 becquerel (Bq) = 1 dis/s

- For example: 1 mCi = 10-3 Ci = 3.7 x 107 Bq = 37 MBq

CHERNOBYL

Students currently in this course along with me mostly are in their final year and thus time constraint is an inevitable factor due to our final year project. For those who are interested in facts on Chernobyl incident and 3 mile island can simply seek it in the web as frankly I haven’t done any studies or literature reviews regarding this matter but if you prefer a story tale, this might be a good start.

Recently I watched a documentary program on national geography channel on Chernobyl accident. Let me try to explain it as easy as possible as far as my understandings according to the program I watched. Chernobyl nuclear power plant uses an active system where all of the power plant including components such as pumps and control rods are powered by electricity. Electricity used is the one produced by the nuclear power plant itself.

One day, they scheduled to run a simple test which is to make sure that the turbine will keep on running when the reactor power level is at its minimum with aid of auxiliary system on reactor number 4. During daylight, at least 3 senior engineers are on duty and therefore the test can be monitored closely by experts but unfortunately this is when the grid demand reaches its peak. Instead they postponed the test decided to run the test past midnight when the grid demand is at minimum, leaving the task to 2 inexperienced young engineers on duty at that time.

Preparation for the test took more than 5 hours to complete and these 2 engineers faces several difficulties during this phase in terms of balancing the reactor. Unfortunately I can’t remember either names and therefore let me designate them as Mr Ku and Mr Bu. As Mr Ku in the control room lowers the core power by inserting 114 control rods, Mr Bu in the other building increase the water (moderator) flow into the reactor causing the reactor power and temperature to decrease rapidly. As more water pumped in, insufficient heat means that not enough steam produced to run the turbine. At this time, moderator to fuel ratio is decreasing meaning that the reactor is under moderated.

Mr Ku in the control room quickly compensate this condition by leaving only 6 control rods in the reactor despite the fact that the manual stated that the reactor should operate with a minimum number of 26 control rods. Due to communication breakdown, Mr Bu who just realized that the steam production is insufficient intends to help by reducing the water (moderator) flow rate and thus causing the power and temperature of the reactor to rise rapidly. At this time, moderator to fuel ratio is increasing meaning that the reactor is over moderated.

Temperature changes in a reactor affect reactivity in a matter of seconds where at this point, catastrophe is inevitable. At a time, the core power stabilized for a while before it suddenly burst up. Mr Ku reacts quickly by inserting all available control rods to reduce the power but unfortunately it is too late. 

Analysis said that the moderator used is made of graphite, while the coolant used is water which means that even without coolant, neutrons are still thermalized causing fission to occur. Furthermore graphite is  a material that can catch fire at extremely high temperature. If it catches fire, it will actually increase temperature and results in positive feedback reactivity coefficients. The reactor exploded and instantly more than 60 firemen came to put off the fire without noticing that it is highly radioactive and results in lost of 31 brave firemen. Residential around 25km radius is evacuated until today. Fortunately no leak detected at the bottom part of the reactor core or it might contaminate soil,ground water and everything available. Why is the damage due to the blow is so vigorous and how can radiation leak so easily? Because they didn't build a reactor containment building yet at that time.

My lecturer will explain the physics behind this incident in the next class.

Class on 28th September

First of all, I will try to keep my post light and easy in accordance to my personal understandings and therefore I do apologize for any inaccurate information. My idea of presentation is to allow people from various backgrounds, and understanding levels to participate in an active discussion in order to trade knowledge on nuclear technology. For those who are craving for information or facts in depth could easily refer to other websites established by nuclear experts.

Today is my first duty to post a topic on this blog in accordance to what we have discussed in class dated 28^th September 2010. Basically what we learnt today, which is reactivity coefficient is continuity from previous lectures on reactor theory whereby each topic is closely related to one another.

Let’s recap and put it in simple words, neutron life cycle on average needs at least one neutron than cause fission of another nucleus expressed in terms of multiplication factor designated as k. Hence k is actually the ratio of neutron produced by fission in a generation over neutron absorbed in the preceding generation (neutron loss). Value of k = 1 means production of neutron is self sustaining (term used as critical), whereby k larger than 1 means that production of neutron is increasing (term used as supercritical), and k smaller than 1 means that production of neutron is decreasing (term used as subcritical).

For those who have a hard time understanding this, say we want to maintain the neutron population just nice. Too many neutron produced means more fission, more power, more heat and therefore could lead towards fuel pellet melting or even reactor meltdown. Decreasing number of neutron production means that the reactor will eventually die. No reactor can be constantly critical (neutron is self sustaining) due to fuel depletion, fission product build-up, and temperature changes and therefore controlling the value of k is essential to control the neutron population in a reactor. This could be achieved by adjusting the fuel concentration and size of the reactor.

After understanding how to control the neutron population, we move on to predict how it will change over time using a concept called reactivity. As discussed by my colleague in the previous post, reactivity is the fractional change in neutron population per generation. Means that reactivity can be expressed in terms of k and hence controlling the value of k directly affects the value of reactivity designated as ρ.

Up to this point, we have assumed that changes in reactivity were achieved by regulation of the system (k value) and could simply analyze the resulting power changes. Unfortunately there are inevitable reactivity changes which occur when the reactor is operating and therefore we need to introduce reactivity coefficients designated as α. Reactivity coefficients are useful in quantifying the reactivity change that will occur due to the change in physical properties inside the reactor.

There are 4 most important α but up to this point, we will look into moderator temperature coefficient of reactivity, α_Tmod. Due to expansion effect, the value could be either positive (over moderated, +α_Tmod) or negative (under moderated) whereby reactors are usually designed to operate in an under moderated condition (-α_Tmod) as it is stable with respect to changes in temperature. Skip all the physics, simply introducing negative reactivity feedback effect means that we can increase power and temperature by increasing the value of k or reduce power and temperature by reducing the value of k. Positive reactivity feedback effect will enhance the effect that produced it and destabilizing.

NAGASAKI AND HIROSHIMA....

During the final stages of World War II in 1945, the United States conducted two atomic bombings against the cities of Hiroshima and Nagasaki in Japan.

          For six months, the United States had made use of intense strategic fire-bombing of 67 Japanese cities. Together with the United Kingdom, and the Republic of China the United States called for a surrender of Japan in the Potsdam Declaration. The Japanese government ignored this ultimatum. By executive order of President Harry S. Truman, the U.S. dropped the nuclear weapon "Little Boy" on the city of Hiroshima on Monday, August 6, 1945, followed by the detonation of "Fat Man" over Nagasaki on August 9. 

          These two events are the only active deployments of nuclear weapons in war^. The target of Hiroshima was a city of considerable military importance, containing Japan's Second Army Headquarters, as well as being a communications center and storage depot.

          Within the first two to four months of the bombings, the acute effects killed 90,000–166,000 people in Hiroshima and 60,000–80,000 in Nagasaki, with roughly half of the deaths in each city occurring on the first day. The Hiroshima prefectural health department estimates that, of the people who died on the day of the explosion, 60% died from flash or flame burns, 30% from falling debris and 10% from other causes. During the following months, large numbers died from the effect of burns, radiation sickness, and other injuries, compounded by illness. 

          In a US estimate of the total immediate and short term cause of death, 15–20% died from radiation sickness, 20–30% from flash burns, and 50–60% from other injuries, compounded by illness. In both cities, most of the dead were civilians.

          Six days after the detonation over Nagasaki, on August 15, Japan announced its surrender to the Allied Powers, signing the Instrument of Surrender on September 2, officially ending the Pacific War and therefore World War II. Germany had signed its Instrument of Surrender on May 7, ending the war in Europe. The bombings led, in part, to post-war Japan adopting Three Non-Nuclear Principles, forbidding the nation from nuclear armament.The role of the bombings in Japan's surrender and the U.S.'s ethical justification for them, as well as their strategical importance, is still debated.



Comdr. A.F. Birch, numbering LB (Little Boy) unit L-11, before loading on trailer in Assembly Bldg. #1. Unit L-11 was the one dropped on Hiroshima. Dr. Ramsey standing nearby. (August 1945)



             

The mushroom cloud over Hiroshima after dropping of Little Boy