Nadia Aaron asked...

Where is it going to be build in Malaysia?

If our neighbouring countries had a NPP reactor blow, will the radiation reach us?

Where to build? Borneo (Sabah or Sarawak)... Not a chance. There's already BAKUN over there to supply electricity. Well not yet but sooner. Ok, suppose our NPP is built in Borneo, how are we supposed to send the electricity to the peninsular where the demand is at peak? Transmission cables ya... Distance more than 1000km so we should use underwater sea cable. Can you imagine how much power loss will occur within 1000km cable? Suppose we increase the power. P=VI. Increase voltage? Not a chance. If there's a leakage then imagine 1000V or more in water. Increase current. Possible but high current flow would produce a strong electromagnetic field around it which could damage the eco system. Perhaps my colleague taking EE could explain more on this as it is neither my expertise nor preferences. It is actually not possible to do such a thing for now. Even if we manage it, the cost will be too high hence leaded to loss rather than profit. Profit (loss) = Total Revenue - Total Cost. I actually heard rumours saying we already have a spot for our NPP. It should be near the sea for the cooling loop system and easy transportation. Not around north east coast (Kelantan, Pahang, Johor, Terengganu) as it will be too "exposed". So the nominations are Penang,kedah, Perlis, Selangor, Perak, Negeri Sembilan, Melaka, and Johor. Owh ya Penang, Kedah and Perlis have a Tsunami history and hence can be excluded. Melaka also as its sea shore are too dense. I think the rumours stated Negeri Sembilan.

Radiation could actually be transferred in almost every possible way you could think of. It only depends on its intensity, shielding, and distance. Chernobyl incident occur in Ukraine, Europe. In several days, the United States of America detected that the radiation reached them followed by Japan. That’s just a simple indication on how far and how fast it can spread. It actually depends on the source radiation intensity where the intensity decreases as distance increase. We are talking about Indonesia and Thailand.

Thank you for your question. A good one actually. Thanks for everyone who had been reading this blog. Thank you all for your precious comments. We do appreciate it but please read through all article posted in this blog before asking as some questions were asked repeatedly. I know it’s kind of boring but that’s why I said just read through or even glance through . Thank you.

Topic for 14th October

How am I suppose to expect readers to get excited with my posts if even myself thinks it's boring. Never mind, people differs in taste and preferences.

Thoughts for the day

We live in a world full of lies. Great people are the ones who can tell lies yet no one can deny. The greatest people are those who can tell lies, not only you can’t deny it but you agrees with it. You can meet most of these great people in the political and business world.

Some example is that how you felt safe when boarding an airplane or when you drive a Volvo or how you felt secure saving your money in a bank.

The truth is that there is nothing in this world that you could say 100% safe yet you choose to be deceived in order to live or otherwise you’ll live in paranoia. The world is harsh. Concluding all the above, Malaysia needs a spokesman… A great one that could deceive us by saying NPP is 100% safe.

Malaysian government concerns about the citizens fear of NPP. Why fear?

Humans by nature fear something that we can’t truly understand, predict, nor control. We don’t fear nuclear power plant (NPP) but its radiation. Why is that? Nuclear scientist and engineers understands about radiation. They also can control and predict it within some limitations so why fear is still creeping? Because it is something that we can’t hear, neither smell, touch, feel, nor see. It’s a mystical killer that we can’t comprehend with.

So if we want to build one, we have to find a way to reduce or even eliminate fear among Malaysians. I suppose that if our government provides each house with a radiometer, then the fear could ease of. With it, people thought that they actually have some senses to detect radioactive. Or even build a 1 meter thick concrete fortress for each residential area in case of a nuclear reactor blows, well that’s paranoid actually. The thing is our neighboring countries (Thailand and Indonesia) are seriously considering having a nuclear power plant (NPP) by 2025. Say the worst that you fear occurs to their NPP, and then we will still be affected by the radiation leak even though we don’t have one.
What we need is some optimistic whereby our leaders can set a good example and influence Malaysians by living in the closest residential area where the NPP is built. If a nation that had been struck by 2 nuclear bombs could still appreciate NPP, then it’s time we learn from them.

All I'm saying is my personal thoughts. No hard feelings.

Topic 13th October 2010

Sorry to consolidate several posts in one go (dated 13^th October) as I will be extremely busy for the week and maybe for the upcoming weeks as well due to my Final Year Project.  It’s been a while since my last post and before this all my post are quite lengthy with some technical information blended together. In fact I hesitate on anyone out there who actually read it. This time I’ll make my posts light and easy. Said that before… As my colleagues had done some interesting job posting pure facts and technical information, today I would post some general issues and nothing much on technical information. (Excuses as to be honest, currently I am lazy to use my brain and hence I wrote down spontaneous thoughts).

Is it myths or fact?

I’ve heard this ever since I was in primary school. Universiti Kebangsaan Malaysia (UKM) has the highest rate of staffs diagnosed to have cancer in Malaysia. They believe that it had something to do with Malaysian Institute of Nuclear Technology (MINT) which is located less than 20km away from UKM. If MINT is emitting substantial amount of radioactive, why didn’t their employees suffer the same consequences? Ask staffs in UKM and their answer would be “people working at MINT are equipped with sufficient protective gears to reduce radioactive exposures while UKM staffs have none of these”. It absolutely makes sense.

But remarkably when you look deep into residential area around MINT (Bandar Baru Bangi, Teras Jernang, Bangi Lama,etc), the statement doesn’t make sense. The numbers of people diagnosed with cancer among these permanent residents are significantly low. They don’t have any protective gears or live in a 1 meter concrete fortress. Maybe electronic devices or any equipments or even the soil in UKM is the source of radiation, which is if you insist to put the blame on radiation.

Class on 12th October 2010

Topic covered today was related to nuclear fuel. We were supposed to do some research via internet about why did the price of Uranium dropped significantly around 1970’s but I am lazy to do some research on that so I guess I’ll put it on hold for now. You could read it from my clasmates' blog instead.

As a replacement, I would just like to nag a bit about this whole course. Here I go… A wise man once said that people who had never done any mistakes in their lives are the ones who actually didn’t do anything. The statement is not completely true from my point of view as we can actually learn from others’ mistakes as well. The truth is we do learn best from our own mistakes but unfortunately we don’t live long enough to do all the mistakes and learn from it plus some mistakes cost too much to bare and thus learning from others’ mistakes is the best alternative. Having said this, take Chernobyl incident as an example. It is a devastating event for the nuclear power plant industry and human history. But looking things from another perspective, the incident is a major breakthrough in nuclear power plant studies. Learning from others’ mistakes ya.

If our lecturer takes a week or 2 during lectures, telling the story of this incident in sequence then relate it to the physics in accordance with its sequence, I’m quite sure that most of the students would appreciate or even understand more on balance of reactor, multiplication factor, reactivity (reactivity coefficients, positive and negative feedback), Xenon poisoning, and control rods. Better than teaching some tedious derivation and mathematical formula which I could assure that most of the students won’t use it for the rest of their natural life. Well how much could we actually cope within 4 months right? It’s better to have a few sharp blades rather than having thousands of blunt blades in our kitchen. Meaning that; let’s not focus too much on learning a lot but learn a bit which we can easily explore in depth. The course syllabus and notes were carefully thought out which is why it is very comprehensive, everything is important, but it is futile if students can’t cope up well. Students can’t even answer general questions asked by the public related to NPP. After all it is an introduction class.

Please don’t get me wrong, this is my personal point of view. I did enjoy attending this course. In fact, it is an exciting course that I only went absent once (which is during quiz 4) in the whole semester despite its harsh schedule (6pm to 730pm even during fasting month). Again, for those who are planning to take this course, don’t change your mind after reading this. If the teaching method doesn’t suits me well, it doesn’t mean that it won’t suits you. The lecturers are quite reasonable, flexible, and very open minded. In addition young and talented (Sir please don’t deduct marks sir please…)

Exam on 11th October 2010 Overview

This is my personal experience which I would like to share. On the last Monday, all students taking Introduction to Nuclear Power Plant in UNITEN had to undergo an extremely harsh test. Topic covered consists more than 500 slides. Imagine how harsh it is just to read through and understand that much amount of knowledge. Not to mention that words used in the slides are technical (Nuclear Jargons). We were quite providential though as our lecturers allow us to bring in one personal cheat sheet per test session, which is quite a useful way to “force” students to read almost everything.

That’s not where it ends though; imagine having an hour and a half test, take a break for 30 minutes then continues for another 2 more hours (total duration including 30 minutes break is 4 hours). Gosh the last time I undergo an examination with duration exceeding 3 hours is when I took my Engineering Drawing exam 7 years ago. Imagine 4 hours answering all subjective questions. From studies, basically normal human brains could focus over 90% of its capacity for 2 hours straight and that explains why most classes were scheduled to be less than 2 hours for a subject, final examinations were scheduled to be 2 hour and a half where 30 minutes extra is actually time for you to squeeze out something from your brains and do some checking, a F1 regulations state that a race could only last for 2 hours max (Despite that exceeding 2 hours of race would leave the driver dehydrated and cause internal damages due to the G-forces exerted on them).

How does it feel… For the 1^st session of the exam, everything feels great. Having a cheat sheet in addition with my understandings, answering was a breeze but entering the 2^nd session disaster strikes. My brain refuses to generate the answers even though I actually knew the answer, my fingers starts to hurt and trembling reluctant to write down anything, my eyes starts to haze (or fuzz or dusky or dim or blur or glaze) reluctant to read the questions nor my cheat sheet, my body temperature starts to fluctuate causing perspiration even in cold …. My whole body was against me. Amazingly the questions set up nicely with the time provided.

Ok I think that describes my experience enough. For those who are planning to take this course, don’t change your mind after reading this. The lecturers are quite reasonable and flexible. I’m sure they will improvise their assessment method in the future. Having 50% objective, 20% structural subjective, and 30% essay might be better. This is my personal prospective and doesn’t speak for the team or the class.

Basic of nuclear physics......

History of nuclear physics

So here...I post again about the basic of nuclear physics....hope you guys can understand a little bit of it....like me also...

@ The history of nuclear physics is

*  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.
*  In 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. 

@ So,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,it has been included under the same term in earlier times.

So,

@ 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. 

And also,

What is the Nuclear (fission) energy:

•The Commercially established since 1956 

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

• and 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 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:

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

- 1 becquerel (Bq) = 1 dis/s

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

Binding energy is the Mass defect???

Let's talk about mass defect and binding energy.Im actually not that person who can write and post this confidently...but I can  try my best to share my knowledge with u guys....so be cool, no offence......let just start the party...

Binding energy is the mechanical energy required to disassemble a whole into separate parts.

So,

Mass defect is the difference between the mass of the atom and the sum of the masses of its constituent parts.

Then,  Binding energy is  the amount of energy that must be supplied to a nucleus  to completely separate its nuclear particles.

And,

Binding energy is the energy equivalent of the mass defect.

Also,

 Mass defect can be calculated by using the equation below. 

  

Dm = [ Z(mp + me) + (A-Z)mn ] - Matom 

  

So,binding energy can be calculated by multiplying the mass defect by the factor of 931.5 MeV per amu.

The reader's and followers....

It is possible to convert  between mass and energy. Instead  of  two  separate  conservation  laws,  a  single  conservation  law states  that  the  sum  of  mass  and  energy  is  conserved.  

So with that...do you think that the binding energy is the mass defect???

  

I let you know that the decrease in mass will be accompanied by a corresponding increase in energy and vice versa.

Mass does not magically appear and disappear at random.....

So with this, can u imagine it? 

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.