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

 



Reactivity on Monday Lectures.....

A very good day to all.....

It is the summarize about what we have learns today(Monday 27th Sept 2010) with Mr Shamsul.

Lets check it out... 

Principles of Nuclear Power 

Atoms are constructed like miniature solar systems. At the center of the atom is the nucleus, and orbiting around it are electrons. The nucleus is composed of protons and neutrons, very densely packed together. Hydrogen, the lightest element, has one proton; uranium, the heaviest natural element has 92 protons.

      The main key of nuclear is Uranium. Nuclear power is generated using Uranium. Uranium can be found easily in any part of the world. In nuclear power plants (NPP), neutrons collide with uranium atoms, splitting them. This split releases neutrons from the uranium that in turn collide with other atoms, causing a chain reaction. This chain reaction is controlled with "control rods" that absorb neutrons.

Reactivity

Reactivity is a measure of the departure of a reactor from criticality. It is a useful concept to predict how the neutron population of the reactor will change over time

Nn = No(Keff)  ----> know number at particular time/generation

 

Accumulation = Production - Absorption - Leakage

 

If accumulation:

o = 0     critical           steady state       static

o >0      supercritical      increasing         kinetic/dynamic

o <0      subcritical        decreasing         kinetic/dynamic

k = 0 -----> neutron population steady

Criticality control

In order to keep an operating nuclear reactor critical must need to 'adjust' terms in the neutron balance

State of criticality:

Keff = 1       critical            p = 0

Keff > 1       supercritical       p > 0

Keff < 1       subcritical         p < 0

No reactor can be constantly critical :-

• Fuel depletion
• Fission product buildup
• Temperature changes

K and Keff are used interchangeably.

Reactivity: dimensionless # (a ratio of two dimensionless quantities) the value is often a small decimal value               

      A parameter called reactivity is positive when a reactor is supercritical, zero at criticality, and negative when the reactor is subcritical. Reactivity can be controlled in various ways: by adding or removing fuel; by changing the fraction of neutrons that leaks from the system; or by changing the amount of an absorber that competes with thefuel for neutrons. Control is generally accomplished

      A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes (e.g. ²³⁵U) or the fusion of light isotopes (e.g. ²H and ³H). The nuclear chain reaction is unique since it releases several million times more energy per reaction than any chemical reaction.

        Reactivity can be controlled in various ways: by adding or removing fuel; by changing the fraction of neutrons that leaks from the system; or by changing the amount of an absorber that competes with the fuel for neutrons. Control is generally accomplished by varying absorbers, which are commonly in the form of movable elements control rods or sometimes by changing the concentration of the absorber in a reactor coolant. Leakage changes are usually automatic.

        Reactivity coefficient, the amount of change in eactivity per unit change in the parameter, to quantity the effect that a variation in parameter.Increasing in temperature, CR insertion, increase in neutron poison.