Monday, November 1, 2010

Back End Nuclear Fuel Cycle.

Assalamualaikum, First of all, congratulation to Kelantan FA for the winning Piala Malaysia 2010. Gomo Kelate Gomo!! : D

 Back end nuclear cycle or in other words, it’s called a radioactive waste. This is the waste product containing radioactive material.  Usually it is comes from the nuclear process such as Nuclear Fission and etc. Radioactivity diminishes over time, in principle the waste needs to be insulated for a period of time until it no longer poses a hazard. The majority of radioactive waste has low levels of radioactivity per mass or volume.

 The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the front end, which are the preparation of the fuel, steps in the service period in which the fuel is used during reactor operation, and steps in the back end, which are necessary to safely manage, contain, and either reprocess or dispose of spent nuclear fuel. If spent fuel is not reprocessed, the fuel cycle is referred to as an open fuel cycle (or a once-through fuel cycle); if the spent fuel is reprocessed, it is referred to as a closed fuel cycle.

 One part of the back end nuclear fuel cycle is reprocessing. Spent fuel discharged from reactors contains appreciable quantities of fissile (U-235 and Pu-239), fertile (U-238), and other radioactive materials, including reaction poisons, which is why the fuel had to be removed. These fissile and fertile materials can be chemically separated and recovered from the spent fuel. The recovered uranium and plutonium can, if economic and institutional conditions permit, be recycled for use as nuclear fuel. This is currently not done for civilian spent nuclear fuel in the US.

 Currently, plants in Europe are reprocessing spent fuel from utilities in Europe and Japan. Reprocessing of spent commercial-reactor nuclear fuel is currently not permitted in the United States due to the perceived danger of nuclear proliferation. However the recently announced Global Nuclear Energy Partnership would see the U.S. form an international partnership to see spent nuclear fuel reprocessed in a way that renders the plutonium in it usable for nuclear fuel but not for nuclear weapons.

 Another part of back end nuclear fuel cycle is waste disposal. A current concern in the nuclear power field is the safe disposal and isolation of either spent fuel from reactors or, if the reprocessing option is used, wastes from reprocessing plants. These materials must be isolated from the biosphere until the radioactivity contained in them has diminished to a safe level. It is also possible to burn rather than bury nuclear waste, for instance in Integral Fast Reactors or in variations of molten salt reactors. 

A proposed type of nuclear reactor called a traveling wave reactor is claimed, if it were to be built, to be able to be fueled by nuclear waste, and to be able to operate for 200 years without needing any refueling.

 So, as a conclusion waste disposal can be reused. Another option is to find applications for the isotopes in nuclear waste so as to re-used them. Already, caesium-137, strontium-90 and a few other isotopes are extracted for certain industrial applications such as food irradiation and radioisotope thermoelectric generators. While re-use does not eliminate the need to manage radioisotopes, it reduces the quantity of waste produced.



When nuclear become part of us....
Nuclear...and the reds warriors are champions....huhuh...

Friday, October 29, 2010

Nuclear Safety

Hi to all readers,

Now, I will explain about nuclear safety… how about you? Do you think it is safe to build nuclear power plant in Malaysia? Basically, the three primary objectives of Nuclear Safety Systems as defined by the Nuclear Regulatory Commission are to shut down the reactor, maintain it in a shutdown condition and prevent the release of radioactive material during events and accidents. There are many aspects we should consider. One from them is defense-in-depth approach. To achieve optimum safety, nuclear plants in the western world operate using a 'defense-in-depth' approach, with multiple safety systems supplementing the natural features of the reactor core. Key aspects of the approach are:
-         high-quality design & construction,
-         equipment which prevents operational disturbances or human failures and errors developing into problems,
-         comprehensive monitoring and regular testing to detect equipment or operator failures,
-         redundant and diverse systems to control damage to the fuel and preventsignificant radioactive releases,
-         provision to confine the effects of severe fuel damage (or any other problem) to the plant itself.

How about nuclear safety when earthquake occur? According to nuclear world website, we find that nuclear power plants are designed with sensors to shut them down automatically in an earthquake, and this is a vital consideration in many parts of the world. Below is the example other country take when earthquake occur:-
-         Japanese, and most other, nuclear plants are designed to withstand earthquakes, and in the event of major earth movement, to shut down safely.
-         In 1995, the closest nuclear power plants, some 110 km north of Kobe, were unaffected by the severe Kobe-Osaka earthquake, but in 2004, 2005, 2007 and 2009 Japanese reactors shut down automatically due to ground acceleration exceeding their trip settings.
-         In 1999, three nuclear reactors shut down automatically devastating Taiwan earthquake, and were restarted two days later.

Nuclear facilities are designed so that earthquakes and other external events will not jeopardize the safety of the plant. Below are the safeties that other countries take into considerations:-
-         In France for instance, nuclear plants are designed to withstand an earthquake twice as strong as the 1000-year event calculated for each site.
-         Because of the frequency and magnitude of earthquakes in Japan, particular attention is paid to seismic issues in the siting, design and construction of nuclear power plants. The seismic design of such plants is based on criteria far more stringent than those applying to non-nuclear facilities. Power reactors are also built on hard rock foundations (not sediments) to minimize seismic shaking.
-         Japanese nuclear power plants are designed to withstand specified earthquake intensities evident in ground motion. These used to be specified as S1 and S2, but now simply Ss, in Gal units. The plants are fitted with seismic detectors. If these register ground during the motions of a set level (formerly 90% of S1), systems will be activated to automatically bring the plant to an immediate safe shutdown.
-         The December 2004 tsunamis following a magnitude 9 earthquake in Indonesia reached the west coast of India and affected the Kalpakkam nuclear power plant near Madras/Chennai. When very abnormal water levels were detected in the cooling water intake, the plant shut down automatically. It was restarted six days later.

One more thing we must take into considerations about nuclear safety is passive safety system. One major feature Japan has in common (beyond safety engineering already standard in Western reactors) is passive safety systems, requiring no operator intervention in the event of a major malfunction. The main metric used to assess reactor safety is the likelihood of the core melting due to loss of coolant. These new designs are one or two orders of magnitude less likely than older ones to suffer a core melt accident, but the significance of that is more for the owner and operator than the neighbours, who - as Three Mile Island showed - are entirely safe also with older types.

Other than that, the Emergency Core Cooling System (ECCS) is one more safety thing to the nuclear. Emergency Core Cooling System (ECCS) comprises a series of systems which are designed to safely shut down a nuclear reactor during accident conditions. Under normal conditions heat is removed from a nuclear reactor by condensing steam after it passes through the turbine. These systems allow the plant to respond to a variety of accident conditions and at the same time create redundancy so that the plant can still be shut down even if one or more of the systems fails to function. In most plants ECCS is composed of the following systems; High Pressure Coolant Injection System (HPCI), Depressurization System (ADS), Low Pressure Coolant Injection System (LPCI), Isolation Cooling System and other else.

Friday, October 22, 2010

Nuclear spent fuel management

The production of nuclear electricity results in the generation of spent fuel that requires safe, secure and efficient management National strategies for the management of spent fuel vary, ranging from reprocessing to direct disposal. This indicates that spent fuel is regarded differently by countries - as a resource by some and as a waste by others. Appropriate management of the resulting spent fuel is a key issue for the steady and sustainable growth of nuclear energy .For the example, at the end of 2005, 443 nuclear power reactors were operating in 30 countries worldwide, providing 16% of the global electricity supply. Over 10 000 t of heavy metal (t HM) are unloaded from these reactors each year, which will increase to ~11 500 t HM by 2010. This is the largest continuous source of civilian radioactive material being generated, and needs to be managed appropriately. At the moment most spent fuel is in storage at nuclear power plants, at a few centralized storage sites and at reprocessing facilities. Originally all spent fuel was expected to be reprocessed within a few years and the remaining fuel material recycled into new fuel. The waste from reprocessing was intended to be disposed of in geological repositories. The next steps towards the disposition of spent fuel are either reuse, through reprocessing, or disposal in geological repositories. Some countries are continuing the recycling route, while others have decided to regard the spent fuel as a waste intended for direct disposal .Because progress on implementing these strategies is slow in most countries; the amounts of spent fuel in storage are increasing. The prospect of a revival of the nuclear power industry in the next decades indicates that even more spent fuel could go into storage. On the other hand, spent fuel has been successfully and safely stored in wet and dry conditions for several decades without serious problems, but without decisions on more permanent solutions there could be the prospect of continued storage for times of up to and beyond one hundred years. The management of spent fuel is, for strategic, economic, safety and security reasons.

Reprocessing of spent fuel used

The used fuel dry storage process

Monday, October 18, 2010

Nurkilan nusantara berangan....

Friday, October 15, 2010

Nuclear Fuel Cycle (3)

Other steps in nuclear fuel cycle is Milling...

Uranium milling is the process of extracting uranium from mined ore. The ore is crushed into sand size particles and the uranium is leached out. The uranium then is precipitated out of the leaching solution and dewatered, dried, and packaged. Through the extraction process, uranium is concentrated into a product referred to as "yellowcake." In situ leaching is a combined mining and milling operation.

The picture show the milling process:
     1. Mined ore is crushed
     2. Crushed ore ground into fine sand
     3. Slurry pumped into leach tanks
     4. Acid dissolves uranium
     5. Uranium filtered from waste
     6. Purified & Concentrated
     7. Uranium extraction
     8. Moisture Removed

Yellowcake is a kind of uranium concentrate powder obtained from leach solutions, in an intermediate step in the processing of uranium ores. Yellowcake concentrates are prepared by various extraction and refining methods, depending on the types of ores. Yellowcake is used in the preparation of uranium fuel for nuclear reactors, for which it is smelted into purified UO2 for use in fuel rods for pressurized heavy-water reactors and other systems that use natural unenriched uranium.

Nuclear Fuel Cycle (2)

There are many steps nuclear fuel cycle…There first step is mining.
Uranium mining is the process of extraction of uranium ore from the ground. As uranium ore is mostly present at relatively low concentrations, most uranium mining is very volume-intensive, and thus tends to be undertaken as open-pit mining. 

There are 3 types of mining which are:
1.      Open cast mining
2.      Underground mining
3.      In situ leaching.

1.      Open cast mining
It’s means mining at the surface rather than underground. Coal, iron ore, and phosphates are often extracted by opencast mining. The mineral deposit is covered by soil, which must first be stripped off, usually by large machines such as walking draglines and bucket-wheel excavators.
      Underground mining
Underground mining is a technique used to access ores and valuable minerals in the ground by digging into the ground to extract them.
       In Situ Leaching
In situ leaching (ISL), also known as solution mining, or in situ recovery (ISR) in North America, involves leaving the ore where it is in the ground, and recovering the minerals from it by dissolving them and pumping the pregnant solution to the surface where the minerals can be recovered.

 From 3 types of mining I prefer underground mining. This is because it have their advantages compared to other type. Advantages of underground mining are:
·         It allows minerals to be extracted from deep underground
·         It doesn't create a mess like open cut or surface mining