Main Story: TECHNICAL ASPECTS OF MOX FUEL IN LIGHT WATER REACTORS/THE REACTOR OPTION
http://www.nirs.org/reactorwatch/mox/puupdat4.txt
1. How Does It Work ? Major Differences From Conventional LWRs
2. Will MOX Option Really Eliminate Plutonium?
3. Impacts on Radioactive Waste Management?
4. The Health and Environmental Effects of the Use of MOX 1. How Does It Work? Major Differences From Conventional LWRsProponents of burning plutonium in mixed-oxide(MOX) fuel in LWRs often say that since plutonium already exists in the burned uranium fuel and is still burning, there will not be a big difference by increasing the amount of plutonium a little bit.
The fact is that great effort is put to make it "not a big difference."
In conventional LWRs, the uranium fuel has about 3% fissile uranium-235 and the rest is non-fissile uranium-238.
When fissile uranium absorbs a neutron, it starts fissioning and releases energy, emitting several neutrons.
One neutron will likely start another fission, creating a chain reaction, but the other neutrons must be controlled so that it will not make a massive reaction which will induce an uncontrolled chain reaction.
Control rods are designed to absorb the extra neutrons.
But some neutrons are also absorbed by the non-fissile uranium-238 and this decays into fissile plutonium-239.
In the beginning, the plutonium content is zero and the fissile uranium is about 3%.
The fissile uranium decreases as they burn, creating plutonium at the same time.
At the end of one reactor cycle, the content of fissile uranium is about 0.7%-0.8%, approximately equal to the content of fissile plutonium-239 that is created. (1)
In the case of MOX fuel used in one-third of a LWR core, the plutonium content is roughly 4% from the beginning, which is approximately 5 times more than that in the end of one cycle of a uranium fuel.
This is a significant difference in terms of core nuclear physics. (2)
In a fast reactor, plutonium content of MOX fuel can be up to 50%.
In the option to burn plutonium in CANDU reactors, the MOX fuel content could be 100% core. (3)
But this has not been tested, nor is there any experience at all of burning plutonium fuel in CANDU reactors.
All light water reactors are designed to burn uranium fuel.
Thus the nuclear physics of MOX fuels must be adapted to be as similar as possible to that of uranium fuel.
The MOX fuel assemblies should be able to be operated as uranium assemblies without any restriction to the level of power, performance or safety.
#Various Types of Fuel Assemblies Necessary To Burn MOXIn order to achieve the same performance as normal LWR, the fuel assemblies are made into various types with different plutonium and uranium contents.
Usually, MOX fuel assembly designs for pressurized water reactors use three types of plutonium contents, 1.9%, 2.3% and 3.3%.(4)
For boiling water reactors, four to six different plutonium contents designs are used.(5)
Another difference is that, because of the intensity of plutonium's thermal energy, plutonium fuel pellets cannot be of the same form as uranium fuel.
It could be in the form of a donut where the central part is void to let the heat dissipate.
But this type of a fuel pellet is likely to collapse.(6)
All these factors make the fuel production extremely complicated and difficult, compared to the one-standard uranium fuel for conventional LWRs.
#Reduced Efficacy of Control RodsControl rods work by absorbing neutrons in the reactor core, so maintaining stable power conditions.
Criticality depends on the small fraction of neutrons produced in the fission of uranium or plutonium which are generated with a delay of about ten seconds. (7)
This time difference makes it possible to control the power level by mechanically inserting additional control rods into the core.
However, the fraction of delayed-neutrons in Pu-239 is about one-third that of uranium-235, which means that the reactor is more sensitive to variations in power. (8 )
In addition, plutonium has a slightly higher propensity to capture thermal neutrons than uranium.
Therefore the efficacy of control rods is somewhat reduced, and safety margins are lower.
The additional demands on control systems are largest for those plutonium fuels in which plutonium-239 content is highest, as in MOX fuel using weapon-grade plutonium. (9)
For these reasons, the MOX fuel assemblies should be placed away from the control rods.
The higher average energy of the neutron spectrum of MOX also increases the rate of radiation damage to structural materials in and around the core.
This could cause embrittlement of the reactor vessel in the end, which is another factor for safety concerns.
#Danger of Losing Control of the Reactor Is Greater with MOXConventional LWRs are designed to decrease the reactivity when the temperature rises.
But when using Pu-239 as fuel, heating of the core from an increase in reaction rate tends to increase the reaction rate still further.
This is called the positive temperature coefficient of reactivity, meaning
there is a danger of losing control of the reactor by accelerated chain reaction of fissioning. (10)
2. Will MOX Option Really Eliminate plutonium? Three options for the disposition of "excess" plutonium are considered.
They are the "reactor option," burning it as MOX fuel in reactors; "immobilization", mixing plutonium with highly radioactive fission products and glassifying it into logs for geological disposal; and "deep borehole," burying plutonium deep enough so that it will be unretrievable.
The proponents of the reactor option using MOX fuel often say that the immobilization and the deep borehole options will merely put the plutonium underground and will not eliminate plutonium.
They claim that the only way to eliminate plutonium is to burn it in reactors. But this is misleading.
It can only be eliminated by repeatedly reprocessing the spent MOX fuel, and reusing the separated plutonium.
But plutonium from reprocessed MOX spent fuel is degraded in quality and cannot be used as fuel.
The scale of effort required to overcome the economics and the technological difficulties is overwhelming.
The objective of burning excess plutonium in reactors is to convert the weapons-grade plutonium into spent fuel, contaminated with other highly radioactive fission products so that it will be difficult to retrieve plutonium without reprocessing.
This is called the "spent fuel standard."
But even if it becomes "spent fuel standard," plutonium could be retrieved by reprocessing, and studies have shown that nuclear bombs could be made from such separated "reactor-grade" plutonium.(11)
The real amount of plutonium will decrease by half by burning it as MOX fuel in LWRs.
The MOX fuel starts with 4% plutonium and 96% uranium-238.
By the end of one fuel cycle, 2% plutonium will remain in the spent MOX fuel together with 94% uranium and 4% fission products. (12)
According to National Academy of Sciences, out of the 50 tons of "excess" weapons plutonium, a substantial amount of plutonium would remain in the spent MOX fuel, between 10 to 40 tons(13) depending on how long the fuel remains in the reactor core and the percentage of MOX the core use.
3. Impacts on Radioactive Waste ManagementMOX spent fuel contains more fission products than uranium spent fuel.
The important factor in managing spent fuel is the heat generation caused by the highly radioactive fission products.
Since spent MOX fuel contains much more fission products, the heat generation from MOX spent fuel is twice as high as that of spent uranium fuel after 10 years and three times as high after 100 years.(14)
What this means is that less spent MOX fuel could be put in a limited repository site, leading to the necessity of more or larger repository sites.
Or, longer periods of centuries for interim storage would be necessary.
Because of the existence of more plutonium, there is a criticality concern for geologic repository, and requires separate licenses for disposal.
This means additional costs and delays.
In other words, spent MOX fuel disposal will require more space, more time, and more substantial costs.
Another typical argument proponents of plutonium "recycling" raise is that the extent of uranium mining, milling, conversion, enrichment and fabrication will be reduced, and thus curb the amount of related radioactive waste as a whole.
This argument ignores the additional waste produced by fabricating MOX fuel, burning in LWRs and the effects of making spent MOX fuel disposal more difficult, not to forget the huge waste produced by reprocessing (which is unnecessary only in the case of disposition of weapons plutonium).
4. The Health and Environmental Effects of the Use of MOX#Specific Dangers of plutoniumplutonium does not exist in the natural environment, and is only produced in nuclear reactors. It is known as one of the most toxic elements.
It emits high energy alpha radiation, and has harmful biological effects.
Alpha radiation has a very short range but very intense ionization power.
If exposed on the surface of the skin, the skin works as a shield and will prevent its penetration into the body, but all of its ionizing power will be focused on the small spot, causing burns and killing the skin tissue.
If inhaled into the body, the alpha particle will go in through the respiratory tract, and enter the lung.
Due to its long half-life, it will stay in the body permanently, emitting alpha radiation, and killing the surrounding tissues by strong ionization.
If plutonium is taken into the body in soluble form (e.g. plutonium nitrate) through food chain, it will enter the blood stream, and into the bones, liver and genital organs where it will be enriched.
Alpha radiation leads to reactions in the cells of living things.
It can cause damage to the nucleus and DNA of the cell, in effect causing genetic damage in descendants, particularly if germ cells are affected. (15)
#Dangers of Resuspension in the Environment In the event of a contamination of the environment with plutonium, the whirling up and inhalation of plutonium particles, known as resuspension, plays an important role.
If there is a road traffic, building work or cleaning work at the plutonium contaminated site, plutonium can enter the body through the respiratory tract.
Generally, the more whirled up, the greater the dose intake per quantity of plutonium on the ground.
If there is fire, and plutonium becomes airborne into fine aerosol particles, plutonium contamination of the environment will extend to a far larger scale, landing on ground, contaminating a vast wider area. plutonium remains effective over very long periods affecting the health of the people and the environment. (16)
#Accident Scenario When Burning MOXAccidents involving overheating and meltdown are possible in any nuclear reactors.
In such accidents, not only would readily volatile noble gases, like iodine and caesium be released to the environment, but a small portion of the actinides, including plutonium and neptunium would be released.
As the activity of the actinides is substantially higher in the case of MOX, the consequences of such severe accidents become more serious.
When MOX fuels are used, the probability of having such serious accidents or trouble would increase due to the high content of plutonium in the fuel.
Even if an accident is not a serious one, it could become serious since even a small portion of the inventory of actinides released to the environment could cause significant radiological consequences.
According to a comparative analysis of possible consequences of a core meltdown accident in the German Kruemmel nuclear power plant with and without the use of MOX fuel (17):
*The radiation exposure from inhalation of radioactive materials during the passage of the radioactive cloud is higher by several dozen percent than if uranium fuel elements were exclusively used.
*Radiation exposure through the route of inhalation of remobilized long-lived actinide isotopes is more than doubled.
*The land areas to become out of use by long-term contamination increases as the resuspension pathway is a limiting factor and the greater part of the dose resulting from the pathway comes from the actinides. (18)
#Accidents at Fabrication PlantsAccidents at MOX fuel fabrication plants have occurred.
In June, 1991, the storage bunker of the MOX fuel fabrication plant in Hanau, Germany was contaminated with MOX.
It occurred after the rupture of a foil for container packaging in the course of an in-plant transportation process.
Five workers were exposed to plutonium.
This accident was the main reason the fabrication plant at Hanau was shut down. (19)
In November, 1992, a rod was broken through a handling error and MOX dust released during the mounting of MOX fuel rods to fuel assemblies in the fuel fabrication facility adjoining the MOX facility in Dessel, Belgium.(20)
In event of such accidents, if the International Commission on Radiological Protection (ICRP) recommendations for general public exposure were adhered to, only about 1 mg of plutonium may be released from a MOX facility to the environment.
As a comparison, in uranium fabrication facility, 2kg (2,000,000mg) of uranium could be released in the same radiation exposure.
A 1 mg release of plutonium from a processing process can easily happen from various smaller incidents.(21)
#Worker HazardsThe National Academy of Sciences concludes that the main environment, safety and health related issues in weapons plutonium disposition that needs special attention with the addition of weapons plutonium is the occupational risk from fuel preparation. (22)
Because plutonium is more radioactive than uranium, greater safety concern is required when handling the material in whatever way.
The ICRP sets a standard for occupational exposure to radiation at 100 mSv over 5 years, with a maximum of 50 mSv in any one year.
If you interpret this in comparison for workers at an uranium fuel fabrication plant with MOX fuel fabrication plant workers,
the standards for protection against inhalation are roughly two Million times stricter in plutonium processing than in uranium processing.(23)
Another factor is the gamma radiation exposure which comes from americium, which accumulates as plutonium decays into americium as time lapses.
Gamma radiation penetrates through almost anything, so it is very difficult to protect workers from this radiation.
