Nuclear Technology Essay Example

other cross-sections increase significantly, increasing the chances of interaction.

In nuclei with odd numbers of neutrons (like U-235), the fission cross-section becomes notably large at thermal energies where neutrons move slowly. It is important to note that both scales in this context use logarithmic measurements. To achieve equilibrium with their surroundings, thermal reactors commonly employ U-235 fuel and a moderator like water since they effectively slow down neutrons to thermal energies. Light Water Reactors available as Pressurized Water Reactors and Boiling Water Reactors are well-known examples.

Under thermal conditions, other heavy nuclei like U-233, Pu-239, and Pu-241 can also undergo fission. These isotopes are artificially produced in nuclear reactors from specific fertile nuclei such as Th-232, U-238, and Pu-240.U-235 is the only naturally occurring fissile isotope in natural uranium, with a concentration of 0.7 percent. The main fertile isotopes found in nature are U-238 and Th-232. The likelihood of fission or any neutron-induced reaction depends on the cross-section area surrounding the target nucleus that the incoming neutron must pass through for the reaction to occur. The fission cross-section increases significantly at lower neutron velocities, increasing the chance of interaction. In nuclei with an odd number of neutrons like U-235, the fission cross-section becomes large at low thermal energies of neutrons. However, high-energy neutrons travel too fast to have significant interactions with fuel nuclei, causing a reduction in fission cross-section compared to thermal energies. Nonetheless, fast fission can still be utilized in a fast neutron reactor designed to minimize moderation of high-energy neutrons generated during fission. The process of nuclear fission starts by capturing a neutron and distributing its total energy among the nucleons forming the compound nucleus

(demonstrated with U-235 in a thermal reactor).The compound nucleus is relatively unstable and likely to split into two fragments, each roughly half the mass. These fragments are typically nuclei found in the middle of the Periodic Table, and due to their stochastic breakup, there are hundreds of potential combinations. The creation of these fission fragments is quickly followed by the release of several neutrons (usually 2 or 3, averaging 2.5), which sustain the chain reaction. To initiate the chain reaction, beryllium mixed with polonium is introduced to generate neutrons as beryllium converts to carbon-12.

Initially, around 85% of the released energy comes from the kinetic energy of the fission fragments. However, in solid fuel, these fragments can only travel short distances before transforming their energy into heat. The remaining energy comes from gamma rays emitted during or shortly after fission and from the kinetic energy of the neutrons.

Some neutrons produced during fission (known as prompt neutrons) are immediately released, while a small percentage (0.x%) is released later through radioactive decay of certain fission products. This delayed release has a half-life of approximately 56 seconds for the longest group of delayed neutrons and plays a crucial role in controlling and maintaining criticality in a chain reaction system or reactor.When the system is in a critical state, there is an equilibrium between the neutrons produced in fissions and those involved in causing further fissions, being absorbed elsewhere, or escaping from the system. This equilibrium ensures that the power generated by the system remains constant. The control system has the ability to adjust the power level by changing this equilibrium - either by reducing or increasing the number of

neutrons present, which consequently alters the rate of power generation. Once the desired power level is reached, the control system restores balance. Although it is not possible to predict how individual nuclei will break up due to statistical probability, conservation laws guarantee that both total number of neutrons and total energy are conserved.

During fission reactions involving U-235, various fission products with atomic masses ranging from 95 to 135 are produced, such as Baa, Kerr, Sir, CSS,I,and Ex.

Typical examples of reaction products include: U-235 + n Baa-144 + Kerr-90 + an + about 200 Move;U-235 + n Baa-141 + Kerr-92 + an + 170 Move;U-235+n zero-94 +-re139+an+197 Move.In these equations,the number of nucleons (protons+neutrons) remains conserved,e.g.,35+1=141+92+3,but there may be a small loss in atomic mass corresponding to released energy.Barium and krypton isotopes then undergo decay and transform into more stable isotopes of neodymium and yttrium while emitting several electrons from their nucleus (beta decays). Over time,beta decays and radioactivity decrease.The total binding energy released in the fission of an atomic nucleus varies depending on the specific breakup. For U-235, it averages about 200 Move* or 3.X10^-11 Joule per fission, while for Pu-239, it is around 210 Move* per fission. In comparison, carbon burned in fossil fuels releases only 4 eve or 6.5x10^-19 per atom. These figures include the kinetic energy values (Eek) of the fission fragments and additional releases such as neutron, gamma, and delayed energy that add approximately 30Move.

Approximately 6% of the heat generated in the reactor core comes from radioactive decay of fission products and transatlantic elements formed by neutron capture, primarily the former.
When the reactor is shut down, heat generation continues due

to decay. This decay accounts for initial heat generation in used fuel and necessitates cooling measures. The Fuchsia accident brought attention to a loss of cooling one hour after shutdown, resulting in the fuel still generating about 1% of its full-power heat. Even after one year, typical used fuel produces approximately 10 k of decay heat per ton, which then decreases to around 1 k/t after ten years.

Neutron capture is another mechanism that produces energy. Nuclei can capture neutrons leading to emission of gamma rays as compound nuclei de-excite. This process may also result in emission of alpha or beta particles for increased stability.
When uranium isotopes capture neutrons, transatlantic elements beyond uranium are formed.
Neutron capture by U-238 and subsequent formation of U-239 are important processes because U-238 is a primary component in thermal reactor fuel material. U-239 undergoes beta decay and transforms into Np-239, which then undergoes further beta decay to become Pu-239. Some Pu-239 nuclei may also capture neutrons and become Pu-240. Through additional neutron captures, certain Pu-240 nuclei may form Pu-241. Pu-241 subsequently undergoes beta decay and becomes Am-241 – a substance commonly used in household smoke detectors.

It should be noted that, like U–235, Pu–239 can undergo fission with thermal neutrons and serves as a major energy source in nuclear reactors. If fuel remains in the reactor for around three years, about two thirds of the Pu–239 undergo fission with U–235, contributing approximately one third of the energy output.

The fission products have masses distributed between 100 and 135 atomic mass units. The main constituents of used fuel are plutonium, curium, neptunium, and americium isotopes. These isotopes are alpha-emitters with long half-lives that decay

at a similar rate as uranium isotopes. This is why secure disposal of spent fuel is necessary beyond the few thousand years required for fission product decay alone.

Neutrons that impact any material surrounding the fuel result in activation products within the reactor including tritium (H–3), carbon–14, cobalt–60 iron–55, and nickel–63.During reactor dismantling, the challenges posed by these four radioisotopes impact recycling efforts. Fast Neutron Reactors utilize Pu-239 as fuel and consist of a core surrounded by a fertile blanket of U-238. Abundant neutrons escaping from the core breed more Pu-239 in the blanket. Although a small fraction of U-238 may undergo fission, most neutrons that reach the U-238 blanket have lost energy and can only be captured to produce Pu-239. Minimal neutron moderation is necessary to cool down the fast reactor core, which is why liquid metals like sodium or sodium-potassium mixtures are used for this purpose.

Fast neutron reactors, such as the Fast Breeder Reactor, possess a more efficient configuration of fissile and fertile materials that enables them to convert fertile materials up to 100 times better than ordinary thermal reactors. Pu-239 produces more neutrons per fission compared to U-235. Despite both isotopes yielding more neutrons per fission with fast neutrons, their fission cross sections diminish at high neutron energies.

Unlike standard reactors with a conversion ratio around 0.6, fast reactors can achieve ratios surpassing 1.0. They can be designed as breeders that generate more fissile material than they consume or as plutonium burners that dispose of excess plutonium without requiring breeding blankets while optimizing the core for plutonium fuel.Countries initially developed fast breeder reactors like the Fast Breeder Reactor to increase uranium resources by about

60 times. However, technical and materials challenges hindered their development programs and none of the designs proved commercially competitive with existing light water reactors. Assessing the economic value of bred plutonium fuel is crucial in evaluating fast reactor economics. If the cost of using this fuel is not advantageous compared to current uranium prices, there would be little benefit in utilizing this type of reactor. This realization became prominent in the years asses and asses when it was recognized that uranium resources were abundant and its price was relatively low at that time.

Fast reactors have a beneficial negative temperature coefficient, meaning that as the temperature rises excessively, the reaction slows down. This characteristic ensures their safety and forms the basis of automatic load-following in certain new designs by regulating coolant flow. The significance of this technology extends to long-term considerations of global energy sustainability.

Additionally, these reactors have potential roles in various areas: firstly, disposing of ex-military plutonium in the short term; secondly, burning long-lived actinides recovered from used fuel of light water reactors; and thirdly, maximizing utilization of the world's abundant uranium resources.

Fission involves releasing an average of 2 or 3 neutrons when U-235 nuclei split.To maintain a steady criticality level in a controlled chain reaction, it is essential to have one neutron while the others either escape or undergo non-billions reactions. To regulate the power output of the reactor, control rods containing boron and/or cadmium with strong neutron-absorbing capabilities are employed. The rods are slightly withdrawn from their critical position to surpass the criticality limit and increase available neutrons for ongoing fission. Once the desired power level is stabilized, the control rods are returned

to their critical position. The presence of delayed neutrons allows for control of the chain reaction and prevents immediate and uncontrollable changes in the neutron population if there is a shift in the critical balance. Deviations from criticality have strict limits integrated into both the overall design and operation of a reactor. As fuel burns in the reactor, fission products and transatlantic elements accumulate gradually, resulting in additional neutron absorption. Consequently, adjustments must be made to account for this increased absorption in the control system. After approximately three years, fuel replacement becomes necessary due to accumulation of absorption and changes in fuel material caused by constant neutron bombardment. This means that only about half of the fissile material can be consumed before fresh fuel needs to replace fuel assemblies.To extend the life of fuel and compensate for neutron absorbers, burnable poisons like gadolinium can be used. Slower neutrons are more effective in causing fission in U-235, so a moderator material consisting of light atoms is placed around the fuel rods in a reactor. The main purpose of the moderator is to slow down fast neutrons through elastic collisions, similar to billiard balls colliding on an atomic scale.

Reactor designs using natural uranium typically utilize graphite and heavy water as moderators due to their low levels of neutron absorption. However, when enriched uranium is used, ordinary water can serve as a moderator. Water also functions as a coolant in many reactors by removing heat and generating steam.

Some reactors may incorporate additional features for controlling the chain reaction, such as adding boron to the cooling water. In emergency situations, the addition of boron can be rapidly increased. Commercial

power reactors are generally designed with negative temperature and void coefficients.

If the temperature or boiling level exceeds normal operating limits in a nuclear reactor, it affects the chain reaction by reducing fission rate and temperature. This disruption is significant because it upsets the balance of the reactionThe disruption is caused by the Doppler effect, where U-238 absorbs more neutrons as temperature rises, shifting the neutron balance towards suboptimal conditions. In light water reactors, steam formation within the water moderator is also an important contributing mechanism. Naval reactors used for propulsion experience a reduction in density and moderating effect which affects the neutron balance and results in suboptimal conditions. To prevent frequent fuel changes, burnable poisons (neutron absorbers) are added to the fuel. Initially, the fuel is enriched to higher levels and as fission products and transatlantic elements accumulate, the neutron absorbers are depleted to counterbalance their effects. Commercial reactors use burnable poisons like gadolinium along with higher enrichment levels of up to 5% U-235 to optimize fuel burn-up. As fuel burns in the reactor, additional neutron absorption occurs due to accumulation of fission products and transatlantic elements. The control system needs adjustment to compensate for this increased absorption. After around three years in the reactor, metallurgical changes occur in the fuel and it accumulates absorption from continuous neutron bombardment. At this point, it becomes necessary to replace the fuel and limit burn-up to half of its fissile material. Fresh fuel assemblies are then installed and burnable poisons like gadolinium can be used to prolong fuel life by offsetting the increase in neutron absorbers.

Traditionally, gamma measurement has been commonly used in uranium exploration to detect levels

of uranium. However, this method relies on decay products rather than directly detecting uranium itself. In cases where uranium and its decay products are leached from their original location and redeposited in buried river channels, relying on gamma measurements may not accurately indicate uranium concentrations. To obtain the most accurate indication, stimulating fission is necessary. A portable prompt fission neutron (PEN) logging tool incorporates a neutron source and detector. The neutron source irradiates the uranium deposit, causing any present uranium to emit prompt or delayed neutrons as a result of fission, which are then detected and recorded. This method is the only reliable means of geophysical measurement for certain types of uranium deposits.

Nuclear fusion utilizes nuclear fusion power. Currently, the deuterium-tritium reaction is considered the most feasible option technologically. The Joint European Torus (JET) reactor successfully achieved this reaction with 16 MM briefly achieved and 5 MM sustained in 1997. Now, this project is being expanded internationally with the construction of TIER in France.The reaction involves the conversion of H-2 and H-3 to He-4, a neutron, and 17.6 Move Tritium. Tritium can be generated by breeding from lithium-6 in a blanket surrounding the torus using neutrons from the reaction. Deuterium is abundant in seawater. Enrichment is a physical process that concentrates one isotope, such as U-235, compared to others. Water serves as both a moderator and coolant commonly used in commercial power reactors. However, achieving criticality with a water moderator requires enriched fuel. Enrichment increases the proportion of U-235 by about five to seven times from the natural uranium's 0.7% content.
Commercial enrichment processes involve converting uranium into a gaseous form using uranium hexafluoride (UF6), which

can be contained as a liquid or solid under pressure in steel cylinders.
The text discusses two main enrichment processes: diffusion and centrifugation, applied in multiple stages called a cascade. Each stage involves some isotope separation, with the product becoming the feed for the next stage.
The stages above the initial feed point are referred to as the enriching section, while those below are known as the striping section. Each stage has double feed - enriched product from below and depleted product from above. The amount of enriched product is approximately one sixth or one seventh of the depleted materialThe material collected at the bottom of the striping section, also known as "tails," has a concentration of residual U-235 called tails assay. Each stage or cascade's separating power is measured in flow, indicating energy consumption. Measurement of feed or product quantities, such as uranium, is usually done in tons or kilograms. Uranium is commonly referred to as keg SSW. For example, if a plant wants to produce one kilogram of uranium enriched to 5% U-235, they would need 7.9 SSW with a tails assay of 0.25%, or 8.9 SSW with a tails assay of 0.0% (yielding only 9.4 keg instead of the natural U feed's 10.4 keg). The cost of enrichment SSW and uranium are always balanced.
Approximately 140,000 SSW is required to enrich the annual fuel loading for a typical 1000 Mew light water reactor at current enrichment levels.The primary cost associated with enrichment is related to electrical energy consumption.The older diffusion process consumes around 2500 kHz (9000 MS) per SSW, while gas centrifuge plants only require about50 kHz/Swell (180 M]).The diffusion process operates by utilizing the

difference in velocity between two types of JIFF molecules to facilitate easier passage for lighter ones through membrane holes due to their faster movement.

Each stage consists of a compressor,diffuser,and heat exchanger for compression heat removalThe JIFF product is extracted from one end of the cascade and the depleted JIFF is removed from the other end. It typically takes around 1400 stages to achieve a U-235 concentration ranging from 3% to 4%. Diffusion plants have multiple stages for separation and can handle large gas volumes.

Centrifuge enrichment involves using the mass difference and peripheral velocity in a rapidly rotating cylinder to separate molecules. Gas is introduced into evacuated cylinders, each containing a rotor that is approximately 3 to 5 meters tall with a diameter of about 20 CM. As the rotors rotate, heavier U-238 molecules concentrate towards the outer edge while U-235 molecules concentrate near the center. A counterculture flow results in axially separated enriched product, with heavier molecules at one end and lighter ones at the other.

To ensure efficient separation, centrifuges operate at high speeds of 50,000 to 70,000 RPM, with the outer wall moving at speeds of 400 to 500 meters per second – providing acceleration equivalent to a million times that of gravity. The production of such equipment presents challenges in terms of materials and engineering; carbon fiber is commonly used as the primary rotor material. Although individual centrifuges have smaller volume capacities compared to diffusion units, they are more effective in separating isotopes.Centrifuge stages usually involve multiple parallel centrifuges (typically 10 to 20 stages), instead of the thousands required for diffusion units.
Laser isotope separation processes have long been of interest due to their

advantages in terms of lower energy inputs, capital costs, and tails assays – resulting in significant economic benefits. These processes use precise beam frequencies unique to lasers that correspond to specific energies.
Currently, none of these processes are commercially viable, although one is nearing completion. Atomic vapor processes utilize a laser that selectively targets atoms in a uranium metal vapor using photo-insulation. By using light with a specific frequency, an atom's electron can be expelled.