Nuclear Fuel Cycle Flashcards
What is the key decision point in the back end of the nuclear fuel cycle?
Following the end of the fuel’s lifetime in the reactor, it can either be reprocessed or stored indefinitely
What are the key steps in the front end of the nuclear fuel cycle?
Mining -> Milling & Extraction -> Enrichment -> Fuel fabrication -> Reactor
What are the key stages of milling?
Uranium ore is crushed and leached with sulfuric acid
Ammonium diuranate is precipitated from the solvent by using gaseous ammonia
Diuranate is dewatered and roasted to produce U3O8
What is the enrichment factor?
The change in molar fraction before and after separation (from feed to product). Analogous to the depletion factor from feed to tails
alpha = Np(1-Nf) / Nf(1-Np)
What is separative work?
Thermodynamic work necessary in order to separate isotopes and enrich fissile material. Measured in mass per unit time.
SW = P V(Np) + T V(Nt) - F V(Nf)
Nx = Concentration V(N) = Value function
What is the value function?
V(N) = (2N-1) ln(N/1-N)
What dilemma arises regarding separative work and the tails assay?
There is a balance between mass of natural uranium mined and the separative work done (in order to enrich to a particular value). Greater SW is necessary to reduce the tails assay (concentration of fissile material in tails), however, less natural uranium would be needed in order to achieve a particular enrichment
What two enrichment processes are most commonly used?
Gaseous diffusion and gas centrifugation
What uranium compound is used for enrichment and what properties of it are desirable for enrichment?
Uranium hexafluoride UF6 is used.
UF6 vaporises at moderate temperatures
Fluorine has only one natural isotope (F-19) hence mass difference between molecules is due only to the isotopic mass difference between U235 and U238
Describe the process of gaseous diffusion enrichment
Effusion rate is inversely proportional to the square root of the gas molecule’s mass. This gives a theoretical separation factor for a single stage of 1.0043. Multiple separation stages are required in order to achieve 3-5% enrichment
Describe the process of gas centrifugation
Centrifugal force creates radial concentration gradient due to molecular mass. As U235 is lighter than U238, U235 is in higher concentration closer to the centre of the centrifuge. A temperature gradient is also applied along the z axis, which causes lighter molecules to rise. The product is collected at the centre of the top of the centrifuge, while the tails are removed at the outer radius at the bottom of the centrifuge.
What properties of gas centrifuges are advantageous over gaseous diffusion plants?
Centrifuges utilise a ‘no maintenance’ design
Power consumption is 5% that of equivalent diffusion plant
More compact layout
Lower capital and operating costs
Flexible performance
What occurs to spent fuel rods immediately after they are removed from the reactor?
Spent fuel rods are placed in a water tank to cool, as a result of continuous heat emission due to decay heat.
What are the current advantages and disadvantages of geological disposal?
Stable, long-term solution to spent fuel management
No need to separate Pu (provides proliferation resistance)
Cheaper in long term than reprocessing, although capital costs are unknown
Difficulty with political and public acceptance (e.g. Yukka Mountain and West Cumbria Country Council)
What is interim storage?
On-site fuel rod storage in naturally cooled concrete casks. Typical lifetime ~50 years. Attractive short-term storage but is not sustainable
