From: Zefram Date: Wed, 13 Aug 2014 01:28:25 +0000 (+0100) Subject: Manual section on uranium enrichment X-Git-Url: http://81.2.79.47:8989/gitweb/?a=commitdiff_plain;h=38e85e9775b132b0ce504d8d795632668eb47852;p=zefram%2Fminetest%2Ftechnic.git Manual section on uranium enrichment --- diff --git a/manual.md b/manual.md index 99ce31f..d356667 100644 --- a/manual.md +++ b/manual.md @@ -343,6 +343,162 @@ There's one more iron alloy in the game: stainless steel. It is managed in a completely regular manner, created by alloying carbon steel with chromium. +### uranium enrichment ### + +When uranium is to be used to fuel a nuclear reactor, it is not +sufficient to merely isolate and refine uranium metal. It is necessary +to control its isotopic composition, because the different isotopes +behave differently in nuclear processes. + +The main isotopes of interest are U-235 and U-238. U-235 is good at +sustaining a nuclear chain reaction, because when a U-235 nucleus is +bombarded with a neutron it will usually fission (split) into fragments. +It is therefore described as "fissile". U-238, on the other hand, +is not fissile: if bombarded with a neutron it will usually capture it, +becoming U-239, which is very unstable and quickly decays into semi-stable +(and fissile) plutonium-239. + +Inconveniently, the fissile U-235 makes up only about 0.7% of natural +uranium, almost all of the other 99.3% being U-238. Natural uranium +therefore doesn't make a great nuclear fuel. (In real life there are +a small number of reactor types that can use it, but technic doesn't +have such a reactor.) Better nuclear fuel needs to contain a higher +proportion of U-235. + +Achieving a higher U-235 content isn't as simple as separating the U-235 +from the U-238 and just using the required amount of U-235. Because +U-235 and U-238 are both uranium, and therefore chemically identical, +they cannot be chemically separated, in the way that different elements +are separated from each other when refining metal. They do differ +in atomic mass, so they can be separated by centrifuging, but because +their atomic masses are very close, centrifuging doesn't separate them +very well. They cannot be separated completely, but it is possible to +produce uranium that has the isotopes mixed in different proportions. +Uranium with a significantly larger fissile U-235 fraction than natural +uranium is called "enriched", and that with a significantly lower fissile +fraction is called "depleted". + +A single pass through a centrifuge produces two output streams, one with +a fractionally higher fissile proportion than the input, and one with a +fractionally lower fissile proportion. To alter the fissile proportion +by a significant amount, these output streams must be centrifuged again, +repeatedly. The usual arrangement is a "cascade", a linear arrangement +of many centrifuges. Each centrifuge takes as input uranium with some +specific fissile proportion, and passes its two output streams to the +two adjacent centrifuges. Natural uranium is input somewhere in the +middle of the cascade, and the two ends of the cascade produce properly +enriched and depleted uranium. + +Fuel for technic's nuclear reactor consists of enriched uranium of which +3.5% is fissile. (This is a typical value for a real-life light water +reactor, a common type for power generation.) To enrich uranium in the +game, it must first be in dust form: the centrifuge will not operate +on ingots. (In real life uranium enrichment is done with the uranium +in the form of a gas.) It is best to grind uranium lumps directly to +dust, rather than cook them to ingots first, because this yields twice +as much metal dust. When uranium is in refined form (dust, ingot, or +block), the name of the inventory item indicates its fissile proportion. +Uranium of any available fissile proportion can be put through all the +usual processes for metal. + +A single centrifuge operation takes two uranium dust piles, and produces +as output one dust pile with a fissile proportion 0.1% higher and one with +a fissile proportion 0.1% lower. Uranium can be enriched up to the 3.5% +required for nuclear fuel, and depleted down to 0.0%. Thus a cascade +covering the full range of fissile fractions requires 34 cascade stages. +(In real life, enriching to 3.5% uses thousands of cascade stages. +Also, centrifuging is less effective when the input isotope ratio +is more skewed, so the steps in fissile proportion are smaller for +relatively depleted uranium. Zero fissile content is only asymptotically +approachable, and natural uranium relatively cheap, so uranium is normally +only depleted to around 0.3%. On the other hand, much higher enrichment +than 3.5% isn't much more difficult than enriching that far.) + +Although centrifuges can be used manually, it is not feasible to perform +uranium enrichment by hand. It is a practical necessity to set up +an automated cascade, using pneumatic tubes to transfer uranium dust +piles between centrifuges. Because both outputs from a centrifuge are +ejected into the same tube, sorting tubes are needed to send the outputs +in different directions along the cascade. It is possible to send items +into the centrifuges through the same tubes that take the outputs, so the +simplest version of the cascade structure has a line of 34 centrifuges +linked by a line of 34 sorting tube segments. + +Assuming that the cascade depletes uranium all the way to 0.0%, +producing one unit of 3.5%-fissile uranium requires the input of five +units of 0.7%-fissile (natural) uranium, takes 490 centrifuge operations, +and produces four units of 0.0%-fissile (fully depleted) uranium as a +byproduct. It is possible to reduce the number of required centrifuge +operations by using more natural uranium input and outputting only +partially depleted uranium, but (unlike in real life) this isn't usually +an economical approach. The 490 operations are not spread equally over +the cascade stages: the busiest stage is the one taking 0.7%-fissile +uranium, which performs 28 of the 490 operations. The least busy is the +one taking 3.4%-fissile uranium, which performs 1 of the 490 operations. + +A centrifuge cascade will consume quite a lot of energy. It is +worth putting a battery upgrade in each centrifuge. (Only one can be +accommodated, because a control logic unit upgrade is also required for +tube operation.) An MV centrifuge, the only type presently available, +draws 7 kEU/s in this state, and takes 5 s for each uranium centrifuging +operation. It thus takes 35 kEU per operation, and the cascade requires +17.15 MEU to produce each unit of enriched uranium. It takes five units +of enriched uranium to make each fuel rod, and six rods to fuel a reactor, +so the enrichment cascade requires 514.5 MEU to process a full set of +reactor fuel. This is about 0.85% of the 6.048 GEU that the reactor +will generate from that fuel. + +If there is enough power available, and enough natural uranium input, +to keep the cascade running continuously, and exactly one centrifuge +at each stage, then the overall speed of the cascade is determined by +the busiest stage, the 0.7% stage. It can perform its 28 operations +towards the enrichment of a single uranium unit in 140 s, so that is +the overall cycle time of the cascade. It thus takes 70 min to enrich +a full set of reactor fuel. While the cascade is running at this full +speed, its average power consumption is 122.5 kEU/s. The instantaneous +power consumption varies from second to second over the 140 s cycle, +and the maximum possible instantaneous power consumption (with all 34 +centrifuges active simultaneously) is 238 kEU/s. It is recommended to +have some battery boxes to smooth out these variations. + +If the power supplied to the centrifuge cascade averages less than +122.5 kEU/s, then the cascade can't run continuously. (Also, if the +power supply is intermittent, such as solar, then continuous operation +requires more battery boxes to smooth out the supply variations, even if +the average power is high enough.) Because it's automated and doesn't +require continuous player attention, having the cascade run at less +than full speed shouldn't be a major problem. The enrichment work will +consume the same energy overall regardless of how quickly it's performed, +and the speed will vary in direct proportion to the average power supply +(minus any supply lost because battery boxes filled completely). + +If there is insufficient power to run both the centrifuge cascade at +full speed and whatever other machines require power, all machines on +the same power network as the centrifuge will be forced to run at the +same fractional speed. This can be inconvenient, especially if use +of the other machines is less automated than the centrifuge cascade. +It can be avoided by putting the centrifuge cascade on a separate power +network from other machines, and limiting the proportion of the generated +power that goes to it. + +If there is sufficient power and it is desired to enrich uranium faster +than a single cascade can, the process can be speeded up more economically +than by building an entire second cascade. Because the stages of the +cascade do different proportions of the work, it is possible to add a +second and subsequent centrifuges to only the busiest stages, and have +the less busy stages still keep up with only a single centrifuge each. + +Another possible approach to uranium enrichment is to have no fixed +assignment of fissile proportions to centrifuges, dynamically putting +whatever uranium is available into whichever centrifuges are available. +Theoretically all of the centrifuges can be kept almost totally busy all +the time, making more efficient use of capital resources, and the number +of centrifuges used can be as little (down to one) or as large as desired. +The difficult part is that it is not sufficient to put each uranium dust +pile individually into whatever centrifuge is available: they must be +input in matched pairs. Any odd dust pile in a centrifuge will not be +processed and will prevent that centrifuge from accepting any other input. + industrial processes -------------------- diff --git a/technic/machines/register/centrifuge_recipes.lua b/technic/machines/register/centrifuge_recipes.lua index 7f252b6..8b447a2 100644 --- a/technic/machines/register/centrifuge_recipes.lua +++ b/technic/machines/register/centrifuge_recipes.lua @@ -20,14 +20,6 @@ local recipes = { { "moretrees:rubber_tree_trunk", rubber_tree_planks.." 4", "technic:raw_latex" }, } --- Refining uranium via centrifuge is intended to make it a practical --- necessity to set up an automated cascade of centrifuges. Once the --- cascade has been primed, production of one 3.5%-fissile dust requires --- input of five 0.7%-fissile dust and 490 centrifuge operations, and --- produces four 0.0%-fissile dust as a byproduct. The busiest stage --- of the cascade is the one taking 0.7%-fissile dust, which performs 28 --- of the 490 operations. The least busy is the one taking 3.4%-fissile --- dust, which performs 1 of the 490 operations. local function uranium_dust(p) return "technic:uranium"..(p == 7 and "" or p).."_dust" end