>>17006388
Nemo is that you?
I told you that poland got back onto the map a century ago

>>the fuel channel was stuffed with powderised uranium
read about peddle bed reactor design
I think it would be better to use liquid bismuth-lead coolant like in these USSR submarines and just use standard rods
>filled with water
it will become radioactive
also water in nuclear reactor decomposes into hydrogen and oxygen slowly (by radiolysis) so you need to remove it
zirconium alloy also can react with steam and produce hydrogen too
obviously both when unmanaged will lead to explosion
>easy refueling
if you managed to do peddle bed design on the ship you would get snatched by closest nuclear power government quickly.
Peddle bed designs have their own problems (friction gives you highly radioactive dust you don't want to breathe in)
Aircraft carriers have a part of the deck that is designed to be removed easily, the deck below that, and yet one below to get access to the reactor.
It would be harder on battlecruiser since you don't have an airstripe on the deck so no big flat space

>>17006500
Nemo? I don't know him but a couple anons from /k/ might recognise me.

What happens to excess radioactive water or H2 / O2 gas from it? Dump it into the sea?
Are reactors ever dried out for maintenance/refueling?

>if you managed to do peddle bed design on the ship you would get snatched by closest nuclear power government quickly.
Why so?
>reactor access
Not impossible, but difficult. It's a long enough hull (for speed, long ships are able to sail faster hydrodynamically) that there is space, the main problem is that there's a few hundred tonnes of armoured decks / reactor armour in the way, and the main armoured deck is structural, but I think I could have a space between reactors (currently thinking 4 units, two on each side) so they could slide out of the reactor chamber on rails and be lifted out from a removable piece of deck in the middle.

But it seems essential to be able to refuel them in place, if not in operation, because I'm Australian and asking for HEU is going to go down like a pork-chop in a synagogue, and using LEU is much cheaper, so refuelling would be approx every 2-3 years instead of every 20-30. Also, pulling them out of the ship has to be a whole lot easier if you can de-fuel them first.
For space reasons I really don't want a tall vertical PWR so being able to refuel horizontally is extremely attractive. I just don't know how to do it without leaking contaminated coolant (with standard rods) or some of the nonsense I suggested in the OP.

>>17006388
oh for fucks sake. starting exercising and get fit enough to join the navy and be a nuke. that way you'll find others just like you and can spend years of your life chattering away with them about it and doing it for real.

I hated every second of being an ELT and I hope you do too.

>>17006784
>Nemo?
it's a joke
the captain Nemo from Verne's book series was supposed to be a Pole seeking vengeance for occupation of his country by attacking random ships with his ultra-tech submarine (it was 19th century).

>What happens to excess radioactive water or H2 / O2 gas from it?
you can burn it safely and make water out of it this way. hydrogen leaks are risky, in the chernobyl they first suspected that the hydrogen in some collector tank exploded.

>Why so?
because it would remove whole "disassemble few layers of deck to get to the reactor"
they replace the whole reactor compartment this way instead of simple refuel (so they don't have to ponder what is inside and how to repair/replace it), this is one of the reason they use highly enriched uranium, they need the fuel to last years before it will need to be replaced (another reason is limited amount of space but reactor could be bigger if it didn't need to be lifted out of the ship's innards)

>the main problem is that there's a few hundred tonnes of armoured decks / reactor armour in the way, and the main armoured deck is structural
yep, this is why you need a different way to refuel a reactor, peddle bed design would be good but it has it's own problems

the main one is that the fuel spheres will move in the reactor and they can lose covering and fuel by friction. it's hard and dangerous to remove that dust (imagine if it got in the vents)

>Also, pulling them out of the ship has to be a whole lot easier if you can de-fuel them first.
defueling is the risky part because you don't have enough shielding and space to carry the waste safely
you would need to use some robots for it and the whole ship would get irradiated

also you could just drop it into the ocean beneath the reactor, water will shield the crew while it falls to the bottom

>refuel horizontally
in windcastle they had experimental air (yes atmospheric air) cooled reactor that had horizontal rods

i'm a physicist so ask

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>>17007146
>Nemo
Ah.

Just did find out the CANDUs (At least the APR-1000, https://youtu.be/-YhHNjjFqb0?t=170) load horizontally. It's similar to my original idea, (see pic), but I thought that wouldn't work because how do you transfer heat from the fuel rods (I assume this is where most heat is generated?) to the coolant when the coolant doesn't run through the fuel channels?
I'm hesitant to go for pebble-beds when they seem pretty unconventional and I already don't really know what I'm doing.

I take it the fuel-tubes are unpressurised, but how do they conduct heat? Those CANDU fuel bundles (pic at 1:59 in video) are a bunch of tubes with air between.
Initially, I thought just stuff uranium pellets in loose, but it has the same air-gap problem. (And maybe use smaller rods, since they're solids instead of a fuel bundle)
Maybe I could size the uranium pellets to fit the fuel-pipes very precisely?
They can't be full of water, or it would heat up and pressurise the rods. Do they rely on blackbody radiation or something?

>access
I put up a drawing of how it might work. Nothing is to scale, because as yet I don't know how to estimate what size reactors I need.
At this point, it's worth noting the ship is pretty big. The space between the torpedo bulkheads is 151' 6" (46.18m), and from the keel to lowest major deck is 47'10", but the reactors are safer the deeper in the ship they are. (Some missiles target the centreline, so better to put them aside)
I think you could have a refueling machine, maybe on the same rails, to insert from the rail-end, and a collector pool on the opposite end. It only takes, maybe, ~6 ft of water to effectively shield spent fuel?
Yes, it's a huge use of internal space, but I needed a lot of empty space anyway for buoyancy. I only really care about reactor/machinery size because the smaller it is, the heavier I can armour it.

Thanks for helping, btw. I'm an engineering student with a thought experiment that has gone a bit far.

>>17007261
P.S.
is it reasonable to mix graphite and light-water moderation? I'd rather not complicate it enough to use heavy water and light water is apparently a bit of an absorber, so maybe adding in some graphite would up the efficiency?

>>17007261
>CANDU fuel rods
the fuel rods have small tunnels inside
I think that the coolant moves through these tunnels while the fuel element sits in larger unpressurized tube
you just place fuel element so both ends are connected to the cooling loop and open the valves I think
I need to read more about this design

>They can't be full of water, or it would heat up and pressurise the rods.
they can because water can escape so it will just expand and flow out instead of pushing too much on the structure (if it heats slowly enough)

>Do they rely on blackbody radiation or something?
nope, power of blackbody radiation is proportional to T^4 (in kelwin) but you would get to melting temperatures before it balances

I think CANDU-similar design would be great, use of heavy water would be OK since you need small but efficient reactor

I think you could load and remove fuel by using some hatch/channel on the bottom of the ship (you would need a dock prepared to catch and load fuel). In this way you get radiation shielding from the water beneath.

>Yes, it's a huge use of internal space, but I needed a lot of empty space anyway for buoyancy.
good idea, you could fill these with some fluid in the dock only when reloading fuel

>is it reasonable to mix graphite and light-water moderation? I'd rather not complicate it enough to use heavy water and light water is apparently a bit of an absorber, so maybe adding in some graphite would up the efficiency?
light-water+graphite reactors have great efficiency, some of them can run on natural uranium
the reason they aren't used so much is safety. graphite is great moderator and the water overmoderates the fuel so lower reaction speed. when the water boils into steam you get more fission, so more heat, so more boiling, it's a positive feedback loop (one of the reasons chernobyl happened as it did since it was this type of design)
>Do they rely on blackbody radiation or something?

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>>17007349
>light-water+graphite reactors
Chernobyl is mostly graphite-moderated. I'm mean more along the lines of a PWR with graphite only a fraction of total moderation, so hopefully safer

I misunderstood the CANDU when I posted, thinking the main vessel was pressurised and the fuel rods weren't. CANDUs run on heavy water moderation? HW absorbs less than LW, so does that mean a LW CANDU would generate more heat/pressure in the moderator?

I've even SEEN this diagram before and didn't twig that I was trying to make an INVERSE-CANDU: fuel-rods at low pressure, main vessel contains the coolant.
Could a version of this work with narrow fuel-tubes filled with solid pellets of cladded UO2? If the pellets were machined slightly under-diameter, they could be inserted, and then expand to fit the tube as they heat up. It isn't full on operational-refueling, since you'd need to let the core cool off to let the spent fuel shrink and move out, but there'd be no need to open the reactor, no coolant valves/pipes to disconnect and no way to leak core-water. (Maybe drill a hole in the fuel rods, so you can pump non-core coolant through to shrink them for extraction?)
I admit to being a total newb to reactors but the idea smells like it could work.

>I think you could load and remove fuel by using some hatch/channel on the bottom of the ship (you would need a dock prepared to catch and load fuel). In this way you get radiation shielding from the water beneath.
you could connect it to the shell handling system and make it somebody else's problem 30 km away
:)

>I think CANDU-similar design would be great, use of heavy water would be OK since you need small but efficient reactor
Might one need to replace heavy water while at sea? AFAIK the machinery to make it is way too big to put to sea.
If not, I worry on the cost. Space is important but not as critical as other naval reactors.
CANDU is probably the best if my inverse idea doesn't work.

Thank you man, you're helping me a lot.

>>17007393
*Forgot: since I'd be using solid fuel rods, the tubes should be smaller and many-er than the tubes for CANDU bundles.

>>17007393
>Chernobyl is mostly graphite-moderated.
yes but there is water inbetween fuel element that overmoderates it
tiny fraction of graphite could work but I'm not sure how it will behave in pressurized environment

>CANDUs run on heavy water moderation?
yes, you can make smaller and more efficient reactors with it
one of the reasons that nazis tried to produce heavy water in huge amounts
it's costly but hey, military use

>LW CANDU would generate more heat/pressure in the moderator?
less and you would need more enrichment to make it run

>Could a version of this work with narrow fuel-tubes filled with solid pellets of cladded UO2?
why use pellets at all? I think you could just copy CANDU design as it is, just think about how to remove safely fuel elements
also the pellets are usually made with protective cladding on it, you don't want it to expand too much since it can break and release material then

>Might one need to replace heavy water while at sea?
not unless you get a leak, but you can just pump it onto the ship with another supply ship then
what matters is the cost since it's expensive

>Thank you man, you're helping me a lot.
reactors aren't my field of study but physicists are interested in anything related (and knowledge from one topic often transfers to another one)
also I like talking about this

>>17007403
i think the cladding could be sized so the uranium only expands to fill it when up to temp. (zirconium has about 1/2 the thermal expansion of uranium oxide)
Mainly I want to simplify CANDU refueling (even though it's already one of the simplest to refuel) because of cost and the fact that refueling machines are HUGE. I could fit a full on RBMK under the deck (barely) except that it would be impossible to refuel.
With a fuel pushthrough design (which the CANDUs already use) AND keeping the fuel channels dry, it could be really simple, literally as simple as a dude with a ramrod and a sledgehammer (obviously with a lot of shielding) shoving the spent fuel out into a containment pool on the other end.

I'm discussing a 350,000 t battlecruiser (5-10 times the size of WW2 entries) but steel is cheap, the real cost is systems, and it's a hard sell today already, so if I can say "hey it costs only 40% more than a 100,000 t carrier" I get people comparing it to 1-2 carriers instead of "mega $$$ wunderwaffe that has to beat everything ever or its a failure"
BTW, the reason she's so big is speed: no point building big guns if the enemy just runs out of their range and shoots missiles. Longer ships have greater hull speed (scales with root length) so, build long and thin. It's a battlecruiser with a length-to-beam ratio of about 12.5:1. The Iowa class are considered pretty slender, and they're at only 8.2:1. I think about 1.4 million shp is enough to achieve 45ish knots.

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>>17007410
>i think the cladding could be sized so the uranium only expands to fill it when up to temp. (zirconium has about 1/2 the thermal expansion of uranium oxide)
yes but you want to avoid moving elements as much as possible
remember each moving part is a point of failure and it's not question if but when it breaks
if you want to use graphite then you will also have to deal with wigner energy (sudden spike in temperature because of the defects in graphite structure caused by radiation) but it could be manageable in your case

> literally as simple as a dude with a ramrod and a sledgehammer
they were doing this and windscale, it's how they removed the fuel from the reactor
insert with long rods from one side, let them fall down on the other
it was plutonium production project tho so lower energy overall

read about this incident and about the design of this reactor
>https://en.wikipedia.org/wiki/Windscale_fire

I think if you were to use more enriched uranium then you could make refueling simpler

>speed of ship
there is also a question of inertia
they can get pretty fast but what about turning?
if it's predictable where the ship will be then it's easier to hit too

also windscale pile on the pic

>>17007451
I want tube-refeuling so that the core itself can be basically a non-moving part, and never opened. (Also running control rods through similar tubes so the mechanism can be simpler and unsealed)

The wiki page says Wigner energy (for graphite) dissipates slowly over 250 C, so it should be safe, as I was planning to run the reactor at about 2.2 ksi / 650 F (343 C). I think even if it's running at low power, you do that by keeping it at temp/pressure, and just having more control rods in. I think.

I also don't understand how the pellets falling out and over the channel was allowed to be such a problem at Windscale, when the fix could be $20 worth of chain-link fence. (Possibly hard to install, but if people were walking through and sweeping the spent rods, it can't be that hard?)

>there is also a question of inertia
>they can get pretty fast but what about turning?
I don't really know. Traditionally, fatter ships tend to turn quicker, but I don't know equations which describe that. I just stuck a 4 rudders on the back and made sure the ratio of rudder area to hullside area was relatively high.
The rudderstocks are specifically reinforced so that you can do a barn-door stop (closing opposing rudders against each other) within the ship's length. USS Wisconsin apparently did such a stop from 33 knots in 600 ft (2/3 her length). On mine, the rudder post is tapered, but at max, 84" wide. (~2.1m?)

Apologies for using only Imperial, I'm trained in metric but decided for the hell of it to use Imperial for this, and now I'm used to it. Overall, it's basically the same, my errors just tend to be powers of 12 instead of power of 10.

btw, if ~194 MeV is released per U235 fission, and ~2.4 neutrons are released at 1-2 MeV each, that means about 98% of the energy is released as heat into the fuel right, only 2% in moderator/absorber? Trying to work out whether the control rods/moderator need much cooling.

>I think if you were to use more enriched uranium then you could make refueling simpler
Certainly less frequent/sizeable, but simpler?

Also, would mixing graphite and water moderation potentially be safer? If you have enough water moderation you could avoid a positive void coefficient but adding in some graphite should make the reactor stabler under conditions like the BWR at La Salle 1988? (Water boiled to steam -> no moderation -> cools -> steam back to water -> repeat)
https://en.wikipedia.org/wiki/LaSalle_County_Nuclear_Generating_Station#Incidents
https://www.youtube.com/watch?v=3ZiKXMgIKGg

>>17007698
>I also don't understand how the pellets falling out and over the channel was allowed to be such a problem at Windscale, when the fix could be $20 worth of chain-link fence.
stupidity and cutting corners
it was weapon production program so each delay had political cost
even installing of the filter on top of the chimney was seen as controversial because cost + delay
>units
don't use imperial please
nobody in academia (even US one) uses imperial units
it's all metric
>btw, if ~194 MeV is released per U235 fission, and ~2.4 neutrons are released at 1-2 MeV each, that means about 98% of the energy is released as heat into the fuel right, only 2% in moderator/absorber?
good first order estimate
some of energy will be carried by neutrinos which will just harmlessly escape the reactor so it should be even lower than that
>Certainly less frequent/sizeable, but simpler?
smaller reactor size for same power so more space for shielding / machinery and stuff
all nuclear subs use more enriched fuel than usual powerplants because of that advantage

>Also, would mixing graphite and water moderation potentially be safer?
yes but I'm not sure about the proportions
you would need to get some specialist data about neutron velocity distribution depending on the thickness of graphite and water
I'm not sure this data is publicly available (but it should be easy to obtain for anyone that has some radioactive material so maybe they don't care about secrecy there)

>I want tube-refeuling so that the core itself can be basically a non-moving part, and never opened
good then but is the system engineered in a way that moving parts getting stuck will block the access to the reactor? that is my concern there

also you can take a look at these
>https://en.wikipedia.org/wiki/Liquid_metal_cooled_reactor
soviets had submarines with reactors cooled by bismuth-lead mixture

>>17006388
>refuel
most modern naval reactors are designed to never be refuelled for their operational lifespan.

>>17008073
well yes because you can lift it out of the ship in the case of carrier (and no other way to safely refuel that), or in the case of submarine you have no way of removing it without cutting out the entire section

in the case of battlecruiser you wouldn't have a large flat space and it would be even less economical to cut the ship apart for the sole purpose of welding in new reactor compartment

hmm what system do nuclear icebreakers use? russia has at least one

>>17007825
>units
I know nobody uses imperial for nuclear stuff (I even asked my dad who did an electrical PhD in the USA, he said it was all metric) but it made a bit more sense for naval design, I think even today wikipedia quotes displacement in long tons.
99% of it is the same as using metric anyway, so many units I use are just the same derivatives of length/mass/force and it's the same except swapping m/kg/kgf for ft/lbm/lbf, maybe with long tons or inches thrown in. I started doing imperial-only for the 'challenge' but it hasn't changed anything much except for that one time I mixed up psf and psi.
then my Aussie uni (officially metric since the 70s) just somehow dumps TSF (short Tons per Square Foot) on us, though the Indian instructor didn't even know what it meant except a conversion factor to kPa.

>good then but is the system engineered in a way that moving parts getting stuck will block the access to the reactor? that is my concern there
Probably, but if so it would only affect individual fuel rods. Probably.

>>17008125
>>17008073
It is still possible to hoist the reactors out (through pre-planned routes) but it'd still be a huge pain. If you can refuel without that, you don't need to use high enrichment uranium, so it's cheaper (main reason) and nobody spergs out about nuclear proliferation. (Assuming you aren't the USN, nobody gives a fuck if they build another ship with 80%+ enrichment)
Because so much of the weight is armour, there's lots of space you need for buoyancy anyway, so setting aside some room to move reactors in/out isn't that hard. It'll probably be free 99% of the time anyway, for storage or as an indoor basketball court or something.