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I was inside RBMK 1500, and have a few questions :)

  1. Apr 16, 2016 #1
    So as I'm interested in nuclear physics , I and a few other friends that work in the tech field we went to a now shutdown and in the process of decommissioning NPP. For various reasons I won't mention the stations name or location but those of you who know much about reactor types etc will probably have a clue where it was.

    So we walked through all the main parts , the reactor hall , controls room , support facilities , feedwater pumps, and the turbine generator or simply machine hall.Long story short , when at site I was amazed at the vast scale and monstrous size of everything starting from corridors and support buildings to the machine hall which was so big I had problems seeing the other end of it even from the middle of that hall.
    the reactor is the RBMK channel type.
    So here's the questions.

    1) In the active zone I spent about 1 hour and that gave me about 2 uSv of dosage , if my calculations are correct that's not much overall although I would like to hear some feedback on this because it doesn't seem alot in terms of average annual dosage for a person but the time scale in which I got that is also very short so how does that add up?

    2) The RBMK and the CANDU (according to wikipedia) are the only two reactors in the world that can refuel while in operation , I saw the huge and tall crane which is at the rector hall which does the refueling, while it's done nobody stays in the reactor hall but instead the operator of the crane watches through a very thick glass window in a room that is sealed off to prevent radiation exposure.the question is what is the most prevelent type of radiation near the fuel cassette when it's taken out of the core and how far it reaches from the source also it's penetrating power , because I got some contradictory opinions about that yesterday.

    3) Question about dust. the reactor hall had a floor entirely out of stainless steel all corridors up to the hall had the same , is that done because such a floor is easier to clean (decontaminate) in case some radiation source dust have settled on it , like for example when refueling?

    4) Radiation being part of the EM field needs a source , say for example that dust particles from a radioactive metal like uranium have scattered across a given territory like in Chernobyl , is it true that the radiation intensity is dependent on the size of the particle/s and their overall quantity in that given area ?
    so a larger single piece of radioactive material would have a larger amount of radiation emitted than a smaller piece of that same material.

    5) the RBMK is said to have been a "good design" for the soviets because it gave them the chance of both producing civilian electricity and creating weapons grade plutonium at the same time hence the refueling while in operation. I read in "world-nuclear.org" that the only naturally fissile metal is uranium which is in the form of uranium ore which consists mostly of fertile U238 which cannot undergo a chain reaction itself.So the U235 does the chain reaction and the neutrons from that reaction hit the U238 which turns into the Pu239 which can then be used for weapons.
    Does it become weapons grade in the reactor or does it need to undergo more "purifying" elsewhere, like the uranium that is enriched ?

    and the last one for now , when they dig out the uranium if most of it's mass consists of the non fissile U238 using a gas centrifuge I read that they can separate the U238 from U235 simply because the U238 is heavier and in high rpm of the gas is located more to the outside , so they have to make the uranium in gaseous form before they can enrich it using this process ?
    Last edited: Apr 16, 2016
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  3. Apr 16, 2016 #2


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    Weapons grade fissile material is usually purified to > 90% of the fissile isotope, either U-235 or Pu-239. In the case of U-235, this process is extremely expensive, involving either electromagnetic separation, separation by gaseous diffusion, or separation by centrifuge. Pu-239 can be separated from U-238 and U-235 using chemical means, since it is a different element altogether. Enrichment in any event does not take place inside the reactor.
    For ease of handling, uranium metal is usually converted to uranium hexafluoride gas (UF6), which substance can be used in a gaseous diffusion plant to separate U-235 from U-238.

    For more information on enrichment technologies, see this article:

  4. Apr 16, 2016 #3


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    Natural background radiation is about 2000µSv/year, or 0.2µSv/hour, but depending on the place where you live. Within this hour your dose rate was higher, but if you compare it to the dose over a year, it is completely negligible. Moving from one place to another gives you larger changes in natural dose rate within a day, and airplane flights give doses significantly above 2 µSv.
    All radiation types contribute, but alpha and beta don't get far enough to be relevant, unless radioactive dust gets distributed. I guess a steel floor makes cleaning easier.
    It does not depend on the shape, it just depends on the amount of radioactive material. More radioactive material, more radiation.
    Most reactor types also produce other isotopes of plutonium, which prevent building a nuclear weapon out of them without expensive enrichment afterwards. If you can take out fuel during operation and extract the plutonium, you can get a better isotopic composition, and save time and money because processing gets easier.
  5. Apr 16, 2016 #4
    2 uSv is around 0.2 mRem. That's about what I get doing a tour in my BWR (not counting lower elevations of containment). So that's pretty typical.

    For spent fuel, using water you need at least 7 feet to reduce it down to "normal"/"occupational" dose levels (on the order of mRem or uSv). It's really nasty stuff, especially when you just pull it out of the core, and when unshielded it has the potential to deliver lethal doses in seconds to minutes.
  6. Apr 16, 2016 #5
    seems like the very enrichment , purifying process is the most complicated in all of the cycle from digging out of ground to putting inside formed pellets into the core until the critical mass and the start of the chain reaction which is possible the easiest step.
    like for example if a criminal entity with limited knowledge and resources wanted to make a bomb , enriching uranium would be the step that would most likely fail them but if they got already enriched uranium I assume it would be comparably easy to then make a bomb if efficiency and maximum yield is not the prime target.

    What do you mean by that , sounds like you have a personal nuclear reactor in your basement ? :D that's probably not what you meant but I just had a little giggle about the idea of that.

    Well yes they had large ponds deep into the sublevels of the RBMK were they keep the rods taken out of the active core and they store them there for 5 years until then they are carried further to the storage facility site.
    also they built a pretty unique thing for transportation of still active and usable fuel rods, since the EU wanted them to shut down the RBMK yet the reactors were in good shape and just bit over half their primary lifetime expectancy of 30 years.a small railroad vessel with radiation absorbing walls they simply places the still usable fuel into it from the first reactor which was to be shut down first and transported it to the second were using the crane they simply inserted the rods into the core while the reactor was working.

    Ok here's a follow up question.If I were to take out a cassette which has something like 36 ,or maybe less, can't remember , fuel assembly rods , what would be the average radiation intensity in the vicinity of such a cassette and if there was no walls or barriers anywhere around such a thing how far away would the radiation still be dangerous from it's source?
    This is probably one of the most confusing things to the general public , the question of how far away from a highly energetic ionizing radiation source the radiation levels falls off to a safe one. I do understand this is dependent as mfb affirmed before on the mass of the source in question and it's atomic mass number properties (either U235 or 238)

    Speaking about different radioactive elements, one more question from me , I have seen the question and thought myself about what elements are more dangerous, the ones with short half life or the ones with a long one.
    From my own thinking and from what I read elsewhere it seems that the ones with a short half life are the most energetic ones since they release more radiation in a shorter amount of time instead of releasing the same amount or less in a very long amount of time, so if you are in their vicinity you get a much higher dose than from a longer half life one correct?
    But they also say that the ones with the long half life are more dangerous in the long run not because you would receive a deadly or dangerous dosage but because they accumulate slowly in the environment if not taken care of and can later have the risk of being digested and accumulate in the human body , is this general viewpoint I just wrote correct?

    And also if U235 has a half life of 700 million years then it means that naturally it's a very long half life metal which makes it a very very low power radiation source , my question is, does enriched uranium have the same half life as it's natural ore partner (I assume it has )
    so that means that it a natural fuel with a high energy density that can be stored for extremely long periods of time without significant degradation in it's energy density up until the point when critical mass is reached and a chain reaction starts ?

    here's the part which I start to misunderstand , what happens with uranium's half life after it is undergoing chain reaction ? I read that only small amount of it's total energy density is used in a typical reactor, which to me would indicate that most of the U235 is still there in the material but somehow unable to undergo further fission only I don't know why exactly , my guess would be because other isotopes of U235 have been formed which catch up the neutrons necessary for further sustained chain reaction ?

    thank you so far for answers :)
  7. Apr 16, 2016 #6
    I'm a senior reactor operator at a BWR. Operators will tour the plant shiftly verifying all the equipment is working properly, making adjustments to valves, adding oil to pumps, etc. I occasionally am out with my operators doing tours, or just do one on my own if I'm not running the control room.

    For spent fuel, I do not have exact numbers. I have been quoted that a spent fuel bundle that is freshly irradiated in open air can deliver a whole body dose of over 1 million rad/hr.

    It's important to distinguish between radiation, and radiation sources. The products inside of nuclear fuel are radiation sources. As long as they remain in the fuel rod, and there is shielding, they will never impact the health and safety of the public. If the shielding is lost, then the area directly around the fuel is dangerous, but it will not migrate outside. If the fuel rod ruptures, then it will go outside, and that's when there is a public health impact.

    As for the fuel itself, my BWR fuel is enriched close to 5% U-235. About 92-94% U-238, and the rest is additives or gadolinium poison. By the time we discharge it from the core, it is around .6-.7% U-235, and around .6-.7% Pu-239 (it breeds some plutonium during operation that is usable as fuel). Fuel depletion is a main part of why we discharge fuel from the core. Other reasons include fission product buildup as you said, tested lifetime limits, and internal changes to the fuel that limit it's linear heat generation rate limits. It has nothing to do with half-life, that's a radioactive decay process, we use fission in power reactors.
  8. Apr 16, 2016 #7


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    They are dangerous in different ways. Per atom, a short-living isotope will lead to more radiation in the short term. Which is bad if you stand next to it, but it is good if the atom decays within the reactor lifetime, so you don't have to worry about containing it for longer timescales.

    The lifetime of nuclei does not depend on their environment (apart from a few exceptions that are not relevant here). A 235U nucleus simply does not care if it is somewhere in a natural uranium deposit or somewhere in a processing plant. For technical applications, the half-life of both 238U and 235U is so long that it does not matter.
    Try to make a fire with a few pieces of wood in a huge pile of ash. It just won't work for similar reasons.

    235U is a single isotope, there are no "other isotopes of 235U".
  9. Apr 16, 2016 #8

    jim hardy

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    I've only been around a PWR so don't know what is a "cassette" .

    Spent fuel fresh out of the reactor is highly active . Here's an image from Forbes magazine and that's about what they look like..... (i was never able to get such a good photo.)

    The radiation from that fuel element as you see makes the water glow blue,
    i daresay nobody would get close enough to measure its level with a meter,

    Probably someone here could give you an educated estimate.
    Must be thousands of Sv ?

    On a practical note - With no water in between it and me, i wouldn't want to be near enough to see it , a few hundred meters.
    I hope that gives you at least a gut feel for your question.
  10. Apr 16, 2016 #9
    That's a great picture of a BWR core. Probably Columbia generating station? It's definitely a "newer" BWR.
  11. Apr 16, 2016 #10


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    U-235, like each radionuclide, has a unique half-life that does not change due to irradiation. A mass of U-235 would decay naturally, whether in a reactor or not. However, in a reactor, the U-235 will absorb a neutron, and either fission (about 84% probability), or emit a gamma and become U-236 in a less excited state. U-236 can absorb a neutron and become U-237, otherwise, U-236 emits an alpha particle producing Th-232, and U-237 emits a beta particle to become Np-237. U-238 may absorb a neutron and become U-239, which emits a beta particle and becomes Np-239, which emits a beta particle becoming Pu-239, which most likely fissions.

    During operation in an LWR and graphite-moderated reactors fueled by enriched uranium, the U-235 is depleted, while some U-238 is converted to transuranics including Pu-239/-240/-241 and higher mass isotopes. Isotopes like Pu-239 and Pu-241 are fissile, so they begin to make up for some of the lost U-235.

    The level of enrichment depends on the power density in the core, how long the reactor operates before refueling, and the fraction of the fuel replaced. Commercial reactors have a regulated/licensed limit of 5 wt% U-235 in U, or equivalent if MOX (U,Pu)O2 is used. The 5 wt% limit is applied on a pellet and fuel rod basis, i.e., no commercial pellet (UO2) is fabricated with more than 5 wt% U-235, and in fact, BWR fuels tend to use a max of 4.90 and PWR fuels a max of 4.95%. Some special fuel can be fabricated with higher enrichment for special tests, and some special fuel for small research reactors use higher enrichments, but less than 20% U-235.

    Commercial plants will shutdown periodically to remove some fuel, usually the most depleted, and add fresh fuel. The depleted or spent fuel not only has less U-235, but it has a lot of fission products, which parasitically absorb neutrons in competition with the fissile species. Typically, commercial plants consume about 5 to 5.5% of the uranium (most of the U-235, and some of the U-238) on an assembly basis. The peak burnup fuel rods consume about 6 % of the uranium, and some pellets consume about 6.5 to 7.5% of the uranium. The relative amounts of fuel rods and pellets depend on the power (and burnup) distribution in the core, which is radially and axially peaked, with the peak shifting according to operation.
  12. Apr 16, 2016 #11


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    That appears to be CGS. I can confirm with the author.
  13. Apr 16, 2016 #12


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    Traditionally, uranium oxide is converted to UF6, which has a relatively low boiling/vapor point. It makes it easy to process the stream. Centrifuge systems are fairly common for gaseous systems, and they make use of the different in mass (3 amu) between U-235 and U-238.
  14. Apr 16, 2016 #13

    jim hardy

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    Makes sense. I'm a PWR guy, and we don't leave the head studs in place.
    I take it your BWR refueling water is not borated?

    Was looking for a good picture of Cherenkov..
  15. Apr 17, 2016 #14


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    I have a question, if the fuel rods are removed, no further reaction in the reactor can take place, right?
    If that's true, why can't you shut down a nuclear reactor when it's not needed? You would just take the fuel rod and put it in a nearby unterground nuclear dump storage. Is it so, that the fuel rod is so active that there is no practical methods for moving them? If they are still critical, you would break them so that they are subcritical.

    And one more question, if the reactors can't be shut down for a little time, can't we basically redirect the energy from the main power line to something else. I mean, it's easy to lose unwanted energy.
  16. Apr 17, 2016 #15
    thank you guys for contributing here , I appreciate that.

    @Hiddencamper , that's interesting to know about your workplace.I'm definitely sure that a spent fuel rod has a very high radiation intensity around it I was more interested in knowing how far this radiation can go and still be dangerous , if there is no obstacle inbetween and the radiation source is not some secondary dust etc but just the spent fuel taken out of the core.
    I ask this because when a catastrophic core meltdown and rupturing of pipes and explosions as a result of all this happens like in Chernobyl the radiation was literally out of this world in terms of dose levels and most of the dosimeters at site gave up and their pointing arrows went "through the roof", but as we know Chernobyl unit 4 aftermath was so deadly mostly because due to fire and at least two powerful explosions much of the radiation source was pulverized and as dust went into the atmosphere were it then traveled huge amounts of territory ,
    now a purely theoretical question if for example there was the same core meltdown and explosions etc but somehow nothing turned to dust and no dust or no radiation source big or small went anywhere outside the reactor , I wonder what would then the radiation levels be say 1km from the site , 5km and so on ?

    @Jim and others , is that the spent fuel pond in the photo , or is that part of the active zone of a BWR ? if it's the second case does the BWR really work something similar like a electrical heating spiral dropped in a kettle which makes the water flow from the lower cold region pas the heating element and then it flows upwards and boils at the surface creating steam ?
    if the fuel is really underwater all the time I guess it has to be tightly and perfectly sealed from water entering the fuel right?

    A cassette is what they call each of the fuel bundles inside the RBMK core , not sure whether it's an official term or something that some workers like to refer to but it's just that and nothing more.

    @Astronuc and mfb , so the spent fuel can no longer be used in a traditional reactor mostly because the U235 which is the main fission target has underwent fission which has turned most of the U235 into other elements with similar yet slightly different atomic masses and most of those elements absorb neutrons without being able to further undergo fission themselves , except for a few like the Pu but since it takes up only a small portion of all the mass of the depleted fuel it's effect is small correct?
    But if the Pu which forms in the fuel while the U235 undergoes fission is there in small quantities and fissions itself then in reactors like the RBMK they would have to take the fuel out before it's completely spent and somehow extract the Pu out of it otherwise if they leave it till the end doesn't the Pu239 degrade much like the U235?

    Also I wonder what is the enrichment level for military naval reactors since limited space is a big factor there and I assume the most energy density per given size is needed.?
  17. Apr 17, 2016 #16
    @Garlic , if all the fuel is removed from the core the core no longer has reactions going on in it.But you can't completely shut down a nuclear reactor like you can simply turn off a light bulb , even when the moderator control rods are fully inserted upon the operator's wish to shut down the reactor there is still some small amount of nuclear reactivity going on.To my knowledge a nuclear reactor is said to be shut down when it's core thermal energy output is at it's minimal possible output with all the moderator rods inserted , no usable electricity can be generated at this point but the reactor still needs active cooling because there is the so called decay heat that needs to be taken away otherwise the core will overheat and melt.
    I assume this is what happened in Japan in Fukushima plant , they lost the backup electrical power for the cooling system after the reactor was shut down and due to this loss of coolant flow the fuel overheated by the heat it still produced even in the shut down state and so core meltdown happened which further released some gasses under pressure which then broke the containment and escaped into the atmosphere.

    P.S. When I was at the RBMK they said that even the fuel which has been fully spent and is inserted into the spent fuel reservoir , the reservoir itself has some active cooling because the fuel still produces some heat overtime due to the decay reactions that are still happening inside of it.ofcourse this cooling is not as big as the one taking place inside the reactor core but it's still there.

    you can move fuel even when it's not fully spent , for example I said earlier they built this railway wagon in which they placed the good still capable fuel rods from the first reactor and transported them to the second reactor and put them there so that they could further fission and produce electricity.

    Why would you need to waste produced electricity ? It's not like we have a tree full of electricity and we don't know were to put it , the grid will always need some extra electricity atleast here so none of it goes to waste and if for some time the grid is full they can simply lower the energy output and produce less at that moment.
    Last edited: Apr 17, 2016
  18. Apr 17, 2016 #17


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    Oh, I diddn't know that.

    Is there a reactor that redirects some of its energy directly to the active cooling system of it, so that the reactor works without needing any electricity source to power its active cooling system?

    And a while ago I have red an article from popular science, there it says there is a reactor type (that hasn't been implemented yet), in which when the meltdown occurs, the molten salts flow into a chamber where it's safely contained, not harming anyone. Why doesn't anyone use this?

    "If a reactor loses power, a freeze plug melts, draining the radioactive fuel into a tank where it cools to a solid."

    Wait.. Can the radioactive fuel cool to a solid despite it's enormous decay heat?
  19. Apr 17, 2016 #18
    all mainstream nuclear reactors that I know of use the electricity they produce to cool themselves with electrical pumps that feed the water through the reactor because that very water not only cools the reactor but also produces the heat and steam necessary to run the turbine in the first place.It's a steam cycle only the heat source is radioactive instead of being coal or gas or anything else.
    the problems begin when the reactor is shut down and doesn't produce it's own electricity and if besides that somehow also the grid to which the station is connected fails somehow then it needs a reserve electricity source for the coolant pumps , which is normally a diesel generator.
    the Chernobyl accident happened because they were testing the ability to power the coolant pumps using the leftover inertia of the turbine to cover the time that it takes from emergency shutdown of the reactor to the full power from the backup diesel generators.

    uranium inside the cores of nuclear reactors, atleast most of which I know, is in solid state , in formed metal pellets not in liquid form , it's melting point is quite high so it normally stays solid and is kept that way.
    as for the molten salt reactors and my questions I will wait and let the more experienced forum members answer and I suggest you do the same.
  20. Apr 17, 2016 #19
    Jim: BWRs don't use boron during refuel. The control rods remain fully inserted in the core, and the spent fuel pool is either spaced to ensure k effective is < 0.95 or boron plated fuel racks are used. During operation, boron injection is only used during ATWS scenarios (failure to scram) at high power/with core instabilities/if containment is challenged. Boron injection can also be used to provide supplemental inventory control during loss of high pressure injection scenarios, and some plants will inject boron to adjust ph of reactor coolant during a LOCA which improves the ability of water to absorb iodine released from fuel damage and minimize offsite release.

    In that picture you can see the feedwater spargers (upper rings), and directly next to the top of fuel you can also see the core spray ECCS spargers. The steam lines are also easily visible. This picture has the steam separator and core shroud head removed (to get access to the fuel).

    Garlic: when you scram a commercial light water reactor, the core is shut down in a couple seconds. RBMKs are slower at scram times, but the average BWR/PWR scram is less than three seconds to put all the rods in. The core is subcritical in less than 3 seconds in these cases.

    When the core is shut down, you get a prompt power drop from full power to around 7%. Neutron flux then rapidly drops with a -80 to -110 second period, reaching the source range within the next 20-30 minutes. The heat from fission is negligible after the first minute or so and nearly all heat is from decay heat.

    Salvador: when temperatures get hot enough to start melting fuel, ceramic fuel pellets begin to almost vaporize. So when the fuel cladding ruptures you end up with a fair amount of radioactive dust. Additionally the noble gas inventory in the fuel rod is not a dust but also gets ejected, which is a large part of the initial release during a LOCA. You also get various noncondensibles or other fission products that can be carried outside by steam in the core.

    That is the BWR core by the way. Water is forced through the bottom using jet pumps driven by recirculation pumps. Water flows upwards past the fuel and boils. Then it passes through an in core steam separator and steam dryer until it gets to the top head of the vessel steam some where it can go down the steam lines. The fuel is under water at all times. Normal water level in a BWR is roughly 20 feet above the top of the fuel. The fuel is only uncovered during accidents when steam cooling is being used to cool the core, or during scram failure when you may partially uncover the top of the core to help lower power and try to bring the core subcritical on temperature and steam voids. BWR cores can remain adequately cooled with the top 1/3rd of the core uncovered, or more, if there is adequate steam flow or if the core spray system is in operation.

    Also nearly all plants have a non electric core cooling system. For example, nearly all BWRs have a steam driven emergency feed pump which can use decay heat to operate for several days. These feed pumps are what kept the four reactors at Fukushima Daini cooled for the first couple days after the March 2011 tsunami.

    Some plants use diesel driven emergency feed pumps. Some plants simply have passive steam generators with gravity makeup tanks. But the bottom line is that you have some form of non electric cooling for a period of time.

    After a scram you don't have enough heat to drive the main turbine/generator. But you can operate large and small turbine driven feed pumps for a few hours, and the small feed pumps for days. If we lose normal pressure control of a BWR, one way we will depressurize the core is to just operate a large turbine driven feed pump on the minimum flow line at high speed to draw steam from the core. This allows us to avoid using relief valves.
  21. Apr 17, 2016 #20
    Seems like the BWR is simpler with respect to CANDU or others that it doesn't have all these small coolant tubes running through the core which complicates the design and also brings in more chance of hardware failure. If I understand correctly the BWR is simply U235 fuel formed in rods which form a core together with the moderator rods which is simply made watertight and placed inside a large cylindrical vessel with thick walls. Correct?
    So the primary loop water is simply pumped in at the bottom of the vertical vessel then it goes through the core and as being heated partly due to the flow partly due to heat flows upwards until boils at the surface where then a steam separator takes away the steam and the water simply condenses and falls back into the vessel ?

    Also because of this system does the BWR operate it's primary loop under pressure and if yes what kind of pressure ?

    The way I see it is that a PWR keeps the coolant under high pressure in the primary to prevent boiling and then simply lets the secondary loop to boil because sooner or later one needs to boil the water to create steam but the BWR (hence the name boiling) boils the water already in he very reactor vessel top aka the primary loop and then directly drives the turbine with that boiled steam after the turbine it's condensed with the help of auxillary water and sent back to the very reactor vessel.
    Seems like this reactor then has one loop less in the steam cycle.Sounds like should give better efficiency ?
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