# Low pressure Fission reactor

## Main Question or Discussion Point

In small Fission reactors it can be hard to get enough heat in order to boil the water inside the boiler, so why don't we create low pressure boiler systems, where we can boil water at slightly above room temperature, use it to turn the steam turbine as it flows to the similarly low pressure condenser chamber, is cooled back down into its liquid form, and pumped back into the boiler?

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etudiant
Gold Member
That would be a pretty inefficient setup, as the temperature drop you're suggesting is very low.
That said, nuclear fuel is cheap, so if the construction and management costs are slashed by this approach, it may have merit.
Do note that cooling will be a real issue in this device, most of the energy will go to heat the condenser water.

Why would it be inefficient?

Astronuc
Staff Emeritus
In small Fission reactors it can be hard to get enough heat in order to boil the water inside the boiler,
Not unless the small reactor is designed to produce low energy. HIFR operates at up to 85 MW, but it is about the size of a large washing machine.
https://neutrons.ornl.gov/hfir
http://large.stanford.edu/courses/2012/ph241/vane2/

Most small reactors are intentionally operated at low temperature, such that they don't boil water in the core.

Why would it be inefficient?
low pressure boiler systems, where we can boil water at slightly above room temperature, use it to turn the steam turbine as it flows to the similarly low pressure condenser chamber,
What temperature differential would one propose? What size generator does one propose? 5 kW, or 10 kW? Slightly above room temperature would have very low Carnot efficiency.

I would recommend looking at the change in specific enthalpy between hot and cold states across a turbine of 10 kW, and in addition to looking at the mass flow rate, look at the practicality of it.

Start with a small gas-fired steam plant and determine the costs of: a small 12-15 kW steam turbine, a 10 to 12 kW generator, a 1 to 2 kW pump, a condenser, and various control systems.

A small core still need shielding of the radiation (primarily gamma), and then there is the matter of control of the special nuclear material. So, it's uneconomical to have a small reactor in the kW range.

I am not very familiar with Carnot Efficiency. I remember watching a video on Carnot engines a while back, basically explaining how an engine operating between a high temperature and a low temperature could not have greater efficiency than a Carnot system at the same temperatures. Could you explain it a bit more and help me understand how it is relevant to this situation?

Astronuc
Staff Emeritus
Could you explain it a bit more and help me understand how it is relevant to this situation?
Carnot efficiency is the theoretical thermodynamic efficiency of a process. In a thermodynamic (Rankine) system driving a turbine, heat is put into a liquid (water) which is turned to steam. Steam passes through a turbine and loses energy (enthalpy). The work out of the turbine is equivalent to the energy decrease in the working fluid and other losses. That energy change is slightly more than the energy increase in the boiler.

Bascially, one needs to produce more energy than one will receive in useful work, and at very low temperatures, that's rather impractical.

In small pool type reactors, they are intentionally operated at low power with sufficient cooling to keep the surrounding coolant at low temperature. That's by design, not a disadvantage.

For small power systems, it is not practical to use fission as an energy source.

I am still not sure I understand. If you have chamber A, where steam is produced, and it flows through a turbine into chamber B, which has a lower pressure than chamber A, how does equally increasing or decreasing the pressure in the chambers effect the amount of energy that is produced by the turbine?

etudiant
Gold Member
Steam by itself is not a very complete description. The amount of energy in the steam depends on the temperature and pressure involved.
The Carnot cycle gives a clear understanding of how much of that energy can be extracted to do useful work.
If the steam is at low temperature and low pressure, very little energy can be extracted. This hold true for any low power and low pressure steam source , whether nuclear or conventional.

Astronuc
Staff Emeritus
I am still not sure I understand. If you have chamber A, where steam is produced, and it flows through a turbine into chamber B, which has a lower pressure than chamber A, how does equally increasing or decreasing the pressure in the chambers effect the amount of energy that is produced by the turbine?
What does one understand about thermodynamic cycles and the production of energy?

Here is a reasonable discussion of the Rankine cycle
https://en.wikipedia.org/wiki/Rankine_cycle
http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node65.html

and Carnot efficiency
https://en.wikipedia.org/wiki/Heat_engine#Efficiency
https://en.wikipedia.org/wiki/Carnot_cycle

Steam by itself is not a very complete description. The amount of energy in the steam depends on the temperature and pressure involved.
The Carnot cycle gives a clear understanding of how much of that energy can be extracted to do useful work.
If the steam is at low temperature and low pressure, very little energy can be extracted. This hold true for any low power and low pressure steam source , whether nuclear or conventional.
Ok, I think I understand, but isn't the amount of energy that you can extract from the steams pressure also dependent on the pressure difference? I mean, if you have a 18 psi boiler and a 14 psi condenser, isn't the amount of energy you can get out of it equal to the amount you can get out of a 8 psi boiler and a 4 psi condenser?

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A small core still need shielding of the radiation (primarily gamma), and then there is the matter of control of the special nuclear material. So, it's uneconomical to have a small reactor in the kW range.
BES-5 produces about 100 kW thermal, and converts just 3 kW of that into electricity.
BES-5 also has mass of under 400 kg including shielding.
Note that BES-5 does not have steam: it uses NaK as coolant, and produces energy by thermoelectric means.

Astronuc
Staff Emeritus
BES-5 produces about 100 kW thermal, and converts just 3 kW of that into electricity.
BES-5 also has mass of under 400 kg including shielding.
Note that BES-5 does not have steam: it uses NaK as coolant, and produces energy by thermoelectric means.
And the cost is? I seriously doubt that it would economical for a typical household, and the fact that it requires "30 kg of uranium more than 90% enriched U235" raises security/proliferation concerns. Ref: https://en.wikipedia.org/wiki/BES-5

The OP was asking about systems using water cooling and steam generation.

And the cost is? I seriously doubt that it would economical for a typical household, and the fact that it requires "30 kg of uranium more than 90% enriched U235" raises security/proliferation concerns.
... issues not inquired about by OP.
The OP was asking about systems using water cooling and steam generation.
Yes. And my point is that for low temperature heat engines, steam is not an efficient option even for the Carnot efficiency.
For example, if you have a heat source at 37 Celsius and no more, and a heat sink at 0 Celsius, you could run a heat engine at a theoretical Carnot efficiency of (37-0)/(37+273)=37/310=12% efficiency. Yet I doubt that a water steam boiler is an efficient way to use a heat source at 37 Celsius. Something else may have a better efficiency, though of course still under 12 %.

Astronuc
Staff Emeritus
... issues not inquired about by OP.
Not explicitly. Economy is a consideration in the design and deployment of any engineered system. The OP inquires " so why don't we create low pressure boiler system . . . ," The answer involves the physics/thermodynamics and economics.

Systems using highly enriched fissile material add another dimension, that of safety and security/non-proliferation.

Yes. And my point is that for low temperature heat engines, steam is not an efficient option even for the Carnot efficiency.
For example, if you have a heat source at 37 Celsius and no more, and a heat sink at 0 Celsius, you could run a heat engine at a theoretical Carnot efficiency of (37-0)/(37+273)=37/310=12% efficiency. Yet I doubt that a water steam boiler is an efficient way to use a heat source at 37 Celsius. Something else may have a better efficiency, though of course still under 12 %.
Odd example, since little in the natural environment is at or below 0 C, except during winter, and in the polar regions, but the BES-5 example given is a high temperature system.

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Not explicitly. Economy is a consideration in the design and deployment of any engineered system. The OP inquires " so why don't we create low pressure boiler system . . . ," The answer involves the physics/thermodynamics and economics.
And a part of the answer is that while low temperature heat engines are inherently low efficiency, low pressure water steam is low efficiency even among them. There are more efficient ways to skim the small amount of energy from low temperature difference.

There are more efficient ways to skim the small amount of energy from low temperature difference.
Like what? A Thermoelectric generator?

In pressure systems, isn't the work done by the pressure difference? Like I said, if you have a 18 psi boiler and a 14 psi condenser, isn't the amount of energy you can get out of it equal to the amount you can get out of a 8 psi boiler and a 4 psi condenser? Regardless of temperature difference?

Like what? A Thermoelectric generator?
Yes.
In pressure systems, isn't the work done by the pressure difference? Like I said, if you have a 18 psi boiler and a 14 psi condenser, isn't the amount of energy you can get out of it equal to the amount you can get out of a 8 psi boiler and a 4 psi condenser? Regardless of temperature difference?
Yes - given same volume of steam.
But same amount of heat will give a much smaller volume of 18 psi steam than 8 psi steam.

Ok, thank you.

etudiant
Gold Member
The idea of exploiting small temperature differentials that are hugely available in nature is an old one.
The best developed afaik is to use the difference between cold deep ocean water and warm tropical ocean surface water to boil and condense ammonia.
Those efforts go back to the 1920s, but Lockheed most recently tried this off Hawaii. That location appeals because Hawaii imports its fuel and because very deep water is accessible from the shore there, which saves on construction costs.
The project did not prove a success. The setup is now used to sell ultra pure( after desalination) water from the deep ocean to the Asian market.
Usually the glitches are biofouling, which impacts the condenser effectiveness, or failure of the intake pipe from the deep.

Hm.... interesting.

In small Fission reactors it can be hard to get enough heat in order to boil the water inside the boiler, so why don't we create low pressure boiler systems, where we can boil water at slightly above room temperature, use it to turn the steam turbine as it flows to the similarly low pressure condenser chamber, is cooled back down into its liquid form, and pumped back into the boiler?
The efficiency of electricity generation from any thermal engine, ideally, 1 - Tcold/Thot, where temperatures are in Kelvins. So, for 400K hot side and 300K cold side, efficiency is only 1/4 = 25%. Whereas for 600K hot side it is 50%. (These are ideal efficiencies, real ones are inevitably worse).

That's why electricity generation from heat always pushes for the maximum attainable temperature difference. At low difference, you lose most of the energy.

etudiant
Gold Member
In theory, efficiency is not that critical for a nuclear plant, as the fuel is cheap. So if the low power reactor could be built and operated much more cheaply, it might well be able to overcome its low efficiency handicap.
However, nuclear operating regulations are pretty draconian and impose high fixed costs on any nuclear site. The proponents of small reactors are currently struggling to alter that, with little visible progress. The same regulatory philosophy would probably block any low power, low cost reactor concept.