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Holtec announces novel combined nuclear/solar power plant design: CSNP

Holtec International announced a new power plant design which melds the benefits of nuclear (high energy density, non-polluting, base load) with those of solar (zero fuel cost, minor regulatory barriers) that is entirely carbon-free and is adaptable for deployment in any country.

Called CNSP, an acronym for Combined Nuclear/Solar Plant, this design employs Holtec’s SMR-300 small modular reactor and the company’s HI-THERM HSP, solar thermal system, along with Holtec’s Green Boiler to provide base load or on-demand power while eliminating the intermittency drawback of solar plants.

The Green Boiler is a three-in-one device that:

  1. can store vast amounts of heat;

  2. receives high temperature heat conveyed to it from the solar collector; and

  3. can make motive steam at the required pressure and superheat to power the turbine.

The most immediate application of the CNSP technology is to facilitate the much-needed inflection in the world’s power generation from “coal to clean.” The coal-fired plants typically have sufficient land area to house the CNSP, which would use the coal plant’s power block minimizing the cost of transition.

The steam production portion of the coal plant will be decommissioned, freeing up most of the plant’s land area where the solar plant would reside. Holtec plans to offer the CNSP technology principally in those regions of the world where solar radiation level is adequate to be harvestable.

The SMR-300 small modular reactor is premised on the most proven type of light water technology used in most land-based reactors and in submarines and aircraft carriers. SMR-300 has added defense-in-depth features that are gravity-actuated that confer fail-safe emergency recovery capability to the nuclear plant qualifying it for the moniker “walk away safe.”

The energy contribution of the sun to the power plant will occur through the HI-THERM HSP hybrid solar plant, which is considerably more efficient than its predecessor technologies, yielding as much as 8 MWH of solar heat per acre in equatorial and subtropical locales.

The nuclear reactor’s steam supply system and the heat from the solar thermal plant are combined in the Green Boiler which is a multi-function device engineered to produce steam at the desired pressure and superheat to run the coal plant’s existing turbogenerator.

Where the site has no pre-existing fossil plant, the solar thermal plant can be as large as the available land area will accommodate. The CNSP will have a much higher thermodynamic efficiency than the nuclear plant alone and would make solar power an integral part of base load supply, Holtec said.

CNSP contains no fragile parts or materials that may limit its service life, which is expected to exceed 60 years.



Here is a bit more on the energy storage, the 'Green Boiler':


' Each Green Boiler module consists of three large cylindrical vessels substantially filled with engineered salts (hereafter called the “Thermal Repository”) and equipped with specially configured tubes to convert feed water into superheated steam. The three vessels are referred to as the pre-heater, the boiler and the superheater, respectively. Each vessel is heavily insulated to limit the loss of heat to the ambient to minuscule values. Each vessel contains a tube bundle configured to permit significant expansion and contraction during the plant’s operation without significant cyclic fatigue and withstand solidification of the enveloping Thermal Repository as its heat is delivered to produce cycle steam. Another essential feature is the extremely corrosive molten Thermal Repository (TR) material remains stationary in each vessel: It delivers its heat to the water circulating in the tubes imbedded in the TR space through radiation and conduction action depending on the thermophysical properties of the TR material. Each vessel is evacuated of air and backfilled with an inert gas to mitigate oxidation of the TR material and corrosion of the tubes. The inside space of the Green Boiler vessels are engineered to be amenable to convenient maintenance and repair, if required.'

Further down they say that their standard size is for 12 hours of full output.

This is a workable number of hours for the move to renewables, batteries can't nearly do the job, as their practical economic limit is around 4 hours of storage,

12 hours is enough to cover diurnal variability, which is pretty much all you need relatively close to the equator where most people in 2050 will live.

The nuclear bit would seem to me to be an optional extra! ;-)

Roger Brown

"The CNSP will have a much higher thermodynamic efficiency than the nuclear plant alone"

I do not understand the above statement. If the efficiency is the percentage of the collected heat which gets converted to electricity why is feeding solar and nuclear into the same boiler going to change the efficiency?

I do not really understand the economic advantages of this hybrid system either. One of the disadvantages of nuclear energy is that it does not do a good job of load following since nuclear power engineers do not like turning the plant power up and down on a frequent basis. Adding thermal energy storage adds load following capabilities even without solar, so what is the role of the solar heat? If the yearly electricity demand profile requires more power in summer than in winter then value of the solar heat is clear. If not then adding solar actually lowers the capacity factor of steam generator. If the solar heat is cheaper than the nuclear heat then maybe you can economically justify this drop in capacity factor.

It is also possible that you could turn off the nuclear plant during a summer peak period. Such an action would lower the capacity factor of the nuclear plant but would improve the energy production profile. Such an action would allow a convenient time for doing any upgrades or maintenance required by the nuclear power plant without a loss of power production.



If the solar plant is more efficient than the nuclear, then you have upped the efficiency.

Having a nuclear component operating 24/7 means that there is no cooling effect when the sun is not shining, so it does not have to work against that cooling.

There is no technical problem with ramping new reactors up and down, unlike the existing US nuclear fleet which simply weren't designed to do so,

In practise though it is not much done, although in France the fleet is a high enough percentage of electric supply to make it necessary sometimes.

The real issue is that almost all the cost of a nuclear reactor is in the build and interest on that build.

The cost of fuel is tiny, and switching the power output down only saves commensurate amounts.

That is why things like hydrogen production are very attractive to nuclear plant operators, as it makes something for otherwise surplus and almost free power.

Not only are there all sorts of potential issues with the nuclear bit, but solar thermal has a horrible record, unlike photovoltaic.


Implement nuclear/solar and start making efuels with the excess electricity and heat to provoke an economic expantion . Also stop subsidizing costly windmills and batteries. Biden and trudeau with their subsidies are killing the economy.

Mike Keller

Efficiency correlates to the steam temperature (and to an extent, the pressure) going into the turbine. The reactor on its own is perhaps around 30 percent efficient. The steam is being superheated by high temperature salt that has been heated by solar energy in some form of solar tower.
“Vast amounts” of energy from solar energy require vast numbers mirrors (and land) to focus the energy onto the salt. Such installations are very expensive, far exceeding the cost of a conventional arrays that simply create electricity.
The cost of the “hybrid” is undoubtedly extremely expensive.
There is a nuclear approach that is a true hybrid. A gas reactor drives the air compressor of a combustion turbine, with both nuclear fuel and natural gas used. The hybrid- nuclear plant can use solar energy to create hydrogen using steam electrolysis to augment the fuel used by the combustion turbine or fire a duct burner associated with the boiler using combustion turbine exhaust gas. The hybrid- nuclear plant is a an advanced combined-cycle plant that uses an advanced reactor. The plant’s output is over 1000 megawatts (electric) and uses a +600 megawatt (thermal) reactor. The hybrid is exceptionally efficient. A number of U.S. patents are involved with the hybrid-nuclear technology.


Mike, Just so.

The proposal we are commentating on seesm to stick together several either dubious or at least unproven techs, to create a sort of 'improbabllity drive'.

Meanwhile the Chinese have up and running an HT PEM perfectly capable of producing hydrogen in its output mix, to be supplemented as required by photovoltaic and burnt or used in a fuel cell for power when renewables output is low.


Sounds like something from Popular Mechanics magazine from the 1970s

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