4.7: NuScale’s Small Modular Reactors – an Oregon-born Project

Nuclear power has not been the “darling” of public opinion. The Three Mile Island Accident of March 28, 1979, and the Chernobyl disaster of April 26, 1986, greatly contributed to the emergence of such negative attitude. The first of those events had created widespread fear, but not really serious consequences. The other was an authentic catastrophe. After that, for a quarter of century there were no serious accidents in the global nuclear power sector and its bad reputation began to slowly improve – until a new blow that hit a Japanese nuclear power complex in Fukushima Daiichi on March 11, 2011. The enthusiasm for nuclear energy almost completely disappeared due to this disaster.

Nonetheless, in the opinion of many experts issuing a “death penalty” on nuclear power would be an unforgivable mistake. As is well known, a goal supported by more and more people is to attain a total “decarbonization” in the 21^st Century. It’s a colossal challenge. To make the Reader aware of the scale of the problem, let’s do some numbers.

Current global consumption of fossil fuels is around 14,000 MTOE per year. MTOE is an energy unit, acronym of “Million Tonnes of Oil Equiv- alent”. It’s instructive to convert MTOE-s to units more commonly used: there are several Web calculators (e.g., this one) for quick conversion to other units. For instance, one can find that 1 MTOE is equivalent to 11,630 GWh = 11.63 TWh. 1 GW is the power generated by a medium-size conventional power plant; there are 24,365 = 8,760 hours in a year, so that the total energy output of such a plant is 8,760 GWh = 8.76 TWh – or 0.753 MTOE. Now, let’s take a look at the data published by the International Energy Agency. One can find that the total global generation of wind energy in 2019 was 1430 TWh, and 900 TWh of solar energy – corresponding, respectively, to 123 and 77 MTOE, or 200 MTOE together. Well, we see solar panels and wind turbines all around, but the data are bit disappointing – in order to replace all CO₂-emitting energy sources, we would need to add 60 times more solar panels and 60 times more windmill than those currently installed! But the aforementioned 14,000 MTOE is the total consumption of en- ergy, which includes all sectors, such as transportation, metallurgy, chemical industry, etc. – the gross electric power generation in 2019 is about 27,000 TWh, corresponding to 2320 MTOE, which is about 16.6% MTOE consumed. Let’s keep in mind that the wind and solar sources generate almost exclu- sively electric power – so that the 200 MTOE of the two sources combined contributes about 8.6% to the gross electricity production.

And now, let’s take a look at the nuclear sector, which generates power that is 100% emission-free. In 2019, an the total global output of those plans is about 2560 TWh = 220 MTOE, or about 9.5% of the gross electric energy production. More than the solar and wind energy combined.

In developed countries the dominance of nuclear energy may be even higher: in the US, nuclear generation, and wind + solar generation were, respectively, 19% and 8.2% of the total, and in France, respectively, 74.5% and less than 1% (2016 data).

In view of the above, many experts in the field and not only them agree that – once we face a “Herculean challenge” of total “decarbonization” of energy acquisition well before the end of the 21^st – it does not make much sense to eliminate the electricity generating branch that is “patently zero- emitting”. Rather, it may be a good idea to build new nuclear power sources in parallel to new wind and solar power sources.

Guidelines for New Reactor Safety Standards

The specter of the Three-Mile Island, Chernobyl and Fukushima Daiichi disasters still haunts our thinking of nuclear power. It is instructive to consider what went wrong in those catastrophic events. In the TMI case, faulty operation of some devices and sensors triggered a chain of malfunctions which, when combined with human errors, led eventually to a partial meltdown of the reactor core. In the case of Chernobyl catastrophe, what contributed were serious errors in the design of the reactor (of a different type than the reactors used in most Western countries), combined with a series of human errors (the whole story cannot be summarized in a few sentences, for details the Reader is referred to one the many reports published by appropriate Agencies – e.g., to this, a relatively compact one – and many other can be found in the Web). Finally, in the Fukushima Daiichi there was no obvious human error made during the disaster itself. The error in question had been made years earlier when it had been decided to build a power reactor complex at the ocean shore, in an area where catastrophic earthquakes and tsunamis may happen.

In all these three cases it was the failure of the core cooling system that initiated the critical phase of the catastrophe, from which“there was no way out”. And the “hearths” of the cooling systems – as in almost all other power reactors in the world – were colossal pumps, driven by electric motors supplied from the power grid.

Another common denominator in all of them was that the malfunction- ing reactors were large units, with net generating capacity close to, or even larger than 1 GW. So, all the reactors contained a large amount of radioactive elements. Only in the case of TMI the release of radioactive materials was relatively modest – in the other two cases large areas of land were contaminated.

The lessons that can be drawn from the facts presented can be summa- rized as follows:

• Future reactors should be designed in such a way that both in normal operation and in emergency situations human intervention would not be necessary.
  
• Make the core cooling system completely autonomous, preferably by complete elimination of pumps supplied by power from external sources, prone to power outages. it is entirely conceivable to build a deep shaft cove
  
• Future generations of reactors should be much smaller than those being currently operated – not units of power of the order of 1 GW (1000 MW), but of 100 MW or even smaller. Why? Even if a reactor is 100% immune to the mechanical failures and human error, there are still unpredictable cataclysmic events such fall of a meteorite, or 9/11. The amount of radiation released in such an event would depend on the mass of the uranium fuel in the reactor. In a typical reactor of 1 GW power there is about 100 tons of fuel, and in a 50 MW reactor there will be only some 5 tons. Small size of a reactor also gives a much higher guarantee that in the case of a cataclysmic event the fuel will not be released and spread out over a large area.
  
  All power reactors are enclosed in a reinforced “containment shells” designed to contain the escape of radioactive substances in the case of an emergency. However, placing such a shell of a 1 GW reactor under- ground would pose an extremely difficult technical problem. Therefore, all such facilities (the existing and the planned ones) are placed above the ground level, thus making possible targets for a meteorite strike , or for a 9/11-style attack. In contrast, for small reactors with power of several dozen MW it is entirely conceivable to build a containment structure in the form of a deep shaft underground shaft.
  
• Finally, the location of the reactor must be very thoughtfully consid- ered. Areas where tsunamis or floods may happen, or where earth- quakes may occur, must be excluded from such considerations.

The NuScale Module

The design of the module created by NuScale Power is consistent with the guidelines outlined in the preceding subsection (only the last item, related to the location, must remain the concern of those who will buy the reactor, and not of them who have designed the reactor).

In fact, what is called “a module” by the company, is a compact nuclear reactor capable of operating all by itself, and of producing steam that can be used (by a separate facility) to generate about 60 MW of electric power. The reactor is slim, only about 9 feet (2.7 m) in diameter and of the height of 65 feet (20 m). The module contains only about 5 percent of the nuclear fuel of a conventional 1,000 MW_e nuclear reactor, which is one factor that contributes to its safety. Another factor is the module’s slimness – it can be housed in an underground cylindrical “well” made of reinforced concrete. Needless to say, such an underground tube is a much sturdier “containment structure” that anything built above the ground level.

One can say that 60 MW of power is not much. Therefore, the design is such that up to twelve individual modules can be teamed up, yielding the output power up to 720 MW, not much less than than the typical power of about 1 GW of a typical conventional power reactor. There are many advantages of such an arrangement. Defects cannot be ruled out entirely, but in a “12-pack arrangement” it is quite unlikely that more that one unit will be malfunctioning – that module can be turned off and taken care off. In the worst scenario, it can be replaced by a “fresh” module. So, the system will be working for a while as an “eleven-pack” with power reduced by less than 10%. Another advantage is there may be no need of shutting down the system for refueling. Most conventional reactors in the 1 GW output power range typically refuel every 18 to 24 months, and during such an operation the reactors needs to be shut down for three weeks, or even longer. In a “12- pack” SMR power plant, the modules will not be refueled in situ – instead, they will be removed and transported to the factory, where the refueling operation will take place. And the removed unit can be replaced by a “fresh one”, brought from the factory. While a single unit is removed and replaced, the remaining eleven may keep working. Accordingly, the power plant will keep working with the power reduced by less than 10% for a relatively short period – but its operation will never need to cease completely.

But the most remarkable technological novelty in the NuScale Power’s SMR design is the core cooling system. The pumps are completely eliminated – instead, the circulation of the cooling agent (water) is maintained by a natural physical phenomenon, known as convection. It’s a heat transfer mechanism, in which heated gas or liquid travels away from the heat source, carrying thermal energy along.

An instructive example – the hot air balloon. A spectacular demon- stration of a convection-like effect is a hot air balloon. In such an aircraft, a large volume (typically 2 - 5 thousand cubic meters) is contained within a “bag”, called the envelope. The density of air decreases with increasing temperature, at constant pressure it is inversely proportional to the absolut temperature (i.e., the one expressed in Kelvins). The air temperature in a balloon is usually kept at about 120^◦C = 393 K. The ambient air temper- ature at hot air balloon events is usually close to 20^◦C = 293 K, and the ambient air density not too high above the sea level (e.g., in Willamette Valley) is about ρ_A = 1.2 kg/m³. Then, the hot air density inside the envelope is ρ_H = 1.2 (293/393) kg/m³ = 0.895 kg/m³.

The buoyant force B acting on the balloon – i.e., on a body of volume V “immersed” in ambient fluid (air) of density ρ_A – can be readily found from the Archimedes Law: B = V ρ_A g, where g = 9.81 m/s² is the acceleration due to gravity. But the hot gas inside the balloon is not weightless, its weight (i.e., the force with which Earth attracts it) is W = V · ρ_H · g. By subtracting it from the buoyant force B we get the lifting force F_L of the balloon:

\[ F_{L} = V \cdot ( \rho_{A} - \rho_{H} ) \cdot g \]

One can readily find that for the conditions outlined above, each cubic meter of hot air contributes a lifting force of (1.2 0.895) 9.81 N = 2.99 N. 2.99 N is the weight of a mass of 0.305 kg. Then, a typical balloon of 2500 m³ volume can lift a total mass of 2500 · 0.305 kg = 763 kg.

Natural convection in gases and liquids. An “envelope” is not needed for a portion of hot air to rise, of course. It happens whenever air at a low altitude is heated – for instance, as is discussed in Chapter 6, it is the global- scale convection that gives rise to the so-called trade winds. Air convection over lands produces “up-blowing winds”, taken advantage of by many bird species, as well as by pilots flying gliders.

An artificial device utilizing convection – known to humans for thousands of years – is a chimney. A vertical pipe in which the convection creates an up-moving air current that captures smoke and combustion products from the inside of a stove or a fireplace and blows them out.

Convection also occurs in liquids. It plays an important role in the heat transportation in oceans and other deep bodies of water (by the way, vertical convection currents in the Oregon’s Crater Lake are studied and monitored by OSU oceanographers, as the Lake can be thought of as a “microscale model” of an ocean).

In a teaching lab scale, convection in liquids can be visualized in simple experiments – e.g., by injecting a “blob” of dyed hot water near the bottom of a beaker filled with clear cold water, and observing how the “blob” raises towards the surface. Many such experiments are presented at YouTube, for instance – in this movie clip, in another one, and in yet another one. The Reader is encouraged to look for other similar movies, there are many other demonstration of convection in the YouTube.

The NuScale module unique convection-based core cooling system. In a reactor using water as a moderator/coolant, the core temperature should be maintained at 300^◦C or slightly higher. But why not at even higher a temperature? Well, we have to keep in mind that substances exist only at liquid state at temperatures lower than the so-called critical point, specific for a given liquid (for a list, see the table at the end of this Wikipedia article. For water, the critical temperature is 374^◦C. But not only the temperature is relevant – another essential parameter is the pressure. As we all know, at normal atmospheric pressure liquid water exists only up to 100^◦C (212 F). At higher temperatures, in order to keep water liquid the pressure has to be increased – for a given temperature, it has to be kept above the so-called boiling point pressure. The boiling point parameters for water up to the critical point can be found in in this Web page (from the graph, or by using the on-line tool). For the temperature of 300^◦C, one gets the boiling point pressure of 86 bara (bara, or bar is a unit of pressure liked by engineers; it’s equal to 100,000 N/m² – which is slightly less than the normal atmospheric pressure, equal to 1.013 bara). Consequently, inside all boiling water reactors (BWRs) and pressurized water reactors (PWRs) the pressure during normal operation must be of the order of 100 bara.

The NuScale modular reactor is essentially a small PWR, so that the core temperature and the pressure inside will be similar as in the “big” reactors of this type. But there is one more temperature dependent parameter of water that is essential for the module operation: namely, the density . As all other gases, liquids and solids, water experiences thermal expansion : the volume of a given mass of water expands in response to increasing temperature – hence, its density decreases. decreases. For a temperature as high as 300^◦C the change of water density is already substantial. At 4^◦C the density of water is exactly 1000 kg/m³. At 100^◦C – as can be found from the graph in Fig. \(\PageIndex{1}\) – it’s 950 kg/m³, and at 300^◦C it is already as low as 720 kg/m³.

Figure \(\PageIndex{1}\): The dependence of water density on temperature (plotted based on the data listed in this Dortmund Data Bank Web page).

Suppose that 100^◦C water enters the core of a NuScale SMR from below and inside the core it is heated up to 300^◦C. Its density – as follows from the numbers given above – dramatically decreases. It gives rise to a strong convection, leading to an upward flow of the heated water and escape through the core’s top – whereby a new portion of cool water is sucked into the core from below. In other words, the convection acts as a pump circulating water through the core and carrying out thermal energy from it.

In order to assure that a large amount of thermal energy is removed from the core every second, the circulation must be made really “vigorous”. Is there any way to speed up the convection? Yes, there is, and people have discovered a proper “trick” centuries ago when experimenting with chimneys. They found out that the higher the chimney was, the stronger was the chim- ney “draft” and the faster was the flow of hot gases in the chimney duct. This is often referred to as the stack effect. If we look at old photos of factory complexes from the “industrial revolution” of the 19th century, there are tall chimneys everywhere. Why? Because before the age of electricity even in large factories there was typically only a single huge steam engine, delivering power to a large number of individual machines through a “cat’s cradle” of shafts and pulleys. Such steam engines consumed tons of coal, the burning of which required an intense intake of air into the boiler furnaces. The rate of air influx is equal to the rate of combustion products (the “flue gas”) removal from the furnace by the chimney. In order to speed up the removal, a strong “draft” was needed and this is why very tall chimneys were built. For the same reason – as one can see on old photos (e.g., that of “Titanic”) – there were very tall “smokestacks” on merchant and navy steamships.

Figure \(\PageIndex{2}\): Photo of factory chimneys from the age of the 19^th century industrial revolution (source: Wikimedia Commons).

The very same “stack effect” is taken advantage of in the NuScale re- actor module to enhance the core cooling. In the Figure 4.11 it’s explained schematically how this system works. The water heated up in the core passes next to long vertical channel acting as a “chimney” for producing an upward convection “drag”, thus speeding up the passage of water through the core. At the top of this “stack” is a “steam generator” – essentially, it’s a heat exchanger in which heat is transferred from water to steam circulating in a separate loop – this steam passes next to the turbines in the electricity- generating unit.

Figure \(\PageIndex{3}\): A scheme explaining how the convection-driven core cooling system works. The red bars are the fuel rods in the core, and the blue ones are the pipes in the “steam generator”. The low-temperature ( 100^◦C) water with higher density is colored dark-green, and the high-temperature ( 300^◦C) water with lower density is colored light-green. The cooler water enters the core from below, where it is heated up and its density drops. When it emerges from the core’s top, it is driven up by convention enhanced by a long “stack” between the core and the steam generator. After giving away its heat in the steam generator, the cooled-down water with higher density “sinks” towards the bottom, where it may reenter the core and go through another heating-up/cooling-down cycle. In the steam generator water injected into it is heated up and it leaves the generator as a high-pressure hot steam, which is sent to a steam turbine driving an electric generator. However, the drawing of the steam generator in the figure is very simplified and the pipes needed to inject water and to lead steam away are not shown.

The cooled-down water emerging from the “steam generator” again has a higher density and therefore it “sinks” tothe bottom of the system from which it may reenter the core, and undergo another heating-up/cooling-down cycle.

Fig. \(\PageIndex{3}\) also explains why such a reactor has to be “tall and slim”

• it’s because the “stack” has to be long enough to produce a strong “draft”
• strong enough to guarantee, in turn, that the water circulation will be fast enough to remove from the core the entire 60 MW of thermal power generated in it.

As follows from above, the water circulation in the system is driven only by convection and is not aided by any pump powered by electricity from an external source. Nate that this is crucial for the system safety! In the Three Mile Island and Fukushima Daiichi disasters it was the failure of the core cooling systems that led to the reactor core meltdown – an irreversible catastrophic event. Even though the chain reactions in the cores had been stopped, the heat was still generated due to the decay of fission products the lifetime of which can be as long as days or weeks. To prevent the core meltdown, it is necessary to keep the circulation of the coolant through the core for a long time. In the NuScale SMR design, the coolant circulation is driven by the heat generated in the core, no matter whether it’s due to the chain reaction, or only due to the decay of the fission products. In other words, the core cooling system is “intrinsically safe”.

It’s of crucial importance, but it still isn’t enough. The coolant circu- lation, yes, removes heat from the core – but, in turn, something has to removed the heat from the coolant. Otherwise, all the water in the SMR would eventually reach the same temperature and the convection-driven cool- ing mechanism would stop. Normally, it’s the steam generator that removes heat from the coolant, after it passes through core. So, suppose that there is a major disaster that knocks down the power generating system: water does not circulate any longer through the steam generator, its cooling action is completely stopped. The temperature inside the closed SMR “capsule” keeps raising, as also does the pressure – eventually, the capsule bursts open, nothing is cooling the core anymore and everything is heading towards a meltdown.

However, the NuScale designers have also taken such a scenario into con- sideration. They added another safety mechanism into the system. How it works is explained in the Fig. \(\PageIndex{4}\). The reactor capsule is installed into another larger “outer” shell and the space between the two vessels is evac- uated during the normal operation. The entire assemble is installed in an underground water-filled “well”. Normally, the vacuum jacket around the inner capsule provides an efficient thermal insulation, so that any heat gen- erated in the reactor is removed by the steam generator only. Yet, if the chain reaction is stopped and, in addition, the steam generator becomes in- active and no longer removes any heat from the reactor, then valves in the outer capsule are opened and the space between the two shells is flooded with water. Now, the water inside the reactor circulates due to convection and heat is transferred out of it across the capsule wall, to the water that replaced the vacuum jacket.

Figure \(\PageIndex{4}\): The reactor unit in the NuScale design is housed inside another “outer” capsuke. The whole device is placed in a deep well-like shaft filled with water. During normal operation, as shown in part (a), the space between the “inner” and “outer” capsule is evacuated, i.e., there is no thermal contact between them. In emergency situations, when the steam generator does not remove heat emerging from the core any longer, an emergency cooling system is activated, as shown in part (b): water from the “well” is let into the space between the two capsules. Now, the hot water flowing down inside the “inner” capsule passes its heat across the capsule wall to the “well water”.

This heat makes the water boil, so that steam escapes from the well and the water level in it gradually lowers. There is enough water in the well to cool the reactor in this way for a month. It is assumed that the system will not stay abandoned for such a long period and some intervention will begin sooner. In fact, the simplest action of such kind is simply to keep adding water to the well. So, if necessary, the emergency cooling of the reactor can be extended by periodically refilling the well for a time period much longer than a month, until other more radical repairing actions become possible.

The diagrams in Figures \(\PageIndex{3}\) and \(\PageIndex{4}\) are pretty simplified – in order to make it easier to explain the general principle on which the reactor cooling works. For much more detailed drawings (like, e.g., this one) and for a much more detailed description of the safety features of the reactors and their functioning, the Reader is referred to the Home Page of the NuScale Power Company, and to many “daughter pages” that can be accessed from this Home Page.