Nuclear Energy: Water-cooled reactors

Welcome the the second installment of my series about nuclear reactor types under active development. This time, we will look at anything that is water-cooled!

Currently deployed reactor systems

Water-cooled reactors are the dominant type of reactors for commercial applications. Normally, the distinction between light and heavy water-cooled reactors is made, but I will not follow that practice. The main reason for my utter disregard for common practices being that an article about heavy water moderated reactor designs would be too short and I’d like to mention them somewhere.

Pressurized Water Reactor’s lineage can be traced back to the early naval use of nuclear power. The commercial power reactors of today dwarf their predecessors and are generally capable of delivering well above 1 GWe. Once enriched uranium become commercially available and advances in material technologies enabled the forging of the massive reactor pressure vessels necessary for this type of reactor, the ability to use natural uranium and to do without a reactor pressure vessel by employing technically less demanding pressure tubes could apparently not balance the cost of the heavy water moderator.
The higher capacity factors of early heavy water reactors relative to their contemporary light water competition has also vanished. The US LWR fleet achieved a capacity factor 93%.
Today, several designs are offered on the market. The Russian VVER series seems to be dominant with active projects or even operational units in challenging environments like India, Bangladesh, Turkey, Belarus and Iran. South Korea has been building their APR1400 successfully domestically and also in the UAE. The EPR, a massive 1650 MWe reactor, has been built in China and is still under construction in France, Finland and the UK. Some politicians have called for using Corona stimulus money for building up to 6 units of this type in France and the UK.
Westinghouse has developed the AP1000 reactor, which has been built in China. In the US, Vogtle 3 and 4 are still under construction.
In India and China various nuclear power plants are under construction and various domestic designs are under development. India in particular has demonstrated a dedication to the construction of heavy water moderated power plants. There are plans for massive constructions of IPHWR-700.
The Canadian CANDU reactor are of course also heavy water-cooled reactor systems that are deployed in China, South Korea, Romania and Argentina.
Some water-cooled, graphite moderated reactors of the RMBK-type are still operational in Russia. The largest reactors of the type had a capacity of 1500MWe and held the record until the EPR was constructed. Power reactors of this design have not been constructed outside of the former Soviet Union.
Boiling Water Reactors represent a smaller part of the active US nuclear fleet and have also operated in Japan, Taiwan, Switzerland, Sweden, Mexico and Germany. New large boiling water reactors of the gigawatt scale, like General Electrics Economic Simplified Boiling Water Reactor have not been built for quite sometime, although one of these reactors had been planned as a third block in the nuclear power plant “North Anna”.

A change of strategy

Almost all construction projects of reactors of these sizes have resulted in cost overruns and delays. More tightly controlled economies and tightly integrated, often government assisted companies seem to have relative advantages in the construction of these projects. They are vertically integrated, sometimes from the level of mining to construction services, with research and engineering companies in between.

Western, more market-driven companies with their reliance on contractors and anti-trust laws against a too deep vertical integration, seem to have trouble competing on these terms internationally. It is my understanding that this is one of the main drivers for a new trend in the world of water-cooled reactors: going small.
While counter-intuitive at first glance, this drive hopes to shift a lot of the work from the construction site to manufacturing factories. This should enable the mass production and quality control of modules, which are then send to the construction site to be assembled.
The idea is that this enables are steeper learning curve, because more, smaller units are built. It is also supposed to reduce the financial risks for utilities, because smaller units cost less in absolute terms. The smaller output is also seen as a way to bring nuclear power to markets yet too small to integrate gigawatt scale reactors.
Going small is also reducing the absolute (but not relative per unit of power) amount of nuclear material in the core, which enables passive modes of cooling, sometimes also during normal operation, reducing or even eliminating the need for costly active safety systems, which have to be redundantly installed to reduce the hypothetical core damage frequencies low enough to be licensed.
It remains to be seen, if the “horizontal scaling”, which worked so well in the cloud computing and solar energy context, can be also applied to the nuclear and energy space. Given current experience of thermal energy systems, the “vertical scaling” of going larger has clearly dominated. I think the answer to this question is not definitely known and wish companies pursuing this approach the best of luck with their endeavors.

NuScale

NuScale is developing a 60MWe integrated pressurized reactor. Up to 12 modules can be deployed in a shared pool building. This contains enough water to act as a heat sink, if for whatever reason heat could not be removed from individual modules. The size of the pool is designed to provide enough cooling, until the residual heat has subsided to levels low enough for indefinite air cooling.

The modules are designed to be fueled with standard LEU. There is no reactor coolant pump. The coolant is moved by differences in densities induced by different temperatures.

NuScale has obtained the first ever SMR design approval by the NRC. A 12-reactor power plant is planned to be realized in Idaho in the 2020s.

General Electric - Hitachi

General Electric – Hitachi: Is developing a 300 MWe boiling water reactor. The BWRX-300 uses some of the passive safety elements of the 1500 MWe ESBWR. The design will use up to 50% less building volume and also concrete per MWe and is supposed to cost 60% less capital cost per MW than other water-cooled SMRs. The idea behind this is that cost-analyses of previous BWR builds indicate that concrete is a major driver of construction cost. The size is designed to make the BWRX-300 a viable replacement for a lot of older coal power plants. It could also be an option for markets that could not afford a full sized nuclear power plant or could not integrate a full sized reactor into their energy infrastructure.
There is no reactor coolant pump. TitansOfNuclear had an interesting episode on this. A private company in Poland has signaled interest in this reactor.

Holtec

Holtec is developing the SMR-160. A lot of equipment has been bought and installed in Holtec’s headquarters, which is currently used for producing and handling fuel storage canisters. This supposed to do double duty for the mass manufacturing of the reactor. It’s a pressurized water reactor, fueld by standard GAIA fuel assemblies. The steam generator is directly welded to the reactor pressure vessel, rendering large LOCA events non-credible. There is no reactor coolant pump.

The off-set design enables access to safety relevant welds and enables refueling and maintenance without the necessity of disassembling an integrated heat exchanger.
The pressure boundary is a steel enclosure, which is embedded in a protective concrete building, which would absorb kinetic energy in the unlikely event of a direct attack on or aviation accident above the power plant.
The reactor pressure vessel is located below ground, as are spent fuel cooling pools.
All of the reactivity control is done by control rods, liquid neutron poisons are not employed, which is supposed to reduce corrosion issues to such a low level that 80 year service lives become possible.

Rolls-Royce

Rolls-Royce is developing a three loop, close-coupled Pressurized Water Reactor (PWR) that is supposed to deliver between 1200 and 1350 MWth, which wll result in approx. 400-450 MWe. It’s calles UK SMR. It is going to be fueled with standard LEU UO2 fuel. Three Reactor Coolant pumps and three vertical steam generators are used to extract heat from the reactor pressure vessel. The steam generators are positioned above the reactor pressure vessel to create a robust thermal driving head between the core and the steam generators for passive decay heat removal. Reactivity will be controlled by burnable poison in the fuel assemblies and control rod movement. Boron will not be present in the primary coolant loop. Modular construction, standardization and commoditization are supposed to decrease the costs of the reactor system significantly. The usage of well proven technologies minimizes the regulatory risk and creates a strong safety case. The stated target is a 500 day build, which also includes the modular buildings around the reactor

CANDU-SMR

SNC Lavalin is developing a 300 MWe version of their CANDU design. The released information is not very detailed at the moment, but it stands to reason that this design will feature all of the unique properties of the CANDU design, which are heavy-water moderator and coolant, a calandria, horizontally oriented independent pressure tubes and the possibility of online refueling. The ability to use natural uranium as fuel is of course dependent on the neutron economy of the whole reactor. It remains to be seen if the needed levels can be realized in this smaller design. The Indian IPHWR-220 systems are able to achieve this feat. The technology is quite mature, which could enable a shorter review process with the Canadian regulator and it’s claimed that the last 7 reactors were completed on budget and time.
The Canadian supply chain is well developed and cited as one of the major strengths of this design.

Open-100

Open-100 is an open-source project aimed at creating a 100MWe-sized PWR reactor. The principal goal is to enforce competition and commoditization to create a source of energy that is cheaper than fossil alternatives. The design relies on proven technologies and – despite its size- relies on reactor coolant pumps. It’s a project of the Energy Impact Center, which is also the parent of TitansOfNuclear.

SMART

The South Korean SMART reactor is a 100 MWe, 330MWth integrated reactor. It received the first-ever Standard Design Approval (SDA) from Korean regulatory body in 2012.  Major components such as pressurizer, steam generator, and reactor coolant pumps are contained in a single reactor pressure vessel, which is supposed to increase the safety case significantly.
Saudi-Arabia and South Korea have established a joint entity for the commercialization and construction of a SMART reactor in Saudi-Arabia.

Supercritical-water cooled

Water, when heated above 374°C and 22.1 MPa becomes supercritical. In this state, it behaves like a gas with the density of a liquid. It doesn’t go through a phase transition.
There are certain theoretical advantages to cooling reactors with water in this state.
In fact the Generation IV International Forum has selected the Supercritical-Water-Cooled Reactor (SCWR) as a candidate design. Canada , Europe and Russia have worked on designs, but it doesn’t seem like there is commercial interest in this type of reactors at the moment.

Summary

Water-cooled reactors are currently the type of reactor with by far the largest amount of experience. Some of their drawbacks, light rather low outlet temperatures, in general a reliance on enrichment facilities, high-pressure operations and poor fuel utilization have not stopped this technology from becoming a major provider of clean electricity around the globe.
It remains to be seen, whether the SMRs will be able to claim more than niches in not yet mature enough energy systems. Currently, it seems like developing countries trying to diversify their electricity generation systems with robust power generation are choosing the same route developed nations chose with respect to nuclear energy: building large, water-cooled reactors. Given the maturity of the nuclear supply chain for these reactors, it seems like a low-risk decision. Unfortunately, it doesn’t seem likely that this will advance the technology far beyond its current price tag.
But cheaper energy is what is needed, if climate change is supposed to be dealt with effectively and efficiently. Some proponents hope to create cost decreases by repeatedly building the same large design, like multiple EPRs in the UK or by using novel financing tools like regulated asset base to lower costs. These approaches need testing. Unfortunately, the size of such projects make failure quite costly.