In this post I will be providing a basic overview of the General Atomic fast neutron breeder reactor. There has been somewhat of a renaissance in Nuclear Engineering and the Energy Multipler (EM2) is a very interesting example of some of the new and innovative designs now in progress. In my view this is the most promising of these new design offerings.
Like all nuclear reactors EM2 generates electrical energy by producing heat. The EM2 utilizes the Brayton cycle, the same cycle used in gas turbines and turbojet engines. This allows for a significantly higher thermal efficiency than achievable in the normally used Rankine cycle. Below is a simplified diagram of this design.
The basic principle is simple. EM2 is a fast neutron nuclear reactor, hence has no moderator to slow down the neutron flux to thermal energies. This allows breeding of new fuel with a factor greater than one from the fertile elements, Uranium and Thorium. In the EM 2 design this capability allows the reactor to operate for 30 years without re fueling and without fuel reshuffling the fuel rods. This offers a very important cost saving feature of this design The general design is not that different than a fossil fuel gas turbine.
However, the role of combustors has been replaced by the nuclear core and unlike fossil fueled gas turbines we have a closed gas system. Hot gas (Helium) flows out the reactor into the turbine portion of a turbo engine. This gas is at 850 degrees C (1562 F) and a pressure of 13.1 Mpa (1900 PSI). The exhaust of the turbine flows into the tube side of the recuperator heat exchanger. The cooled gas flows into shell side of the precooler heat exchanger to get the Helium to a low enough temperature to allow efficient compression by the turbo engine compressor. The compressed gas flow out the compressor into the shell side of the recuperator heat exchanger recovering some of the energy lost to allow efficient compression and then into the inlet of the nuclear reactor.
If the EM2 is cited where there is a river, lake or ocean ultimate heat sink, it’s possible to extract additional useable energy by including an organic Rankine cycle. This cycle utilizes an organic liquid with a low enough boiling point to undergo a phase change at a fairly low temperature to drive an additional turbine. The nuclear reactor produces a design maximum thermal power of 500 MW, resulting in a 240 MWe output in the Brayton cycle generator and 25 MWe in the Rankine cycle generator giving an overall efficiency of 53%.
Both generators are Asynchronous permanent magnet three phase generators. This allows them to be high speed which is especially important for the Brayton cycle. These generators are coupled to the AC grid using Load Commutator Inverters. (LCI) These convert the high frequency generator output to DC and then invert it to grid compatible AC. (To digress, LCI inverters are somewhat different the inverters I posted on previously. They don’t use seg fire mode for wave shaping, they depend on the load to wave shape. They therefore are able to use SCR switches rather than IGFETS because they don’t need switching off capability. However, when LCI units are used to drive an electric motor they need to feed into a reactive generator to have a wave shaping source. Therefore they can only drive synchronous motors. However at startup they must implement a seg fire mode by turning the source side AC to DC converter on and off)
The Reactor Core
The EM2 core utilizes a fuel composed of Uranium Carbide which has a melting point of 2350 degrees C. ( 4262 F ) This fuel is clad with Silicone carbide which is physically stable up 2730 Degrees C ( 4946 F) This is far higher than current reactor designs. Also the cladding used can never be a source of hydrogen as is the case for light water reactors. The fuel is enriched to an average of 6.5 % (5.5-15%) with the lower enrichment at the edge of the fuel assembly. The core contains a large quantity of fertile isotope and EM2 can use both a Thorium and Uranium fuel cycle. If the fuel utilized is reprocessed spent fuel no uranium enrichment is required. EM2 has a 97% fuel utilization, equal or better than LFTR.
Reactor control utilizes control rods and rotating core barrels around the core made of 90 % enriched Boron 10- carbide which has a melting point of 2763 C. (5005 F). To insure maximum neutron economy the control rods are fully withdrawn during reactor operation and reactivity control rests solely with the core barrels.
In order to use core barrels the EM2 reactor is designed to require reflected neutrons to achieve criticality. This is accomplished by the relativity low enrichment for a fast neutron reactor. The core is surrounded by a Beryllium oxide reflector. The core barrel can rotate its Beryllium oxide surface or Boron carbide surface to control reactivity. The Beryllium Oxide has a melting temperature of 2100 C. (3812 F)
Achieving a 30 year refueling cycle is accomplished by having a large fertile Uranium or Thorium core load and a system to remove fission product gasses from the fuel during power operation. The Uranium Carbide fuel is porous to fission product gasses and the fuel rod design is annular, so fission product gasses can collect in the center void in the fuel rod to be removed by a Fission gas trapping system. This fission gas removal has two advantages, the removal of this gas prevents fuel swelling and deformation, a process that limits fuel life in other nuclear reactors and the fission gas isotopes have large neutron absorption cross sections. This improves the neutron economy helping to achieve a 30 year fuel load.
There is no source of energy that is 100 % risk free. Risk is the price we must pay to have the benefits of technology. The only reasonable question that can be asked is if the risk is low enough in proportion to the benefit. In my opinion, assuming the correctness of the current safety analysis and a demonstrated need for an energy source beside renewable energy sources the answer is yes for EM2.
Nuclear power plants present all the same risks as other sources of energy. But they also a present a special risk that is unique. This is the risk of a large scale radioactive material release to the environment. I hope to show that for EM 2 this risk is low enough to make this design an attractive option to generate power from nuclear energy.
There are two different types of events that cause a large scale release of radioactive material into the environment. These are a prompt criticality accident and a core meltdown due to loss of reactor cooling and the presence of decay heat.
Prompt Criticality accident
To understand this risk a little physics is needed.
To understand the nuclear process, a basic description of the neutron chain reaction is needed. Some perhaps unfamiliar terms will need to be introduced. These are;
Microscopic cross section, This parameter relates to the probability that a neutron will be absorbed by a given nucleus. It is not a constant, it can be affected in somewhat complex ways by the energy of the nucleus and the neutron. The units for this parameter is area.
Number density N, This parameter is the value of the density of a given micro entity. Its units are atoms/ unit volume.
Macroscopic cross section. This parameter relates the probability that a neutron will be absorbed by a given collection of atoms. Its value is given by Eq 1
With units of atom/unit distance. Flux (Neutron generally)
This is the produce of the number density and mean velocity. Its units are neutrons/Unit area- second.
Reaction rate R. This is given by; Eq 2
With units of interactions/ Unit volume – time interval Geometric Buckling
This parameter represents the geometry of the critical assembly, for our purposes we can think of it as the ratio of the effective surface area and volume of the critical assembly, though its calculation is somewhat complex for a nuclear reactor core.
The neutron multiplication factor.This is the ratio of the number of neutrons in one generation and the number of neutrons in the previous generation.
Reactivity. This is the parameter that defines the degree of departure from criticality and is given by; Eq 4
Reactor Period, The time it takes the reactor to change in power by a factor of e.
Delayed Neutron fraction
In the fission process, most neutrons used to support the chain reaction are released during the fission event. However, for a select group of fission products, additional neutrons are released during the beta decay process. This occurs because in some cases the neutron rich fission product is an excited state, which causes it to emit a neutron rather than a gamma photon as it falls back to a less excited state. In defining the overall neutron generation time, we must average over all the neutrons that are needed to sustain fission. These delayed neutrons make the neutron generation time long enough to allow control of the chain reaction. A reactor must be designed to avoid having so much reactivity that these delayed neutrons are not needed. Such a condition is called prompt critical and is fatal for almost any reactor. By summing over the sources of neutrons used to sustain the nuclear reaction we get the mean generation time Eq 5A &5B
Where are number of precursor atoms and are number of prompt neutrons per generation
A closely related term is delayed neutron fraction which is given by Eq 6
Except in cases of very rapid power changes these terms are essentially equal to each other and will be considered so here.
Effective Delayed fraction
Normally this value is somewhat higher than the delayed neutron fraction because delayed neutrons are born at a lower energy and therefore more likely to avoid escape or absorption by non-fuel materials. However, in a fast breeder there is very little neutron energy lost in the neutron cycle. So for fast breeder reactors the effective delayed neutron fraction tends to be smaller.
Power level of the reactor is given by: Eq 7
Where’ Eq 8
For all value of reactivity less than Here is the Delayed neutron precursor decay fraction, which is the ratio of precursor atoms that decay within a given time and all precursor atoms. This value ranges generally from 0.08 steady state, 0.1 for increasing power and 0.05 for decreasing power.
For values of reactivity greater than the equation is; Eq 9
Where the cycle time is equal to sec We can easily see the problem here. If we can somehow get enough positive reactivity the exponential rate of power increase becomes uncontrollable. This is what happened at the Chernobyl and SL1 accidents.
Nuclear reactors can explode. They can’t explode like a nuclear bomb, they disassemble too fast, but they can release enough energy to breech any containment. So how does EM2 stack up concerning this risk? Well it can never explode the way Chernobyl did. This is because when the reactor power is sufficient to increase the fuel temperature, which is called above the point of adding heat, the natural processes of the reactor adds negative reactivity, making prompt criticality impossible to achieve.
However, what about reactor operation below the point of adding heat, the situation that occurred at SL1. Here EM2 has no advantage over any other reactor and in fact except for the special case of the TRIGA reactor, no other nuclear reactor is immune from this risk. So how large a risk is this. Very small. This type of accident can only occur at reactor start up and during reactor start up highly redundant safety systems and rigorous procedures prevent such an event. I can go into more detail on this for those interested but I won’t here.
When a Nuclear reactor is shut down, the chain reaction is stopped, but the reactor core does not stop producing heat.
Decay Heat. The power level of a shutdown nuclear reactor can be calculated by summing over all the fission product isotopes decaying. Each isotope adds energy, with the shorter have life isotopes contributing the most energy. Eq 10
Summing over all the fission products we get; Immediately after Shut Down 6.5% total of the average power over the history of the reactor before shutdown.
One hour after shutdown 1.5% of average power before shutdown
One day after shutdown 0.4% of average power before shutdown
One week after shutdown 0.2% of average power before shutdown
After one week the fall off in decay heat is very slow, the spent fuel will produce just under 0.2% for years.
So how does EM2 cope with this issue? First we will look at the case where the load is lost (generator trip) and all the reactor shutdown systems fail. Given that these are highly redundant systems that need no power to operate this is very unlikely event. Under these conditions the reactor core would rapidly heat up. However, because the EM2 core can reach such a high temperature before any fuel damage occurs this design can utilize the change of core geometry to shut down the chain reaction. In this design a geometric reactivity factor becomes important. Looking at the core the probability of neutron escape is given by; Eq 11
Where is a constant based on core design that is less than one and is the mean free path of a neutron through the core. This is based on the geometry of the core and is given by approximately; Eq 12
Where V and A are volume and surface area. So that; Eq13
Giving us; Eq 14
This factor becomes small enough to terminate the chain reaction before the fuel temperature reaches its meltdown temperature. However, this doesn’t account for decay heat which does not shutdown when the chain reaction stops. Therefore the EM2 reactor incorporates a Direct Reactor Auxiliary Cooling System.
A very simplified diagram of DRACS is below.
The operation of this system is simple. For normal operation this is an active system using blowers and pumps to effect the cooling of the reactor core. (There are two redundant DRACS in the EM2 design) However upon complete loss of power, valves and dampers automatically position to allow the reactor to cool by a natural convection process. This is sufficient to prevent the core from exceeding its maximum design temperature due to decay heat. However, consideration of a loss of coolant accident, that is the rupture of the primary system must be considered. This is easily demonstrated by looking at the thermodynamic relationships in the cooling process. We see that approximately we can use the equation’ Eq 15
Where heat transfer rate, is the mass flow rate and are the Helium and Fuel temperature and is a constant based on the heat exchangers parameters. For natural circulation flow we have; Eq 16
Here P is coolant pressure. Therefore, a loss of pressure can reduce the coolant mass flow rate causing a higher fuel temperature. If the coolant pressure dropped to atmospheric from its operating pressure of 13.1 Mpa fuel temperature would exceed safe limits. This problem can be dealt with in two ways or a combination of these two ways. One is to limit the containment volume which limits the lowest pressure the coolant will reach on a primary system rupture. However, the external equipment installation makes this a challenge. Therefore this can be augmented by using an inert gas to pressurize the containment volume. This has the disadvantage that the reactor must be shut down and cooled before any maintenance can be performed inside the containment , but given the design of this reactor this should only occur rarely.
This has been a rather broad stroke description of EM2. But I think this gives some basic understanding of this design and its innovative features