Power supply to remote locations like Antarctica, Alaska, Ladakh, remote islands, strategic defence locations, remote mining sites, natural disaster site etc. have historically relied on diesel power which is quite expensive. If the supply chain of diesel is interrupted, there will be no electricity. With proper safeguards, 1 to 10 MWe mobile reactor system could provide robust, self-contained and long-term power. Heat pipe reactors are ideally suited for application in remote areas, because they have characteristics such as self-regulation and high reliability. These reactors cost about $ 25 – 30 million each. These small reactors are well suited for transportation to and installation at remote site where they can provide heat and electricity for years at a time without refuelling. They are capable of operating independently of external electricity grids that could vulnerable to threats, natural and otherwise. The obvious drawback is that each kWh of electricity to cost between 20 per cent – 70 per cent more than a kWh of electricity produced in the conventional way.
These nano nuclear generators are powered by standard nuclear fission that would generate the heat which is recovered with the help of heat pipes. As the Uranium inside the reactor breaks apart naturally, it generates heat and sends neutrons (tiny particles that exist in the nucleus of atoms) blasting out. If those neutrons hit other Uranium atoms they break apart as well, creating even more heat and more neutrons. Many modern nuclear facilities moderate the reaction with control rods that, when inserted into the nuclear fuel, slow down and moderate the reaction. Hence chain reaction cannot go faster than the designed rate.
Heat pipes are one of the most efficient ways to move heat from one point to another. These devices are sealed vessels that are evacuated and backfilled with a working fluid in small quantity. The pipes use a combination of evaporation and condensation of this working fluid to transfer heat in an extremely efficient way. Its cross section is generally cylindrical, with a wick on the inner diameter (Fig. 1 and 2). Cold working fluid moves through the wick from the colder side (condenser) to the hotter side (evaporator) where it vaporizes. This vapor then moves to the condenser’s heat sink, releasing its latent heat in the condenser, and then repeats the cycle to continuously remove heat from the part of the system. Capillary action moves the condensed liquid back to the evaporator through the wick structure. The temperature drop in the system is minimal due to the very high heat transfer coefficients during boiling and condensation. The flow circulation in the heat pipe will continue as long as there is large temperature difference between the evaporator and condenser sections. The fluid stops moving if there is no temperature difference. No power source other than heat is needed.
Design concept of the reactor
The basic system of a portable reactor is substantially different from other power reactor system. Basic features are:
• A typical reactor of 5 MW thermal will have ~ 2 MWe – i.e. electric.
• No water is used. It is heat pipe cooled.
• Low enriched ~ 19.75 per cent enriched UO2 fuel is used.
• Stainless steel monolith to contain UO2 pellets and heat pipes, is provided.
• No moving parts, valves, pumps or very high-pressure systems are necessary.
• Self-regulating law of physics in the core regulates the active control system.
• There is passive decay heat removal.
A typical portable reactor with 5 MW thermal capacity can generate around 2 MWe for 5 years with less than 2 per cent U235 depletion. As shown in Fig 3, the unique core design is built around a solid steel monolith of mass ~ 2.6 MT, with channels for both heat pipes and fuel pellets housed in 6 blocks of 60 deg. sector wedges. The heat pipes remove heat from the block as the potassium metal in liquid form inside the heat pipe is vaporized. For the ease of manufacture, the monolith core is fabricated in 6 identical segments forming a hexagon with two emergency shutdown control rods. There are about 352 fuel holes and 204 HP holes per block. This means there are total 3336 drilled holes serving all 6 sectors in a monolith core. Type 316 stainless steel is used for the construction of monolith structure, which contains 5.22 MT of uranium oxide (UO2) fuel pins and liquid metal potassium (K) suitable for operating heat pipes at 675 deg C. Corrosion is not a significant issue. Number of fuel rods in core are ~2112. Fuel rod height ~ 1.5 metres and fuel channel hole dia is ~ 14 mm. The monolith acts as a structural support for the fuel pellets and a containment barrier to fuel fission product gases. Solid monolithic core prevents voids in the core. This eliminates issues with positive reactivity being introduced by voids. There is no dedicated or conventional fuel cladding. The heat pipes remove the heat from the monolith core as the potassium liquid in the heat pipes is vaporized; no valves or pumps are necessary. The total potassium mass in all the heat pipes is about 123 kg and has no impact on reactivity. About 1,224 nos of heat pipes (dia. about 15.7 mm) will be required to transfer all the heat of core fission to a power conversion heat exchanger. The heat is subsequently transferred to the condenser region of the heat pipes. The condenser region can be sized to accommodate multiple heat exchangers, like one primary heat exchanger for power conversion and one or two additional heat exchangers for redundant decay heat removal. The reactor uses an alumina (Al2O3) neutron side reflector, with 12 embedded control drums that contain an arc of boron carbide (B4C) poison for reactivity control. The outer diameter of the Al2O3 reflector is 1.5 metres. Heat pipe reactors can be physically smaller than the other advanced reactor concepts. Enrichment of the fuel to nearly 20 per cent, the use of a fast neutron spectrum and the use of a highly reflected core allow for a very small reactor core and weight. Heat pipes will remove heat effectively including decay heat in any orientation and this characteristic is a must for safe transportation of the reactor.
Use of heat pipes addresses some of the most difficult reactor safety issues and reliability concerns, particularly loss of primary coolant. Heat pipes operate in a passive mode at relatively low pressures, less than an atmospheric pressure using simple law of physics like capillary action, boiling and condensation. No pumps are used, hence less chance of failure. The failure of multiple heat pipes is quite low. Each heat pipe contains a small amount of about 100 gm working fluid encased in a sealed pipe. The relatively uniform temperature distribution throughout the core and relatively small temperature drop from the fuel pin to heat pipe makes the heat transfer from the core without any problem, even if some heat pipes fail. The high thermal conductivity of the steel monolith ensures efficient heat conduction to the heat pipes. But it has to be ensured that the webbings between fuel and heat pipe channels have sufficient factor of safety. Decay heat can also be removed by the heat pipes with the decay heat exchanger during shut down or emergency condition. The core contains no moderating material. Exploded view of a portable nuclear station is shown in Fig 4.
Any transient power excursions would be mitigated quickly by the negative temperature feedback. The strong negative reactivity feedback, the use of control drums and the relatively high beta effective of U235 provide easy control of the reactor power under both normal and accident conditions. These reactors are self regulating and load following as they are controlled by thermal expansion and subsequent negative reactivity feedback. Thermal feedback will lower the reactor power if less heat is extracted by the power conversion system. This makes the system more tolerant to power conversion failure.
Heat pipe reactors operate at ambient pressure which eliminates issues with high pressure as might occur in a high-pressure system such as gas cooled reactor design. Depressurisation accidents are a major concern for high pressure system.
Most power producing reactors pass a fluid through the reactor core and then transfer the heat to a working fluid. As shown in Fig 5, primary heat exchanger is of shell and tube type built over the condenser end of heat pipes which is situated outside the core. Here at no stage the working fluid is in direct contact with the core. Hence less chance of contamination with radioactive material. Primary heat exchanger is used for heating the working fluid i.e. air that produces energy. Heat exchange is done by air convection over heat pipes. With this configuration the heat transfer to the working fluid (air) takes place outside the core. So, chance of contamination is less. Power conversion system uses open air Brayton cycle. Heat pipe length – evaporator in – core is 1.5 metres and condenser ex – core is 2.5 metres. HP total length is 4 metres. HP material is SS 316 and its thickness is ~ 2 mm. HP isothermal temp is 675 deg C, whereas monolith maximum temperature is 700 deg C.
A neutron reflector reflects neutron back into the fuel rod thereby making an otherwise subcritical mass of fissile material critical, or increase the amount of nuclear fission that a critical mass will undergo. It is made of alumina and installed like a ring round the monolith core. Reactivity control system consists of 6 to 12 numbers of rotable control drums with arc of boron carbide strategically embedded in the reflector region and operating in banks. It provides overall reactor power level and excess reactivity control during normal operation. It reduces the non-uniformity of the power distribution in the peripheral fuel assemblies and reduces neutron leakage. By reducing neutron leakage, the reflector increases reactivity of the core and reduces the amount of fuel necessary to maintain the reactor critical for a long period. The neutron reflectors also serve as a thermal and radiation shield of a reactor core.
Salient features are:
• Side reflector material – Alumina (Al2O3)
• Side reflector outer diameter – 1560 mm
• Side reflector radial thickness – 250 mm
• Radial reflector length – 2000 mm
• Mass of side reflector – 8.4 MT,
• Top and bottom reflector material – SS 316 + BeO (above and below fuel).
The best method to control and tame the wild nuclear power is the use of control drums. The control drum material should have heavy absorption capacity for neutrons and also should not start fission reaction despite the heavy absorption of neutrons.
Salient features are:
• Number of control drums – 6 to 12, located inside reflector
• Drum OD -250 mm
• Drum axial length – 2000 mm
• Control material is B4C with Boron – 10 enrichment of 90 per cent.
Emergency Control Rod
The reactor core includes shutdown systems consisting of two numbers of shutdown rod along with its driving mechanism. While there is a need to have rapid emergency shutdown they are independently and adequately activated.
Salient features are:
• Number of emergency control rods (of diameter 56 mm encased in a tube) are 2. They are located inside core hexagon volume.
• Control material is B4C with 90 per cent enriched Boron – 10.
• Length – 2,000 mm.
The aforesaid design concept might have the following weak points:
• There is no fuel cladding, inclusion of which might give additional safety.
• The maximum calculated thermal stress in the thin 1.75 mm steel monolith webbing between some fuel pin channels at ~ 700 deg C operating temp may cross the allowable limit of 29 MPa and may cause web failure.
• Single heat pipe failure may happen due to thermal stress and localised high temperature of monolith.
• Making drilling holes in the 1,500 mm long solid monolith block is very difficult. However, for ease of manufacturing, if the webbing thickness is increased, then there would be severe core reactivity penalty.
In order to plug these loop holes, a new improved design alternative – A has been envisaged and is under development. In this design concept – A, no stainless steel monolith would be used. Pre-fab HPs and pre-fab cladded fuel elements in hexagonal tubes as shown in Fig – 6 would be installed in a stainless-steel tank. There would be double tank containment. The improved fuel pin has a volume of 435 cc and can accommodate more UO2. HP cascade failure will also be reduced. HP with bigger diameter can be used. Configuration of heat pipe/ fuel rod assembly in design Alternative – A is shown Fig – 7.
Power Conversion Unit
The flowsheet of the power conversion process is shown in Fig – 8. For power generation optimal heat recuperated air Bryton cycle with compressor and turbine isentropic efficiencies of 90 per cent has been envisaged. The isentropic efficiency has a strong impact on thermal efficiency. The thermal efficiency drops to 34 per cent at 85 per cent isentropic efficiency compared to 40 per cent at 90 per cent isentropic efficiency. So, it is important to use turbines and compressors that have high isentropic efficiencies. From heat pipe heat exchanger hot air at 675 deg. C,237 kPa enters the turbine where it expands and does the work that generates electricity. After doing the work, exhaust air at 511 deg. C, 103 kPa from the turbine exhaust enters the recuperator where its residual heat is utilised for preheating the compressed air supplied by the air compressor to 486 deg. C, 246 kPa. It then enters the HP heat exchanger where it is heated to 675 deg. C by reactor heat and then again fed to the turbine for power generation.
Portable power reactor using heat pipes developed by Los Alamos National Laboratory, USA is passively safe in operation and it is self-contained. It provides readymade plug-and-go power supply. This route of energy production has potential applications at strategic defence locations, theatres of battle, emergency locations and remote communities living in the mountainous barren terrains. Another big advantage is that, being very light and small in size they can be easily transported by truck. Although cost of generation is higher than conventional mode, but there are so many added advantages. Unlike conventional nuclear power plants, it eliminates severe accident possibilities due to loss of forced-cooling plus the large number of in-coreheat pipes provide lot of safety margin in the event of a few heat pipe failures. It can provide round the clock electricity even under adverse weather conditions for at least 5 years. It is not susceptible to vagaries of fuel supply as in DG set. India’s mountainous regions have lot of barren lands as well as snow fed rivers where greenhouse can be constructed and its climate control and irrigation system can be powered by this system. As the production from greenhouses under controlled condition is ~ 8-10 times more than conventional farming it will bring prosperity to the local people.
(Image/diagram courtesy: Author)
Rathindra Nath Biswas,