1. Nuclear Power Reactors
Sources of Radiation
Nuclear Power Reactors
Day 4 – Lecture 3

2. Objective
To discuss about Nuclear Power Reactors including their Types and Basic Elements

3. Contents Types of Nuclear Reactors Components of a Nuclear Power Plant
PWRs
BWRs
CANDU
Advanced Nuclear Reactors
Components of a Nuclear Power Plant

4. The Beginning
Enrico Fermi led the team which produced the first sustained controlled nuclear chain reaction.

5. Fossil vs Nuclear

6. Nuclear Reactors Types of Nuclear Reactors: Light Water Reactors (LWR)
Heavy Water Reactors (HWR)
High-Temperature Gas-Cooled Reactors
Fast Neutron
Fast Breeder

7. Primordial Nuclides Nuclide Half-life Natural Activity 235U
7.04 x 108 yr
0.711% of all natural uranium
238U
4.47 x 109 yr
99.275% of all natural U; 0.5 to
4.7 ppm total U in common rocks
232Th
1.41 x 1010 yr
1.6 to 20 ppm in common rocks
Three very important naturally occurring terrestrial radionuclides include U-238, U-235, and Th They are actually parents of three long radioactive decay chains, which end in stable lead.
Some nuclides, like Th-232 have several members in their decay chains. You can roughly follow the chain starting with Th-232
Th > Ra-228--> Ac >Th > Ra >
Rn-220--> Po > Pb-212--> Bi > Po > Pb-208 (stable)
Some other primordial radionuclides are: V-50, Rb-87, Cd-113, In-115, Te-123, La-138, Ce-142, Nd-144, Sm-147, Gd-152, Hf-174, Lu-176, Re-187, Pt-190, Pt-192, and Bi-209.

8. Slow Neutron Interactions
Fission
1n U  fission products
available for more fission
A very important neutron interaction mechanism is fission, which is the basis for nuclear power reactors. More neutrons are relased in this reaction than are absorbed (the mean number of neutrons released per fission for U-235 is 2.5). This leads to a self-sustaining chain reaction or “critical mass.”
the mean number of neutrons released per fission for U-235 is 2.5). This leads to a self-sustaining chain reaction or “critical mass.”

9. Boiling Water (BWR) Nuclear Reactors
In a BWR the water is boiled by the core, turned to steam and that steam is used to drive the turbines which generates the electricity. The spent steam is cooled back to liquid and recycled through the core.

10. Pressurized Water (PWR)
Nuclear Reactors
In a PWR, water is heated in the core and converted to superheated steam. This is a closed system and is called the primary loop. This contaminated water/steam does not exit the containment. The heat from the steam in the primary loop is transferred to a separate water supply (the secondary loop) causing it to boil and turn to steam. This is done by using “steam generators” which have many small tubes inside. The steam from the primary loop travels through the tubes giving up heat to the water surrounding the tubes. The steam in this secondary loop is used to run the turbines to generate the electricity. In this way, the contaminated water supply is always maintained inside the containment unless of course the steam generator tubes leak causing cross contamination in the secondary loop. After passing through the turbines, the spent steam in the secondary loop is cooled back to water and run through the steam generators again.

11. Components of a Nuclear Plant
The next five slides display the main components of a Nuclear Power Plant:
Control Building
Containment Building
Turbine Building
Fuel Building
Diesel Generator Building
Auxiliary Building

12. Control Building
From this location, the operator controls the reactor.

13. Containment Building
This is the the location of the core and primary components including the steam generators if it is a PWR.

14. Turbine Building
This is where the steam is converted to electricity. In a PWR it is “clean” whereas in a BWR, the steam is contaminated since it is produced from water which has been in contact with the core. Thus the turbine floor in a BWR has elevated radiation levels.

15. Fuel Building
This is where the spent fuel is stored onsite in a pool.

16. Diesel Generator and Auxiliary Buildings
This is the location of the generators which supply emergency power and the other components which support the water/steam system.

17. Protective Barriers
The fuel pellets are protected by the fuel rod which is in turn protected by the reactor vessel which is in turn protected by the reactor containment. This affords 3 levels of “containment” for the radioactive material.

18. Steam Generator
For a PWR, heat from the steam in the primary loop is transferred to the secondary system via a heat exchange system. The primary steam (which may contain radioactive contamination) travels through “U” tubes similar to the ones pictured here (these are actually from a heat exchanger rather than a PWR steam generator). The tubes are immersed in clean water from the secondary loop which is then turned to steam. Since this device generates steam in the secondary loop it’s called a “Steam Generator”

19. Nuclear Reactors
Reactor building don’t always look the same. Many people in the US believe the cooling towers are the reactors. They are not. They provide cooling for the lake or river water which is used to condense the spent steam back to water in the closed system. They typically exhaust condensation from the cooling process. This condensation cloud is sometimes mistaken for leakage from the reactor.

20. Advanced Reactors The first advanced reactors now operating in Japan
Nine new nuclear reactor designs either approved or at advanced stages of planning
Incorporate safety improvements and are simpler to operate, inspect, maintain and repair
Advanced Reactors
Today's nuclear reactor technology is distinctly better than that represented by most of the world's operating plants, and the first advanced reactors are now in service in Japan.
Reactor suppliers in North America, Japan and Europe have nine new nuclear reactor designs either approved or at advanced stages of planning, and others at a research and development stage. These incorporate safety improvements including features which will allow operators more time to remedy safety problems and which will provide greater assurance regarding containment of radioactivity in all circumstances. New plants will also be simpler to operate, inspect, maintain and repair, thus increasing their overall reliability and economy.

21. Advanced Reactors The new generation of reactors have:
a standardised design to expedite licensing, reduce capital cost and reduce construction time
higher availability and longer operating life, will be economically competitive in a range of sizes, further reduce the possibility of core melt accidents
higher burn‑up to reduce fuel use and the amount of waste
The new generation reactors:
have a standardised design for each type to expedite licensing, reduce capital cost and reduce construction time,
-are simpler and more rugged in design, easier to operate and less vulnerable to operational upsets,
-have higher availability and longer operating life,
-will be economically competitive in a range of sizes,
-further reduce the possibility of core melt accidents,
-have higher burn‑up to reduce fuel use and the amount of waste.

22. Advanced Reactors
More 'passive' safety features which rely on gravity, natural convection to avoid accidents
Two broad categories:
Evolutionary - basically new models of existing, proven designs
Developmental - depart more significantly from today¹s plants and require more testing and verification before large‑scale deployment
The greatest departure from most designs now operating is that many new generation nuclear plants will have more 'passive' safety features which rely on gravity, natural convection, etc, requiring no active controls or operational intervention to avoid accidents in the event of malfunction.
The new designs fall into two broad categories: evolutionary and developmental. The evolutionary designs are those which are basically new models of existing, proven designs. The developmental designs depart more significantly from today¹s plants and require more testing and verification before large‑scale deployment.

23. CANDU Reactors CANDU stands for "Canada Deuterium Uranium“
It is a pressurized‑heavy‑water, natural‑uranium power reactor designed first in the late 1950s by a consortium of Canadian government and private industry
All power reactors in Canada are CANDU type
The CANDU designer is AECL (Atomic Energy of Canada Limited), a federal crown corporation

24. CANDU Reactors
The CANDU reactor uses natural uranium fuel and heavy water (D2O) as both moderator and coolant (the moderator and coolant are separate systems). It is refuelled at full‑power, a capability provided by the subdivision of the core into hundreds of separate pressure tubes.
Each pressure tube holds a single string of natural uranium fuel bundles (each bundle half a meter long and weighing about 20 kg) immersed in heavy‑water coolant, and can be thought of as one of many separate "mini‑pressure‑vessel reactors" ‑ highly subcritical of course. Surrounding each pressure tube a low‑pressure, low‑temperature moderator, also heavy water, fills the space between neighbouring pressure tubes.
The cylindrical tank containing the pressure tubes and moderator, called the "calandria", sits on its side. Thus, the CANDU core is horizontal.

25. CANDU Reactors
In the CANDU design, as with the PWR design, the heat of fission is transferred, via a primary water coolant, to a secondary water system. The two water systems "meet" in a bank of steam generators, where the heat from the first system causes the second system (at lower pressure) to boil. This steam is then dried (liquid droplets removed, since they can damage turbine blades) and passed to a series of high‑pressure and low‑pressure steam turbines. The turbines are connected in series to an electrical generator. The primary water system, which becomes radioactive over time, does not leave the reactor's containment building.
It is a highly complex system from start to finish, involving a series of energy transformations with associated efficiencies. The potential energy of nuclear structure is converted first to heat via the fission process, then steam pressure, kinetic energy (of the turbine and generator), and ultimately to electrical energy
Fueling is accomplished by a fuelling machine which visits each end of the core, one fuelling and the other de‑fuelling, allowing operators to insert fresh fuel at alternate ends for neighbouring fuel channels. From six to ten bundles are "shuffled" each day.
Flux‑shaping is provided by fuel management. Long‑term reactivity control is also achieved through fuel management. Short‑term reactivity control is provided by controllable light‑water compartments, as well as absorber rods.
Thermalhydraulically, the core of most CANDU reactors is divided into two halves, with the divider line running vertically down the centre of the reactor face. Each half represents a separate coolant circuit. Heavy water coolant is supplied to the pressure tubes in each circuit via large headers at each end of the calandria, one pair of headers (inlet/outlet) for each half of the core. The subdivision of the core into two circuits, plus the fine subdivision into hundreds of interconnected pressure tubes, greatly reduces the effect of a potential LOCA (Loss‑of‑Coolant Accident).

26. High Temperature
Gas Cooled Reactors

27. High Temperature Gas Cooled Reactors
Building on the experience of several innovative reactors built in the 1960s and 70s, development is proceeding on new high‑temperature gas‑cooled reactors which will be capable of delivering high‑temperature (up to 950oC) helium either for industrial application or directly driving gas turbines for electricity.
The small High‑Temperature Test Reactor (HTTR) in Japan started up at the end of Its fuel is ceramic‑coated particles incorporated into hexagonal graphite blocks or 'prisms', giving it a high level of inherent safety.
China's HTR‑10 demonstration reactor started up in 2000 and has its fuel particles compacted with the graphite moderator into spherical balls, collectively known as a 'pebble bed'.
South Africa's Pebble Bed Modular Reactor (PBMR) is being developed by a consortium led by the utility Eskom, drawing on German expertise, and aiming for a step change in safety and economics. Modules with a direct‑cycle gas turbine generator will be of 110 MWe and thermal efficiency about 42‑50%. Fuel consists of billiard ball sized pebbles of graphite moderator each containing 1500 particles of 8% enriched UO2 and coated with silicon carbide. Up to 450,000 fuel pebbles recycle through the reactor continuously until they are expended, giving an average enrichment in the fuel load of 5‑6% and burn‑up of 80,000 MWday/t U. Each unit will finally discharge about 19 tonnes of spent pebbles per year to ventilated on‑site storage bins.
Construction cost (for clusters of 10 ‑ 14 units) is expected to be US$ 1000/kW and generating cost 1.6 US cents/kWh. Eskom and the South African Industries Development Corporation hold 55% the project, with BNFL 20% and Exelon (USA) 12.5%. A prototype is due to be built in 2002 for commercial operation in 2006.
A larger US design, the Gas Turbine ‑ Modular Helium Reactor (GT‑MHR), will be built as modules of 285 MWe each. It has prismatic fuel elements like the HTTR and will directly drive a gas turbine at almost 50% thermal efficiency. It is being developed by General Atomics in partnership with Russia's Minatom, and initially will be used to burn pure ex‑weapons plutonium in Russia. In 1996‑97 Framatome (France) and Fuji (Japan) joined the development consortium. The detailed design stage is complete and some componenet testing has started. Plant costs are expected to be less than US$ 1000/kW.

28. Pebble Bed Reactor
In the 1950s, Dr Rudolf Schulten ( 'father' of the pebble bed reactor) had an idea. The idea was to compact silicon carbide coated uranium granules into hard billiard-ball-like
graphite spheres to be used as fuel for a new high-temperature, helium-cooled type of reactor. The idea took root, and in due course, the AVR, a 15 MW (megawatt) demonstration pebble bed reactor, was built in Germany. It operated successfully for 21 years.

29. Pebble Bed Reactor
The pebble bed modular reactor (PBMR) is designed to skirt some of the biggest headaches of nuclear power: Pausing to refuel (which takes on average, about 40 days), complex piping and melting down. But the design does not address every objection to nuclear, and it raises some problems while solving others.
Small pebble bed reactors ran in Germany in the 1970s, and China has recently started one. A larger version is now being designed by South Africa's state utility, with investments from British Nuclear Fuels, owner of the reactor maker Westinghouse, and Exelon Corp., the largest U.S. operator of power reactors.
The design uses advances that appear to produce a small reactor that can be built cheaply and operated safely. Instead of the typical rod-shaped fuel, the fuel is formed into "pebbles" about the size of a tennis ball. Each pebble is made of grains of uranium sheathed in heat-resistant graphite and silicon carbide. The 100 million-watt reactor is supposed to use 310,000 fuel pebbles.

30. Pebble Bed Reactor Potential Problems (according to some groups)
It has no containment building
It uses flammable graphite as a moderator
It produces more high level nuclear wastes than current nuclear reactor designs
Although the arguments against current nuclear reactors have been addressed for some time, the PBMR is relatively new. The comments on this and the next slide are taken from a web site sponsored by an anti-nuclear group indicating that opposition is already forming to this new technology.
1. The lack of a containment building is a necessity because cooling is by natural convection. Also, a containment building would hinder the modular design - that is - no additional reactors could be added onto the plant after initial construction. This modular capability is what is so appealing to utilities because it requires less investment from the beginning.
Frankly, this single point is enough to conclude that this reactor design is unsafe. The United States has criticized Soviet reactor designs for not having containment buildings. It is the last line of defense for containing a radiological release.
Furthermore, the lack of a containment building leaves the reactor(s) wide open to a terrorist attack.
2. The uranium is covered by a layer of graphite. The graphite is covered by several other layers of materials including a silicon carbide. The graphite could burn if defects in the fuel defeat the outer coverings. The industry acknowledges that there is approximately 1 defect per pebble associated with these layers. There are approximately 370,000 pebbles in a pebble bed reactor. One tennis ball sized pebble comes out the bottom of the reactor every 30 seconds. It can be returned to the top of the reactor for additional use. The 1957 Windscale accident and the 1986 Chernobyl accident both involved burning graphite. The burning graphite dispersed radioactivity. At Chernobyl, the burning graphite released radiation for ten days.
3. Although the volume by "configuration for long term storage" is lower than current design, the actual amount of high level waste by weight is higher. The pebbles are less radioactive than conventional fuel assemblies and more pebbles are required to produce the needed heat inside the reactor. There will be many more truck and railroad transports needed to remove the wastes. This will increase the numbers of vehicle accidents and the odds of another radiological accident involving these vehicles traveling across the country.

31. Pebble Bed Reactor Potential Problems (according to some groups)
It relies heavily on nearly perfect fuel pebbles
It relies heavily upon fuel handling as the pebbles are cycled through the reactor
4. The industry acknowledges that "fuel pebble manufacturing defects are the most significant source of fission product release." Recent history shows that some companies have falsified fuel quality. In fact, there have been instances of fuel sabotage and tampering over the last few decades. Germany and Japan have shut down plants or refused fuel shipments once the problems were discovered. The industry can't produce "defect-free" fuel and therefore it is a certainty that a pebble bed reactor will experience an accident. The industry acknowledges that there is approximately 1 defect per pebble associated with these layers.
This shows an actual photograph of defective fuel pebble cross-section - nuclear industry reports admit the US had been unable to manufacture satisfactory pebbles
5. & 6. There was a pebble bed reactor accident at Hamm-Uentrop West Germany nine days after the Chernobyl accident. On May , a pebble became lodged in a feeder tube. Operators subsequently caused damage to the fuel during attempts to free the pebble. Radiation was released to the environs. The West German government closed down the research program because they found the reactor design unsafe.
There's already been an accident at a pebble bed reactor in Germany due to fuel handling problems

32. Where to Get More Information
Cember, H., Johnson, T. E, Introduction to Health Physics, 4th Edition, McGraw-Hill, New York (2009)
More information at: