Nuclear power plant myths

1) Uranium is running out

There is 600 times more uranium in the ground than gold and there is as much uranium as tin. There has been no major new uranium exploration for 20 years, but at current consumption levels, known uranium reserves are predicted to last for 85 years. Modern reactors can use thorium as a fuel and convert it into uranium  and there is three times more thorium in the ground than uranium.

Uranium is the only fuel which, when burnt, generates more fuel. In short, there is more than enough uranium, thorium and plutonium to supply the entire world’s electricity for several hundred years.

2) Nuclear is not a low-carbon option

During its whole life cycle, nuclear power releases three to six grams of carbon per kiloWatthour (GC kWh) of electricity produced, compared with three to 10 GC/kWh for wind turbines, 105 GC/kWh for natural gas and 228 GC/kWh for lignite (‘dirty’ coal).

3) Nuclear power is expensive

With all power generation technology, the cost of electricity depends upon the investment in construction , fuel, management and operation. Like wind, solar and hydroelectric dams, the principal costs of nuclear lie in construction. Acquisition of uranium accounts for only about 10 per cent of the price of total costs, so nuclear power is not as vulnerable to fluctuations in the price of fuel as gas and oil generation.

4) Reactors produce too much waste

Production of all the electricity consumed in a four-bedroom house for 70 years leaves about one teacup of high-level waste, and new nuclear build will not make any significant contribution to existing radioactive waste levels for 20-40 years.

5) Building reactors takes too long

The best construction schedules are achieved by the Canadian company AECL, which has built six new reactors since 1991, from the pouring of concrete to criticality (when the reactors come on-line), the longest build took six-and-a-half years and the shortest just over four years.

6) Leukemia rates are higher near reactors

Childhood leukemia rates are no higher near nuclear power plants than they are near organic farms. ‘Leukemia clusters’ are geographic areas where the rates of childhood leukemia appear to be higher than normal, but the definition is controversial because it ignores the fact that leukemia is actually several very different diseases with different causes. Men who work on nuclear submarines or in nuclear plants are no more likely to father children with leukaemia than workers in any other industry.

7) Reactors lead to weapons proliferation

More nuclear plants would actually reduce weapons proliferation. Atomic warheads make excellent reactor fuel; decommissioned warheads (containing greatly enriched uranium or plutonium) currently provide about 15 per cent of world nuclear fuel. Increased demand for reactor fuel would divert such warheads away from potential terrorists

8) Reactors are a terrorist target

Terrorists have already demonstrated that they prefer large, high visibility, soft targets with maximum human casualties rather than well-guarded, isolated, low-population targets. Any new generation of nuclear reactors  will be designed with even greater protection against attack than existing plants, and with ‘passive’ safety measures that work without human intervention or computer control.


reference: www.spiked-online.com/index.php?/site/article/4259/
reference: www.wna.org
reference: www.washingtonpost.com/wp-dyn/content/article/2009/03/20/AR2009032001781.html
reference: www.cleanenergyinsight.org/interesting/wednesday-fact-series-npps-dont-cause-cancer/

Nuclear safety systems

There are 3 primary objectives of Nuclear Safety Systems which are:

• to shutdown the reactor
• maintain it in a shutdown condition
• prevent the release of radioactive material during events and accidents

One of the safety system implemented is the Reactor Protection System. It is composed of systems which are designed to immediately terminate the nuclear reaction. While the reactor is operating, the nuclear reaction continues to produce heat and radiation. By breaking the chain reaction the source of heat can be eliminated and other systems can then be used to continue to cool down the core. All plants must have some kind of these systems.

Another important system is the Emergency Core Cooling System (ECCS). The system comprises a series of systems which are designed to safely shut down a nuclear reactor during accident conditions. These systems allow the plant to respond to a variety of accident conditions and at the same time creates redundancy so that the plant can still be shutdown even if one or more of the systems fails to function.

Under normal conditions, nuclear power plants receive power from off-site. However, during an accident a plant may lose access to this power supply and thus may be required to generate its own power to supply its emergency systems. These electrical systems usually consist of diesel generators and batteries.

There's a containment systems are designed to prevent the release of radioactive material into the environment. It includes fuel cladding, reactor vessel and primary and secondary containment.

In case of a radioactive release, most plants have a system designed to remove radiation from the air to reduce the effects of the radiation release on the employees and public. This system usually consists of containment ventilation and control room ventilation.

reference: www.ask.com/wiki/Emergency_Core_Cooling_System

Reactivity Coeffcients and the Chernobyl Incident

The reactor involved in the Chernobyl nuclear accident was a 3,200-MWt RBMK (High Power Channel-type Reactor), a boiling-water pressure-tube, graphite moderated power reactor that was developed and operated in the former Soviet Union.

 

The Chernobyl Nuclear Power Plant.

Because of the pressure-tube design using water as coolant within channels in the graphite moderator, RBMK reactors have a significant positive void coefficient of reactivity in which a reduction in the coolant density results in an increase in the system reactivity due to a reduction in neutron absorption by the coolant. This reactor also has a positive moderator coefficient of reactivity in which the reactivity increases as the temperature of the moderator increases. Both of these operating characteristics are compensated by the negative temperature coefficient of the fuel which loses reactivity as the fuel temperature increases.

 Schematic diagram of an RBMK reactor.

At 00:28 on the day of the accident, the monitoring systems were adjusted to the lower power levels, but the operators failed to reprogram the computer to maintain power in the 700 to 1000 MWt range. The power fell to 30 MWt. The majority of the control rods were withdrawn to counteract the negative reactivity effect of xenon (fission product) poison which built up during the delay in power reduction. The power climbed and stabilized briefly at 200 MWt. At 01:03 All eight pumps were activated to ensure adequate cooling after the test.

The control room inside reactor 4 at Chernobyl

This violated two rules, one on high flow rate, the other protecting against pump cavitation. The resulting high flow rate increased heat transfer and thereby maximized coolant (neutron) absorption to require still more (prohibited) control rod withdrawal. It also maximized the reactivity increment available from the change in neutron absorption associated with coolant voiding. The combination of low power and high flos produced instability and required many manual adjustments. The operators turned off other emergency shutdown signals.

Engineers test a reactor's control panel at the Chernobyl nuclear power plant control room in Chernobyl

At 01:22 The computer indicated excess reactivity. Under pressure to complete the test, the operators reserved the possibility of rerunning the test by blocking the last remaining trip signal just before it would have shut down the reactor. "01:23 The test began. As power started to rise, coolant voiding increased and, through the positive reactivity feedback mechanism, led to accelerated power increase. Recognizing the potential consequences, the operators began insertion of all control rods.

 

Inside Chernobyl number 3 reactor unit

The power surged to 100 times the reactor's normal capacity in the next four seconds. A second pulse may have reached nearly 500 times full power and caused the fuel to disintegrate, breach the cladding and enter the water coolant. A steam explosion was caused by contact of the fragmented fuel with the water-steam coolant mixture.

The resulting force lifted the massive top shield, penetrated the concrete walls of the reactor building, and dispersed burning graphite and fuel. Oxidation of zirconium and graphite produced combustible hydrogen and carbon-monoxide gases that may have contributed to additional explosions. The initial excursion by itself was well beyond the containment design basis. It blew off the building roof and sent a plume of radioactive gases and particulates high into the atmosphere.

Aerial view of the damaged core. Roof of the turbine hall is damaged 

Short documentary on the Chernobyl Accident

reference: http://en.wikipedia.org/wiki/RBMK
reference: https://hps.org/publicinformation/ate/q1743.html
reference: http://en.wikipedia.org/wiki/Chernobyl_disaster 

Reactivity

Reactivity is the measure of the departure of a reactor from criticality. In the nuclear reactor, the control rods are being used to control reactivity.

For example, we take Uranium-235 that is being used as fuel in a nuclear reactor. When the nucleus of U-235 are struck by a slow-moving neutron, they will under go fission reaction. As by products of the reactions, it will release fragments, radiation, and neutrons. If these neutrons are slowed down  and hit another U-235 nucleus, that nucleus will also fission and  continues the chain reaction.

 Figure showing the fission of U-253 nucleus

For the chain reaction to be self-sustaining, each generation of fission events has to produce enough neutrons so that there are enough are left to cause just as many fission events in the next generation.

Positive reactivity causes power to rise exponentially proportional to the reactivity. Negative reactivity causes power to decrease. To change power in a planned manner, reactivity is adjusted by moving the control rods, either manually or by means of automatic controls. Partially removing a control rod is expected to increase the reactivity, causing the power to rise to a new level.

 

Figure above shows the effect of a relatively large initial reactivity leading to a rapid rise to a blowup.

The effective neutron multiplication factor, k, is the average number of neutrons from one fission that cause another fission. The value of k determines how a nuclear chain reaction proceeds: 

if k < 1 (subcriticality): The system cannot sustain a chain reaction, and any beginning of a chain reaction dies out over time.  

if k = 1 (criticality): Every fission causes an average of one more fission, leading to a fission (and power) level that is constant. Nuclear power plants operate with k = 1 unless the power level is being increased or decreased.

if k > 1 (supercriticality): The result is that the number of fission reactions increases exponentially.  

reference: www.wordiq.com/definition/Nuclear_chain_reaction 
reference: www.sizes.com/properties/reactivity