Drakkith said:
I think there's a fundamental concept missing from these explanations.
The basic reason why fuel rods don't heat up and melt themselves, and the reason that the fuel is useable in the first place is because certain materials undergo a Chain Reaction.
The reason that the fuel doesn't melt is that cooling is provided, i.e., there is an adequate flow of coolant such that the thermal energy is carried away from the fuel rods (core) and deposited elsewhere (turbines (~32-37%) and condensers (68-63%) for most steam plants) during normal operation.
This means that the fuel absorbs a neutron that has been emitted from a decaying atom, and this splits apart, releasing more neutrons to do the same thing. The core uses fuel rods and neutron moderators to control the reaction to avoid too much or too little. However, as the fuel is decaying and reacting, it is releasing the decay products. These build up inside the core during normal operation and can impede the chain reaction if they readily absorb neutrons themselves.
In a fission chain reaction, the fast or prompt neutrons are released from the fissioning nucleus. Those account for about 0.993 of neutrons born. The smaller fraction (~0.007) of 'delayed' neutrons actually do come from decay of certain fission products. It is the presence of delayed neutrons that allows for 'control' of the fission reaction rate. At criticality, k = 1 (k = k effective), the amount of neutrons produced from fission and decay is balanced by the neutrons absorbed and leaking from the system. When k = 1, the reactor power is constant. When k < 1, power decreases, and when k > 1, power increases. The difference, Δk = k-1, determines the rate at which power changes. When k-1 > 0.0065, or the fraction of delayed neutrons, the reaction is prompt critical and power can increase orders of magnitude in a matter of seconds. This situation (the basis of a reactivity insertion accident) is to be avoided.
Normal operation of the core results in an equilibrium where the decay products build up to the point where they are decaying as fast as they are being produced. (Some can absorb neutrons and turn into products that no longer affect the core as well)
More or less correct. Those nuclides with short half-lives, i.e., shorter than the operating time of the fuel, reach equilibrium. Nuclides such as Cs-137 and Sr-90 continue to accumulate with burnup (or time of operation).
The problem, like in japan recently, was that these decay products produce heat during their natural decay. Normally the reactor has cooling systems that remove this heat from both the fuel and the decay products and use it to power turbines. Even during a core shutdown the cooling system is active because of the need to control the decay heat. However, if the cooling system doesn't work, then the heat builds up to the point that the fuel starts to melt and bad things happen.
The problem at Fukushima was loss of cooling. The decay power is much lower than operating power, but cooling is still required.
A fuel rod on it's own does not have enough material in the right physical configuration to sustain a chain reaction. It most likely produces a small amount of heat, but not much.
The first statement is correct. Fuel rods are arranged in regular arrays that constitute fuel assemblies, and a collection of fuel assemblies constitutes the core. The core is arranged to allow for criticality, energy generation and cooling.
Each fuel rod produces a fair amount of heat. Each foot (30 cm) of fuel produces between 3 to 6 kW of thermal energy - on average. Some section of fuel operate at a power of up to 10 kW/ft in a 17x17 PWR fuel rod, or up to 14 kW/ft for a 10x10 BWR fuel rod. The maximum allowable linear heat generation rate is determined by design (enrichment, enrichment distribution, fuel rod geometry) and operation.
In a PWR the fuel rod surface temperature can be as high as 350°C, but usually slightly lower, and the surface of BWR fuel rod cladding about 290°C (saturated steam conditions). During operation, the surface of the ceramic fuel pellet is around 370-400°C, while the centerline temperature around 900-1300°C.
The water in contact with the fuel in the core is contaminated by the decay products. Each decay product is another element with its own half life. Some are very short, on the order of days, others are very long, on the order of years or hundreds of years or more. An isotope of Iodine is commonly produced with a halflife of somewhere around a week I believe. While very dangerous if ingested or inhaled, it decays fairly quickly, so if you can keep it isolated for a short period of time it will decay to a much less hazardous material.
The water in core is only contaminated with fission products if the fuel rod cladding is breached. Otherwise, the corrosion products that accumulate on the fuel become 'activated' by neutron absorption, and some small amount of hydrogen may become tritium.
To avoid leaking the radioactive waste products AND the fuel itself into the environment. Normally the material that is used to make fuel rods is in a very low concentration, so it does relatively little harm. However in a fuel rod it is highly concentrated and any leakage can result in a much higher than normal amount contaminating the environment.
Ideally, during operation, fuel does not fail, i.e., the cladding is not breached. However, fuel rods do occassionally fail. There are treatment systems to collect the fission products and hold them until they decay.
This page describes the fission reaction and the distribution of fission energy.
http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/u235chn.html
The part about Neutrons 12 should be Neutrinos 12.
