In this article we will discuss about the classification and safety of nuclear reactors.
Classification of Nuclear Reactors:
Nuclear reactors may be classified in several ways i.e., on basis of their applications, type of fission, fuel used, state of fuel, fuel cycle, arrangement of fissile and fertile material, arrangement of fuel and moderator, moderator material, cooling system employed, coolant used etc.
I. According to the applications the reactors are classified as:
(a) Research and Development Reactors:
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These reactors are used for testing new reactor designs and research.
(b) Production:
These reactors are used for converting fertile materials into fissile materials.
(c) Power:
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These reactors are used for generation of electrical energy.
II. According to the type of fission the reactors are classified as fast reactors, slow reactors and intermediate reactors.
In fast reactors the fission is caused by fast neutrons whereas in thermal reactors it is caused by slow or thermal neutrons. Initially all the neutrons are fast when emitted in a reactor and in thermal reactor their speed is reduced with the help of moderator. For natural uranium, graphite moderated reactor, the ratio of moderator to fuel volume is between 50 and 80; for heavy water moderated reactor this ratio lies between 20 and 40 while for enriched uranium light water moderated reactor this ratio is between 1.5 and 2.5.
Thus the reactor core of a thermal reactor is very much larger than that of a fast reactor which has no moderator. Obviously heat generated per unit volume of the reactor core in a thermal reactor is very much less than that in a fast reactor with the result that the cooling problems are much simpler in a thermal reactor.
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In intermediate reactors, most of fission events are caused by neutrons in the process of slowing down. Such systems are very difficult to design because of the resonance cross- section structure in this energy range. Slightly enriched fuels are used in these reactors.
The main advantages and disadvantages of fast reactors are given below:
Advantages:
1. Fast reactors can convert more fertile material to fissile material with the result that the net fuel consumption for such reactors is much less. As a matter of fact more fissile material could be produced in a fast reactor than would be consumed by it.
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2. These reactors are small and compact and so easier to shield.
3. Since absorption cross sections are small, any structural material can be used for reactor core.
Disadvantages:
1. Heat transfer and cooling problems in the core are complicated due to high power density (thermal power to core volume ratio in kW/m3).
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2. High fuel loading requirement.
3. The core of a fast reactor requires high enrichment (above 10% of fissile material).
4. Radiation damage (embrittlement and swelling) of the structural materials in the core due to fast or high energy neutrons.
5. The fast neutrons having much shorter “neutron lifetimes” than thermal neutrons cause some control problems under certain conditions.
The advantages and disadvantages of thermal reactors are enumerated as below:
Advantages:
1. Ease of control because of relatively low power densities and longer neutron lifetimes.
2. Greater inherent safety.
3. Low fuel loading.
Disadvantages:
1. Very much restricted choice of fuel when uranium is used as fuel.
2. Higher size and weight of reactor per unit power due to low power density.
3. More fissile material consumption.
4. Requirements of small absorption cross section structural materials.
III. According to the type of fuel used the reactors may be classified as:
(a) Natural uranium.
(b) Enriched uranium.
(c) Plutonium.
IV. According to the state of fuel the reactors may be classified as:
(a) Solid.
(b) Liquid.
V. According to the fuel cycle the reactors may be classified as:
(a) Burner (Thermal) Reactor:
Such reactors are designed for generating heat only without any recovery of converted fertile material.
(b) Converter Reactor:
Such reactors convert fertile material into fissile material different from the one initially fed into the reactor core. We know that natural uranium as the nuclear fuel U-238 is converted into plutonium.
(c) Breeder Reactor:
Such reactors convert fertile material into a fissile material, which is similar to one initially supplied to the reactor core. A breeder reactor is also one in which the fertile material is converted into a fissile material at a rate higher than at which the fissile material is consumed.
VI. As per arrangement of fissile and fertile material the reactors may be classified as:
(a) One region (fissile and fertile material mixed).
(b) Two region (fissile and fertile material separate).
VII. According to the arrangement of fuel and moderator the reactors may be classified as:
(a) Homogeneous and
(b) Heterogeneous.
In homogeneous reactors, the nuclear fuel and the moderator represent a uniform mixture in the fluid form, including gases, liquids and slurries whereas in heterogeneous reactors, separate fuel slugs or rods are inserted in the moderator in some sort of regular arrangement forming a so called lattice.
In a homogeneous reactor, the mixture of nuclear fuel and moderator is circulated from the reactor to an external heat exchanger, then to a pump and back to the reactor. The major drawback of homogeneous reactor is that the fuel solution also contains the highly radioactive fission products. Any leakage or component failures in the primary-reactor coolant system are extremely difficult to repair because of the presence of these fission products.
Otherwise, a homogeneous reactor has some significant advantages such as excellent in core heat transfer because of generation of the fission energy in the fuel-coolant solution itself. Also, the reactor fuel can be added, removed and reprocessed during reactor operation without shutting it down. Most of the present reactors are heterogeneous type.
VIII. On the basis of moderator material used the reactors may be classified as:
(a) Heavy water,
(b) Graphite,
(c) Ordinary water,
(d) Beryllium, and
(e) Organic reactors.
The most commonly used moderator materials are graphite, ordinary or natural water and heavy water. Graphite has got higher atomic weight than water and, therefore, the reactors employing graphite as moderator will be very bulky. Natural water gives a small and compact reactor, but the reactor would have to be pressurized and use enriched fuel. With heavy water, ordinary natural fuel can be used but it is very expensive.
IX. On the basis of coolant used the reactors may be classified as:
(a) Gas
(b) Water
(c) Heavy water, and
(d) Liquid metal reactors.
X. On the basis of cooling system employed the reactors may be classified as:
(a) Direct and
(b) Indirect reactors.
In direct system of cooling, the fuel is in the liquid form and it acts as a coolant. It is circulated through the reactor core and a heat exchanger in which its heat is transferred to the circulating water to produce steam. In this system reactor may be either of the aqueous homogeneous type or liquid metal fuelled type and the heat exchanger should be located within the biological shield because the circulating liquid fuel is highly radioactive.
In indirect system of cooling, the coolant may be a gas, water (light or heavy), a liquid metal or an organic coolant. In this system of cooling, coolant is pumped through pipes.
Safety of Nuclear Reactors:
The reactor cannot explode like an atom bomb because of the following reasons:
1. In the atom bomb, the nuclear material is almost pure and highly fissionable. The interior mechanism of the bomb is designed so as to bring the material together instantly and compress into a dense mass. But in the power reactor, the nuclear fuel is always in the form of a chemical compound or alloy. Internal design of the power reactor is also such that it would be impossible for the fissionable material in the reactor core to be concentrated and held in place such that it could explode like a bomb.
2. Another inherent safeguard lies in the nature of fuel itself. Absorption of neutrons by U-238 is much larger at high temperature. So, in case of an occurrence of a sudden increase in reactivity, with a consequent increase in neutron flux and heat generation, the U-238 in the uranium alloy core immediately steals neutrons otherwise available for fissionable U-235 in the core and the fission rate falls rapidly. The self-sustaining reaction is thus no longer possible and the reactor automatically shut down, without any assistance from control rods or other devices. This effect is referred to as “Doppler effect or coefficient.”
Along with these natural safeguards, there are various sensitive and sophisticated systems that monitor and control the operation of the reactor.
Reactor Operating Faults Affecting Reactor Safety:
These faults may be:
i. Uncontrolled reactivity release.
ii. Loss of coolant or coolant flow.
iii. Local overheating leading to a fire in the core.
The “fail to safety” principle i.e., loss of power or malfunctioning of equipment results in its operating to a trip condition. The safety protection is provided in triplicate and is arranged to trip the reactor on a two out of three operating coincidences.
The following safety protections are provided:
(i) Fuel Element Temperature Trip – An acceptable trip setting is 40°C above the nominal maximum core temperature.
(ii) Outlet Duct Gas Temperature Trip – Thermocouples are used as the detecting elements.
(iii) Cooling Flow Failure Detection – Loss of coolant flow will result in because of failure of one or more circulators or closure of an inlet or outlet duct valve, the most probable cause of the former being loss of electric supply. The current in each circulator motor is monitored by means of a CT (current transformer) operating a relay. The loss of two out of eight blowers result in slight temperature rise, and therefore, failure of any two blowers is arranged to trip the reactor.
(iv) Rate of Change of Pressure Trips.
(v) Excess Flux Trip – Trip level is set at 20% above the operating level.
In case all the above mentioned accident prevention systems should all fail, there are a series of mechanical barriers between the reactor core and the outside world:
1. Fuel element cladding.
2. Reactor vessel.
3. Reactor shielding.
4. Containment shell.