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Chernobyl catastrophe in 1986 in USSR and then the Three Mile Island near disaster in 1997 in USA, has cast a depressing shadow on the possibilities of electric power generation from nuclear energy. Since then the prospect of power generation from nuclear reactor has become more or less taboo! The nuclear power plants now running in EU; understandably maintain a very low-key profile. Activities in this sector are more or less unofficially kept under wraps and voluntary self-censor. Given this somewhat justified fear and well publicized fear and apprehension, most countries tend to give the prospect of nuclear power generation a wide berth! Added to it is the problem of nuclear waste, disposal; that needs various safeguards in handling and containment
Given this historical background, the possibility of having nuclear power stations in Bangladesh, naturally appears to be a dangerous, suspect and unproven approach. However nuclear technology for power generation has undergone fundamental change in the safety oriented design, concept and operation of the plant. Modern unclear power plants have fundamentally been reinvented, and made especially safe and disaster proof, for normal operation without special safeguards required. Currently designed nuclear power plants must satisfy a number of critical fail-safe criteria that are:
a) It must be technically impossible to have a runaway chain reaction irrespective of any accident, system failure or any conceivable human carelessness in operation. This is the critical and essential criterion of the new generation plants! Simply stated it should not produce mass radiation; irrespective of any operational error or negligence; the power plant must be inherently safe!
b) Nuclear power fuel for these plants is totally different from nuclear weapon grade fuel! It has to be impossible (repeat impossible) to use this fuel to make any kind of nuclear weapon!
c) Spent fuel disposal must be simple and easy. It must not leave any future radioactive problems in any manner! The possibility of uranium coming in contact or being exposed to atmosphere must not be possible.
d) The cost of generating electric power should be lower then that of convention fuels like coal, oil or gas.
A number of designs have been developed in various countries; meeting the above demanding safety criteria. Among these, the “pebble-bed” reactor is the most interesting and safest version. In this case, uranium fuel is enclosed in a minute spherical casing like a very tiny ball bearing around 0.03 inch in diameter! The enclosures of the sphere is ultra strong capable of sustaining very high pressures and temperatures; with no possibility of uranium dust spreading! Like the containment dome of traditional nuclear power stations; the containment zone of this nuclear enclosure of the sphere is a four layer shell, below one millimeter in diameter!
During operation, neutrons pass thong this enclosure casing shell, breaking up the uranium atom to two atoms of lower atomic weight and releasing heat. This fission reaction releases two or three neutrons that fly out of this shell and enter other shells. Thus the chain reaction begins; but the uranium always remains inside the individual enclosure shell. In a pebble-bed reactor around 15,000 such shells exist in a fuel ball (pebble), similar but slightly larger than a billiard ball! Each such “pebble” generates around 500 watt of heat when the reactor is fully operational. The essential safety feature is in this design of the small enclosure shells that make up the pebbles, so that the balls decrease neutron production once the reactor temperature reaches a maximum of 1600°C; without any human intervention. This ensures that the reactor is meltdown proof.
The nuclear reaction occurs in an underground pressure vessel, about 20 feet in diameter and 65 feet in height. Inside it are about 310,000 billiard ball-like fuel pebbles, and the same number of graphite balls to moderate the reaction. The reactor is continuously refueled by adding new balls at the top and removing the spent balls from the bottom. This process eliminates the costly shutdowns needed for recharging traditional nuclear reactors!
Fuel balls in turn heats helium gas entering at the top of the pressure vessel and leaving after passing through the “pebbles” at about 900°C. This volumetric expansion of the gas is transformed into rotating motion of the alternator to produce electricity. With very high exhaust temperatures; these power plants have higher electric power generation efficiency. The helium gas is used to turn the electric power generator turbine and the turbine exhaust gas (helium) also acts as coolant, eliminating the need of most of the auxiliary and support equipment, essential in a conventional nuclear power plant! These auxiliary equipment are usually the Achilles' heel of conventional nuclear power plants!
In pebble-bed reactors, uranium remains sealed inside the super strong shell and cannot leach out when spent balls are stored. These tiny shells are designed to last a million years! Comparing overall plant sizes; the “pebble-bed” reactor also mini-sized! A reactor generating up to 200 megawatts is about the size of a standard 40 feet shipping container! Large sized plants can have a number of such reactors in modular formation; operating from a common control room.
Future nuclear power plants will be totally isolated from the possibility of nuclear weapon production! The “pebble-bed” reactor fuel has only 9 percent uranium 235 and 91 percent uranium 238. With such composition it is impossible to make weapon grade nuclear material! Weapon grade uranium is highly enriched (or use plutonium). In essence the operation of such “pebble-bed” reactors are considered to be safer than driving a car! The safety and simplicity of operation of a “pebble-bed” reactor based power plant therefore offer a pragmatic, safe and economic power generation alternative for Bangladesh. It needs to be explored further as a potential option for mitigating the eclectic power shortfall in Bangladesh as a realistic and viable option.
S.A. Mansoor: Technical Adviser, Spectra Group |