Activities in this sector are more or less unofficially kept under wraps, and voluntary self-censored. 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 this is the problem of nuclear waste disposal, which 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 prospect. However, nuclear technology for power generation has undergone fundamental changes in the safety-oriented design, concept and operation of the plant. Modern nuclear power plants have fundamentally been reinvented, and made especially safe and disaster proof for normal operation without special safeguards.

Currently designed nuclear power plants must satisfy a number of critical fail-safe criteria:

It must be technically impossible to have a runaway chain reaction, irrespective of any accident or system failure, or any conceivable human carelessness in operation. This is a 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.

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.

Spent-fuel disposal must be simple and easy. It must not cause any radioactive problems in any manner in the future. The possibility of uranium coming in contact with, or being exposed to, the atmosphere must not exist.

The cost of generating electric power should be lower than that of convention fuels like coal, oil or gas.

A number of designs meeting these demanding safety criteria have been developed in various countries. Among them, the "pebble-bed" reactor is the most interesting and the 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 enclosure 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 is a four-layer shell, less than one millimeter in diameter.

During operation, neutrons pass through this enclosure casing shell, breaking up the uranium atom into 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 to but slightly larger than a billiard ball!

Each such pebble generates around 500 watts of heat when the reactor is fully operational. The essential safety feature is in the 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 remains 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!

The fuel balls, in turn, heat helium gas entering from 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 is mini-sized! A reactor generating upto 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; operated 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 (nine) percent uranium 235 and 91(ninety one) percent uranium 238. With such a 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 is 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.

The writer is Technical Adviser, Spectra Group.