Wednesday, 24 September 2008

OPAL's minor flaw

OPAL Reactor

A recent article quoted Greens Senator Scott Ludlum as calling for the 20 MW OPAL research reactor to be shutdown until the reflector leakage can be repaired.

Ludlum has called ANSTO's comments on the technical nature of the leak, "spin" and links today's technical concerns to potential safety concerns in the future.

The facts are that the OPAL core sits in the centre of the reflector vessel - but not within it. The fuel is not cooled by the reflector's heavy water, but instead by the significantly more massive quantity of regular [light] water sitting in the reactor pool. In the photo above the reflector is the circular tank in the centre [in a way resembling a large wheel of Swiss cheese]. The box in the centre of the reflector is the array of 16 fuel assemblies. The axial penetrations in the reflector [large and small holes that pass through the entire height in the vertical direction] support the generation of products such as neutron doped silicon or medical and industrial isotopes as well as the completion of irradiation experiments. Neutron beams [or rather their massive shutters] can be seen exiting the pool at the periphery of the reflector.

Power reactors are economic/business machines that receive compensation directly proportional to the electrons their turbines pump out onto the grid. Unlike a power reactor, a research reactor's lifeblood is neutron production. Heat in most cases is typically an unused byproduct [occasionally it is used for district heating]. The purpose of the reflector is to improve neutron economy.

As neutrons burst onto the scene from a 'split' U-235 atom [average is about 2.5 neutrons per thermal fission], they typically have too much energy to be useful. They must be 'slowed' or moderated. Think of a billiard ball flying down the table. Unless you're very brave, you would not try to catch it unless it was travelling relatively slow. It's similar with neutrons. They must be slowed so target material [U-235 atoms, silicon, neutron beam lines, etc.] can better use them. Also, if the neutrons are slowed within a minimum distance from the core there are increased odds that this neutron will travel back into the core to be used for the fission of fuel [i.e. they are reflected].

Heavy material [such as lead, steel or concrete] is used to shield radiation . But other materials such as graphite and water are much better neutron moderators. Heavy water is a better moderator than light water. With one more neutron in its neucleus the heavy water deuterium atom absorbes slightly more energy than a normal hydrogen atom per collission. [From above analogy, imagine slowing down our billiard ball with impacts from ping-pong balls or slightly heavier golf balls]. So to maximise the neutron economy in OPAL [provide the most usable neutrons per fission], heavy water is used to moderate the neutrons in the reflector. However, heavy water is very expensive.

The reflector is kept at a lower pressure than the reactor pool. Any leakage path will allow light water into the reflector. When this happens to a significant extent, some fraction of available neutrons will not be slowed enough to be used in the target material. Neutron efficiency will have decreased. As a research or isotope production machine - OPAL will become slightly less effective.

Therefore leakage into the reflector vessel has no safety consequence. There appears to be no grounds for Ludlum's "spin" accusation.

I believe OPAL staff are planning to construct a heavy water purification system to process a slip stream of the reflector circulation loop. This slip stream will then be purified to remove light water [this is possible, for example, because heavy water has a boiling point slightly above light water: 101.4 C].

The ANSTO response to Ludlum's claim may be found here.

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