MAPLE reactors

The following info was sent by my friend Fred Schneider:

AECL (Atomic Energy of Canada Limited) began work on the MAPLE reactors in the mid-1990s. These are small “isotope production” reactors located at Chalk River Ontario (the Canadian nuclear research establishment which is run like a high-security military/industrial fortress).

The MAPLE reactors (1 and 2) use highly enriched weapons-grade uranium as target rods to produce molybdenum-90 (Mo-90) and other isotopes that are sold to customers all over the world by a private company, Nordion, in Kanata, just on the outskirts of Ottawa. The commercial use of weapons-grade uranium is widely regarded as a highly undesirable practice which should be terminated and proscribed due to weapons proliferation concerns.

MAPLE-1 was supposed to be in operation by 1999. Eight years later it is still not operational because of a critical safety feature which is behaving exactly opposite (that is, in the unsafe direction) to what AECL designers had promised. According to plans, any loss-of-coolant accident in a MAPLE reactor should automatically REDUCE the power in the reactor by slowing down the fission process.

Technically, this is called a “negative void coefficient of reactivity”. But instead, MAPLE-1 is showing a POSITIVE void coefficient of reactivity, which in plain English means that a loss of coolant leads to an INCREASE in the power level of the reactor.

This is a nasty feature for any reactor to have, because it means that one of the worst kinds of accident (loss of coolant) is made even worse.

A prolonged loss of coolant to the core of a reactor could provoke overheating of the nuclear fuel, leading to fuel damage — and in the worst case fuel melting — with consequent releases of highly radio-active gases and vapours (à la Three Mile Island or even Chernobyl, but on a much smaller scale because MAPLE-1 is a much smaller reactor).

The danger of fuel melting remains even after the reactor is shut down. But a positive void coefficient of reactivity makes this kind of nasty accident even nastier, because the extra power surge contributes in a big way to the overheating of the nuclear fuel. It is bad enough that all commercial CANDU reactors have this undesirable characteristic (positive void coefficient of reactivity), including the new ACR (Advanced CANDU reactor) designed but not yet built by AECL.

But the MAPLE reactors do not have the same elaborate containment systems as the CANDUs have, and all the safety analysis that has been carried out for the MAPLE reactors assumes exactly the opposite — a NEGATIVE void coefficient of reactivity. The 8-year delay is likely to become a 10-year or 12-year delay, or it may be a fatal design flaw and the reactors may never be licensed for commercial production.

The ACR uses light water (i.e. ordinary water) as a coolant instead of the expensive heavy water (i.e. deuterium oxide) used in all existing CANDU reactors, but it nevertheless has a positive void coefficient of reactivity like the older CANDUs. This nasty feature affects all reactors that use a “pressure-tube” design; it was an important contributing factor to the Chernobyl catastrophe over 20 years ago.

The only other power reactor that AECL ever designed to use light water as a coolant was the Gentilly-1 reactor at Bécancour in Québec. It was a technical and economic fiasco, operating for only about 180 days over a period of a few years, and never producing a single useful kilowatt of electricity for offsite use. Ironically, one might say that Gentilly-1 was the safest reactor AECL ever built, because it was so unstable that it automatically shut itself off every time it was started up. The reason? The positive void coefficient of reactivity.

Related article in the Ottawa Citizen, August 12, 2007:

Chalk River delays risk $3.7-billion medical isotope industry

A key safety feature in two nuclear reactors near Ottawa isn’t working despite years of attempted fixes, according to a new Canadian Nuclear Safety Commission report.

The resulting eight-year delay in putting the MAPLE reactors into commercial production for life-saving medical isotopes is potentially threatening Canada’s world dominance in the $3.7-billion global molecular imaging and radiotherapeutics market, led by Ottawa’s MDS Nordion.

It also risks Canada’s international reputation as a nuclear technology exporter at a time when concerns about greenhouse gases and oil and gas energy supplies are making nuclear power an attractive option once again.


Read more here.


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