An important issue bearing on the safety of today’s generation of CANDU reactors concerns the void reactivity as expressed by the CVR (Coefficient of Void Reactivity).
In light water reactors (LWR’s), most of the world’s operating reactors, the formation of coolant bubbles (voids) around the fuel can occur for example when the coolant gets too hot. This causes the nuclear chain reaction to shut down very rapidly. In effect, this is a negative feedback mechanism which is good because it achieves the desired effect of stopping the reaction in an anomalous situation. Negative is good in this context. This negative void reactivity is an inherent safety feature of light water reactors (LWR’s) because it comes from the basic physics of the reactor in contrast to an engineered system.
In CANDU reactors, the void reactivity is positive. In other words the chain reaction increases when bubbles form and the potential for a reactor runaway or Loss of Regulation Accident is present. A positive CVR is often associated with pressure tube reactors including the Soviet RBMK design that failed catastrophically in the Chernobyl accident of 1986. Combustion of the carbon moderator of that reactor was a major factor in the severity of the accident, whereas existing CANDU reactors have heavy water coolant.
The calculation of the void reactivity can be very complicated since for instance the CVR can change as the fuel is consumed. For a full technical explanation of the complex issues involved in the void reactivity and its role in CANDU and in the Chernobyl accident please refer to the book by Daniel Rozon, Introduction to Nuclear Kinetics, Polytechnique International Press, Montreal, 1998.
A positive void reactivity has always been a concern for CANDU’s. In today’s CANDU reactors the positive void reactivity is compensated by having two completely independent shut down systems so that the probability of a reactor runaway is acceptably low. This has so far proven to be an effective solution to the problem. In fact the CNSC’s current draft regulations state that reactors built in Canada must have the two shutdown systems. This incidentally would be a great difficulty for licensing LWR’s in Canada since they have only one shutdown system. However, their negative CVR might be counted as a second “shutdown system”. More about regulatory problems in future posts.
AECL’s design for the ACR-1000 predicts a negative CVR and AECL must convincingly demonstrate that this will in fact be achieved. The sign of the CVR for its MAPLE isotope production reactors was a continual bone of contention between AECL and the CNSC. AECL had designed MAPLE for a negative void reactivity but the reactors as built showed a slightly positive one. Intense efforts at AECL and at two US laboratories have not identified how this came about in the design computer codes.
Unless the void reactivity of MAPLE is fully and satisfactorily explained, AECL may have great difficulty in convincing the CNSC that the calculated CVR of its ACR-1000 is indeed negative. The MAPLE reactors are now abandoned but I would hope that AECL would continue to research the problems calculating the MAPLE void reactivity. To do otherwise would be to incur a large credibility deficit for the ACR-1000.