ACR-1000 – the AECL Design

Timing is everything – not only in comedy but also in life. I leave it to you to decide whether they are one and the same thing.

It’s unfortunate for AECL that the AP1000 and EPR are so far ahead of the ACR-1000 in their development. As noted in previous posts, EPR reactors are being built in Finland and France and construction of the first AP1000 design has just started in China with orders on the books from two US utilities.  It appears that the ACR–1000 design is not yet complete and the final design will not be ready for construction until 2012.

This timing means that there will be real construction and operating experience for both its competitors before construction even starts on the first ACR-1000.

This reactor will be heavy-water moderated and light-water cooled with a design power of 1,050 MW (electrical).  Precise details of the design are either not publicly available or haven’t been fixed yet.  For example, there is an optimisation process between the burnup (energy produced from a given quantity of fuel), CVR (void reactivity) and fuel enrichment (percentage of uranium in the fuel that is uranium-235).  In the past AECL has been reluctant about specifying an exact enrichment for ACR fuel.  I’ve heard 2.1% but I have also been told other numbers.

There are several design options that could be used to reduce costs and enhance performance compared to previous CANDU reactors. For example, it is likely that new fuel will be used. The CANFLEX fuel bundle has 43 fuel elements, instead of 37, and has been undergoing tests for several years. The elements have two different diameters and projections to improve the heat transfer into the reactor coolant. Using slightly-enriched uranium fuel will yield more power from each fuel channel giving a smaller reactor core and other reactor systems could also be scaled down. A smaller and simpler reactor should reduce maintenance and capital costs.

As noted in an earlier post, a CANDU 6 reactor requires 265 Mg (metric tons) of heavy water for its moderator and 192 Mg for its coolant, a total of 460 Mg per reactor. The ACR-1000 as currently envisaged will require 250 Mg, all for its moderator but none for its coolant which will be light water. This even though the ACR-1000 has significantly higher output power, 1050 MW (e), versus an average of 640 MW (e) for the CANDU 6. This again represents a substantial capital cost reduction.

However, like its CANDU predecessors, the ACR-1000 will require replacement of its pressure tubes after thirty years.  This would extend its life by another 30 years but still would require a major expenditure even if the projected one year time frame for refurbishment could be maintained.   

The ACR-1000 has the possibility to be an excellent next step in the evolution of the CANDU design. We will only know when the first ACR-1000 is built and operated.

Privately many familiar with the Canadian nuclear industry dismiss the ACR-1000 as a viable contender for Ontario’s new reactors simply because it is so far behind its competitors. I think that’s a shame on patriotic grounds even though Canadians generally don’t like flag waving. On the other hand, as a citizen of Ontario who fully expects a reliable and sufficient electricity supply I would be very nervous about staking the future on the success of the ACR-1000.      


Void Reactivity – a key issue for the ACR-1000

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.

Is there enough heavy water for more CANDU reactors? (Upated March 11, 2014)

Update: This is one of the most popular posts on this blog and so merits an update. Since 2008 Canada’s nuclear situation has radically changed. There are now no prospects for more domestic CANDUs and any more CANDU exports are also very doubtful. The Quebec reactor, Gentilly II, has been shut down and in addition to the two reactors already shutdown, the remaining six at Pickering will be taken out of service by about 2020. Therefore, lots of heavy water will be available in future but likely no more new CANDUs to use it.

Heavy water (deuterium oxide) is fundamental to CANDU reactors.  For example, a CANDU 6 reactor requires 265 Mg (metric tons) of heavy water for its moderator and 192 Mg for its heat transport system (coolant) making a total of 460 Mg per reactor. In comparison a Darlington reactor needs 592Mg. The Advanced CANDU Reactor (ACR-1000) will require 250 Mg all for its moderator but none for its coolant which will be light water.


Approximately 3Mg of heavy water are needed to make up the annual losses in operating CANDU reactors. For example, Ontario Power Generation (OPG) is committed to providing Bruce Power with 18 Mg to make up losses in the reactors it leases. The 18 Mg was based on the 6 reactors operating at the time of the agreement.  


The last heavy water production plant in Canada, at the then Ontario Hydro Bruce site, closed in 1997 and has since been decommissioned. A 1 Mg per year prototype plant in Hamilton Ontario operated for about two years ending in 2002. Since then there has been no heavy water production plants in Canada nor have plans for new plants been announced.


Heavy water may be available from sources outside Canada principally India which produces about 600 Mg per year mainly for its own heavy water reactors.


The financial statements in Atomic Energy of Canada Ltd (AECL) Annual Reports for the past few years list an asset of $300M as “heavy water inventory”, mostly in storage at a site in LaPrade, Quebec adjacent to the Gentilly reactors. If the price of heavy water is assumed to be $600/kg, a reasonable ballpark number, then the asset would consist of 500 Mg and $300/kg would give 1,000 Mg. However, these simple minded calculations are unlikely to be correct because of the eccentricities of government book keeping in terms of asset valuation as the next paragraph shows.


Notes to the AECL financial statements indicate that the $300M includes the 1,003 Mg belonging to AECL on loan to the Sudbury Neutrino Observatory (SNO), a physics experiment. Therefore, the heavy water price used to value the inventory must be $300/kg or less.


Paraphrasing a recent OPG document, OPG owns 14,440 Mg of heavy water, of which 13,440 Mg is radioactive, and 1,000 Mg is non-radioactive.  Most of the radioactive heavy water is in OPG (6,300 Mg) and Bruce Power (6,000) reactors; the other 1,140 Mg of radioactive heavy water are available in the closed down Pickering 2 and 3 reactors. With a provision of 500 Mg for make-up to operating reactors OPG also sells modest quantities from its stocks and leases some heavy water to AECL and other utilities.  


At this time the maximum potential for building ACR-1000 reactors in Canada would appear to be two in Ontario, two in Alberta and one in New Brunswick for a total of five.  It would appear that OPG’s existing stock of heavy water could cover the 1,250 Mg required, provided other arrangements for sale or lease don’t complicate the matter. However, it is doubtful that sufficient heavy water would be available to supply five Enhanced CANDU 6 reactors requiring 2,300 Mg.


The foregoing contains too much guess work and speculation to form a prudent basis on which to order heavy water reactors.


The proponents of CANDU reactors need to clarify the issue of heavy water supply by publicly quantifying the inventories they have on hand.