Alex Goodwin wrote:
how would the dupic cycle alter the CANDU's void coefficient?
Definitely in a good way -- ie. reducing it significantly.
The details are spelled out nicely in Jeremy's FAQ post, referenced by NNadir:
NNadir wrote:
My first guess would be "add plutonium." (One can see more here:
Positive Void Coefficient, CANDUs.)
As stated in the linked text, in addition to Pu, an increase in U enrichment above natural level (NU = 0.71% U235) has a similar effect.
The U235 content of LWR SNF varies between 0.8% to 1.0% (by weight) -- this would end up in the DUPIC fuel, along with about 0.8% fissile Pu (or ~1.15% total Pu).
I would also add that there has been much interest in the potential capital-cost reduction by replacing the D2O coolant with H2O and by reducing the overall inventory of D2O (such as by using a smaller lattice pitch and overall calandria size).
This is much tougher to achieve, since the void coefficient with H2O coolant in the fuel channels can be a lot worse, than the classic CANDU design (as a numerical example, the void reactivity of a standard CANDU lattice, 28.575 cm pitch, with H2O PHT, is +70 mk, whereas going from NU to SEU only reduces that to about +67 mk; ....by contrast, reducing lattice pitch from 28 cm to 20 cm reduces void reactivity to ZERO, with the same fuel channels etc.).
From "Reactor Physics of NG CANDU" by Chan, P.S.W., Tsang K.T., Buss, D.B. (Proceedings of the Twenty Second Annual Conference of the Canadian Nuclear Society. Toronto, June 2001) :
Quote:
4.0 Comparison of NG CANDU with Other Reactor Designs
Figure 3 compares the NG CANDU reference lattice with the NU CANDU lattice, the
Japanese FUGEN reactor lattice, and the UK SGHWR lattice. The NG CANDU, the FUGEN
reactor and the SGHWR are all channel-type reactors that use H2O as coolant and D2O as
moderator. FUGEN and SGHWR are vertical reactors with boiling H2O coolant. NG CANDU
is a horizontal reactor with pressurized H2O coolant. All three reactor-designs suppress the
coolant-void reactivity by reducing the moderator-to-fuel volume ratio. Furthermore, all three
designs use relatively large calandria tubes to fine-tune this ratio.
FUGEN and the SGHWR use different methods to achieve slightly negative coolant-void
reactivity. The SGHWR used interstitial floodable moderator-displacing tubes to achieve a very
low moderator-to-fuel volume ratio at a relatively large lattice pitch of 26 cm. FUGEN uses
Mixed Oxide (MOX) fuel, with sufficient plutonium content to give a slightly negative
coolant-void reactivity at a lattice pitch of 24 cm.
In more general terms, an inherent reactivity characteristic of CANDU reactors is a positive void reactivity coefficient, which leads to a power increase following a large LOCA.
For comparison, light water reactors are susceptible to reactivity increases when the coolant density in the reactor increases. This effect can occur if a pipe breaks on the secondary side of the steam generator in the case of a pressurized water reactor, or if steam flow out of the piping is suddenly interrupted in the case of a boiling water reactor.
The discharged secondary side coolant removes heat and causes the primary reactor coolant temperature to decrease, resulting in a power increase due to the negative coolant temperature coefficient.
The rate of power increase is governed by the rate of coolant expulsion, which depends not only on the size of the pipe break but also on the assumptions regarding the closure of fast-acting main steam isolation valves and the shutoff of feed-water to the steam generators. Modern light water reactor analyses assume that both of these beneficial actions take place. If they did not, the speed of the calculated positive power transient would be significantly higher.
Another inherent characteristic of CANDU design is its longer neutron lifetime.
CANDU reactors have a small power coefficient. As a result they can be shut down easily and quickly by small changes of control absorber level.
In contrast, light water reactors have a large power coefficient and so require a large amount of control movement to render the reactor safe after a postulated initiating event (or PIE, as defined in IAEA Safety Standard NS-R-1)
The Doppler effect (decreased power change rate due to increased fuel temperature) is very important in limiting power changes if the prompt neutron lifetime is short as it is in a light water reactor, and less important if the prompt neutron lifetime is long as it is in the CANDU system, thereby allowing more time for control response.
Doppler feedback is the only effective means of controlling power rise in a light water reactor if reactivity exceeds the “cliff-edge” of prompt criticality. …..due to its long neutron lifetime, a CANDU reactor is not nearly as sensitive to this condition, since engineered control action is still effective within a limited range above prompt criticality.