RE: CANDU coolant reactivity feedback

[BERNARD L COHEN <blc+@PITT.EDU>]

	--I make the simplifying assumption that it is better for the

chain reaction if the neutrons hit deuterium or graphite which moderate

without absorbing, than if they hit ordinary water which absorbs while

moderating. If I oversimplify, I apologize.



On Thu, 14 Feb 2002, Franta, Jaroslav wrote:



>

> 	--To make things more clear, if a better moderator is available --

> graphite in RBMK and heavy water in CANDU -- ordinary water serves as a

> poison.

>

> ************************************************************************

> Its not always that simple.

> To quote from a CNS conference paper last year,

>

> <begin quote>

> Coolant-void reactivity is determined by the net result due to the loss of

> absorption (a positive

> reactivity change) and the loss of moderation (a negative reactivity

> change). Spatial and spectral

> changes of the neutron flux in the lattice cell due to voiding of the

> coolant, as well as the nuclide

> composition in the fuel, also affect the coolant-void reactivity to a lesser

> extent.

> ......

> The neutron spectrum "hardens" when the coolant is lost and the dips at low

> epithermal neutron

> energies become more pronounced. [there is] a general increase in the fast

> and epithermal

> neutron flux in the moderator region as a result of increased escape of fast

> neutrons from the

> voided channel, and an associated decrease in the thermal neutron flux due

> to the lack of moderation.

> [.....there is a] large resonance absorption cross section of 238 U between

> 1 eV and 1 keV, [and a] prominent broad resonance of 239 Pu fission cross

> section centred at 0.3 eV. The behaviours of these cross sections have a

> major influence on the coolant-void reactivity in the ..... lattice.

> .....

> Voiding of the H2O coolant removes its moderation effect, which renders the

> lattice

> even more under-moderated. The shift in neutron spectrum upon coolant

> voiding increases the

> absorption in 238 U in the resonance region. There is also a significant

> reduction of the fission-yield

> in 239 Pu due to the decrease in neutron flux near 0.3 eV. These negative

> reactivity effects

> reduce the overall coolant-void reactivity in the lattice cell that would be

> expected from the loss

> of substantial absorption in the H2O coolant.

> <end quote>

>

> Hope this helps.

>

> Jaro

>



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