As the the protection of the low level of DNBR and the surveillance of the As the the protection of the low level of DNBR and the surveillance of the
LCO of the DNBR are based on the value of DNBR elaborated by the LCO of the DNBR are based on the value of DNBR elaborated by the
simplified algorithm, the threshold of automatic shutdown of the simplified algorithm, the threshold of automatic shutdown of the
reactor at low level DNBR and the threshold of the LCO with regard to reactor at low level DNBR and the threshold of the LCO with regard to
THERMAL-HYDRAULIC IN NUCLEAR REACTOR
taking into account the uncertainties related to the elaboration of the taking into account the uncertainties related to the elaboration of the
DNBR and the accuracy of the measurements.
DNBR and the accuracy of the measurements.
The boiling crisis (DNBR criteria been reached) is avoided by maintaining The boiling crisis (DNBR criteria been reached) is avoided by maintaining
the calculated values of DNBR in the operation under the thresholds.
the calculated values of DNBR in the operation under the thresholds.
The uncertainties could be different for the two systems because they are The uncertainties could be different for the two systems because they are
associated to the accuracy of the system to the relative operation associated to the accuracy of the system to the relative operation
conditions respectively to the latter.
conditions respectively to the latter.
To define the two thresholds, it is necessary to repartite the transients in To define the two thresholds, it is necessary to repartite the transients in
two classes:
two classes:
**The transients where the automatic shut-down of the reactor is effective The transients where the automatic shut-down of the reactor is effective (transients of type I):
(transients of type I):
They are characterized by the following conditions:
They are characterized by the following conditions:
The parameters affecting the DNBR during the transient are utilized in the The parameters affecting the DNBR during the transient are utilized in the
low level DNBR protection chain;
low level DNBR protection chain;
The evolution of parameters is sufficiently low to be correctly registered The evolution of parameters is sufficiently low to be correctly registered
by the low level DNBR protection chain.
by the low level DNBR protection chain.
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The automatic shut-down of the reactor at low level DNBR is initiated The automatic shut-down of the reactor at low level DNBR is initiated
each time that the DNBR has reach the safety DNBR criterion.
each time that the DNBR has reach the safety DNBR criterion.
The variable Y = DNBR/ DNBR(PS) is characterised by a mean value The variable Y = DNBR/ DNBR(PS) is characterised by a mean value
(m(Y,PS) and a deviation standard (σ(Y,PS).
(m(Y,PS) and a deviation standard (σ(Y,PS).
DNBR(PS) is the value of the predicted DNBR by the algorithm of the DNBR(PS) is the value of the predicted DNBR by the algorithm of the
protection system.
protection system.
The automatic shut-down of the reactor at low level DNBR is then The automatic shut-down of the reactor at low level DNBR is then
initiated at the calculated value of the DNBR by the protection system initiated at the calculated value of the DNBR by the protection system
reaches the threshold of the calculated DNBR(PS) as follow:
reaches the threshold of the calculated DNBR(PS) as follow:
DNBR(PS) = SC/ m(Y,PS)(1 – 1.645 V(Y,PS) DNBR(PS) = SC/ m(Y,PS)(1 – 1.645 V(Y,PS)
This means that the probability to avoid the boiling crisis is then 95%
This means that the probability to avoid the boiling crisis is then 95%
with a confidence of 95%.
with a confidence of 95%.
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* The transients on which the protection at low level DNBR is not effective
* The transients on which the protection at low level DNBR is not effective (transients type II):
(transients type II):
In this case, the automatic shut-down is obtained unically by utilizing the In this case, the automatic shut-down is obtained unically by utilizing the specific parameters (in several cases, only one parameter, per specific parameters (in several cases, only one parameter, per example low mass flow rate of the pump). Consequently, it is example low mass flow rate of the pump). Consequently, it is necessary to survey other parameters which are not taking into necessary to survey other parameters which are not taking into account in the elaboration of the automatic shut-down and to account in the elaboration of the automatic shut-down and to determine the threshold of the survey system of the limit of operation determine the threshold of the survey system of the limit of operation
conditions.
conditions.
The most critical point is associated to the power distribution and the The most critical point is associated to the power distribution and the
best method is the surveillance of the initial DNBR.
best method is the surveillance of the initial DNBR.
In normal operation, the value of the calculated DNBR by the algorithm In normal operation, the value of the calculated DNBR by the algorithm must be maintained above the surveillance threshold of the must be maintained above the surveillance threshold of the DNBR(LCO) in order to avoid the boiling crisis during the considered DNBR(LCO) in order to avoid the boiling crisis during the considered
transient.
transient.
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The most characteristic transient of such an accident is the total loss of The most characteristic transient of such an accident is the total loss of
the primary mass flow (4 pumps).
the primary mass flow (4 pumps).
To dimension the surveillance chain of LCO with regard of the DNBR for To dimension the surveillance chain of LCO with regard of the DNBR for
this type of transients, one calculates the maximal DNBR for all this type of transients, one calculates the maximal DNBR for all
transients of that category.
transients of that category.
The transient which lead to the maximal variation of DNBR permits the The transient which lead to the maximal variation of DNBR permits the
determination of the low threshold of DNBR of the surveillance of LCO determination of the low threshold of DNBR of the surveillance of LCO
(DNBR(LCO)).
(DNBR(LCO)).
DNBR (LCO) = [SC + (
DNBR (LCO) = [SC + (ΔDNBR)max]/[m(Y,SS) (1 – 1.645V(Y,SS)]ΔDNBR)max]/[m(Y,SS) (1 – 1.645V(Y,SS)]
Where m(Y,SS) and V(Y,SS) are the mean value and the standard Where m(Y,SS) and V(Y,SS) are the mean value and the standard
deviation of the vriable Y = DNBR/DNBR(SS) and DNBR(ss) is the value deviation of the vriable Y = DNBR/DNBR(SS) and DNBR(ss) is the value
of DNBR calculated by the surveillance system.
of DNBR calculated by the surveillance system.
In normal operation, the DNBR must be always above the low threshold In normal operation, the DNBR must be always above the low threshold DNBR (DNBR(LCO)), the DNBR criterion will be respected if that type DNBR (DNBR(LCO)), the DNBR criterion will be respected if that type
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