This NUKEFACT identifies more test bank item deficiencies.
Question 4: Reactor power increases from 5% to 20% in 3 minutes. The reactor period for this transient is:
a. +2 seconds.
b. +80 seconds.
c. +130 seconds. *
d. +300 seconds.
Comment: The asterisk indicates the intended correct answer is choice "c". This question is a variation on Question 3 in NUKEFACT #13, but is included again because so many versions of this error exist. As before, the question is flawed because the initial power level, 5%, is above the point-of-adding-heat (POAH). Typically, the POAH is between 0.1% and 1% of rated power on any plant. Above the POAH the negative temperature coefficients of reactivity preclude a constant period. The proposed resolution is to reduce the initial power level below the POAH. Changing Power Range notation, %, to Intermediate Range units or to Kwt is one way to accomplish this. Restate the question as follows:
Question 4: (revised) Reactor power increases from 5 Kwt to 20 Kwt in 3 minutes. If reactivity is constant during this interval, what is the reactor period for the transient?
a. +2 seconds
b. +80 seconds
c. +130 seconds *
d. +300 seconds
Question 5: Delayed neutrons have a dominant effect on reactor period (startup rate) because of the:
a. short delayed neutron generation time.
b. long delayed neutron generation time. *
c. short delayed neutron lifetime.
d. long delayed neutron lifetime.
Comment: The asterisk indicates the intended correct answer is choice "b". The question is flawed because of incorrect terminology. Delayed neutrons have no "generation time." A generation of neutrons includes all neutrons in the population, both the prompt and delayed. Prompt neutrons have an average lifetime of the order of 1x10-4 seconds from fission to fission. Delayed neutrons have an average lifetime of 12.5 seconds. Generation time is a weighted average of the lifetimes of the prompt and delayed neutrons, typically of the order of 0.08 seconds. The proposed resolution is to change the correct answer, as follows:
Question 5: (revised) Delayed neutrons have a dominant effect on reactor period (startup rate) because of the:
a. short delayed neutron generation time.
b. long delayed neutron generation time.
c. short delayed neutron lifetime.
d. long delayed neutron lifetime. *
Question 6: Keff is increased from 0.8300 to 0.9425 by withdrawal of control rods. The amount of reactivity added to the core is:
a. +0.1125 delta-k/k.
b. +0.1438 delta-k/k. *
c. +0.2048 delta-k/k.
d. +0.2658 delta-k/k.
Comment: The arrow indicates the intended correct answer is choice "b". The question is well stated but the answer choices are not in correct units. Both keff conditions have an associated reactivity value. Moving from one k-effective to another involves a reactivity change, i.e. an increment of reactivity, which is expressed as delta-rho = rhof - rhoi . Or, we can write delta-rho = delta(delta-k/k). Reactivity and reactivity change are two distinct entities. The proposed resolution is to restate the question and change the units on the answer choices to delta-rho or delta(delta-k/k), as follows:
Question 6: (revised) Keff is increased from 0.8300 to 0.9425 by withdrawal of control rods. The reactivity change is:
a. +0.1125 delta(delta-k/k).
b. +0.1438 delta(delta-k/k). *
c. +0.2048 delta(delta-k/k).
d. +0.2658 delta(delta-k/k).
Question 7: From an initial steady state power level of 15 Kwt, a control rod is inserted which results in a stable period of -80 seconds. Three minutes later, the power level is:
a. 1.6 Kwt. *
b. 5.5 Kwt.
c. 7.2 Kwt.
d. 9.7 Kwt
Comment: The arrow indicates the intended correct answer is choice "a". The question is flawed because it provides insufficient information for solution. Specifically, the value of the negative reactivity insertion must be known. This reactivity reduces source multiplication, which produces a power decrease that may well be greater than caused by the negative period. The proposed resolution is to restate the question, as follows:
Question 7: (revised) After completing control rod insertion from criticality, the power is 15 Kwt and the stable period of -80 seconds. Three minutes later, the power level is:
a. 1.6 Kwt. *
b. 5.5 Kwt.
c. 7.2 Kwt.
d. 9.7 Kwt.
Are these questions, or similar ones, in your exam bank? Don't be too sure that they aren't. These questions have been widely disseminated. Check your exam bank.