Several earlier NUKEFACTs have dealt with the superiority of a unique Reactor Rate Diagram and its application in training. There is more to come on the Rate Diagram but for now we turn to an important new subject, namely deficiencies in test bank items.
Reactor Operator training challenges many students because it confronts them with subjects outside their previous learning experience, e.g. the subject of Reactor Theory. Then, if the subject matter is of poor quality and laced with flawed concepts, several of which have been identified in NUKEFACTs, the instructor must employ smoke and mirrors in the ClassRoom, and the student struggles to gain even a muddled understanding of the subject. Finally, as the icing on the cake, a license exam must be passed that may contain erroneous test items. Specific examples of erroneous test items are provided herein and in future NUKEFACTs. Use the comments and proposed resolution after each erroneous example to upgrade your own exam bank, as appropriate.
Question 1: A reactor is categorized as supercritical when keff is ________________ and neutron population is ________________ .
a. greater than 1, increasing *
b. less than 1, increasing
c. greater than 1, decreasing
d. less than 1, decreasing
Comment: The asterisk indicates the intended correct answer is choice "a". The question is flawed because the only criterion necessary for defining supercriticality is that keff be greater than 1. The added stipulation of increasing neutron population is not only irrelevant but incorrect. As illustrated in NUKEFACT #11, Example 11.1, the neutron population can decrease in a supercritical reactor if a negative rho-dot of sufficient magnitude exists, which means that keff is greater than one but decreasing with time. As written, both "a" and "c" are correct answers. The proposed resolution is to restate the question and choices as follows:
Question 1: (revised) A reactor is supercritical when:
a. keff is greater than 1.0000. *
b. power is increasing with time.
c. keff is less than 1.0000.
d. reactivity is 0.0000.
Question 2: With keff = 1.0000, the subcritical multiplication factor is:
a. 0.0000.
b. 0.5000.
c. 1.0000.
d. infinity. *
Comment: The asterisk indicates the intended correct answer is choice "d". The question is flawed because the subcritical multiplication factor referred to, -1/(keff - 1), is only valid for values of keff that are less than 1.0000 (NUKEFACT #3). The stipulated value, keff = 1.0000, is not less than 1.0000 and the condition is not subcritical. If the subcritical multiplication were infinite, the power level would also be infinite. The proposed resolution is to restate the question and choices as follows:
Question 2: (revised) With keff = 0.9999, subcritical multiplication of the non-fission source is by a factor of:
a. 0.0001.
b. 2000.
c. 5000.
d. 10000. *
Question 3: A reactor is at a power level of 64 megawatts thermal (Mwt) with a constant period of +87 seconds. If the power increase continues for 3.1 minutes, what is the final power?
a. 66 Mwt
b. 544 Mwt *
c. 600 Mwt
d. 1421 Mwt
Comment: The asterisk indicates the intended correct answer is choice "b". The question is flawed because the given initial power level, 64 Mwt, is above the point-of-adding-heat (POAH) for even a large 3000 Mwt unit. Typically, the POAH is between 0.1% and 1% of rated power, or between 3 Mwt and 30 Mwt for a large unit. Above the POAH the negative temperature coefficients of reactivity preclude a constant period. The proposed resolution is to reduce the initial power level so that the POAH is not exceeded. Changing Mwt to Kwt is one way to accomplish this. Restate the question and choices as follows:
Question 3: (revised) A reactor is at a power level of 64 kilowatts thermal (Kwt) with a constant period of +87 seconds. If the power increase continues for 3.1 minutes, what is the final power?
a. 66 Kwt
b. 544 Kwt *
c. 600 Kwt
d. 1421 Kwt
In these three examples, all taken from exam banks, we have a misdefinition of keff , a misapplication of subcritical multiplication, and a misuse of reactor rate, all fundamental concepts of reactor behavior. Is there any doubt as to why the students might be a little confused?
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. .... More examples in the next issue.