Reactor Engineering Qn's
What type of fuel, with which degree of enrichment is used in
a)PWR
b)AGR
c)BWR
d)Candu
e)VVER
f)RBMK
g)LMFBR - correct answer-PWR - 4.4% U235
AGR - 3.78% U235
BWR - 2.5-3.5% U235
CANDU - Natural uranium ( 99.3% U238)
VVER - 2.4-4.4% U235
RBMK - natural uranium (99.3% U238)
LMFBR - Compatible with many nuclear fuels, depending on design
What are the moderator, primary and secondary coolant used in the following reactor types?
a)BWR
b)Candu
c)VVER
d)RBMK
e)LMFBR - correct answer-BWR - Moderator and primary coolant - water dosed with boric
acid
Candu - Moderator and primary coolant - Heavy Water (D20), secondary coolant light water
(H20)
VVER - Primary coolant - water, moderator - water, secondary coolant, water
RBMK - Primary coolant - light water, moderator - graphite, secondary coolant - water
LMFBR - primary coolant - liquid metal (often sodium) - moderator -none (as it is a fast
reactor), secondary coolant - water
What were the world's three most significant civil nuclear accidents? Do you understand why
they happened, what went wrong, what protection systems failed, how the operators
misunderstood the state of the plant, and what protection worked as intended, if any? -
correct answer-Chernobyl, Fukushima, Three Mile Island
What is the principle purpose of a moderator? - correct answer-to slow down, or thermalise
neutrons, increasing the probability of fission
,What is the principle fissile isotope used in AGRs and most PWRs? (Ignore PWRs burning
MOX). - correct answer-U-235
What proportion of natural uranium is fissile? - correct answer-0.711%
What degree of enrichment is usual in AGRs and PWRs - correct answer-PWR - 4.4% 235
AGR - 3.78% 235
Why do AGRs used stainless steel cladding rather than Zircaloy? - correct answer-Does not
produce explosive gases in AGR faults, Creep resistant, Good chemical stability even at
fault temperatures
What chemical threat is posed by LOCAs in respect of Zircaloy cladding in a steam
environment? - correct answer-Zircaloy can react chemically with hot water or steam to form
hydrogen, which is potentially explosive.
Is a good moderator likely to consist of light atoms or heavy atoms? - correct answer-Light
atoms, as they will slow the neutrons more, thermalizing them faster
Hydrogen has atomic mass 1 whereas carbon has atomic mass 12. Bearing in mind the
simple approximate formula keaft/kebef = ((M-m)/(M+m))^2 is water or graphite likely to be
the more effective moderator? - correct answer-water, as hydrogen has a much lower atomic
mass, hence it will reduce KE faster
PWRs have a more effective moderator (water) than AGRs (graphite). But what is the
principal drawback of using a water moderator in PWRs? - correct answer-Water absorbs
neutrons to form heavy water (D2O)
Reacts with zircalloy under severely high temps to form hydrogen
The moderator reduces the energy of the neutrons. Where does this energy go? - correct
answer-Into the moderator as kinetic energy, heating it
Why must the primary coolant temperature of a PWR operating at 155 barg be kept below
345oC? - correct answer-In order to avoid the coolant becoming supercritical, dramatically
changing it's behaviour
On the basis of the approximate formula keaft/kebef = ((M-m)/(M+m))^2 how many elastic
collisions of a neutron with U238 are required to reduce the neutron energy by the same
amount as a single collision with C12? - correct answer-C - KEaft/KEbef = 0.716
U - Keaft/KEbef = 0.983 = 0.017 fractional decrease
Therefore 42.12 collisions to reduce neutron energy by same amount, rounded to 43.
What are the typical primary coolant inlet and outlet temperatures, and pressure, for a PWR
and an AGR? - correct answer-AGR - inlet - roughly 300 degrees c
AGR - outlet - 650 degrees c
, PWR - inlet - 290 degrees
PWR - outlet - 324 degrees
A particular fission reaction, together with subsequent beta decays, is given by the overall
reaction equation,
U(235,90) + n(1,0) -> Mo(95,42)+Xe(136,54) + Ne(e^-1) +No(n(1,0)) + 4antielectron
neutrinos
State how many beta-particles (electrons) and how many neutrons are produced, i.e., what
are the numbers and ? - correct answer-4 electrons, 2 neutrons
Roughly how much energy, in MeV, is produced per fission of U-235, including the
subsequent beta decays to stable isotopes. - correct answer-200MeV
What is beta decay? Why is it of crucial importance in reactor safety provision? - correct
answer-A neutron decays to a proton, and a electron and anti electron neutrino is emitted.
Crucially important as it is responsible for a large amount of post-trip decay heat
Where is most of the heat produced in the reactor deposited? Where is most of the rest
deposited? - correct answer-Most deposited In the fuel
Most of the rest in the moderator
Does all the energy released by nuclear processes in the reactor end up as heat? If not, how
much does not and where does it go to? - correct answer-5% of the total nuclear energy,
from antineutrino's is lost to space, as they do not interact with any part of the reactor
From what nuclear processes does decay heat arise? - correct answer-decay of fission
products
In a U-235 fuelled reactor, what is the power due to decay heat as a proportion of the reactor
power in operation,
(i) Immediately after shut-down;
(ii) 1 minute after shut-down;
(iii) 1 hour after shutdown;
(iv) 24 hours after shut-down;
(v) 10 days after shut-down? - correct answer-i) 6.5%
ii) 5%
iii) 3%
iv) 2%
v) 1%
A reactor fuelled with 4% enriched U-235 with thermal power rating 1500 MWth was
shutdown 24 hours ago. What is the current rate of decay heat production? How long will it
take to reduce to half this level? - correct answer-15MWth
For a U-235 fuelled reactor, what is the average number of neutrons produced per fission?
What is the "neutron multiplier, k" and why is this different from the average number of
What type of fuel, with which degree of enrichment is used in
a)PWR
b)AGR
c)BWR
d)Candu
e)VVER
f)RBMK
g)LMFBR - correct answer-PWR - 4.4% U235
AGR - 3.78% U235
BWR - 2.5-3.5% U235
CANDU - Natural uranium ( 99.3% U238)
VVER - 2.4-4.4% U235
RBMK - natural uranium (99.3% U238)
LMFBR - Compatible with many nuclear fuels, depending on design
What are the moderator, primary and secondary coolant used in the following reactor types?
a)BWR
b)Candu
c)VVER
d)RBMK
e)LMFBR - correct answer-BWR - Moderator and primary coolant - water dosed with boric
acid
Candu - Moderator and primary coolant - Heavy Water (D20), secondary coolant light water
(H20)
VVER - Primary coolant - water, moderator - water, secondary coolant, water
RBMK - Primary coolant - light water, moderator - graphite, secondary coolant - water
LMFBR - primary coolant - liquid metal (often sodium) - moderator -none (as it is a fast
reactor), secondary coolant - water
What were the world's three most significant civil nuclear accidents? Do you understand why
they happened, what went wrong, what protection systems failed, how the operators
misunderstood the state of the plant, and what protection worked as intended, if any? -
correct answer-Chernobyl, Fukushima, Three Mile Island
What is the principle purpose of a moderator? - correct answer-to slow down, or thermalise
neutrons, increasing the probability of fission
,What is the principle fissile isotope used in AGRs and most PWRs? (Ignore PWRs burning
MOX). - correct answer-U-235
What proportion of natural uranium is fissile? - correct answer-0.711%
What degree of enrichment is usual in AGRs and PWRs - correct answer-PWR - 4.4% 235
AGR - 3.78% 235
Why do AGRs used stainless steel cladding rather than Zircaloy? - correct answer-Does not
produce explosive gases in AGR faults, Creep resistant, Good chemical stability even at
fault temperatures
What chemical threat is posed by LOCAs in respect of Zircaloy cladding in a steam
environment? - correct answer-Zircaloy can react chemically with hot water or steam to form
hydrogen, which is potentially explosive.
Is a good moderator likely to consist of light atoms or heavy atoms? - correct answer-Light
atoms, as they will slow the neutrons more, thermalizing them faster
Hydrogen has atomic mass 1 whereas carbon has atomic mass 12. Bearing in mind the
simple approximate formula keaft/kebef = ((M-m)/(M+m))^2 is water or graphite likely to be
the more effective moderator? - correct answer-water, as hydrogen has a much lower atomic
mass, hence it will reduce KE faster
PWRs have a more effective moderator (water) than AGRs (graphite). But what is the
principal drawback of using a water moderator in PWRs? - correct answer-Water absorbs
neutrons to form heavy water (D2O)
Reacts with zircalloy under severely high temps to form hydrogen
The moderator reduces the energy of the neutrons. Where does this energy go? - correct
answer-Into the moderator as kinetic energy, heating it
Why must the primary coolant temperature of a PWR operating at 155 barg be kept below
345oC? - correct answer-In order to avoid the coolant becoming supercritical, dramatically
changing it's behaviour
On the basis of the approximate formula keaft/kebef = ((M-m)/(M+m))^2 how many elastic
collisions of a neutron with U238 are required to reduce the neutron energy by the same
amount as a single collision with C12? - correct answer-C - KEaft/KEbef = 0.716
U - Keaft/KEbef = 0.983 = 0.017 fractional decrease
Therefore 42.12 collisions to reduce neutron energy by same amount, rounded to 43.
What are the typical primary coolant inlet and outlet temperatures, and pressure, for a PWR
and an AGR? - correct answer-AGR - inlet - roughly 300 degrees c
AGR - outlet - 650 degrees c
, PWR - inlet - 290 degrees
PWR - outlet - 324 degrees
A particular fission reaction, together with subsequent beta decays, is given by the overall
reaction equation,
U(235,90) + n(1,0) -> Mo(95,42)+Xe(136,54) + Ne(e^-1) +No(n(1,0)) + 4antielectron
neutrinos
State how many beta-particles (electrons) and how many neutrons are produced, i.e., what
are the numbers and ? - correct answer-4 electrons, 2 neutrons
Roughly how much energy, in MeV, is produced per fission of U-235, including the
subsequent beta decays to stable isotopes. - correct answer-200MeV
What is beta decay? Why is it of crucial importance in reactor safety provision? - correct
answer-A neutron decays to a proton, and a electron and anti electron neutrino is emitted.
Crucially important as it is responsible for a large amount of post-trip decay heat
Where is most of the heat produced in the reactor deposited? Where is most of the rest
deposited? - correct answer-Most deposited In the fuel
Most of the rest in the moderator
Does all the energy released by nuclear processes in the reactor end up as heat? If not, how
much does not and where does it go to? - correct answer-5% of the total nuclear energy,
from antineutrino's is lost to space, as they do not interact with any part of the reactor
From what nuclear processes does decay heat arise? - correct answer-decay of fission
products
In a U-235 fuelled reactor, what is the power due to decay heat as a proportion of the reactor
power in operation,
(i) Immediately after shut-down;
(ii) 1 minute after shut-down;
(iii) 1 hour after shutdown;
(iv) 24 hours after shut-down;
(v) 10 days after shut-down? - correct answer-i) 6.5%
ii) 5%
iii) 3%
iv) 2%
v) 1%
A reactor fuelled with 4% enriched U-235 with thermal power rating 1500 MWth was
shutdown 24 hours ago. What is the current rate of decay heat production? How long will it
take to reduce to half this level? - correct answer-15MWth
For a U-235 fuelled reactor, what is the average number of neutrons produced per fission?
What is the "neutron multiplier, k" and why is this different from the average number of
