2b. Energy, Decay Heat, Criticality

Question 1

Fission vs fissionable vs fissile

Answer 1

Fission - Heavy nucleus breaking down into two or more daughter nuclei

Fissionable - a nucleus is fissionable if it can be made to fission by the impact of a neutron

Fissile - nuclei which are not only fissionable but fissions under suitable circumstances and results in a chain reaction

Question 2

What are the typical energy released per fission event?
Ex. U-235 + 1n —> Mo-42 + Xe-54 + 4e + 4 V/ + 2n + gamma

Answer 2

n - 939.6 MeV
e - 0.5 MeV
U - 218,869 MeV
Mo - 91,169 MeV
Xe - 126,554 MeV

Net Energy = 204 MeV (won’t vary much)

Neutrons carry away several MeV
Beta decay electrons “”
Antineutrons from beta decay ~ 11MeV
Gamma ray photons several MeV
Daughter nuclei ~ 80%

Question 3

Heat release and Decay Heat, what percentage of decay heat is produced

Answer 3

93.5% of the heat generated is produced immediately from the initial fission event

6.5% results from beta decays after some delay - this is from the inventory of unstable nuclei produced from the fission reactions

After 1 hour, decay heat produces 1.9% of previous core power

10 days this falls to 0.5% (all double for PWR’s)

Question 4

Neutron multiplier, prompt criticality, delayed critical

Answer 4

Neutron multiplier, k - average number of neutrons produced by each fission
• k >=1 required for sustained fission (critical)

Prompt critical - if k = 1 without any contribution from delayed neutrons
Delayed critical (prompt sub-critical) - k < 1 without delayed neutrons, using delayed neutrons turns it critical

Fission reactions happen in picoseconds and K multiples the rate in seconds, hence a small increase in k means a large increase in reactivity.

Question 5

Reactivity, delayed neutrons, typical mNiles magnitudes, what makes a stable and controllable reactor?

Answer 5

Reactivity, rho = (k-1)/k
Reactivity in Niles = 100,000[(k-1)/k]
Prompt critical = 650miliNilies (runaway chain reaction)
Typical operating reactivity =~ 200mNiles

Delayed neutrons - makes a reactor controllable
Negative temperature coefficient of reactivity - makes a reactor stable

Question 6

Asymmetry of nuclear events, what is the cause and result this?, what are the highest yield daughter nuclei?

Answer 6

“Driving force for any physical or chemical reaction is the tendency to lower the potential energy and increase the stability of the system”

Elements with atomic numbers closest to the ‘magic numbers’ are most stable - 2,10,18,36,54 etc

Daughter nuclei typically comprise of significantly different atomic numbers - with the highest and lowest having the highest yield around Mass numbers 137 and 95 respectively.

Most common daughter nuclei - Xe-140 and Sr-94