Sources of radiation exposure from NNPP sources

Question 1

Type of fuel used in Nuclear Reactors

Answer 1

Most used Fissile isotopes are Uranium-235 and Plutonium-239

These fuels are in ceramic form

Question 2

Type of fuel used in Naval Nuclear Reactors

Answer 2

NNPP reactors require very-highly enriched Uranium 235. Navy Submarine >90%

Question 3

Types of radiation emitted during the fission process

Answer 3

thermal neutron is absorbed by U-235 nucleus and forms an excited nucleus U-236* (*) means excited

U-236* vibrates until fission resulting in:

release of gamma rays

release of 2-3 fast neutrons per fission

Question 4

Types of radiation emitted from fission can be classified as penetrating and non-penetrating radiation

Answer 4

Penetrating- can pass through the entire body causing ionizations and may escape the reactor core

ex. Neutrons and Gammas

Non-Penetrating radiation cannot pass through the entire body

ex. fission fragments

Question 5

2-3 Neutrons which are produced during fission may

Answer 5

Cause neutron activation of plant components inside the core and in the reactor compartment or beyond

escape to the atmosphere and interact with components external to the reactor or vessel

be captured by U-235 and result in U0236 with no fission

survive to cause fission in another U-235 atom

Question 6

Critical conditions of a nuclear reactor

Definition of criticality

Answer 6

Condition such that for each fission, at least one neutron survives to produce fission.

Question 7

Sub-critical reactor

Answer 7

(Neutron loses are greater than production)

Question 8

Critical Reactor

Answer 8

(Neutron losses equal production)

Question 9

Super-critical reactor

Answer 9

(Neutron losses are less than production)
(All reactors must go super critical to start up)

Question 10

Neutron losses can be caused by absorption of neutron by other core structures.

this results in activation of the core structures

Answer 10

238U (n,gamma) 239Pu in fuel filler
55Mn(n,gamma) 56Mn found in steel
59Co(n,gamma) 60Co found in cobalt-based alloys

Question 11

Purpose of control rods in a nuclear reactor

Answer 11

Control the rate of the fission chain reaction. Are constructed of metals that readily absorb neutron

examples- hafnium (Hf) : Cadmium (Cd) : Boron (B)

SCRAM- control rods moved rapidly down to automatically shut down reactor

Question 12

Purpose of water in the reactor vessel

Answer 12

Reflector- keeps neutrons in the core

coolant- removes heat from fission and radioactive decay

moderator- thermalizes neutrons to promote fission

shielding- attenuates radiation emitted from fission and radioactive decay

Question 13

Major nuclide that causes radiation exposure during reactor operations

Answer 13

N-16 gamma’s account for 80% of operational exposure

Fast Neutron- 10%

Gammas from neutron-activated components inside the reactor compartment- 10%

Question 14

Major Nuclide of concern during reactor maintenance

Answer 14

Co-60

due to:

Half-life 5.27y
decays in pipes outside the primary shielding
can be inhaled or ingested during maintenance

Question 15

Purpose of the primary shield

Answer 15

Primary shield surrounds the reactor vessel. Made of steel, lead and water. Water is treated with Potasium chromate to prevent and minimize corrosion

reduces- gamma/neutron radiation

limits neutron activation of other material inside the reactor compartment

limits gamma radiation in the reactor compartment after shutdowns so that personnel can enter if necessary

Question 16

Purpose of secondary shield

Answer 16

Limits personnel exposure to both Gamma and Neutron radiation from reactor and coolant.

Incorporated into the reactor compartment bulkheads and surrounds primary shield.

composed of steel, lead, borated polyethylene and virgin polyethylene.

On submarine- Normal Fuel Oil Tank (NFO)