Radiochemistry

Question 1

Describe the two mechanisms of fission product releases to the reactor coolant.

Answer 1

• Tramp Uranium - uranium oxide imbedded in fuel cladding; zircaloy also contains 0.1-1.0 ppm uranium “impurity.”
• Cladding Defect - pinholes, cracks, etc. through which fuel generated fission products can leave the fuel and enter the reactor coolant system.

Question 2

Describe two methods for monitoring fuel cladding integrity during power operations.

Answer 2

• Gross activity - or gross beta-gamma activity; a sample is taken, degassed and counted at exactly 20 minutes after sampling. This eliminates contribution or variance caused by short half life isotopes.
• Iodine 131/133 ratio - Because of the difference in half life between iodine 131 and 133 a “normal” ratio will develop. If iodine 133 increases with respect to iodine 131, it is likely that a “fresh” fuel leaker exists.

Question 3

Define Dose Equivalent I-131.

Answer 3

• Concentration of iodine-131 equivalent to the mixture of all radioiodines present;
• Concentration that would produce a thyroid dose as if all iodines were Iodine-131.
• Conversion factors of iodines reflect their half life and volatility, etc.

Question 4

Define E-Bar and explain its relationship to the Technical Specifications gross activity limit.

Answer 4

The average energy of the sum of all beta and gamma emitters (in MEV) for isotopes with half lives greater than 15 minutes, excluding radioiodines. Technical Specifications limits the reactor coolant system gross activity to 100/Ebar MCi/g (T.S. 3.4.8).

Question 5

Define CRUD and explain the CRUD cycle.

Answer 5

• CRUD is/are the metal oxides deposited or suspended in the reactor coolant system.
• The six steps in the CRUD cycle are: 1) corrosion, 2) release, 3) deposition, 4) activation, 5) release,
6) deposition.
• CRUD can be released by power changes, pH changes or by rapid oxidation via air or hydrogen peroxide

Question 6

State the three major classifications of activation products, and list two examples of each.

Answer 6

 Activation of corrosion products: Co60, Ag110M

27Co59 + 0n1  γ + 21Co60 47Ag109 + 0n1γ + 47Ag110M

 Activation of water/water impurities: N16, Na24

8O16 + 0n1  1P1 + 7N16 11Na23 + 0n1  γ + 11Na24

 Activation resulting in tritium production

3Li6 + 0n1  24 + 1H3 5B10 + 0n1  2 24 + 1H3

Question 7

Describe the production of, and concerns associated with, Nitrogen-16.

Answer 7

8O16 + 0n1  1P1 + 7N16

N16 is a very high gamma emitter (6.13 MEV); it is the most abundant activation product and is the most limiting nuclide for shielding installation around the reactor coolant system. Fortunately it has a short (7.13 second) half life so it is not a concern after a reactor shutdown.

Question 8

Describe the hazards associated with tritium and the process responsible for the majority of the tritium in the reactor coolant.

Answer 8

5B10 + 0n1  1H3 + 224 produces about 80% of all tritium in the reactor coolant system. Tritium is not removed by filtration, ion exchange or evaporation. It is a low level beta emitter which becomes a radiological concern if it is inhaled, ingested or absorbed thru the skin. It cannot be detected by a whole body count, only by urinalysis. It has a biological half life of 8-14 days.